Genetic evaluation of the ratio of calf weaning weight to cow weight.

The phenotypic ratio of a calf's weaning weight to its dam's weight is 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 200-d calf 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 (100 kg/ kg; n = 4,184), birth weight (kg; n = 5,083), 200-d weight (kg; n = 4,902), 365-d weight (kg; n = 4,626), and mature-equivalent cow weight at weaning (kg; 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 and 3.90 +/- 0.08 yr in the ratio selected line and control line, respectively. The ratio selection line (n = 895) accumulated approximately 4.7 SD more selection differential than the control line (n = 912) over 2.5 generations. Data were analyzed with 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 ratio, birth weight, 200-d weight, 365-d weight, and cow weight, respectively. Estimates for heritability of maternal effects were 0.58 +/- 0.05, 0.10 +/- 0.02, 0.13 +/- 0.02, and 0.10 +/- 0.02 for 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 a trait of the calf, estimated maternal genetic effects on the ratio contained both genetic effects due to dams that environmentally affected progeny performance and 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 kg/generation. 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.