Author
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Macneil, Michael |
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Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 1/4/2005 Publication Date: 4/1/2005 Citation: Macneil, M.D. 2005. Genetic evaluation of the ratio of calf weaning weight to cow weight. Journal of Animal Science 83:794-802. Interpretive Summary: Nutritional cost of maintaining females is a substantial portion of input to cow-calf production and cow weight is significantly associated with that cost. Weaning weight represents the output from cow-calf production systems. Thus, the phenotypic ratio of the weaning weight of a calf to the weight of its dam may be an indicator of efficiency, and has been postulated to be a reasonable measure of economic utility of a commercial beef cow. However, the utility of this ratio has been questioned from both statistical and biological perspectives and no partitioning of genetic effects on the ratio has been reported. Thus, objectives of this research were to estimate genetic parameters for the ratio, its components, and other growth traits; and to evaluate responses to selection based on the ratio. Selection for a ratio is complicated by shifting emphasis given to the component traits Phenotypic selection for the ratio of calf weaning weight to cow weight is further complicated by measurement of the two component phenotypes on different individuals and the consequent confounding of direct and maternal genetic effects on these phenotypes. Thud, results contraindicate use of the ratio of calf weaning weight to cow weight as a selection criterion and selection index procedures may yield a better defined criterion with clear and consistent emphasis on the traits and their genetic components. Technical Abstract: The phenotypic ratio of a calf 's weaning weight to its dam's weight is often thought to be an indicator of efficiency of the cow. Thus, the objectives of this research were to 1) estimate genetic parameters for the ratio of calf 200-d weight to mature-equivalent cow weight at weaning, its components, and other growth traits; and 2) evaluate responses to selection based on the ratio. Phenotypes evaluated were the ratio (n = 4,184), birth weight (n = 5,083), 200-d weight (n = 4,902), 365-d weight (n = 4,626), and mature-equivalent cow weight at weaning (n = 4,375). In 1989, a randomly selected and mated control line and a line selected for greater values of the ratio were established. Average generation intervals were 3.39 ' 0.05 yr and 3.90 ' 0.08 yr in the ratio selected line and control line, respectively. The ratio selection line (n = 895) accumulated approximately 30 ratio units more selection differential than the control line (n = 912) over 2.5 generations. Data were analyzed using a multiple-trait Gibbs sampler for animal models to make Bayesian inferences. Heritability estimates (posterior mean ' SD) for direct effects were 0.20 ' 0.03, 0.46 ' 0.04, 0.48 ' 0.03, 0.58 ' 0.04, and 0.76 ' 0.02 for the ratio, birth weight, 200-d weight, 365-d weight, and cow weight, respectively. Estimates for the heritability of maternal effects were 0.58 ' 0.05, 0. 10 ' 0.02, 0.13 ' 0.02, and 0.10 ' 0.02 for the ratio, birth weight, 200- d weight, 365-d weight, respectively. Significant response to selection was limited to maternal effects; 1.32 ' 0.38 ratio units per generation. As the ratio was considered a trait of the calf, estimated maternal genetic effects on the ratio contained not only true maternal effects but also direct effects on the reciprocal of cow weight. In the control line, genetic trends in direct and maternal 200-d weight were -1.28 ' 0.91 and 0.62 ' 0.92 kg/generation respectively and the genetic trend in direct effects on cow weight was -5.72 ' 2.80 kg/generation. In the selection line genetic trends in direct and maternal 200-d weight were 1.43 ' 0.79 and 2.90 ' 0.80 kg/generation and the genetic trend in cow weight was -2.79 ' 2.43. Significant correlated responses were observed in direct effects on birth weight and maternal effects on 365-d weight. Results contraindicate use of the ratio of calf weaning weight to cow weight as a selection criterion. |
