RED CLOVER 33
Trifolium pratense L., family LeguminosaeRed clover is a highly important forage legume, although much of the acreage formerly in red clover has been diverted to alfalfa growing. The acreage in red clover seed production has dropped from over 2 l/2 million acres in 1950 to about 1/2 million acres per year for 1967-71.
The principal seed-growing area is the Central and North Central States, although highest per acre production is in the Western States. In 1969, production in the four Western States, California, Idaho, Oregon, and Washington, ranged from 305 to 410 pounds per acre, with Oregon fourth in total production of clean seed. Michigan (5.5 millions pounds), Indiana (5.2 million pounds), and Illinois (5 million pounds) lead in total seed produced. These seven States produced more than half of the 43.9 million pounds.
E. A. Hollowell (personal commun., 1971) expressed the belief that red clover will be used more in the future than it is at present. This, he believed, was because farmers had concentrated on production of high- priced corn and soybean crops and had long neglected a crop rotation program to replenish the soil. He believed that with the inevitable return to such a program, red clover will regain its popularity.
Red clover is a short-life herbaceous perennial plant that grows to a height of 15 to 36 inches. It is easily recognized by its fine leafy stems, its trifoliate leaves, and rose-pink oval flower heads that are 1 to 1 l/2 inches in diameter (fig. 168). When the crop is harvested for hay, the plants are cut during early bloom. If seed is desired, the plants are usually left after a first cutting, until all of the seed heads are mature.
__________
33 See "Clovers, General."[gfx] FIGURE 168. - Red clover blossoms.
Inflorescence:
The compact flower head, borne on the tip of the branch or stem, is made up of 55 to 275 florets (Williams 1930), which open over a period of 6 to 8 days from the base toward the top (Pammel and King 1911). An acre of red clover in full bloom will have an estimated 300 million florets (Hollowell and Tysdal 1948). (There are 250,000 or more seeds per pound.) Depending on the vigor of the plant, the floret may be 1/4 to l/2 inch long (7.5 to 12.4 mm) but only 1/12 inch in diameter (1.6 to 2.5 mm) (Akerberg 1953, Dennis and Haas 1967b). Within the ovary of the floret are two ovules, but rarely more than one develops. Dijkstra (1969) showed that when two-seededness occurred it had no influence on total seed yield. The staminal column, with its 10 stamens and the slightly longer stigma, extends to the mouth of the corolla tube, but is enclosed within the keel petals. When the bee exerts pressure with its head on the keel petals, the stigma and the anthers are excerted or "tripped" and come in contact with the bee, usually on the posterior part of the head (Woodrow 1952b ). When the pressure is removed the staminal column returns to its former position within the keel, but can be tripped repeatedly.
Nectar is secreted at the base of the corolla tube but only extends 1.35 to 1.47 mm up the tube. Tetraploid red clover produces more nectar per floret than diploid, but because of the longer corolla tube the nectar is no more accessible to the honey bee (Dennis and Haas 1967b), which has a "tongue" or proboscis length of only 5.90 to 6.25 mm (McGregor 1938). Thus, only with the shortest corolla tube (7.5 mm) filled to the highest (1.5 mm) would the 5.90 to 6.25 mm honey bee tongue reach the red clover nectar. Hawkins (1969) stated that the honey bee can reach to a depth of 7 mm in the corolla tube. Dennis and Haas (1967b) stated that the honey bee is able to push its head about 1.4 mm into the corolla tube thereby increasing the effective length of the tongue (fig. 169). It is well known that the corolla tube of late-season red clover is usually much shorter than at the first flowering. Although surplus red clover honey production is uncommon, beekeepers frequently report that bees work red clover late in the season. Holm (1972) caused a reduction in corolla tube length by spraying the plants with a growth retarding chemical. This resulted in a higher frequency of honey bee visits and increased seed production on the treated plants.
Bukhareva (1960) showed that nectar secretion was influenced by plant nutrients. Plots treated with 8 oz boron and 3 oz ammonium molybdate showed a 53 percent increase in nectar secretion, 7 to 11 percent increase in sugar concentration, 17 to 32 percent increase in bee visitation, and 14 to 15 percent increase in seed production. Killinger and Haynie (1961) associated lack of boron with low seed yields in Florida. Unfortunately, too little attention has been paid to the influence of soil nutrients on floral attractiveness of red clover as well as on many other plants.
MacVicar et al. (1952) obtained no benefit from spraying plots with dilute honey to attract honey bees, and, in general, the baiting of bees into the field has not proven practical in the United States.
Hawkins (1969) showed that the corolla varied considerably in length, and he believed that the development of either short corolla strains of red clover or long-tongued bees might be feasible. He also believed that breeding clover cultivars with more nectar would contribute to better pollination. Akerberg et al. (1966) found that, in general, the corolla tubes were shorter towards southern Europe than to the north, an indication that location of growth might influence bee visitation.
There seems to be no question that some species of bumble bees can reach the red clover nectar with ease. Under certain conditions, all species of bumble bees and honey bees can reach the nectar (Bond 1968, Bond and Fyfe 1968).
Woodrow (1962a) concluded that the "depth of the red clover corolla in relation to the length of the honey bee's tongue appears to be unimportant to mechanics of the pollination act," because the tongue plays no part in the transfer of pollen. The question then becomes one of the degree of visitation by the honey bee if it were not rewarded with nectar. Woodrow (1952a) believed that most of the honey bee visits to red clover were for pollen, although some bees collected nectar also. Bond (1968) showed that tetraploid red clover produced more nectar than related diploid cultivars, but the nectar was lower in the corolla tube and more difficult for honey bees to reach.
Specific races of honey bees have been mentioned by various writers as being better pollinators of red clover than other races (Alpatov 1946, 1948; Smaragdova 1956; and Hammer 1950). There is no agreement as to which race is superior in this regard, and no recent attempt has been made to breed such a bee. In the late 1890's, beekeepers made attempts to select superior red clover honey-producing bees but failed. However, Stahlin and Bommer (1958) concluded that breeding clover to suit the bees would be more profitable than breeding bees to fit the clover. Starling et al. (1950) concluded that short corollas alone offered no advantage for increased seed production.
Woodrow (1952b) described in detail the method the honey bee follows in pollinating red clover. He stated that the length of the tongue of the bee need be no handicap in pollination of this flower because the sexual parts of the flower are at the tips, and the bee's tongue is not used in transferring pollen from flower to flower.
[gfx] FIGURE 169. - Bee head (enlarged) with proboscis extended into red clover bloom.
Pollination Requirements:
The pollination of red clover has probably been more extensively studied than that of any other plant, not only in the United States but also in many other countries. Excellent reviews have been made by Bohart (1957, 1960 *), Dennis and Haas (1967a, b), Free (1970*), Gubin (1947), Stahlin and Bommer (1958), and Umaerus and Akerberg (1959). The discussion on red clover pollination by Free (1970*) is extensive and thorough. Krishchunas and Gubin (1956) also devoted about 25 pages of their book to this crop.
Self-sterility in red clover has been known since Darwin (1889*) showed that caged plants would not set seed unless they were cross- pollinated. The pollen must come from another plant if commercial production of seed is anticipated (Williams 1931, Westgate and Coe 1916). Martin (1913) demonstrated that self-pollen tubes penetrated the style toward the ovary more slowly than foreign pollen (from another red clover plant). Whether or not this time factor is critical has not been demonstrated, but the floret must be pollinated within 2 to 4 days after it opens (Free 1965, Umaerus and Akerberg 1959). The appearance of the flower heads is a strong clue to the adequacy of pollination. If pollination does not occur, the florets remain turgid, the head is soon covered with the colorful florets, and the field takes on a flower-garden appearance.
If pollination is adequate, an individual head in flower will have the lower florets pollinated and wilted, with the position of the florets changed from upright to drooping, the color changed from rose-pink to rusty brown, those in the center of the head attractive to pollinators, and the uppermost ones still in the bud stage (Woodrow 1952a). When this situation exists, the field takes on a greenish-brown cast. Naturally, the seed-grower should strive for this situation and should be concerned if the field has the flower-garden appearance.
Pollinators:
The relative value of the pollinating insects on red clover has been debated for decades. Many references attest to the value of bumble bees. Others support honey bees, and some support other genera of bees. Hawkins (1962a) found a correlation between bumble bee populations and red clover seed production in England 2 years out of 3, but no correlation between honey bees and seed production. He (1962b) proposed that an organization be formed to make annual counts of bumble bees, similar to bird counts, to stimulate public interest in these insects. Bird (1944) also considered bumble bees much more important than honey bees.
Lindhart (1911) concluded that honey bees occasionally aid in red clover pollination. This evaluation of honey bees has consistently increased over the years in many subsequent papers. For example, Anderson and Wood (1944) obtained one seed per head of red clover where bees were excluded but 56 seeds per head where honey bees were caged on the plants.
Butler (1941), Valle (1959), and Valle et al. (1960) after thorough studies gave credit to both honey bees and bumble bees. The other genera of bees that have been mentioned, but in general considered of little importance, include Andrena (Benoit et al. 1948), Eucera (Yamada and Ebara 1952), Halictus (Maurizio and Pinter 1961), Megachile (Akerberg et al. 1966), Melissodes (Folsom 1922), Osmia (Maurizio and Pinter 1961, Akerberg et al. 1966), Psithyrus (Sculler 1930), and Tetralonia [Synhalonia] (Folsom 1922).
Dennis and Haas (1967b) also observed the action of bumble bees on red clover and learned that Bombus terrestris (L.), with a short (6.8 mm) tongue, obtained red clover nectar only by cutting a hole in the base of the corolla tube. B. lapidarius (L.), with an 8-mm tongue, collected nectar normally from diploid red clover. B. distinguendus F. Morawitz, with an 8.8-mm tongue, and B. hortorum (L.) (11.1 mm) were more frequent on tetraploid red clover.
Bohart (1957) and van Laere and Martens (1962) concluded that bumble bees, except for a few nectar-thieving species, are ideal pollinators of red clover although their populations are unpredictable and usually insufficient to adequately pollinate all the blossoms in a large field. Bohart (1957) considered honey bees satisfactory if they are sufficiently concentrated in the area and the competing pollen and nectar sources are kept at a minimum. In Canada, Peterson et al. (1960) also concluded that honey bees were best. In Russia, Gubin (1947) considered bumble bees to be the best pollinators of red clover on a bee-for-bee basis but that overall they provided only 3.5 percent of the pollination service. The value of honey bees was expressed by Hopkins (1896a, b), Pieters (1924), and Stapel (1934) and demonstrated by Richmond (1932), Dunham (1932, 1939a, b, c), and Armstrong and Jamieson (1940a, b).
Bumble bees were considered of such importance that they were transported from England and established in New Zealand for the express purpose of pollinating red clover (Belt 1876, Hopkins 1914). However, Forster and Hadfield (1958) showed that 35 colonies of honey bees placed adjacent to a 10-acre field of Montgomery red clover in New Zealand provided 77 percent of the pollinating insects in 1954 and 89 percent in 1955. They stated that this was a fair cross-section of pollinator activity on red clover crops in South Canterbury. Morrison (1961) found inconsistencies over the years between the efficiency of honey bees and bumble bees in New Zealand and considered both groups of value. Hills (1941), Palmer-Jones et al. (1966), and Palmer-Jones (1967) considered honey bees of greater value than bumble bees. Bond and Fyfe (1968) showed that seed production in a cage with one strong colony of bees was more than twice that in a cage with a weak colony.
Hollowell (1932) proposed the introduction of additional honey bee colonies into clover fields to increase seed production. This action has now become a common practice. Walstrom et al. (1951a, b) proposed 400- to 600-foot intervals as an economical distance between groups of colonies used for red clover pollination. Jamieson (1955) showed that only 63 lb/acre of seed were obtained with local wild bees, but 307 lb/acre were obtained with two colonies of honey bees per acre.
The production of red clover seed is directly proportional to pollinator activity. Everly (1950) associated reduced native pollinators with decreased seed yields in Indiana, and stated that red-clover pollen- collecting honey bees were effective in setting a good crop of seed. Walstrom et al. (1951a, b) showed that seed production decreased 6.4 lb/acre with each 100 feet of distance from the apiary. Walstrom (1958) showed that differences in seed yields at 100-foot intervals from apiary sites were significant at the 1-percent level of probability. Zivov and Skvorcov (1951) also showed that seed production decreased with increased distance: 246 lb/acre when the field was only 0.5 km from the bee source, 158 lb/acre at 1 to 1.5 km, and only 90 lb/acre beyond 1.5 km. Jamieson (1956) obtained 307 lb/acre with two colonies per acre but only 63 lb/acre when honey bees were not provided. Thomas (1961) and Braun et al. (1963) obtained similar results.
The data leave little doubt that if bumble bees are not sufficiently abundant (and they usually are not), their services can be supplemented and seed production stabilized by the use of honey bees. Other pollinators are of little significance.
Pollination Recommendations and Practices:
Hogborg (1966) considered pollinators in terms of "positive bee pollinating units," and calculated that adequate pollination would be given by 20,000 units per hectare (using the value of 2.5 units for one bumble bee). This amounts to about one bumble bee or two nectar-collecting honey bees per square yard. Akerberg (1947) calculated that 1,100 bumble bees per hectare (about 0.1 bee/yd2 ) were sufficient to produce 300 kg alfalfa seed per hectare (about 300 lb/acre). These estimates are below the amounts specified in the formula of Stanley Roadfeldt (McGregor 1966) of one honey bee per 4 yd2 per minute equals 300 pounds red clover seed, or one bee per square yard per minute equals 700 pounds red clover seed.
Dennis and Haas (1967a) used a numerical rating on the values of bees on diploid red clover, based on the bees' working speed, as follows:
[gfx] fix table:
Apis.................................................................................................1.0 Short-tongued Bombus .........................................................1.5 Long-tongued Bombus............................................................2.5 However, their remanipulated data gave the following values: Nectar-collecting honey bee ................................................1.0 Pollen -collecting honey bee ...............................................1.3 Nectar-collecting bumble bee .............................................1.6 Pollen-collecting bumble bee ..............................................1.9
By this method, they considered pollen-collecting, long-tongued bumble bees to be about twice as efficient as honey bees.
The pollination recommendations for red clover revolve around bumble bees and honey bees. The presence of bumble bees can be encouraged by providing them with domiciles, by protecting them from pesticides (rye and Medler 1964, Hobbs 1967, Holm 1966), and by planting off-season flowering plants to provide nectar and pollen. Honey bee colonies can be transported and placed in or adjacent to red clover fields in any number desired and when desired. This is a more dependable practice than "encouraging" the bumble bees.
In most cases, the number of colonies of honey bees that has been recommended per acre has ranged from one to three, but a few recommendations have mentioned four, five, six, and up to 10 colonies per acre. Some urge that the colonies be placed adjacent to the field, others recommend that the colonies be placed within the field in groups of 10 or more 100 to 400 yards apart. Some recommendations stress bees per square yard, the number of bees ranging from 1 to 18. The use of strong colonies is urged. E. A. Hollowell (personal commun., 1971) stated that two bees per square foot (18 per square yard) should set an abundant seed crop. This may require the use of several strong colonies per acre.
If visitation in the field is adequate, the field will have a rusty- brown hue instead of the rose-colored flower-garden appearance. The number of colonies of honey bees per acre necessary to provide this visitation will vary with condition of the colonies, placement pattern, climate, crop, and competing plants. The important point to remember is that there should be sufficient bees on the flowers to keep the florets tripped as rapidly as they appear.
LITERATURE CITED:
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1959. POLLINATION AND SEED SETTING IN TETRAPLOID RED CLOVER IN FINLAND. Suomen Maataloustieteellisen Seuran Julkaisuja 95(4): 1-32.______SALMINEN, M., and HUOKUNA, E.
1960. POLLINATION AND SEED SETTING IN TETRAPLOID RED CLOVER IN FINLAND. II. Suomen Maataloustieteellisen Seuran Julkaisuja 97(1): 1-62.WALSTROM, R. J.
1958. EFFECTS OF FLIGHT DISTANCES FROM HONEY BEE COLONIES ON RED CLOVER SEED YIELDS. Jour. Econ. Ent. 51: 64-67.______PADDOCK, F. B., PARK, O. W., and WILSIE, C. P.
1951a. RED CLOVER POLLINATION AT VARIOUS DISTANCES FROM HONEY BEE COLONIES. In lowa State Apiarist Rpt. 1950. Pp. 51-52.______PADDOCK, F. B., PARK, O. W., and WILSIE, C. P.
1951b. RED CLOVER POLLINATION. Amer. Bee Jour. 91: 244-245.WESTGATE, J. M., and COE, H. S.
1915. RED CLOVER SEED PRODUCTION: POLLINATION STUDIES. U.S. Dept. Agr. Bul. 289: 17-18.WILLIAMS, R. D.
1930. SOME OF THE FACTORS INFLUENCING YIELD AND QUALITY OF RED CLOVER SEEDS. Welsh Plant Breed. Sta., Aberystwyth, Bul. Ser. H. 11: 60-91.______ 1931. SELF- AND CROSS-STERILITY IN RED CLOVER. Welsh Plant Breed. Sta., Aberystwyth, Bull Ser. H. 12: 181-208.
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1952a. EFFECT OF TIME OF POLLINATION BY HONEY BEES ON RED CLOVER SEED YIELDS. Jour. Econ. Ent. 45: 517-519.______ 1952b. POLLINATION OF THE RED CLOVER FLOWER BY THE HONEY BEE. Jour. Econ. Ent. 45: 1028-1029.
YAMADA, I., and EBARA, K.
1952. [STUDIES ON THE FERTILIZATION OF RED CLOVER TRIFOLIUM PRATENSE, BY BUMBLE BEES AND HONEY BEES.] Hokkaido Nat. Agr. Expt. Sta. Rpt. No. 45, 33 pp. [ln Japanese, English summary.]ZIVOV, V., and SKVORCOV, S.
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