Nutritional and endocrinological influences on the composition of animal growth.

Several domesticated species of mammals are raised to provide protein for human consumption. The goal of producers is to generate a product, lean meat, efficiently. Much of the weight gain of animals is fat, a large portion of which is not needed by the animal and unwanted by the human consumer. Thus, improvement in efficiency of animal growth must consider not only rate of weight gain and use of feed to support the gain but also the composition of the gain. One method to improve efficiency of growth, including body composition is to lower the energy intake of animals; feeding less than ad libitum is particularly effective for some species. There are three types of endocrines, anabolic steroids, growth hormone and adrenergic agonists, that have demonstrated ability to change mammalian growth and body composition. Exogenous anabolic steroids increase weight gain, improve efficiency of feed utilization and change body composition toward more muscle and less fat. They are especially effective in ruminant species and are used in beef cattle. Exogenous growth hormone has the same effects as anabolic steroids but is effective in nonruminant species as well. Exogenous adrenergic agonists, analogs of norepinephrine, have marginal effects on weight gain and feed efficiency but favorably change body composition in all species. As more is understood about the control of endocrine secretion, the possibility arises for animal manipulation by administration of exogenous releasing factors or release-inhibiting factors or by changing central nervous system neurotransmitter control of the hypothalamic production and release of these factors. It is also possible to use selective immunization to modify endogenous hormones. Thus, biological knowledge coupled with modern technology continues to open many frontiers to favorably change animal growth and composition. Some of these approaches are in use, several are in experimental use and others await experimental demonstration of utility.