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it is the leading legume in forage production, and it maintains a significant

position in forage production in the United States. Red clover is adapted to a

wide range of soil types, pH levels, and environmental conditions. This plasticity

has enabled red clover to retain its usefulness for hay, silage, pasture, and soil

improvement in much of the temperate region of the world.


The overall economic importance of red clover to world agriculture is difficult

to assess. Red clover is consumed by animals in the form of hay, silage, or

pasture fodder, and is marketed to the consumer in the form of meat, fiber, and

dairy products. Its introduction to central and northern Europe and the United

States had a profound impact on civilization and agriculture by providing a

stabilized supply of feed to livestock (Piper, 1924; Gras, 1940; Fergus and

Hollowell, 1960).

Red clover is generally grown with timothy, Phleumprutense L. In the United

States, red clover is grown for hay, silage, pasture, and soil improvement on

about 5.4 million hectares. According to United States Department of Agriculture Crop Production and Agricultural Statistics Reports, the annual yield is

approximately 21.5 million metric tons, or about 4.0 metric tons per hectare.

Maximum yields using improved varieties should easily be twice these values.

The average value received in 1976 per metric ton of hay was $66.44. Therefore,

the economic worth of red clover is approximately 1.4billion dollars annually, or

$265 per hectare. This does not consider the value of the red clover as pasture.

The value of the annual red clover seed crop in the United States is between 23

and 27 million dollars.

Red clover is a nitrogen-fixing legume, thus contributing to the supply of

nitrogen available in the soil for subsequent crops. It is estimated that red clover

provides between 125 and 200 kg of nitrogen per hectare (Rohweder et ul.,



Red clover is thought to have originated in southeastern Europe and Asia

Minor. Local ecotypes probably developed over 2000 years ago, but documentation is limited. A review of the earliest references to the utilization of red clover

as forage is provided by Fergus and Hollowell (1960) and Pieters and Hollowell

(1937). Red clover was probably introduced to northern Europe during the fifteenth century A.D. and into North America in the late seventeenth or early

eighteenth century A.D. Today, it is an important forage legume throughout the



temperate regions of Europe, the Soviet Union, Australia, New Zealand, Argentina, Chile, Canada, Japan, Mexico, Columbia, southern Canada, and eastern

and central United States.

II. Taxonomy

Red clover is a species of the genus Trifolium, which contains 8 subgenera or

sections (Hossain, 1961). The subgenus Trifolium, containing about 70 species,

was divided into 17 subsections by Zohary (1971, 1972). Within T . pratense,

two main types of clover are grown in the United States: medium or double-cut,

and mammoth or single-cut. Mammoth usually flowers later, is taller, yields

more in the first growth, and produces less aftermath growth than medium.

Double-cut red clover, contrary to its name, may produce several flushes of

growth per year, depending on the length of the growing season. Most red clover

grown in the United States is of the medium type.


Red clover has a diploid chromosome number of 14 ( n = 7). Classified by

Zohary (1971) as closely related to red clover are species with base numbers of 8.

Trifolium difisum Ehrh. and T . pallidum Waldst. and Kit., two annual species,

have a diploid number of 16. A closely related perennial is T . noricum Wulf.,

which also has 16 diploid chromosomes. Somewhat more distantly related are T.

medium L. (zigzag clover, 2n = 64-80), T . alpestre L. (2n = 16), T . rubens L.

(2n = 16), and T . heldreichianum (h= 16). Another species sometimes considered to be a form of T . medium is T . sarosiense Hazsl. (2n = 48). Two other

annual species perhaps more distantly related are T . hirtum All. and T . cherleri

L. (2n = 10).


Probably the best evidence on the evolution of red clover is provided by

interspecific hybridization. Although many interspecific crosses have been attempted, the only verified hybrids involving red clover were obtained with the

diploid annuals T. difisum and T . pallidum. The diploid hybrid of T. pratense

x T . difisum was sterile, but the amphidiploid of 4x T . pratense x 4x T .

diffusum was fertile (Taylor et al., 1963). Pollination of 2n T . pratense with 2n

T . pallidum yielded only shriveled seeds, but the cross of 4x T. pratense x 2n T .

pallidum resulted in a sterile triploid hybrid (Armstrong and Cleveland, 1970).



Meiotic analyses of diploid and tetraploid hybrids of T. pratense X T . diffusum suggested that the chromosomes of the two species differ by a series of

complex structural interchanges. This is based on the presence of chain multivalents in the diploid and amphidiploid hybrids (Schwer and Cleveland, 1972a,b).

Analyses of chromosome pairing in the triploid hybrid of 4x T. pratense X 2x T .

pallidurn revealed mostly bivalents and univalents, but the presence of multivalents suggested some homology between the two species. These results apparently indicate that red clover is more closely related to annuals than to perennials,

and to T. difisum than to T . pallidum. Cytological analyses of interspecific

hybrids suggest that speciation has resulted from a complex series of structural

interchanges causing chromosome differentiation and eventual loss of one

chromosome pair in T. pratense.

Crosses among perennial species related to red clover have been reported by

Maizonnier (1972) and Quesenberry and Taylor (1976, 1977, 1978); they are

summarized in Table I. Observation at metaphase- 1 of diploid hybrids indicated

good pairing, with most PMCs having eight bivalents. Pollen stainability was

about 50%, indicating that the chromosomes of the diploid species differ by

small structural changes. Although crossability barriers among these species

were weak, barriers after fertilization were strong, ranging from embryo abortion

to poor germination and low F2 survival. Analysis of meiosis of polyploid hybrids also showed a low frequency of multivalents. Fertility of pollen from hybrids

was 70-85%, and no major barriers to hybridization were exhibited except for

partial chlorosis.


Interspeciflc Hybrids of Perennial Trifolium Species in Section Trifolium Zoh









Species and chromosome

number (2n)"

Hybrid chromosome

number (2n)



sar (48) x alp (16)

med (80) x sar (48) & rec.

med (64)x sar (48) & rec.

alp(16) x held(16)&rec.










alp (16) x rub (16)


med (72) x sar (48)


sar (48) x alp (32)














"Species abbreviations: sar = T . sarosiense Hazsl.; alp = T. alpesfre L.; med = T. medium L.;

held = T. heldreichianum Hausskn.; rub = T . rubens L.

Male and female fertility as measured by cross-pollination among hybrid plants.

'Refers to literature cited: (1) Maizonnier (1972); (2) Quesenberry and Taylor (1976); (3) Quesenberry and Taylor (1977); (4) Quesenberry and Taylor (1978).



Trifolium pratense produced no hybrids with the perennial species, indicating

that little gene exchange has occurred. Hybridization may have been involved in

the evolution of the perennial species, but the presence of F, sterility and the

absence of natural polyploids among the species most closely related to red

clover indicate that little hybridization has occurred in the more recent evolution

of T. pratense.

111. Reproduction


Red clover has the complete leguminous flower, consisting of calyx, corolla,

ten stamens, and a pistil. The calyx tube terminates in five lobes, or teeth. Five

petals unite at the base to form a corolla tube consisting of a standard petal, two

wing petals, and two keel petals. Nine stamens and the one stigma unite to form

the sexual column. The tenth anther is free. Up to 125 or more flowers are

situated on peduncles to form a single head or, in rare cases, multiple heads.

Flowers usually are reddish-pink but vary from white to deep reddish-purple.


Anthers of red clover consist of four microsporangia. Prior to dehiscence the

walls between the microsporangia break down, forming a common cavity on

each side of the anther. Dehiscence of the anther occurs before the petals have

reached their full size. Pollen fills the space between the fused keel petals and is

held in place until disturbed (Hindmarsh, 1963).

Transfer of pollen generally is by bees, but the flowers of red clover are so

large that honeybees (Apis mellifera L.) will avoid them if other nectar sources

are available. Honeybees may collect pollen, however. Bumblebees (Bombus

spp.) are efficient pollinators of red clover. Alkali bees (Nomia melanderi Ckll.)

and leaf cutter bees (Megachile rofundata [F.]) also have been used for red

clover pollination. When red clover is pollinated by a bee, the sexual column

protrudes from the interior of the flower, with the pistil extending slightly beyond

the stamens. When the weight of the bee is removed, the sexual column returns

to its original position. Each flower may be pollinated several times, as long as

the stigma remains receptive.


The stage of bloom for optimum seed set appears to be when flowers are about

half open. Stigma receptivity and pollen viability continue for about 10 days



under greenhouse and field conditions. In liquid nitrogen, pollen may be stored

up to 26 weeks without loss of viability (Engelke and Smith, 1974). The length

of time between pollination and fertilization of the egg cell in diploid red clover

is between 28 and 35 hours, and between 17 and 26 hours in tetraploids (Mackiewicz, 1965). Seeds are physiologically mature 14 days after pollination and

are dry enough for harvesting at about 21 days. Two ovules occur in each ovary,

but, except for some bred strains, one usually aborts (Van Bogaert, 1958; Schieblick, 1966; Bingefors and Quittenbaum, 1959). In rare cases, up to four ovules

per ovary occur (Povilaitis and Boyes, 1959).


Red clover is a self-incompatible cross-pollinated species that under natural

conditions produces very few self-seed. However, self-compatible, autogamous

genotypes have been found (Diachun and Henson, 1966). The mechanism of

self-incompatibility (reviewed by Fergus and Hollowell, 1960) is the one-locus,

gametophytic S-allele system in which plants with the same S-alleles are prevented from selfing by slow growth of pollen tubes through styles. The system

also prevents cross-fertilization of plants that have the same S-allele genotype.

Details of the S-allele systems in red clover have been investigated by inbreeding via pseudo-self-compatibility (PSC), a term used to indicate the production of

self-seed by normally self-incompatible genotypes. Leffel (1963) showed that

selfing heterozygous (S,S,) clones produced a 1- 1 ratio of homozygotes (SJ,

and/or SzS2) to heterozygotes (S,Sz). However, the progeny of one clone significantly deviated from this ratio, producing fewer homozygotes than expected.

Johnston et af. (1968) also found a deficiency of homozygotes in I, (selfed)

progenies of 15 heterozygous (S,S2) clones. Detailed examination of three

families indicated that two possessed both homozygous classes (S ,S and S4S2)as

well as the heterozygous class (S J2), and one possessed only one homozygous

class and the heterozygous class. Consequently, it appears that the S-allele system in red clover deviates in some plants from the classical system. The reason

for the deviation is unknown.

An extensive series of S-alleles exists in red clover, and much research has

centered on the means by which such a large number are generated. Irradiation

induces mutations to self-compatibility (Sf)rather than to new S-specificities (de

Nettancourt, 1969). Denward (1963) observed what were thought to be new

S-alleles among I, sib plants from selfed (I,) clones, but he could not be certain

that they were not contaminants. Johnston et al. (1968) found no changes in 1,’s

of three red clover families. Anderson et al. (1974a) reported one changed

S-specificity in eight I2 families. Pandey (1970) speculated that the principle

method of generation of new S-alleles during inbreeding was intracistronic re-




combination in the structural cistrons of the S-gene complex, with a minor role of

point mutations and deletions. Regulatory genes in the heterozygous condition

during outbreeding suppress recombination. Inbreeding produces homozygous

recessive regulatory genes, thus eliminating suppression of recombination. Experimental verification of this hypothesis has not been obtained, in part because

of the low frequency of recombinants.

IV. Heritability and Gene Action


Polyphylly, leaves with five leaflets, is conditioned by two recessive genes, f

and n (Simon, 1962). Polyphylly results when either or both of these genes are

present in the homozygous condition. Artemenko (1972) developed a population

with four to nine leaflets on 89% of the plants.

One of the first reports of the inheritance of male sterility was that of Smith

(1971), who described a male-sterile plant with shriveled and dark-orange anther

sacs. The male-sterile condition is controlled by a single recessive gene designated m s l m s l . A second gene for male sterility isolated by Taylor et al. (1978~)

was designated mszmsz. The two loci interacted to give a duplicate recessive

epistatic type of gene action. Macewicz (1976a,b) reported a male-sterile type in

which the anthers were either incompletely developed or entirely absent. Male

sterility was controlled by the complementary action of a recessive gene, rf,, and

a dominant one, Rf,, in a sterile cytoplasm. Restorers of fertility were readily

detected in the population. Shcheglov and Zvyagina (1975) and Zvyagina (1973)

reported cytoplasmic male sterility induced by colchicine. Whittington ( 1958)

described a male-sterile type resulting from asynapsis. Since the original source

was also female-sterile, no genetic information was obtained.

Liang (1965) reported that leafmarking is inherited on a monofactorial basis

with developmental and suppressor genes active. Annual habit of growth was

reported to be controlled by a single recessive gene in some cases, and by two or

more in others (Strzyzewska, 1974).


Inheritance of a gibberellin-responsive dwarf mutant in red clover was described by Smith (1974). The dwarf characteristic is controlled by one gene,

dw,,and is recessive to normal growth.

Nutman (1968) described two independent recessive host genes, n and d, each

of which prevents nitrogen fixation in the nodules of red clover. Each factor is



specific in its ineffectiveness of nitrogen fixation for specific strains of the

bacteria Rhizobium trifolii. The two factors, n and d, are independent and

nonallelic to factors i l and ie reported earlier by Nutman (1954, 1957).

Graham and Newton (1971) reported internal necrosis of crown pith tissue of

red clover, which they called internal breakdown (IB). Selection studies by

Zeiders et al. (1971) have shown that resistance or susceptibility to IB can be

readily obtained, suggesting that few genes control this character.


Disease resistance in red clover generally is controlled by one to a few genes.

Inheritance of resistance to southern anthracnose caused by Colletotrichum

trifolii B. & E. is conditioned by one recessive gene, according to Athow and

Davis (1958). However, resistance to most diseases in red clover is conditioned

by dominant genes. For northern anthracnose, caused by Kabatiella caulivora

(Kirch.) Kavak., two (Sakuma et al., 1973) or more than three (Smith and

Maxwell, 1973) dominant genes are involved. Resistance to powdery mildew

caused by Erysiphe polygoni DC is dominant and, for five races of the fungus, is

monogenic. In two other races, resistance seems to be controlled by two genes,

and in a third race, inheritance of resistance varies among red clover clones

(Hanson, 1966; Stavely and Hanson, 1967). Inheritance of resistance to rust

caused by Uromyces rrifolii var. fallens Arth. is controlled by a single dominant

gene (Diachun and Henson, 1974a,b). However, this source of resistance cannot

be used for cultivar development because it is linked with a seedling lethality

factor (Engelke et al., 1977). Other types of resistance to rust are inherited in a

quantitative manner (Engelke et al., 1975). Although earlier investigations indicated that resistance to crown rot caused by Sclerotinia trifoliorum Eriks. is

heritable, no detailed inheritance investigations have resulted. Autotetraploid

cultivars are more resistant to crown rot than are comparable diploids (Vestad,

1960). The effect of induced tetraploidy differs by genotype, suggesting that

dosage effects of genes for resistance may be important.

Resistance to virus in red clover also appears to be qualitatively inherited.

Diachun and Henson (1974a,b) reported three types of resistance to bean yellow

mosaic virus, each controlled by a different dominant gene: necrotic local lesion

(hypersensitive) reaction; resistance to mottling and systemic necrosis; and resistance to general mottling, controlled by a gene that appears to be epistatic to the

gene for hypersensitive reaction. Khan et al. (1978) determined that the resistance to red clover vein mosaic virus was controlled by a single dominant gene,

Rc *

Resistance to the stem nematode (Ditylenchus dipsaci) was reported by Nordenskiold (1971) to be regulated by two dominant genes. One of the genes is

closely linked to the S-locus (self-incompatibility).

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