Genetic Parameters of Age at Conception in Nellore Females Using Threshold Models.

Age at conception is a critical factor in the intensification of production systems. It is crucial for intensifying production, optimising reproductive efficiency and boosting Nellore female productivity for genetic progress. However, the low heritability estimates currently limit the effectiveness of selection responses, thereby underscoring the necessity for more precise genetic parameter estimation methods. The aim of this study was to evaluate the genetic associations between body weights at weaning and yearling, and age at conception, in Nellore females. The methodology employed in this study involved the utilisation of a mixed polychotomous threshold model, a methodological framework that has been deemed appropriate for the analysis of categorical variables characterised by an underlying continuous distribution. The records of body weights at weaning and yearling, in conjunction with female age at conception, from 796 animals, were treated as a categorical variable associated with conception success. The (co)variance components were estimated via Bayesian inference using a Gibbs sampler. The mean heritability values were 0.43 (0.27; 0.60) for weaning weight, 0.63 (0.46; 0.81) for yearling weight, and 0.19 (0.06; 0.40) for the categorical variable of age at conception. While body weights exhibited a high additive genetic correlation were (0.79; 95% CI: 0.57; 0.96), correlations were lower between the categorical variable and weaning (-0.21; 95% CI: -0.75; 0.28) and yearling (0.34; 95% CI: -0.14; 0.71) weights. The study concluded that indicators of age at conception should incorporate additional selective criteria beyond body weight in order to improve the probability of conception.