Effect of previous reproductive status of dam on the preadjustment of weaning weight for genetic evaluation in tropical beef breeds

The effects of dam previous reproductive status (PRS) on the pre-adjustment of weaning weight for genetic evaluation (WWT) was examined for Santa Gertrudis (SANTA), Brahman (BRAH) and Tropical Composite (TCOMP) breeds of beef cattle. Weaning weight records were classified into 3 groups according to the dam's PRS: whether in the last year, she had reared a bull calf (PBC), a heifer calf (PHC) or no calf (PNC). Least squares means showed that calves born to PNC dams had consistently higher WWT than those which had previously reared a calf. Calves born to PBC cows had the lowest weaning weight across the 3 groups and were 5.9 to 16.6 kg lighter than the calves born to PNC cows across the 6 dam age classes studied for SANTA. When age of calf at weaning was fitted as a covariate, the differences between the PRS groups reduced, with calves born to PBC cows being 0.5 to 4kg lighter than the calves born to PNC cows for SANTA. For BRAH and TCOMP the differences were 1.2 to 3.4kg and 5.6 to 10.1 kg respectively. For TCOMP, adjusting for weaning age reduced the effect, though WWT differences between PNC and the PHC and PBC categories remained significant. These results demonstrate that differences in weaning weight across the 3 PRS groups were due primarily to PNC cows calving earlier, and producing older and heavier calves at weaning than dams which had reared calves in the previous year.