Feeding
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Animals used as livestock are predominantly herbivorous, the main exceptions being the pig and the chicken which are omnivorous. The herbivores can be divided into "concentrate selectors" which selectively feed on seeds, fruits and highly nutritious young foliage, "grazers" which mainly feed on grass, and "intermediate feeders" which choose their diet from the whole range of available plant material. Cattle, sheep, goats, deer and antelopes are ruminants; they digest food in two steps, chewing and swallowing in the normal way, and then regurgitating the semidigested cud to chew it again and thus extract the maximum possible food value.[30] The dietary needs of these animals is mostly met by eating grass. Grasses grow from the base of the leaf-blade, enabling it to thrive even when heavily grazed or cut.[31]
In many climates grass growth is seasonal, for example in the temperate summer or tropical rainy season, so some areas of the crop are set aside to be cut and preserved, either as hay (dried grass), or as silage (fermented grass).[32] Other forage crops are also grown and many of these, as well as crop residues, can be ensiled to fill the gap in the nutritional needs of livestock in the lean season.[33]
Extensively reared animals may subsist entirely on forage, but more intensively kept livestock will require energy and protein-rich foods in addition. Energy is mainly derived from cereals and cereal by-products, fats and oils and sugar-rich foods, while protein may come from fish or meat meal, milk products, legumes and other plant foods, often the by-products of vegetable oil extraction.[34] Pigs and poultry are non-ruminants and unable to digest the cellulose in grass and other forages, so they are fed entirely on cereals and other high-energy foodstuffs. The ingredients for the animals' rations can be grown on the farm or can be bought, in the form of pelleted or cubed, compound foodstuffs specially formulated for the different classes of livestock, their growth stages and their specific nutritional requirements. Vitamins and minerals are added to balance the diet.[35] Farmed fish are usually fed pelleted