Biological basis for variation in residual feed intake in beef cattle. 2. Synthesis of results following divergent selection

Experiments on Angus steer progeny following a single generation of divergent selection for residual feed intake suggest that there are many physiological mechanisms contributing to variation in residual feed intake. Difference in energy retained in protein and fat accounted for only 5% of the difference in residual feed intake following divergent selection. Differences in digestion contributed (conservatively) 10% and feeding patterns 2% to the variation in residual feed intake. The heat increment of feeding contributed 9% and activity contributed 10%. Indirect measures of protein turnover suggest that protein turnover, tissue metabolism and stress contributed to at least 37% of the variation in residual feed intake. About 27% of the difference in residual feed intake was due to variation in other processes such as ion transport, not yet measured. It is hypothesised that susceptibility to stress is a key driver for many of the biological differences observed following divergent selection for residual feed intake in beef cattle. Further research is required to accurately quantify the effect of selection for improved residual feed intake on protein turnover, tissue metabolism and ion transport, and to confirm the association between stress susceptibility and residual feed intake in beef cattle.