Response in female fertility and calf growth to selection for pregnancy rate in tropical beef cattle

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.