Nutritional constraints to lean tissue accretion in farm animals.

INTRODUCTION During the last decade the emphasis in animal production has switched from promoting maximum or near maximum growth to altering the efficiency and composition of growth, and the enhancement of lean tissue growth or protein accretion. These changes have been brought about by increasing consumer demand for leaner meat products, and the realization that at the same level of feed or energy intake enhancement of lean tissue growth is associated with improvements in the rate and efficiency of liveweight gain, and with reduced lipogenesis and carcass fatness. The extent to which lean tissue growth is influenced by nutrition is reasonably-well established. The basic principles determining the response of lean tissue growth to change in nutrient intake also appear to be qualitively the same for most domestic animal and avian species. Nevertheless, animal species differ in their capacity for lean tissue growth and some are more limited by nutrition than others. Because of their inherently-low appetite ruminant species probably never consume sufficient energy to express fully their genetic potential for lean tissue growth. On the other hand, the appetite of growing pigs may exceed their inherent capacity for lean tissue growth, and energy intake often has to be restricted to prevent excessive fat development. Even within a species the capacity of the growing animal for lean tissue growth is influenced by factors such as live weight, sex and breed or genotype, and any discussion of lean tissue growth would be incomplete unless the interrelationships between nutrient