Abstract:
Direct response to divergent selection based on estimated breeding value for female pregnancy rate in a herd of Bos indicus cross cattle was evaluated along with correlated responses in calf survival, weaning rate, cow liveweight and calf growth. Pregnancy rates were significantly higher ( P < 0.05) in heifers of the High line compared with the Low line (High-Low=15�_ 5%) and in lactating 4-year-old cows (25�_9.5%) and in 3 of the 5 years of the study (average difference 24%). The Low line had significantly ( P < 0.05) higher pregnancy rates in 1 year (-16�_5.4%). These differences in pregnancy rates were reflected in weaning rates, but calf survival was only different between the lines in 2 of the 4 years analysed (-6�_ 3.0% in 1990 and 18�_6.5% in 1991). The High line had a significantly ( P < 0.05) shorter days to calving than the Low line in heifers (High=309�_2.2 days v. Low= 315�_2.5 days), lactating 3-year-old cows (3l5�_ 3.6 days v. 331�_4.1 days), non-lactating 3-year-old cows (301�_5.4 days v. 320�_4.6 days) and lactating 4-year-old cows (311�_3.9 days v. 336�_4.9 days). Differences between the lines in pre-weaning growth varied according to generation with significantly lower (P < 0.05) weaning weights (182�_3.1 kg v. 189�_3.3 kg) and pre-weaning growth (790�_17 g/day v. 826�_ 17 g/day) in first generation High line calves from foundation dams and no difference between the lines in second generation calves from first generation dams. Post-weaning growth was significantly higher ( P < 0.05) in High line calves (350�_ 5 g/day v. 327�_5 g/day). The results indicate that selection of sires on EBV for pregnancy rate leads to improved fertility in heifers and 4-year-old lactating cows without adverse effects on progeny growth.