The effects of age, weight, and sire on pregnancy rate in cattle

The goal was to estimate the heritabilities and genetic variances for pregnancy rate (PR) and calving date (CD) in Angus cattle along with the effect of weight, age, and sire on PR and CD. The data consisted of 4,999 records on PR and CD. Statistical models included year as a fixed effect; premating/postmating weight and age as covariates; and sire of embryo, maternal grandsire (MGS), and permanent maternal environmental effects as random effects. The models also included the interactions between herd and weight (weight change). Direct and maternal effects on PR and CD were estimated using sire MGS and animal models in REML. Pregnancy rate increased from age 2 to 6 and decreased from age 7 to 11 (P < 0.01) and this effect was independent of the culling strategy. There was a quadratic effect of premating cow weight independent of age on PR, with lower PR for low weights (P < 0.01). Overall, cows with a premating weight of 550 kg had the greatest PR. Cows that lost weight during mating had lower PR (P < 0.01). The maternal additive heritability for PR was 0.001 +/- 0.012 and the direct additive heritability was 0.024 +/- 0.020. The ratio of permanent maternal environmental variance to phenotypic variance was significant (0.048 +/- 0.017; P < 0.01). This demonstrates that permanent maternal environmental effects play a major role in the repeatability of PR (0.049 +/- 0.015; P < 0.01). The maternal additive heritability for CD was 0.040 +/- 0.022, and the direct additive heritability was 0.076 +/- 0.045. The ratio of permanent maternal environmental variance to phenotypic variance was low (0.014 +/- 0.017) and the repeatability for CD was significant (0.0544 +/- 0.0180; P < 0.01). This suggests that maternal genetic effects are as important as direct genetic effects on CD. There was a positive quadratic relationship between premating cow weight and CD with delayed calving for low/high weights (P < 0.01). Cows that lost weight over mating also had a later CD (P < 0.01). Comparisons of a weight-selected herd to the control herd showed differences (P < 0.05) in the optimal premating weight for early calving (control, 480 kg, and weight selected, 615 kg). Calving date was also more sensitive to changes in weight over mating in the weight selection herd (P < 0.05). Therefore, the set point in the weight-fertility axis and the sensitivity of fertility to changes in weight both changed in the weight selection herd.