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There is a growing body of evidence that there is genetic variation in beef cattle feed intake relative to their liveweight and weight gain. Difference in feed intake, above and below that expected or predicted on the basis of size and growth, is measured as residual feed intake. Variation in residual feed intake must be underpinned by measurable differences in biological processes. This paper summarises some plausible mechanisms by which variation in efficiency of nutrient use may occur and presents several testable hypotheses for such variation. A companion paper [Richardson and Herd (2004) Aust. J. Exp. Ag. 44, 431–441] presents results from experiments on cattle following divergent selection for residual feed intake. There were at least 5 major processes identified by which variation in efficiency can arise. These are associated with variation in intake of feed, digestion of feed, metabolism (anabolism and catabolism associated with and including variation in body composition), activity and thermoregulation. The percentage contribution of different mechanisms, to variation in residual feed intake, was: 9% for differences in heat increment of feeding; 14% for differences in digestion; 5% for differences in body composition; and 5% for differences in activity. Together, these mechanisms may be responsible for about one-third of the variation in residual feed intake. The remaining two-thirds were likely to be associated with heat loss due to variation in other processes, such as protein turnover and ion transport. There is no shortage of candidate mechanisms that, singularly or in combination, might contribute to genetic variation in energy utilisation in ruminants. Further research in beef cattle, to better define these mechanisms and enable their incorporation into breeding programmes, may lead not only to cattle which eat less for the same performance, but are superior in other traits as well. |
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