Bayesian threshold-linear model for genetic evaluation of direct and maternal calving traits in Iranian primiparous Holstein cattle

The main objective of this study was to estimate the genetic parameters for multi-trait evaluation of birth weight (BW), gestation length (GL) and calving difficulty (CD) in first-parity Iranian Holstein dairy cattle. The data included 29,950 calving records collected during 1995 to 2014 by the Animal Breeding and Improvement Center of Iran. A threshold-linear sire-maternal grandsire model was fitted. The model included the effect of sex of calf born, age at first calving and calving month (for GL and BW) as fixed effects and sire, maternal grandsire, herd-year-season of calving and residual effects as random effects. Posterior means (posterior standard deviations) of direct heritabilities were 0.12 (0.01) for BW, 0.40 (0.03) for GL and 0.07 (0.01) for CD. The maternal heritability estimates for BW, GL and CD were 0.04 (0.01), 0.07 (0.01) and 0.04 (0.01), respectively. Direct-maternal genetic correlations were -0.15 (0.04) for BW, -0.36 (0.08) for GL and -0.53 (0.14) for CD (P<0.01). Direct additive genetic correlations for BW-GL and BW-CD were 0.39 (0.06) and 0.43 (0.09), and the corresponding maternal additive genetic correlations were 0.58 (0.08) and 0.47 (0.13), respectively. Direct and maternal additive genetic correlations for GL-CD were non-significant. The estimated posterior means for phenotypic and environmental correlations were positive and low to medium. Non-linear phenotypic relationships were found for GL-CD and BW-CD. The existence of corresponding non-linear and also cause-and-effect (which is usually ignored in traditional genetic evaluation programs) relationships between CD-GL and CD-BW may influence the genetic evaluation of these traits under standard mixed models.