^3 --'^

WEED CONTROL

IN U.S. RICE PRODUCTION

Agriculture Handbook No. 497

C3

cy>

Agricultural Research Service

UNITED STATES DEPARTMENT OF AGRICULTURE

in cooperation with

Arkansas, California, and Texas Agricultural Experiment Stations
WEED CONTROL IN U.S. RICE PRODUCTION

ROY J. SMITH, JR., research agronomist

Agricultural Research Service, US, Department of Agriculture

Stuttgart, Arkansas

WAYNE T. FLINCHUM, assistant professor of weed science

Texas A&M University Agricultural Research and Extension Center

Beaumont, Texas

DON E. SEAMAN, specialist in agronomy

University of California Rice Experiment Station

Biggs, California

Agriculture Handbook No. 497

Agricultural Research Service

UNITED STATES DEPARTMENT OF AGRICULTURE

in cooperation with

Arkansas, California, and Texas Agricultural Experiment Stations
Th.s publication contains the results of research only. Mention of pesticides does not constitute a recommendation for use by the

U.S. Department of Agriculture, nor does ft imply that the pesticides are registered under the Federal Insecticide, Fungicide, and Rodenticide Act as amended. The use of trade names in this publication does not constitute a guarantee, warranty, or endorsement of the

products by the U.S. Department of Agriculture.

Caution: Pesticides can be injurious to humans, domestic animals, desirable plants, and fish or other wildlife-if they are not handled

or applied properly. Use all pesticides selectively and carefully. Follow recommended practices for the disposal of surplus pesticides and

pesticide containers.

FOLLOW THE LABEL

U.S. DEPARTMENT OF AGRICULTURE

This handbook supersedes Agriculture Handbook No. 292, "Weeds and Their Control in Rice Production,"

by R. J. Smith, Jr., and W. C. Shaw.

Washington, D.C.

For sale by the Superintendent of Documents, U.S. Government Printing Office

Washington, D.C. 20402 - Price $1.50

Stock No. 001-000-03656-4

Issued March 1977
CONTENTS

Page

THE WEED PROBLEM 1

Effect of Weed Competition on Rice 2

Rice Culture and Weed Problems 3

Development of Herbicides and Changes in Rice Culture and

Weed Control Practices 4

PRINCIPLES OF WEED CONTROL 7

Crop Rotation 9

Land Leveling and Levee Construction 9

Seedbed Preparation 10

Seeding 10

Water Management 11

Fertilizer Management 11

Cultivation 12

Herbicides 14

EQUIPMENT FOR WEED CONTROL 45

Hand Sprayers 45

Tractor Sprayers 45

Fixed-Wing Airplane Sprayers 46

Helicopter Sprayers 47

Calibration of Tractor and Aerial Sprayers 48

WEEDS THAT CAUSE ECONOMIC LOSSES IN RICE 49

green and blue-green algae, alligatorweed, arrowhead,

barnyardgrass, beakrush, broadleaf signalgrass, bulrush,

burhead, cattail, common waterplantain, dayflower, ducksalad,

eclipta, false pimpernel, fimbristylis, gooseweed, hemp sesbania,

horned pondweed, jointvetch, knotgrass, mexicanweed,

morningglory, naiad, panicum grasses, pondweed, red rice,

redstem, smartweed, spikerush, sprangletop, umbrellaplant,

waterhyssop, waterprimrose

SELECTED REFERENCES 74

iii
THE WEED PROBLEM

Weeds reduce the yield and quality of rice

in the United States by an estimated 15 percent

each year on approximately 2.5 million acres;

this loss is valued at about $165 million. The

cost of using herbicides to prevent greater

losses was about $55 million in 1974. In the

same year the cost of cultural practices (including rotations, land preparation, irrigation, and

fertilization), prorated to control weeds, was

estimated at $70 million. Thus, the total estimated direct losses from weeds and expenditures for their control are $290 million

annually.

Herbicide usage in rice has steadily increased

as effective herbicides have been developed.

About 81 percent of the commercial rice in

the United States was treated with one or more

herbicides in 1968, up from 78 percent in 1965

and 53 percent in 1962 {13Í, 136, 138) J Since

1968, herbicide usage in rice has continued to

increase to where an estimated 95 percent of

the acreage was treated in 1974. Custom aerial

applicators apply herbicides to 87 percent or

more of the rice acreage, while farmers apply

the remainder (138),

Effective weed control systems combine preventive, cultural, mechanical, chemical, and

biological methods. Nonchemical methods may

include some or all of the following practices:

planting weed-free seed, crop rotation, leveling

land, seedbed preparation, selecting the proper

seeding method, and managing water and

fertilizers properly. Chemical methods involve

the use of herbicide treatments that selectively

control weeds in rice when applied correctly.

The system that omits any one of these components is often inadequate.

In this handbook we emphasize the importance of combining various cultural and chemical practices to effectively control weeds. This

1 Italic numbers in parentheses refer to "S'elected

References/' page 74.

handbook is designed as a guide for research

scientists, extension specialists, county agents,

vocational agricultural teachers, and personnel

of private and public organizations concerned

with rice production, research, and education

in the United States and abroad.

Since the use of herbicides for weed control

in rice changes and since new herbicides or new

uses for older ones are continually being developed, it is important to obtain the latest

information on herbicides before using them.

All pesticide products must be registered by

the Environmental Protection Agency (EPA)

before .they can be legally used in the United

States. Use of nonregistered pesticides, or use

in violation of the directions on the label, is

illegal and punishable. Since the status of

chemical weed control may change rapidly, it

is important to keep in touch with the EPA,

the State experiment stations, the cooperative

extension services, or manufacturers of specific

products for up-to-date information.

Weeds compete with rice for light, nutrients,

space, water, and other growth requirements.

Experiments in the United States {107),

Australia (P), Japan {83), the Philippines

{30) y and Taiwan {18) vividly demonstrate

that weeds reduce grain yields and lower the

market value by reducing quality. Furthermore,

weeds increase the cost of production, harvesting, drying, and cleaning, and they increase

infestations of other pests by harboring insects

and diseases.

Conditions favorable for growing rice are

also favorable for the growth and reproduction

of many terrestrial, aquatic, and semiaquatic

weeds. Weeds in rice produce an abundance of

viable seed, and once these seed infest the soil

they are difficult to remove and may remain

viable in the soil for many years. The broadcast

seeding and drill seeding of rice reduce the

opportunity for cultivation after emergence to

remove weeds.
WEED CONTROL IN U.S. RICE PRODUCTION

EFFECT OF

WEED COMPETITION ON RICE

Field trials with barnyardgrass, hemp sesbania, northern jointvetch, and ducksalad—

among the most serious and widely distributed

weed pests in rice—have demonstrated that

weed competition substantially reduces rice

yields. In trials conducted in 1961 chiefly with

barnyardgrass (other weed grasses, sedge,

broadleaf, and aquatic weeds were present),

yields were reduced about 74 percent in Arkansas, 35 percent in Louisiana, 64 percent in

Mississippi, 48 percent in Texas, and 36 percent

in California (^-4).

Both the density of weeds in rice and the

duration of weed-rice competition affect rice

yields. In numerous field experiments with

various rice varieties, yields decreased with

increasing weed density and increasing duration of competition (barnyardgrass, tables 1

and 4; hemp sesbania, tables 2 and 5; northern

jointvetch, tables 3 and 6; ducksalad, table 7).

Even one barnyardgrass plant in 31 rice plants

per square foot reduced yields; moreover, the

thinner the stand of rice, the greater the loss

in yield (table 1).

Barnyardgrass and ducksalad compete more

with rice than hemp sesbania and northern

jointvetch during the early growing season

(tables 4-6) ; hemp sesbania and northern jointvetch compete most during the late season by

shading the crop.

The seed of such weeds as dayflower, hemp

sesbania, northern jointvetch, mexicanweed,

and red rice reduce the value of the grain by

lowering the market grade. All are difficult

to remove. Red rice, which belongs to the same

species as white rice, has a red bran coat.

When red and white rice grains are mixed, the

close milling required to remove the red coat

breaks many white rice kernels. Broken rice

has less value than whole milled rice. Moreover,

if the red bran coat is not entirely removed,

the milled rice is unattractive, and its market

value is reduced.

Seeds of hemp sesbania, northern jointvetch,

and morningglory are not permitted in foundation, registered, and certified rice seed. Red

rice seed is not permitted in foundation rice

TABLE 1.—Effect of barnyardgrass density on

yield of 'Bluebonnet 50' rice, Stuttgart,

Ark,, 1962-1968'^

1Plants/ Rough Yield per acre5 lost

square foot rice to grass competition

Rice Grass

yield

(lb/acre) Pounds Percent

Dollar

value2

3 0 4,460 ... ... ...

3 1 1,930 2,530 57 126

3 5 890 3,570 80 178

3 25 240 4,220 95 211

10 0 5,160 ...

10 1 3,110 2,050 40 102

10 5 1,760 3,400 66 170

10 25 580 4,580 89 229

31 0 5,580 ...

31 1 4,190 1,390 25 70

31 5 2,840 2,740 49 137

31 25

(5 pet)3 .

1,180 4,400 79 220

LSD .... 750 ... ... ...

1 Data taken from Smith (109). Barnyardgrass competed with rice all season.

2 Rough rice valued at $5/hundredweight.

3 LSD =least significant difference at level given.

TABLE 2.—Effect of hemp sesbania density on

yield of 'Bluebonnet 50', *Nato', and 'Northrose' rice, Stuttgart, Ark,, 1960-62^

Rough Yield per acre lost

Hemp sesbania rice to sesbania competition

plants/acre yield Dollar

(lb/acre) Pounds Percent valued

0 5,520 ... ... ...

5,445 4,970 550 10 28

10,890 4,690 830 15 42

21,780 4,030 1,490 27 74

43,560 3,310 2,210 40 110

LSD (5 pet) 3 .. .... 610 610 11 ...

LSD (1 pet) 3 .. .... 880 880 16 ...

1 Data taken from Smith (109), Hemp sesbania competed until rice was mature.

2 Rough rice valued at $5/hundredweight.

3 LSD =least significant difference at level given.

seed, and only a small quantity (0.5 to 1 seed

per pound) is allowed in registered and certified seed. The maximum quantity of all weed

seed that includes beakrush, dayflower, mexicanweed, and smartweed cannot exceed 0.05

percent in foundation and registered seed or 0.1

percent in certified. Specific standards are es-
THE WEED PROBLEM

TABLE 3.—Effect of northern jointvetch density on yield of 'Bluebonnet 50' and 'Northrose' rice, Stuttgart, Ark., 1963-6J^^

Rough Yield per acre ]iost to

Northern jointvetch rice jointvetch competition

plants/acre yield Dollar

(lb/acre) Pounds Percent value^

0 6,380 ... ... ...

5,445 6,120 260 4 13

10,890 5,930 450 7 22

21,780 5,680 700 11 35

43,560 5,170 1,210 19 60

LSD (5 pet)3 ... 510 510 8 ...

LSD (1 pet) 3 ... 760 760 12 ...

1 Data taken from Smith {109), Northern jointvetch

competed until rice was mature.

2 Rough rice valued at $5/hundredweight.

3 LSD = least significant difference at level given.

TABLE 4.—YieM of 'Bluebonnet 50' and 'Northrose' rice as influenced by duration of barnyardgrass competition, Stuttgart, Ark.,

1962-65^

Rough Yield per acre lost

Duration of rice to grass competition

competition yield DÔÏÏâ?

(lb/acre) Pounds Percent ^^^^^^

7-10 days 4,900

15-20 days 4,470 430 9 22

22-26 days 4,520 380 8 19

37-40 days 3,920 980 20 49

51-54 days 3,180 1,720 35 86

65-68 days 2,800 2,100 43 105

All season 1,010 3,890 79 194

LSD (5 pet)3 ..400 ... 8

LSD (1 pet)3 ..540 ... 11

1 Data taken from Smith {109). Rice contained about

50 barnyardgrass panicles/square foot.

2 Rough rice valued at $5/hundredweight.

2 LSD =least significant difference at level given.

tablished for each State, and a list of these is

available from each of the official certifying

agencies {6).

RICE CULTURE AND

WEED PROBLEMS

Rice production in the United States is centered in the Southern States of Arkansas,

TABLE 5.—Yield of 'Bluebonnet 50' and 'Northrose' rice as influenced by duration of hemp

sesbania competition, Stuttgart, Ark.,

1961-62^

Rough Yield per acre lost

Duration of rice to sesbania competition

competition yield "~ ~ T . Dollar nK/oo>^^\ Pounds Percent ^^^^'^^ (lb/acre) value^

0 (check) 5,710

4 weeks 5,600 110 2 6

8 weeks 5,370 340 6 17

12 weeks 5,200 510 9 26

All season 4,630 1,080 19 54

LSD (5 pet)3 ..460 460 8

LSD(1 pet) 3 .. 630 630 11

1 Data taken from Smith {109). Hemp sesbania population was 21,780 plants/acre.

2 Rough rice valued at $5/hundredweight.

2 LSD = least significant difference at level given.

TABLE 6.—Yield of 'Bluebonnet 50' and ^Northrose' rice as influenced by duration of

northern jointvetch competition, Stuttgart,

Ark., 1963-6J,^

Rough Yield per acre lost to

Duration of rice jointvetch competition

competition yield DÖUär

(lb/acre) Pounds Percent ^^^^^2

0 (check) 6,100 77. TT^ TT^

4 weeks 5,980 120 2 6

8 weeks 5,610 490 8 24

12 weeks 5,610 490 8 24

An season 5,060 1,040 17 52

LSD (5 pet) 3 . 490 490 8

LSD (1 pet) 3 . 730 730 12

1 Data taken from Smith {109). Northern jointvetch

population was 21,780 plants/acre.

2 Rough rice valued at $5/hundredweight.

3 LSD =least significant difference at level given.

Louisiana, Mississippi, and Texas and in California. It is the principal cash crop in many

counties and parishes. Small amounts of rice

also are grown in Missouri, Oklahoma, South

Carolina, and Tennessee. Some rice has been

grown in each of the States in Southeastern

United States. A handbook compiled by the

U.S. Department of Agriculture discusses rice

production in the United States {139).
WEED CONTROL IN U.S. RICE PRODUCTION

Algae, grass, and broadleaf and sedge weeds

infest ricefields. Both annuals and perennials

in terrestrial and aquatic sites are problems

in rice culture. Terrestrial weeds germinate in

moist soil, and most tolerate flushing after

emergence; aquatic weeds germinate and grow

in flooded soil. Weed species making up the

total weed complex in a field vary among ricegrowing regions and areas and often differ

between adjacent fields. Methods that control

one complex of weeds may fail completely in

another field that is infested with a different

weed complex.

Several methods are used to seed rice in the

United States. Different weed complexes are

associated with each seeding method. Rice

seeded in soil is usually infested with terrestrial

weeds such as barnyardgrass, dayflower, and

morningglory that tolerate flooding after

emergence; aquatic weeds germinate after flooding, when rice is about 3 weeks old. Rice seeded

in water is usually infested with aquatic weeds

such as algae, ducksalad, American and horned

pondweed, and redstem that germinate in

flooded soil; terrestrial weeds may germinate

during drained periods after rice emerges. Each

weed complex that develops in the various

cultures requires a particular weed control program.

DEVELOPMENT OF

HERBICIDES AND CHANGES IN

RICE CULTURE AND

WEED CONTROL PRACTICES

Herbicides are required to control weeds in

rice when preventive, cultural, and mechanical

methods fail. Combinations of herbicides, in

mixtures or in sequential applications, are often

more effective than a single treatment with

only one herbicide. The principal herbicides in

use are propanil, molinate, and the phenoxy

herbicides, chiefly 2,4-D, 2,4,5-T, silvex, and

MCPA. In 1973 about 95 percent of the U.S.

rice crop was treated with propanil or molinate

to control grass and broadleaf and sedge weeds;

about 30 percent was sprayed with phenoxy

herbicides to control aquatic, broadleaf, and

sedge weeds (table 8).

Propanil has increasingly replaced the

TABLE 7.—Yield of 'Nato' and 'Zenith' rice as

influenced by duration of ducksalad competition, Stuttgart, Ark,, 1957-61^

Rough Yield per acre lost

Duration of rice to ducksalad competition

competition yield DÔÏÏar

(lb/acre) Pounds Percent ^^^^^^

0 (check) 5,220

2 weeks 4,910 310 6 16

4 weeks 4,440 780 15 39

8 weeks 3,810 1,410 27 70

All season 4,120 1,100 21 55

iData taken from Smith (109).

2 Rough rice valued at $5/hundredweight.

TABLE 8.—Rice area treated with propanil,

molinate, and phenoxy herbicides and

rough rice yields in the United States,

1960-73^

Percentage of rice area treated with^—

Ph "^^^^

Year Propanil Molinate ^erbkidYs^ ^^^^^^ nerbicides (ib/acre)

1960 0

1961 1

1962 15

1963 50

1964 60

1965 75

1966 80

1967 85

1968 85

1969 70

1970 70

1971 70

1972 75

1973 80

0

0

0

0

0

0

0

0

2

15

15

15

20

30

70

70

60

50

40

40

40

40

40

40

30

30

30

30

3,420

3,410

3,730

3,970

4,100

4,260

4,320

4,540

4,420

4,320

4,620

4,720

4,680

4,280

1 Estimates based on reports by the Cooperative Extension Service in rice-producing States and by herbicide manufacturers.

2 Includes 2,4-D, 2,4,5-T, silvex, and MCPA.

3 Yields for 1960-72 were taken from Agricultural

Statistics, 1973 {IJfO) ; yields for 1973 were taken from

Rice Situation, March 1974 (IWphenoxy herbicides. The first experiments in

Arkansas in 1959 showed that propanil controlled certain young grasses and broadleaf

weeds in rice. It was very selective, and early

pcstemergence treatments at high rates of 8 to

12 lb/acre did not injure rice. Rice farmers,

in cooperative trials with chemical companies.
THE WEED PROBLEM

used it in 1961 on about 20,000 acres (i^). In

1962 propanil was recommended for the first

time by the Cooperative Extension Service in

Arkansas, Mississippi, and Texas, and farmers

treated approximately 250,000 acres with this

herbicide. Good to excellent grass control and

significant improvements in yields were obtained on more than 90 percent of the fields

(15). Molinate was first used in commercial

rice in 1968, after experiments conducted from

1964 through 1967 indicated that it controlled

barnyardgrass and did not injure rice (86, 108,

112),

Researchers continue to find improved herbicides. Among the promising new ones are

bentazon, benthiocarb, bifenox, butachlor,

endothall, oxadiazon, KN3, and NaN3.

A newly discovered endemic fungus disease

has killed northern jointvetch in ricefields {28).

Aerial sprays of the spores to ricefields in

1972 and 1973 controlled weed plants effectively

by 3 to 6 weeks after treatment. Although weed

control by the fungus was less rapid than by

herbicides such as 2,4,5-T or silvex, activity was

fast enough for practical field control. Rice,

soybeans, cotton, and other field crops grown

in Arkansas were not injured by the fungus.

The development of efficient and economical

herbicides and combination methods for fullseason control of weeds in rice has had a

significant impact on rice production.

Improved yields and quality.—Since 1961 the

use of herbicides such as propanil and molinate

for the control of weed grasses in rice has

steadily increased. Commercial ricegrowers in

the United States are now treating most of their

rice with herbicides (table 8). Frequently, both

herbicides are applied in sequence to the same

ricefield. During this period, per-acre yields

have increased by approximately 1,200 lb/acre.

Better weed control, although not solely responsible, was a vital factor in bringing about

increases in yield and improvements in the

quality of rice. Also, herbicides, properly used,

facilitate good management of other production

practices.

Changes in seeding methods and tvater management,—Ricegrowers in the South have

changed from water seeding to dry seeding

of rice, because safe and effective herbicides

are available to control weed grasses. Water

seeding controls weed grasses but fails to prevent infestations of aquatic weeds. Dry seeding,

combined with effective herbicides for grass

control, reduces problems with weed grasses

and aquatic weeds.

In California, a deep flood of 6 to 8 inches

is used primarily to control barnyardgrass. A

deep flood, however, inhibits growth of young

rice seedlings, reduces tillering, and lowers

grain yield {85), A shallow flood of 1 to 4

inches, combined with the use of effective herbicides, controls weeds and increases yields of

rice about 850 lb/acre, compared with deepflood culture.

Use of improved cultivars.—New high-yielding, lodging-resistant cultivars that are responsive to high levels of nitrogen are practical for

commercial production only because effective

weed control is possible (55). The new shortstatured cultivars with upright leaves allow

more sunlight to penetrate the crop canopy,

and they respond better to nitrogen than taller,

leafier cultivars. However, higher light penetration through the crop canopy, combined with

high levels of nitrogen, stimulates weed growth.

Cultivars that mature about 115 days after

seeding are now used for commercial production {55), Frequently two crops are harvested

each year from a single early-spring planting

of short-season cultivars in the South. Also,

short-season cultivars seeded late in the spring

extend the spring planting season because they

mature before low temperatures damage the

crop. These short-season cultivars do not compete well with barnyardgrass and other weeds.

If new, short-strawed, short-season, highyield cultivars are to be grown efficiently,

weeds must be controlled, because they do not

compete with weeds as well as older varieties

that are taller and leafier or have a longer

growing season {29), In experiments in Arkansas in 1969, yields obtained when barnyardgrass competed with 'Bluebelle', which matures

in 115 days, and with 'Starbonnet', which matures in 140 days, were compared {117), Allseason competition reduced grain yields of

'Bluebelle' by 64 percent, compared with 40

percent for 'Starbonnet'. Although effective

weed control practices are essential for economical production of all new high-yielding

cultivars, they are even more important for
6 WEED CONTROL IN U.S. RICE PRODUCTION

short-season cultivars, which have less time

to grow after the weeds mature than longseason cultivars.

Timely fertilizer management,—New highyielding rice cultivars adapted for the South

are most responsive to nitrogen when part is

applied just before seeding or up to 15 days

after crop emergence, followed by one or two

applications at midseason {96), Weeds must be

controlled early if the rice is to get maximum

benefits from the nitrogen. Where weed grasses

are not controlled, nitrogen applied before

planting or just after rice emergence often

stimulates weed growth and reduces grain

yields even more than when no nitrogen is

applied at this time {102),

Reduction of insect problems,—In Arkansas,

rice stink bug {Oebalus pugnax Fabricius)

populations may be directly related to barnyardgrass infestations {8i), Fields free of

weed grasses may have low populations of stink

bugs all season long. Conversely, fields with

heavy infestations of barnyardgrass may develop peak stink bug populations about the

time rice is most susceptible to damage. Control

of barnyardgrass, therefore, reduces stink bug

infestations.

Weed infestations affect populations of the

rice water weevil {Lissorhoptrus oryzophilus

Kuschel). A seed treatment of aldrin failed to

control rice water weevils in rice heavily infested with barnyardgrass {126), Aldrin controlled the insect in rice if barnyardgrass was

controlled with herbicides. Therefore, use of a

herbicide to control barnyardgrass plus an effective insecticide treatment can reduce rice

water weevil infestations more than the use of

an insecticide only.

In California, rice water weevil infestations

retard vegetative growth of rice during the

early growing season and reduce the competitiveness of rice with aquatic weeds. Hence,

these weeds grow rapidly and reduce grain

yields.

Interdependence of cultural practices and

herbicides,—When cultural-mechanical systems

fail to control weeds in rice, herbicides are

necessary to reduce losses from weeds. Combinations of herbicides in a mixture or in sequential applications are frequently more effective

than treatment with only one herbicide.

When weed grasses develop in ricefields because of improper management of culturalmechanical systems, timely applications of

effective rates of propanil or molinate reduce

losses from these weeds. Likewise, when aquatic

and broadleaf weeds infest ricefields with improperly managed cultural or mechanical control method, timely treatments with phenoxy

herbicides can significantly reduce yield and

quality losses by these weeds.

By combining control methods into effective

systems, most weeds in rice can be controlled.

Consequently, high yields of good-quality rice

can be produced with a minimum of labor and

machinery. Effective weed control also permits

the farmer to select seeding methods, varieties,

water and fertilizer management practices, insect control programs, and perhaps other rice

production practices that favor rice growth and

production.
PRINCIPLES OF WEED CONTROL

Weed management systems combine cultural,

mechanical, chemical, and biological control

programs. Management of cultural and mechanical weed control practices may be used

effectively to control specific weeds (table 9).

Preventive methods of weed control are required to avoid weed problems before they

begin in ricefields. Preventive methods include

use of weed-free crop seed, use of irrigation

water free of weed seed or other propagation

parts, and use of clean equipment. Conformance

to certified seed regulations and use of certified

seed are related ways of avoiding weed-seedcontaminated rice seed.

Weeds are disseminated in rice seed and by

irrigation water, farm machinery, and birds

and animals. Many weeds such as red rice,

hemp sesbania, northern jointvetch; dayflower,

and mexicanweed are spread almost entirely by

rice seed. Even cleaned rice seed may contain

these weed seeds because they are difficult to

remove.

Red rice, a problem weed in the South, is

spread by rice seed contaminated with grains

of this weed. Use of rice seed free of red rice

seed prevents establishment of red rice on land.

Once established, red rice is difficult to control

because its seeds shatter easily and can persist

in the soil for many years. Since the herbicides

presently available for use in rice are not selective enough for control of red rice in the crop,

cultural practices that combine crop rotation,

fallowing, cultivation, and hand weeding can

help to reduce red rice problems.

Seeds of barnyardgrass, northern jointvetch,

ducksalad, redstem, and sprangletop are disseminated by irrigation water. When these

weeds grow to maturity on canal banks and

in ricefields their seeds drop into the water

and a]^e scattered through the ricefield.

Machinery carries weed seeds from field to

field. Dissemination of weed seeds by machinery may be reduced by cleaning equipment

thoroughly after use. Control of weeds in areas

where equipment is stored prevents contamination of machinery. Mowing, burning, oiling,

grazing, and using soil sterilants help control

weeds in these areas.

Weed seeds are carried and disseminated by

birds, livestock, and wild animals. Some species

of blackbirds feed mainly on weed grasses, such

as paspalums (Paspalum spp.), barnyardgrass,

crabgrass {Digitaria spp.), broadleaf signalgrass, and foxtail {Setaria spp.). The diet of

many blackbird species contains as much as 10

percent barnyardgrass seed (81), Livestock

that graze ricefield stubble scatter weed seeds

from their hoofs and hide and in manure.

Livestock that graze weed-infested pastures

also contribute to dissemination of weed seeds.

Because weed seeds remain viable in the soil

for many years, preventing the establishment

of weed plants is essential in a weed control

program. In experiments initiated in 1879 in

Michigan, freshly gathered seeds of 20 species

of weeds were mixed with sand, placed in pint

bottles, and buried 18 inches below the ground

surface (7, 93). After 40 years, seeds of 12

species were viable; after 80 years, seeds

of three species—common eveningprimrose

(Oenothera biennis L.), curly dock (Rumex

crispus L.), and moth mullein (Verhascum

blattaria L.)—were viable. Although these

weeds are not problems in ricefields, research

indicates that seeds of hemp sesbania and

northern jointvetch remain viable in the soil for

at least 10 years (R. J. Smith, Jr., unpublished

data).

Sometimes, viable weed seeds are incapable

of germinating immediately after maturity,

even when environmental conditions are favorable. This characteristic is called dormancy.

Seeds of cocklebur (Xanthium spp.), wild oat

(Avena fatua L.), and redroot pigweed

(Amaranthus retroflexus L.) exhibit dormancy

(93). In experiments in Japan barnyardgrass

seed lay dormant in the soil for 10 to 48 months

after maturity (i, 5). Secondary dormancy oc-
WEED CONTROL IN U.S. RICE PRODUCTION

TABLE 9.—Control of common ricefield weeds by selected cultural practices

Cultural practice^

„ ^ __ ^ Clean Seedbed Timely Timely Crop Weed Hand ^.^^ p^^^^. Water Dry ^^^^^_ ^^^^_ Rice Summer ^^^^_

weeding ^^^^, ^^^-^^ seeding seeding .^^3 .^^3 stand^ fallow ^.^^

Algae (green and

blue-green) Poor Poor Good Poor Good Poor Good Good Poor Good

Alligatorweed Poor Poor Good Poor Poor Poor Poor Good Good Good

Arrowhead Poor Good Good Poor Good Poor Fair Good Good Good

Barnyardgrass Poor Good Good Good Poor Fair Poor Fair Fair Fair

Beakrush Poor Good Good Poor Fair Poor Fair Good Good Good

Broadleaf signalgrass Poor Good Good Good Poor Good Poor Fair Fair Fair

Bulrush Poor Poor Good Poor Good Poor Good Poor Good Good

Burhead Poor Poor Good Poor Good Poor Good Good Good Good

Cattail Poor Poor Good Poor Good Poor Good Poor Good Good

Common waterplantain Poor Poor Good Poor Good Poor Good Good Good Good

Dayflower Poor Good Good Poor Poor Fair Poor Good Fair Fair

Ducksalad Poor Poor Poor Poor Fair Poor Good Good Poor Poor

Eclipta Poor Poor Poor Good Poor Fair Poor Good Fair Fair

False pimpernel Poor Poor Poor Poor Fair Poor Good Good Poor Poor

Fimbristylis Poor Poor Poor Poor Good Poor Good Good Poor Poor

Gooseweed Poor Poor Good Poor Good Poor Good Good Good Fair

Hemp sesbania Good Good Good Fair Poor Fair Poor Fair Fair Fair

Horned pondweed Poor Poor Poor Poor Good Poor Fair Good Fair Fair

Jointvetch Good Good Good Fair Poor Fair Poor Fair Fair Fair

Knotgrass Poor Poor Good Poor Fair Poor Fair Good Good Good

Mexicanweed Good Good Good Fair Poor Fair Poor Fair Fair Good

Morningglory Poor Good Good Good Poor Good Poor Good Good Good

Naiad Poor Poor Fair Poor Good Poor Fair Good Fair Fair

Panicum grasses:

Annuals Poor Poor Fair Good Poor Fair Poor Good Good Good

Perennials Poor Poor Good Poor Poor Fair Poor Good Good Good

Pondweed Poor Poor Good Poor Good Poor Fair Good Good Good

Red rice Good Good Good Fair Poor Poor Poor Fair Good Good

Redstem or purple

ammannia Poor Poor Poor Poor Fair Poor Good Good Poor Poor

Smartweed Poor Good Good Poor Fair Poor Poor Good Good Good

Spikerush :

Annuals Poor Poor Fair Poor Fair Poor Good Good Good Good

Perennials Poor Poor Good Good Poor Poor Poor Good Good Good

Sprangletop Poor Good Fair Poor Poor Poor Poor Good Good Good

Umbrellaplant :

Annuals Poor Poor Fair Poor Fair Poor Good Good Good Good

Perennials Poor Poor Good Fair Poor Fair Poor Good Good Good

Waterhyssop Poor Poor Poor Poor Fair Poor Good Good Poor Poor

Waterprimrose Poor Good . Good Poor Fair Poor Fair Good Good Good

1 Ratings for classes of cultural practice: Good—Practice can be used effectively in commercial rice to prevent

or reduce weed infestations. Fair—Practice can be used in commercial rice but usually gives only fair weed control. Poor—Practice cannot be used economically in commercial rice or fails to control the weed.

2 Seeding weed-free crop seed reduces problems with all weeds. A poor rating indicates that weed seeds do not

usually contaminate seed rice. (Weed seeds are not harvested with the crop or can be removed early with

commercial cleaning equipment.) A good rating indicates that the weed seeds are difficult to remove from the

rice seed and special effort is required to remove the weed seeds.

3 After crop emergence.

4 A good rice stand of 12 to 20 plants per square foot helps reduce problems with many weeds.
PRINCIPLES OF WEED CONTROL

curred after a period in which the seed germinated. Barnyardgrass seeds were dormant for

10 months immediately after they matured on

the plant. Then they went through a period in

which they germinated. After submergence in

the soil for 4 or 5 months, these seeds went

through a period of secondary dormancy in

which they failed to germinate. Environmental

factors, such as variations in drying and wetting and changes in temperatures, help to break

primary and secondary dormancy of barnyardgrass seed.

Experiments therefore indicate that weed

seed can remain dormant in soil for long periods

and will germinate many years later when environmental and physiological conditions are

favorable.

CROP ROTATION

The occurrence of some weed species in rice

is often associated with crop rotation. Properly

managed rotations combined with the use of

herbicides are important for controlling such

weeds in rice. Keeping all crops in the rotation

free of weeds reduces weeds in the rice crop.

In Louisiana and Texas, a rice-pasture rotation reduces weeds in rice. Rice following

poorly drained native pasture frequently becomes heavily infested with perennial weeds

such as spikerush and jointed flatsedge. Rice

following drained and heavily fertilized improved pasture may become infested with barnyardgrass, but not with perennial weeds such

as spikerush, jointed flatsedge, bulrush, or cattails, or with annual broadleaf weeds such as

hemp sesbania, mexicanweed. or northern

jointvetch. If hemp sesbania, mexicanweed.

northern jointvetch, or annual sedges infest the

pasture crop, they can be controlled by timely

applications of phenoxy herbicides.

In Arkansas, rice-soybean or rice-soybean-oat

rotations reduce infestations of barnyardgrass,

hemp sesbania, northern jointvetch, spikerush,

umbrellaplant, broadleaf signalgrass, beakrush,

knotgrass, and red rice. The soybean crop in a

rice-soybean-oat rotation can be kept free of

weeds by combining cultural practices with the

use of preemergence herbicides. The oat crop

can be kept free of broadleaf weeds by the use

of 2,4-D or MCPA. The land may also be

summer-fallowed after oat harvest to control

weeds not controlled in the soybeans and oats.

Summer fallow is especially effective in controlling barnyardgrass, broadleaf signalgrass,

beakrush, knotgrass, and red rice.

Crop rotation coupled with hand pulling of

scattered plants is important in controlling red

rice. Pasturing, cultivation of soybeans, grain

sorghum, or cotton, and summer and fall fallowing after harvesting small-grain crops are some

rotation practices that reduce problems of red

rice in the rice crop. Herbicides applied preemergence or postemergence to row crops help

control red rice in the row crop and reduce infestation in the rice crop.

In California, rotation of rice with fallow or

an unirrigated row crop such as safflower controls cattail, bulrush, spikerush and other

perennial weeds with large rootstocks. Plowing

the ricefield to a depth of 12 inches during the

fallow season improves control.

The combination of crop rotation and the

use of herbicides is more effective in controlling

weeds in rice than either practice used alone.

Preemergence and postemergence herbicides

applied to the row and pasture crops rotated

with rice also reduce weed populations in the

rice crop. Likewise, chemical control of grass

and broadleaf and sedge weeds in the rice crop

reduces weed infestations in the crops rotated

with rice.

LAND LEVELING AND

LEVEE CONSTRUCTION

Land leveling and proper construction of

levees reduce weed infestations. Since level

land requires fewer levees, there are fewer

sites for weed growth. Barnyardgrass, hemp

sesbania, mexicanweed, and northern jointvetch

may be more abundant on ridges where water

does not cover the land adequately. Weed

grasses such as sprangletop and aquatic weeds

such as algae, bulrush, ducksalad, gooseweed,

redstem, spikerush, umbrellaplant, and waterhyssop grow best in shallow water, 3 inches

deep or less, and are most numerous in low

areas where surface drainage is inadequate.

Thus, land leveling and levee construction to

eliminate low and high areas in the ricefield
10 WEED CONTROL IN U.S. RICE PRODUCTION

and to maintain 4 to 8 inches of water on the

land reduce problems with weeds.

Plastic levees satisfactorily control water in

ricefields {Jf6, 70) and eliminate barnyardgrass,

broadleaf signalgrass, hemp sesbania, morningglory, northern jointvetch, and smartweed,

which are problem weeds on soil levees. Covering soil levees with black polyethylene plastic

achieves the same purpose.

Herbicides reduce weed problems in ricefields

that are not level or where levees are improperly constructed. Silvex, 2,4-D, 2,4,5-T, and

MCPA control weeds associated with low areas

in ricefields. Likewise, propanil or molinate reduces barnyardgrass infestations, and 2,4-D,

2,4,5-T, and silvex control hemp sesbania, mexicanweed, and jointvetch infestations that are

found on ridges in ricefields where 4 to 8

inches of water cannot be maintained. Infestations of barnyardgrass, hemp sesbania, mexicanweed, northern jointvetch, and smartweed

on levees are reduced by propanil or phenoxy

herbicide treatments.

SEEDBED PREPARATION

Thorough seedbed preparation helps to control most weeds that infest ricefields. The goal

is the elimination of weed growth up to the

time of planting.

The seedbed may be prepared in many ways—

plowing, disking, harrowing, rotary tilling, and

combinations of these. The best method to use

depends on the soil type and condition, other

crops in the rotation, seeding method, climate,

and perhaps other considerations. When a rotation includes a spring-harvested crop, the land

may be cultivated during the summer and fall

by repeated and timely cultivation or disking to

rid the field of many weeds. Then the soil may

be disked or harrowed again in the spring to

prepare a weed-free seedbed for rice.

Repeated cultivations in the spring at 1- to

3-week intervals before seeding rice reduce

barnyardgrass and other annual grass infestations. The last cultivation should be shallow—

usually not more than 2 or 3 inches deep—to

avoid bringing buried weed seeds to the soil

surface, where they may germinate. This method does not kill aquatic weeds such as algae,

ducksalad, redstem, and annual spikerush or

annual species of umbrellaplant because they

do not germinate until the soil is flooded. When

rice is seeded on heavy clay, a roughly prepared

seedbed discourages germination of barnyardgrass, umbrellaplant, spikerush, waterplantain,

burhead, and seedling cattail.

In California, thorough plowing, followed by

complete drying of soil before seeding rice in

water, controls established perennial weeds

such as cattail, spikerush, and knotgrass. In the

South, spring rains prevent sufficient drying

of the soil to kill these weeds.

SEEDING

Rice may be drill-seeded, broadcast-seeded in

dry soil and disked or harrowed to cover, or

water-seeded. The method of seeding influences

subsequent weed growth and weed control.

Barnyardgrass, for example, is difficult to control by cultural methods in drill- or dry-broadcast-seeded rice. Seeding in water was therefore

developed in California because this method

inhibits its germination and growth (i).

Water seeding may be used to selectively control barnyardgrass, broadleaf signalgrass, hemp

sesbania, northern jointvetch, sprangletop, and

red rice. To be effective the water must be

held at 4 to 8 inches for 3 to 4 weeks after

seeding.

Experiments have shown the relation of

water depth, submergence time, and weed control. In pot-culture tests common barnyardgrass

seeded on puddled soil was not controlled, but

90 to 100 percent control was obtained by

seeding in 2 to 10 inches of water {57), Prolonged submergence reduced barnyardgrass

infestations and delayed germination and emergence. Some barnyardgrass types, however,

germinated in submerged soil. In pot-culture

experiments white barnyardgrass, a type of

Echinochloa crus-galli, germinated when seeded

in 10 inches of water, but submergence did

delay germination.

In a field experiment where 6 inches of water

was held all season, weeds such as barnyardgrass and spikerush were controlled and the

rice yield was 5,500 lb/acre {UO). Where the

water was lowered from 6 inches to 2 or 3

inches at 2, 3, and 4 weeks after seeding, the

rice yields were 3,500, 2,900, and 3,800 lb/acre,
PRINCIPLES OF WEED CONTROL 11

respectively. Where the water was lowered at

2 weeks, barnyardgrass infested the plots.

When the water was lowered at 3 or 4 weeks,

barnyardgrass was controlled, but spikerush

infested the plots and reduced the yield of rice.

Sometimes, however, the water is lowered

because it stretches and weakens the rice plant.

But weeds such as barnyardgrass, dayflower,

hemp sesbania, northern jointvetch, sprangletop, and spikerush germinate when the water

is drained. Conversely, water maintained on

the ricefield for several weeks after seeding

may injure the rice and intensify problems with

some aquatic weeds, such as algae, ducksalad,

redstem, and waterhyssop.

Dry-seeded culture is more prevalent than

water-seeded in the South because a continuous

flood during seedling establishment frequently

injures the rice. Weeds that develop in dryseeded rice are more easily controlled with

herbicides than are those that develop in a

water-seeded culture. Combination treatments

of propanil and molinate, applied during the

first 3 weeks after crop emergence, control

barnyardgrass, broadleaf signalgrass, dayflower, hemp sesbania and northern jointvetch,

the prevalent weeds in dry-seeded rice.

WATER MANAGEMENT

Flooding ricefields early in the season to a

depth of 4 to 8 inches reduces infestations of

barnyardgrass and other weeds, but it may

stretch and weaken the rice. Water controls

grass plants in the one- to four-leaf stages

better than it does larger ones; deep water

usually fails to control barnyardgrass that is

in the tillering or older stages of growth.

High water temperatures, 95° F or above, facilitate control of barnyardgrass, presumably

because of the low oxygen content of warm

water, but the rice may be injured as well.

Rice grown on problem (calcareous, saline, and

sodic) soils may be injured severely by deep

and prolonged flooding even when water temperatures are optimum (75° to 90° F). Earlyseason flooding to control weed grasses, therefore, is not always effective and selective.

Timely and thorough draining of the water

from the ricefield helps control many aquatic

weeds. Experiments in Arkansas showed that

blue-green algae, or ''scum,'' are controlled by

draining water from the field as soon as green

areas of algal growth appear on the soil (87),

Water is manipulated on and off the field to

keep algal growth to a minimum until the rice

plants are 12 to 18 inches tall and unaffected

by algae. Young ducksalad, redstem, waterhyssop, and other emersed or submersed aquatic

weeds are effectively controlled by draining

the water and thoroughly drying the field. The

rice, however, may be injured if the soil dries

and sunbakes. Drying the ricefield to kill

aquatic weeds without injury to the crop has

a narrow margin of selectivity. Rain may prevent sufficient drying of the soil to control the

weeds. Barnyardgrass, broadleaf signalgrass,

dayflower, hemp sesbania, northern jointvetch,

and other weed grasses and broadleaf weeds

may germinate during drainage for the control

of aquatic weeds. Weeds may germinate if the

water is left off the field for more than 3 or 4

days. Manipulation of the water to control

algae, to prevent desiccation of rice, and to

forestall germination of barnyardgrass and

other weed grasses is difficult to manage in

commercial ricefields.

FERTILIZER MANAGEMENT

Weed growth is usually stimulated by applications of phosphate and nitrogen to ricefields,

but the ensuing weed competition with the crop

can be reduced by selection of times and

methods of fertilizer application that are more

advantageous to the rice than to the weeds.

There are several ways to apply phosphate

fertilizer without excessive stimulation of weed

growth. Since seeds of most weeds germinate

and grow near the soil surface, phosphate may

be placed below the rice seed during drill seeding, where it is less available to the weeds.

In fields lightly infested with barnyardgrass,

phosphate fertilization may also be delayed

until just before the first flooding to take advantage of the inhibition of weed growth by the

floodwater. Application of phosphate after

flooding stimulates the growth of algae and

aquatic weeds, which compete with rice for

this nutrient and reduce its availability and

benefits to the crop. In fields severely infested

with barnyardgrass and other weeds, phosphate
12 WEED CONTROL IN U.S. RICE PRODUCTION

is usually applied to another crop in the rotation

to avoid the stimulation of weed growth expected after direct application to the rice crop.

Applications of nitrogen can also be made

at times when rice benefits more than weeds.

In experiments in Arkansas, nitrogen applied

before seeding rice or after emergence of weeds

stimulated growth of barnyardgrass and

aquatic weeds. Nitrogen applications that were

delayed until after barnyardgrass headed benefited rice more than earlier applications (table

10). However, nitrogen applications can be

managed most efficiently only when weeds are

controlled. Nitrogen applied early and at midseason is usually more effective than applications made at midseason only (^7, 96).

CULTIVATION

Although hand weeding is the main method

of weed control in Asian countries (where rice

is transplanted into rows), in the United States

it is used only to remove scattered infestations

in rice grown for seed, and other cultivation

methods, except for rotary hoeing, to remove

TABLE 10.—Effect of time and rate of nitrogen

application on yield of rice heavily infested

with barnyardgrass, Stuttgart, Ark., 1957

and 1959^

Stage of Nitrogen Rice yield (lb/acre)

barnyardgrass2 ^^-^^^^^^ 1957 1959 Average

Untreated check .... 0 2,090 1,900 1,990

Heading 60 2,900 1,780 2,340

Do 120 3,270 2,730 3,000

Do 180 3,470 3,380 3,430

Mature 60 2,390 1,840 2,120

Vegetative and

headings 120 2,360 1,370 1,860

Vegetative and

matures 120 2,500 1,530 2,010

Heading and

matures 120 3,070 2,470 2,770

LSD (5 pet) 4 900 720 810

LSD (1 pet) 4 1,200 950 1,080

iData taken from Smith (102).

2 Vegetative, heading, and mature stages were 3, 8,

and 12 weeks after emergence of rice and barnyardgrass, respectively.

s 60 lb/acre applied at each stage.

4 LSD = least significant difference at level given.

weeds after the rice crop has been seeded are

just about nonexistent. In drill-seeded rice, cultivating between rows to remove weeds is difficult, and in dry-broadcast or water-seeded rice,

cultivation is impossible.

Rotary hoeing soon after crop emergence controls small weeds in dry-seeded rice. It is the

only practical method of cultivation after seedTABLE 11.—Common and chemical names of

herbicides, insecticides, and fungicides discussed in this handbook

Common name or ^, . ,

designation Chemical name

aldrin 1,2,3,4,10,10-hexachlorol,4,4a,5,8,8a-hexahydro-l,4-e?icioe£t;o-5,8-dimethanonaphthalene.

bentazon 3-isopropyl-lH-2,l,3-

benzothiadiazin-4 (SH) -one 2,2-

dioxide.

benthiocarb 5-[ (4-chlorophenyl) methyl]

diethylcarbamothioate.

bifenox methyl 5- (2,4-dichlorophenoxy)-2-

nitrobenzoate.

butachlor N- (butoxymethyl) -2-chloro-2',6'-

diethylacetanilide.

carbaryl 1-naphthyl methylcarbamate.

carbofuran 2,3-dihydro-2,2-dimethyl-7-

benzofuranyl methylcarbamate.

copper chelate copper hydroxide-triethanolamine

complex.

copper sulfate cupric sulfate pentahydrate.

endothall 7-oxabicyclo [2.2.1] heptane-2,3-

dicarboxylic acid,

fensulfothion 0,ö-diethyl 0-[p-

(methylsulfinyl) phenyl]

phosphorothioate.

KN3 potassium azide.

MCPA [(4-chloro-o-tolyl)oxy] acetic acid.

molinate iS-ethyl hexahydro-lH-azepine-lcarbothioate.

NaNa sodium azide.

oxadiazon 2-íerí-butyl-4-(2,4-dichloro-5-

isopropoxyphenyl) -A2-l,3,4-

oxadiazolin-5-one.

parathion ö,ö-diethyl 0- (p-nitrophenyl)

phosphorothioate.

pebulate »S-propyl butylethylthiocarbamate.

propanil 3',4'-dichloropropionanilide.

silvex 2-(2,4,5-trichlorophenoxy)propionic

acid,

thiram bis (dimethylthiocarbamoyl)

disulfide.

2,4-D (2,4-dichlorophenoxy) acetic acid.

2,4,5-T (2,4,5-trichlorophenoxy) acetic acid.

Panogen cyano (methylmercuri) guanidine.
PRINCIPLES OF WEED CONTROL 13

TABLE 12.—Control of common ricefield weeds by selected herbicides^

Herbicide

Weed ^——— — —— Propanil Molinate 2,4-D MCPA Silvex 2,4,5-T Copper

Algae :

Blue-green Poor Poor Poor Poor Poor Poor Good

Green Poor Poor Poor Poor Poor Poor Good

Alligatorweed Poor Poor Fair Fair Fair Fair Poor

Arrowhead Poor Poor Fair Fair Fair Fair Poor

Barnyardgrass Good Good Poor Poor Poor Poor Poor

Beakrush Fair Poor Good Good Good Good Poor

Broadleaf signalgrass Good Fair Poor Poor Poor Poor Poor

Bulrush Poor Poor Poor Fair Poor Poor Poor

Burhead Poor Poor Good Good Good Good Poor

Cattail Poor Poor Fair Fair Fair Fair Poor

Common waterplantain Poor Poor Good Good Good Good Poor

Dayflower Fair Fair Good Good Good Good Poor

Ducksalad Fair Poor Good Fair Fair Fair Poor

Eclipta Good Good Good Good Good Good Poor

False pimpernel Good Poor Good Good Good Good Poor

Fimbristylis Good Poor Good Good Good Good Poor

Gooseweed Poor Poor Fair Fair Fair Fair Poor

Hemp sesbania Good Poor Good Fair Good Good Poor

Horned pondweed Poor Poor Good Good Fair Fair Poor

Jointvetch Good Poor Fair Fair Good Good Poor

Knotgrass Fair Poor Poor Poor Poor Poor Poor

Mexicanweed Poor Poor Fair Fair Good Good Poor

Morningglory Fair Poor Good Good Good Good Poor

Naiad Poor Poor Good Good Good Good Poor

Panicum grasses:

Annuals Good Good Poor Poor Poor Poor Poor

Perennials Fair Poor Poor Poor Poor Poor Poor

Pondweed Poor Poor Fair Fair Fair Fair Poor

Red rice Poor Fair Poor Poor Poor Poor Poor

Redstem or purple

ammannia Fair Poor Good Good Good Good Poor

Smartweed Fair Poor Fair Fair Fair Fair Poor

Spikerush :

Annuals Good Good Good Good Good Good Poor

Perennials Fair Poor Fair Fair Fair Fair Poor

S'prangletop :

Bearded Fair Poor Poor Poor Poor Poor Poor

Tighthead Good Poor Poor Poor Poor Poor Poor

Umbrellaplant :

Annuals Fair Fair Good Good Good Good Poor

Perennials Fair Fair Fair Fair Fair Fair Poor

Waterhyssop Good Poor Good Good Good Good Poor

Waterprimrose Poor Poor Fair Fair Fair Fair Poor

1 Susceptibility of weeds based on observations made in greenhouse and field experiments and in ricefields from

general applications. Good—1 application at susceptible stage of growth and at normal rates kills 70 to 100 pet

of the weeds with little or no injury to rice. Fair—1 application at normal rates at susceptible growth stages

kills only 40 to 60 pet of the weeds with little or no injury to rice. Poor—Weeds not controlled with normal rates

applied at susceptible growth stages. Normal rates for propanil, 3 to 6 lb/acre; for molinate, 2 to 3 lb/acre;

for 2,4-D, MCPA, silvex, and 2,4,5-T, 0.5 to 1.5 lb/acre; for copper sulfate, 0.5 part per million by weight of copper.
14 WEED CONTROL IN U.S. RICE PRODUCTION

ing, but it is seldom used because it is only

effective when the soil is neither too dry nor

too wet.

HERBICIDES

Some herbicides are registered for use in

rice and others that have been researched show

promise for control of weeds in rice. Herbicides

and other pesticides and chemicals discussed

in this handbook are presented in table 11.

Weeds, which vary in their susceptibility to

herbicides, may be controlled by proper herbicide management. Table 12 gives relative susceptibility of problem ricefield weeds to various

registered herbicides.

General Information About

Herbicide Use

Drift

Field crops such as cotton and soybeans and

certain horticultural, fruit, and ornamental

plants may be injured by drift from herbicide

spray. Size and density of spray droplets, herbicide formulation, vapor pressure of spray solution, wind velocity and direction, height of

spray nozzle above the treated surface, air

temperature and relative humidity, and possibly

other factors influence drift of herbicides. The

effect of droplet size on drift of sprays is shown

in table 13.

Many different distribution systems are used

for spraying herbicides by aircraft. They range

from the most common boom-nozzle system to

various spinning brushes, disks, and screens.

TABLE 13.—Drift of spray as influenced by

droplet diameter^

Drop Distance

diameter Particle type particles

(^m) carried

400 Coarse aircraft sprays 8 ft

150 Medium aircraft sprays 22 ft

100 Fine aircraft sprays 48 ft

50 Very fine aircraft sprays 178 ft

20 Fine sprays and dusts 1,100 ft

10 Usual dusts and aerosols 4,500 ft

2 Fine aerosols 21 mi

1 Water solutions of herbicides sprayed from 10 ft

above ground in a wind of 3 mi/h. Adapted from

Brooks (16) and Akesson and Yates (S),

Chemical formulations vary from water-soluble

materials to thick, viscous liquids. Regardless

of the distribution system or the chemical

formulation used, droplets vary from aerosols

to coarse. However, the distribution system and

chemical formulation used may affect the

amount of the spray found in each droplet-size

class. For example, experiments in California

indicated that although aerial applications of

viscous invert (water-in-oil) emulsions contained fine droplets which drifted, as did oil-inwater emulsion sprays, they contained more

large particles that were deposited in the application swath.2 Therefore, the amount of

spray that drifted from the target area was

less with invert-emulsion than with normalemulsion sprays.

Atomization of aerial spray is affected by

the type of nozzle used. Hollow-core nozzles produce fine sprays, but flood or jet nozzles produce coarse sprays. The operating pressure of

the system and discharge angle of the nozzle in

relation to the airstream of the airplane also

affect droplet size. Atomization increases as

the operating pressure increases; it also increases as the nozzle discharge outlet is angled

into the airstream. Physical properties of the

spray solution, such as density, surface tension,

and viscosity, also affect atomization.

States regulate the spraying of herbicides by

ground and aerial equipment. These regulations,

which deal with equipment, herbicide formulation, wind velocity, records, responsibility, and

liability, should be consulted before spraying

ricefields with herbicides.

Phenoxy herbicides.—Cotton and soybeans

are highly susceptible to phenoxy herbicides.

Cotton is injured more by 2,4-D than by silvex,

2,4,5-T, or MCPA (table 14). The growth stage

at treatment, environment, soil, formulation of

herbicide, and possibly other factors affect the

response of cotton to phenoxy herbicides. Silvex

and 2,4,5-T injure soybeans more than 2,4-D.

Soybeans treated in the vegetative stage are

injured more than those treated in the bloom

2 Akesson, W. B., and Yates, W. E. Application

aspects of viscous emulsions as an agricultural spray

formulation. Paper presented at the American Society

of Agricultural Engineers meeting, Dec. 12-15, 1961,

Chicago, 111.
PRINCIPLES OF WEED CONTROL 15

stage (table 15). Cotton and soybeans are usually injured less by amine salt than by ester

formulations.

Spray-drift injury of susceptible crops or

plants by amine salts and low-volatility esters

usually occurs during spraying, not afterward.

However, at temperatures above 95° F, even

low-volatility esters may vaporize after spraying, then drift and injure nearby susceptible

crops (27),

Propanil.—Drift from propanil spray may

injure susceptible crops. Cotton plants 2 to 4

inches tall are injured more than those 10 to

22 inches tall (table 16). Applications at rates

of 0.5, 1, and 2 lb/acre at both stages of growth

TABLE 14.—Yield reduction of cotton after

treatment ivith phenoxy herbicides

[Percent]

Location and Cotton stage at treatment

herbicide Vegetative Square Flower BoH

California:^

2,4-D 94 78 0

Silvex 52 55 11

2,4,5-T 61 46 3

Louisiana: 2

2,4-D 43 32 ... 9

Silvex 14 11 ... 9

2,4,5-T 16 14 ... 9

Mississippi: 3

2,4-D 62 66 34 16

2,4,5-T 34 20 16 10

MCPA 24 21 8 10

Texas: 4

2,4-D 84 70 54 15

Silvex 51 28 18 10

2,4,5-T 69 44 42 13

MCPA 78 65 17 1

^ Average yield reduction from plots treated with

low-volatility esters applied at 0.01 lb/acre in 1954 and

1955 (77). Untreated checks yielded an average of

2,490 lb/acre of seed cotton.

2 Average yield reductions from plots treated with

amine salts applied at 0.0001 to 0.1 lb/acre in 1955,

1956, and 1957 (91). Untreated checks yielded an average of 2,200 lb/acre of seed cotton.

3 Average yield reductions from plots treated with

amine salts applied at 0.01 and 0.1 lb/acre in 1951,

1952, and 1953 (^^). Untreated checks yielded an average of 1,810 lb/acre of seed cotton.

* Average yield reductions from plots treated with

low-volatility esters applied at 0.05 and 0.1 lb/acre at

2, 3, or 4 locations in 1954 (8). Untreated checks yielded

an average of 330 lb/acre of lint cotton.

reduced yields significantly. At rates of 0.25

lb/acre or more, propanil delayed maturity of

cotton. When cotton plants 10 to 22 inches tall

were treated with 2 lb/acre only 3 percent of

the seed cotton was harvested the first picking

compared with 53 percent for untreated cotton.

Propanil at 1 or 2 lb/acre applied to soybeans

2 to 5 and 10 to 25 inches tall reduced yields,

but rates of 0.5 lb/acre or less did not (table

17). Propanil at 2 lb/acre reduced soybean

yields most when applied to 2- to 5-inch-tall

plants. Cotton was more susceptible than soybeans to propanil, but yields of both crops were

reduced when propanil reduced stands.

Propanil injury was manifested on both

crops as brown and yellow mottling on leaves,

necrosis along leaf margins, reduction in height,

TABLE 15.—Yield reduction of 'Lee' soybeans

after treatment with phenoxy herbicides,

Stuttgart, Ark., 1958 and 1959"^

[Percent]

Herbicide^

and rate

Soybean stage at

treatment^ Average

(lb/acre) Vegetative Bloom

2,4-D :

0.01 5 10 8

.05 19 8 14

.10 28 7 18

.25 50 10 30

Average • 26 9 18

2,4,5-T:

0.01 14 4 9

.05 30 26 28

.10 45 41 43

.25 74 60 67

Average • 40 33 37

Silvex:

0.01 10 8 9

.05 28 26 28

.10 48 34 41

.25 90 m 78

Average • 44 34 39

1 Untreated weed-free checks yielded an average of

44 bu/acre of soybeans. Data taken from Smith {106).

2 Amine salts of 2,4-D and 2,4,5-T used; low-volatility

ester of silvex used.

3 Plants in the vegetative stages were 6 to 12 inches

tall ; those in the early bloom stage were 30 to 36 inches

tall.
16 WEED CONTROL IN U.S. RICE PRODUCTION

and death of plants. Plants that were not killed

grew new leaves, recovered vegetatively, and

produced nearly normal yields, but maturity

was often delayed. Older plants recovered more

quickly and completely than younger ones.

Molinate.—Drift of molinate from spray or

granular application may dam.age susceptible

crops. Soybeans, injured vegetatively by direct

applications of granular molinate at 3 to 6

lb/acre, were not reduced in yield (R. J. Smith,

Jr., unpublished). Treated plants exhibited leaf

crinkling, chlorosis, and necrosis.

Residues

Federal laws provide for establishing maximum levels, called tolerances, of residual herbicides on and in raw agricultural products.

Rough and milled rice are classed as raw agricultural products. Tolerances of 2 parts per

million in rice grain and 75 parts per million

in straw have been established for propanil by

the Federal Environmental Protection Agency.

A tolerance of 0.1 part per million in rice grain

and straw has been established for molinate.

Tolerances for 2,4-D, 2,4,5-T, MCPA, and silvex

have not been established, but the use of these

herbicides for weed control in rice is permissible until tolerances are set.

The label directions on pesticide containers

are subject to clearance under the Federal

Insecticide, Fungicide, and Rodenticide Act as

amended. Clearance is withheld if, on the basis

of experimental evidence, the use proposed may

result in residues exceeding established tolerances. Container labels, therefore, furnish a

reliable guide for using pesticides in a way that

will avoid excessive residues.

Some herbicides persist at toxic levels in soil;

others are degraded, altered, or inactivated by

micro-organisms or by absorption to soil colloids, leaching, volatilization, photodecomposition, and perhaps other ways {67). Factors

that affect persistence include soil type, moisture and organic matter in soil, rainfall, temperature, herbicide type, cropping system, and

cultivation procedures. Applied to ricefields at

normal rates, the phenoxy herbicides 2,4-D,

MCPA, and 2,4,5-T persist in soil for 1, 3, and

5 months, respectively {6í)\ molinate may persist for about 1 month; and only small amounts

(0.2 part per million or less by weight) of

propanil are found 2 to 4 weeks after treatment

{65), Silvex is more persistent than 2,4-D but

less persistent than 2,4,5-T. Although propanil

may convert to 3,3',4,4'-tetrachloroazobenzene

(TCAB), only small quantities (less than 0.2

part per million by weight) are found {65),

Herbicides used for control of weeds in rice are

sufficiently short-lived that their residues are

not injurious to other crops planted after harvesting rice.

Storage

Some agricultural chemicals may be dangerTABLE 16.—Effect of time and rate of propanil

applications on yield of seed cotton^ Rohwer, Ark., 1961-63^

Propanil ^^^^^ ^^ ^^^^ ^^^^^^ '(lb/acre) Reduction

in yield

(pet) 2

(lb/acre) iggi 1962 1963 Average

COTTON 2 TO 4 INCHES TALL WHEN SPRAYED

0.01 1,520

.05 1,980

.1 1,260

.25 1,430

.5 750

1.0 570

2.0 120

2,360 2,890

2,360 2,400

2,120 2,540

1,920 2,760

1,760 2,270

1,510 2,100

940 930

2,260

2,250

1,970

2,040

1,590

1,390

660

2

3

15

12

31

40

72

Average 1,740 25

COTTON 10 TO 22 INCHES TALL WHEN SPRAYED

0.01 1,390

.05 1,610

.1 1,770

.25 1,620

.5 870

1.0 880

2.0 570

2,410 2,650

2,210 2,550

2,290 2,680

2,080 2,580

2,160 2,430

2,160 2,210

1,710 2,060

2,150

2,120

2,250

2,090

1,820

1,750

1,450

7

8'

3

10

21

24

37

Average ... 1,950 16

UNTREATED CHECK

0 2,020 2,330 2,580 2,310 0

LSD (5 pet) 3 500 280 480 420 18

1 Weeds controlled in all plots. Data taken from Smith

{108).

2 Yield expressed as percentage reduction compared

with yield of the untreated check.

3 LSD (least significant difference) applies to stage

by rate averages (**Average" column), but not to stage

averages.
PRINCIPLES OF WEED CONTROL 17

DUS in storage if formulated with solvents having low flash points (140° F or less). The

herbicides used for weed control in rice have

relatively high flash points; therefore, spontaneous combustion is not likely to occur during

storage or use of these chemicals.

Proper storage is essential to prevent the

deterioration of herbicides and their containers.

Dust and wettable powders stored in a dry

place do not deteriorate rapidly, but if they

get wet, they may cake and the packages may

deteriorate. Water-soluble solids may cake

when wet and when subjected to great changes

in temperature. If packages are left open,

hygroscopic chemicals become wet by absorbing

TABLE 17.—Effect of time and rate of propanil

applications on yield of soybeans, Stuttgart,

Ark., 1961-63^

Propanil Yield of soybeans (bu/acre)

(lb/acre) "Wdi Ï962 1963 Average

Reduction

in yield

(pet) 2

SOYBEANS 2 TO 5 INCHES TALL WHEN SPRAYED

0.01

.05

.1

.25

.5

38

37

36

34

30

27

26

26

26

25

35

37

35

31

34

33

33

32

30

30

1.0

2.0

28

25

21

18

36

24

28

22

15

33

Average • • • 30 9

SOYBEANS 10 TO 25 INCHES TALL WHEN SPRAYED

0.01

.05

.1

.25

.5

1.0

2.0

36

32

38

38

34

32

34

26

27

26

24

24

23

23

34

33

34

33

33

29

25

32

31

33

32

30

28

27

3

6

0

3

9

15

18

Average 30 9

UNTREATED CHECK

0 37 27 34 33 0

LSD (5 pet) 3 3 4 4 4 12

1 Weeds controlled in all plots. Data taken from Smith

{108),

2 Yield expressed as percentage reduction compared

with yield of untreated check.

3 LSD (least significant difference) applies to stage

by rate averages (^'Average" column), but not to stage

averages.

water from moist air. It is essential, therefore,

to store containers tightly sealed in cool, dry

areas.

Liquid formulations can be stored on pallets

or duckboards to prevent water condensation

and rusting of metal containers. Tightly closed

containers reduce herbicide deterioration. Small

amounts of water introduced into emulsifiable

concentrates or oil solutions may cause jelling

or corrosion of the container.

Liquid formulations of phenoxy herbicides,

propanil, and molinate may crystallize or precipitate at temperatures below 30° F. The

crystals can usually be redissolved by rolling

or shaking the container after the liquid has

been warmed to 40° F or slightly higher. If the

crystals dissolve, the herbicide is usable; if

they do not, the herbicide cannot be applied

properly.

Liquid herbicides may become unusable if

stored at temperatures above 95° F. Herbicides

stored in drums expand upon warming, and the

increased internal pressure may cause the containers to leak. High temperatures may also

reduce the effectiveness of emulsifiers and

hasten the corrosion of containers. In direct

sunlight in hot weather, herbicide drums become excessively hot. Therefore, storage of

herbicides in the shade or in well-ventilated

buildings or sheds during hot weather reduces

the chances of deterioration.

Handling

Although the herbicides used in rice are

comparatively low in oral and dermal toxicity

to humans and animals, contact should be

avoided. Ingestion of even small quantities of

some herbicide concentrates or spray solutions

can cause severe acute or chronic illness or

even death. They may also be irritating to the

skin and eyes. Washing the skin or eyes

thoroughly with clean water immediately after

accidental contact can prevent severe injury.

If repeated and prolonged exposure is likely,

goggles and protective clothing should be worn.

Poison control centers located throughout the

United States disperse information on treatments to administer if pesticides are ingested

or contacted.

Fish and wildlife may be injured if ponds,
18 WEED CONTROL IN U.S. RICE PRODUCTION

lakes, or streams are contaminated with herbicides. Feed or food stored near herbicides may

become contaminated. Fumes from volatile

herbicide containers are likely to offset the

viability of seed, including rice seed, stored

nearby.

Cleaning spray equipment

Phenoxy herbicides are difficult to remove

from spray equipment. Since even traces of

these chemicals may severely injure cotton,

tomatoes, and other sensitive plants, equipment

used to spray phenoxy herbicides should never

be used to apply other chemicals to these plants.

After thorough cleaning, a sprayer used for

phenoxy herbicides can be used to apply fungicides, insecticides, and other herbicides to

moderately sensitive crops such as soybeans,

clover, and lespedeza. Thoroughly cleaned

sprayers previously used for applications of

propanil and molinate can be used to apply

other pesticides to most plants, including cotton,

soybeans, clover, lespedeza, small grains, and

vegetable crops.

Thorough cleaning of a sprayer with warm

water and detergent removes most of the herbicide in the sprayer. Further cleaning is usually

necessary and may be accomplished with a

solution of 1 part household ammonia in 100

parts of water. Cleaning is enhanced when

this solution remains in the tank, hose, and

boom for 12 to 24 hours before rinsing the

equipment with clean water.

Finely ground activated charcoal is one of

the best agents for cleaning herbicides from

spray equipment; it is faster and more effective

than household ammonia. Phenoxy herbicides

can be removed by rinsing the sprayer for

about 2 minutes with a 1-percent suspension of

activated charcoal (for example, 1 pound of

powdered activated charcoal in 12 gallons of

water), followed by rinsing with clean water.

The addition of a detergent to the water aids

in wetting and suspending the activated charcoal so that it can be rinsed easily from the

tank. Residues of activated charcoal in the tank

could reduce the activity of the next chemical

to be sprayed.

To be sure the sprayer has been adequately

cleaned, add water to the tank and spray it on

seedlings of a sensitive plant, such as bean,

tomato, or morningglory. If the seedlings are

not injured after 1 or 2 days, the equipment is

probably clean enough for use on moderately

sensitive plants.

Propanil

Propanil, which is only slightly soluble (500

p/m) in water, but highly soluble (about 25

percent) in most common organic solvents, is

formulated with organic solvents and emulsifiers and is slightly volatile at high temperatures (95° to 100° F). The acute oral LD50 of

technical propanil for rats is low, 1,384 mg/kg.

Mode of selectivity

Propanil applied to emerged rice and weeds

selectively kills barnyardgrass and many other

grass, aquatic, broadleaf, and sedge weeds,

while rice is only slightly injured (table 12).

The basis of this selectivity is biochemical. Experiments with a radioactive isotope of propanil

indicate that both rice and barnyardgrass absorb and translocate propanil rapidly and equally (150), An enzyme, aryl acylamidase, in the

leaves of rice plants rapidly detoxifies propanil

by oxidative metabolism and hydrolysis to 3,4-

dichloroaniline (DCA) and propionic acid (38).

Young rice leaves contain approximately 60

times as much aryl acylamidase as young barnyardgrass leaves. Roots of rice and barnyardgrass contain the same low concentrations of

the enzyme.

In propanil-treated rice, the DCA hydrolysis

product becomes tightly bound in carbohydrate

and lignin constituents of the cell walls of the

rice plant, from which it can be released only

by extreme caustic treatment (130, 151), DCA

was found at higher concentrations in the straw

than in the grain. The propionic acid, formed

by the hydrolysis of propanil, is further

metabolized to carbon dioxide and water by

beta-oxidation (129),

Scientists in Japan found a rice mutant that

is susceptible to propanil (75), Research indicated that this susceptible rice hydrolyzes

propanil at a much slower rate than the tolerant rice. Apparently, the susceptible rice

contained a very low concentration of the

propanil-hydrolyzing enzyme.
PRINCIPLES OF WEED CONTROL 19

TABLE 18.—Effect of propanil application rate on barnyardgrass control and rice yields, Stuttgart, Rohiver, and Reiser, Ark., 1960-6í

[Propanil applied to 1- to 4-leaf grass, 1 to 3 inches tall, and 1- or 2-leaf rice, 3 to 4 inches tall]

Barnyardgrass control^ (pet) Rough rice yield (lb/acre)

Year and

location Propanil rate (lb/acre) Lgj) Propanil rate (lb/ acre) LSD

2 3 4 5 7 9 12(5pct)2 Q ^ ^ ¡ I ^ ^ 12~(5pct)2

1960

Stuttgart 80 .. 84 88 .... 8 2,640 4,640 4,700 4,630 330

1961

Stuttgart ...... 57 .. 70 77 73 .. 15 2,340 3,520 3,670 4,050 3,630 550

Rohwer 70 85 .. 85 88 83 87 (0 2,460 3,110 3,320 3,380 3,400 3,410 3,730 (*)

Keiser 70 78 .. 86 91 96 99 10 3,490 3,840 3,700 4,000 3,860 3,930 3,590 (*)

1962

Stuttgart 65 .. 85 90 89 .. 12 2,590 3,930 4,610 4,850 4,750 650

Do 72 80 85 (5) 2,960 4,690 5,020 5,540 980

Rohwer 99 .. 100 (s) 2,860 4,180 4,100 420

Keiser 70 .. 85 (s) 1,260 2,950 3,450 390

1963

Stuttgart ....35 85 78 23 3,150 4,020 5,180 4,620 670

Keiser 96 .. 98 (5) 2,770 3,870 3,700 450

196J^

Stuttgart ....90 100 98 (s) 3,460 6,060 6,450 6,400 740

Rohwer 85 .. 97 {-) 2,900 5,570 5,770 420

Average ...67 82 87 88 87 85 93 (5) 2,740 4,340 4,360 5,520 4,150 4,160 3,930 3,660 (s)

1 Measurements made at midseason after grass matured.

2 LSD == least significant difference at level given.

^ No significant differences at the 5-pct level.

^ Yield of untreated rice was significantly less than that of treated rice at the 5-pct level, but yields of plots

treated at 2 to 12 lb/acre were not significantly different at the 5-pct level.

^ Not determined.

TABLE 19.—Effect of time and rate of propanil application on barnyardgrass control and rice

yield, Stuttgart, Ark,, 1960

Barnyardgrass control (pet) Rough rice yield (lb/acre)

Grass stage Propanil rate Propanil rate

at treatment (lb/acre) Average (lb/acre) Average^

1 5 7~ 3 5 7

After seeding 17 18 15 17 2,320 2,660 2,930 2,640

1 leal 1/2 ineh2 58 52 64 58 4,100 3,720 4,100 3,970

2 or 3 leaves, 2 inches .... 80 84 88 84 4,640 4,700 4,630 4,660

LSD (5 pet)3 ... ... 8 ... ... ... 330

LSD (1 pet)3 ... ... 11 ... ... ... 440

1 The hoed and unhoed checks yielded 4,720 and 2,640 lb/acre, respectively.

- Barnyardgrass reinfested plots after herbicide treatment.

3 LSD = least significant difference at level given.
20 WEED CONTROL IN U.S. RICE PRODUCTION

PN-4936 PN-4936

FIGURE 1.—Control of bamyardgrass in rice with propanil. Left, plot sprayed with propanil at 4 lb/acre when

barnyardgrass had two or three leaves. Right, unsprayed plot.

'JSMÉ

PN-4937

FIGURE 2.—Growth stages of barnyardgrass. Left to right, plants in one-, two-, three-, four-, and five-leaf and

tillering stages.

Time and rate of application

Propanil applied at 3 to 5 lb/acre controls

barnyardgrass 1 to 3 inches tall, in the one- to

four-leaf stage of growth, with little or no

injury to rice (tables 18 and 19, figs. 1 and 2).

These rates are considered optimum for weed

control in rice. It is usually ineffective as a

preemergence application or when applied to

barnyardgrass in the tillering stage of growth

or later (table 20).

Ducksalad 1 inch tall or less is controlled

by propanil, but plants 2 to 6 inches tall are

usually resistant (table 21). Likewise, young
PRINCIPLES OF WEED CONTROL 21

gooseweed, northern jointvetch, and purple

ammannia plants 1 to 3 inches tall are controlled, but older ones, 12 to 24 inches tall, are

resistant. Propanil controls blunt spikerush 3

inches tall, dwarf spikerush 1 inch tall, hemp

sesbania 2 to 48 inches tall, tighthead sprangletop 2 inches tall, waterhyssop 4 inches tall, and

yellow foxtail [Setaria glauca (L.) Beauv.] 4

inches tall. It does not control dayflower or

morningglory plants that are 2 to 4 inches tall

or taller.

Temporary chlorosis and tipburn on rice

leaves may occur soon after treatment with

propanil at rates of 3 to 5 lb/acre, but permanent injury usually does not occur even at

rates as high as 12 lb/acre. Such temporary

injury is caused mainly by formulation solvents.

Injury of rice is more pronounced and persists

longer when temperatures are either extremely

low (daily lows below 50° F) or extremely high

(daily highs above 95° F) at the time of treatment. Even when injury seems severe, rice

plants usually recover within 2 weeks after

treatment, and new leaves that emerge after

treatment show no injury symptoms.

Propanil controls rapidly growing weeds effectively, but frequently fails to control weeds

growing slowly under conditions of low moisture or low temperatures. It controls barnyardgrass seedlings growing rapidly in moist soil

better than slow-growing ones in dry soil. In

a 1961 field experiment propanil became decreasingly effective against barnyardgrass as

the weed plants increased in size (table 20).

In 1962 the effectiveness of propanil again

decreased as the grass plants increased in size

from 1/^ to 2 inches to 6 to 13 inches tall. But

applications to grass 10 to 36 inches tall (tillering-to-jointing and jointing-to-heading stages

of growth) controlled grass better than treatment of plants 6 to 13 inches tall (tillering

stage). The increased control of 10- to 36-inchtall grass was caused by an application of nitrogen 1 to 2 weeks before treatment, which

stimulated growth of the grass. Propanil controlled the larger fast-growing grass better

than it controlled the smaller slow-growing

grass. The results of this experiment indicate

that the growth rate of weeds can affect the

efficacy of propanil.

Propanil kills barnyardgrass plants that have

emerged, but it fails to control those that

emerge after treatment (table 19). Weed

grasses and aquatic, broadleaf, and sedge weeds

that germinate after treatment become troublesome and reduce yields. Usually all weed grasses

have emerged by the time the oldest grass

reaches the four-leaf stage, but sometimes they

emerge after treatment if the floodwater is not

applied in time or if the flood has to be drained.

Propanil is usually applied at the most susceptible growth stage of the grass. When barnTABLE 20.—Effect of time of propanil application on barnyardgrass control and rice

yields, Stuttgart, Ark., 1961 and 1962

[Values are averages from plots treated with 3, 5, 7,

and 9 lb/acre]

Grass stage at treatment 1961 1962 Average

Grass control (pct)i

1/2 to 2 inches or 1 to 4 leaves • • 69 ÎÔÔ W

V2 to 7 inches or 2 leaves to

tillering 60 82 71

2 to 8 inches or tillering 48 50 49

6 to 13 inches or tillering 17 22 20

10 to 20 inches or tillering to

jointing 0 42 21

15 to 36 inches or jointing to

heading 10 46 28

LSD (5 pct)2 ñ 16 Ï4~

LSD (1 pet) 2 16 23 20

Rice yield (lb/acre) ^

1/2 to 2 inches or 1 to 4 leaves • • 3,720 4,680 4,200

V2 to 7 inches or 2 leaves to

tillering 3,570 4,530 4,050

2 to 8 inches or tillering 3,440 3,950 3,700

6 to 13 inches or tillering 2,700 3,070 2,890

10 to 20 inches or tillering to

jointing 2,310 3,510 2,910

15 to 36 inches or jointing to

heading 2,530 3,380 2,950

LSD (5 pct)2 440 890 660

LSD (1 pct)2 630 1,060 840

1 In 1961 nitrogen fertilizer was applied after the

last spraying; in 1962 it was applied 7 and 14 days

before the tillering-to-jointing and jointing-to-heading

treatments, respectively. Measurements were made at

midseason after grass matured.

2 LSD = least significant difference at level given.

^ There was no grass-free check plot, but the untreated check plots yielded 2,340 and 2,010 lb/acre, respectively, in 1961 and 1962.
22 WEED CONTROL IN U.S. RICE PRODUCTION

TABLE 21.—Control of weeds in various stages with propanil, Stuttgart,

Rohwer, and Reiser, Ark., 1960-6U

Weed

Ducksalad

Northern jointvetch

Purple ammannia \ 19

Í 3

Gooseweed •! 04

Waterhyssop 4

2

2

2

Hemp sesbania < 2

24

36

48

20

20

Weed Percentage of weed control with propanil

height Year applied at (lb/acre)—

(inches) 0.5 1 2 3 4 5

1962

1962

1960

1961

1962

1963

1960

1961

1962

1962

1960

1962

1960

1962

1961

1962

1963

1964

1961

1962

1960

1962

1963

1964

1962

1962

1963

1963

1963

1962

Tighthead

sprangletop.

Blunt spikerush • • •

Dwarf spikerush . •

Yellow foxtail

Tall morningglory {

50

70

... 40 ... ...

0 20

10

40 50

80

10

20 ... ••

0 20 ... 10 ... 50

40 30

20 20

90

80

...

0 0

90

20 40

10 50

70 80

40 80

70 80

90

90

100

...

80

100

90

40 90

90 90

100

20 100

90

100

90

90

90

90

100

30

30

100

90

100

100

100

80

yardgrass has reached the one- to four-leaf

stage of growth, rice usually has one or two

leaves, but it may be smaller or larger. A satisfactory stand of rice before propanil treatment

is desirable. The propanil treatment would be

wasted on a poor stand of rice that had to be

replanted after treatment.

Water management

Irrigating dry ricefields 2 to 5 days before

propanil treatment increases weed control. In

an experiment at Stuttgart, Ark., barnyardgrass grew slowly and was not controlled by

propanil when rice was seeded in soil sufficiently moist for germination of rice and grass, but

which did not receive any rain during the

11-day period from planting to spraying. Irrigation water applied 3 days before spraying

stimulated growth of barnyardgrass and increased control. In this experiment, propanil

controlled 90 percent of the barnyardgrass that

grew in moist soil, but it controlled only 70

percent of the grass that grew in dry soil.
PRINCIPLES OF WEED CONTROL 23

Propanil does not kill weeds covered with

water at the time of treatment because it cannot

contact the leaves of the weeds. Flooding rice

3 to 4 inches deep 1 to 5 days after treatment

prevents the germination of more barnyardgrass plants. In experiments at Stuttgart, Ark.,

propanil controlled 80 percent of the grass in

plots flooded 5 days after spraying, but only

10 percent in plots flooded 22 days after treatment. Grass plants that emerged during the

17-day delay of flooding reduced rice grain

yields.

Some commercial ricefields are so large that

timely flooding after a propanil treatment may

be difficult. If a large ricefield is sprayed all

at once, weed grasses may germinate after the

treatment in some areas where flooding is delayed. Separate treatment of portions of large

fields to permit timely flooding with the available water supply prevents weed grasses from

reinfesting sprayed fields.

Method o£ application

Propanil may be applied by ground or aerial

spray equipment. Boom-nozzle sprayers are

commonly used for ground treatments. Boomnozzle, venturi, or rotary-atomizer sprayers,

when properly adjusted, are satisfactory for

aircraft spraying. Fixed-wing and helicopter

aircraft can be used to apply propanil. Aircraft

spraying is usually better than by ground rigs

because levees do not reduce spraying efficiency, and fields too wet to support ground

equipment can be sprayed rapidly and at the

correct time. Propanil sprays applied in medium-fine droplets (200 to 300 micrometers in

diameter while in the air) cover weeds sufficiently for effective control and are less likely

to drift than finer droplets.

The spray pattern of aircraft and tractor

sprayers affects the efficacy of propanil treatments. Propanil is most effective when the

spray covers the weeds well. When fixed-wing

aircraft sprayers are properly adjusted, a swath

width of 30 to 40 feet (equal to or slightly less

than the wingspan of the aircraft) usually

gives good spray coverage. When tractor or

helicopter sprayers are properly adjusted, the

length of the spray boom governs the swath

width. Operation of aircraft 8 to 10 feet above

the ground usually permits satisfactory spray

coverage and minimizes spray drift.

Spray volume

Adequate coverage of leaves with propanil

spray is required for effective weed control.

Normally propanil applied with water carrier

at 15 to 20 gal/acre controls weeds satisfactorily

when tractor sprayers are used, and volumes

of 10 to 15 gal/acre are sufficient when spraying by aircraft. Propanil sprays applied in volumes below 15 gal/acre with tractor sprayers

or below 10 gal/acre with aircraft sprayers

frequently control weeds less effectively and

are more susceptible to drift than applications

at higher spray volumes. Aerial applications of

propanil at 12 to 15 gal/acre give better coverage and control of dense stands of large weeds

(6 to 12 inches tall) than lower volumes.

In a field experiment in Arkansas, propanil

at 3 lb/acre applied at the proper time with a

tractor sprayer at volumes of 5, 10, 20, or 40

gal/acre controlled barnyardgrass effectively

at all volumes without rice injury. Conversely,

propanil controlled weeds less effectively as the

spray volume decreased in experiments in

Texas. Propanil at 3 lb/acre applied with a

tractor sprayer controlled 99, 90, and 66 percent

of the barnyardgrass when applied in water

volumes of 10, 8, and 5 gal/acre, respectively.

Further research showed that tractor-sprayer

applications of propanil at 3 to 5 lb/acre controlled barnyardgrass and other weeds effectively when applied at the correct time at

volumes of 15 to 20 gal/acre.

At Stuttgart, Ark., aerial applications of

propanil at 3 lb/acre in a 5-gal/acre water

emulsion controlled weed grasses less effectively than at the same rate in 10-gal/acre emulsion. In another experiment at Stuttgart,

propanil applied by a boom-nozzle aircraft

sprayer at 3 or 4 lb/acre in 5 or 10 gal/acre of

spray controlled barnyardgrass effectively and

did not injure rice. Spray drift, however, appeared to be less with 10 than with 5 gal/acre.

In a third experiment at Stuttgart, propanil

applied by aircraft with a boom-nozzle sprayer

at 3 lb/acre in 10 gal/acre controlled 95 percent

of the barnyardgrass, broadleaf signalgrass,

and northern jointvetch plants and did not
24 WEED CONTROL IN U.S. RICE PRODUCTION

injure rice. Propanil applied by aircraft at the

same rate in 3 gal/acre of emulsion with a

rotary-atomizer sprayer controlled only 70 to

80 percent of the weeds and injured rice

leaves. Adding surfactant L^ at 2 percent

(volume/volume) to the 3-gal/acre treatment

increased weed control to 95 percent, but this

treatment injured rice leaves severely. Adding

surfactant to the 10-gal/acre treatment did not

alter the activity of the spray on weeds or rice.

Presumably, the surfactant increased foliar retention and penetration of the 3-gal/acre treatment by both weeds and rice plants. The

concentration of the propanil in the spray of

3 gal/acre was three times that at 10 gal/acre,

which would probably increase the injury by

solvents in the formulation.

Method and depth of seeding

Propanil at 2 to 16 lb/acre applied to wateror dry-seeded 'Nato' or 'Bluebonnet 50' rice

grown in the greenhouse injured only slightly

rice plants in the one- to two-leaf stages of

growth, but 32 lb/acre injured rice moderately

(table 22). Propanil applied in the field at 3

to 12 lb/acre did not reduce grain yields of

'Bluebonnet 50' rice that was water- or dryseeded (table 23). This information indicates

that field rice plants as well as succulent greenhouse plants are not injured by high rates of

propanil.

Propanil properly applied usually controls

^ Petroleum sulfonate, free and combined fatty acid,

and petroleum oil.

TABLE 22.—Effect of seeding method and depth

and rate of propanil applied early postemergence on growth of 'Nato' and 'Bluebonnet 50' rice in greenhouse conditions,

Stuttgart, Ark,, 1959

[Percent injury based on visual rating]

Seeding method Propanil rate (lb/acre)

and depth 0 2 4 8 16 32

'Nato':

Water-seeded 0 0 0 13 17 60

Dry-seeded, 1/2 inch •. 0 20 0 0 13 37

Dry-seeded,

11/^ inches 0 0 0 0 13 27

'Bluebonnet 50':

Water-seeded 0 0 0 0 23 37

Dry-seeded, 1/2 inch • • 0 0 0 7 10 43

Dry-seeded,

11/2 inches 0 0 0 0 0 13

susceptible weeds in dry- and water-seeded rice.

The method of seeding the rice crop does

not alter the effectiveness of propanil for weed

control, but it does influence the populations of

weed species infesting fields. Dry seeding increases problems with weed grasses, and water

seeding enhances growth of aquatic and sedge

weeds while decreasing grass populations.

Rice cultivars

Most rice cultivars grown commercially in

the United States tolerate propanil applied at

effective rates for weed control. In one field

experiment at Stuttgart, Ark., propanil at 3

or 6 lb/acre applied as a postemergence treatTABLE 23.—Yield of 'Bluebonnet 50' rice grown in the field as influenced

by seeding method and depth and rate of propanil applied early postemergence, Stuttgarty Ark,, 1960^

Seeding method

and depth

Rough rice yield,

untreated plots

(lb/acre)

Yield increase,

treated plots (pet) 2

Propanil

rate (lb/acre)

1 6 12~

Average

Water-seeded : Soil surface 5,630

Dry-seeded :

Drin-seeded ; 11/2 inches 5,310

Broadcast-seeded; ^2 to 1^/4 inches 5,220

1 Rice essentially weed free.

2 Percentage increase in yield over untreated plots.
PRINCIPLES OF WEED CONTROL 25

ment did not injure the cultivars listed in

table 24. Applications of propanil at normal

use rates in field experiments and in commercial

ricefields have not injured the following cultivars: 'Arkrose', *Belle Patna', 'Bluebelle',

'Nova', 'Nova 66', and 'Starbonnet' in the South

and 'Calrose', 'Colusa', 'Earlirose', and 'CSM3' in California.

Nitrogen fertilization

Propanil controlled barnyardgrass effectively

when urea nitrogen (45 percent N as granules)

was applied at any time from the day of herbicide treatment until 15 days after spraying

(table 25). None of the treatments injured rice.

In field experiments and in commercial ricefields, applications of nitrogen as urea or ammonium sulfate applied any time before or

after propanil treatments did not affect the

selectivity of propanil to rice or the control of

rapidly growing, susceptible weeds 3 inches tall

or less.

In field experiments propanil mixed with

water injured rice less than propanil applied

in nitrogen solution.^ Rice treated with propanil

in 20 gal/acre of nitrogen solution was injured

4 Contains 45 percent ammonium nitrate, 35 percent

urea, and 20 percent water.

TABLE 24.—Effect of postemergence treatment

with propanil on yields of rice cultivars,

Stuttgart, Ark., 1960 and 1961''

Rough rice yield Yield increase

(lb/acre) or decrease,

^^^^'^^'' Untreated Treated treated plots

plots plots2 (pet) 3

^Bluebonnet 50^ 5,070 4,950 -2

^Caloro' 5,900 5,760 -2

^Century Patna 231' . 5,440 5,530 +2

*Gulfrose' 5,230 4,890 - 6

^Nato' 5,800 5,920 +2

^Northrose' 6,270 6,320 +1

Tainan-iku 487'

(PI215936) 6,150 6,080 -1

^Zenith' 5,290 5,100 -4

1 All plots kept weed free by hoeing.

2 Plots treated at 3 and 6 lb/acre.

3 Yields from treated and untreated plots did not

differ significantly for any variety at the 5-pct level.

severely within 2 days after treatment, although

it recovered after 10 days. Rice treated with

propanil in 20 gal/acre of water emulsion was

not injured. Nitrogen solution alone at 20 gal/

acre injured the leaves of rice and barnyardgrass moderately within 2 days after treatment,

but they recovered after 10 days. Propanil

mixed with water controlled barnyardgrass

better than propanil in mixtures of water and

TABLE 25.—Effect of time of applying urea nitrogen after propanil treatments on barnyardgrass control, Stuttgart, Ark,, 1963

[Propanil applied at 4 lb/acre to 1- to 4-leaf grass, ^/i to 2 inches tall, and to 1- and

2-leaf rice, 2 to 4 inches tall]

Grass control (pct)^

Method of Time of urea application

grass control after propanil treatment (days) Average

~Ö 5 ñ 15"

Hand-hoed2 80 77 82 85 81

Propanil2 93 83 78 92 86

Hand-hoed check^ •. • • • • 92

Propanil check^ .. .. .. 86

None (untreated check) * • • . • • • 7

^ Measurements made at midseason after grass matured.

- Values are averages from plots that received 40 and 80 lb/acre nitrogen at 0, 5, 11,

or 15 days after propanil and an additional 40 lb/acre 60 days after rice emergence.

Grass control on hand-hoed or propanil-treated plots did not differ significantly at the

5-pct level for all times of nitrogen application.

"' No nitrogen was applied early, but 40 lb/acre was applied at 60 days.

' No propanil or hand hoeing; nitrogen applied as explained in footnote 2.
26 WEED CONTROL IN U.S. RICE PRODUCTION

nitrogen solution or in nitrogen solution alone

(table 26).

In another field experiment mixtures of

propanil and water or of propanil and nitrogen

solution controlled one- to four-leaf barnyardgrass about equally (table 27). Mixtures of

propanil and nitrogen solution injured rice

more than mixtures of propanil and water, but

rice recovered 1 to 2 weeks after treatment.

Barnyardgrass with five or more leaves was not

controlled by either mixture.

Mixtures of propanil and nitrogen solution

controlled barnyardgrass no better and usually

injured rice more than propanil and water

mixtures. When nitrogen solution is used as

the nitrogen fertilizer, an application 1 to 3

days after a propanil treatment should cause

fewer complications than an application of both

as a tank mixture.

Soil type

Soil type does not affect the activity of

propanil on rice or weeds (table 28). Correctly

timed propanil applications at 3 to 5 lb/acre

TABLE 26.—Effect of nitrogen solution used as

a carrier for propanil on barnyardgrass

control and rice yields, Stuttgart, Ark.,

1963

[Propanil applied with ground equipment at 3 lb/acre

to 1- to 4-leaf grass, i/4 to 2l^ inches tall, and to 1- and

2-leaf rice, IV2 to 4 inches tall]

Carrier and spray volume

(gal/acre) Grass

control

(pet) 2

Rough rice

yield

Water (lb/acre) Nitrogen

solution^

30 99 5,870

40 0 1,900

20 0 80 5,230

10 10 35 3,420

5 15 42 3,140

0 20

ct) 5

30 2,770

LSD (5 p ...14 1,290

1 Plots treated with nitrogen solution alone at 10, 15,

and 20 gal/acre were not significantly different from

the weedy check in grass control or rice yield at the

5-pct level.

2 Measurements made at midseason after grass matured.

3 Hoed check.

4 Unhoed check.

5 LSD = least significant difference at level given.

to actively growing plants controlled barnyardgrass effectively on Crowley silt loam, Sharkey

clay, and Perry clay. Treated rice yielded significantly more grain than untreated. Where

barnyardgrass infestations were heaviest, yield

increases from the treatments were greatest.

Rainfall and temperature

Rainfall and air temperature affect the herbicidal activity of propanil on weeds. At least 8

hours without rain after propanil treatment is

usually required for effective weed control. In

experiments at Stuttgart, Ark., when a 2-inch

rain occurred 3 and 9 hours after treatment,

propanil at 5 lb/acre controlled 40 and 95 percent of the barnyardgrass and treated rice

yields were 2,830 and 6,260 lb/acre, respectively. Rain 3 hours after the treatment apparently

washed off the propanil before it was absorbed

by the weeds and reduced activity.

Best control of barnyardgrass, other weed

grasses, and broadleaf weeds by propanil occurs

TABLE 27.—Effect of carrier and of time and

rate of propanil treatments on barnyardgrass control, Stuttgart, Ark., 196A

[Propanil applied in 20-gal/acre emulsion with ground

equipment]

Grass control (pet) 2

Time propanil applied Propanil rate

after grass emergence /IK/O^T.^\ and carrier! Ub^acre) Average3

0 2 4 6

1 week, water 0 53 70 73 65

1 week, N solution 0 53 73 70 65

2 weeks, water 0 17 37 30 28

2 weeks, N solution .... 0 20 30 23 24

3 weeks, water 0 0 7 13 7

3 weeks, N solution .... 0 10 20 13 14

5 weeks, water 0 3 17 23 14

5 weeks, N solution .... 0 10 27 23 20

7 weeks, water 0 0 10 17 9

7 weeks, N solution .... 0 0 7 3 3

1 Stage and height of grass when sprayed : 1 week,

1 to 4 leaves, i/4 to IV2 inches tall; 2 weeks, 1 to 5

leaves to tillering, ^4= to 3 inches tall ; 3 weeks, tillering,

2 to 5 inches tall; 5 weeks, tillering and jointing, 8 to

12 inches tall; 7 weeks tillering, jointing, and heading,

12 to 16 inches tall.

2 Measurements made at midseason after grass matured.

^ Value for zero rate excluded from average.
PRINCIPLES OF WEED CONTROL 27

TABLE 28.—Effect of soil type on harnyardgrass control and rice yield in

plots treated with propanil, Stuttgart, Ark., 1961

[Values are averages from plots treated with 3 and 5 lb/acre propanil applied to 1- to

3-leaf grass]

Grass Rough rice yield (lb/acre)^ yield increase,

Soil control Untreated Treated treated plots

(pet) plots plots (pet)

Crowiey silt loam 82 2,610 4,670 79

Sharkey clay 82 3,490 3,850 11

Perry clay 85 2,460 3,350 36

1 Yields from treated and untreated plots differed significantly at the 5-pct level on

all soils.

when daily maximum air temperatures range

from 70° to 90° F and daily minimums are

above 60° F. Propanil is most active at these

temperatures when cloudy weather prevails for

several days before and after herbicide treatment. Presumably, cloudy weather retards

cuticle development on grass leaves, slows detoxification of propanil in the plant by enzymes

{50) and decreases evaporation of the herbicide, all of which make the herbicide more

effective.

When daily low and high temperatures are

below 50° and 70° F, respectively, for a few

days just before treatment, propanil treatments

are frequently ineffective. Propanil activity

may be reduced by temperatures above 95° F.

Extremely low and high temperatures slow the

growth rate of grass plants, making them less

susceptible to propanil. When temperatures are

unfavorable for propanil activity, it would be

advantageous to delay spraying.

Multiple applications

Flooding ricefields soon after propanil applications may damage rice grown on problem

(calcareous, saline, and sodic) soils {89), If

the water is withheld to prevent rice injury,

barnyardgrass and other weeds usually reinfest

the field. Sometimes sequential propanil treatments can be used to advantage in these situations. The first treatment of 2 to 4 lb/acre is

made when the largest weed grasses are about

2 inches tall; a second treatment of 3 to 4 lb/

acre is applied after late-germinating weeds

are about 3 inches tall. Flooding is then started

as soon as the rice is large enough to tolerate

the water.

Sequential propanil treatments also control

weed grasses and aquatic weeds on good soils.

The first treatment of propanil at 3 to 5 lb/acre

applied postemergence to drained fields controls annual grasses 2 to 3 inches tall (table 29).

TABLE 29.—Effect of sequential propanil treatments applied tvith ground

equipment on control of barnyardgrass and aquatic weeds, Stuttgart,

Ark,, 1969 and 1970

Weed control (pet)

Propanil treatment Barnyardgrass Aquatic weeds^

1969 1970 1969 1970

lb/acre applied when barnyardgrass was 3 inches

tall, but before germination of aquatic weeds 96

lb/acre .applied when barnyardgrass was 3 inches

tall, but before germination of aquatic weeds;

3 lb/acre applied when aquatic weeds were 1 inch

tall and after death of barnyardgrass 96

92

96

33

85 82

1 Ducksalad, redstem, and waterhyssop
28 WEED CONTROL IN U.S. RICE PRODUCTION

TABLE 30.—Effect of time and rate of propanil treatments on barnyardgrass control and rice yields, Stuttgart, Ark., 1963

[Propanil applied in 20-gal/acre water emulsion with ground equipment]

Propanil applied

(days after

grass emergence)!

Propanil rate (lb/acre)

11/2 2y^

Grass control (pet) 2

All at 8

1/2 at 5; l^ at 8

1/3 at 2, Vs at 5,

45

35

89

1/3 at 8 60 96 98

85

98

100

Rough rice yield (lb/acre)'

All at 8 ...

1/2 at 5, 1/2 at 8

at 5,

4,210

1/3 at 8 4,380 5,360

4,020

5,230

5,180

5,180

5,330 5,300

78

4,620

1 At 2 days grass had 1 leaf (i/4 inch tall); at 5 days grass had 1 to 3 leaves

(1/4 to 1 inch tall) ; at 8 days grass had 1 to 4 leaves (i/4 to 2 inches tall).

- Grass control averaged 96 and 0 pet on the hoed and unhoed checks, respectively.

Least significant difference at 5-pct level = 23 pet. Measurements made at midseason

after grass matured.

'^Yields from the hoed and unhoed checks averaged 5,200 and 3,150 lb/acre, respectively. Least significant difference at 5-pct level = 670 lb/acre.

TABLE 31.—Effect of time and rate of ground

propanil treatments on harnyardgrass control, Stuttgart, Ark,, 196í

[Propanil applied in 20-gal/acre water emulsion]

Propanil applied Grass control (pct)^

(days after Propanil rate (lb/acre)

grass emergence)! "2 3 4 6 verage

All at 0 25 55 32 62 44

All at 4 88 88 75 88 85

All at 7 82 92 80 88 86

V2 at 0, 1/2 at 4 68 82 65 68 71

y2 at 0, 1/^ at 7 78 88 82 78 82

V2 at 4, 1/2 at 7 68 88 78 85 80

1/3 at 0, 1/3 at 4,

1/3 at 7 85 82 82 73 80

1 At the zero-day treatment, grass had 1 leaf ( i/4 to

l^ inch tall) ; at 4 days the grass stage depended on

time and rate of previous applications, but generally

it had 1 to 2 leaves (i/4 to V2 inch tall) ; at 7 days the

grass stage depended on time and rate of previous

application, but generally it had 1 to 3 leaves (^t to

1^/^ inches tall). Amount of grass present depended on

time and rate of previous applications.

2 Grass control averaged 100 and 0 pet on the hoed

and unhoed check plots, respectively. Measurements

were made at midseason after grass matured.

If fields are flooded within 1 week after treatment, aquatic weeds may germinate. An additional treatment of propanil at 3 to 4 lb/acre

applied to drained fields controls these aquatic

weeds.

Multiple applications of propanil at low rates

beginning when the first barnyardgrass

emerged and ending when the oldest plants had

TABLE 32.—Effect of time and rate of aerial

propanil treatments on harnyardgrass control, Stuttgart, Ark., 196i

[Propanil applied in 10-gal/acre water emulsion]

Propanil applied Percentage of grass controP

(days after ^^ propanil rate (lb/acre) of—

grass emergence) 1 1 1^/^ 2 3

All at 11 80

y2 at 1, 1/2 at 6 50 ... 50

1/3 at 1, 1/3 at 6,

1/3 at 11 65 ... 90

1 At 1 day grass had 1 leaf (^/4 to ^/^ inch tall) ; at

6 days it had 1 to 3 leaves (l^ to 1 inch tall) ; at 11

days it had 2 and 3 leaves (1 to 2 inches tall).

2 Measurements made about 1 month after propanil

treatments.
PRINCIPLES OF WEED CONTROL 29

TABLE 33.—Effect of propanil-adjuvant mixtures on rice injury and weed

control, Stuttgart, Ark., 1968^

[Propanil and mixtures applied in 10-gal/acre water emulsion when rice had 3 to 5

leaves, 6 to 8 inches tall, and when barnyardgrass had 1 to 3 leaves, V2 to ly^ inches tall]

Mixture

and application

rate

Percentage area

of rice leaves

burned at

indicated days

after treatment

~~5 12~

Weed control 12 days

after treatment (pet)

Barnyard- Yellow

grass nutsedge

3 lb propanil/acre 10 0

3 lb propanil + 0.05 gal surfactant L/acre • • 40 0

3 lb propanil + 0.46 lb adjuvant D/acre .... 10 0

3 lb propanil + 1 gal phytobland oil/acre ... 40 0

80

95

95

95

50

75

75

70

1 Data taken from Smith {108).

three or four leaves usually controlled weeds

no better than a single treatment of 3 or

4 lb/acre applied when the largest grass had

three or four leaves (tables 30-32). Multiple

treatments applied with ground equipment were

more effective than aerial applications, presumably because 20 gal/acre of spray emulsion

was used in ground applications while only

10 gal/acre was applied by aircraft. Wind and

rain frequently interfered with timely ground

and aerial applications of the second and third

treatments.

Propanil mixtures with adjuvants

The addition of adjuvants, such as surfactants, phytobland oils,^ emulsifiable oils, and

thickening agents, to spray emulsions of propanil may affect the activity of the herbicide on

rice and weeds. In experiments at Stuttgart,

Ark., some surfactants mixed with propanil and

applied with ground equipment in 20 gal/acre of

water emulsion increased rice injury when compared with propanil alone, but others had no

effect {108). Surfactants A and G^ increased

rice injury, but surfactant L did not. Surfactant

L mixed with propanil increased weed control in

some experiments, but not in others. Surfactant

^ An emulsifiable, highly refined paraffin oil containing 98 percent base oil and 2 percent emulsifier, with a

minimum unsulfonated residue of 92 percent. Saybolt

viscosity of 77.5 seconds at 100° F.

6 Surfactant A—polyoxyethylene with ethylene oxide

side chains varying in length. Surfactant G—sulfonated

alkyl ester in a light petroleum distillate solvent.

L increased control of barnyardgrass when applied aerially with propanil in 3 gal/acre of

water emulsion (108); it increased control of

barnyardgrass and yellow nutsedge when applied aerially with propanil in 10 gal/acre of

water emulsion (table 33). In 1969 surfactant L

did not increase the control of barnyardgrass,

but rice treated with propanil plus surfactant L

yielded more than rice treated with propanil

alone (table 34). Propanil plus surfactant L

injured the rice less initially than propanil

alone, which may have contributed to this

increased yield.

Adjuvant D^ or phytobland oil,^ mixed with

propanil and applied aerially in 10 gal/acre of

water emulsion, increased control of barnyardgrass and yellow nutsedge in 1968, but phytobland oil did not increase control in 1969

compared with a standard treatment of propanil alone (tables 33 and 34). Mixtures of

phytobland oil and propanil injured rice more

than propanil alone both years. The rice, however, recovered from the initial burn 10 to 12

days after treatment. Mixtures of adjuvant D

and propanil did not injure rice any more than

the standard treatment of propanil.

In experiments in 1969 through 1971 at

Stuttgart, Ark., mixtures of propanil with surfactant L or with phytobland oil applied with

^ Polysaccharide gum mixture.

8 An emulsifiable, highly refined paraffin oil containing 98 percent base oil and 2 percent emulsifier with a

minimum unsulfonated residue of 92 percent. Saybolt

viscosity of 77.5 seconds at 100° F.

229-238 O - 77
30 WEED CONTROL IN U.S. RICE PRODUCTION

TABLE 34.—Effect of propanil-adjuvant mixtures on barnyardgrass control and rice

injury and yield, Stuttgart, Ark., 1969^

[Propanil and mixtures applied in 10-gal/acre water

emulsion when rice ranged from 4-leaf to tillering stage,

6 to 8 inches tall, and when barnyardgrass ranged from

2-leaf to tillering stage, 1 to 5 inches tall]

Percentage area

of rice leaves Rough

Mixture Grass burned at rice

and application control indicated days yjeld^

rate (pet) after treatment (lb/acre)

4 lb propanil/acre • • 92

4 lb propanil +

0.05 gal surfactant

L/acre 92

4 lb propanil +

1 gal phytobland

oil/acre 92

30 0 5270b

20 0 5700a

40 0 5340b

1 Data taken from Smith {108).

^ Measured 10 days after treatment.

3 Yields followed by the same letter are not significantly different at the 5-pct level by Duncan's multiplerange test.

ground equipment did not control barnyardgrass any better than propanil-water mixtures

{118). Propanil at 3 or 6 lb/acre was applied

alone or in mixtures with surfactant L (0.5, 1,

and 2 percent, volume/volume) or phytobland

oil (2.5, 5, and 10 percent, volume/volume) in

20 gal/acre of water emulsion to barnyardgrass

in the four-leaf, tillering, or jointing stages of

growth. The most important factors were the

rate of propanil and the stage of barnyardgrass

at treatment. Rates of 3 or 6 lb/acre applied to

four-leaf grass or 6 lb/acre applied to tillering

grass controlled 80 to 90 percent of it, but

neither rate controlled grass in the jointing

stage satisfactorily. However, 3 lb/acre applied

to tillering grass and 6 lb/acre applied to jointing grass reduced weed competition enough to

increase grain yields significantly (by 1,000

to 3,000 lb/acre of rough rice) compared with

untreated rice.

Adjuvants added to propanil spray emulsions

did not consistently increase the activity of

propanil on barnyardgrass. Sometimes adjuvants mixed with propanil increased grass

control by 10 to 20 percent compared with

propanil alone, but often they failed to increase

control at all. Time and rate of applying propanil affected weed control considerably more

than the addition of an adjuvant.

Propanil mixtures with other herbicides

Mixtures of propanil and molinate control

certain hard-to-kill weeds better than either

herbicide alone (R. J. Smith, Jr., unpublished

data). This mixture applied early postemergence at 3 lb/acre of each herbicide controlled

bearded sprangletop, mexican sprangletop, and

spreading dayflower as well as more susceptible

weeds such as barnyardgrass, hemp sesbania,

and northern jointvetch.

Mixtures of propanil with either benthiocarb

or butachlor applied postemergence to weeds 2

to 3 inches tall control weed grasses and broadleaf, aquatic, and sedge weeds that have

emerged at treatment and those that germinate

for up to 4 weeks after treatment (R. J. Smith,

Jr., unpublished data). Because of their

residual activity, these herbicide mixtures control weed grasses and aquatic and broadleaf

weeds in rice grown on problem (calcareous,

saline, and sodic) soils, and flooding can be

delayed until the rice is large enough to tolerate

it. Weeds that germinate during the drained

period are controlled by the residual action of

the herbicides. These mixtures control many

hard-to-kill weeds, for example, dayflower and

sprangletop, better than standard treatments of

propanil or molinate.

Interactions with other pesticides

Rice may be injured when certain insecticides

and propanil are applied together or within a

few days of each other. Carbamate and organophosphate insecticides consistently cause leaf

necrosis and kill rice plants when applied as a

seed treatment or directly to rice before or

after a propanil treatment {11,13). Chlorinated

hydrocarbon insecticides applied as a seed

treatment or within a few days of a propanil

treatment are less phytotoxic to rice than

carbamate or organophosphate insecticides.

Parathion at 0.1 and 1 lb/acre injected into

water 1 and 8 days ifter propanil treatment

injured rice moderately to severely (table 35).

When injected into water at 15 to 22 days after
PRINCIPLES OF WEED CONTROL 31

TABLE 35.—Effect of propanil and parathion

treatments on rice grown in the greenhouse, Stuttgart, Ark., 196U^

Days from

propanil to

parathion

treatment

3 lb/acre propanil,-

parathion at—

No propanil,^

parathion at —

0.1 lb/acre lib/acre 0.1 lb/acre lib/acre

Percentage loss in green weight of treated

rice compared with untreated rice^

1

8

15

22

53

30

17

14

100

60

38

31

29

29

2

5

76

53

28

13

Percentage rice leaf chlorosis

15

22

40

30

40

20

80

60

40

10

10

20

10

0

30

40

20

10

1 Data taken from Smith {108).

2 Propanil at 3 lb/acre reduced green weight by 14

pet and caused 20 pet chlorosis, compared with untreated rice.

^ Parathion applied at comparable times as in the

propanil and parathion treatments.

4 Least significant difference at 5-pct level = 11 pet.

Measurements made 63 days after seeding.

propanil treatment, it injured rice only slightly

to moderately. Parathion at about 0.1 lb/acre

injected into the water as ricefields are flooded

controls mosquitoes {69). Effective mosquito

control treatments consist of applying parathion

in the first floodwater after propanil treatment.

Mixtures of propanil and carbaryl injure rice

moderately to severely and reduce grain yields.

In a field experiment at Stuttgart, Ark., a

mixture of propanil at 3 lb/acre and carbaryl

at 0.5 lb/acre applied to rice 4 inches tall caused

55 percent injury, compared with 5 and 0

percent injury from either propanil or carbaryl,

respectively (108). In a field experiment at

Beaumont, Tex., mixtures of propanil at 4 lb/

acre and carbaryl at 1 lb/acre applied early

postemergence reduced yields 37 percent compared with those from plots treated with either

pesticide alone (12).

Carbofuran applied after propanil does not

interact to adversely injure rice, but propanil

applied after carbofuran does (127). Propanil

at 4 lb/acre was sprayed on 'Starbonnet' rice

Carbofuran Percentage rice

rate leaf chlorosis

(Ib/cwt 9 days after

rice seed) propanil treatment

0 0

.5 0

0 0

0 0

.5 40

.5 80

TABLE 36.—Effect of propanil and carbofuran

treatments on rice groivn in the greenhouse, Stuttgart, Ark., 1966^

[Carbofuran applied to rice seed as a dressing; propanil

applied in 20-gal/acre water emulsion when rice plants

had 2 leaves, 4 to 6 inches tall]

Propanil

rate

(lb/acre)

0

0

3

6

3

6

1 Data taken from Gifford et al. {Jf2).

at intervals of 3 to 56 days after applying

granular carbofuran at 0.6 lb/acre. Interaction

of carbofuran and propanil did not cause

chlorosis or necrosis of rice leaves when the

insecticide was applied 5 days after the herbicide. Leaf chlorosis and necrosis developed

within 7 days after propanil treatment and

lasted for 2 to 4 weeks. As the time interval

between carbofuran and propanil treatments

increased, leaf injury decreased. Older rice

recovered more quickly than younger plants.

Mixtures of propanil and carbofuran sprayed

directly on young rice plants injured them

severely. In greenhouse experiments propanil

at 3 or 6 lb/acre applied postmergence injured

rice moderately to severely when the seed had

been treated with carbofuran (table 36).

Neither pesticide applied alone injured rice.

In field experiments propanil at 4 lb/acre applied postemergence 1 to 3 weeks after seeding

rice treated with carbofuran (0.25 lb/100 lb

seed) as a seed dressing injured rice and reduced grain yields (11) ^

Carbamate and organophosphate insecticides,

such as carbaryl, carbofuran, and parathion,

inhibit the action of the enzyme aryl acylamidase in the rice plant. This enzyme hydrolyzes

and detoxifies propanil to 3,4-dichloroaniline

and propionic acid (38, 7Jp). Different carbamate and organophosphate insecticides affect

this enzyme differently. In greenhouse experiments at Fargo, N. Dak., carbaryl reduced the

activity of aryl acylamidase by about 90 per-
32 WEED CONTROL IN U.S. RICE PRODUCTION

cent. Organophosphate insecticides inhibit aryl

acylamidase less than carbamate insecticides do,

but they are still active enough to produce adverse interactions with propanil in rice

seedlings {7Í). Chlorinated hydrocarbon insecticides apparently do not inhibit the activity

of aryl acylamidase; thus, they usually do not

interact adversely with propanil in the rice

plant.

Molinate

Molinate controls barnyardgrass and certain

sedge and broadleaf weeds without injury to

rice (table 12). It does not control broadleaf

signalgrass and sprangletop and aquatic weeds,

such as ducksalad, false pimpernel, redstem,

and waterhyssop. Growth stage of weeds, rate

of herbicide, and water management influence

the effectiveness of molinate on susceptible

weed species.

Molinate, a thiocarbamate herbicide, is slightly soluble in water (1,000 p/m at 68° F) and

highly soluble in common organic solvents. It

is formulated with organic solvents and emulsifiers as an emulsifiable concentrate or as a

granular formulation on clay or other suitable

granular carriers. Molinate is rapidly lost by

volatilization or by codistillation with water

from moist surfaces; volatilization is practically

stopped by incorporating it into soil or irrigation water. Soil micro-organisms contribute

significantly to the disappearance of molinate

herbicides incorporated in soil. When incorporated into warm, moist soil it is normally

decomposed 4 to 6 weeks after treatment.

Technical molinate has an acute oral LD50 of

720 mg/kg for rats, which is low.

Mode of selectivity

Molinate is rapidly metabolized in plants and

soils {36, 79, 80). It may be absorbed by weed

seed during the early stages of germination.

Seeds that are not germinating apparently do

not absorb molinate. It may also be absorbed by

roots, coleoptiles, and basal shoot tissue. Some

plants absorb molinate more rapidly than

others. Excised barnyardgrass coleoptiles absorbed more molinate than rice coleoptiles did

{19), Once molinate is absorbed into the plant,

it rapidly translocates throughout the plant.

It accumulates, however, in the meristems of

the root and shoot. Molinate is rapidly metabolized in the plant to carbon dioxide metabolites;

these are assimilated in natural plant constituents such as organic acids, carbohydrates,

cellulose, amino acids, and proteins. Some plant

species detoxify thiocarbamates more rapidly

than others. For example, mung bean degrades

S-propyl butylethylthiocarbamate (pebulate),

which is chemically similar to molinate, faster

than does wheat. Mung bean, therefore, is

more tolerant than wheat to thiocarbamates.

Probably for the same reason rice is more

tolerant than barnyardgrass to molinate. In

experiments in California, molinate at 0.3 p/m

reduced the length of barnyardgrass shoots

39 percent, but 7 p/m reduced the length of

rice shoots only 30 percent {86).

Time and rate of application

Granular molinate, applied in the floodwater

by aircraft after crop emergence and when

barnyardgrass plants are 3 to 6 inches tall,

controls barnyardgrass in dry- and water-seeded rice (tables 37 and 38, fig. 3). Rice is usually

injured little, if any, by molinate treatments

made when grass plants are 3 to 6 inches tall.

Ordinarily, 3 lb/acre controls barnyardgrass

better than 2 lb/acre, and usually this rate is

as effective as 4 or 6 lb/acre. Molinate applied

at 4 or 6 lb/acre to tillering rice and barnyardgrass may injure rice and usually controls

barnyardgrass less effectively than lower rates

applied earlier (table 38). Rice treated With

molinate yields more grain than untreated rice

when competitive weed grasses are controlled.

Granular molinate at 3 lb/acre applied into

the flood suppressed growth of tillering barnyardgrass (plants up to 12 inches tall) and

improved rice yields; this was an economical

treatment (R. J. Smith, Jr., unpublished data).

In experiments at Biggs, Calif., granular or

emulsifiable molinate applied at 3 lb/acre to

dry soil immediately before flooding the ricefield and seeding rice controlled barnyardgrass

effectively (88 to 96 percent); rice plots so

treated yielded 6,670 lb/acre, compared with

1,320 lb/acre for untreated plots {86).

Water management

When molinate is applied in irrigation water

after the emergence of rice and weed grasses.
PRINCIPLES OF WEED CONTROL 33

TABLE 37.—Effect of method of seeding rice and time and rate of molinate treatment on rice

injury, bamyardgrass control, and rice yield, Stuttgart, Ark., 196V

Rice injury^ (pet) Grass control** (pet) Rough rice yield (Ib/aere)

Seeding method and time Molinate rate , Molinate rate ~~ Molinate rate

of molinate (lb/acre) ^^«^- (Ib/aere) ^ver- (ib/aere) ^verapphcation^ „ , ^ g age^ -^—^ ^-^ age^ —^ ageDry-seeded :

Drill, preplanting 0 0 10 40 20 0 40 70 100 70 30 3,740 4,480 4,420 4,210

Drill, postemergence 0 0 0 30 10 0 90 100 100 97 990 5,530 5,580 5,300 5,470

Broadcast, preplanting .... 0 0 10 40 20 0 90 100 100 97 150 3,690 4,750 4,420 4,280

Broadcast, postemergence . 0 0 20 30 20 0 90 100 100 97 1,730 3,240 4,330 3,940 3,840

Water-seeded :

Preplanting 0 0 0 30 10 40 100 100 100 100 3,330 4,860 3,740 4,990 4,530

Postemergence 0 0 30 50 30 60 100 100 100 100 3,080 4,380 4,130 3,850 4,120

Average' 0 0 10 40 .. 17 85 95 100 ... 1,560 4,240 4,500 4,480

1 Data taken from Smith (108).

2 Preplanting treatments incorporated into soil just before seeding. Postemergence treatments applied to the

floodwater when rice had 1 or 2 leaves and when bamyardgrass had 1 to 3 leaves.

2 Based on visual rating of growth inhibition.

^ Measurements made at midseason after grass matured.

^ Value for zero rate excluded from averages.

t^ Least significant difference at 5-pct level = 920 lb/acre.

' Least significant difference at 5-pet level for rice yields =470 lb/acre; at 1-pct level, 620 lb/acre.

its effectiveness for control of bamyardgrass yardgrass and broadleaf signalgrass, the latter

is influenced by the number of days that the of which is somewhat tolerant to molinate, were

water remains on the ricefield after molinate controlled more effectively when the flood was

application and the length of time that the rice- maintained for 7 or 12 days after an applicafield remains drained after molinate has con- tion of molinate than when it was held only 2

trolled the initial weed infestation. days (table 39). Plots that remained flooded

In field experiments at Stuttgart, Ark., barn- for 12 days after application of molinate yielded

PN-4938 PN-4939

FIGURE 3.—Control of bamyardgrass in rice with molinate. Left, drill-seeded rice treated with granular molinate

at 3 lb/acre when bamyardgrass had three or four leaves. Right, untreated plot.
34 WEED CONTROL IN U.S. RICE PRODUCTION

TABLE 38.—Effect of time and rate of molinate treatments on rice injury^

barnyardgrass control, and rice yield, Stuttgart, Ark,, 1965 and 1967^

Plant height Molinate

at treatment (inches) rate

Rice Barnyardgrass (lb/acre)

Rice

injury-

(pet)

Grass

control^

(pet)

Rough rice

yield

(lb/acre) 4

1-5

4-6

4-8

56-10

0.25-2

1-4

n-6

53-10

0 0 0 5720b

2 0 98 6910a

3 6 98 6860a

4 0 94 6640a

6 0 98 6870a

0 0 0 5480b

2 0 91 6710a

3 0 95 6980a

4 0 96 6740a

6 0 97 6700a

0 0 0 4600c

2 0 76 6600a

3 0 94 6650a

4 0 92 7010a

6 0 95 6740a

0 0 0 3810c

2 4 80 6740a

3 8 82 6180a

4 6 90 6340a

6 14 90 6400a

1 Data taken from Smith (. . 'Bluebonnet 50' and 'Starbonnet' were drill-seeded

in 1965 and 1967, respectively.

- Based on visual rating of growth inhibition and chlorosis.

2 Ratings taken at midseason after heading of barnyardgrass.

4 Yields followed by the same letter are not significantly different at the 5-pct

level, by Duncan's multiple-range test.

5 Largest rice and barnyardgrass plants were tillering.

TABLE 39.—Control of weed grasses as influenced by length of period flooded after

treatment with molinate, Stuttgart, Ark.,

1966 and 1967^

TABLE 40.—Barnyardgrass control as influenced by period drained after treatment

with 7nolinate, Stuttgart, Ark,, 1966 and

1967^

Flood

period

(days)

Weed grass control (pet)

Barnyardgrass^ Broadleaf

signalgrass^

2

7

12

70

86

96

27

53

80

1 Granular molinate at 3 lb/acre was applied in irrigation water when drill-seeded rice was 7 to 8 inches

tall and when weed grasses were 2 to 5 inches tall.

Data taken from Smith (108).

" Average values for 1966 and 1967.

3 Values for 1967 only.

Days kept drained

Barnyardgrass control (pet)

1966 1967

0

5

10

20

29

83

91

80

95

63

90

88

70

63

57

1 Granular molinate at 3 lb/acre was applied in the

irrigation water when drill-seeded rice was 2 to 6 inches

tall and when barnyardgrass was 0.5 to 3 inches tall.

Plots were kept flooded for 7 days after applying

molinate and then drained for number of days indicated.

Data taken from S'mith {115).
PRINCIPLES OF WEED CONTROL 35

better than plots drained 2 or 7 days after

treatment. This yield increase was attributed

to more effective control of weed grasses by

the longer flood period.

In other experiments postemergence applications of molinate at 3 lb/acre to flooded rice

controlled emerged barnyardgrass and prevented reinfestations (table 40). After molinate

had controlled the emerged grass (usually about

7 days after treatment), the flood could be removed for up to 10 days without reinfestation

by weed grasses. Barnyardgrass reinfested rice

that had been kept drained for 20 or 29 days;

these late-germinating plants competed with

rice sufficiently to reduce grain yields.

Method of application

Emulsifiable molinate may be applied with

tractor or aircraft equipment. Applications by

tractor in water emulsion of 15 to 20 gal/acre

or by aircraft in 8 to 10 gal/acre, with equipment calibrated and adjusted accurately, control weeds satisfactorily. Emulsifiable sprays

or granular formulations applied to dry soil

before seeding or granular formulations applied postemergence in floodwater control weed

grasses more effectively than emulsifiable

sprays applied postemergence in floodwater.

Molinate applied before seeding is more effective when it is incorporated into the soil by

disking or by flooding immediately after application.

Applications of granular or emulsifiable

molinate to dry soil before seeding may be made

Vvdth tractor or aircraft equipment. Postemergence treatments in floodwater are applied

with aircraft equipment. Venturi and rotary

spreaders that are accurately calibrated and

adjusted provide even distribution of molinate,

which results in effective weed control.

Method of seeding

Seeding method may affect the tolerance of

rice to molinate. In experiments at Stuttgart,

Ark., molinate at 8 lb/acre applied postemergence to flooded, drill-seeded rice or incorporated in the soil before water seeding of rice

did not injure rice enough to reduce grain

yields (table 37). But the same amount incorporated before drill seeding of rice or applied

postemergence to flooded, water-seeded rice injured the crop sufficiently to reduce grain

yields. Even when molinate injures rice slightly

to moderately, grain yields increase wherever

molinate controls weed infestations that would

otherwise reduce yields. In experiments at

Biggs, Calif., molinate applied at rates as high

as 12 lb/acre before flooding controlled weed

grasses effectively and did not injure waterseeded rice (78). Differences in cultivar, environment, soil, and water depth at the two

locations may have contributed to differences

in rice susceptability.

Rice cultivars

Commercial rice cultivars respond in the

same way to treatments of molinate. Molinate

at 2 or 3 lb/acre applied immediately before

water seeding of rice or applied in irrigation

water soon after emergence of dry-seeded rice

injures these cultivars little, if any. In field

experiments and in commercial ricefields, *Bluebonnet 50\ ^Bluebelle', 'Bonnet 73^ 'Brazos\

'Caloro', 'Dawn^ 'Labelle^ 'Lebonnef, 'Nato^

'Nova 66', 'Saturn', and 'Starbonnet' were not

injured significantly by postemergence treatments of molinate at 3 lb/acre applied in floodwater before rice began tillering (R. J. Smith,

Jr., unpublished data). However, these cultivars

were frequently injured by molinate at 6 lb/

acre.

Soil type

Soil type has little influence on the effectiveness of molinate for control of barnyardgrass.

Molinate controls barnyardgrass effectively

when applied to clay, clay loam, and silt loam

soils. However, soil type may affect injury of

rice from molinate treatments. Rice grown on

clay was injured less by postemergence treatments applied in floodwater than rice grown on

silt loam.

Temperature

Low temperatures affect the activity of

molinate on barnyardgrass less than they influence the activity of propanil. In a growthchamber experiment at Stuttgart, Ark.,

molinate and propanil, each at 3 lb/acre, were

applied to barnyardgrass plants 1 to 2 inches
36 WEED CONTROL IN U.S. RICE PRODUCTION

tall grown at low (75° day and 55° F night)

and high (88° day and 70° F night) temperatures. Granular molinate was applied to

flooded plots, but propanil was sprayed on

unflooded plots to simulate the way these

herbicides are normally applied to commercial

ricefields. At the low temperatures molinate

reduced growth of grass 100 percent, compared

with only 75 percent for propanil. At the high

temperatures both herbicides controlled 97 percent of the barnyardgrass. Neither herbicide

injured rice at either temperature regime, but

the rice grew faster at the higher temperatures.

In field experiments and in commercial ricefields molinate controlled barnyardgrass effectively when night temperatures were as low as

45° F and day temperatures were as low as

60° F. Propanil usually fails to control barnyardgrass when temperatures are this low.

Sequential applications with propanil

Sequential treatments of propanil and

molinate control barnyardgrass better than

either herbicide applied alone. Propanil applied

postemergence controls plants 2 to 3 inches

tall. Additional barnyardgrass may germinate

and infest the ricefield if irrigation water is

not applied within 5 days after the propanil

treatment. These infestations are controlled

with 2 or 3 lb/acre of molinate applied after

the ricefield has been flooded. When propanil

is used instead of molinate, the ricefield has to

be drained before the second treatment.

Molinate can be applied directly in the irrigation water, so the cost of draining and reflooding the ricefield for a second herbicide

treatment is avoided.

Interaction with other pesticides

Molinate and other pesticides, such as insecticides and fungicides, do not interact to adversely affect rice or alter the effectiveness of

molinate on weeds. In field experiments in

Louisiana, tank mixtures of molinate at 4 lb/

acre and insecticides such as carbofuran or

fensulfothion, each at 1 lb/acre, did not affect

rice adversely {U2), Field observations indicate

that molinate and insecticides such as aldrin,

parathion, and carbaryl or fungicides such as

cyano(methylmercuri)guanidine and thiram do

not interact unfavorably on rice or weeds.

Phenoxy Herbicides

Postemergence treatments of 2,4-D, MCPA,

2,4,5-T, and silvex control many aquatic, broadleaf, and sedge weeds, but not weed grasses

(table 12). They are applied at rates of 0.25

to 1.5 lb/acre of acid equivalent. The rate depends on the formulation to be applied, the

weed species, stage of growth of the rice, air

and water temperatures, and perhaps other

conditions.

Phenoxy herbicides usually are formulated

and marketed as two basic types—salts and

esters. Salt formulations are available as

amines, such as diethanol amine, triethanol

amine, dimethyl amine, triethyl amine, and

isopropyl amine, and as alkali-metal salts, such

as lithium, potassium, and sodium. Amine salts

are available chiefly as water-soluble liquids,

and inorganic salts are available as watersoluble powders; both types are practically

nonvolatile.

Esters of phenoxy herbicides are high or low

in volatility. The highly volatile esters include

the methyl, ethyl, isopropyl, butyl, and amyl.

The low-volatility esters include the butoxyethanol, butoxyethoxypropanol, capryl, ethoxyethoxypropanol, isooctyl, and propylene glycol

butyl ether. These esters of 2,4-D, MCPA, 2,4,5-

T, and silvex are liquids that, when properly

formulated, form oil-in-water or water-in-oil

emulsions. Low-volatility esters are less likely

to vaporize and drift to susceptible crops than

highly volatile ones when temperatures are

95° F or less. When temperatures exceed 95° F,

even the low-volatility esters may vaporize,

drift, and injure susceptible crops. States that

produce rice prohibit the use of highly volatile

esters, but low-volatility esters are permitted

in some States for weed control in rice. Silvex,

2,4-D, 2,4,5-T, and MCPA range from low to

intermediate in acute oral toxicity (LD.,o=300-

1,000 mg/kg for rats) {79).

Mode of selectivity

The herbicidal action of phenoxy herbicides

in plants has been discussed frequently {22,
PRINCIPLES OF WEED CONTROL 37

67, 79), Entry and translocation of phenoxy

herbicides occur through leaves, stems, and

roots of rice and weeds. They are translocated

from sites of absorption to sites of action. The

movement of phenoxy herbicides may be upward into young leaves, downward into young

roots, or to meristems anywhere in the plant.

Kind of plant, growth stage of plant, rate of

plant growth, environment, and soil—all affect

the responses of plants to phenoxy herbicides.

Phenoxy herbicides affect many interacting

plant processes to cause death of plants. Effective herbicidal action involves penetration of

plant membranes, absorption and assimilation

by cells, translocation through tissues and by

the vascular system, and finally a toxic action

usually involving the living protoplasm (22),

Experiments show that the herbicidal action

of phenoxy herbicides may be influenced by

each of these physiological and biochemical

processes and that the herbicides in turn affect

the processes. At lethal rates, phenoxy herbicides upset the balance between production and

use of plant foods. Respiration increases and

photosynthesis decreases, with the consequent

depletion of carbohydrates. Plants treated with

phenoxy herbicides may also contain high levels

of phytotoxic breakdown products.

Phenoxy herbicides induce structural changes

in plants. After treatment, plants may become

desiccated and brittle, or they may develop malformed and twisted leaves, stems, roots, and

inflorescences. Typical responses of sensitive

plants to phenoxy herbicides include cell enlargement, increased cell division, development

of malformed tissue, production of numerous

root primordia in the stem, and finally death.

Stage of rice growth

The stage of growth greatly influences the

response of rice plants to phenoxy herbicides.

Very young rice (from emergence up to 3 weeks

after emergence) may be injured severely or

even killed by phenoxy herbicides at rates required to control weeds. Rice treated at the

early-tillering, late-jointing, booting, or heading

stages also may be seriously affected. Rice in

the late-tillering to early-jointing stages is

usually not injured by phenoxy herbicides. The

tolerant stage can be positively identified when

the basal internode begins to elongate from

0.25 to 0.5 inch long. Rice may be injured when

the internode is longer than 0.5 inch. Most

commercial U.S. varieties of rice are tolerant

at this stage of development regardless of maturity-group classification. For very-short-season varieties ('Bluebelle', *Belle Patna') this

tolerant stage usually occurs 5 to 6 weeks after

emergence; 7 to 9 weeks for short-season varieties (*Nato', 'Nova 66'); and 8 to 10 weeks

for midseason varieties ('Starbonnet', *Bluebonnet 50', Toro'). The California varieties

('Calrose', 'Caloro', 'Colusa', and 'CS-M3') reach

this tolerant stage from 7 to 8 weeks after

seeding. In a 6-year investigation at Stuttgart,

Ark., 2,4-D applied at 0.75 and 2 lb/acre to rice

in the early-tillering and booting stages reduced

yields substantially, but applications in the latetillering and early-jointing did not (table 41).

Kind o£ phenoxy herbicide

Rice responds differently to different kinds

of phenoxy herbicides. When rice is in the earlytillering stage of growth, 2,4,5-T and silvex are

less phytotoxic than 2,4-D or MCPA (table 41).

MCPA is even less toxic than 2,4-D when applied during early stages of growth. Silvex and

2,4,5-T did not reduce rice yields when applied

at early-tillering, late-tillering, and early-jointing stages of growth. However, all these

phenoxy herbicides reduced yields when applied

during the booting stage.

Several characteristic injury symptoms appear when rice is injured by phenoxy herbicides. Rice treated with 2,4-D at the

early-tillering stage grows tubular leaves

("onion leaf" symptoms) and malformed panicles (fig. 4), but rice treated with MCPA,

2,4,5-T, and silvex at the same stage appears

normal. All these herbicides applied at any

stage may injure roots moderately to severely,

may cause the plant to turn dark green soon

after spraying early-tillering rice, or cause

chlorosis or downward positioning of the leaves

if applied to booting or heading rice. Treatments of phenoxy herbicides may affect plant

height and bushel weight of rough rice (table

42) or delay rice maturity.

Weeds also respond differently to the different kinds of phenoxy herbicides (table 12).
38 WEED CONTROL IN U.S. RICE PRODUCTION

TABLE 41.—Effect of time of application of phenoxy herbicides on rice

yields, Stuttgart, Ark., 1955-61

[Values are averages from plots treated with 0.75 and 2 lb/acre of phenoxy herbicides]

Herbicide and Change in yield (pet/acre) 2

stage of ricei 1955 1956 ~l957 Ï958 Ï959 196Î Ä^.

2,4-D: ~ '

Early-tillering _13 _20 _24 -16 -4 _27 _i7

Late-tiHering +1 _6 +22 +5 +8 _4 +4

Early-jomting _lo _9 +12 _2 -8 IQ

^°°""S -14 -18 -21 -19 _9 _"iV _i6

LSD (5 Pct)3 ¡ 8 Ü TQ Í¡ 8 ñ

2,4,5-T: ==========================

Early-tillering _7 ,3 ,„ ,

i:^^r'-"--^ -5 +13 il Z2 +2 Early-jomtmg +3 ^g -4 n

^°°""^ -21 -14 -14 -29 -20

LSD (5 pct)^ TT^ ::: 2^ io Í2 8 TS

MCPA: • Early-tillering ... ... _ 20 _ 7

Late-tillering ... .. _2 _q '' ''' —14

Early-jointing ... 4.1 _^^ '" '" ~^

ß°°ting _7 _j9 ;•• _••• _+^

LSD (5 pet)3 ... Z^ ¡^ ]^^ " ~

Silvex : ^^^====^^=^^=^=====^

Early-tillering +2 -2 +11 +4

Late-tillering _1Q _2 _|.2 " _^

Early-jointing 0 -2 +6 '" -4-1

B°°**"^ -18 -4 -11 ;.■■ _n

LSD (5 pct)3 ~7 ~ 21 W í¡ r: ü

^'^'tf'.^f'^:^' ^""^ ^^^^ formulated as amines. Silvex formulated as the propylene

glycol butyl ether, ester. Herbicides applied about 5, 8, 11, and 14 weeks after rice

T^ZlüZy -rly-tillering, late-tillering, eariy-jointing, and booting stages,

^ LSD —least significant difference at level given.

Hemp sesbania is very susceptible to 2,4-D and

2,4,5-T (fig. 5), but northern jointvetch is

somewhat resistant to 2,4-D and susceptible to

2,4,5-T (table 43). Ducksalad, however, is more

susceptible to 2,4-D than to 2,4,5-T (table 44).

Where several species of weeds varying- in susceptibility to a herbicide are present, mixtures

of phenoxy herbicides are often more effective

than one herbicide used alone.

Formulations

Silvex is applied as an ester because this

formulation controls weeds better than amine

or inorganic salts. Inorganic salts of 2,4-D,

2,4,5-T, and MCPA, such as sodium, potassium,

and lithium, are less effective on weeds than

amine or ester formulations. Esters usually

control weeds better than amines.
PRINCIPLES OF WEED CONTROL 39

jmmm mmmm lllillHIt

PN-4940

FIGURE 4.—Two untreated plants (right) and rice with

malformed leaves and panicles caused by injury

from 2,4-D applied at the early-tillering stage of

growth (left).

:ÄÜl|iil

PN-4942

FIGURE 5.—Control of hemp sesbania with phenoxy

herbicides in rice. Left, plot sprayed with 2,4-D at

1 lb/acre at midseason. Right, unsprayed plot.

Rice is most tolerant to all formulations of

phenoxy herbicides during the late-tillering and

early-jointing stages of growth. However, at

susceptible stages of growth, esters frequently

are more toxic than amines. Amines of 2,4-D

TABLE 42.—Effect of time- of application of

phenoxy herbicides on plant height and

bushel weight of rice, Stuttgart, Ark.,

1957-59

[Values are averages from plots treated with 0.75 and

2 lb/acre of phenoxy herbicides]

Stage of rice when treated

Herbicide'^ Early- Late- Earlytillering tillering jointing °° ^^^

Percentage change in height of

mature rice plants2,4-D -3 -1-1 +2 +2

2,4,5-T 0 -1-1 -f-5 -1-12

MCPA -2 -1 -1-2 -1-5

Silvex -1-2 0 -1-4 -1-10

LSD (5 pet)3 .... 3 3 3 3

Percentage change in bushel weight*

2,4-D 0 -0.2 -M.O -0.9

2,4,5-T -1-0.6 -1-.4 +.2 -1.7

Silvex +A +.2 0 -1.1

LSD (5 pet) 3 9 .9 .9 .9

1 2,4-D, 2,4,5-T, and MCPA applied as amines. Silvex

applied as the proplyene glycol butyl ether ester.

^Expressed as percentage reduction ( — ) or increase

( + ) as compared with untreated weed-free check.

Values are averages of 3 years of data, 1957-59. Average height for untreated plots was 44.8 inches for 2,4-D,

2,4,5-T, and silvex comparisons, and 44.4 inches for

MCPA comparisons.

3 LSD =least significant difference at level given.

* Expressed as percentage reduction ( — ) or increase

( + ) as compared with untreated weed-free plots.

Values are averages of 1 year of data, 1959. Average

bushel weight for untreated plots was 46.2 lb.

TABLE 43.—Control of hemp sesbania and

northern jointvetch with 2,U-D and 2,i,5-T,

Rohwer, Ark., 1960 and 1962

HerbiWeed control (pet)

Hemp sesbania Northern jointvetch

1960 1962 Average 1960 1962 Average

2,4-D .... .95 98 96 25 30 28

2,4,5-T .. • 100 95 98 75 95 85

2,4-D +

2,4,5-T • 100 100

1 Applied postemergence at midseason as amines :

2,4-D and 2,4,5-T applied at 1 lb/acre; 0.5 lb/acre each

of 2,4-D and 2,4,5-T applied in a mixture.
40 WEED CONTROL IN U.S. RICE PRODUCTION

and 2,4,5-T, formulated as water-in-oil or

invert emulsions, have been developed to control aquatic, broadleaf, and sedge weeds in rice.

These formulations are more active on weeds

and drift less than regular ester or amine

formulations (88). Invert emulsions are applied

with specially designed equipment.

Although invert emulsions, properly applied,

drift less than regular emulsions or watersoluble sprays, their sprays can drift sufficiently to damage nontarget susceptible crops.

In experiments in Texas (88) invert emulsions

sprayed aerially had only 0.1 percent of the

spray volume composed of droplets smaller than

200 micrometers. By comparison, regular, or

oil-in-water, emulsions had 30 percent of the

spray volume composed of droplets smaller than

200 micrometers. In other experiments in

Texas, invert emulsions sprayed aerially had

less than 1 percent of spray volume deposited

beyond 100 feet downwind from the treated

area, compared with 18 percent for regular

emulsion sprays.

In experiments in Arkansas, invert emulsion

sprays of 2,4-D or 2,4,5-T at 0.5 to 1 lb/acre

applied by aircraft controlled weeds effectively

with only slight rice injury. The treatments

caused brown spots on rice leaves, but the rice

recovered within 2 weeks after treatment. A

herbicide-water spray volume of 4 to 8 gal/acre

applied in spray droplets that ranged from 200

to 1,000 micrometers in diameter covered weeds

and drifted little if any.

Granular formulations of phenoxy herbicides

frequently control aquatic, broadleaf, and sedge

weeds less effectively than sprays.

Method of seeding and water management

When phenoxy herbicides are applied at 0.25

to 1.5 lb/acre during tolerant stages of rice,

the method of seeding and water management

usually do not influence the response of rice

to these herbicides. Seeding method and water

management may significantly affect the response when the herbicides are applied in susceptible stages, especially rice 3 to 6 weeks old.

When water-seeded rice 3 to 6 weeks old is

to be sprayed with phenoxy herbicides, a shallow flood on the ricefield reduces damage. The

stage of rice development at the time of applyTABLE 44.—Control of ducksalad with 2,i'D

and 2,i,5'T, Stuttgart, Ark., 1958-60

Herbicide and Ducksalad control (pet)

"^P^^^,^f^" 1958 Ï959 Î96Ô Â^^^ÏS^e

2,4-D:

1 lb/acre 80 100 ... 90

2 lb/acre 83 100 80 88

2,4,5-T :

1 lb/acre 13 25 ... 19

2 lb/acre 47 75 30 51

1 Applied postemergence at midseason as amines.

ing phenoxy herbicides, however, is much more

important than seeding method or water management.

Water management may affect the response

of weeds to phenoxy herbicides. If water covers

low-growing aquatic weeds such as ducksalad,

redstem, purple ammannia, or waterhyssop at

spraying time, the weeds may not be controlled

because the herbicide does not contact them.

If weeds are growing slowly because of dry

soil, control may be poor. When draining of

the ricefield begins 3 to 6 days before herbicide

treatment, weeds are exposed to the herbicide,

and soil moisture is sufficient for rapid growth

of weeds.

Nitrogen fertilization

When phenoxy herbicides are applied at the

tolerant stages of rice development, the time

of applying nitrogen fertilizer often affects the

response of rice to the herbicides. Rice may be

injured when nitrogen is applied 10 to 15 days

before or 10 to 15 days after the herbicides.

At Stuttgart, Ark., applications of nitrogen

15 days before the herbicide stimulated growth

of rice in 4 or 5 days. At the time herbicides

were applied, rice was green and growing

rapidly. The herbicides caused chlorosis and

reduced yields. The rice was not injured when

nitrogen was applied from 5 days before to 5

days after the herbicide, but yields were reduced when nitrogen was applied 15 days after

applying a phenoxy herbicide in the tolerant

or early-jointing stage.

Phenoxy herbicides applied to rice in the

early-jointing stage when basal internodes

range from 0.25 to 0.5 inch long injure rice
PRINCIPLES OF WEED CONTROL 41

little, if any, when some nitrogen is applied

within 5 days after treatment. Indeed, nitrogen

may help the rice recover from the herbicide

injury.

Environmental factors

In greenhouse experiments rice treated with

2,4-D, MCPA, and 2,4,5-T was injured more

when subjected for 3 days (2 days before and

1 day after herbicide treatment) to 97° F

than when subjected to 62° or 72° F (61), Rice

2 to 8 weeks old exposed to a temperature of

97° F was injured severely by these herbicides,

but as the rice grew older than 2 weeks, injury

decreased. At 97° F rice was injured most by

2,4-D, followed by MCPA and 2,4,5-T in decreasing order.

The effect of temperature in increasing the

activity of phenoxy herbicides on rice is greatly

influenced by the rate of plant growth at

different stages of rice development. Even when

rice is treated during the tolerant stages, high

temperatures may increase rice injury by

phenoxy herbicides if plants have received untimely applications or excessive rates of nitrogen and are growing rapidly. The stage of

development and rate of growth of the rice

plant at time of herbicide treatment are more

important than temperature before, at, or after

treatment.

Weeds respond best to phenoxy herbicides

when temperatures range from 70° to 90° F.

Temperatures below 60° F during the week

before treatment may slow weed growth and

reduce herbicide activity. Temperatures above

95° F before treatment may increase superficial contact injury by the herbicide and reduce

its absorption, which in turn would decrease

its efficacy on the weeds.

Rainfall within 3 hours after postemergence

treatments of phenoxy herbicides may reduce

weed control, because the herbicides may be

washed from the plant before it is absorbed.

Phenoxy herbicides usually control weeds effectively if rain occurs no sooner than 6 hours

after treatment. Ester formulations resist

washing from weed plants more than amine

or inorganic salt formulations.

Moderate to high relative humidities (ranging from 60 to 90 percent) increase the effectiveness of phenoxy herbicides. The humidity

is usually in this range in ricegrowing areas

when phenoxy herbicides are applied. Rice is

usually sprayed in the early morning or late

afternoon and sometimes it is sprayed at night.

At these times the relative humidity is higher

than at other times of the day.

Spray volume and method

Phenoxy herbicides are applied with lowgallonage sprayers mounted on tractors or

aircraft. Spray volumes of 5 to 15 gal/acre

for tractor application or 3 to 10 gal/acre for

aircraft treatments usually cover weeds sufficiently for satisfactory control. Regardless

of the equipment used, weeds are controlled

only when the herbicides contact the weeds

and are absorbed by them.

Sequential applications with other herbicides

Sequential treatments of phenoxy herbicides

with propanil or molinate frequently control

weed grasses and aquatic, broadleaf, and sedge

weeds more effectively than a treatment of

only one herbicide. Propanil at 3 to 5 lb/acre

or molinate at 3 lb/acre applied at early postemergence controls young weed grasses. When

aquatic, broadleaf, or sedge weeds infest the

ricefield after flooding, 2,4,5-T or silvex applied

to drained fields at 0.5 to 1 lb/acre 2 to 3 weeks

after the propanil or molinate treatment controls aquatic weeds that are less than 2 inches

tall with only slight injury of rice. The ricefield is flooded again within 5 days after the

phenoxy herbicide treatment.

More aquatic, broadleaf, and sedge weeds

may germinate after early applications of these

herbicides. If they become a problem at midseason another phenoxy herbicide treatment

with either 2,4,5-T, silvex, 2,4-D, or MCPA

can be applied during the tolerant stage of rice

growth.

Interaction with other pesticides

Phenoxy herbicides and other pesticides, such

as insecticides and fungicides, do not interact

to adversely affect rice or alter the effectiveness of phenoxy herbicides on weeds. Rice

treated with aldrin, thiram, or cyano (methyl-
42 WEED CONTROL IN U.S. RICE PRODUCTION

mercuri) guanidine as seed dressings is not

injured by postemergence treatments of

phenoxy herbicides applied to rice in the tolerant stage. Rice treated with postemergence

applications of insecticides such as carbaryl,

carbofuran, and parathion is not damaged by

proper postemergence treatments of phenoxy

herbicides.

Copper Herbicides

Copper sulfate

Copper sulfate (pentahydrate) at 0.5 p/m

by weight of copper (approximately 2 lb copper

sulfate per 4 acre-inches of water) applied in

irrigation water as soon as algal colonies form

on the soil frequently controls green and bluegreen algae in ricefields and irrigation systems

and does not injure rice (87). It is more effective against green than against blue-green

algae, and it does not control chara or aquatic,

broadleaf, and sedge weeds or weed grasses

(table 12). It is most effective when applied

to acid soils low in organic matter or to water

low in carbonate or bicarbonate salts. These

salts in the soil-water medium inactivate copper

sulfate by forming insoluble basic copper

carbonates. Copper sulfate at concentrations

as low as 0.33 p/m by weight in the water

may injure fish. If water with copper sulfate

is drained from the ricefield into irrigation systems, fish, shellfish, and other aquatic animal

life may be injured. Quick-dissolving, fine-mesh

(32-100 mesh) copper sulfate is applied aerially

in California to control the tadpole shrimp

(Triops longicaudatus Le Conte), which damages rice seedlings.

Copper complexes

Chelated copper formulations such as coppertriethanolamine complexes control green and

blue-green algae more effectively than copper

sulfate (R. J. Smith, Jr., unpublished data).

These formulations are not inactivated as easily

as copper sulfate by carbonate and bicarbonate

salts. A rate of 0.75 to 1 lb/acre of elemental

copper controls blue-green algae in ricefields

when applied 1 to 3 days after permanently

flooding the rice. It is most effective when

applied as soon as algal colonies form on the

soil; if applications are delayed until colonies

form a floating mat, control may be poor.

Treatments applied aerially in 3 to 5 gal/acre

of solution kill the algae within 2 to 5 days of

application.

New Experimental Herbicides

Benthiocarb

Benthiocarb applied at 3 to 4 lb/acre or

tank mixtures of benthiocarb and propanil at

3 or 4 lb benthiocarb/acre and 3 lb propanil/

acre control many weed grasses and aquatic,

broadleaf, and sedge weeds 2 to 3 inches tall

and those that germinate for up to 4 weeks

after treatment. Benthiocarb at 4 lb/acre applied after the soil has been sealed by rain or

irrigation and just before emergence of weeds

and rice controls many weeds effectively and

does not injure rice. Because benthiocarb alone

and in mixtures with propanil controls weeds

residually, it is effective for weed control in

rice grown on problem soils. Flooding immediately after treatment is not required to prevent

reinfestation of weed grasses.

Benthiocarb alone or mixed with propanil

sprayed early postemergence injures dry-seeded

rice only slightly, but such treatments may

damage water-seeded rice. However, granular

formulations applied at 2 to 3 lb/acre 10 to 20

days after seeding controlled aquatic weed complexes and did not injure water-seeded rice.

Benthiocarb-propanil mixtures control weed

grasses, including barnyardgrass, broadleaf

signalgrass, and sprangletop, and aquatic weeds

such as dayflower, ducksalad, redstem, and

waterhyssop. Benthiocarb alone frequently fails

to control broadleaf signalgrass and broadleaf

weeds such as morningglory, northern jointvetch, and hemp sesbania.

In California, applications of benthiocarb at

rates as low as 3 lb/acre made 10 to 14 days

after water seeding controlled barnyardgrass,

bearded sprangletop, and ducksalad effectively.

There was no significant rice injury at rates

as high as 12 lb/acre. Applied before flooding,

benthiocarb controlled weeds ineffectively at

rates less than 6 lb/acre, but this rate controlled

barnyardgrass effectively. In most instances

where benthiocarb controlled weed grasses at
PRINCIPLES OF WEED CONTROL 43

3 or 6 lb/acre, control of California arrowhead,

roughseed bulrush, and creeping spikerush was

very poor. Soil incorporation practically eliminated the herbicidal activity of either emulsifiable or granular formulations applied at 3 or

6 lb/acre before flooding and water seeding of

rice.

Oxadiazon

Oxadiazon applied at 0.5 to 1.25 lb/acre or

tank mixtures of propanil at 2 or 3 lb/acre and

0.5 to 0.75 lb/acre of oxadiazon control many

weed grasses and aquatic, broadleaf, and sedge

weeds 2 to 3 inches tall and those that germinate for up to 4 weeks after treatment. Oxadiazon applied after the soil has been sealed by

rain or irrigation and just before emergence

of weeds and crop controls many weeds effectively and is adequately selective for rice.

Oxadiazon is somewhat more phytotoxic to

rice than benthiocarb. If water stands on treated rice, oxadiazon may reduce rice populations

and inhibit early growth of the crop. This

herbicide is selective on dry-seeded rice but not

on water-seeded rice.

Oxadiazon alone and in mixtures with propanil applied postemergence controls weed

grasses, including barnyardgrass, broadleaf

signalgrass and sprangletop, aquatic weeds such

as dayflower, ducksalad, redstem, and waterhyssop, and broadleaf weeds such as hemp

sesbania and northern jointvetch. Morningglory

species are not effectively controlled.

Butachlor

Tank or formulated mixtures of butachlor

and propanil applied at 3 lb butachlor/acre and

3 or 4 lb propanil/acre control many weed

grasses and aquatic, broadleaf, and sedge weeds

2 to o inches tall and those that germinate for

4 or 5 weeks after treatment. Because of its

residual activity, butachlor is very effective

for controlling weeds in rice grown on problem

soils. When the mixture is used on problem

soils, flooding can be delayed until the rice is

large enough to tolerate the water or for up to

4 weeks after emergence; weed plants that

germinate during the drained period are controlled by the residual action of butachlor. Because the mixture controls weeds during the

critical early part of the growing season on all

soils, subsequent treatments with propanil,

molinate, or phenoxy herbicides often are not

required.

Butachlor and propanil mixtures applied

early postemergence injure dry-seeded rice

little, if any, but they may damage water-seeded

rice. These mixtures control weed grasses such

as barnyardgrass, broadleaf signalgrass, and

sprangletop. They control many aquatic weeds

such as dayflower, ducksalad, redstem, and

waterhyssop that are not controlled by propanil

or molinate. However, some broadleaf weeds,

such as morningglory, northern jointvetch, and

hemp sesbania, are not controlled.

KN3 and NaNg

Sequential treatments of propanil and KN3

or NaNs control weed grasses and aquatic,

broadleaf, and sedge weeds in dry-seeded rice

in the South (110), Propanil applied at early

postemergence controls grass weeds 2 to 3

inches tall. Small aquatic weeds that germinate

after treatment with propanil and after flooding are controlled with granular KN3 and NaNa

at 3 to 4 lb/acre applied in the floodwater about

2 weeks after the propanil treatment; weeds 1

to 2 inches tall are most susceptible. KN3 or

NaNs alone fails to control weed grasses and

broadleaf weeds. They control aquatic weeds

such as ducksalad, redstem, waterhyssop, and

false pimpernel, however, that have emerged

at treatment and those that germinate for up

to 10 days after treatment, provided the water

is held at a depth of 2 to 4 inches. Additional

aquatic weeds may germinate after this period.

If the rice stand is good, these late-germinating

weeds usually do not compete significantly with

rice.

Sequential treatments of molinate and KN3

or NaN3 control many weed grasses and aquatic

weeds in dry-seeded rice in the South. Granular

molinate at 3 lb/acre applied in the floodwater

after emergence of the crop and weed grasses

controls barnyardgrass and other annual weed

grasses. Small aquatic weeds, 1 to 2 inches tall,

that germinate after molinate treatment and

flooding are controlled with granular KN3 or

NaNs applied in the water. KN3 and NaN3
44 WEED CONTROL IN U.S. RICE PRODUCTION

treatments applied about 2 weeks after a molinate treatment have the same limitations as

those following a propanil treatment.

Granular KN3 or NaNs applied at 3 to 4

lb/acre in floodwater 15 to 20 days after water

seeding controls many aquatic weeds in the

South. Aquatic weeds less than 2 inches tall,

such as ducksalad, false pimpernel, fimbristylis,

redstem, and waterhyssop, are controlled without injury to rice; however, waterhyssop and

spikerush are less susceptible than the others.

Since KN3 and NaN:, do not control weed

grasses, a propanil or molinate treatment may

be combined with the azide treatment if weed

grasses are problems in water-seeded rice.

In dry- and water-seeded rice, KN3 and NaN,^

do not injure young rice plants growing in a

favorable environment, but they may injure

seedlings growing under adverse conditions.

Rice growing on problem soil during periods

of cool temperatures is especially susceptible.

Bifenox

Granular bifenox applied at 2 or 3 lb/acre

just before or just after permanent flooding

controls germinating aquatic weeds and does

not injure rice. Bifenox treatments applied

sequentially with propanil or molinate effectively control most grass and aquatic weeds all

season. Although preemergence sprays of

bifenox injure rice moderately, they control

bearded and other species of sprangletop.

Propanil treatments applied postemergence control barnyardgrass and broadleaf weeds that

escape bifenox treatment.

Bentazon

Bentazon applied at 0.5 to 1 lb/acre controls

spreading dayflower and many species of smartweed and does not injure rice. It controls

spreading dayflower plants from 3 to 12 inches

tall and smartweeds from 2 to 6 inches tall.

Propanil or molinate treatments, applied sequentially with bentazon, control grass, broadleaf, and sedge weeds that escape bentazon

treatment.

Bioherbicide

An endemic anthracnose disease of northern

jointvetch incited by the fungus Colletotrichum

gloeosporioides (Penz.) Sacc. f. sp. aeschynomené was discovered in 1969 at Stuttgart, Ark.

(28), Water sprays of the fungus spores at

2 million spores/ml in 10 gal/acre controlled

95 to 100 percent of the northern jointvetch

plants in field trials. The fungus controlled

plants in the bloom stage of growth, up to 5

feet tall. The disease developed on northern

jointvetch most rapidly at temperatures between 70° to 90° F and at relative humidities

above 80 percent; the ricefield environment is

ideal for development of the disease on weed

plants. An incubation period of 4 to 7 days

and up to 5 weeks is usually required to kill

the weeds. The fungus is very virulent on

northern jointvetch and only slightly virulent

on Indian jointvetch. It does not affect rice,

soybeans, or cotton or common field forage and

vegetable crops or other weeds.

Commercial ricefield application in 1973 and

1974 demonstrates that an endemic fungus can

be used effectively to control a severe weed.

Research and development will determine if the

fungus can be produced in sufficient quantities

for general use for control of northern

jointvetch.
EQUIPMENT FOR WEED CONTROL

Herbicides are applied to rice by hand and

tractor sprayers and by fixed-wing aircraft and

helicopter sprayers (figs. 6 and 7). Aerial

equipment is used more often than ground

equipment because timely and quick applications can be made when the soil is too wet to

support a tractor. In addition, levees are obstacles for tractors.

HAND SPRAYERS

Several types of hand sprayers are used in

rice, including regular and constant-pressure

knapsack sprayers. If uniform and accurate

herbicide applications are required, the constant-pressure knapsack sprayer is best. It

consists of two tanks, one for the spray solution

and the other for compressed air or other compressed gases. The tanks are connected with

a hose through an air-pressure regulator that

keeps the gas pressure constant in the liquid

tank. The knapsack sprayer can be calibrated

for applying herbicides at precise rates.

TRACTOR SPRAYERS

Tractor-mounted boom sprayers, the most

common type, consist of a power-takeoff-driven

pump, tank or tanks, solution delivery hose,

pressure regulator, pressure gage, nozzles,

strainers, and booms. Boomless sprayers are

sometimes used over rough terrain and where

even distribution of spray is not essential.

Pumps.—Several types of pumps, including

centrifugal, piston, gear, roller, and rubber

impeller, are available for spraying herbicides.

They are rated in gallons per minute of discharge at specific speeds. The capacity of the

pump is about twice that of the total nozzle

delivery rate; this provides adequate flow

through the bypass line for hydraulic agitation

in the tank.

Tanks.—Tanks are available in mild steel

(such as oil drums), stainless steel, aluminum,

and plastic reinforced with fiberglass. Much of

the trouble with clogged nozzles is caused by

rust and scale that accumulate in oil-drum

tanks between spraying seasons. The other

three materials are more expensive than mild

steel, but they withstand the corrosive action

of most herbicides. Nozzle stoppage is much

less when tanks are constructed of corrosionresistant materials.

Hoses and connections.—A durable and

strong spray hose, resistant to sunlight, oil, and

chemicals, is usually used for spraying herbicides. A hose made of neoprene or base rubber

combined with tough rayon and nylon cord gives

good service. Hose connections are made of corrosion-resistant materials and have inside

diameters large enough to avoid restrictions

in the lines.

Pressure regulators.—Two types of bypass

pressure regulators are generally used for lowvolume, low-pressure spraying. One type consists of a spring over a.ball and seat; the other

consists of a spring, diaphragm, plunger, and

seat. Both types have an adjusting screw to

regulate pressure. Sometimes a pressure-reducing regulator is used in conjunction with the

bypass pressure regulator. The bypass valve

may be set at 10 to 15 lb above the desired

pressure at the nozzle tips. The reducing regulator may then be set to maintain a uniform

pressure on the nozzle tips even though the

pump may cause some fluctuation in the

delivery pressure at the bypass valve.

Pressure gages.—Accurate pressure gages

are essential for delivery of herbicides at precise rates and volumes. They are placed in the

hose between the pressure regulator and the

delivery nozzles. Gages with a range of 0 to

100 Ib/in^ are satisfactory. A dial scale with

1- or 2-lb divisions facilitates easy and accurate

calibration of the sprayers.

Nozzles.—Broadcast booms used for spraying

rice are usually fitted with fan nozzle tips that

produce a heavy center spray. The size of the

nozzle is determined by the particular spraying

job. The manufacturer's guide may be consulted

for information.

45
46 WEED CONTROL IN U.S. RICE PRODUCTION

PN-4M1

FIGURE 6.—Application of 2,4-D to rice for control of broadleaf weeds with a fixed-wing airplane sprayer.

Booms.—Broadcast spray booms are classed

as wet or dry. A pipe or metal tube transports

the liquid in the wet boom. Usually a pipe

coupling or eyelet fitting is welded or placed

around small holes spaced 18 to 20 inches apart

in the pipe. The materials used for making

the boom are resistant to corrosion to prevent

nozzle stoppage. In the dry boom the supporting

member does not carry the liquid. The liquid

is transported by hoses and fittings, which consist of pipe tees bolted to the supporting frame.

This type of boom is very satisfactory because

corrosion is minor, and straightening the boom

after accidental bending does not cause leaks.

Strainers.—Line and nozzle-tip strainers are

used in the sprayer to remove solid particles

that may clog nozzle tips. Usually 50- or 100-

mesh screens are satisfactory. All strainers are

removed, cleaned, and inspected for holes every

few days while in use.

FIXED-WING AIRPLANE

SPRAYERS

Most airplane sprayers are equipped with

booms and nozzles that are similar to those on

tractor sprayers. Other distribution systems

include rotation brushes or screens, disks, hollow propellers, bifluid and foam nozzles, and

the venturi type. The latter may be a single

large unit under the fuselage or several small

units mounted under each wing. The boom

distribution system is usually used for spraying

herbicides on rice.

Granular herbicides are applied aerially with

the venturi type or with rotating disks. The

distribution device is located under the fuselage

and the chemical hopper.

The same types of pumps used with tractors

are used with airplanes, but the centrifugal

pump is most common. The pump is mounted

outside the fuselage and driven by power takeoff from the engine or by a small propeller.

With the latter, the size of the propeller may

be changed for the desired volume of spray.

The capacity of the pump is about twice the

nozzle output. This allows sufficient liquid

through the bypass valve for hydraulic agitation.

Tanks are usually made of corrosion-resistant

materials, such as aluminum or fiberglass, and

they carry 90 to 250 gallons of herbicide mixture. Pressure regulators, pressure gages, hoses,

and strainers used for aerial spraying are

similar to those used on tractors.

Booms are usually made of a corrosion-resistant material such as aluminum. To miniMMjji^. ^« ' *"^ .

1

riiiksi 3?MHBÍHHÍ^H^HHHHHBBBIêà.

PN-494S

FIGURE 7.—Application of 2,4-D to rice for control of

broadleaf weeds with a helicopter sprayer.
EQUIPMENT FOR WEED CONTROL 47

mize drift of spray, the boom is placed as far

below the wing as is practical, usually about

1 foot, and is extended within about 2 feet of

the vingtip. If the boom extends to the wingtip,

the spray may be whirled upward in the wingtip vortexes to cause excessive spray drift.

Nozzles for fixed-wing aircraft sprayers are

made of corrosion-resistant materials such as

aluminum, brass, or nylon. Each is equipped

with a quick-cutoff diaphragm, screen, and jet.

Spray droplet size is greatly affected by the

angle at which the nozzles discharge into the

airstream. Smaller droplets occur when the

nozzles are directed against or across the airstream than when they are directed with it.

Droplet size is also affected by pump pressure

and nozzle-orifice diameter. Most ricegrowing

States have regulations that specify the maximum operating pressure for aerial spraying

of phenoxy herbicides; this usually does not

exceed 20 Ib/in^ A compromise is usually made

between small droplets, which give thorough

coverage but have a tendency to drift, and

large ones, which settle fast but do not give

adequate coverage. Sprays usually give adequate weed control if droplets range from 100

to 300 micrometers in diameter.

Spray pattern, or distribution, is important.

Proper placement and spacing of nozzles along

Example 1.—Calibration of Tractor Sprayer Discharge Rate

1. Select an area for a test run that is similar to the field to

be sprayed. Measure off a distance of 660 feet.

2. Place the sprayer on level ground and fill the spray tank

with water. Make sure it is full.

3. Spray the 660-foot test run. Operate the sprayer at exactly

the same pressure and tractor speed that will be used in

the field—usually about 3 or 4 miles per hour.

4. Upon reaching the 660-foot mark, stop spraying. Then

measure carefully the number of quarts needed to refill the

tank.

5. Convert the number of quarts to gallons of water required

to refill the spray tank by dividing by 4. Then multiply

this figure by 66. Divide the results by the width, in feet,

of the strip sprayed. The answer obtained is the number

of gallons of spray liquid the sprayer will apply on 1 acre

when it is operated at the same settings.

6. Suppose the sprayer boom sprayed a strip 30 feet wide.

After 660-foot strip is sprayed, 10 quarts are required to

refill the tank. Ten quarts divided by 4 equals 2.5 gallons.

Multiply 66 by 2.5, which equals 165. Then divide 165 by

30 (the width in feet of the sprayed strip). The answer is

5.5, which is the gallons of spray liquid applied per acre.

the boom helps to distribute the spray evenly.

Usually the spray pattern is improved if more

nozzles are placed on the right side of the

plane than on the left. The air is swirled from

right to left by the counterclockwise rotation

of the propeller (facing the propeller). Spraying the proper swath width for the particular

aircraft also improves spray distribution. The

wingspan and the flying height of the airplane

govern the swath width. The width of the

swath is usually about equal to the wingspan

of the plane. Proper flying height improves

spray pattern and reduces spray drift. Spray

distribution is best when fixed-wing airplanes

fly at 10 to 15 feet above the crop, but spray

drift is less when they fly at 5 to 10 feet.

Phenoxy herbicides are released at the lowest

possible height, but propanil or molinate is

released at a height that gives best spray

distribution.

HELICOPTER SPRAYERS

Helicopters are sometimes used to apply

herbicides to rice, but they are more expensive

than fixed-wing aircraft. Some users claim that

spray applications from helicopters provide

better coverage and control of weeds than do

those from fixed-wing planes because the downwash from the helicopter rotors forces the

spray down into the crop. In closely bounded

fields a helicopter can be used advantageously

to give better weed coverage near the edge

of the ricefield and to reduce spray drift to

nearby susceptible crops. The helicopter can

stay within the ricefield while turning at the

ends of the field. This eliminates flight over

nearby susceptible crops and thus prevents accidental contamination of these crops by spray

trailing the aircraft or by leakage of spray.

Boom sprayers are used on helicopters. The

pumps, tanks, hoses, pressure regulators, pressure gages, nozzles, booms, and strainers used

on helicopters are similar to those used on

fixed-wing planes.

A helicopter carries about 60 gallons of liquid

spray in two spray tanks; one is located on

each side of the aircraft for balance. The flying

height is usually 2 to 5 feet above the rice.

The swath width is about equal to the length

of the boom.
48 WEED CONTROL IN U.S. RICE PRODUCTION

Example 2.—Calibration of Airplane Sprayer Discharge Rate

1. Measure off a distance of 1 mile (5,280 feet).

2. Fill the spray tank with water. Make sure it is full.

3. When the weather is similar to that which will prevail while

spraying the rice, spray the 1-mile test run. Operate the

sprayer at exactly the same pressure and airplane speed

(ground speed) that will be used in the field—usually about

80 to 85 miles per hour for fixed-wing planes and about

40 miles per hour for helicopters.

4. Upon reaching the 1-mile mark, cut off the sprayer.

Measure carefully the gallons of water needed to refill

the tank.

5. Multiply the number of gallons of water needed to refill

the tank by 8.25. Divide the results by the width, in feet,

of the strip sprayed. The answer obtained is the gallons

of spray liquid the sprayer will apply on 1 acre when it

is operated at the same settings.

6. Suppose the strip sprayed is 33 feet wide. After the 1-mile

strip is sprayed, 12 gallons are required to refill the tank.

Multiply 12 by 8.25, which equals 99. Then divide 99 by

33 (the width in feet of the sprayed strip). The answer is 3,

which is the gallons of spray liquid applied per acre.

CALIBRATION OF TRACTOR

AND AERIAL SPRAYERS

Proper calibration of sprayers is essential

for good results with herbicides. Too much

herbicide can injure the crop; too little will

not control the weeds.

First, select the kind of nozzle that will give

the desired volume of spray per acre. In rice,

herbicide solutions are applied at low rates—5

to 20 gal/acre by tractor and 1 to 15 gal/acre

by aircraft. Each nozzle of a sprayer must discharge uniformly. Check this by measuring the

volume of water discharged by each nozzle in

1 minute. Replace or clean nozzles that vary

excessively in volume discharged.

After initial selection and adjustment of the

spraying equipment, determine the actual peracre output of the sprayer by making a trial

run. This is done by spraying a measured strip

of land with water at the tractor or airplane

speed that best suits the contemplated job. The

amount of water used in the run is measured,

and the per-acre discharge rate is calculated

as illustrated in examples 1 and 2. Once this is

done, the area that can be covered by one

tankful of spray is calculated by dividing the

Example 3.—Calculation of Area Covered by

One Tank of Spray

Problem: Your tank holds 90 gallons. The sprayer is calibrated to apply 3 gal/acre (example 2). How

many acres will 1 tank cover?

Calculation: Divide tank capacity by sprayer discharge rate:

90-^-3 = 30 acres covered by 1 tank.

Example 4.—Calculation of Herbicide-Water Ratio

Problem: To kill ducksalad infesting a ricefield requires

1.5 lb/acre of 2,4-D acid equivalent. Your tank

holds enough to cover 30 acres at a 3-gal/acre

rate (example 3). The commercial 2,4-D formulation you are using contains 4 lb/gallon of acid

equivalent. How much commercial 2,4-D formulation is required per tank to achieve a 1.5

lb/acre 2,4-D application?

Calculation: Multiply the number of acres 1 tank will cover

(30) by the required herbicide rate (1.5 lb/acre).

Divide the answer by the acid equivalent in the

commercial formulation (4 lb/gal). 30X1.5 =45

pounds of 2,4-D required per tank. 45-^4 =

11.25 gallons of commercial 2,4-D formulation

required per tank.

Mix 11.25 gallons of commercial formulation with enough

water to fill the tank, and the spray is ready to use.

capacity of the tank by the sprayer discharge

rate, as illustrated in example 3. Finally, calculate the amount of commercial herbicide and

water needed to make the desired strength of

liquid spray for the size of tank as illustrated

in example 4.

It is advisable to check the spray distribution

pattern of both tractor and aerial sprayers.

This may be done by placing water-soluble

dye in the tank. Place a strip of white paper

on the ground across the swath to be sprayed

and, operating the tractor or aerial sprayer

exactly as it will be operated in the field, make

a test spray run. Thé spray pattern will be

shown on the paper. Height, nozzle, and other

adjustments may be required to obtain the desired spray pattern.

Slight pressure adjustments may be required

to obtain the desired output of liquid spray.

Each time the sprayer is readjusted, it should

be recalibrated as described. It is a good idea

to make frequent checks for nozzle orifice wear

and other factors that may change the rate of

spray output.
WEEDS THAT CAUSE ECONOMIC LOSSES IN RICE

Accurate weed identification is essential to

control because weeds differ in their response

to various herbicides and cultural practices

(tables 9 and 12). Only weeds that cause major

losses in U.S. rice production are described in

this section, although there are others that reduce yields. They are listed alphabetically by

common name. The background board in some

pictures is marked in 6-inch squares.

GREEN AND BLUE-GREEN

ALGAE

Principal genera.—The green algae (Chlorophyceae) include Chara spp. (fig. 8) and filamentous Hydrodictyon spp., Pithophora spp.,

and Spirogyra spp. The blue-green algae

(Cyanophycaea) include Anabaena spp., Lyngbya spp., Nostoc spp., and Phormidium spp.,

which frequently form "scum'' in ricefields.

Description.—Algae reproduce vegetatively

by cell division and by spores. Vegetative reproduction is characterized by accidental

separation of colonies or filaments. These small

units are distributed by flowing water; they

establish new colonies by cell division. The

microscopic seedlike spores may be carried by

wind and flowing water. Each spore germinates

and develops into a new plant much like a seed

develops into a plant.

Chara algae are large species that anchor

themselves to bottom mud. Symmetrically

branched from evenly spaced, cylindrical leaf

whorls at the nodes, the plants are usually

encrusted with calcareous deposits that give

them a coarse, gritty feeling. When crushed,

they emit a musky odor similar to garlic or

skunk. Usually gray-green, they may have a

slightly different color owing to calcareous

deposits and accumulated dead plants.

Hydrodictyon algae develop as saclike nets.

Numerous small units can be observed growing

on the mud and debris in canals in the initial

stages of development. These cylindrical sacs,

which contain thousands of cells arranged in a

mesh, may elongate into heavy masses of

growth.

Pithophora algae are irregularly branched

and filamentous. Initially, the plants grow on

mud and debris in irrigation systems, but the

dense growth of later development captures

gases that raise mats of plants to the water

surface. These floating mats often resemble

wet wool, with individual strands of growth resembling coarse thread.

Spirogyra algae grow in long threadlike

filaments. Normally bright green, they may

be coated with a rust-colored deposit from iron

in the water. Typically the chloroplasts can be

observed under the microscope as elongated

spiral bands extending from one end of the cell

to the other; they are embedded in the layer of

cytoplasm just inside the cell wall.

Anabaena, Lyngbya, Nostoc, and Phormidium

algae are filamentous and usually grow in colonies resembling tufts of soft wool. These

colonies develop on mud after spore germination. A gelatinous material around the filaments

collects gases liberated in growth, and after the

gas bubbles accumulate in the colony, small

units float to the water surface, where they

continue to grow. Typically, the algal units

unite to form a solid mass of scum, or '*jellyballs,'' on the water surface. The bottom layer

of the scum is covered with soil, but the surface, exposed to air and sunlight, drys to a

tough film. Development of scum is usually

rapid and varies with soil and environmental

conditions.

Distribution.—Green and blue-green algae

are troublesome in all rice-producing areas.

Chara spp. grow in ricefields where stands are

thin, in barrow ditches, and in irrigation systems. Green filamentous algae, such as the

species of Hydrodictyon, Pithophora, and

Spirogyra, grow most abundantly in irrigation

and drainage canals and ditches, where they

impede waterflow, but they may enter ricefields

from canals and reduce the growth and tillering

of young rice plants. Blue-green filamentous

49
50 WEED CONTROL IN U.S. RICE PRODUCTION

.-i^à^'

A^m- - .¿* ',y

PN-4944

FIGURE 8.—Chara {Chara sp.).

algae, such as the species of Anabaena,

Lyngbya, Nostoc, and Phormidium, are most

abundant early in the growing season in newlyflooded ricefields. They are especially troublesome in water-seeded rice. Algal growth pulls

young rice plants down into the water or mud,

thus reducing the stand. Tillering of the rice

plants is reduced because the young tillers cannot penetrate the scum. Rice plants may become

diseased where they contact the scum, probably

from fungi and bacteria in it.

ALLIGATORWEED

Altemanthera philoxeroides (Mart.) Griseb.

Other name.—Alligatorgrass.

Description.—Alligatorweed (fig. 9) is an

aquatic and terrestrial perennial that reproduces vegetatively. The stems, 3 to over 24

inches long and often hollow, grow prostrate

on water or soil. They are glabrous except at

the leaf axils and sometimes near the tips. The

nodes usually have two buds, each of which may

sprout to produce a shoot. Roots may form at

every node in water or on moist soil. On soil

the roots enlarge and thicken; they look like

the roots of perennial terrestrial plants. The

leaves are opposite, narrowly elliptical to

lanceolate or oblanceolate, 1 to 5 inches long,

and about 0.25 to 1 inch wide; they taper at the

base to a very short petiole or are sessile, and

have distinct midveins. From 6 to 20 white,

chaffy flowers in cloverlike heads are borne on

axillary peduncled spikes. Viable seeds have not

been found in the United States.

Distribution.—Alligatorweed probably grows

in a wider range of water and soil conditions

than any other plant. When it grows as a

rooted terrestrial plant along the banks of

canals and creeks and in cultivated fields, it

has a deep and extensive root system. Plants

also grow in free-floating mats; if such plants

contact unflooded soil, they root. Emersed
WEEDS THAT CAUSE ECONOMIC LOSSES IN RICE 51

plants may grow from mud up through 9 feet

of water. Alligatorweed grows in and along

irrigation systems and in ricefields in the

South. Small isolated infestations have been

found in southern California, but not in ricegrowing areas. Because alligatorweed responds

vigorously to fertilization, it grows especially

well in fertilized, moist ricefields, where it

reduces yields, interferes with harvesting, prolongs drying, and impedes the flow of water

in canals and ditches.

ARROWHEAD

Sagittaria spp.

Other names.—Swamp-potato and arrowheadlily.

Principal species.—S. montevidensis Cham.

& Schlecht, (fig. 10), S. latifolia Willd., called

common arrowhead, S. cuneata Sheldon, S.

graminea Michx., and S. calycina Engelm.,

called California arrowhead.

Description.—Perennials that reproduce by

seed, underground rhizomes, and tubers, arrow- FiGURE 9.—Alligatorweed (Altemanthera

philoxeroides).

PN-4945

FIGURE 10.—Arrowhead (Sagittaria montevidensis).

PN-4946
52 WEED CONTROL IN U.S. RICE PRODUCTION

PN-4947

FIGURE 11.—Mature barnyardgrass (Echinochloa crusgalli).

heads are rooted aquatics, usually emersed. The

common species have arrow-shaped leaves, but

some species have linear ones. The normal

leaves are all basal, with petioles usually as

long as the water is deep. Plants are normally

1 to 2 feet tall.

Distribution.—Arrowheads grow in all riceproducing States, and several species grow in

most States. They grow in shallow water in

irrigation and drainage canals, in ricefields

where stands are thin, and in ricefield barrow

ditches.

PN-4948

FIGURE 12.—Collar region of barnyardgrass (left) and

of rice (right). Barnyardgrass has no ligule or

auricle; rice has ligules and hairy auricles.

BARNYARDGRASS

Echinochloa spp.

Principal species, varieties, and common

names.—E. crus-galli (L.) Beauv. (fig. 11),

called watergrass, millet, bluestem, and baronetgrass; E. colonum (L.) Link, called junglerice,

little barnyardgrass, and shortmillet; and E.

crus-pavonis (H.B.K.) Schult., called gulfcockspur and cattailgrass. There are many varieties

of E. crus-galli, including zelayensis (H.B.K.)

Hitchc, longiseta (Trin.) Farw., and those

commonly known as baronetgrass and white

barnyardgrass.

Description.—Species and varieties of barnyardgrass that infest ricefields are annuals;

they reproduce by seed. Species differ greatly.

All species tiller abundantly and produce abun-
WEEDS THAT CAUSE ECONOMIC LOSSES IN RICE 53

FIGURE 13.—Panicles of several varieties of barnyardgrass {E. crus-galli).

PN-4949

dant seed that may remain viable in the soil

for many years. Young barnyardgrass plants

often are difficult to distinguish from rice.

The collar region on the leaves may be used

to differentiate them. Barnyardgrass has no

ligules or auricles; rice has membranous ligules

and hairy auricles (fig. 12).

E. crus-galli has more intraspecific morphological variation than most other species. It is

erect to decumbent and from 3 to 6 feet tall

at maturity. Some varieties are awnless; others

have awns, the length of which is characteristic

of each. Varieties differ greatly in time to

mature, height, and size of stems, heads, and

seeds. (Panicles of several varieties of E. crusgalli are shown in fig. 13.) Whereas white

barnyardgrass, which grows in California,

germinates under water, most varieties do not.

E. colonum is prostrate to erect and from

1 to 2 feet tall at maturity. Compared with

E. crus-galli, it branches more at the base,

grows more openly, and has smaller culms,

heads, and seeds.

E. crus-pavonis is erect or sometimes decumbent and about 3 to 4 feet tall at maturity,

with large conspicuous panicles. The spikelets

have long awns.

Distribution.—E. crus-galli is a troublesome

weed in all ricegrowing States. E. colonum and

E. crus-pavonis grow mostly in Louisiana and

Texas, but E. crv^s-pavonis is not common even

in these States. All species grow well in rice,

on levees, along irrigation and drainage canals,

on reservoir banks, in sloughs, and in other

wet areas. Barnyardgrass also grows in row

crops such as soybeans, corn, and cotton. Seed

may be spread by irrigation water, animals,

birds, crop seed, farm machinery, and other

carriers. Barnyardgrass infestations greatly

reduce rice yields.

BEAKRUSH

Rhynchospora comiculata (Lam.) Gray

Other names.—Spearhead, hornedrush, umbrellaweed, tadpole-sedge, and ace-of-spades.

Description.-—Beakrush (figs. 14 and 15)

is a perennial that reproduces by seeds and

rhizomes. The stems are stout, leafy, and triangular and from 2 to 6 feet tall. The leaf

blades have a rough margin. The inflorescence

is widely branched. The seeds are about 0.25

inch long, have beaks 0.5 to 0.75 inch long,

and are extremely difficult to separate from

rice seed.

Distribution.—Beakrush is a troublesome
54 WEED CONTROL IN U.S. RICE PRODUCTION

PN-496I

Beakrush panicle.

PN-49B0

FIGURE 14.—Mature beakrush (Rhynchospora

eomiculata).

weed in the South. It grows in ricefields, canals,

ditches, and other wet areas. Infestations in

rice reduce yields, interfere with harvesting,

prolong drying, and reduce quality.

BROADLEAF SIGNALGRASS

Brachiaria platyphylla (Griesb.) Nash

Other names.—Brachiaria and signalgrass.

Description.—Broadleaf signalgrass (fig. 16)

is an annual that reproduces by seed. The young

plants are similar to crabgrass (Digitaria spp.)

seedlings, but they have shorter, wider leaves.

The seeds are about twice as large as crabgrass

seed. The plants are about 1 to 2 feet tall at

maturity.

Distribution.—Broadleaf signalgrass grows

only in the South—in rice, on levees, and along

edges of canals. It reduces yields by competing

with rice early in the growing season.

BULRUSH

Scirpus spp.

Principal species and common names.—S.

mucronatus L. (fig. 17), called roughseeded

bulrush; S. fluviatilis (Torr.) Gray, called river

bulrush; and S. acutus Muhl. (fig. 18), called

hardstem bulrush and great bulrush.

Description.—Most of the bulrushes that are

troublesome in rice are perennials, reproducing

by seed, rhizomes, or rootstocks. S. mucronatus

and S. fluviatilis have triangular, stout culms

that grow 2 to 4 feet tall. S. acutus has round,

erect culms from 3 to 6 feet tall.

Distribution.—Bulrushes are troublesome in

most rice-producing States. S. mucronatus and

S. fluviatilis are problems only in California,
WEEDS THAT CAUSE ECONOMIC LOSSES IN RICE 55

ml

•i -, ^' ¿CM i^V*^":*m^-

PN-49B2

FIGURE 16.—Broadleaf signalgrass (Braehiaria

platyphylla).

and the former is more troublesome because

it infests large acreages of rice. Bulrushes in

the South may infest poorly drained ricefields,

drainage and irrigation canals, ditches, reservoirs, and other wet areas. In competing with

rice, they lower yields, interfere with harvest,

and impede the flow of water in canals and

ditches.

BURHEAD

Echinodortis cordifoUus (L.) Griseb.

Other names.—Mudbabies and creeping

waterplantain.

Description.—Burhead (fig. 19) is an

aquatic, emersed, erect annual that reproduces

by seed. The plants are 24 to 48 inches tall

and bear many whorls of flowers. The leaves

have long petioles; the leaf blades are egg

PN-495S

FIGURE 17.—Roughseed bulrush (Seirpus

mucronatua).

shaped, have heart-shaped bases, and are up

to 10 inches long by 5 to 6 inches wide. The

fruit, which resembles a bur, is 0.25 to 0.5

inch in diameter. The seeds are about 0.1 inch

long, sharply ridged, and short beaked.

Distribution.—Burhead grows in most riceproducing States. It frequently grows in ricefields, shallow irrigation and drainage canals,

ditches, potholes, sloughs, and other wet areas.

It is most troublesome in areas where rice

stands are thin or where cold water enters the

ricefield. Burhead germinates under water.
56 WEED CONTROL IN U.S. RICE PRODUCTION

H5

»'>

»» •f»

PN-4954

FIGURE 18.—Hardstem bulrush (Scirpits acutus).

CATTAIL

Typha spp.

Other names.—Flags, tules, and reedmace.

Principal species.—T. angustifolia L., T.

domingensis Pers., T. glauca Godr., and T.

latifolia L. (fig. 20).

Description.—Cattails are perennials that reproduce by rootstocks and by minute airborne

seeds that germinate readily in mud and occasionally in shallow, clear water. The plants,

which have an extensive rhizome system, are

erect and have long, narrow leaves. The flowers

are in a long and very dense cylindrical spike

terminating the stem. The plants are 4 to 8

feet tall and usually grow in colonies. The

seeds may remain viable for more than 5 years

in the soil.

Distribution.—Cattails grow in all riceproducing States, but all species are not found

PN-4965

FIGURE 19.—Burhead (Echinodorus cordifolius).

in every State. They grow in shallow water in

irrigation and drainage ditches and may grow

in rice where drainage is inadequate. They

compete with rice and impede the flow of

water in irrigation systems.

COMMON WATERPLANTAIN

Alisma triviale Pursh

Other names.—Waterplantain and waterlily.

Description.—Common waterplantain (fig.

21), an emersed, erect, aquatic perennial that

reproduces by seed and rootstocks, may grow

48 inches tall. The elliptical leaf blades, up to

10 inches long and 6 inches wide, grow on long

petioles. The inflorescence is pyramid shaped

and branched. The seeds are round, reddish

brown, and about 0.1 inch long and may remain

viable in the soil for many years.

Distribution.—Common waterplantain is
WEEDS THAT CAUSE ECONOMIC LOSSES IN RICE 57

PN-4956

FIGURE 20.—Cattail (Typha latifolia), showing leaf,

spike, stem, and root parts.

found in rice in California only. It grows in

ricefields where stands are thin, on banks and

edges of irrigation and drainage canals, and

in most wet areas; it often grows abundantly

where cold water enters the ricefield. The seeds

germinate under water and at a lower temperature than rice.

DAYFLOWER

Commelina diffusa Burm. f.

Other names.—Spreading dayflower, wandering-Jew, batwing, and water parsley.

Description.—Spreading dayflower (fig. 22)

is an annual that reproduces by seeds. Its

succulent stems, 1 to 3 feet long, grow erect

in the seedling stage but begin to spread or

creep in midsummer or later. Roots may grow

from the lower nodes. The alternate, lanceshaped leaves, 2 to 4 inches long and about

half as wide, clasp the stem. Irregularly shaped

flowers, blue or purplish blue and about 0.5

inch wide, are borne on boat-shaped, leaflike

structures about 1 inch long and half as wide.

PN-49B7

FIGURE 21.—Common waterplantain (Alisma triviale).

PN-49B8

FIGURE 22.—Spreading dayflower (CommeZina diffusa).
58 WEED CONTROL IN U.S. RICE PRODUCTION

PN-4960

FIGURE 24.—Mudplantain (Heterwnthera reniformü).

PN-49B9

FIGURE 23.—Ducksalad (Heteranthera limosa).

The seeds are wrinkled and are about 0.2 inch

long.

Distribution.—Spreading dayflower grows in

the South. It is particularly troublesome in the

ratoon rice crop in Louisiana and Texas. It

grows profusely in late July or early August,

after the first crop has been harvested and

when the ratoon crop is watered and fertilized.

Spreading dayflower reduces yields, lowers the

market value of rough and milled rice because

seeds are difficult to remove, interferes with

harvesting, and slows drying.

DUCKSALAD

Heteranthera spp.

Other names.—Mudplantain and waterlily.

Principal species.—H. limosa (Sw.) Willd.

(fig. 23) and H. reniformis R.&P. (fig. 24).

Description.—Species of ducksalad are

aquatic annuals that reproduce by seed. Leaf

shape and flower number differentiate H.

lirnosa and H. reniformis. The leaves of H.

limosa are oval, narrow, and 0.5 to 5 inches

long; the inflorescence has only one flower.

In contrast, the leaves of H. reniformis are

kidney shaped or heart shaped, 1 to 3 inches

long, and about as broad; the inflorescence

has two or more flowers. Ducksalad seeds

PN-4961

FIGURE 25.—Eclipta {Eclipta alba).

which are minute, ridged, and black, are borne

copiously in a dehiscent capsule.

Distribution.—Ducksalad is a troublesome

weed in all ricegrowing areas. It grows in rice,

in shallow water in irrigation and drainage

canals, and in ditches, potholes, sloughs, and
WEEDS THAT CAUSE ECONOMIC LOSSES IN RICE 59

PN-4962

FIGURE 26.—False pimpernel (Lindernia pyxidaria).

other wet areas. It is particularly troublesome

in water-seeded rice. The seeds germinate only

in saturated or flooded soil in an anaerobic environment. The young plants compete with rice

early in the growing season and reduce yields.

ECLIPTA

EcUpta alba (L.) Hassk.

Other names.—Tagoweed and yerba-de-tago.

Description.—Eclipta (fig. 25) is a weakly

branched annual that reproduces by seeds. Its

fine-haired stems, 8 to 36 inches tall, grow

erect; the base, however, may recline, and roots

often emerge from the nodes. The leaves, 1 to

4 inches long and slightly toothed along the

margins, are lance shaped, sessile, and opposite.

Several bell-shaped flower heads are borne on

elongated stalks that grow from the leaf axils;

they contain many white ray flowers. The seeds

PN-4963

FIGURE 27.—Fimbristylis {Fimbristylis miliacea).

are triangular to rectangular and somewhat

flattened.

Distribution.—Eclipta grows in the South

in ricefields where stands are thin, on levees,

and along the banks of irrigation systems. It

reduces yields, interferes with harvesting, and

slows drying.

FALSE PIMPERNEL

Lindernia spp.

Principal species.—L. anagallidea (Michx.)

Pennell and L. pyxidaria L. (fig. 26).

Description.—False pimpernel is a loosely

branched aquatic annual, 4 to 8 inches tall,

that reproduces by seed. L. pyxidaria is more

branched than L. anagallidea. The smoothmargined leaves, 0.2 to 0.8 inch long, are

elliptical, but broader at the base than at the

tip. White or pale lavender flowers, 0.2 to 0.3

inch long, are borne on axillary stalks about

twice as long as the leaves. The flowers of

L. pyxidaria are shorter (usually less than 0.2

inch) than those of L. anagallidea. The sepals

of L. pyxidaria are usually as long as the corolla,

but those of L. anagallidea are usually shorter.

The seed pods, 0.1 to 0.2 inch long, contain

many brown or yellow seeds.

Distribution.—False pimpernel grows in the
60 WEED CONTROL IN U.S. RICE PRODUCTION

PN-4964

FIGURE 28.—Gooseweed (Sphenoclea zeylanica).

South, usually with other aquatic weeds such as

ducksalad, redstem, and waterhyssop. It is most

troublesome where stands of rice are thin or

where rice is seeded in water. It grows in

rice, shallow ditches, canals, and most wet

areas. It competes with rice during the early

growing season and may reduce tillering and

yield.

FIMBRISTYLIS

Fimbristylis spp.

Other name.—Hoorahgrass.

Principal species.—F. autumnalis (L.) R.&S.

and F. miliacea (L.) Vahl (fig. 27).

Description.—Species of fimbristylis are

turflike annuals that reproduce by seed. The

culms are slender, erect to decumbent, leafy at

the base, 12 to 18 inches tall at maturity, and

weak; they lodge easily. The leaf blades are

narrow and short. The inflorescence is open

.surf.

%"'>

**- ^^, ^ ^

^91

'^

PN-496B

FIGURE 29.—Hemp sesbania (Sesbania exáltala).

and has several small foliagelike bracts of

variable length at its base.

Distribution.—Fimbristylis is troublesome

only in the South. It grows in rice, on levees,

along canals, and in most wet areas. It competes

with rice in thin stands and reduces yields.

GOOSEWEED

Sphenoclea zeylanica Gaertn.

Other name.—Hollowstem.

Description.—Gooseweed (fig. 28) is an

aquatic annual that reproduces by seed. Its

stems are erect, smooth, hollow, and 1 to 5 feet

tall. The leaves are elliptical to linear, smooth

margined, 1 to 4 inches long, and light green

(paler beneath than above). Flowers are borne

on a continuous dense spike and are white or

greenish. The bucket-shaped seed pods, about

0.2 inch in diameter, contain many small red or

brown seeds.
WEEDS THAT CAUSE ECONOMIC LOSSES IN RICE 61

PN-4966

FIGURE 30.—The needlelike stipules of hemp sesbania

(left) distin^ish it from northern jointvetch

(right).

Distribution.—Gooseweed grows in the South

in rice where stands are thin, in irrigation and

drainage canals, in ditches, along levees, and

in most wet areas. It competes with rice, interferes with harvest, and prolongs drying when

it is green and succulent.

HEMP SESBANIA

Sesbania exaltata (Raf.) Cory

Other names.—Coffeebean, coffeeweed, tallindigo, sennabean, and sesbania.

Description.—Hemp sesbania (fig. 29) is a

tall, erect, smooth, leguminous annual that reproduces by seed. It may grow 12 to 15 feet

tall. The leaves are compound, with 12 to 25

pairs of leaflets. The flowers are yellow and

usually mottled with purple. The narrow, compressed pods, 6 to 12 inches long, contain numerous small, oblong seeds separated by

transverse partitions. The seeds may be viable

in the soil for many years. Young plants of

hemp sesbania resemble those of northern

jointvetch, but they may be identified by the

\

PN-4967

FIGURE 31.—Homed ponáweed (Zannichellia palustris).

small, narrow bract or stipule, the smooth stem

(fig. 30), and the simple leaf that emerges just

after the cotyledons.

Distribution.—Hemp sesbania is a problem

weed in the South in uncultivated fields, waste

areas, row crops, and rice, on levees, and along

canals and ditches. It does not grow in California. It competes with rice, interferes with

harvest, and reduces the value of rough and
62 WEED CONTROL IN U.S. RICE PRODUCTION

milled rice because its seeds are difficult to

remove.

HORNED PONDWEED

Zannichellia palustris L.

Description.—Horned pondweed (fig. 31) is

a submersed aquatic perennial that reproduces

by seed and slender rhizomes. Slender, branched

stems, 10 to 30 inches long, root at the node

or may be free swimming. Leaves 1 to 4 inches

long by 0.02 inch wide emerge from the nodes

in a whorl. Numerous unisexual flowers are

borne in the leaf axils. The achenial seed, 0.1

inch long, is sessile or short stalked, flat,

slightly curved, and beaked at the tip.

Distribution.—Horned pondweed is found in

all ricegrowing States, but it is a troublesome

weed only in California in water-seeded rice.

It grows in rice during the early season, especially in fields with deep floods of 6 to 8

inches, in irrigation systems, and in most wet

areas. It competes with rice during the early

growing season and may reduce tillering and

yield.

JOINTVETCH

Aesckynomene spp.

Principal species and common names.—A.

virginica (L.) B.S.P. (fig. 32), called northern

jointvetch, sensitive jointvetch, indigo, curly

indigo, short indigo, bashfulweed, and coffeeweed; and A. indica L., called Indian jointvetch,

sensitive jointvetch, indigo, curly indigo, and

short indigo.

Description.—A. indica and A. virginica are

leguminous annuals that reproduce by seeds.

The stems of both species are branched, sparsely bristled at the base but abundantly bristled

near the apex, and 2 to 5 feet tall. The compound leaves have short petioles, and 25 to 55

linear, oblong leaflets that fold at night or

when touched. The leaves of A. indica taper

more at the tip and are a paler green than

those of A. virginica. The stipules are large

and deciduous, especially those below the tip

of the stem. The inflorescence is axillary, often

leafy, and contains one to six yellow, red-veined

flowers. The pods, 1 to 3 inches long, are

segmented into 4 to 10 nearly square joints

PN-4968

FIGURE 32.—Northern jointvetch {Aesckynomene

virginica).

that separate easily at maturity. The pods of

A. indica are smaller than those of A. virginica.

The seeds are kidney shaped and remain viable

for many years in the soil. The seeds of A.

indica are smaller than those of A. virginica.

The young plants resemble hemp sesbania and

may be identified by their large stipules (fig.

30) and bristly stems near the shoot apex.

Distribution.—A. indica and A. virginica are

troublesome weeds in the South. A. virginica

is more common in ricefields than A. indica;

the estimated distribution ratio in ricefields

for the two species is 19 to 1. Jointvetch grows

in rice, on levees, in uncultivated fields and

waste areas, and along canal and ditch banks.

Both species compete with rice, interfere with

harvest, and reduce the value of rough and

milled rice because their seeds are difficult to

remove.
WEEDS THAT CAUSE ECONOMIC LOSSES IN RICE 63

PN-4969

FIGURE 33.—Knotgrass (Paspalum distichum).

KNOTGRASS

Paspalum spp.

Principal species and common names.—P.

distichum L. (fig. 33), called knotgrass, water

bermudagrass, jointgrass, and lakegrass; P.

lividum Trin., known commonly as longtom,

doggrass, and watergrass; and P. acuminatum

Raddi., called brookpaspalum, watergrass, and

crawlinggrass.

Description.—The principal species are

perennials that reproduce by seed and creeping

runners. P. distichum has long stolons, often

25 feet long; relatively short, 1- to 3-inch leaf

blades; and two ascending 1- to 2-inch racemes

or seed heads, often incurved. P. lividum is a

smooth, creeping grass with 20- to 40-inch

culms, 6- to 10-inch purple-tipped leaf blades,

and usually four to seven dark-purple racemes

on the seedstalk. P. acuminatum resembles P.

lividum except that it has shorter and wider

leaf blades with very light-colored midribs, and

its seed heads bear three to five (usually three)

erect or descending racemes 2 to 3 inches long.

PN-4970

FIGURE 34.—Mexicanweed (Caperonia castaneaefolia).

Distribution.—Knotgrass grows in all riceproducing areas, but longtom and brookpaspalum grow only in southern Louisiana and Texas.

They are usually most troublesome in young

rice and in irrigation and drainage canals»

where they impede the movement of water.

These weeds compete with rice and reduce

yields.

MEXICANWEED

Caperonia castaneaefolia (L.) St. Hil.

Other names.—Birdeye and Texas-weed.

Description.—Mexicanweed (fig. 34) is a

tough, fibrous-rooted annual that reproduces

by seed. The leaves are alternate, about 2 inches

wide, 3 to 5 inches long, prominently veined,
64 WEED CONTROL IN U.S. RICE PRODUCTION

FIGURE 35.—Small white morningglory (Ipomoea obscura).

PN-4971

coarsely toothed on the margin, and petioled.

The stem is rough, hairy, and 2 to 3 feet tall.

Mexicanweed has small white flowers and a

burlike fruit. The seeds are spherical, gray with

distinctive light-colored markings, and about

0.12 inch in diameter.

Distribution.—Mexicanweed is troublesome

in Louisiana and Texas. It grows in rice, on

levees, and in most wet areas. It competes with

rice, interferes with harvesting, and lowers

the value of rough and milled rice because its

seeds are difficult to remove.

MORNINGGLORY

Ipomoea spp.

Principal species and common names.—I.

• obscura Hassk., called small white morningglory

(fig. 35) ; /. hederacea (L.) Jacq., called ivyleaf

morningglory; /. purpurea (L.) Roth, called

tall morningglory; and /. wrightii Gray, called

willowleaf morningglory.

Description.—The species of morningglory

are twining annuals that reproduce by seed.

The stems are glabrous to pubescent and 3 to

15 feet long. The leaves of both /. obscura and

/. purpurea are heart shaped, but those of /.

obscura have purple margins; those of /. hederacea have three deeply cut lobes with rounded

sinuses; and those of /. wrightii are divided

into three to seven palmlike leaflets. The flowers

of /. obscura, I. hederacea, and /. purpurea are

solitary or in clusters on stalks arising from

the leaf axils, funnel shaped, and white, purple,

or pale blue; those of /. wrightii are borne on

long axillary stalks terminating with one to

several flowers and are orange or red. The

corollas of /. hederacea and /. purpurea are

about 2 inches long; those of /. obscura and

/. wrightii are about 1 inch long. Morningglory

seeds are about 0.1 to 0.25 inch long, are dark

brown or black, and have one round and two

flat sides.

Distribution.—Morningglories are troublesome in ricefields in the South, where they

grow on levees and on banks of irrigation and

drainage canals. They do not grow between

levees when water is managed well. They interfere with harvest and lower the value of rough

and milled rice because the seeds are difficult

to remove. They reduce grain yields of rice on

levees.
WEEDS THAT CAUSE ECONOMIC LOSSES IN RICE 65

NAIAD

Najas spp.

Principal species.—N. flexilis (Willd.) Rostk.

& Schmidt, called slender naiad; and N. guadalupensis (Spreng.) Magnus, called southern

naiad (fig. 36).

Description.—Naiads are rooted, submersed

aquatic annuals that reproduce by seed and

vegetatively. The stems, 10 to 25 inches long,

are slender and branched. The narrow (0.01

to 0.1 inch wide) leaves, with minute spines

along the margins, are 0.5 to 1.5 inches long,

opposite or crowded into whorls of three, and

tapered from the base to the tip. Small axillary

flowers emerge from the sheath at the base of

the leaf. The brown or straw-colored seeds,

covered by a thin pericarp, are about 0.1 inch

long and are marked with numerous lengthwise

rows of minute spaces.

Distribution.—Naiads grow in all ricegrowing States, but are troublesome weeds only in

California in water-seeded rice. They grow in

rice early in the season, especially in fields

with deep water, in irrigation systems, and in

flooded areas. They compete with rice during

the early growing season and may reduce the

number of tillers and thus yield.

PANICUM GRASSES

Panicum spp.

Principal species and common names.—P.

dichotomiflorum Michx., called fall panicum

or spreading panicum (fig. 37); P. capillare

L., called witchgrass, ticklegrass, or tumbleweed-grass; P. texanum Buckl., called Texas

panicum, Texas millet, or Colorado grass; and

P. purpurascens Raddi, called paragrass.

Description.—P. diohotomiflorum, P. capillare, and P. texanum are annuals that reproduce

by seeds; P. purpurascens is a robust, aquatic

perennial that reproduces by seeds and stolons.

P. dichotomiflorum, P. capillare, and P. texanum are branched at the base; their stems are

erect and usually 15 to 60 inches tall, but

robust plants of P. dichotomiflorum and P.

texanum may be 6 to 10 feet tall. The leaves,

4 to 20 inches long by 0.2 to 0.8 inch wide,

vary in pubescence from soft hairs in P.

dichotomiflorum, and P. texanum to stiff hairs

PN-4972

FIGURE 36.—S'outhem naiad (Najas guadalupensis).

in P. capillare. The panicles of P. capillare and

P. dichotomiflorum, 4 to 16 inches long, are

loosely spreading and may be terminal or

axillary, whereas the panicles of P. texanum

are somewhat shorter, 3 to 8 inches long, and

more compressed. P. purpurascens stems, 6

to 15 feet long, recline on the ground and root

at the basal nodes; its leaves, 4 to 12 inches

long by 0.4 to 0.6 inch wide, are flat and
66 WEED CONTROL IN U.S. RICE PRODUCTION

FIGURE 37.—Fall panicum {Panicum dichotomiflorum).

PN-497S

smooth; and its panicles, 5 to 8 inches long,

are raceme shaped. The spikelets of all four

species are about 0.1 inch long.

Distribution.—P. capillare and P. dichotomiflorum are troublesome in most ricegrowing

areas, but P. texanum and P. purpurascens

grow more abundantly in Louisiana and Texas.

Panicum grasses mature in late summer or

early fall and grow in rice, in and along irrigation canals, and on levees. They reduce yields

by competing with rice late in the growing

season and interfere with harvesting; some

species impede the flow of water in irrigation

systems.

PONDWEED

Potamogetón spp.

Principal species.—P. nodosus Poir. (fig.

38), called American pondweed, and other species.

Description.—The species of pondweed are

PN-4974

FIGURE 38.—American pondweed (Potamogetón

nodosus) leaves with distinct parallel veins.
WEEDS THAT CAUSE ECONOMIC LOSSES IN RICE 67

PN-497B

FIGURE 39.—Red rice (Oryza sativa) panicles. Left, grains have purple or black hulls. Right, grains have strawcolored hulls.

aquatic perennials that reproduce by underground winter buds or tubers and also by

seed. They have simple or slightly branched

stems. The leaves are usually alternate, but the

upper leaves may be opposite; the submersed

and floating leaves on the same plant are

often very different in appearance. Leaf structure varies among species from grasslike to

broadleaf with many distinct veins. The flowers

in elongated spikes are borne on axillary stalks

that vary in length. Whorls of egg-shaped,

beaked seed, called achenes, develop in the

spikes.

Distribution.—Species of pondweed grow in

all rice-producing States. They usually grow

in irrigation canals, ditches, and reservoirs,

but may be a problem in rice when stands are

thin or when rice is grown on land previously

used as a reservoir for storage of irrigation

water or fish production. Pondweeds compete

with rice to lower yields, interfere with harvesting, slow drying, and impede the flow of

water in irrigation systems.

RED RICE

Oryza sativa L.

Other names.—Vermillion red rice, Italian

red rice, and common red rice (fig. 39).

Principal types.—The many types include

those with short, medium, or long grains; those

with straw-colored, red, or black hulls; and

those with short or long awns on the spikelet.

Description.—Red rice is an annual that reproduces by seed. The plants, which tiller

profusely, are difficult to distinguish from

white rice plants until they are large; then their

brushier growth is discernible. The taller plants

shade the crop and often mature and lodge

before commercial rice. The leaves have short,

stiff hairs on their upper and lower surfaces;

the panicles are loose and open and droop

slightly. The grain shatters easily when ripe.

Seeds buried in the soil remain viable for

several years.

Distribution.—Red rice is a problem weed in

all rice-producing States, where it grows in rice,

in irrigation and drainage canals, on levees, and

in most wet areas. It reduces yields and lowers

the market value of rough and milled rice

because the seeds are difficult to remove.

REDSTEM

Ammannia spp.

Principal species and common names.—A.

auriculata Willd., called redstem; A. coccinea
68 WEED CONTROL IN U.S. RICE PRODUCTION

PN-4976

FIGURE 40.—Purple ammannia {Ammannia coccínea).

Rothb., called purple ammannia or redstem

(fig. 40); and A. teres Raf., called pink ammannia, redstem, or toothcup (fig. 41).

Description.—All species of redstem are

aquatic annuals that reproduce by seed. The

stems are erect and 8 to 40 inches tall; those of

A. auriculata and A. coccínea are often freely

branched, but those of A. teres are sparsely

branched. The leaves are narrow and 0.5 inch

wide by 1 to 4 inches long; the leaves of A.

auriculata and A. coccínea are clasping at the

base, but those of A. teres taper. The flowers

are borne in the axils of the leaves; the flowers

of A. coccínea and A. teres are attached directly

to the stem, but those of A. auriculata are on

stalks. Small red or brown seeds are produced

PN-4977

FIGURE 41.—Pink ammannia (Ammannia teres).

in spherical capsules 0.1 to 0.2 inch in diameter.

The leaves and stems turn red at maturity.

Distribution.—Redstem is a problem weed in

all rice-producing States, where it grows in

rice, along levees, ditches, and canals, and in

most wet areas. It grows especially well where

rice stands are thin. It reduces yields, interferes with harvesting, lowers the value of rough

rice, and increases the cost of drying when

immature pods are present because the seed

pods are difficult to remove.

SMARTWEED

Polygonum spp.

Other names.—Knotweed and pepperweed

(fig. 42).

Description.—Numerous species infest rice.

Some are annual herbs and others are perennial; some species grow erect and others are

prostrate. Thé plants are usually 2 to 6 feet

tall, with thick-jointed stems and alternate,

linear, short-petioled leaves. The flowers are

in axillary clusters or in nodding spikes or

panicles. The seeds are small, three-angled

achenes with dark-brown or black, hard coats.

Distribution.—Smartweeds grow in ricefields

where cultivation and drainage are inade-
WEEDS THAT CAUSE ECONOMIC LOSSES IN RICE 69

PNr4978

FIGURE 42.—Smartweed {Polygonum sp.).

quate, in drainage and irrigation ditches,

potholes, and sloughs, on levees, and in most

wet areas. Some smartweeds are true aquatics

and grow in shallow water; others are upland

weeds. They reduce rice yields, interfere with

harvesting and drying, and lower the value of

rough and milled rice because their seeds are

difficult to remove.

PN-4979

FIGURE 43.—Blunt spikerush {Eleocharis obttisa).

SPIKERUSH

Eleocharis spp.

Principal species and common names.—E.

obtusa (Willd.) Schultes (annual), called blunt

spikerush (fig. 43) ; E. párvula (R.&S.) Link

(annual), called dwarf spikerush; E. macrostachya Britt. (perennial), called creeping

spikerush (fig. 44), common spikerush, or wiregrass; E. quadrangulata (Michx.) R.&S. (perennial), called squarestem spikerush or foursquare; and others.

Description.—The species of Eleocharis are

annuals or perennials that reproduce by seed

or rhizomes or both. The plants are turflike in

growth and have simple round, erect or slightly

reclining culms terminating in seed heads. The

leaves are bladeless sheaths. E. obtusa is 12

to 20 inches tall at maturity and has blunttipped sheaths. E. párvula is one of the smallest

spikerushes; it grows only 1 to 4 inches tall.

E. macrostachya grows about 1.5 to 3 feet tall

and creeps. E. quadrangulata grows to a height

of 3 feet and has stout, thick, sharply square

culms.

Distribution.—Spikerushes grow in all riceproducing States. E. macrostachya is especially

troublesome in California. Spikerushes grow in

rice, on levees, in shallow ditches, and on poorly drained soil. E. obtusa and E. párvula are

especially troublesome in water-seeded rice.

They compete with rice in the early growing

season and may reduce yields greatly.
70 WEED CONTROL IN U.S. RICE PRODUCTION

PN-4980

FIGURE 44.—Creeping spikerush (Eleocharis

macrostachya).

SPRANGLETOP

Leptochloa spp.

Other names.—Raygrass and feathergrass.

Principal species and common names.—L.

panicoides (Presl) Hitchc, called tighthead

sprangletop, Amazon sprangletop, and Christmastreegrass (fig. 45); L. fascicularis (Lam.)

PN-4981

FIGURE 45.—Tighthead sprangletop (Leptochloa

panicoides).

Gray, called bearded sprangletop (fig. 46) ; L.

filiformis (Lam.) Beauv., called red sprangletop; L. nealleyi Vasey, called Nealley sprangletop; and L. uninervia (Presl ) Hitchc. & Chase,

called Mexican sprangletop.

Description.—The species of sprangletop are

annuals that reproduce from very small seeds

that are seldom found in rough rice. The plants

grow 2 to 3 feet tall, tiller heavily, and have

fine stems that terminate in panicles.

Distribution.—Except for L. panicoides,

which does not grow in California, sprangletop

is found in all rice-producing States. It grows

in shallow water along canals, in rice, and on

levees. It is particularly troublesome where

stands of rice are thin. It grows especially well

in the heavy clay soils of the Mississippi River

bottom lands.

UMBRELLAPLANT

Cyperus spp.

Other names.-

brellasedge.

-Nutgrass, nutsedge, and um-
WEEDS THAT CAUSE ECONOMIC LOSSES IN RICE 71

PN-4982

FIGURE 46.—Bearded sprangletop (Leptochloa

fascicularis).

Principal species and common names.—C.

erythrorhizos Muhl. (annual), called redroot

flatsedge (fig. 47) ; C. articulatus L. (perennial), called jointed flatsedge, jointed sedge,

and onions; C. iria L. (annual), called rice flatsedge (fig. 48) ; C. difformis L. (annual), called

smallflower umbrellaplant; C. strigosus L.

(perennial), called false nutsedge and strawcolored sedge (fig. 49); and C. esculentus L.

(perennial), called yellow nutsedge.

Description.—The species of Cyperus are annuals and perennials that reproduce by seed or

tubers or both. The general appearance of

plants within each species varies greatly. Most

species have a triangular flower stalk bearing

PN-4983

FIGURE 47.—Redroot flatsedge (Cyperus erythrorhizos).

PN-4984

FIGURE 48.—Rice flatsedge (Cyperus iria).
72 WEED CONTROL IN U.S. RICE PRODUCTION

PN-4985

FIGURE 49.—False nutsedge (Cyperus strigosus).

a headlike or branched inflorescence. At the

base of the inflorescence there are foliagelike

bracts that vary in size and number. Most umbrellaplants grow 1 to 2 feet tall.

Distribution.—Umbrellaplants grow in all

rice-producing States. C. erythrorhizos and C.

iria are troublesome in the South, and C. difformis is prevalent in California. They grow

in rice, in and along irrigation and drainage

canals, in sloughs and other poorly drained

areas, and on levees. The perennials, except

C esculentus, usually are prevalent in poorly

PN-4986

FIGURE 50.—Waterhyssop (Bacopa rotundifolia).

drained areas, such as idle ricefields, but may

infest ricefields when seedbed preparation is

inadequate. The annual umbrellaplants are

more troublesome in the rice crop than the

perennials. C. erythrorhizos, C iria, and C. difformis are especially troublesome where rice

stands are thin; they ordinarily appear late in

the season, after the land has been submerged

for some time. Umbrellaplants reduce yields

and interfere with harvesting.

WATERHYSSOP

Bacopa rotundifolia (Michx.) Wettst.

Description.—Waterhyssop (fig. 50) is an

aquatic annual that reproduces by seed. The

stems, 8 to 24 inches long, are submerged with

the tips floating; they are pubescent when

young. The leaves are thin, round, clasping at

the base, and palmately veined. The flowers,

two to four at each node, are white with a

yellow throat, pediceled, and axillary.

Distribution.—Waterhyssop grows in all riceproducing States. It is usually found with other
WEEDS THAT CAUSE ECONOMIC LOSSES IN RICE 73

PN-4987

FIGURE 51.—Winged waterprimrose {Jttssiaea

decurrens).

aquatic weeds, such as ducksalad, and it is

most troublesome where stands of rice are thin

or where rice is seeded in water. It grows in

rice, shallow ditches and canals, and most wet

areas. It competes with rice during the early

growing season and may reduce tillering and

yield.

WATERPRIMROSE

Jussiaea spp.

Other name.—Primrose-willow.

Principal species and common names.—J.

decurrens (Walt.) DC. (fig. 51), called winged

PN-4988

FIGURE 52.—Creeping waterprimrose (Jussiaea repens

var. glabrescens).

waterprimrose; and /. repens L. var. glabrescens Ktze. (fig. 52), called creeping waterprimrose.

Description.—J. decurrens is an aquatic annual that reproduces by seed. The stems are

erect, 3 to 6 feet tall, smooth and square; the

leaves are narrow, linear, and sessiled; and the

capsules are slender, square, and 0.5 to 1 inch

long. /. repens var. glabrescens is an aquatic

perennial that reproduces by seed, stems, and

rootstocks. The stems are smooth, prostrate or

floating, and ascending at the tips; the leaves

are smooth, oblong, 1 to 4 inches long, and

petioled; the capsules are almost cylindrical

and 1 to 2 inches long. Roots form at the

nodes.

Distribution.—Waterprimroses grow mainly

in the South in irrigation and drainage canals,

ditches, and most wet areas. /. repens var.

glabrescens grows in dense beds in ponds and

ditches. Waterprimroses are not generally

troublesome in rice, except where stands are

thin. They compete with rice, interfere with

harvesting, slow drying, lower the value of

rough and milled rice, and impede the flow of

water in canals and ditches.
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