Meat
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Meat, mainly from farmed animals, is a major source of dietary protein and essential nutrients around the world, averaging about 8% of man's energy intake.[60] The actual types eaten depend on local preferences, availability, cost and other factors, with cattle, sheep, pigs and goats being the main species involved. Cattle generally produce a single offspring annually which takes more than a year to mature; sheep and goats often have twins and these are ready for slaughter in less than a year; pigs are more prolific, producing more than one litter of up to about 11[61] piglets each year.[62] Horses, donkeys, deer, buffalo, llamas, alpacas, guanacos and vicunas are farmed for meat in various regions. Some desirable traits of animals raised for meat include fecundity, hardiness, fast growth rate, ease of management and high food conversion efficiency. About half of the world's meat is produced from animals grazing on open ranges or on enclosed pastures, the other half being produced intensively in various factory-farming systems; these are mostly cows, pigs or poultry, and often reared indoors, typically at high densities.[63]