D.H. “Denny” Crews Jr
Published April 28, 2004
Genet. Mol. Res. 4 (2): 152-165 (2005)
About the Authors
D.H. “Denny” Crews Jr
Corresponding author
D.H. Crews Jr.
Email: dcrews@agr.gc.ca
ABSTRACT
Selection for the wide range of traits for which most beef breed associations calculate expected progeny differences focus on increasing the outputs of the production system, thereby increasing the genetic potential of cattle for reproductive rates, weights, growth rates, and end-product yield. Feed costs, however, represent a large proportion of the variable cost of beef production and genetic improvement programs for reducing input costs should include traits related to feed utilization. Feed conversion ratio, defined as feed inputs per unit output, is a traditional measure of efficiency that has significant phenotypic and genetic correlations with feed in take, growth rate, and mature size. One limitation is that favorable decreases in feed to gain either directly or due to correlated response to increasing growth rate do not necessarily relate to improvement in efficiency of feed utilization. Residual feed in take is defined as the difference between actual feed in take and that predicted on the basis of requirements for maintenance of body weight and production. Phenotypic independence of residual feed in take with growth rate, body weight, and other energy depots can be forced. However, genetic associations may remain when a phenotypic prediction approach is used. Heritability estimates for phenotypic residual feed in take have been moderate, ranging from 0.26 to 0.43. Genetic correlations of phenotypic residual feed in take with feed in take have been large and positive, suggesting that improvement would produce a correlated response of decreased feed in take. Residual feed in take estimated by genetic regression results in a zero genetic correlation with its predictors, which reduces concerns over long-term antagonistic responses such as increased mature size and maintenance requirements. The genetic regression approach requires knowledge of genetic covariances of feed in take with weight and production traits. Cost of individual feed in take measurements on potential replacements must be considered in implementation of national cattle evaluations for efficiency of feed utilization. These costs need to be compared to expected, and, if possible, realized rates of genetic change and the associated reduction in feed input requirements.
Key words: Beef cattle, Feed efficiency, Genetic evaluation.