Data from a multibreed composite beef cattle population managed under intensive irrigated grazing conditions were used to estimate direct additive heritabilities (h2a) and maternal heritabilities (h2m) for birth weight (BW) and weaning weight (WW) of the calf and cow efficiency of the dam (CE; WW/dam. weigh 0.75). Calves born between 1968 and 1993 (n = 52 628) had varying levels of Simmentaler genes, ranging from 0% to 97%, with an average of 33.4%. Direct heritabilities fitting unitrait models were 0.66, 0.53 and 0.21 for BW, WW and CE respectively, with corresponding estimates of h2m being 0.22, 0.36 and 0.59. Genetic correlations between direct and maternal effects (ram) were negative for all three traits, varying from -0.32 to -0.63. Direct breeding values for BW and WW decreased and maternal breeding values increased with increasing proportion of Simmentaler. Cow efficiency was unaffected by an increase in the proportion of Simmentaler.
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