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Abstract
Genetic variations for feed utilization efficiency widely exist in meat-type chickens, even in populations that have undergone selection for the trait. The trait has moderate heritability and is moderately correlated with growth rate. The genetic basis for the trait was thought to be different from that for growth even though they may be related. As a result, the improvement in feed efficient through selection for body weight should be limiting. Fast growing birds require more nutrients and energy for maintenance. When age-fixed measure is used, part of the genetic variation of feed efficiency for growth may be masked. For this reason, adjusted age-fixed feed efficiency should be used in commercial breeding. The combination of restricted maximum likelihood (REML) method with an animal model can greatly account for the effect of selection of breeding population, leading to more accurate estimates of genetic parameters. The identification of quantitative trait loci (QTL) for feed efficiency in chickens has begun. The incorporation QTL in the prediction of breeding value will further improve selection efficiency for feed efficiency. While it was generally assumed that there is little variation in digestibility and metabolizability of feed within and between different strains or breeds, selection for growth rate has resulted in increased digestive enzyme activity. This suggests that the improvement in digestive ability could not be ignored in investigating new approach for increasing feed efficiency in meat-type chickens. Selection for improved FCR makes birds deposit less of the retained energy as fat and convert more energy and nitrogen into body protein. A negative effect of selection for feed efficiency is that, the combination of fast growth and improved FCR is related to increased occurrence of ascites. Index selection based on weight gain and feed intake could be better for improving FCR.
