Livestock production is a part of agriculture throughout the world, in regions ranging from the most intensive type of scientific development to the crudest form of scavenging. Livestock production is the largest where per capita supply of developed agricultural and industrial resources also is the largest. Greater and/or more efficient productivity from farm animals is partly related to the genetic potential and the husbandry system under which they are raised. The animal that is free to roam expends a portion of its food energy in locomotion. It does, however, have access to soil, sunlight, varied plant life, etc. and thus augments and varies his feed intake to suit his nutrient needs and desires. On the other hand, animals confined to limited quarters, usually indoors, convert feed energy to animal weight gains more efficiently, but this husbandry practice places a greater burden on the feeder since all nutrients must be supplied in the ration or feed in adequate amounts for the optimum physiological animal response and in a palatable and physically acceptable form. Furthermore, under the latter system, the animal is physiologically challenged, stressed by population pressures, more subject to disease exposure, and must be under close scrutiny for problem prevention. While the higher levels of production of animals of better quality have been influenced by improved breeds and strains through genetic selection, by a more intensive system of husbandry, and by greater protection, prophylactically as well as therapeutically, against disease, the recognition of farm animal's practical needs for nutrients and the formulation of rations containing these nutrients to achieve production goals has been a major factor in the year-round economy and efficiency of livestock production.