Crops
Trifolium pratense L. - Red clover
Taxonomic position:
Family: Fabaceae Lindl., genus: Trifolium L., species: Trifolium pretense L. (Cherepanov, 1995).Biology and morphology:
2n=14, 28. Red clover's tetraploid varieties have been artificially bred. Clover is a biennial or perennial plant. Two varieties of cultivated red clover are distinguished: late-maturing or single-crop winter clover (var. serotinum) and early-maturing or double-crop spring clover (var. praecox). Clover has a well-developed taproot system. It has numerous lateral and hypocotylous adventitious roots. They have nodules with bacteria that draw nitrogen from the air. Clover's nitrogen content depends on its biotype composition. It increases with the age of the plants. Clover has a cylindrical stem. It is not very pubescent and is differently colored depending on its anthocyan content (from deep red to green). In addition to the short main stem, there are lateral stems growing from the axils of seed leaves, from epicotyl and from the axils of lower nodes. Clover leaves are compound, trefoil and pubescent. The leaflets are wide and obovate. The stipules are tunicate and constricted. Clover inflorescences are capitate and multiflorous (with 30-70 flowers). Closer to their bottoms, they are surrounded by apical leaves. Clover flowers are sessile and of the papilionaceous type. Their corollae are red, more rarely purple. Lower corolla's petals are connate and form a tube 8-10 mm long. With tetraploid varieties, it is up to 14 mm long. Each clover flower has 10 stamens (9 accrete and 1 free) and a pistil with a rounded stigma. Anther and pistil stigma do not emerge from the flower. Ripe clover anthers are kidney-shaped with a lengthwise groove. They burst when touched by pollinating insects, mostly by bees and bumble-bees. Ripe clover pollen is reddish yellow. Its grains are fine but difficult to spill and are covered with spines. Clover pollen's fertility may reach 90%. The gynaecium is superior, unilocular with two ovules, of which usually only one develops. At the base of each clover flower, there is a nectary in a tube. Nectar quantity and quality are critical for successful entomophily. In rainy weather, clover nectar has a higher moisture content, while during droughts, it gets dehydrated, changes its consistency, and bees are not attracted to it any longer. Clover fruit is a pod. It is usually monospermous, rarely dispermous. Depending on particular varieties' features and on weather conditions, 0.4-20% of pods may be dispermous. Clover seeds are small, egg-shaped, slightly flattened and many-colored (yellowish, yellow violet and violet). 1000 seeds weigh 1.5-2.5 g. With tetraploid varieties, 1000 seeds weigh 2.5-3.5 g.Ecology:
Maturing rate, yields and hardiness of red clover populations vary greatly depending both on their genetic features and on environmental conditions. Clover plants reaching their generative stage in their first year of life (spring biotypes), plants that do not blossom during the year when they were sown but pass the winter in the form of a foliose rosette shoot (winter biotypes), and plants that form in their first year of life both short and long vegetative shoots (intermediate biotypes) may be found within the same population. Early-maturing double-crop red clover populations include spring biotypes with different maturing rates (90-100%). Late-maturing clover populations consist of spring, intermediate and winter biotypes. During their first year of life, 4-47% of plants blossom in late-maturing populations, 13-96% in intermediate populations, and 95-100% in early-maturing populations. Late-maturing populations blossom for 3-4 weeks. Early-maturing clover varieties begin to blossom 1-2 weeks earlier than late-maturing ones. Weather conditions in particular years may considerably shift clover blossoming dates. The blossoming date of late-maturing clover allows us to clearly distinguish three main biotypes, while with early-maturing ones it only allows us to distinguish one biotype. The blossoming times of 2-year-old clover plants are influenced by these plants' degree of development to the same extent as with 1-year-old plants. Within any given population, greater yields are to be expected from winter and intermediate biotypes. Yields and plant height also vary between winter and intermediate biotypes more than with spring ones. During its vegetation period, late-maturing clover varieties yield only one seed-harvest or a full-scale hay-and-fog harvest, with clover fog consisting mainly of leaves. 2-year-old early-maturing clover grows fast and can yield during its vegetation period two full-scale hay-harvests or one hay-harvest and one seed-harvest. In the north during cold winters with little snow, late-maturing clover freezes less and less often perishes. Late-maturing red clover can survive frosts as cold as -20°C in the area of its root collar provided it is hardy enough. However, the temperature of -15°C can be considered critical. Low moisture content in the soil during the autumnal and first winter months has a negative influence on herbage's hardiness in winter. Clover's ability to survive in unfavorable environmental conditions is reduced when its herbage is harvested for seeds or is mown often during the early stages of its development, especially in poor soils or when it is mown using improper agricultural techniques. Clover thrives in loamy soils, loamy sands, gray wooded and leached soils, as well as in ordinary and deep black earth. With irrigation, clover yields increase on all kinds of soils. With land-improvement, yields increase in peat-bogs as well. Clover requires a lot of moisture but cannot survive its excess. Red clover thrives within 70-75% of the full moisture capacity of the soil. Tetraploid and late-maturing clover varieties are especially sensitive to lack of moisture. Clover plants' yield depends on rainfall in April, May, and June. With low temperatures, clover grows well, even with low soil moisture content. Clover cannot survive prolonged flooding and quite often perishes on flood-plains. In the north, clover thrives when the subterranean waters begin at a depth of at least 1.5 m from the surface. In the south, this depth should be at least 75 cm. Clover dislikes soils with low humus content. In acidic and saline soils, clover plants grow farther from each other and yield less hay and fewer seeds. Late-maturing varieties of red clover react better to fertilizers than early-maturing ones. At the early stages of their growth, clover plants need more phosphorus. Upon application of NPK, the bushing-out stage of clover plants is accelerated by 5-6 days, their budding stage by 8-9 days, and their ripening stage by 5 days. The optimal temperature for clover seed germination is 5-7°C. The optimal temperature for clover plant growth and development is 17-20°C. Red clover is a long-daylight plant with a well-pronounced photoperiodic reaction. With longer daylight, late-maturing clover varieties accelerate their development much more than early-maturing clover varieties. During its first year of growth (when sown without cover), the period between the first sprouts and blossoming with early-maturing clover varieties lasts 95-125 days. During their second year of growth, this period lasts 50-60 days, depending on the cultivation area. When clover plants are cultivated for their seeds, their vegetation period is 35-40 days longer. With late-maturing clover varieties, the period from the beginning of their growth in the spring to the beginning of flowering lasts 60-75 days. The effective heat sum necessary for the ripening of early-maturing clover varieties is 1200-1400°C; with late-maturing clover varieties, this value is 1400-1500°C.Distribution:
Clover is cultivated in all agricultural areas of the world. Red clover is cultivated in all of Europe and Western Siberia. There are large plantations of red clover in the Caucasus, in the mountainous areas of Central Asia, in the Levant and around the Mediterranean. In Russia, it is cultivated in all wooded areas and in many regions of forest-steppes, in the foothills and in the mountains. In 2004, 75 red clover varieties were approved for cultivation in various areas of the Russian Federation (Altyn, Vesna, Drakon, Zarya, Ermak, Martum, Ranny 2, Sedum, Smolensky 29, Suydiets, Topaz, etc.). 13 varieties of white clover were also approved (Bitunay, VIK 70, Nanuk, Smena, etc.), as were 9 varieties of alsike clover (Mayak, Pervenets, Smolensky, Faley, etc.). 1 variety of annual clover (Maykopets 12) and 1 variety of honey clover (ZGID) have been approved. The main breeding agencies include the V.R. Williams Research Institute for Fodder, the Northwestern Research Institute for Agriculture of Russian Academy of Agricultural Sciences, the N.V. Rudnitsky Zonal Research Institute for Agriculture of the Northeast and Falen Breeding Station, the Morshansk Breeding Station, the Research Institute for Agriculture of Northern Transuralia, the Perm Research Institute for Agriculture, the All-Union Research Project and Technology Institute for Rape, the Urals Research Institute for Agriculture, and the A. Engelgardt Smolensk State Regional Agricultural Experimental Station.Economic value:
Clover is an important fodder crop. It is widely used in fodder grass cultivation in order to create cultivated hayfields and pastures. Clover considerably increases soil fertility and protects the soil from wind and water erosion. It raises soil nitrogen content and is one of the best precursors in field-crop rotation. Clover is used as green fodder, hay, grass meal, haylage, and silage. 100 kg of clover herbage contain 19.8 fodder units and 2.7 kg of digestible proteins. Clover contains more indispensable amino acids (cystine, tryptophan and leucine) than maize and oat grains. Clover is high in carotin, C, D, E, K and B-group vitamins and trace elements (copper, manganese, molybdenum, cobalt, sulfur, and boron). Within field-crop rotation, clover is sown both separately and mixed with Gramineae or leguminous herbs. When sown separately, 14-16 kg of clover seeds are sown per hectare. When sown in a mix, 13-15 kg of clover seeds are sown per hectare. Clover seeds are planted at a depth of 1-3 cm. Clover is gathered for fodder at the stage of budding and in the beginning of flowering. Clover yields up to 60,000 kg of herbage per hectare, 6,000-10,000 kg of hay per hectare, and 300 kg or more of seeds per hectare.References:
Cherepanov S.K. 1995. Vascular Plants of Russia and Neighboring Countries. St. Petersburg, pp. 494-496.Konovalov Y.B., ed. 1990. Particular Breeding of Field Crops. Moscow, pp. 513-518.
Novoselova, A.S., Konstantinova, A.M., Kuleshov, G.F. et al. 1978. Breeding and Seed-Farming of Perennial Herbs. Moscow, pp. 121-191.
Suvorov, V.V. 1961. Botany. Moscow, pp. 312-314.
State Register Breeding Achievements Approved for Practical Application. 2004. Moscow, p. 34-36.
Suvorov, V.V. Botany. Leningrad: 1961. p. 421.
Zhukovsky, P.M. 1971. Cultivated Plants and Their Congeners. Leningrad, pp. 661-667.