The bulk of the protein of ruminants comes from plant sources. The protein content of plant varies considerably from one type to another. Even within the same plant, there is considerable variation from one stage of maturity to another or from one part of the plant to another. Proteins in plants are primarily associated with the tissues which are actively metabolizing such as leaves, centers of growth and the seeds. Eventhough they are not especially high in protein by comparison with other feedstuffs, the vegetative portions of many plants supply an extremely large portion of the protein in the total ration of livestock, simply because these portion of feeds are consumed in large quantities. Needed protein not provided in these feeds is commonly obtained from one or more of the oilseed by-products –soybean meal, cottonseed meal, groundnut meal, safflower meal, sunflower meal, rapeseed meal linseed meal, sesamum meal and coconut meal. The protein content and feeding value of these products vary according to the seed from which they are produced, the amount of hull and/or seed coat included and the method of oil extraction used. Sometimes the unprocessed seed is used to provide both a source of protein and a concentrated source of energy. The oil bearing seeds are especially high in energy because of the oil they contain.
Additional plant proteins are obtained as by-products from grain milling, brewing and distilling and starch production. Most of these industries use the starch from grains and seeds, then dispose of the residue which contains a large portion of the protein of the original plant seed.