Legumes

Plants of the legume group belong to the family Fabaceae. Their fruit is a pod. Seeds are called pulses. They are grown mainly for feeding purposes, as food to a lower extent only. In animal nutrition dry seeds and whole plants are used (often in mixture with cereals) for green feeding, ensiling and drying. For human nutrition dry or immature seeds are used, or immature fruits (green beans). From the cultivation viewpoint they are appreciated for their high value as previous crops.

Nutrient content in grain:

Legumes have a high content of crude protein (about 20-35%).

Biological value is higher than that of cereals, unlike cereals they contain more lysine, but less sulphur amino acids. Fat content is lower than in cereals (about 1.5%). An exception is soya that contains about 20% fat. Fat is used for food production purposes, therefore soya is also ranked among oil crops. Carbohydrate content is about 50%, it is mainly starch. Crude protein content is relatively low (5-7%). In legumes there are higher contents of minerals than in cereals, from macroelements, the highest contents are achieved by potassium, phosphorus and calcium. Also calcium to phosphorus ratio is more favourable that in cereals (Ca : P = 1 : 3-4). From microelements, there are higher contents of iron, zinc and manganese. From vitamins there is the highest content of B vitamins, and small amounts of vitamins C, D, and b-carotene.

Legumes have some antinutritional properties. Oligosaccharides contained in legumes are not digested by host digestive enzymes and proceed to the large gut where they are degraded by microorganisms forming large amounts of gas. Therefore they may cause bloating. These oligosaccharides disappear during seed germination. In legumes there are also other anitinutritional substances (tannins, protease inhibitors, lectins, antigenic proteins, saponins).

 Tannins (“bitter substances“) belong to phenolic substances. They have a typical tart taste. They decrease feed palatability and feed intake. Their biological effect is based on the ability to react with proteins. They form complexes with food dietary proteins thereby decreasing their resorption. The greatest decrease in absorption is in essential amino acids methionine and lysine. Beside dietary proteins they also react with digestive enzymes. This leads to decreased digestibility of other digesta components, too. Tannins may also inhibit ruminal microflora. Tannins are not degraded in the digestive tract under normal conditions and they do not penetrate through the gut wall. When high amounts of tannins are ingested, however, the gut epithelium can be irritated because tannins react with the gut wall proteins. Under such circumstances tannins can be absorbed. If detoxification mechanisms of the host cannot cope with the absorbed phenols, the liver and kidneys may be impaired.

Tannins can also exert favourable effects. In ruminants they prevent bloating because they decrease the stability of foam formed in the rumen when feeding some kinds of feed. They may also increase protein utilization because protein complexes are protected against degradation by rumen microflora. In the acid environment of the abomasums, the complexes are broken and proteins can be digested.

Protease inhibitors are polypeptides and proteins forming stable complexes with proteolytic enzymes. These complexes have no more enzyme activity. In animals they cause growth retardation, due to decreased protein availability. There can also be hypertrophia or hyperplasia of the pancreas, due to an increased enzyme secretion. Young animals are very sensitive to protease inhibitors. Protease inhibitors are usually thermolabile, therefore they can be inactivated by heat.

Lectins are proteins of glycoproteins capable of binding to carbohyrates. Binding to the gut villas cells increases their turnover rate. Irritation of the gut wall may cause the gut hypertrophy. Lectins also decrease activity of many digestive enzymes, as a result of increased secretion the pancreas hypertrophy may develop. Mainly young animals are sensitive to lectins, their growth is slowed down and body reserves of lipids and glycogen may drop. Lectins are more thermostable than protease inhibitors, therefore higher temperatures are needed for their inactivation.

Antigenic proteins are resistant against breaking by digestive enzymes. They may penetrate the gut barrier uncleaved and induce the immune response. Their occurrence in feed causes a damage of the gut mucosa in sensitized individuals. This may lead to a decrease digestive and absorption function of the gut. Also the passage of digesta may speed up and diarrhea may occur. Young animals are sensitive to the effects of antigenic protein. Antigenic proteins are thermostable. Other technological treatments than a heat treatment can pose other risks, therefore the best measure to take is to exclude legumes from the diets for very young animals.

Saponins are glycosides that received their name due to the ability to form foam and their wetting effect. They have a bitter or tart taste and their effect consists in the formation of complexes with sterols present in cell in membranes, thereby increasing the membrane permeability. Thus they may damage the small intestine mucosal cells, but these are probably removed during normal gut epithelium exchange. An unfavourable effect is that through the impaired membranes some undesirable components of digesta may penetrate. With a high intake of saponin containing feed the growth is retarded and availability of basic nutrients is decreased, as well as other nutrients, e.g. of zinc. The mechanism of these phenomena is not known, however. Saponins are toxic for poikylothermic animals (fish, gastropods, insects). Saponines can also have favourable effects because they react with sterols in the digesta, particularly cholesterol, forming insoluble complexes that cannot be absorbed.

Seed structure:

Seeds of most legumes have a similar anatomy. On the surface there is a skin-like episperm, in some legume species there is the endosperm underneath. Inside the seed there is a sperm consisting of two cotyledons and the germ.

Episperm is covered with a thin cuticle on the surface. Underneath there is a layer of high column-like cells placed next to each other, so called palisade cells. These cells give hardness to the episperm and can contain pigments that give colour to seeds. Under those there is a layer of cup-shaped cells. The cup-shaped cells have both ends widened, so between their middle parts intracellular spaces are formed that enable the flexibility of episperm. Underneath there is a thin call wall parenchyma with conducting strands which changes into spongy parenchyma in deeper layers.

Endosperm is under the parenchyma and is present only in proteinaceous legumes. It makes up only a small part of the seed and can be either fully developed , consisting of the outer layer of aleurone cells and the inner layer of mainly slime cells or there can be its remainder only.

Cotyledons take up the biggest part of the seed. On the top they have a thin cuticle. According to their composition, legumes are divided into amylaceous and proteinaceous. In most legumes (e.g. green peas, lentils, horse bean, bean, vetch) they are filled with starch grains that are similar to each other and have an oval shape and an S- shaped, often branched, fissure in the middle. These legumes are amylaceous ones. The other ones are proteinaceous legumes. Their cotyledons are filled with protein and they contain only a few starch grains.

Agronomic importance:

Legumes favourably influence soil fertility. Their positive properties include:

-        The ability to bind atmospheric nitrogen by means of symbiosis with nodular bacteria. Nitrogen received in this way covers nearly all nitrogen requirement by the plant and enriches soil for subsequent crops.

-        Improvement of soil structure by means of a massive root system

-        Suitable composition of post harvest residues (N : C = 1 : 6-8), it is not necessary to apply inorganic nitrogen to support mineralization

-        Suppression of light-requiring weeds, because legumes shade soil with their abundant foliage

-        Ability to obtain nutrients from deeper layers of the soil profile that are not easily accessible for other plants, and their partial release to the soil

 Legumes also have some negative cultivation properties, namely:

-        Sensitivity to weather changes resulting in different yields between the years

-        Sensitivity to low temperatures (particularly thermophilic species – soya, beans)

-        Procumbency of some species

-        Dehiscence of pods, which leads to higher harvest losses

-        Susceptibility to mechanical damage of seeds during harvest

-        Low competitiveness with weeds in early growth phases caused by their slow initial growth

-        Intolerance to being grown in the same field for successive years