UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

WASHINGTON D.C. , 20460 

MAY 2 8 2015 

OFFICE OF CHEMICAL SAFETY 

AND POLLUTION PREVENTION 

MEMORANDUM 

SUBJECT: 

FROM: 

THRU: 

TO: 

Rice Production Practices in the United States and Use of Cyhalofop-butyl for 

Weed Control in Rice Production, Revised (DP# 420989) 

Sunil Ratnayake, PhD, Biologist ~ Ak-VVV\~ Biological Analysis Branch ~ '"'" ' ~ __ l 

Biological and Economic Analysis Division (7503P) 

Arnet 

Biological 

Jones, 

Analysis 

Chief 

Branch 

~ 

Biological and Economic Analysis Divisi 

Jolene Trujillo, Chemical Review Manager 

Pesticide Re-evaluation Division (7508P) 

(7503P) 

Peer Review Panel date: March 3, 2015 

SUMMARY 

This document revises and replaces a previous version from July, 2014. 

Environmental Fate and Effects Division (EFED) is currently working on a risk assessment for 

mitigate risks involved with the use of cyhalofop ( cyhalofop-butyl) in rice production. To 

facilitate this process, BEAD is providing information on drill (dry) seeded and water (wet) 

seeded rice production, use of cyhalofop for weed control, and duration of holding water 

following the applications of cyhalofop in these rice production systems. 

Cyhalofop is a post-emergence herbicide used for selective control of grass weeds in rice 

production. Under drill seeded conditions cyhalofop is applied as post-emergence pre-flood and 

post-emergence post-flood (post-flood early and post-flood late) treatments. Early post-flood 

applications of cyhalofop are made 7 to 10 days after the establishment of permanent flood and 

Page 1 
this timing may vary by several days depending on the weather conditions and level of weed 

infestation in the field. Also, late post-flood treatments may be made in the mid-season to save 

the rice crop from higher weed infestations (salvage treatments). Sequential applications of 

cyhalofop are also made under heavy grass weed infestations (Clincher® SF Specimen Label, 

2009). Sequential application can be made at 15 fl oz per acre 7 to 10 days after the permanent 

flood followed by second application at the rate of 10 fl oz per acre 10 to 15 days later. 

According to available usage information (2008 to 2012), most cyhalofop (83%) is applied as 

post-flood applications. 

Duration of water holding following a cyhalofop application varies depending on the timing of 

cyhalofop application, time needed for the maturity of planted rice variety and draining time of 

water in the field before harvesting. A longer duration of water holding following a cyhalofop 

application is needed with rice varieties with longer maturation time, late draining of water 

before harvest, and earlier applications of cyhalofop in the growing season. In this analysis, the 

longest duration of water holding (95 days) is estimated with postemergence pre-flood 

application of cyhalofop on a 130 day rice variety which is drained 14 days before harvest. The 

shortest duration of water holding (43 days) is estimated with post-flood late applications (midseason) of cyhalofop on a 120 day variety which is drained 21 days before the harvest. 

Calculations for these estimates are shown on pages 7 to 13. 

Rice Production in the United States 

According to USDA NASS, Arkansas, California, Louisiana, Mississippi, Missouri, and Texas 

are major rice producing states, (Table 1 ). Arkansas has the highest acreage in rice production 

and California produces the highest yields in rice production in the United States (Table 1). Also, 

usual planting and harvesting dates of rice are shown in Table 2. 

Table 1. Rice Production in the U.S. in 2013 

Total 

Acres Harvested Production 

State (in 1000 acres) Yield/ Acre (lbs) (1000 cwt) 

Arkansas 1,070 7,560 80,888 

California 561 8,480 47,574 

Louisiana 413 7,300 30,135 

Mississippi 124 7,400 9,176 

Missouri 156 7,030 10,968 

Texas 144 7,740 11,145 

United States 2,468 7,694 189,886 

Source: USDA, NASS, 2014. 

Page 2 
Table 2. Typical Planting and Harvesting Dates of Rice in Different.Rice Producing States 

State r· Plantin2 Harvestin2 

Begin End Begin End 

Arkansas April6 June 2 Aug. 29 Oct. 24 

California April 20 June 10 Sep.5 Nov. 10 

Louisiana March20 May 15 July 27 Sep.28 

Mississippi April6 May24 Aug. 29 Oct. 20 

Texas March 17 May 10 July 29 Sep.20 

Source. USDA, NASS, 2010 

Rice Production Systems in the United States 

Drill Seeding (Dry Seeding) 

Under drill seeding, dry seeds are planted in well leveled fields in rows using mechanical 

planters. Pre-plant herbicides are applied in the field before planting.seeds. After planting, levies 

are built and fields are flushed (irrigating and draining of excess water in the field) in order to 

soak seeds to facilitate seed germination. Before the emergence of seedlings, pre-emergence 

herbicides are applied. Depending on the soil temperature and soil moisture, it takes about 7 to 

12 days for seedling emergence following flushing of rice fields. After the emergence of rice 

seedlings, fields are maintained under a wet condition using rain water or by quick irrigation 

(flushing). The permanent flood will be established when the seedlings are with 4 to 5 leaves or 

at the development of 2nd tiller. Most rice varieties achieve this stage within 4 to 6 weeks after 

planting. This timing may vary depending on the location, type of the planted rice variety, (late 

or early), soil moisture, soil fertility, and weather conditions. At the establishment of permanent 

flood, rice fields are flooded to a depth of 4 to 10 inches of water. Maintaining a water level 

about 113 to 112 of seedling height is important and seedlings should not be submerged. 

Following the establishment of permanent flood, post-flood herbicide treatments may be applied 

to control the existing weeds in the field. Generally, permanent flood is maintained until 2 to 3 

weeks before harvesting and then drained. This also helps reduce the moisture level in maturing 

seeds. 

Water Seeding (Wet Seeding) 

Water seeding is a common production method in southern Louisiana and areas in Texas and 

California (Louisiana Rice Production Handbook, 2014; Texas Rice Production Guidelines, 

2012; Crop Profiles for Rice in California, 1998). In this production system, following the land 

preparation and pre-emergence treatments, fields are flooded before seeding and pre-germinated 

or dry seeds are broadcasted over the flooded fields using an airplane. Fields with emerged 

seedlings are maintained with a low level of water (shallow flood) until the permanent flood is 

Page 3 
established. The primary reasons for water-seeding are suppression of red rice and other weeds, 

reduced labor and reduced inputs. Also, it can be used when heavy rainfall interferes with drill 

seeding. In addition, water seeding can be used to control weeds, especially, broadleaf and grass 

weeds, including red rice. However, water seeded production system tends to favor sedges and 

other aquatic weeds such as hydrilla (Hydri/la verticillata) waterhyssop (Bacopa rotundifolia), 

duck salad (Heteranthera limosa), purple ammannia (Ammannia coccinea), goose weed 

(Spehenoclea zelanica), and arrowhead. (Sagittaria montevidensis) (University of AR, Rice 

Production Handbook, Undated). 

On the basis of water management, water-seeded rice can be categorized as delayed flooding, 

pinpoint flooding, and continuous flooding (Louisiana Rice Production Handbook, 2014). 

Delayed Flooding 

Under the delayed flooding system, fields are flooded before seeding and after seeding, fields are 

drained and maintained to a shallow level (usually 3 to 4 weeks) until the permanent flood is 

established. Delayed flooding is adopted in fields where red rice is not a serious problem 

(Louisiana Rice Production Handbook, 2014). Finally, flooded water is drained 2 to 3 weeks 

before harvesting. 

Pinpoint Flooding 

Under pinpoint flooding, a thin layer of water is maintained in the field before pre-germinated or 

dry seeds are broadcasted aerially over the fields. The thin layer of water is maintained until the 

seedlings are well established (7 to 10 days). Then the permanent flood is established and 

maintained until seeds reach the maturity. Finally the flooded water is drained to dry the fields. 

Draining of flooded water facilitates the use of mechanical harvesters and reduces seed moisture 

in maturing seeds before harvesting. 

Continuous Flooding 

Under continuous flooding, fields are flooded before seeding and continuously maintained until 

2 to 3 weeks before harvesting. The continuous flooding system is the best water management 

system for red rice suppression. However, proper seedling establishment is difficult under this 

method. 

Weed Control in Rice Production 

Weed management in rice production is a significant challenge for rice farmers because water 

can carry pesticides, and it is difficult to use ground application equipment. Both pre-emergent 

and post-emergent herbicides are used to control weeds in rice production. Mostly, preemergence herbicides are applied in the field before planting rice se~ds to kill the germinating 

weed seeds in soil. However, under certain situations they can be applied after the planting of 

rice. Use of a pre-emergence herbicide with residual activity prolongs the duration of weed 

control after the crop establishment. Herbicides such as pendimethalin, clomazone, thiobencarb, 

and quinclorac are some of the pre-emergence herbicides used in rice production. 

Page4 
After planting of rice, levies are built and fields are irrigated and drained to facilitate a uniform 

germination of rice seeds. Depending on the moisture level in soil, rice fields may be irrigated 

several times before the establishment of permanent flood. Permanent flooding helps in 

controlling many grass and broadleaf weeds. The drill seeded production system favors grass 

and broadleafweed species. Generally, barnyard grass (Echinochloa crus-galli), broadleaf 

signal grass (Brachiaria platyphyUa), crabgrass (Digitaria sanguinalis and Digitaria 

ischaemum), panicum (Panicum dichotomiflorum), and sprangletop (Leptochloa spp.) are the 

troublesome grass weeds found in the rice production. Carfentrazone-ethyl, bentazon, 

acifluorfen, cyhalofop, propanil + bensulfuron, quinclorac, triclopyr, bensulfuron, 

dichloropropionanilide, and halosulfuron are some of the post-emergence herbicides used in rice 

production (Rice Production Guidelines, Texas Agric. Experiment Station, 2012). 

Usage of Cyhalofop in the United States 

Table 3. Average Usage of Cyhalofop in the United States (2008 to 2012) 

~ - ~ ~- ~ 

I 5-Year Average - ~~ 

State i (Pounds of Active 1: 5-Y ear Average ' 

I lngtedient) II Percent Crop Treated (%) 

- - ~ 

-·- --· 

Arkansas 32,910 5 

California 23,800 10 

Louisiana 7,920 5 

Mississippi 9,950 11 

Missouri 2,480 2 

Texas 4,090 6 

(Proprietary Data, 2008- 2012) 

Table 3 shows the percentage of crop treated with cyhalofop in different states. On average, 

California and Mississippi have the most use with 10 and 11 % of the crop treated, respectively. 

Table 4. Average Usage and Percentage of Cyhalofop Applied at Different Application Timings 

Application Timing' - Av. Usage (a.L lb~) - Percentage (%} 

Pre-flood 13,150 17 

Early post-flood 5,850 7 

Late post-flood 60,270 76 

(Proprietary Data, 2008 - 2012). 

Based on the proprietary data, about 1 7% of total pounds of cyhalofop is applied before 

establishing the permanent flood 17% (Table 4). The total% of cyhalofop applied after the 

establishment of permanent flood (post-flood) was 83%. These herbicide applications are made 

Page 5 
in the field over the crop and generally flooded water is drained two to three weeks before 

harvesting. 

Table 5. General Timeline for Rice Production in AR, LA, MS and TX1 

Cultural Practice Duration Herbicide Treatment 

and preparation Early March to end of March 

Pre-plant treatments Mid-March to Mid-April 

Planting (seeding) First week of April 

Pre-emergence treatments After seeding in April 

Flushing After seeding, fields are irrigated to 

wet the soil and then the water is 

drained to facilitate seed germination 

Seedling emergence Second week of April 

Pre - permanent flood Mid to end of April Cyhalofop 

treatments Application 

Permanent flooding First week of May 

Post- permanent flood early Second week of May Cyhalofop 

treatments Application 

Post-permanent flood late Early to Mid-June Cyhalofop 

treatments (mid-season)2 Application 

Draining of flooded Water Second or Third week of July 

Harvesting First week of August to end of 

August 

1

Please note that this timeline for cultural practices may vary depending on the location, weather conditions (rainfall, air 

temperature) soil conditions (soil temperature, soil moisture) water management, and the rice variety used for planting. Timeline 

described is for a 120 day variety planted in first week of April. 

2Pre-harvest interval for this application is 60 days. 

Cyhalofop Applications in Dry Seeded Rice Production 

Pre-permanent Flood Application 

Application of cyhalofop is made before establishing the permanent flood. Flushing the field 

may be needed to have a good soil moisture level prior to herbicide application. If the field is 

irrigated to enhance seed germination, water must be drained before the herbicide application 

and to expose weeds for good herbicide coverage. Fields can be re-flooded three hours after the 

herbicide application and best results can be achieved by flooding fields 24 to 48 hours after the 

herbicide application. 

Post-permanent Flood Early Application. Application of cyhalofop is made 7 to 10 days after the 

establishment of permanent flood. This timing may vary few days depending on the field 

situation. 

Page 6 
Field must be drained prior to herbicide application to expose the weeds (at least 70%) for a 

good herbicide coverage (Clincher® SF. Specimen Label. 2009). 

Permanent-Flood (mid-season) Application. Application of cyhalofop is made 6 to 8 weeks after 

planting to control existing grass weeds in the field. This application is also considered as a 

salvage treatment to minimize the yield reduction due to a higher weed infestation. The preharvest interval for this application is 60 days. 

Estimated duration of water holding in drill seeded rice following the herbicide 

applications in AR, LA, MS and TX (please note that estimated durations may vary 

depending on the weather condition, duration of the rice variety, and location). 

Duration of Holding Water in the Field under Drill Seeding (Dry Seeding) 

1. Duration of holding water in the field following the pre-flood applications 

Estimated upper range 

120 - 21 - 14 = 85 days (120 day variety. cyhalofop is applied 21 days after planting and before 

establishing the permanent flood. Field is drained 14 days before harvesting) 

Estimated lower range 

120 - 21 - 21=78 (120 day variety. cyhalofop is applied 21 days after planting and before the 

permanent flooding. Field is drained 21 days before harvesting) 

2. Duration of holding water after post-flood early applications 

Estimated upper range 

120 - 32 -14 = 74 days (120 day variety, cyhalofop is applied 32 days after planting. Field is 

drained 14 days before harvesting). 

Estimated lower range 

120 - 32 - 21 = 67 days (120 day variety, cyhalofop is applied 32 days after planting. Field is 

drained 21 days before harvesting). 

3. Duration of holding water after post-flood late (mid-season) applications 

Estimated Upper Range 

120 - 56 - 14 = 50 days (120 day variety, cyhalofop is applied 56 days after seeding and field is 

drained 14 days before harvesting). 

Estimated Lower Range 

120 - 56 - 21 = 43 days (120 day variety, cyhalofop is applied 56 days after seeding and field is 

drained 21 days before harvesting). 

Page 7 
Table 6. Timeline for Water (Wet) Seeding and Cyhalofop Applications 1 

Cultural Practice Duration - Herbicide Application 

Land Preparation In March 

Flooding Early April before seeding 

Seeding (broadcasting of Early April after flooding 

germinated seeds or dry seeds 

over the flooded fields) 

Pre-permanent flood In late April before establishing Application of cyhalofop 

treatment the permanent flood 

Permanent flooding Early May 

Post permanent flood early 7 to 10 days after establishment of Cyhalofop Application 

treatments permanent flood. This timing may 

vary depending on the level of 

weed infestation. 

Post-flood-late treatments 6 to 8 weeks after seeding. Cyhalofop Application 

(mid-season applications) 2 

Draining of flooded Water Second or third week of July 

Harvesting Beginning to end of August 

'Please note that in water seeded rice, only liquid formulation of cyhalofop is applied. This timeline for cultural practices may 

vary depending on location, weather conditions (rainfall, air temperature) soil conditions (soil temperature, soil moisture) water 

management, and the rice variety used for planting. Timeline described is for a 120 day variety planted in first week of April. 

2Preharvest interval for this application is 60 days. 

Cyhalofop Applications in Water Seeded Rice Production 

Pre-permanent flood application 

Application of cyhalofop is made after flooding and before the establishnient of permanent flood. 

Post permanent flood early application 

A single post flood application is recommended 7 to 10 days after permanent flooding. This 

timing may vary depending on the level of weed infestation. The field must be drained before 

the application to expose weeds for a better coverage 

Post permarient flood late application 

Application of cyhalofop is made 6 to 8 weeks after planting to control existing grass weeds in 

the field. This application is also considered as a salvage treatment to minimize the yield 

reduction due to a higher weed infestation. 

Page 8 
Estimated duration of water holding (in wet seeded rice) after herbicide applications in AR, 

LA, MS and TX (please note that estimated durations may vary depending on the weather 

conditions, duration of the rice variety used for planing and location). 

1. Duration of holding water following the pre-permanent flood applications 

Estimated upper range 

120 - 21 - 14 = 85 days (120 day variety. Cyhalofop is applied 21 days after seeding and field is 

drained 14 days before harvesting). 

Estimated lower range 

120 - 21 - 21=78 days (120 day variety. (Cyhalofop is applied 21 days after seeding and field is 

drained 21 days before harvesting). 

2. Duration of holding water after post- permanent flood early applications 

Estimated Upper Range 

120 - 32 - 14 = 74 days (120 day variety. Cyhalofop is applied 32 days after seeding and field is 

drained 14 days before harvesting). 

Estimated Lower Range 

120 - 32 - 21=67 days (120 day variety. Cyhalofop is applied 32 days after seeding and field is 

drained 21 days before harvesting). 

2. Duration of holding water after post-flood late (mid-season) applications 

Estimated Upper Range 

120 - 56 - 14 = 50 days (120 day variety. Cyhalofop is applied 56 days after seeding and field is 

drained 14 days before harvesting). 

Estimated Lower Range 

120 - 56 - 21=43 days (120 day variety. Cyhalofop is applied 56 days after seeding and field is 

drained 21 days before harvesting). 

Page 9 
Figure 1. Growth Stages of a Rice Variety Gown in California (Drill Seeded Variety) 

II 

L..,rsi- 1 2 J 4 5 

Days: Early variety 25 - 30 

Days: Lale ""(ielY 25 - 30 

1~ Tier 

lnlUallon 

Pre-flood 

cyhalofop 

Applications 

Post flood 

applications 

55-60 

65-75 

Mid-season 

cyhalofop 

Applications 

Source: Crop Profile for Rice in California. 1988. 

BS-100 

100-115 

130-145 

140-165 

~1 

This figure shows different growth stages of rice after planting and stages of cyhalofop 

applications. The different timing may vary depending on the weather, location, and the rice 

variety. 

Page 10 
Table 7. General Timeline in California Rice Production Drill (dry) Seeding 

Cultunl Pra~ti~e - Timmi Herbicide Aoolication ··-

Land Preparation Early March 

Pre-plant treatments Mid-March to Mid-April 

Planting Third week of April 

Pre-emergence treatments Third week of April 

Flushing In the third week of planting, 

fields are irrigated to wet the soil 

and drained to facilitate seed 

germination. 

Seedling Emergence Fourth week of April 

Post-emergence pre-flood First week of May Cyhalofop Application 

treatments 

Permanent flooding Third week of May 

Post-flood early treatments Fourth week of May Cyhalofop application 

Post-flood late applications Mid-June Cyhalofop application 

(mid-season) 2 

Draining of flooded water Mid to end of July 

Harvesting Mid-August to Mid-September 

1

Please note that this timeline for cultural practices may vary depending on location and weather conditions (rainfall, air 

temperature) soil conditions (soil temperature, soil moisture) water management, and the rice variety used for planting. Timeline 

described is for a 130 day variety planted in third week of April. 

2Pre-harvest Interval for this application is 60 days. 

Duration of Holding Water under Drill Seeding 

1 ). Duration of holding water in the field following the pre-permanent flood application of 

cyhalofop. 

Estimated upper range 

130 - 21 - 14 = 95 days (130 day variety. cyhalofop is applied 21 days after planting and field is 

drained 14 days before harvesting). 

Estimated lower range 

130 - 21 - 21 = 88 days (130 day variety. cyhalofop is applied 21 days after planting and field is 

drained 21 days before harvesting). 

2. Duration of holding water following the post-flood early applications 

Estimated upper range 

130 - 32 -14 = 84 days (130 day variety. Cyhalofop is applied 32 days after seeding and field is 

drained 14 days before harvesting). 

Page 11 
Estimated lower range 

130 - 32 - 21 = 77 days (130 day variety. Cyhalofop is applied 32 days after seeding and field is 

drained 21 days before harvesting). 

3).Duration of holding water following the post-flood late (mid-season) applications 

Estimated Upper Range 

130 - 56 -14 = 60 days (130 day variety). Cyhalofop is applied 56 days after seeding and field is 

drained 14 days before harvesting). 

Estimated Lower Range 

130 -56 - 21 = 53 days (130 day variety. Cyhalofop is applied 56 days after seeding and field is 

drained 21 days before harvesting). 

Table 8. General Timeline in California Rice Production Water 1 wet) Seeding 

Cultural Practice Timing · Herbicide Application - '' ,( 

Land Preparation Early March 

Pre-plant treatments Mid-March to Mid-April 

Flooding Third week of April 

Broadcasting of germinated Third week of April 

seeds or dry seeds (seeding) 

Pre-permanent flood Second week of May Cyhalofop application 

treatments 

Establishment of permanent Third week of May 

flood 

Post flood early treatments Fourth week of May Cyhalofop application 

Post-flood, late treatments Third week of June Cyhalofop application 

(mid- season) 2 

Draining of water Second or third week of July 

Harvesting In September 

1

Please note that this timeline for cultural practices may vary depending on location and weather conditions (rainfall, soil 

moisture, soil temperature and the rice variety used for planting). Timeline described below is for a 130 day variety. 

2Pre-harvest interval for this application is 60 days. 

1). Duration of holding water in the field following the pre-permanent flood application of 

cyhalofop. 

Estimated upper range 

130 - 21 - 14 = 95 days (130 day variety. cyhalofop is applied 21 days after planting and field is 

drained 14 days before harvesting). 

Page 12 
Estimated lower range 

130 - 21 - 21= 88 days (130 day variety. cyhalofop is applied 21 days after planting and field is 

drained 21 days before harvesting). 

2. Duration of holding water following the post-flood early applications 

Estimated upper range 

130 - 32 -14 = 84 days (130 day variety. Cyhalofop is applied 32 days after seeding and field is 

drained 14 days before harvesting). 

Estimated lower range 

130 - 32- 21= 77 days (130 day variety. Cyhalofop is applied 32 days after seeding and field is 

drained 21 days before harvesting). 

3).Duration of holding water following the post-flood late (mid-season) applications 

Estimated Upper Range 

130 - 56 -14 = 60 days (130 day variety). Cyhalofop is applied 56 days after seeding and field is 

drained 14 days before harvesting). 

Estimated Lower Range 

130 -56 - 21= 53 days (130 day variety. Cyhalofop is applied 56 days after seeding and field is 

drained 21 days before harvesting. 

As shown in above examples, duration of water holding after applications of different herbicides 

can be calculated for different rice varieties with known maturation time, known herbicide 

application timings, and known draining timings of water before ha.rVesting of the rice crop. 

REFERENCES: 

Arkansas Rice Production Handbook 192. Undated. Online Publication. Available at: 

http://www.arkansas-crops.com/wp-content/uploads/2014/02/MPl 92.pdf. Accessed on 

June 6, 2014. 

Clincher® SF. Specimen Label. 2009. Dow AgroSciences. Online publication Available at: 

http://www.cdms.net/LDat/ld5SF009.pdf. Accessed on June 5, 2014. 

Crop Profiles for rice in California. 1988. Online publication. Available at: 

http://www.ipmcenters.org/cropprofiles/docs/carice.pdf. Accessed on June 9, 2014. 

Louisiana Rice Production Handbook. 2014. Publication of Louisiana Ag Center. Available on 

line at: http://www.lsuagcenter.com/NR/rdonlyres/F7E930BF-346A-45DA-A989-

37FE3DOBOF1 l/59612/pub2321RiceProdHdbkJune09.pdf. Accessed on June 6. 2014. 

Page 13 
Proprietary Data, 2008 - 2012. 

Rice Background. 2008. USDA Economic Research Service. 

http://www.ers.usda.gov/topics/crops/rice.aspx. Accessed ori June 30. 2014. 

Shipp, M. Rice Crop Timeline for the Southern States of Arkansas, Louisiana, and Mississippi. 

Undated. Online Publication. Available at: 

http://www.cipm.info/croptimelines/pdf/Rice.pdf. Accessed on June 10, 2014. 

Texas Rice Production Guidelines. 2012. Online Publication. Available at: 

http://varietytesting.tamu.edu/rice/varietyfiles/2012 Rice Production Guidelines.pdf 

Accessed on June 6, 2014. 

Page 14