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WHITE CLOVER

Agriculture Handbook No. 314

V''

Agricultural Research Service

UNITED STATES DEPARTMENT OF AGRICULTURE
CONTENTS

Page

Value and use 1

Plant characteristics 2

The seed 2

The seedling plant 2

The growing plant 4

Roots 4

Leaves 4

Stolons 4

Flowers 5

Persistency of stands (longevity) 5

Genetics and cytogenetics 7

Breeding 7

Source nurseries 8

Evaluation of clones 9

Synthetic varieties 10

Mass selection and natural selection, 10

Inbreeding 10

First-generation and double-cross hybrids 10

Induced polyploidy and species hybridization._ 10

Mutational breeding 10

Varietal release and seed increase 10

Techniques 10

Emasculation, pollination, and seed production 10

Induction of flowering 11

Vegetative propagation 11

Propagation with seed H

Field plots 11

Measuring yields 12

Measuring stands 12

Alternate approach to improvement 12

Types and varieties 12

Types 12

Varieties 12

Variety adaptation 13

Description of varieties 13

Common White 13

State-named varieties 13

Louisiana SI 14

Regal 14

Ladino 14

Merit 14

Pilgrim 14

Espanso 15

Nordic 15

New York Wild White___ 15

Foreign varieties and strains 15

Diseases 15

Pepper spot 15

Page

Diseases—Continued

Cercospora leaf and stem spot 15

Curvularia leaf spot 16

Sooty blotch 17

Rust 17

Viruses 17

Stolon and root rots 17

Nematodes 18

Insect pests 19

Grasshoppers 20

Clover leaf weevil 20

Potato leafhopper 20

Meadow spittlebug 20

Garden ñeahopper 20

Spider mites 20

Clover root curculio 20

Green June beetle 20

Lesser clover leaf weevil 20

Clover head weevil 21

Clover seed weevil 21

Ladino clover seed midge 21

Other damaging species 21

Control 21

Requirements for growth and soil adaptation 21

Soil adaptation 21

Fertilizers and limestone 22

Inoculation 22

Climate 22

Production practices 23

Management in the North . 23

Seedbed preparation and seeding 23

Mixtures 23

Time of seeding 23

Defoliation practices 23

Renovation of pastures 24

Ratio of clover to grasses 24

Management in the South 24

Seedbed preparation and planting 24

Associated grasses 25

Time of seeding 25

Utilization 25

Fall management 25

Practices affecting botanical composition of

clover-grass association

Seed production

Responses of white clover to climatic factors

Nutritive value and chemical composition

Bloat

Literature cited

Washinston, D.C. Issued August 1966
WHITE GLOVER

By PRYCE B. GIBSON, research agronomist, and E. A. HOLLOWELL, eollahorator {formerly clover investigations leader),

Crops Research Division, Agricultural Research Service

White clover {Trifolium repens L.) is one of the

most nutritious and widely distributed forage legumes of the world. History and the presence of

diverse forms in the areas indicate that white

clover originated in the eastern Mediterranean

countries or in Asia Minor. Its spread to other

continents was rapid and apparently was associated with early colonization and the presence of

domesticated grazing animals. It now is a member of the flora of every continent, where it is

widely distributed. In the Western Hemisphere,

its distribution extends from Alaska to southern

South America.

White clover apparently was included in the

"hayloft seed" that early colonists brought with

them to America (33),^ Historical records indicate that it was one of the first forage plants to

form dense stands in pastures that followed cultivated crops or cleared forests. Being highly

prolific in producing seed and having characteristics that favor dissemination, it preceded the

white man into new areas. This was natural, as

the small seeds are disseminated easily by wind,

water, grazing animals, and birds.

White clover grows best in humid sections of the

Temperate Zone during the cool, moist seasons.

Periods of prolonged high temperatures with

either high or low rainfall are unfavorable for its

growth and maintenance of stands. It is an important forage crop in most countries within the

humid sections of the Temperate Zone, e.g., Denmark, England, and Italy. The luxuriant growth

of white clover is largely responsible for New

Zealand's preeminence in dairying.

Today white clover in one or more of its morphological forms is the most widely used pasture

legume in the humid and irrigated sections of the

United States (^, 63).

VALUE AND USE

The value of the white clover crop cannot be

estimated accurately because of its volunteering

characteristic and the seemingly spontaneous development of stands in pastures, lawns, turfs,

orchards, and other areas if cultural practices permit its growth. Plants frequently appear where

lime and other required minerals are present or

after their application, where there is adequate

moisture, and where associated grass and other

vegetation are grazed or cut frequently to reduce

competition for light. It is estimated that at least

half of the 105 million acres of humid or irrigated

pastureland has varying amounts of white clover

(fig. 1). Besides its pasture value, it is esteemed

by some in lawns and orchards and as a ground

cover. The annual value of its seed crop in the

united States is about $5,500,000.

^ ItaUc numbers in parentheses refer to Literature Cited,

p. 30.

White clover is extensively and effectively used

in mixtures or alone. Its contribution to agriculture is as follows :

(1) As a forage legume,—It provides a highly

nutritive feed as pasture, hay, and silage for livestock and poultry. Although it usually is grown

in a mixture with grasses for grazing, it may be

seeded alone, particularly for poultry and swine.

(2) By fixing atmospheric nitrogen,—If white

clover is effectively inoculated with symbiotic bacteria, the amount of nitrogen made available for

associated plants may range from 50 to 250 pounds

per acre per year. The amount of nitrogen fixed

depends on density of stand, growth produced,

length and nature of growing season, soil fertility,

and degree of effective inoculation.

(3) As a cover crop,—The growth of stolons

soon after the seedlings are well established provides a ground cover that promotes soil stabilization and reduces erosion.
AGRICULTURE HANDBOOK 314, U.S. DEPT. OF AGRICULTURE

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FIGURE 1.—Mixture of clover and grass makes excellent pasture. This combination is unsurpassed as source of

highly nutritious, palatable forage. It results in heavy milk production of dairy cattle, high daily gains and

good carcass grade of beef cattle, and a high weaning percent for brood cows.

PLANT CHARACTERISTICS

The Seed

White clover seeds are small, approximately

700,000 per pound. Len^h and width are approximately equal, averagmg about li/^ mm. (fig.

2). The embryos fill the entire cavity of the seed

and there is no visible endosperm. The tip of the

thick radicle is strongly divergent (89). The arrangement of the radicle and cotyledons forms a

broadly heart-shaped, triangular, or oval seed.

The radicle may be equal or almost equal in length

to the cotyledons with the hilum between the two

lobes on the end, or the radicle may be much

shorter than the cotyledons with the hilum on one

side in a broad, shallow indentation.

The seed surface is smooth and yellow or sometimes reddish. It changes to brown with age and

weathering. The seedcoat may be impermeable,

thereby causing "hard" seed. The percentage of

hard seed varies among seed lots. Hard seed may

remain viable in the soil for 30 years or longer

{ß8,m).

In general, seeds of varieties and types are alike.

No method has been developed for reliable identification of varieties or types based on visual characteristics of the seeds.

Seeds germinate rapidly at temperatures from

50° to 68° F. Scarification and also cool night

and warm day temperatures tend to cause the hard

seeds to germinate (93). High temperatures inhibit germination. In the South, this characteristic serves as a protective mechanism by reserving viable seed for the more favorable fall months.

The Seedling Plant

The seedling is epigeal. Swelling of the cotyledons and growth of the radicle break tlie seedcoat. The primary root rapidly penetrates the

soil, forming a slender taproot. The cotyledons

and terminal bud are raised above the ground as
WHITE CLOVEB

BN-14345, BN-14344, BN-14S43, BN-14349

FiGUBE 2.—White clover seed compared with seed of other common forage legumes: Á, White clover; B, alsike

clover ; C, red clover ; D, alfalfa.
AGRICULTURE HANDBOOK 314, U.S. DEPT. OF AGRICULTURE

the hypocotyl elongates and straightens. Elongation of the hypocotyl stops after exposure and

straightening of the arch in bright light. Excessive elongation of the hypocotyl may occur in

greenhouses when shaded or during dull, short

days of winter.

The first leaf to develop from the terminal bud

is simple, but thereafter the leaves are normally

trifoliolate. Leaves have progressively longer

petioles until the normal length is attained. The

internodes of the primary stem normally are very

short. Under conditions favorable for growth,

the lower axillary buds of the primary stem may

grow within 6 to 8 weeks into stolons, which develop radially from the primary stem. Elongation of the primary stem stops or is limited after

stolon growth begins.

The Growing Plant

Growth after stolon initiation is rapid. Leaves

arising from the stolons are borne alternately, one

per node. One or more adventitious roots may

develop at each node. Two partially developed

root initials—one on each side of the attachment

of the vascular bundles of the petiole—usually

protrude through the stipule. The axillary bud

may remain dormant or may develop into either

a branch stolon or a flow^er. The growth of the

axillary bud depends on environment and genotype. Development of branch stolons is important

in persistence of stands and continued production

of leaves near the center of the plant.

Roots

The importance of the primary root rapidly

diminishes. This root usually dies before or during the second year, and thereafter the life of the

plant depends on the adventitious nodal roots

{100^ 109), In general, most of the roots are in

the top 6 inches of the soil, although in soil of

good tilth many roots reach a depth of 3 feet.

Boot development during cool weather is proportionately greater than top growth {96), This

abundant root growth during cool weather enables plants to withstand moisture stresses of the

following summer.

Leaves

The trifoliolate leaves are long petioled and

glabrous. Leaflets are sessile with finely serrated

margins. Leaves vary widely in shape and size

depending on type, variety, and environment.

Normally each leaflet has a V-shaped white mark

near the middle {32). This mark varies in intensity, size, shape, and position or may not be

present at all. Also, leaflets may be variously

marked with red pigment, and the number of

leaflets is not always three. One supernumerary

leaflet produces the "four-leaf" clover legendarily

associated with good luck. The number of leaflets

undoubtedly is genetically controlled; however,

in most cases the expression of numbers other

than three is affected by nutrition and

environment.

A white clover leaf has a relatively short life.

In a cool, moist environment the life oi a leaf from

bud to senescence is about 40 days, but is much

shorter under stresses of high temperature and

water deficiency. The leaf area index (LAI) of

thick stands may reach a ceiling value of 5.5

twenty days after defoliation and change little

thereafter {28), In a cool, moist environment a

LAI of 5.5 is composed mostly of fully opened

leaves, with smaller quantities of bud leaves and

leaves in various stages of senescence. The continuous replacement of leaves enables this plant to

tolerate grazing.

Stolons

The first stolons arise from axillary buds of the

primary stem (fig. 3). If not obstructed, they

extend radially in all directions. The stolons vary

in size and length of intemode. Growth is apical

and indeterminate.

An axillary bud of a stolon may remain dormant or develop into either a flower or a branch

stolon. The flower bud appears soon after the

young subtending leaf is visible. Branch stolons

appear later. The small leaves that appear at the

nodes are the first leaves of branch stolons. Occasionally a small unifoliolate leaf develops instead of the trifoliolate leaf. The axillary bud

at the node more nearly resembles a small seedling

than does the apical bud of the stolon. These

small buds are excellent for rooting. They make

plants that are more like plants from seed than

those made by the larger apical buds.

Since the primary stem frequently dies before

or during the second year, the life of the plant

soon depends on the stolons and their adventitious

roots. The survival of the younger tissue serves

as a natural means of vegetative reproduction.

Production of branch stolons increases the chance

of plant survival {17), Since each flower head

reduces the potential branch stolons by one,

branching during periods of profuse flowering is

greatly reduced or not present {103), The precarious condition of a plant composed only of

nonbranching stolons radiating from the primary

stem is obvious. Defoliation will result in leafless stolons except near the stolon tips, where new

leaves are forming. Stolon branching near the

center of the plant provides new nodes for the replacement of leaves and aids in maintenance of

uniform stands by providing meristems, which

may form the centers of new plants.
WHITE CLOVER

FIGURE 3.- -Young white clover plant : A, Primary stem ; li, stolon from basal node of primary stem ; C, branch

stolon from axillary bud. (Courtesy Clemson University.)

Flowers

Intensity of flower production varies among

varieties and appears to be affected by photoperiod, temperature, nutrition, and available moisture. Flower production usually is greatest in

early summer, whereas stolon branching is greatest during cool, moist periods. The flower heads

are borne singularly on stems that slightly exceed

the leaf petioles in length (fig. 4). The heads are

mostly round and are composed of individual perfect flowers or florets, each of which is short

stalked. The papilionaceous corolla is normally

white and is composed of five petals. The 10

stamens are diadelphous. The one stigma is borne

on a simple style. The small, normally three- to

four-seeded pod is membranous and is included in

the five-pointed calyx.

The number of florets per head ranges from 20

to 150, but averages about 75. These open progressively from the bottom to the top of the head

m approximately a week, although the rapidity

of opening depends upon the environment, particularly the temperature. Tlie florets remain

open for about 4 to 5 days or until fertilized.

High temperatures shorten this period. Upon

fertilization, which occurs about 18 hours after

cross-pollination, the corolla collapses, and as the

embryo develops the florets reflex, giving the maturing seed head an umbrellalike shape. The

seeds mature in 22 to 30 days after fertilization.

The time required to complete the entire process

varies, with temperature exerting the greatest effect. High temperatures accelerate the process

and low temperatures retard it.

Persistency of Stands (Longevity)

The most serious weakness of white clover is its

frequent failure to maintain thick stands for several years {100). White clover is classified as a

perennial, but plants may behave mostly as winter

annuals in the South and as biennials or shortlived perennials in the North. Undoubtedly sev-
6 AGKICULTUEE HANDBOOK 314, U.S. DEPT. OF AGRICULTURE

FiGUBE 4.—White clover flowers profusely if environmental requirements are met. Honey bees are best pollinators.

Florets mature from bottom to top and reflex soon after fertilization. (Courtesy Clemson University.)

eral factors contribute to the thinning of stands.

The available information on the physiology,

morphology, and longevity of the primary stem

does not clearly establish the role and importance

of this stem in the persistence of stands. Apparently the seedling taproot does not serve as a

storage organ for metabolites to support growth

of leaves and stolons after defoliation. Tests in

the field and the greenhouse have shown that the

original seedling taproots of plants in which nodal

rooting was prevented by elevation of the stolons

lived only slightly longer and were only slightly

larger in volume than seedling taproots of plants

whose nodal roots were allowed to develoj) normally. Elaborated metabolites and absorbed minerals translocate mostly to the young meristematic

tissue of the stolons and nodal roots {90, 99).

Insects and soilborne pathogenic fungi contribute to the early death of the seedling taproot

(44) 56). The older tissues of the taproot, nodal

roots, and the stolons appear to be more susceptible

to infection by destructive root and stolon rots.

The taproot seldom lives longer than 2 years, even

in the most favorable environment (109).

The primary stem does not form a "crown" tliat

produces new growth in the second and later years

as occurs in alfalfa. Most new growth comes

from axillary and terminal buds of the stolons.

If the primary stem persists, it contributes little.

Profuse flowering and heavy seed setting, particularly under strong environmental stresses,

shorten the life of the stolons (51 ).

The continued presence of white clover in a

pasture may be the result of (1) annual volunteering from seed, as is the case with true reseeding

annuals, and (2) vegetative propagation, i.e.,

growth of rooted stolons after death of the

primary stem.

The maintenance of good, continuous stands of

wliite clover over several years remains an unresolved problem. Thicli stands of an established

grass competing for moisture, nutrients, and light

become a significant barrier to the young, develop-
WHITE CLOVEIR

ing nodal roots of the young white clover stolons,

especially during periods of environmental stress.

Genotypes (new varieties) that have the required

morphological and physiological characteristics,

together with disease and insect resistance, are

needed to prolong the life of stands.

GENETICS AND CYTOGENETICS

White clover is a tetraploid (2n=32 chromosomes). It is normally self-incompatible. Disomic inheritance and the prevalence of regular

pairing and disjunction of the chromosomes in

meiosis indicate that it is an amphidiploid (8).

At least 64 oppositional "/S'" alíeles have been identified. This number is well above the minimum

required for a high degree of seed setting. A fertility factor "Sf^"' is a member of the allelomorphic

series. Plants having this factor are self-compatible. The ability of some plants to set seed

following selfing without the Sf alíele has been

designated as pseudoself-compatibility. Thus,

plants may vary from complete self-incompatibility through intermediate grades of pseudoselfcompatibility to a high degree of true self-compatibility.

Eleven variations of the white V-leaf marking

have been reported {2S). This mark is conditioned by an allelic series. Eed leaves, red flecking, red midrib, and a red spot are other hereditary

leaf characteristics that have been described.

These, as is true of the white V, are dominant to no

mark and are simply inherited.

The presence of HCN (hydrocyanic acid) in

white clover depends on the presence of the precursor glucoside and the hydrolyzing enzyme.

Both glucoside and enzyme are determined by separate single dominant factors, which segregate

independently.

The recent successful species hybrids T, repens

X T, nigrescens Viv. and T, repems X T, imiflorum

L. have opened the door for the plant breeder to

variation beyond that existing in the species {66^

91). Improved techniques for growing excised

embryos and the use of grafts may extend the

range of usable variation to additional species

The inheritance of qualitative characters in

white clover is summarized in table 1.

TABLE 1.—Summary oj inheritance oj qualitative characters in white clover

Character ^ Inheritance

FLOWER

Cyanidin-red corolla, black seed

Blush-colored corolla {24)

Nonclasping bracts {24)

Vestigial florets 2

LEAFLET

Red leaf {26, 32)

Red flecking {25, 32)

Red midrib {26, 32)

Mottling (virosis) {9)

White V (varies in location and expression) {26, 32)

Red spot (controlled by or closely linked to V locus) {67) _

CHEMICAL OR PHYSIOLOGICAL CHARACTERS

Self-incompatibility {6)

Cyanogenetic glucosides {10)

Linamarase (enzyme) {10)

HCN {10)....

Pigmentation= c^jc^, c^ld^.

Blush = 6/6.

Nonclasping=6r/6r.

Vestigial florets =FörV-, Vg'^^jRedleaf= ÄV-.

Flecking= ß//-.

Ä-/-.

Mottling=MV-, M^l-.

Multiple alíeles: i;/v=solid green.

Multiple alíeles.

Presence of glucosides = ^c/ —

Presence of enzyme = Lij —.

Presence of acid = ^c/—, Li/-

1 Numbers in parentheses indicate references in which

inheritances of characters are reported.

2 Based on authors' unpublished preliminary data.

BREEDING

Breeding techniques must be adapted to the

characteristics of white clover and to its use.

Frequent reference to the species as bein^ highly

polymorphic or heterogenous recognizes the existence of great variation (^, 61). Differences in

morphological characteristics are obvious. Other

differences, such as variation in chemical composition and reactions to diseases, insects, different

temperatures, photoperiods, and moisture stresses,

are less obvious but may be more important from

the breeding aspect.

Several species of bees and other insects pollinate
AGRICULTURE HANDBOOK 314, U.S. DEPT. OF AGRICULTURE

FIGURE 5.—Plant breeders may use honey bees for controlled pollinations by placing small hives inside cages.

Lights may be used above plants to induce uniform and abundant flovyering. (Courtesy Clemson University.)

white clover (23). Honey and other bees may be

used under cages to produce the amounts of seed

needed in a breeding program (fig. 5). Heterozygous genotypes can be maintained or increased

easilyl)y vegetative propagation. The reproduction and other characteristics of white clover are

such that it is well adapted to the polycross method

of testing the combining ability of clones.

The ability to tolerate competition is necessary

for the continued existence of a white clover plant

growing either in pure stands or in association with

a grass. Since white clover is used almost exclusively in association with a grass, selections and

evaluations should be made by studying tlie clover

in a stand of the grass with which it will be used.

Since the most common criticism of white clover

is the loss of stands in pastures, improving persistence is a logical goal of most wiiite cloverbreeding programs. Because many factors contribute to persistence, one needs to first determine

the relative importance of these factors. Then,

appropriate specific objectives can be used, e.g., resistance to a specific pathogen. Increased production, at least of older stands, should accompany

improvement in persistence.

Characteristics of white clover that affect the

choice of techniques and methods to be used in developing improved varieties are as follows:

(1) Plants are highly self-incompatible—enforced by a series of ^S alíeles.

(2) Plants are highly heterozygous as a result

of enforced cross-pollination.

(3) Selfing is possible by using either the Sf

gene or the pseudoself-compatibility characteristic.

(4) Plants normally bloom and set seed every

year.

(5) Flowering for making crosses may be induced easily in the greenhouse.

(6) Hand pollinations are easy to make and result in a good seed set.

(7) Vegetative propagation to the extent

needed in a breeding program is easily done by

rooting stolons.

(8) Plants of clones may be grown and maintained in the greenhouse in 6-inch pots or smaller

containers.

Source Nurseries

Screening diverse germ plasm is essential. In

this screening program, collections from areas

having growing conditions that approximate the

environment in which improved varieties will be

used should be investigated extensively.

One method of screening seed lots consists in

planting broadcast with a competing grass, clip-
"WHITE CLOVER

ping to simulate grazing, then selecting superior

plants after the occurrence of environmental

stresses of magnitudes that cause differential

survival.

Plants selected from old stands and from source

nurseries frequently are infected with one or more

viruses. Seedling plants normally are virus free.

The virus status of plants must be considered in

their evaluation.

Plants selected for further evaluation as clones

should have medium-size stolons that branch frequently and should flower sparsely in the environment where the variety will be used for

forage {S3).

Evaluation oí Clones

Phenotypes selected from broadcast-planted

source nurseries or other sources, such as old pastures, are subjected to varying degrees of evaluation. These selections generally are increased

vegetatively and established (1) as spaced plants

for specific characterization or (2) in small plots

or beds—propagules interplanted on narrow centers with a grass to simulate a, pasture (7). The

plots are managed to provide evaluation of the

clones growing with a grass under environmental

stresses.

Bee cages may be used to produce seed for

clonal evaluations. Cages are necessary, because

white clover is so widely and generally distributed

that the use of isolated crossing blocks is rarely

feasible. Hand pollinations are seldom used because of the time required to manipulate the small

floral organs and because of the number of seed

needed to establish plots. Since it is often necessary to minimize the use of bee cages, white clover

breeders compared to breeders of many other crops

probably put more effort into reducing the number

of clones to be evaluated by test crosses. The correlation between the responses of parental clones

and those of their progenies indicates that this can

be done. Evaluating selections first by planting

as spaced plants then by planting on narrow centers in plots permits progressive elimination of

inferior germ plasm.

Concurrently with the field evaluation of clones

by plantings of propagules, the clones should be

screened in the greenhouse or laboratory for as

many characteristics as possible, such as resistance

to specific diseases (fig. 6). Considerable progress has been made in recent years in developing

techniques for locating resistance to soilborne

pathogens and nematodes {S8,59).

After careful elimination among the original

selections, based on observations and measurements

in replicated tests, progenies of the remaining

clones are compared. Polycross progeny testing

usually is employed because it combines economy

FiGXTRE 6.—Greenhouse screening to obtain plants resistant to root-lsnot nematodes. Left, susceptible white

clover plants; right, resistant plants. (Courtesy

Clemson University.)

of effort with estimates of combining ability and

performance in synthetic varieties {69, 105).

This is accomplished by choosing a number, usually between 10 and 20, of superior clones that

flower at the same time and planting these in a

crossing block. Polycross seeds from each clone

are harvested and used to determine the combining

ability of the clone.

Eesults of recent research indicate the feasibility of greater use of single crosses or clonal crosses.

Small plots of a clover-grass mixture established

by check-planting clover seedlings on 6-inch centers and by broadcast seeding the clover gave similar results {52). Thus, by setting seedlings in the

field, a small quantity of seed is adequate to evaluate a strain of clover {4-1). The individual plants

may be classified for growth type and some other

characteristics before they grow together to form

a solid stand. Establishing plots by transplant-
10 AGRICULTURE HANDBOOK 314, U.S. DEPT. OF AGRICULTURE

ing seedlings makes feasible the use of hand pollinations to produce seed for strain evaluations.

Synthetic Varieties

Superior clones may be combined into an experimental synthetic variety. In white clover this can

be done easily by planting the clones together in a

crossing block. The bulk seed is harvested for

testing or increase in successive generations.

Ultimately the evaluation program should include

plantings of seed of the generation to be used in

the seed trade. The parent clones can be maintained indefinitely by vegetative propagation.

However, prior to the release of such a variety it

is desirable to make diallel crosses of the parent

clones to avoid the use of clones that in specific

combinations produce inferior plants.

Mass Selection and Natural Selection

Mass selection is a plant-breeding procedure

that may employ various artificial, or manmade,

selection pressures in addition to those of nature.

The breeding procedures referred to as "strain

building" and "maternal-line selection" may logically be considered modifications of mass selection.

These procedures include a continuous synthesis

of a strain by various methods of breeding {105),

The variability in the species plus the high degree of cross-pollination favor shifts in response

to natural or artificial selection pressures.

Strains of white clover have been naturally and

artificially made and maintained under specific

environmental conditions. Many ecotypes exist.

Some have made valuable contributions as local

varieties. Management practices may be employed in a given environment to create selection

pressures and thus maintain varietal characteristics.

Inbreeding

Selfing is possible by using the Sf gene or

the pseudoself-compatibility characteristics. Inbreeding results in the appearance of chlorophyll

deficiencies, leaf abnormalities, and other offtypes

similar to those that have been obtained by inbreeding other normally cross-pollinated crops.

The use of inbreeding in white clover breeding has

been limited. It should be explored to determine

its ultimate value (^, 6).

First-Generation and Double-Cross Hybrids

The ease of vegetative propagation and the

prevalence of self-incompatibility make firstgeneration and double-cross hybrids definite possibilities. If hybrids are used, their performance

must justify the additional expense involved in

seed production. To date, such a hybrid variety

has not been developed.

Induced Polyploidy and Species

Hybridization

The use of induced polyploidy in white clover

improvement has been attempted. An experimental variety 8x = 64 developed at the Vermont Agricultural Experiment Station was widely tested but

never released. Probably the greatest contribution of polyploids is the increased possibility of

fertile crosses with other species. A fertile Fi

from the cross 8x=64 T, repens X 4x=32 T.

nigrescens has been obtained {26^ 66). Although

this cross has not contributed to an improved variety, it is proof that species crosses are possible

in the genus Trifolium, The development of improved techniques for handling excised embryos

offers greater possibilities for making wide crosses

and for obtaining desired characters from other

species {70).

Mutational Breeding

Changes in white clover chromosomes have been

induced by radiation and other mutagenic agents.

The use of mutagenic agents on other plants has

produced predominantly deleterious changes. On

the other hand, a few changes have been considered

desirable. Regardless, the number of varieties

containing changes attributed to induced mutations is small and their acceptance or use has been

limited. It seems logical to explore thoroughly

existing variability prior to trying haphazardinduced mutations.

Varietal Release and Seed Increase

Procedures for the orderly release, increase, distribution, and maintenance of varieties have been

developed in most countries and States. These are

applicable to white clover. The desirability of a

variety with a wide range of adaptation is evident.

This is especially true for varieties of which seed

must be produced outside the area of utilization.

This frequently applies to white clover varieties.

A wide range of adaptation and the resulting

greater demand for seed favor a greater, more

economical, and more reliable seed supply.

Techniques

Emasculation, Pollination, and Seed Production

Emasculation is not necessary in much of the

crossing included in a breeding program. Emasculation should be practiced in making critical

crosses or when working with self-compatible
WHITE CLOVEiR 11

plants. Several emasculation techniques have

been successfully used. The two best known are

removal of the anthers by suction and with forceps.

One of the most rapid methods consists in (1)

removing all open florets from a young flower

head, then removing all florets above the whorl of

florets that are about to open; (2) seizing the

corolla on the underside at a point midway between the tip of the calyx and the tip of the standard, using only the tip of the forceps; and (3)

withdrawing the entire corolla together with the

staminal tube and all anthers by pulling slowly

and steadily. The stigma is left exposed and

ready for pollination. By using this technique,

emasculation and pollination can be performed as

quickly as pollination without emasculation. To

reduce the chance of selfing as a result of anthers

dehiscing during emasculation, the stigmas may

be atomized with water immediately after emasculation and pollination delayed until the stigmas

have dried {111).

An ordinary lead pencil is convenient for transferring pollen. The pollen can be easily seen on

the black lead.

Bee cages are used to produce polycross seed and

seed of crosses if the amount needed is more than

can be produced by hand pollinations. For convenience in maintaining plant identities, the area

to be covered by the bee cage may be divided into

"cells" by using boards or pieces of sheet metal for

low partitions that confine the plants but do not

obstruct the bees. One plant is then set in each

cell. Slight differences in time of flowering may

be corrected by (1) letting plants grow until

flower buds are forming on plants of all clones,

(2) removing all flower heads containing open

florets, (3) erecting the cage, and (4) moving bees

into the cage when plants of all clones have open

flowers.

In addition to these steps, the authors have used

electric lights inside the cages to induce more

flowering. The lights are set to extend the natural daylight to about 16 hours. This has given

a flush of flowers during the time the bees are used.

Seeds are harvested about 28 days after the bees

are removed.

Induction of Flowering

Flowering is affected by temperature, moisture

stress, light intensity, day length, and the age, condition, and genotype of the plant (i^, ^7, 102).

The authors have induced flowering in the greenhouse both by interrupting the night period by 1

hour of light and by extending the daylight period.

The photoperiod required for flowering varies

from 13 to 16 hours depending on the genotype of

the plant (82), Some plants will produce one

flush of flowers and then produce few if any more ;

other plants will produce flowers over a period of

time.

Vegetative Propagation

Vegetative propagation merely consists in rooting stolon pieces in sand, soil, or rooting media.

Plant hormones to stimulate rooting are not

needed. Stolon tips or stolon segments with developing vegetative buds are preferred ; however,

other segments will root. A satisfactory procedure that results in a high percentage of uniform

plants is as follows: Take stolon tips including

three internodes, remove all leaves except the terminal one, and place the stolons in sand, the cut

end about one-half inch deep and the bud end on

the surface. If a layer of soil is used under the

sand, the cuttings can be left in the sand

until several new leaves are formed prior to

transplanting.

Propagation With Seed

Germination of freshly harvested white clover

seed varies. In addition to hard-seeded dormancy,

some seeds apparently require a period for postharvest maturation. Germination can be increased by scarification and by altering the temperature (93),

The seeds may be germinated on blotter paper

and moved to the soil as they germinate and before

the roots become tangled. This method is very

satisfactory for handling small numbers of seed.

Larger amounts of seed probably can be more

efficiently handled by planting on soil, covering with sand, and transplanting to small pots

at about the time the third trifoliolate leaf appears.

Ground sphagnum moss may also be used instead

of soil, or a %-inch layer of sphagnum moss may

be placed on the soil. Sphagnum moss reduces

seedling loss caused by the damping-off diseases.

Plants kept in greenhouses for seed production

should be in 6- or 8-inch pots. Plants to go into

the field may be started in pots as small as 21/^

inches.

Field Plots

The ultimate goal of improvement programs is

to develop a variety superior in such characteristics as total production, seasonal distribution of

production, and persistence under competition.

Field plots should permit evaluation of selections

and varieties in regard to their yield and persistence when grown in association with a grass.

Plots about 5 feet by 5 feet have been used by the

authors to evaluate both seedlings and propagules.

These plots were separated by grass alleys and had

little if any border effect. However, if larger plots

are preferred, dimensions of 5 by 20 and 10 by 20

feet are frequently used. However, the final eval-
12 AGRICULTURE HANDBOOK 314, U.S. DEPT. OF AGRICULTURE

uation should be based on large plots grazed by

animals.

Measuring Yields

Eotary mowers equipped with catch baskets permit yield determinations at frequent intervals

during the growing season. They enable one to

simulate grazing more closely and to measure the

distribution of production more accurately. Sickle

mowers equipped with catch baskets may also be

used and may be necessary if large plots are used.

Since part of the clover's contribution to forage

production is to furnish nitrogen for the grass,

measurement of the combined yield of the clover

and grass is sometimes made. Botanical separations are expensive, but may be necessary in critical

tests. Preferably, seedings for varietal yield evaluations should be made both with and without a

grass.

Height and frequencj^ of cutting have been

studied by several investigators {31^ 97). Close,

frequent clipping favors the clover. The authors

consider that clipping at a height of 3 inches above

the soil surface at 4-week intervals throughout the

growing season is satisfactory for most tests.

Measuring Stands

Stands may be estimated or measured by various

techniques. Visual estimates have been used successfully, as well as an adaptation of the line transect. This latter technique consists in determining

the average number of live stolons transecting a

meter line in the test area. It recognizes the stolon

as the source of growth. The measurements may

be made in pastures or in plots. The number of

leaves present does not affect the reading. Measurements may be made after harvesting plots or in

grazed pastures.

Alternate Approach to Improvement

The discussion of breeding white clover for increased persistence of stands as a perennial does

not preclude the possibility of breeding as an annual, with or without natural reseeding. For

some areas or uses, breeding as an annual may be

the correct approach. Also, it has the advantage

of starting annually with a large part of the clover

population being virus free. Much of the previous discussion is applicable, with only slight

modification, to breeding as an annual.

TYPES AND VARIETIES

Types

The great range of morphological forms in

white clover is recognized in the classification into

the agronomic types—small, intermediate, and

large. These are represented by the varieties Kent

Wild White, Louisiana White, and Ladino,^ respectively (63). This classification was particularly valuable in describing early varieties and

ecotypes. Care must be exercised in attempting

classification by type, particularly under some environmental conditions, such as optimum temperature and moisture and a short winter photoperiod

in a greenhouse. Variation in size characteristics

of plants is continuous. Differentiation between

the larger plants of an intermediate type and the

smaller plants of a large type is almost impossible.

Since types intercross freely and do not have

specific limits, classification problems are common

(^). For example, a plant may have the large

leaf and flower-head characteristics of the large

type but stolons belonging to the intermediate

^ The U.S. Department of Agriculture Service and Regulatory Announcement No. 156, issued March 1940 and reprinted with amendments June 1960, lists ladino as a kind

of clover not a variety. The Crop Science Society of

America recognizes that ladino is used also as a varietal

name and distinguishes this usage by capitalization. In

this publication, ladino is considered a variety of white

clover. This usage has been followed in the U.S. Department of Agriculture Yearbooks.

type. An intermediate-type variety that has been

grown for an unlimited number of generations and

has been subject to outcrossing may have some

plants of both large and small types. Seed lots of

both the small and the large types may have some

plants belonging to the intermediate type. A field

classification, by the authors, of several hundred

plants of a naturalized intermediate Louisiana

variety gave approximately 7, 65, and 27 percent

of large, intermediate, and small types, respectively.

Varieties

There are not many varieties of white clover of

United States and Canadian origin, and many of

them are ecotypes and are not bred varieties of a

specific plant-breeding program. Two variety

names used in writings and advertisements are

misleading and erroneous. These are Dutch

White clover and Wild White clover. Both names

were used by the seed merchants in the 19th and

early 20th century, probably for customer appeal.

The name "Dutch" was applied to any white

clover, the seed of which was produced in the Low

Countries. Such seed has not been imported into

this country in quantity for many years. Dutch

in common usage is a synonym for Common

White, which may be any one of the three "agronomic types" or mixtures thereof.
WHITE CLOVEIR 13

The term "Wild White" apparently originated

in Great Britain and was used there to designate

the small type. The connotation that Wild White

is the prototype of white clover cannot be substantiated. Wild White clover most likely arose from

the survival of the fittest in pastures under the continuous selection pressure of close grazing by domesticated animals.

Studies of plant characteristics have been made

in efforts to delineate the Ladino variety from

intermediate-type varieties, particularly in the

advanced seedling stage. Length and diameter of

leaf petiole, size and shape of leañets, and number of petiole vascular bundles have been studied.

Not one of these characteristics has been a satisfactory index for differentiation. Diameter of the

leaf petiole has been the most consistent characteristic of those studied under specific environment and management practices. Growth responses to variations in nutrition, soil moisture,

temperature, frequency of defoliation, and photoperiod may make classification difficult. For

example, Ladino when defoliated to simulate a

lawn may produce short, stubby leaf petioles.

Although there are a large number of foreign

varieties of white clover, most of them do not appear on the domestic market in quantity and will

only be mentioned by name, origin, and type.

Most of these foreign varieties are not adapted to

the environments of the United States nor to the

management practices in use.

Variety Adaptation

Many factors contribute to variety adaptation.

Diseases, insects and other pests, climate, photoperiod, soil, and management collectively determine whether a variety is superior, similar, or inferior to others when planted in comparative tests.

Only by testing varieties over a period of years

can the superior ones be identified. Even within

a single State, the use of more than one variety

may be necessary.

In the search for superior germ plasm, most

white clover varieties and many common seed lots

of worldwide origin have been and are now being

tested extensively in the United States. With the

exception of Ladino, none have given superior performance or have proved to be widely adapted

when compared with those of domestic origin.

Occasionally a foreign variety may be outstanding

in a very restricted area. For example. New Zealand white clover is productive and persistent in

northwestern Oregon. Its resistance to injury

from slugs and its adaptation to the cool, moist

climate of the area appear to be the principal reasons for its superiority.

Even within the United States the varieties originating in the Northern States are not adapted to

conditions of the lower South. In the shorter

photoperiod, they flower sparingly, produce insufficient seed for volunteering, and produce low

yields of forage when adapted varieties are making their most rapid growth. As varieties

adapted to the South are moved northward, they

flower profusely and early. Later vegetative development is stimulated by the longer day. They

are adversely affected by low winter temperatures.

The incidence and severity of root and stolon rots

are earlier and greater after profuse flowering.

Variety recommendations are the function of

State agricultural experiment stations, which in

their research programs conduct variety tests

under the different soil and climatic conditions of

their respective States. Such tests should be conducted before a variety is officially named and released for commercial seed production. This

practice is highly desirable as it synchronizes

seed availability with farm demand. Since varieties are continually changing as new, superior

ones replace older ones, many State agricultural

experiment stations annually publish a list of those

recommended.

Description of Varieties

White clover varietal distinctions have only become important since 1935. The continued common usage of the terms "Dutch White" clover and

"Wild White" clover is a vestige from the time

when there were few varieties.

Coimmon Whiu

Common White as the name implies is indeed

common. The seed may come from any source.

In general, the plants are of the intermediate

type. Practically all the so-called varieties given

State names originated from plantings of such

seed. Unadapted seed planted in a State and

harvested for seed unfortunately may be labeled

as a naturalized State variety where grown, even

though it is inferior.

State-Named Vaneties

Of the so-called State varieties, Louisiana White

is the best known. This is a natural consequence

of the position Louisiana holds in white clover

seed production. Although plants from a seed lot

of Louisiana White clover are predominantly of

the intermediate type, some plants may be of the

large or small type or both types. The frequencies of the different type plants vary among seed

lots. The planting in Louisiana of seed from

different places of origin and natural selection

under different management practices account for

much of the variation.
14 AGRICULTUEE HANDBOOK 314, U.S. DEPT. OF AGRICULTURE

Louisiana White plants flower profusely

throughout the South. Seed production is ample

for a volunteer crop even under close grazing.

As a result of the very general requirements for

white clover seed to be classified as Louisiana

White, various producers and groups of producers

have applied their names or brand names to seed

that otherwise would be sold as Louisiana White.

Some of these seeds are produced on large acreages

maintained by following management practices

that should insure a maximum purity of plants

of a desirable size adapted to the southern climate.

Nolin White maintained near Hamburg, La., is an

example.

Louisiana SI

Louisiana Si, a synthetic variety, was developed

at the Louisiana Agricultural Experiment Station

and is very similar to Louisiana White (fig. 7).

The five parental clones were selected from a

source nursery of Louisiana White plants. The

principal objective of the breeding program that

culminated in the release of this variety was to increase heat and drought tolerance.

Regal

Regal is a synthetic variety of five clones

selected for persistency, vigor, and sparse blooming under Alabama conditions. This variety,

developed at the Alabama Agricultural Experiment Station, was released in 1962.

Lad'ino

Ladino, as recorded in 1848, was first found

growing on an estate in the Po Valley of Italy.

The first record of its introduction into the United

States appears to be in 1891 (SI). Introductions

occurred between 1900 and 1910, but the first significant establishment of Ladino fields occurred

in the Western States in 1912.

Ladino is a variety of the large type of Trifolium repens (fig. 7). In general, all its vegetative parts are larger than those of intermediate

white. There is no difference in seed size. The

considerable variation in size characteristics has

probably resulted from outcrossing with the intermediate and small types. Fi plants from crosses

of Ladino and plants of the small type are intermediate in size.

Observations by the authors over a period of

years on spaced-plant populations of domestic and

Italian lots of Ladino indicate that the diameter

of the stolons of the lots of domestic origin is

slightly smaller than that of lots of Italian origin. This could have happened through selection

pressure from the tramping of grazing animals,

since most of the early Ladino seed fields in OreFiGTJBE 7.—Left, Ladino clover; right, Louisiana SI.

Note absence of flowers on Ladino. Sliort photoperiod in lower South prevents flowering and seed

production of northern white clover varieties necessary for volunteer stands In fall. Picture taken

April 12.

gon were grazed several times each year. In

Italy, practically all the Ladino is harvested as

green feed and thus eliminates any injury to plants

with large stolons. It should be noted that Ladino originated in Italy at approximately 45° N.

and that it first flourished in the United States at

approximately 43° N.

tÄerit

Merit is a synthetic variety of 30 clones selected

from Ladino. After 3 years of testing under Iowa

conditions, it was released by the Iowa Agricultural Experiment Station in 1961. Although

Merit is similar to Ladino in appearance, in Iowa

it has shown superiority in forage yield, winter

survival, and summer drought tolerance.

Pilgrim

Pilgrim is a synthetic variety of 21 clones originating from seed collected from older Ladino pastures in the Northeastern States and Canada.

The selected clones were progeny tested for the

large-type characteristics. It was named and

released in 1953 by several northeastern State agricultural experiment stations, the U.S. Regional

Pasture Research Laboratory, and the Division of

Forage Crops and Diseases, U.S. Department of

Agriculture. The N"ursery Division, Soil Conservation Service, assisted by producing the first

breeder seed from a clonal planting. The principal objectives in the development of Pilgrim were

to purify a seed stock of the large type and to

establish a continuing source of breeder seed of the

same genetic composition. The clones are main-
WHITE CLOVER 15

tained and breeder seeds produced under the auspices of the Washington Agricultural Experiment

Station.

Espanso

Espanso, formerly called Granladino, was developed in Italy. It is claimed to have improved

winter and drought tolerance together wath good

persistence. Plants are of the large type.

Nordic

Nordic was developed in Germany from a cross

between Ladino and Morso, a Danish variety. It

is being grown in the United States for the export

seed trade.

New York Wild White

New York Wild White was grown and used in

New York. It was a naturalized small-type variety selected from an old pasture that w as continuously and closely grazed. It is no longer used.

Foreign Varieties and Strains

For many years the U.S. Agricultural Kesearch

Service has collected and distributed seed for testing. Many foreign sources have been included in

this search for superior germ plasm. Some of

these collections, with their sources, are as follows :

Australia : Caboolture, Dingee, Garlanton, Giganteum. Irrigation, Mankland, Mary E., Namlesur,

Nappan Wild, Pyramid, Kedcliffe, Reeli Ck.,

Sanford, S. Maroochy E.; Canada: Canadian

Ladino Synthetic; Denmark: Adefa II V.,

Adefa IV, Daehnfeldt, Daeno II K., Danish

Stryne, Ladino Synthetic, Lodi I., K., and V.,

Lodi Otofte, Lodi Otofte I., K., and V., Milka,

Morso, Morso Otofte, Morso Otofte I. and K.,

Pajbjerg Milka; Finland: Kemi; Grermany:

Nordic; Great Britain (England): Bedfordshire Wild, Cotswold Wild, East Anglia, Essex

Wild, Kent Wild, Kersey, New Suffold, Northants Wild ; Great Britain (Wales) : Aberystwyth S.IOO, Aberystwyth S.IOO (Nomark),

Aberystwyth S.184; Holland: Cultnur KlaverCB leg type, De Weledelzear CB, Witte Weideklavar CB, W.M.K. 46.12, W.M.K. 47.7, W.M.K.

47.9, W.M.K. 47.10; New Zealand: Certified,

Certified Mother, New Zealand, Pedigree, Permanent Pasture, Wild; Sweden: Hero, Nora,

Robusta, Svalof Kivi, V. 22.

Many more seed lots from these and other

countries have been introduced and examined.

They have been obtained from the New Crops Research Branch of the Agricultural Research Service, the Soil Conservation Service, and foreign and

domestic members of the seed trade.

DISEASES'

Several pathogens attack white clover, reducing its quality, yield, and persistence. Pathogens

attacking leaves produce conspicuous symptoms;

however, foliar diseases are usually not so destructive as diseases of roots and stolons.

Only the most prevalent and economically important diseases are discussed (75). With few exceptions, they are widely distributed in the United

States. Their prevalence depends on many factors, including presence of susceptible plants, favorable temperature and moisture conditions, age

of plants, and for some virus diseases, presence of

suitable vectors.

Pepper Spot

Pepper spot is incited by the fungus Leptosphaerulina trifolii (Rost.) Petr. The disease is

most prevalent and destructive during cool, wet

weather. In the Northern States, infection may

occur throughout the entire growing season. In

the South, it is most prevalent and destructive

during the winter and spring {Jf9),

Tiny, black, sunken spots develop on leaves and

petioles, hence the name pepper spot. The spots

may become so numerous that infected leaves appear darkened or grayish (fig. 8). The tiny spots

rarely enlarge beyond a millimeter or two. Diseased leaves rapidly turn yellow, wilt, and die during warm weather. Although related biotypes of

the fungus attack other legumes, the biotype attacking white clover is largely restricted to TrifoliuTYh spp. {Bli). The fungus survives in specialized fruiting structures on dead leaves and

petioles during periods of the year unfavorable

for infection.

Lines more resistant to the disease are being developed. When heavy infection occurs, removing

diseased foliage by grazing or mowing helps to

reduce infection on new growth.

Cercospora Leaf and Stem Spot

Cercospora leaf and stem spot is caused by the

fungus Cercospora zehrina Pass. The disease occurs on white clover throughout its range of adaptation and in some seasons it can be very prevalent

and destructive. The disease is particularly se-

^ Prepared by K. W. Kreitlow, research pathologist,

Crops Research Division.
16 AGRICULTURE HANDBOOK 314, U.S. DEPT. OF AGRICULTURE

FIGURE 8.—Advanced stage of pepper spot infection on Ladino wliite clover leaf. Pepper spot is one of widespread

serious leaf diseases that reduce growth and longevity of leaves.

vere on white clover in the Soutli during tlie summer and early fall {65). Infection occurs

throughout the growing season on plants of any

age.

Leaf lesions ai'e usually rectangular and more

or less limited by the veins. They vary from

light brown to purplish black. When dew is still

on the leaves, the lesions appear silvery from the

numerous spores produced by the fungus. lesions

on stems are usually somewhat sunken and dark.

New infections result from spores spread by air

currents and rain. Heavy infection causes leaves

to wither prematurely. Cercospora spores from

white clover can infect red and alsike clovers and

possibly other species of legumes as well {19).

There is little information on the control of

Cercospora leaf and stem spot. Removing old

crop residues, combined witli crop rotation, helps

to reduce damage. Clipping or grazing when tJie

disease appears destructive may reduce damage to

succeeding crops {Jß). Newer varieties usually

have been selected for greater resistance to the

disease {75).

Curvularia Leaf Spot

Curvularia leaf spot is incited by Curvvlaria

trifolii (Kauff.) Boed. It causes wilting and dying of leaves. Ladino white clover seems to be

more susceptible to infection than varieties of the

intermediate type. The disease is most prevalent

during warm, humid weatlier, when 20 to 25 percent of the leaves may be attacked and damaged

{79\.

Diseased leaves are usually characterized by the

presence of a large yellowed area that turns watery gray to tan, translucent, then liglit brown.

A yellowish band generally outlines the advancing

edge of the lesion. Diseased areas originating at

a leaf tip often are V-shaped. As the injured

area dies, the leaflet frequently curls downward.

The fungus can invade entire leaflets and attack

petioles, causing wilting and death.

Since the disease mainly affects leaves, grazing

or cutting the clover removes diseased foliage and

reduces danger to new growth. Experimental

data indicate that resistance to Curvularia leaf

spot can be readily obtained {71).
WHITE CLOVER 17

Sooty Blotch

Sooty blotch, caused by the fungus Cymadothea

trifolii (Pers. ex Fr.) Wolf, is most prevalent during cool seasons. The disease generally occurs

during late summer and fall in the Northern States

and during the winter and early spring in the

South.

The granular, black blotches are usually angular

and are most conspicuous on the underside of

leaves. Later, when overwintering fruiting

bodies of the fungus develop, the blotches appear

raised, black, and warty. Infected leaves turn

yellow and wither prematurely {60).

The disease is not so destructive on white clover

as it is on some other clover species. Grazing or

cutting the foliage removes the fungus inoculum

and reduces subsequent infection.

Rust

Kust, caused by üromyces trifolii var. trifoliirepentis (Liro) Arth., attacks white clover

petioles and leaflets, resulting in premature shriveling and death. The most conspicuous symptom

occurs during the brown rust stage, usually in late

summer and fall.

Kust pustules are generally deep reddish brown.

They occur principally on the underside of leaflets

and along petioles. If pustules are numerous and

well developed, the upper surface of infected leaflets turns yellow or red. Infected leaves often become distorted. Sometimes during wet weather

in the fall or early spring, small, swollen, yellow

clusters of aecia, or tiny cuplike structures, develop on leaf petioles and the veins of leaflets.

This kind of infection markedly distorts leaves

and petioles {60).

Most of the improved varieties of white clover

are not damaged so severely by rust as their predecessors. Grazing and clipping stands help to

reduce injury. Stands left for seed can be severely damaged by rust infection.

Viruses

The viruses commonly attacking white clover

are widespread and prevalent. Some of them also

attack other related and unrelated hosts, such as

alfalfa, peas, beans, potatoes, strawberries, and

some weeds. The most common viruses, like alfalfa mosaic, bean yellow mosaic, and white clover

mosaic {78^ 92)^ are spread by grazing, mowing,

and insect vectors, principally species of aphids.

The virus causing a condition known as phyllody

is spread only by certain species of leafhoppers.

Absence of virus symptoms does not mean a plant

has recovered from virus infection. Symptoms

merely become so inconspicuous that they are temporarily undiscernible. Baxter and McGlohon

{IS) reported a method of freeing some clones of

one virus. Their method may be valuable to plant

breeders for eliminating the virus from parental

clones to be used for seed production.

Viruses causing mosaic symptoms generally induce mild to severe mottlmg in leaves (fig. 9).

Sometimes the veins become light colored and

conspicuous. Usually light-green to yellowish

blotches develop as patches between the veins. In

some cases, leaves curl downward or appear ruffled.

Severely diseased plants are frequently stunted.

Phyllody is induced by a strain or strains of

aster yellows virus. The same or closely related

strains of the virus induce symptoms m strawberry known as green petal disease {48). To date,

infection has been reported only in California,

Oregon, and Maine. Phyllody symptoms are

characterized by transformation of floral organs

into dwarfed leaves. The flower head may be

comprised entirely of abnormally elongated florets

or of leaves in various stages of normal or abnormal development {67).

Virus infection generally stunts plants, reducing yield and seed production up to 50 percent

{76^ 77). Diseased plants are usually weakened.

This predisposes them to killing by pathogenic

fungi and adverse environmental conditions.

Virus infection increases in a clover stand from

season to season, often reaching 50 to 75 percent

of the plants in 3 years. Longevity of stands may

be seriously reduced by virus infection.

Virus infection is difficult to control, because

once a plant becomes infected it remains diseased

until death. At present, no virus-resistant white

clover plants have been found; however, some

lines tolerate virus infection better than others.

Fortunately the viruses of white clover are not

seedborne. If possible, fields should not be sown

adjacent to other leguminous crops. When a crop

is being grown for seed, insecticides may be judiciously applied to reduce prevalence of insect

vectors.

Stolon and Root Rots

Stolon and root rots are sometimes not so conspicuous as foliar diseases, but they are frequently

more destructive and responsible for reduced

yields and stand persistence. In the North, yield

of white clover is usually satisfactory the first

harvest season, but it decreases substantially in the

second and third seasons {72). The yield decline

is correlated wdth stand thinning during the

second and subsequent growing seasons. In the

South, wählte clover failure occurs mostly during

the summer, and it is attributed largely to disease

attacks {S). Stolon damage occurs from May

until September and is closely associated wûth high

summer temperatures {49^50^83).
18 AGRICULTURE HANDBOOK 314, U.S. DEPT. OF AGRICULTURE

BN-27595

FIGURE 9.—Yellow streaking and mottling in leaves of Ladino white clover caused by virus infection. Viruses are

transmitted in plant juice by insects and mechanical injury.

Stolon and root rots are dependent on the interaction of several agents, including fungi, nematodes, insects, environmental conditions, and age

of tissue. Among the most prevalent fungi associated with diseased stolons and roots are species

of Fusarium^ Rhizoctonia, Golletotrichum, LeftodisGus, and Curvularia. Sometimes the charcoal rot fungus MacrofkomAna pfiaseoli (Maubl.)

Ashby damages fields in the South {69).

Disease symptoms on roots usually consist of

brown to black sunken lesions that may be discrete or they may coalesce and girdle or discolor

the entire root. Lesions are sometimes the result

of insect feeding {Sitona spp.). Fungi enter tissues through the wounds and destroy the root

{65,72).

Fungi invading roots often penetrate into

stolons causing their deatli. The same fungi that

invade roots can enter stolons through wounds induced by grazing animals, farm machinery, and

exposure to the sun. Stolon internodes die and

wither, leaving a mass of dead stolons and bare

soil where abundant foliage formerly occurred.

Although most stolon and root injury occurs

during the summer, damage from the fungus

Sclerotinia trifolioruvi Eriks, sometimes occurs

during the winter or under heavy and prolonged

snow cover (fig. 10). Diseased plants generally

are in patches, and dead leaves and petioles mat in

a papery crust of tan to white dead plant tissue.

Diseased stolons are often flaccid and brown.

Later, hard black sclerotia form, which may adhere to the surface of or be imbedded in stolons.

The fungus generally becomes inactive when the

temperature reaches 80° F. {7J^).

Because of the numerous agents involved in

stolon and root diseases, no eiïective diseasecontrol measures are presently available. Damage can be lessened somewhat by growing adapted

varieties and following recommended cultural

practices.

Nematodes

Nematodes feed on and damage roots of white

clover in many parts of the LTnited States. Most

widespread damage occurs in the South, princi-
WHITE CLOVEiR 19

FiGUBE 10.—Sclerotia (resting or carryover stage) of fungus Sclerotinia trifoliorum causing crown and stem rot disease

of white clover. They germinate and spores infect planta in fall. Plants are killed during winter and early spring.

pally from infestations of root-knot nematodes

{Meloidogyne spp.) {12). Localized damage

often results from feeding by other kinds of nematodes, such as sting, meadow, and clover cyst

nematodes. Surveys suggest that many kinds of

nematodes probably feed on roots of white clover,

but the amount of damage incited by each has not

yet been adequately assessed (54).

Symptoms of nematode injury include plant

stunting, unthrifty growth, and untimely death of

plants during hot, dry periods in the summer.

Eoot-knot and cyst nematode infestations induce

swellings or gall formation on roots. Nematode

cysts and galls often resemble nodules of Rhizohiwm spp. Meadow and sting nematode injury is

frequently indicated by shortened and thickened

roots that may be uniformly discolored brown or

contain discrete brown lesions. Not only does

nematode feeding weaken plants but soilbome

fungi may invade roots through nematode-induced

wounds, causing additional injury.

Crop rotation is not always effective for nematode control, since many species attack unrelated

hosts. Cyst nematodes can survive for long

periods in soil in the absence of a preferred host.

Fumigation with nematocides is presently too expensive and impractical for white clover. Selecting white clover plants resistant to nematodes

appears promising (11).

INSECT PESTS*

Many species of insects attack and damage white

clover. They may prevent or reduce plant

growth, destroy plant tissue, or damage the seed.

Some of them are general feeders on many species

of plants, whereas others largely restrict their

feeding to a few species of legumes. A few of

'Prepared by B. A. App, entomologist, Entomology Research Division.
20 AGRICULTURE HANDBOOK 314, U.S. DEPT. OF AGRICULTURE

these pests attack almost any part of the plant

above ground, but others attack the roots, and

still others confine their feeding to the ñowers or

seed. The successful clover grower must learn to

recognize and control destructive insect pests. A

brief description of the more important species

follows.

Grasshoppers

Many species of grasshoppers attack many

kinds of plants and may sometimes damage white

clover by feeding on the leaves and stems and often

on the flowers and seed.

Clover Leaf Weevil

Larvae of the clover leaf weevil {Hypera jmnctata (F.)) often cause considerable loss of white

clover leaves early in the spring. When full

grown, these larvae are about one-half inch long

and green, with a white or pinkish line down the

center of the back. The adult is about one-fourth

inch long and one-eighth inch wide. It is covered

with brown, yellow, and gray scales, which give it

a mottled appearance. Fortunately the larvae are

attacked by a fungus disease if humidity is sufficiently high to favor spore germination and infection. Control methods are not usually needed.

Potato Leafhopper

Feeding by the potato leafhopper {Empoasca

fabae (Hams) ) causes a reddemng bronzing and

browning of the clover leaves and may stunt the

growth. The adult leafhoppers are pale-green

wedge-shaped insects about one-eighth mch long.

Eggs are deposited within the stems and larger

leaf veins. The eggs hatch into small whitish

nymphs, which later become pale green. Several

generations are produced each year. This insect

overwinters only in the extreme Southern United

States and migrates north each season. When

plant growth is retarded by unfavorable weather,

leafhopper damage may become severe.

Meadow Spittlebug

Almost all damage by the meadow spittlebug

(Philaenus spumarius (L.) ) is done by the nymphs

sucking the plant juices. The eggs are laid in the

fall and hatch the following spring into nymphs,

which are protected by a mass of spittle. Their

feeding stunts the clover plants and often causes a

rosetting of the terminal growth. From one to

several nymphs may inhabit a single spittle mass.

There is only one generation a year.

Garden Fleahopper

The garden fleahopper {Halticus hracteatus

(Say)) sucks plant juices and causes small discolored areas on the clover leaves, which give the

plant a mottled appearance. Heavy feeding may

result in death of the leaves.

Spider Mites

Spider mites of several species often damage

white clover. They feed on the underside of leaves

and spin delicate webs. Infested leaves have yellow blotches, which range in size from small specks

to large areas. In the Western States, particularly in areas under irrigation, these pests are very

serious when the crop is grown for seed.

Clover Root Curculio

The clover root curculio {Sitona hispidula (F.) )

is probably the most economicalljr important insect that infests the roots of white clover. The

adults are small brownish-gray snout beetles about

three-sixteenths inch long. They cut small circular notches in the leaves. The most significant

damage is caused by the larvae gouging the roots.

Larvae are about three-sixteenths inch long with

yellowish-brown heads. There is only one generation a year. The eggs are laid in the fall, and

the young larvae overwinter in the soil.

Green June Beetle

Larvae of the green June beetle {Cotinis nítida

(L.) ) damage white clover by uprooting the plants

as they burrow through the soil feeding on dead

organic matter. They measure nearly 2 inches

long when full grown. Generally they are glossy

white and have a rather small brown head. A

peculiarity of the larvae is that, although they have

legs, they travel on their backs by means of stiff

bristles. The adult beetles are velvety green with

margins of orange yellow and are about threefourths inch long. There is only one generation a

year. Adults are active from June to September

and lay eggs in August and September. In 10 to

15 days the eggs hatch into grubs, which pass the

winter in the soil and become full grown by the

following May.

Lesser Clover Leaf Weevil

Both adults and larvae of the lesser clover leaf

weevil {Hypera nigrirostris (F.)) feed on clover,

but most of the damage is done by the

larvae. They feed in the axil of the leaves,

terminal buds, and developing heads. In severe

infestations many terminal and side buds may
WHITE CLOVER 21

be destroyed. The adults are small snout beetles

about one-eighth inch long with iridescent,

greenish, scalelike hairs. The larvae are legless,

grayish grubs about three-sixteenths inch long

with brownish-black heads. There is a single generation annually. The adults overwinter in trash

and debris on the ground. Eggs are laid in early

spring, and most of the larvae are full grown by

mid-June.

Clover Head Weevil

The clover head weevil {Hypera mêles (F.) ) is

closely related to the lesser clover leaf weevil. The

larvae usually occur in the clover heads, where

they feed on the florets and developing seed.

Their feeding causes small misshapen flower heads

and reduced seed fjroduction. These two species

are largely responsible for the poor seed yields of

^white clover in the South. The adult is brown,

about three-sixteenths inch long, with three longitudinal whitish stripes. The full-grown larva is

legless, about five-sixteenths inch long, with a

rusty-brown head. Generally it is pearljr white

or grayish with faint-brown or bluish longitudinal

stripes. The insect overwinters as an adult and

lays its eggs during the spring in slits in the leaf

petioles or stems. Larvae become full grown

sometime in midsummer. There is a single generation a year, but some evidence of at least a

partial second generation has been observed.

Clover Seed Weevil

The clover seed weevil {Miccotrogus picirostris

(F.) ) was first noted in the United States in the

State of Washington in 1929, and has since been

found over much of the clover-growing area. Its

preferred hosts are white and alsike clovers. The

adults are small gray weevils about one-tenth inch

long. They feed at the base of the calyx and lay

their eggs in punctures on the calyx. The larvae

feed on the developing seeds, and each larva may

destroy from two to fourseeds. When full grown,

the larvae are small, plump, white grubs about

one-eighth inch long with light-brown heads and

no legs. Winter is passed as an adult. Egg laying occurs in June and July. There is one full

generation and possibly a partial second generation

each year.

Ladino Clover Seed Midge

The Ladino clover seed midge {Dasineura

gentneri Pritchard) is a pest of white and alsike

clovers in Oregon and Washington. The larvae

overwinter in tiny cocoons on or slightly below the

soil surface. They are white, changing to yellowish pink as they mature. The larvae feed on the

flower parts and prevent seed formation. Adults

emerge in the spring and lay eggs in the immature

flowers. They are small mosquitolike flies. Several generations a year are produced.

Other Damaging Species

Several other insects may sometimes damage

white clover. Lygus bugs {Lygus spp.) and

other plant bugs attack the buds, blossoms, and

seeds and may lower seed yields. The clover

aphid {Nearctaphis iakeri (Cowen)) may sometimes be abundant. The green cloverworm

{Plathypena scabra (F.)) is often found on the

foliage. Cutworms may cause some injury at

times. White clover, particularly that grown in

small plots, is occasionally damaged by other insects not normally considered pests of this crop.

Some of these are the lesser cornstalk borer {Elasmopalpus lignosellus (Zeller) ), the alfalfa weevil

{Hypera postica (Gyllenhal)), blister beetles

{Epicauta spp.), and the fall armyworm {Spodoptera frugiperda (J. E. Smith) ).

Control

Many of the more serious pests of w^hite clover

can be controlled with insecticides. Eecommendations are constantly being improved, and county

agricultural agents and State agricultural experiment stations should be consulted for the latest

recommendations for their areas.

REQUIREMENTS FOR GROWTH AND SOIL ADAPTATION

White clover grows in every State. Thick productive stands depend on the use of adapted varieties, proper amounts and kinds of fertilizers and

limestone, and good grazing and clipping management. Successful inoculation, favorable weather,

and correct planting with the associated grass on

an adapted soil also are essential (i).

Soil Adaptation

White clover is best adapted to soils ranging

from clays to silty loams. It will grow in lowlying sandy soils that have a high water table, but

it is not recommended for soils that are droughty,

swampy, or low in fertility. It is not tolerant of
22 AGRICULTITRE HANDBOOK 314, U.S. DEPT. OF AGRICULTURE

saline soils nor does it thrive on high alkaline

soils. Highly alkaline soils have been made suitable for the growth of white clover by applications

of relatively large amounts of superphosphate.

Annual applications are generally more effective

than correspondingly heavier applications once

every second or third year. The high phosphorus

requirement for growth of white clover on these

soils is probably explained by the high phosphorus

fixing capacity of the soils and the crop's requirement for a continuous, readily available source of

phosphorus.

Fertilizers and Limestone

Available supplies of phosphorus, potassium,

and calcium are essential to stand establishment

and good growth (<^Ö, p). Phosphorus and potassium are supplied in various formulations of

commercial fertilizers. Calcium is supplied as

agricultural ground limestone. On certain soils,

particularly those of a sandy texture, magnesium

and sulfur may be deficient. The use of dolomitic

limestone will supply both calcium and the magnesium. The use of superphosphate as the source

of phosphorus will supply the sulfur. In a few

areas, some of the minor elements must be applied

for the highest yields of forage and seed. Of the

major minerals, the amounts needed will generally

range from 300 to 600 pounds of 20-percent superphosphate and from 60 to 250 pounds of muriate

of potash per acre or their equivalents.

In comparison with other clover species, white

clover needs more potassium {22). It frequently

responds to greater amounts of potassium than are

customarily applied {29^ SO). Finely ground

limestone at rates of 1 to 4 tons per acre is generally applied when needed.

The county agent. Soil Conservation Service

technician, or State agricultural experiment station should be consulted on kinds of minerals and

amounts needed, since these vary widely depending on the natural fertility of the soil and previous

fertilizer treatments. These agencies normally

base their recommendations upon the results of

soil tests.

Inoculation

Although white clover will grow in slightly to

medium acid soils, higher yields, longer lived

stands, and better inoculation are obtained if the

pH is from 6 to 7. The symbiotic relationship

of nitrogen-fixing bacteria and the clover is most

effective at a pH near 6.5 and enables the clover

plants to utilize atmospheric nitrogen. Wellinoculated plants are not benefited by the application of nitrogenous fertilizers. Normally the

application of nitrogen fertilizer complicates

management of a clover-grass pasture and does not

cause economical increased production of forage

{91^, 108).

In the North, where white clover has recently

grown, bacteria generally are present in the soil

to bring about good inoculation. However, reduced production is so great in the absence of good

inoculation and the cost of inoculating so small

that it is wise to inoculate whenever a new seeding is made. Also, there may be another advantage in inoculating the seed regardless of the

presence of bacteria in the soil. Laboratories of

educational and research agencies and most commercial companies manufacturing cultures are

continually working to isolate and culture the

most efficient strains of bacteria. These superior

cultures increase the amount of nitrogen fixed.

In the South, it is even more important to practice

inoculation at every seeding, because high soil temperatures over several months may seriously reduce the number of bacteria at or near the soil

surface.

Climate

White clover grows best during cool, moist

weather. Good growth occurs at 50° to 85° F.

{15^ 88j 96). White clover is seldom found above

timberline in the western mountains, and the

authors believe that the low summer temperatures

are the limiting factors rather than low winter

temperatures. On the other hand, high summer

temperatures with drought conditions or with

high humidity are unfavorable for persistence of

stands. Interesting temperature relations occur

in both Hawaii and Alaska. In Hawaii, white

clover does not grow under tropical conditions at

low altitudes, but on adjacent mountains at elevations above 3,000 feet it thrives under the more

temperate conditions {27). In Alaska, it grows

near Anchorage, Palmer, and Fairbanks at low

winter temperatures associated with long periods

of winter dormancy.

A readily available supply of moisture in the

top 6 inches of soil is essential for good growth

of white clover. When grown under irrigation,

frequent applications of water are needed, but the

amount applied at any one time may be less than

is used with deeper rooted crops. Wilting of

white clover is not a sure sign of deficient soil

moisture. In the hot inland valleys of the Westem States, white clover frequently wilts in the

afternoon when the temperature is high and the

humidity is low even though the soil is moist.
WHITE CLOVER 23

PRODUCTION PRACTICES

Management in the North

Practically all the white clover seeded for pasture, hay, or cover in the Northern and Western

States is the Ladino variety or others of the large

type. However, many pastures that are unimproved or that are used for night pasture with close

continuous grazing are of Common White. Keference here will be to Ladino and other improved

varieties of the large type unless otherwise stated.

This does not mean that Common White is without

importance, for without this clover most unimproved pastures would be nothing but exercise

areas for animals. Fertilizer and lime requirements are somewhat less for Common White than

for the varieties of the larger type, since it is less

productive. Seeding rates and mixtures are approximately the same.

Seedbed Preparation and Seeding

Lime and fertilizers may be applied before seedbed preparation or, better yet, worked into the soil

surface of the seedbed. The seedbed should be

very firm, smooth, and free of weeds. This may

be accomplished by the use of a corrugated roller

following the usual plowing, disking, and harrowing operations. A loose, cloddy seedbed and a deficiency of required minerals are probably the most

frequent reasons for failure to establish stands.

The seed may be broadcast on the surface,

drilled, or planted by a combination corrugated

roller-seeder. Since white clover seeds are very

small, they must be planted at or near the soil surface not more than one-eighth inch deep, preferably less on most soils. Depending on the mixture used or if seeded alone, the rate of seeding

varies from one-half to 5 pounds per acre, respectively. The seed should be inoculated with a white

clover culture before planting by following instructions given on the container.

Mixtures

Except for swine and poultry pastures and seed

crops, which are seeded alone, most white clover is

seeded with one or more grasses—orchard, timothy, brome, tall fescue—and the legumes red

clover and alfalfa. Kentucky bluegrass should be

avoided where it is highly competitive with white

clover. Strong competition makes maintenance of

clover stands difficult. Of first importance in the

establishment and particularly the maintenance of

good stands of clover is a light rate of seeding for

the grasses. This is somewhat contrary to former

practice.

Eecent research has indicated that the seedling

taproot of white clover does not persist much

longer than a year and that elaborated metabolites

and absorbed minerals are mainly translocated to

newly developing meristematic tissue, that is, toward the stolon tips and nodal roots. The authors

believe that the older roots and stolons become

senescent, fail to grow, and are subject to attack by

many pests, particularly viruses and soilborne

fungi, which lead to their deterioration and death.

The new developing plants must compete with the

older established plants in the association, and the

survival of a stand may be jeopardized by even one

unfavorable environmental factor. This competition is suggested as the principal reason for early

stand losses. Additional research to determine the

effect of grass populations on white clover development and survival is needed. Until more is

known, thick stands of grass should be avoided

(63).

Time of Seeding

Spring seeding may be made alone or with a

companion grain crop and should be done early.

Late-spring seedings are less successful. Frequently the companion grain is seeded at one-half

the normal rate to reduce its competition for light,

moisture, and nutrients. If the legume-grass

seeding is of first importance, the grain crop may

be grazed, starting when it is approximately 12

inches high, or cut for hay or silage when the heads

are in the soft dough stage. It is not advisable

to practice close grazing or let animals trample the

young seedlings when the soil is wet. The young

clover may be grazed when growth permits and the

weeds clipped or sprayed with a specific herbicide

for control. If the grain crop is harvested for

grain, the stubble should be clipped and all straw

removed as soon as possible after harvest.

July and August seedings are successful, particularly under humid, cool conditions. They

have the advantage of coming at a time when there

is less competition with other farm operations and

more time can be spent preparing a good seedbed.

The seeding must be made suiRciently early for the

plants to become well established before the advent

of winter. Small plants are readily heaved out of

the soil by freezing and thawing.

Defoliation Practices

One of the excellent characteristics of white

clover is its remarkable ability to make rapid regrowth after defoliation (62). This characteristic

enables the plants to tolerate the competition of as-
24 AGRICULTURE HANDBOOK 314, U.S. DEPT. OF AGRICULTURE

sociated plants. Depending on the degree of

favorable environmental factors and the associated

plants, the forage may be either grazed or cut at

mtervals of 15 to 30 days during the spring and

summer.

White clover also has been used in green feedmg

practices. Unfortunately the heavy machinery

necessary for this method of utilization severely injures the plants, providing an entrance for diseases. This leads to early loss of stands.

Where winter conditions are severe, white clover

should not be closely grazed or frequently cut in

the fall, as such a practice favors winter injury

{101), Many successful growers follow the practice of topdressing white clover with barnyard

manure soon after the soil surface becomes frozen

in late fall.

When white clover is grown in association with

other legumes or grasses for hay or silage, it is

highly desirable to harvest the crop early even

though some yield is sacrificed. Delayed harvesting favors the development of leaf diseases, reduces light reaching the clover, and consequently

is detrimental to rapid recovery and growth of the

clover. Regrowth of clover is dependent on the

amount of functional leaf surface that is left after

grazing or cutting. For this reason, the forage

should not be grazed or cut shorter than 2 inches

{29), Rotational grazing is more favorable for

clover than continuous grazing, for under continuous grazing some areas are grazed too closely

and forage is not adequately utilized from other

areas. White clover has been successfully used

under daily ration grazing, although care should

be exercised to avoid excessive trampling on the

fleshy stolons.

Renovation of Pastures

If an established stand of white clover is lost,

it may be reestablished in either of two ways. If

there is a thick stand of grass, it must be killed

either by cultivation or by the use of selective

herbicides. Depending on the kind of surviving

grass and the weather, it majr be necessary to plow

or disk and harrow several times. After the grass

is killed, it is necessary to prepare a very firm seedbed as previously described.

Where the clover has set good quantities of seed

during the years before the loss of the stand and

where the associated grass has not formed a thick

turf, volunteer clover stands can be established

under favorable climatic conditions by continuous

close cutting or grazing of the grass, or by opening

the turf by disking. Until the young seedlings

are established, the intensity of defoliation should

be modified to favor their survival. This method

has been very successful when timothy is the associated grass.

If the seeding is to be primarily used for grazing, alfalfa is not a good companion legume in

the mixture, as it will not survive under the frequent defoliation that produces maximum yields

of the clover.

Ratio of Clover to Grasses

Maintaining a desirable ratio of clover to grass

is both challenging and rewarding (107), A

sward of 50- to 70-percent clover is considered

good by beef cattle producers and dairymen. In

general, dairymen prefer higher percentages of

clover. Most poultry and swme producers prefer

all or almost all clover. The ratio will vary with

seasons and years, since all changes in growing

conditions affect the ratio. Management practices can be followed to prevent an excessive shift

as a result of climatic conditions.

Manasement in the South

White clover is used primarily as a pasture crop

throughout the southern region. The varieties

used range from predominantly Louisiana White

or other improved varieties that produce good seed

crops in the Gulf Coast area to predominantly

Ladino near the northern boundary of the region.

In general, the varieties that flower profusely and

consequently produce considerable seed are used

in areas and on soils where maintenance of the

clover stand depends on volunteer plants (18, ^1),

Ladino is used where the clover normally persists

more than one winter. In Louisiana and adjoining States a considerable acreage is managed for

both seed production and grazing. A small acreage in Texas, Louisiana, and Arkansas is used in

rotation with rice. Information concerning these

and other special uses can be obtained from local

agricultural agencies. This discussion will be

limited to its use in clover-grass pastures.

Seedbed Preparation and Planting

Methods of preparing and the characteristics of

the seedbed are the same as those described for the

North. Early and thorough preparation that results in accumulation of soil moisture and the formation of a firm seedbed prior to planting favors

success in establishing stands. Most States recommend a seeding rate of 2 to 3 pounds per acre

and double this amount if seeded alone for poultry, swine, or seed production. Seed should be inoculated before planting. The seed may be broadcast or drilled. The equipment used for planting

should assure shallow coverage—not more than

one-eighth inch, preferably less on most soils.

The rate and method of planting the grass

should assure that space is available for growth

of the clover (fig., 11). Drilling grass in adequately spaced rows is one method of providing

sufficient space for the clover.
WHITE CLOVER 25

FIGURE 11.—Good stand of white clover has persisted in

this pasture for 3 years. Planting tall fescue in rows

18 inches apart helped provide space for clover. ( Courtesy Clemson University. )

Associated Grasses

White clover should be seeded with a grass to

make its greatest contribution to supplying forage

for livestock. The grasses most frequently used

in the South are orchard, tall fescue, Kentucky

blue, dallis, and bermuda. Other grasses used are

varieties of bahia, pangóla, Johnson, and carpet.

The thick turf by Pensacola and Paraguay varieties of bahia, bermuda, carpet, and St. Augustine grasses is not compatible with white clover.

On the other hand, the open growth of dallis and

common bahia grasses permits white clover to

thrive. Since these grasses are aggressive, under

conditions favorable to the grass and unfavorable

to the clover, the association may shift to all grass.

Intelligent management is essential to maintain

a desirable ratio of clover to grass. Periodic renovation or, better yet, the practice of rotation with

a cultivated crop usually results in higher production of forage and a better ratio of clover to grass.

Also the inclusion of a nonclover crop for 1 or

more years after several years of clover should reduce damage from clover diseases and aid in the

control of troublesome weeds.

Time of Seeding

The optimum planting date varies with the location, soil type, and soil moisture. Late-summer

and early-fall seedings are generally preferred,

since rapid root development occurs during cool,

moist periods and thus favors survival the following summer. In areas where freezes occur in November and particularly on soils that heave badly,

seedings should be made the last part of August or

early September. Where the winter is milder and

heaving is less important, seedings made as late as

December may be successful. Spring seedings are

hazardous; however, on moist soil with a low weed

population, spring seedings often are successfully made.

Utilization

White clover may be grazed when the sward

reaches a height of 3 to 4 inches. If temperature

and moisture are favorable, this will occur 2 to 4

months after seeding. Light grazing at first is

recommended to permit good establishment and

the development of stolons. Thereafter, the grazing may be more intense.

The height of the associated grasses must be kept

relatively short for the best development of the

clover. If growth is more rapid than can be

utilized by grazing, the excess forage can be made

into either hay or silage. An occasional clipping

of the pasture to make the growth more uniform

and to control weeds is desirable.

Animal droppings should be scattered to obtain

uniform growth of the pasture, except where a

seed supply for fall volunteering is needed. The

undisturbed droppings make seed islands, which

are valuable for reseeding.

Forage should be utilized soon after it is produced. This may be done by rotational grazing or

by controlled continuous grazing during the growing season. If continuous grazing is used, stocking should be controlled so that the height of forage is maintained at 2 to 6 inches. On heavy soils,

grazing during wet periods should be avoided if

practical. Accumulating large amounts of forage

to be used during hot, dry periods or during cold

weather is hazardous. Clover leaves that touch

the ground because of wilting or other causes are

soon lost. Leaves on the ground favor the development of diseases that may destroy the stand.

Undergrazing more often than overgrazing

properly fertilized pastures results in the loss of

clover.

Fall Management

Fall management may profoundly affect the

growth of white clover the following year. Stolons formed in the fall often become strong growth

centers by the next spring. Growth at low temperatures is characterized by short internodes,

profuse branching, and extensive root development (96). These characteristics contribute to

forage production and persistence of stands the

following spring and summer. To favor high

production the following year, the management

practices employed in the fall should provide the

conditions necessary for growth and rooting of

new stolons.
26 AGRICULTUHE HANDBOOK 314, U.S. DEPT. OF AGRICULTURE

Practices Affectins Botanical Composition

oí Clover-Grass Association

The following list is a summary of practices or

conditions that affect the botanical composition of

a clover-grass association.

Initial population:

(1) Seeding rates,—The number of viable seed

per acre of each crop planted obviously determines the initial balance. Avoid high seeding

rates of grass, as the resulting stand will crowd

out the clover.

(2) Time of seeding,—On clay soils, delaying

the fall seeding date tends to increase the loss of

clover seedlings from heaving.

(3) Nitrogen fertilizer.—Applying nitrogen

fertilizer favors the establishment of the grass

seedlings and retards the clover.

(4) Covering seed,—Clover seed should barely

be covered. Grass seed will push through from

greater depths.

( 5 ) Seedhed,—It must be firm. This can be accomplished by rolling before and after seeding.

Practices tending to shift plant population

toward grass:

(1) Applications of nitrogen fertilizer.

(2) Undergrazing, thus permitting grass to

grow above the clover.

(3) Lack of lime, phosphorus, and potassium.

(4) Permitting grass to mature seed.

Practices tending to shift plant population

toward clover:

(1) Adequate utilization so that grass does not

grow tall. It may be necessary to harvest forage

for hay or silage to prevent excessive accumulation

of growth.

(2) Withholding nitrogen fertilizers.

(3) Adequate applications of lime, phosphorus,

and potassium.

(4) Mowing to control weeds and to remove

islands of undergrazed tall grass.

Practices aiding in establishment of volunteer

clover stands on existing summer perennial

(1) Eemoval in fall of accumulated grass by

close grazing, harvesting, disking, or burning.

(2) Ffîll application of lime, phosphorus, and

potassiu

Seed Production

Seed production of white clover often is outside

the area of utilization, since the environment favoring profuse flowering and seed harvesting may

differ considerably from that of the area where

the variety is used for forage. Naturalized varieties and ecotypes usually flower profusely in

areas where the photoperiod is as long as or longer

than that of the place of origin {98^ 102). Sparseflowering varieties require longer photoperiods for

high seed yields.

The practice of producing seed of a variety outside its area of adaptation or use frequently results in a genetic shift (^^). This is particularly

important in a synthetic variety composed of

clones having different requirements for flowering

{98),

An abundance of bees for cross-pollination and

dry, warm weather are necessary for high seed

yields. Bees work white clover flowers for both

nectar and pollen. The honey bee is by far the

most important species for cross-pollination {23^

64), The honey bee population can be controlled

readily by moving colonies in and out of seed fields.

Prolonged cloudy, rainy weather reduces flowering

and retards bee visits. For this reason, most of

the white clover seed is produced in the Western

States, where clear, bright weather prevails. Such

weather favors flowering and the development of

large plump seed, facilitates harvesting, and reduces harvesting losses (86).

Seed yields may vary from 30 to 200 pounds per

acre under humid eastern conditions, whereas in

the West, yields range from 150 to more than 600

pounds per acre depending on the variety and

environmental conditions.

There is some evidence that Ladino and other varieties of the large type produce fewer flowers and

less nectar per floret than the intermediate- or

small-type varieties. This makes them less attractive to the bees and adversely affects seed production (23),

The production of large yields of high-quality

seed is specialty agriculture. Important factors

are control of destructive insects, cautious use of

insecticides to protect pollinating insects, and wise

use of selective herbicides and defoliants (fig. 12).

Timeliness is a prime necessity in seed production.

Since white clover is indeterminate in flowering,

ripe seed and florets in full bloom may be seen on

the same plant. The seed crop should be harvested

when most seed heads are a light brown. This

condition usually occurs from 25 to 30 days after

full bloom. In western areas under irrigation,

where weeds and destructive insects have been controlled on the first crop, a second seed crop may be

produced and both crops harvested in one operation. The seed crop is cut with a mower and is

cured either in the swath or in small windrows

and is picked up out of the swath or windrow with

combines equipped with pickup attachments.

Careful operation of the combine, both in the rate

of forward speed and in the adjustment of the

cylinder speed and of the concaves and screens, is

important for good harvesting with the least loss.

During showery weather, curing in the swath facilitates combining, whereas during clear, warm

weather, windrowing reduces harvesting losses.

The cut crop should be handled as few times as
WHITE CLOVER 27

ifc' *w

FIGURE 12.—Seed field of Ladino clover In Northwest, showing profuse flowering and growth with irrigation.

Obtaining good seed yields includes wise use of Irrigation water, pollinating insects, fertilizers, insecticides,

selective herbicides, defoliants, and seed-harvesting equipment.

necessary, since each handling causes shattering

losses. Under humid conditions, artificial drying

or spreading the seed thinly under cover for complete drying improves the seed quality. If the

latter method is followed, the seed should be turned

every few days until dry. Rough cleaning both

before and after drying increases the rate of drying and improves the quality.

Defoliation of the seed crop in humid States has

not proved to be a dependable practice, because

rainy, damp weather may occur after defoliation

and before the crop is combined. Under the dry,

bright conditions of the West, defoliation is successfully followed. Suction harvesters are widely

used in the West both on defoliated fields and

where the plants have been windrowed.

RESPONSES OF WHITE CLOVER TO CLIMATIC FACTORS

Environmental factors profoundly affect the

chemical composition and growth of white clover.

HCN (hydrocyanic acid) content was found to

decrease with increases in both latitude and altitude {37-40). In one experiment, plants were

grown in controlled-environment chambers under

different temperatures. The percent of maltose

was highest at low temperature (50° F.) and decreased with increasing temperature; at 86°, maltose was present only in trace amounts. In con-
28 AGRICULTURE HANDBOOK 314, U.S. DEPT. OF AGRICULTURE

trast, sucrose concentrations were as high in plants

at high temperatures as in plants at 50° (16).

Wide variation exists in growth response among

different genotypes under various temperatures.

One clone failed to grow at 50° F., whereas others

made considerable growth. This same clone persisted under extremely high field temperatures,

whereas others died. Apparently white clover

makes greater growth between 60° and 75°. Stolon branching is greater atlower temperatures and

high light intensity (15), In the field, the

branches are important in the rapid spring production of. forage.

At high temperatures, white clover rapidly disappears if grown in association with a thick stand

of grass. Although it will persist longer at lower

temperatures, little growth is made. In plantgrowth chambers, approximately the same amount

of dry matter per unit of leaf area was produced

under light conditions of 600, 900, and 2,000 footcandles. The higher light intensity increased leaf

production and plant weight but decreased the

functional period of the leaf (15). It is apparent

that profuse flowering and seed setting are directly

associated with increases in light intensity and day

length. High light intensity is probably the principal reason for the high yields and large seed

produced in the Western States.

Most literature references on white clover indicate that winter hardiness is important in adaptation under low winter temperatures. Although it

is true that field differences in survival are evident

in the spring, the reasons for these differences are

usually obscure (95), The authors believe that

the interactions among physiological, pathological,

agronomic, and climatic factors causing stand

losses in the spring after severe winters have a

greater effect on survival than the direct effect of

low temperatures. Predisposing factors in the

fall show their effects in early spring. Ladino,

considered by some to lack winter hardiness, has

survived under extreme temperatures ranging

from —40° to 40° F. during the winter, even with

little snow cover.

NUTRITIVE VALUE AND CHEMICAL COMPOSITION

White clover is probably the most nutritious

legume for livestock and poultry, either in the

green state or as hay ( 73^ 85^ 106). Only the leaves

are grazed or harvested, and they are the most

nutritious part of the plant. For this reason,

yields are relatively low. Under close grazing

some animals and poultry feed on the stolons.

This is harmful to the stand and reduces the rapid

regrowth of the plants. White clover is a qualityconcentrated feed if utilized before the leaves have

aged. It is very palatable, although cattle and

sheep grazing on practically pure stands of lush

white clover have been observed searching for and

eating coarse, fibrous plants or plant parts.

Since nutritive value depends on both chemical

composition and digestibility, numerous analyses

and digestion tests of white clover have been made.

The chemical composition of the leaves varies depending on their age, cultural practices, and environmental conditions; however, there is less variability when compared with other forages whose

stems are included {110), Samples have assayed

over 30 percent crude protein.

Compared with other forages, white clover rates

high in amino acids. Loper et al. {80) reported

that at the one-tenth bloom stage white clover generally contained a higher percentage of each of

16 amino acids than birdsfoot trefoil, alfalfa, or

red clover. Their analyses of the spring growth of

Ladino clover and alfalfa in 1959 and 1960 are

given in table 2.

The total sugar content has been found to be

over 12 percent. The phosphorus, calcium, and

potassium contents are over 0.35, 1.50, and 1.50

percent, respectively {86). Vitamins of the B

TABLE 2.—Dry-weight concentration oj amino adds

in Ladino clover and aljalja

Amino acid

Ladino clover Alfalfa

1959 1960 1959 1960

Alanine

Percent

L24

.86

2.04

2. 19

1.25

.43

1.05

1.88

.54

. 37

1.21

1.05

.94

1.03

.85

1.29

22.63

Percent

1. 15

L27

3.00

1.65

1.06

.58

1. 15

1.77

1. 54

.29

1. 14

.98

1.00

1.18

.85

.56

24.88

Percent

0.80

.71

L43

L43

. 75

.34

.67

L 15

.72

.20

.80

.75

.70

.74

.50

.93

16.31

Percent

0.80

Arginine .86

Aspartic acid

Glutamic acid

Glycine

2. 27

1.42

.78

Histidine .41

Isoleucine .72

Leucine 1.25

Lysine L09

Methionine

Phenylalanine

Proline _

.23

.83

.75

Serine _ _ _ .78

Threonine

Tyrosine__

.83

.58

Valine _ _ LOI

Crude protein 19. 50

complex, C, E, K, and provitamin A are present in

fresh white clover in sufficient quantities to supply

animal requirements. Trace elements are usually

present in sufficient amounts, particularly when the

plants are grown in soils where these are not deficient. Otherwise, these elements should be added as supplements in the feed.

Coumestrol, a natural estrogen, was first isolated

from white clover {20), Variety, stage of maturity, and environment appear to affect the quantity. Coumestrol of white clover may be a quality
WHITE CLOVEE 29

factor in feed for certain kinds of livestock and

poultry. It may be more than of academic interest

to note the similarity in molecular structure of

coumestrol and diethylstilbestrol and their beneficial effect on milk and meat production. Coumestrol in excess may also be objectionable, causing

infertility in animals.

The presence of HCN (hydrocyanic acid) in

white clover has been known for many years (87).

The plants contain the cyanogenetic glucosides

lotaustralin and linamarin and the hydrolyzing

enzyme linamarase resulting in the formation of

HCN (SS, 36). The amount of HCN varies

widely depending on genotype, stage of maturity,

and environmental factors. Even with a relatively high concentration in the white clover plant,

HCN has not proved to be toxic to animals or

poultry. Ladino clover is free of or low in HCN,

whereas New Zealand white clover is high. The

presence of HCN in New Zealand white clover

appears to be the reason for its resistance to slugs

or snails, which greatly injure Ladino. The presence of these substances may be the reason for the

reported salivation of horses grazing white clover,

although no evidence has been presented on this

point.

Saponins and sapogenins are present in white

clover. The number and their identity are not

known. At one time they were considered to be the

principal cause of bloat in ruminating animals

(^). Further investigations have not substantiated the earlier conclusion. They may affect the

incidence of bloat or they may not be associated

with it.

BLOAT

White clover is one of many legumes that cause

bloat of cattle and sheep (^). Lush forage appears to be more incitive than older leaves. How^-

ever. Ladino clover pastures under irrigation may

cause bloat at any time. The causes of bloat are

not known. Pasture bloat and dry-lot bloat are

two kinds. Because of its high nutritive value,

white clover is a concentrated feed and may induce

bloat when consumed in large amounts. Many

livestock producers believe that a mixture of 50

percent clover and 50 percent grass is the solution.

Such a mixture may be less likely to bloat animals,

but the authors have seen animals preferentially

graze the clover fraction of the mixture with

disastrous results.

Many livestock producers claim a reduction in

the bloat hazard by keeping racks of dry straw

or coarse grass hay in every field readily available

to the grazing animals. Antibiotics, antifoaming

agents, and different methods of grazing have been

tried with variable results. Precautions should

be taken to reduce the incidence of bloat and provision made to cope with it when it occurs. The

recommendations of the State agricultural experiment stations on the use of antibiotics and antifoaming agents and on the feeding of oils and fats

to control bloat should be followed.

The following practices may be helpful in

reducing the incidence of bloat:

(1) Feed animals straw or grass hay before

turning them out on lush clover pasture.

(2) Keep racks of dry straw or grass hay

readily available in every pasture.

(3) Plant mixtures of grass and clover.

Choose a grass that has the same season of growth

as the clover.

(4) Keep grass highly palatable by frequent

clipping if animals preferentially graze clover.

(5) Use intensive grazing to prevent the accumulation of excessive amounts of lush forage.
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