High lean content or high lean growth rate - implications for nutrition

The effects of selection for high lean content in the carcass versus selection for high lean growth rates, on parameters such as voluntary food intake, relationship between protein deposition and liveweight, the net efficiency of utilization of apparently digested ideal protein, maintenance energy requirements and changes in the body water:protein ratio were examined. It was concluded that selection for high lean content had been effective in producing animals which attained high lean growth rates at lower energy intakes than those observed where high lean growth rate had been the main basis for selection. Consequently there were marked differences in body composition. Animals selected for high lean content had high maintenance energy requirements which could be “explained ” by the higher proportion of lean tissue and possibly by higher rates of protein turnover. They also had higher water:protein ratios in the empty body at slaughter presumably reflecting a younger physiological age. Pigs treated with exogenous growth hormone (GH) showed high rates of lean growth similar to those observed with the best selected animals. The effects on maintenance energy requirements and water:protein ratios were not so marked, presumably due to the limited periods of treatment imposed. It appeared that the efficiency of utilization of absorbed ideal protein (e,) was constant for a particular weight range when energy intake was restricted. However, there is clearly an effect of liveweight/age and possibly an effect of genotype which require further research before the (e,) term can be fully defined. The approach of using a minimum 1ipid:protein ratio related to genotype is not completely satisfactory and it is probably more scientifically correct to consider lipid deposition purely as the end product of the residual energy available for production after protein gain and maintenance needs have been met, provided that a sufficiently accurate means of predicting ep can be established. Thereafter the main factor separating different genotypes m terms of growth rates, feed efficiency and body composition is the voluntary food intake