REVIEW OF SOME ASPECTS OF GROWTH AND DEVELOPMENT OF FEEDLOT CATTLE

Growth in animals is defined as accretion of protein, fat, and bone. A lthough growth typically is measured as the change in Live weight, nut rient retention is estimated more precisely by measuring empty body we ight and composition, whereas production economics are measured ideall y through carcass weights and quality. As a percentage of Live weight gain, carcass weight gain usually is a much higher percentage during t he feedlot phase than during the growing phase of production because d ressing percentage (ratio of carcass:live weight) increases with matur ation and is greater with concentrate than with roughage diets. At a g iven fraction of mature body size (maximum body protein mass), body fa t percentage seems to be a constant. Mature size may be altered geneti cally and nutritionally. Protein accretion declines to zero when cattl e reach their mature body size (approximately 36% fat in empty body we ight in modern cattle) even though mature animals can continue to accr ete fat. Although fat accretion can be reduced by limiting the supply of net energy, rate of fat accretion by finishing steers given ad libi tum access to high-concentrate diets seems to reach a plateau at appro ximately 550 g daily. Protein mass, in contrast, increases in proporti on to empty body weight. The protein:fat ratio of the carcass can be i ncreased through increasing mature size, by administering hormones or hormonal modifiers, by limiting energy intake during the growing perio d or finishing period, or by slaughtering cattle at an earlier stage o f maturity. Energetically, efficiency of accretion of fat is approxima tely 1.7 times that of protein. But because more water is stored with deposited protein than with deposited fat, lean tissue gain is four ti mes as efficient as accretion of fat tissue. Conversion of protein to fat is very inefficient, suggesting that excess protein is utilized in efficiently.