Nutritional factors affecting growth of muscle and adipose tissue in ruminants.

Patterns of tissue growth determine composition of growth, and over intervals of time, these patterns of tissue growth will also be reflected in total tissue accumulated in the animal. Although patterns of tissue growth may be modified by the effects of specific carbon sources through direct impact on lipogenesis, the primary avenue through which nutritional factors regulate or direct protein and fat deposition is through effects of total absorbed energy provided on rates of growth in relation to an animal's daily needs for maintenance and protein growth. Even though mature size and genetic potential establish the maximum upper limits for daily protein growth, other factors determine the extent to which these theoretical limits will ever be achieved. The actual partitioning of absorbed nutrients between protein and fat deposition also depends on other factors, including stage of growth, sex class, hormonal regulation (endogenous and exogenous), and intake of required nutrients. The sex class of an animal establishes different biological limits for bulls, steers, and heifers that are otherwise similar in mature size, genetics, and nutritional adequacy. Also, the use of anabolic agents alters the physiological limit for daily protein growth at the cellular level and allows an animal to deposit protein at rates closer to theoretical genetic limits. Energy that is provided above maintenance and protein growth requirements enhances rates of lipid deposition and accumulation of fat. As a consequence, the rate of growth will direct these tissue responses, and effects of diet, energy level, level of nutrition, and plane of nutrition will impact composition of growth primarily through acceleration of rate of growth above daily limits for protein growth. Changes in tissue storage will impact total body composition to the extent of time and differential in composition of gain and of tissue present.