Sweet clover
White sweet clover (Melilotus albus).
Sweet clover — a forage agricultural crop belonging to perennial legumes.
Economic importance
The green biomass of sweet clover is used as livestock feed, for preparing hay, haylage and silage. By the beginning of flowering, the protein content in dry matter reaches 19%. For fodder purposes, pure stands or grass mixtures are used.
100 kg of sweet clover hay harvested at the flowering phase contains 44.0–44.5 feed units and 11.1–11.9 kg of digestible protein. 100 kg of fresh grass — 18.2–19.5 feed units and 3.1–3.2 kg of protein.
It is also suitable for grazing livestock from early spring to late autumn in the first and second years of life, with 3–4 grazings per year. With sufficient moisture, it regrows after 18–20 days. During the first 3–4 days of grazing, animals eat it poorly because of the bitter taste and pungent odor caused by up to 1.5% coumarin in the plants. However, later the animals become accustomed to it and readily eat it, especially before budding begins. The coumarin content in yellow sweet clover plants is higher than in white sweet clover. It is also higher in the dry steppe zone than in the moist forest-steppe zone. The peak coumarin content in plants occurs during full flowering to the beginning of ripening. During the day, the peak occurs at noon, so grazing and mowing for fodder harvesting are best done in the early morning or evening. Coumarin-free forms of sweet clover exist. According to studies, no harmful effect of coumarin on animal health has been established. Feeding sweet clover improves digestion in animals; in some cases, milk may acquire a specific odor.
Like alfalfa and clover, sweet clover often causes bloat in animals, so cattle are turned out to graze this crop after natural meadows or cereal grasses, so that the animals do not eat a large amount of fresh grass at one time.
In chemical composition, sweet clover is similar to alfalfa, clover, and sainfoin, but is somewhat inferior in feeding value and palatability.
For making silage, mixtures of sweet clover with grasses are better suited.
Sweet clover is characterized by high drought resistance, frost resistance and salt tolerance, as a result of which it is considered one of the best phytomeliorants. Its cultivation promotes soil desalinization and enrichment with nitrogen, making the soil suitable for growing more demanding crops, such as spring wheat, millet, sugar beet, etc. It is suitable for restoring the fertility of solonetzes, sands, mine dumps, quarries, ravines and eroded slopes, lands with a completely or partially removed fertile soil layer, and lands poorly suitable for other plant species.
Sweet clover is one of the best crops used for green manure due to its high yields of green biomass. When it is plowed in, 150–200 kg/ha of nitrogen is added to the soil with the green biomass, which approximately corresponds to applying 30–40 t/ha of manure.
Honey plant. In the quantity and quality of honey collected, sweet clover surpasses phacelia and other honey crops; the harvest from 1 ha reaches 600 kg.
The disadvantages of the crop include: coumarin content in the aboveground part of plants, coarsening of stems after flowering, shedding of seeds and leaves during hay drying, and weed infestation of fields by volunteer plants.
History of cultivation
Sweet clover was first introduced into cultivation as a green manure in the eastern Mediterranean more than 2,000 years ago.
In Western Europe and the USA, it began to be grown in the 18th century.
In Russia, sweet clover was known as a medicinal and honey plant.
Growing regions
Currently, sweet clover is grown in North America and Europe as a green manure and honey crop. In Russia, it is cultivated in the arid regions of Western Siberia, the Volga region, and the Central Chernozem zone.
Yield
Hay yield of sweet clover on light chestnut soils is 2–3 t/ha; on elevated river floodplains and chernozems, 4–6 t/ha. Green biomass yield is 18–20 t/ha; seed yield, 400–800 kg/ha.
Under irrigation, green biomass yield is 39.6–42.7 t/ha (Omskoye farm, Siberian RI of Fodder).
Botanical description
There are 20 species; in agriculture, white sweet clover and yellow sweet clover are used.
Promising sweet clover species also include sweet-scented sweet clover (Melilotus suaveolens Ledeb.), dentate sweet clover (Melilotus dentatus Pers.), Volga sweet clover (Melilotus wolgicus Poiz.), and Polish, or Caspian, sweet clover (Melilotus polonicus (L.) Pall.).
White sweet clover
White sweet clover (Melilotus albus Medik.) is a biennial plant.
The root is a taproot; lateral roots are well developed; it penetrates to a soil depth of up to 1.5 m in the first year of life and up to 2–3 m in the second.
The stem is straight, less often ascending, 75–300 cm tall, hollow. Stem branching begins at a height of 25–30 cm. The upper part of the stem is short-pubescent; the lower part is bare.
The leaves are trifoliate; leaflets of the lower and middle tiers are rounded; those of the upper tier are linear or narrowly elliptic, serrated along the edges.
The inflorescence is an axillary raceme. The flowers are white. Pollination is cross-pollination and self-pollination.
The fruit is a one-seeded, or less often two-seeded, bean of elliptical shape, reticulate-wrinkled, with a short sharp beak. The seeds are oval, yellow or yellowish-green. The mass of 1000 seeds is 1.5–2.4 g.
It is cultivated throughout the European part of Russia: from the Caucasus and Crimea to Arkhangelsk and Murmansk, as well as in Siberia and Central Asia.
Yellow sweet clover
Yellow sweet clover (Melilotus officinalis Pall.) is a biennial plant, rarely annual.
The root is a taproot.
The stem branches strongly, is erect or ascending, with anthocyanin coloration.
The leaflets of the lower tier are obovate or rounded; those of the upper tier are oblong-lanceolate. Leafiness is less than in white sweet clover.
The inflorescence is a long raceme. The flowers are yellow.
The fruit is an ovate bean, transversely wrinkled. The seeds are yellow, yellow-green. The mass of 1000 seeds is 1.5 g.
It is cultivated mainly in the southern steppe regions of the country, although its range almost coincides with the range of white sweet clover.
Biological features
Temperature requirements
Sweet clover is a winter-hardy plant; in this property it is equivalent to alfalfa. Winter hardiness depends on the location of the root collar in the soil; the deeper it lies, the higher the winter hardiness of the plants.
The higher the winter hardiness of a particular sweet clover variety, the higher its drought resistance. Therefore, varieties of steppe origin are more winter-hardy than varieties or forms of more northern origin.
It is ecologically very plastic, tolerates high summer temperatures and low winter temperatures, including spring frosts down to −3…−5 °C.
Seeds germinate at a temperature of 3–4 °C; the optimum is 7–8 °C.
Moisture requirements
Sweet clover is a drought-resistant plant. The critical period in relation to drought is the flowering phase. A moisture deficit during this period leads to leaf drop and deterioration of the fodder qualities of hay. Yellow sweet clover is more drought-resistant than white sweet clover.
Light requirements
Sweet clover is a light-loving plant.
Soil requirements
It is undemanding to soils but prefers loose loamy and chernozem soils rich in lime, with a neutral or slightly alkaline reaction. In the Non-Chernozem zone it gives good yields on production fields with a pH of the soil solution of not less than 5.6. It can grow on light sandy, solonetzic, moistened solonchak, heavy clay, strongly calcareous, stony, and structureless soils.
It tolerates acidic, swampy, strongly moistened, strongly podzolic soils and loose sands poorly.
White sweet clover can withstand flooding for up to 12 days.
Plant nutrition
The removal of nutrients from the soil by sweet clover is higher than by cereal crops: in calcium removal it surpasses them by 3–5 times, potassium by 2–3 times, and phosphorus by 1.5–2.5 times.
For the formation of 1 t of hay, 3–3.5 kg P2O5, 20–24 kg K2O, and 15–16 kg CaO are removed from the soil.
Vegetation
White and yellow sweet clover are plants of the spring type of development, represented predominantly by biennial forms.
Biennial sweet clover, when sown under the cover of another crop, forms a rosette in the first year of life and does not flower. However, with early harvesting of the cover crop and good preservation of the stand, a small cut can be obtained by the end of vegetation. With coverless sowing and sufficient moisture, sweet clover grows to 45–75 cm; in some years it is capable of giving even two cuts in the first year of life.
In the second year of life, sweet clover starts growing earlier than winter rye, awnless brome, or alfalfa. Already at the beginning of May, plant height can reach 25–30 cm.
In the second year of life, white sweet clover grows to 150–350 cm, yellow to 100–150 cm. The first cut is, as a rule, used for hay, haylage, or silage, and the aftermath for green fodder. With early first mowing, it is possible to obtain seeds from the second cut. With sufficient moisture, sweet clover can give 2–3 cuts, provided the first cut is early (not later than budding) and the cut height is not less than 12–15 cm to preserve the renewal buds, which are located relatively high on the stem.
When obtaining seeds from plants of the second year of life after the first cut, sweet clover quickly reaches the fruiting phase, but its fodder qualities decrease.
In spring, regrowth begins from renewal buds located on the root collar. Regrowth after cuts occurs from buds located in the leaf axils of the stem. Therefore, the more axillary buds remain in the uncut part of the stem, the faster regrowth occurs after mowing.
The maximum increase in above-ground mass occurs in the period from the beginning of regrowth to flowering. In the budding phase it averages 3–5 cm per day.
In early northern forms, the duration of the flowering phase is 14–28 days; in late southern forms, 30–45 days. Flowering of an individual raceme lasts 8–14 days; the flowers of the lower part of the raceme bloom first. After flowering, the stems quickly become coarse.
When ripening, the beans easily shatter, which is taken into account when determining optimal harvesting dates.
The vegetation period of sweet clover is 80–140 days. By vegetation duration it is divided into:
- early (80–95 days);
- mid-late (95–110 days);
- late (110–125 days);
- very late (125–140 days).
The earliest forms are cultivated in the forest zone of Russia (Arkhangelsk, Vologda, Leningrad oblasts); mid-late forms in the forest-steppe and steppe; late forms in dry steppes; very late forms in the Mediterranean countries. The longer the vegetation, the taller the plants grow. Early forms of yellow sweet clover are found in the Caucasus (vegetation period 85–90 days); late-ripening forms in the valleys of Azerbaijan (vegetation period 130–140 days).
The height of plants of late forms reaches 3.5 m; of early forms, 1.5 m, sometimes 2 m.
Crop rotation
The best predecessors of sweet clover in crop rotation are considered to be row crops, for example, potato, beet, corn. However, it is an undemanding crop, so it can be grown after any crops.
Fertilizer
The application of phosphorus fertilizers under sweet clover in the northwestern oblasts of Russia increases yield by 1.5–2 times (Leningrad Agricultural Institute). This crop is also distinguished by the ability of its root system to absorb hard-soluble phosphorus compounds, so phosphorite meal shows high effectiveness, especially on acidic soils.
Under the conditions of Omsk Oblast, the application of phosphorus and potassium fertilizers increased hay yield from 4.1 to 4.9 t/ha.
Recommended fertilizer rates: 300–400 kg/ha of superphosphate, 150–200 kg/ha of potassium salt. Fertilizers are applied under the cover crop or after its harvesting in late summer.
Under the conditions of the Non-Chernozem zone, liming of soils to full hydrolytic acidity (1.5–2 t/ha) sharply increased yield.
Soil cultivation
Usually sweet clover is undersown under cover, so soil cultivation corresponds to cultivation for the cover crop.
With coverless sowing, cultivation is carried out in the same way as for small-seeded perennial grasses.
Annual sowing of sweet clover in a mixture with Sudan grass or corn for silage is also practiced.
Sowing
Under the conditions of the arid steppe, sweet clover is recommended to be grown without cover to make it possible to obtain green mass in the sowing year. Under more moist conditions of the forest-steppe, it is sown under the cover of spring cereals, millet, mohar, or fodder crops.
Sweet clover is distinguished by a large number of hard seeds (70–80%), which, retaining germination for up to 10 years, may not produce seedlings in the sowing year, which becomes a cause of weediness in subsequent crops. This feature is due to a very dense seed coat that does not allow water to reach the embryo. Therefore, to break it, a special method of seed preparation is used — scarification, or scratching, with scarifiers or by passing seeds 1–2 times through a clover huller.
Before sowing, the seeds are cleaned and, on the day of sowing, treated with bacterial preparations, for example, nitragin.
The seeding rates of sweet clover for fodder and green manure are 15–20 kg/ha in the steppe zone, 20–25 kg/ha in the Non-Chernozem zone, and 7–8 kg/ha for seeds. On eroded soils, to form a long-lasting stand, the seeding rate is increased to 25–30 kg/ha.
Sowing depth is up to 3 cm.
Sowing time is simultaneously with the sowing of spring cereals, or, less often, after the cover crop has formed 2–4 leaves.
The sowing method for fodder and green manure is ordinary row using disk seeders; for seeds, wide-row (7–8 kg/ha).
If sowing is carried out under annual grasses, it is recommended to harvest them for green fodder rather than hay, in order to free the field earlier. If the cover crop is barley, it is better to harvest at a high cut with combines with choppers, immediately removing straw from the field.
Crop care
When the cover crop ripens, it is harvested immediately. The earlier it is harvested, the more renewal buds form on the root collar of sweet clover, and the better the plants develop in autumn and the higher the yield of hay or seeds in the second year of life.
If necessary, after harvesting the cover crop, mineral fertilizers are applied.
Harvesting
Harvesting sweet clover for hay begins in the budding — beginning of flowering phase. Delay in harvesting leads to coarse hay with a high coumarin content. The cutting height is 12–18 cm; at such a cutting height, better and faster drying of the green mass is achieved, losses during harvesting are reduced, and good regrowth of sweet clover is ensured.
When making hay, large losses often occur due to leaf drop. Therefore, for drying the wilted mass, it is recommended to use active ventilation units.
Growing for seeds
Sweet clover is distinguished by high seed productivity. One plant can produce up to 17 thousand seeds; from one hectare, up to 1.5–1.7 t/ha. This is connected with the possibility of cross-pollination and self-pollination, as well as pollination of flowers not only by bees, wasps, and bumblebees, but also by other insects. Even in unfavorable years with a cold and wet summer, it can produce up to 700–800 kg/ha of seeds. Therefore, large areas of seed plots are not required for production sowings. One hectare of a seed plot can provide seeds for 100 hectares. Seeds can be obtained from ordinary crops or special seed stands, in which sweet clover is planted in a wide-row method with row spacing of 45–60 cm. Since the seeds easily shatter and weed subsequent crops, for seed production it is advisable to allocate out-of-rotation plots for sweet clover.
Seeds can be obtained from ordinary stands from the first or second cut. Mowing of seed stands begins when 1/3 of the beans turn yellow, but not later than when 2/3 of the beans turn brown. The beans ripen well in windrows. The dried mass is harvested with combines with a rubbing device and threshed. The seeds are cleaned, dried to a moisture content of not more than 15%, and stored for long-term storage.
In Western Siberia under irrigation, summer sowings of sweet clover are not inferior to early spring sowings in yield. Thus, seed yield when sown in May was 10.5 c/ha, in June — 11.1 c/ha, in July — 11.2 c/ha.
Growing for green manure
Sweet clover is recommended to be grown for green manure in occupied fallows in northern regions, where lupine and serradella do not manage to produce ripe seeds. This crop grows well on saline soils of the steppe zone.
For green manure, crops of the first or second year of life can be used at a plant height of 30–35 cm and more. Usually it is grown in occupied or green-manure fallows; incorporation into the soil is carried out in the second year of life.
The agrotechnics are usual. The plowing-in dates depend on cultivation conditions. In northern regions, plowing-in is carried out in June–July; in regions with a long summer period, the green mass of the second cut is plowed in, while the first cut is used for hay or haylage.
Before plowing-in, the green mass is rolled and chopped with disk harrows or harrows. Sometimes, for this purpose, a small wooden roller is installed behind the tractor in front of the first body of the plow, pressing the stand before plowing-in. The plowing-in depth is 13–15 cm on heavy soils in granulometric composition and 16–18 cm on medium and light soils.
On saline lands, non-moldboard treatment, milling, or plowing with three-tier plows is used. Saline soils are usually little infested with weeds, so under these conditions coverless sowing is carried out in early spring. Seeding rates for the continuous sowing method are 20–25 kg/ha; for every-other-row sowing, 10–12 kg/ha.
The yield of spring wheat sown after sweet clover was 2.1 t/ha (Leninsky state farm, Kokchetav Oblast, Kazakhstan). The yield of wheat in Irkutsk Oblast after sweet clover increased by 0.4 t/ha, barley by 0.4–0.6 t/ha. The grain yield of rye under the conditions of Estonia after sweet clover fallow, on average over 5 years, was 2.9 t/ha, which is higher than after black fallow fertilized with manure.
Varieties
Varieties of white sweet clover: Iney (1998, Altai RIA), Privolzhsky (1998, Volga Research Institute of Breeding and Seed Production), Om 2 (1999, Siberian RIA), Meshchersky 99 (2000, ARRI of Organic Fertilizers and Peat), Volzhanin (2004, Volga Research Institute of Breeding and Seed Production), Medet, Sretensky 1, Sayansky.
Varieties of yellow sweet clover: Sibirsky 2 (2000, Siberian RIA), Sudogodsky (2002, ARRI of Organic Fertilizers and Peat), Alsheevsky, Omsky skorospely.
Variety of sweet-scented sweet clover: Ononsky (2000, Transbaikal RIA).
References
1. Vavilov P. P., Gritsenko V. V., Kuznetsov V. S. et al. (1986). Crop Production. Ed. Vavilov P. P. 5th ed., revised and enlarged. Moscow: Agropromizdat.
2. Kolomeichenko V. V. (2007). Crop Production/Textbook. Moscow: Agrobusinesscenter.
3. Niklyaev V. S. (2000). Fundamentals of Agricultural Production Technology. Agriculture and Crop Production. Moscow: Bylina.
