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SOYBEAN
Glycine max (L.) Merr., family Leguminosae

Soybeans comprise the most important oilseed crop grown in the United States. Approximately 41 million acres of soybeans were grown with a farm value of $2.6 billion. The crop is widely grown in all areas except New England and the West. The primary reason for growing the crop is for the beans, which are processed to obtain oil and a high protein meal.

Plant:

The soybean is a bushy, upright, annual, summer legume, 1 to 4 feet tall, with dense, alternate, and usually trifoliate oval leaves 2 to 4 inches across. The plant is not frost resistant so it must be grown after spring frosts are past and have time for the seeds to mature before the first fall frost. Many selections or cultivars are bred for production under different daylength or other environmental conditions. They range in maturity from 75 to 200 days. There are hundreds of selections under test; however, only 17 cultivars make up 88 percent of the acreage in the 14 major producing States.⁴⁰

The seeds are borne, one to five (usually two or three) to a pod (Weiss 1949), the 3 to 15 pods are in a cluster on the short seed stalk in the rachis or base of the leaf. A productive plant may have as many as 100 seed clusters. The seeds are mechanically harvested after the plant sheds its leaves as it matures.
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⁴⁰ UNITED STATES DEPARTMENT OF AGRICULTURE. Highlights of U.S. Crop Report as of November 1, 1971. U.S. Dept. Agr. Crop Prod. CrPr 2-2, 33 pp. 1971.

Inflorescence:

There may be from 1 to 35 purple or white florets, three-eighths of an inch long, on each short raceme or flower cluster. A single plant may bear as many as 800 florets, but may set only 13 to 57 percent (van Shaik and Probst 1958). The floret has the characteristics and shape of many other legume flowers - a large standard petal, two small wing petals, and a keel petal that encloses the staminal column (fig. 174). The calyx is relatively large in proportion to the flower or even to the calyx of other legumes. Each floret is capable of producing a bean pod. Southern grown cultivars stop growing when flowering begins. Flowering usually continues for 4 to 6 weeks. There may be one-half million florets per acre. There are no extrafloral nectaries (Jaycox 1970).

Soybean flowers attract relatively few bees (Blickenstaff and Huggans 1962); however, bees do visit the flowers for both pollen and nectar. The anthers dehisce before the flower opens so that the stigma is in contact with and receptive to the pollen on the anthers. Pollination and fertilization is usually accomplished before the flower opens. Beekeepers in some areas frequently report honey crops from soybeans ( Hambleton 1936, Jaycox 1970, Pellett 1947*). Beekeepers in other areas report that bees never visit the flowers, or if they do no honey is obtained.

[gfx] FIGURE 174. - Longitudinal section of soybean flower, x 25.

Pollination Requirements:

The soybean is considered to be self-fertile and not benefited by insect pollination (Morse and Cartter 1937, Rubis 1970). Although there is no experimental evidence to support them, some soybean growers in Arkansas have indicated that bees increase production of beans, and they encourage the presence of apiaries near their fields. Tests with plants caged to exclude bees have shown no decrease in production over exposed plots (Woodhouse and Taylor 1913, Milum 1940, Piper and Morse 1910). Culter (1934) obtained an estimated 5 percent cross-pollination in open plots surrounded by 36-inch cheesecloth fences with an apiary nearby. Gordienko (1960) used honey bees in cages with two cultivars of soybeans. He fed the bees a scented sirup to stimulate the floral visitation and obtained 29 percent hybrids on one cultivar and 44 percent on the other.

Various tests have been conducted to determine the amount of cross-pollination that occurred at different locations, but the agents responsible for the crossing obtained were usually not determined, possibly because the tests were conducted by agronomists who did not consider themselves qualified to record these observations (Caviness 1966,1970; Caviness et al. 1970; Garber and Odland 1926; Woodworth 1922).

Weber et al. (1970) stated that, for a highly self-fertilizing species such as soybeans, two requirements must be met for successful production of hybrid seed. First, there must be heterosis (or an increase in production, or some other beneficial effect, of the cross over the best parent), and there must be an economical, large-scale method of producing the hybrids. Brim and Young (1971) have reported that male sterility, although not cytoplasmic, has been found and that 99 percent of the seed set on the male-sterile plants was due to cross-pollination; however, the total seed set was extremely low.

Veatch (1930) showed that hybrid soybeans out-yielded the average parent variety up to 95.9 percent and the higher parent variety up to 71.5 percent. Weiss et al. (1947) showed a range of 14.2 to 71.3 percent heterosis over the best parent in the greenhouse, and 5.9 to 38.6 percent increase over the best parent in the field. Weiss (1949) showed an increase of 19.6 to 117 percent increase of the hybrid over the best parent. Wentz and Stewart (1924) showed increases of 60 to 397 percent. Weber et al. (1970) showed an average for all tests of the hybrids over the best parents of 13.4 percent.

Mention might be made that the increased yields for hybrids are all based upon a relatively low number of widely spaced plants. Because of the problems in making a sufficient number of hand pollinations to satisfactorily evaluate F₁'s in a normal-type planting, essentially no such studies have been conducted. The extremely large increases that have been reported are assumed to be partly a result of the extra space allotted to the plants.

Bradner (1969) stated that all of the breeding components have been described for the production of hybrid soybean seeds, except that a suitable pollen vector has not been found, although the honey bee "looks encouraging." Piper and Morse (1923) noted that soybean flowers were "much visited by bees." They also noted that three of their varieties, when in full flower at Jackson, Tenn., were very fragrant - the odor suggesting that of lilacs. Although they gave no indication that these varieties were more attractive to bees than other varieties, the information indicates that cultivars might be selected with aroma or attractiveness that when incorporated in a hybrid seed program might efficiently attract pollinating insects. Because of the potential value of hybrid soybeans, the breeders might watch for selections that show attractiveness to bees.

Pollinators:

Little attention has been paid to the prevalence of pollinating insects on soybeans. Jaycox (1970) reported only seven honey bees per 100 yards of row, but he gave no indication as to number of honey bee colonies and the acres of soybeans in the area. Blickenstaff and Huggans (1962) recorded only one honey bee, three leafcutter bees, and nine halictids in 25,346 individual insects collected on soybeans with a sweepnet, but this is a poor method for determining pollinator populations in dense foliage such as soybeans. Culter (1934) and Gordienko (1960) showed that honey bee populations can be built up on soybeans. Beard and Knowles (1971 ) showed that crossing at different locations in California varied: 10 percent at Five Points, 1 to 5 percent at Davis, and O to 7 percent at Shafter. No studies have been made on the value of concentrating the gregarious types of wild bees on soybeans, although such insects might prove to be more efficient than honey bees.

Woodworth (1932) indicated that thrips and honey bees might be responsible for some transfer of pollen; however, Caviness (1970) showed that thrips are uneffective as pollinators of soybeans, but that honey bees were repsonsible for 7.7 percent crossing.

Pollination Recommendations and Practices:

There are no recommendations for the use of bees in pollination of soybeans. The subject is reviewed, however, because of the interest in hybrid soybeans and the possibility of using pollinating insects in hybrid soybean production.

LITERATURE CITED:

BEARD, B. H., and KNOWEES, P. F.
1971. FREQUENCY OF CROSS-POLLINATION OF SOYBEANS AFTER IRRADIATION. Crop Sci. 11: 489-492.

BEICKENSTAFF, C. C., and HUGGANS, L. J.
1962. SOYBEAN INSECTS AND RELATED ARTHROPODS IN MISSOURI. Mo. Agr. Expt. Sta. Bul. 803, 51 pp.

BRADNER, N. R.
1969. HYBRID SOYBEANS: FICTION OR FACT? Soybean Digest 29(7): 16-17. 338

BRIM, C. A., and YOUNG, M. F.
1971. INHERITANCE OF MALE-STERILE CHARACTER IN SOYBEANS. Crop Sci. 11: 564-566.

CAVINESS, C. E.
1966. ESTIMATES OF NATURAL CROSS-POLLINATION IN JACKSON SOYBEANS IN ARKANSAS. Crop Sci. 6(2): 211.

____ 1970. CROSS-POLLINATION IN THE SOYBEAN. In The Indispensable Pollinators, Ark. Agr. Ext. Serv. Misc. Pub. 127, pp.33-36.

____ WALTERS, H. J., and JOHNSON, D. L.
1970. A PARTIALLY MALE STERILE STRAIN OF SOYBEAN. Crop Sci. 10(1): 107-108.

CULTER, G. H.
1934. A SIMPLE METHOD FOR MAKING SOYBEAN HYBRIDS. Amer. Soc. Agron. Jour. 26: 252-254.

GARBER, R. J., and ODBAND, T. E.
1926. NATURAL CROSSING IN SOYBEANS. Amer. Soc. Agron. Jour. 18: 967 - 970.

GORDIENKO, V.
1960. [SEXUAL HYBRIDS OF SOYA BEANS OBTAINED BY DIRECTED BEE POLLINATION.] Pp.400-407. In Mel'nichenko, A. N., [Pollination of Agricultural Plants by Bees.] Izd-vo Minist. Sel'sko Khoz. SSSR. [In Russian.]

HAMBLETON J. I.
1936. SOY BEAN FOR POLLEN AND NECTAR. Gleanings Bee Cult. 64: 431.

JAYCOX, E. R.
1970. ECOLOGICAL RELATIONSHTPS BETWEEN HONEY BEES AND SOYBEANS. Amer. Bee Jour. 110(8): 306-307, (9): 343-345, (10): 383-385.

MILUM, V. G.
1940. BEES AND SOYBEANS. Amer. Bee Jour. 80: 22.

MORSE, W. J., and CARTTER, J. L.
1937. IMPROVEMENT IN SOYBEANS. U.S. Dept. Agr. Yearbook 1937: 1154-1189.

PIPER C. V. and MORSE, W. J.
1910. THE SOYBEAN: HISTORY, VARIETIES AND FIELD STUDIES. U.S. Dept. Agr. Burl Plant Indus. Bul. 197, 84 pp.

PIPER, C.v. and MORSE, W.J.
1923. THE SOYBEAN. 329 pp. McGraw-Hill Book Co., Inc., New York.

RUBIS, D. D.
1970. BREEDING INSECT POLLINATED CROPS. In The lndispensable Pollinators. Ark. Agr. Ext. Serv. Misc. Pub. 127, pp.19-24.

SHAIK, P. H. VAN, and PROBST, A. H.
1958. EFFECTS OF SOME ENVIRONMENTAL FACTORS ON FLOWER PRODUCTION AND REPRODUCTIVE EFFICIENCY IN SOYBEANS. Agron. Jour. 50: 192-197.

VEATCH, C.
1930. VIGOR IN SOYBEANS AS AFFECTED BY HYBRIDITY. Jour. Amer. Soc. Agron. 22: 289-310.

WEBER, C. R., EMPTG, L. T., and THORNE, J. C.
1970. HETEROTIC PERFORMANCE AND COMBINING ABILITY OF TWO- WAY F₁ SOYBEAN HYBRIDS. Crop Sci. 10: 159-160.

WEISS M. G.
1949. SOYBEANS. Adv. in Agron. 1: 77-157.

____ WEBER, C. R., and KALTON, R. R.
1947. EARLY GENERATION TESTING IN SOYBEANS. Jour. Amer. Soc. Agron. 39: 791-811.

WENTZ, J. B., and STEWART, R. T.
1924. HYBRID VIGOR IN SOYBEANS. Jour. Amer. Soc. Agron. 16: 534-540.

WOODHOUSE, E. J., and TAYLOR, C. S.
1913. THE VARIETIES OF SOYBEANS FOUND IN BENGAL, BIKAR AND ORISSA AND THEIR COMMERCIAL POSSIBILITIES. India Dept. Agr. Mem. Bot. Ser. 5: 103-175.

WOODWORTH, C. M.
1922. THE EXTENT OF NATURAL CROSS-POLLINATION IN SOYBEANS. Jour. Amer. Soc. Agron. 14: 278-283.

____ 1932. GENETICS AND BREEDING IN THE IMPROVEMENT OF THE SOYBEAN. Ill. Agr. Expt. Sta. Bul. 384: 297-404.

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