Additive genetic relationships between scrotal circumference, heifer pregnancy and stayability in Nellore cattle.

Abstract

Scrotal circumference data from 47,605 Nellore young bulls, measured at around 18 months (SC18), were analyzed simultaneously with 27,924 heifer pregnancy (HP) and 80,831 stayability (STAY) records, in order to estimate their additive genetic relationships. Additionally, the possibility that economically relevant traits measured directly in females could replace SC18 as a selection criterion was verified. HP was defined as the observation that a heifer conceived and remained pregnant, which was assessed by rectal palpation at 60 days. Females were exposed to sires for the first time at about 14 months of age (between 11 and 16 mo). STAY was defined as whether or not a cow calved every year up to 5 yr of age, when the opportunity to breed was provided. A Bayesian linear-threshold-threshold analysis via Gibbs sampler was used to estimate the variance and covariance components of the multi-trait model. Heritability estimates were 0.42 +/- 0.01, 0.53 +/- 0.03 and 0.10 +/- 0.01, for SC18, HP and STAY, respectively. The genetic correlation estimates were 0.29 +/- 0.05, 0.19 +/- 0.05 and 0.64 +/- 0.07 between SC18 and HP, SC18 and STAY, and HP and STAY, respectively. The residual correlation estimate between HP and STAY was -0.08 +/- 0.03. The heritability values indicate the existence of considerable genetic variance for SC18 and HP traits. However, genetic correlations between SC18 and the female reproductive traits analyzed in the present study can only be considered moderate. The low residual correlation between HP and STAY suggests that environmental effects common to both traits are not major. The high heritability estimate for HP and the high genetic correlation between HP and STAY obtained in the present study confirm that EPD for HP can be used to select bulls for the production of precocious, fertile and long-lived daughters. Moreover, SC18 could be incorporated in multi-trait analysis to improve the prediction accuracy for HP genetic merit of young bulls.