For example, isolated fast and slow growing muscles from broiler and layer-type poultry may exhibit similar rates of protein anabolism and catabolism when maintained in vitro, despite the occurrence of extreme differences in muscle growth in vivo. However, when myoblasts from embryos of broiler and layer-type chickens are compared in vitro, the greater capacity of cells from broilers to accumulate protein may be attributable to a lower rate of protein degradation. Another subtlety that complicates the picture is that animals with a rapid rate of protein accumulation also may have an elevated rate of muscle protein degradation and it has been proposed that selection for meat yield in poultry should be based on decreasing the rates of degeneration, rather than attempting to increase the rate of synthesis. However, in beef, one would not want to jeopardize post mortem proteolysis because it makes an important contribution to taste and tenderness. Energy content Body weight is partitioned between several commercially important compartments.
Less energy is required to form a kilogram of lean meat than to form a kilogram of dissectable fat.
Thus, a progressive diversion of feed energy away from protein accumulation and towards fat deposition may contribute to the decelerating curve of body weight growth in meat animals, although in consistently fat animals such as pigs this effect may be only slight. The decrease in feed efficiency that occurs with increased live weight in pigs is primarily caused by increased maintenance cost and not increased fat deposition. The time at which the diversion to fat deposition occurs may depend on the breed of animal, and is late in animals with a large adult size. The relative amount of water in the lean tissues of the body decreases with age. The rate of decrease in water content may be altered at weaning and at the onset of sexual maturity.