Written by: Dr. Jacquie Jacob, University of Kentucky
The banning of meat and bone meal in animal diets has led to increased interest in vegetable protein sources. Soybean meal is the predominant protein source for animal diets worldwide, but the increased use of genetically-modified (GMO) soybeans has resulted in an interest in alternative sources of vegetable protein, especially among organic poultry producers. Some other legumes can be used as alternative feed ingredients.
Legumes are plant species of the family Leguminosae. They have seed pods that, when ripe, split along both sides. Legumes are noted for their ability to use nitrogen from the air. This ability is the result of a symbiotic relationship between the plants and bacteria (rhizobia) found in root nodules. This ability to use atmospheric nitrogen reduces fertilization costs and allows legumes to be used in crop rotation to replenish soil that has been depleted of nitrogen. Legumes break the annual cycle of cereals, reducing the buildup of cereal weeds and pests.
The term grain legumes is used to refer to those crops cultivated for immature or mature grain. In contrast, the term forage legumes is used to refer to legumes consumed as forage. Forage legumes can also be used as a source of biomass and green manure.
Legume seeds have twice as much protein as grains. Crude protein content of legume grains ranges from 27% in peas and faba beans to almost 50% in soybeans. Legume grains are also high in iron and B vitamins.
One of the factors limiting the use of grain legumes as feed is the presence of antinutritional factors in legumes that decrease the nutritive value of the grain and, if consumed in large amounts, cause health problems for animals. These antinutritional factors include protease inhibitors, lectins, oligosaccharides, phytate, antivitamins, L-canavanine, tannins, and isoflavones.
Chickpeas (Cicer arietinum) are one of the world's most important grain legumes. More than 70% of the world's chickpea production is in India, where the chickpea plays a valuable role in the human diet. Like other legumes, chickpeas contain such antinutritional factors as protease and amylase inhibitors, lectins, tannins, and oligosaccharides. These antinutritional factors interfere with nutrient absorption from the digestive tract. Most of the antinutritional factors in chickpeas can be deactivated by heat treatment.
Cowpeas (Vigna unguiculata), also called black-eyed peas, are an important grain legume in tropical and subtropical regions. Cowpeas are heat- and drought-tolerant crops. Cowpeas have an amino acid profile that is similar to that of soybeans.
Faba beans (Vicia faba) are grown in several countries, especially in the Mediterranean area. The nutrient content of faba beans makes them look like a suitable substitute for soybean meal, but the presence of antinutritional factors has limited their use in poultry diets.
Field peas (Pisum sativum) are grown in several countries, including Canada. Field peas have been referred to as "feed peas" in Canada and as "protein peas" in Europe. The relatively low levels of antinutritional factors in pea grains eliminates the need for heat treatment of field peas prior to inclusion in poultry diets.
Australia is the dominant world producer of lupins, accounting for around 85% of world production. Lupins are also produced in the United Kingdom and western Canada. The high price of organic feed has been hampering the development of organic poultry production. This has resulted in an increased interest in lupins, which have the advantage of not requiring roasting prior to feeding. There are two classifications lupins (Lupinus species): bitter and sweet. Bitter types are high in alkaloids, compounds that have been bred out of the sweet varieties.
Lentils (Lens culinaris) are grown primarily for human consumption, but lentils that fail to meet food-grade standards are available for use in livestock feeds. Lentils have a relatively high protein content and few digestive inhibitors.
Globally, soybeans (Glycine max) are the most important feed grain legume.
Common vetch (Vicia sativa) is an annual climbing legume. Common vetch originated in southern Europe but is now grown all over the world. Common vetch has many valuable agronomic characteristics: It is resistant to drought and adapted well to semiarid regions. It can also grow in poor soils. The presence of cyanoalanine toxins has limited the use of common vetch seed in poultry diets.
Bitter vetch (Vicia ervilia) is an old grain legume that originated in the Mediterranean and is now grown around the world. It has high yields and is resistant to droughts and insects. It is a good source of energy, and its amino acid profile is similar to that of soybeans. The seeds have been used in animal diets, but the presence of canavanine has limited the use of bitter vetch in poultry diets.
The grain of woollypod vetch (Vicia villosa ssp.) cannot be used in any livestock feed, including poultry feed. The plant does have high feeding value in ruminant diets.