CLOVERS, GENERAL
Family LeguminosaeNumerous species of plants in different genera of the family Leguminosae are called clover. They have certain characteristics in common. The leaf is normally composed of three leaflets. The papilionaceous (butterfly-like) or pealike floret of the flower head consists of a large dorsal standard petal, two lateral wing petals, and two lower keel petals. It usually has 10 stamens, one of which is free and the other nine united, that form a tube enclosing the long ovary. The flower trips exposing the stigma, which returns to its original position after pressure on the petals is removed. The plants have the ability to take nitrogen from the air and, by Rhizobium fixation, store it within nodules on the roots. This contributes to the value of the plant to the soil. The plants provide excellent forage for livestock, and they help in erosion control. Many of the species are good sources of honey and pollen for bees.
The United States Government considers legume crops so important that in 1946, when seed stocks were in short supply, Congress appropriated funds to encourage the harvesting of seeds of these crops (Johnson and Loomer 1948).
The hay crop from clover and clover mixtures in 1969 was harvested from slightly more than 13 million acres.
There are about 250 species of Trifolium, the True Clovers, but only four make up the bulk of the acreage. They are alsike clover (T. hybridum L.), crimson clover (T. incarnatum L.), red clover (T. pratense L.), and white clover (T. repens L.).
There are about 20 species of Melilotus, the sweetclovers, but only three species make up the bulk of the acreage. They are biennial yellow sweetclover (M. officinalis (L.) Lam.), biennial white sweetclover (M. alba Desr.), the annual white subspecies (M. a. var. annua Coe), and yellow annual sourclover or sour sweetclover (M. indica (L.) All.).
There are about 65 species of Medicago, some species of which are referred to as clover, for example, the burclovers, two species of which are important. They are toothed burclover (M. hispida Gaertn.) and spotted burclover (M. arabica Huds.). Alfalfa (M. sativa L.) is the most important species.
There are 16 species of Alysicarpus, or Alyce clover, which is not a True Clover, but none are of great economic importance.
There are about 70 species of Lespedeza or bush clover, several of which are of economic importance.
Weaver and Weihing (1960) concluded, with limited cage tests, that pollinating insects were essential for adequate seed production of the experimental species Trifolium isthmocarpum Brot., T. michelianum Savi, T. pallidum Waldst. & Kit., and T. xerocephalum Fenzl.
A memorandum to USDA cooperators, from R. C. Leffel (USDA, Clover Investigations, 1971), listed the following cultivars of clover available for agronomic evaluation: Cluster clover (T. glomeratum L.), Kura clover (T. ambiguum Bieb.), Lappa clover (T. Iappaceum L.), Large Hop clover (T. campestre Schreb.), Small Hop clover (T. dubium Sibth.), and Striate clover (T. striatum L.). Leffel mentioned that other species may also be present in agronomists' test plots, but none are currently grown commercially. Their pollination requirements are unknown but should definitely be evaluated by the agronomists along with their other characteristics, if release of the species for commercial production appears likely.
The important species of these different genera that are known to be dependent upon or benefited by insect pollination are discussed as separate crops herein.
LITERATURE CITED:
JOHNSON, N. W., and LOOMER,C.W.
1948. THE HELP THE GOVERNMENT OFFERS. U.S. Dept. Agr. Yearbook 1948: 34-44.WEAVER, N., and WEIHING,R.M.
1960. POLLINATION OF SEVERAL CLOVERS BY HONEYBEES. Agron. jour. 52: 183-185.
