Generic analysis of age at first calving and scrotal circumference in Nellore cattle breed

Aiming to study age at first calving (AFC) and scrotal circumference (SC) and their relationship to weighttraits in a Nellore herd, 2, 796 observations for AFC, 2, 746 for SC, 3,657 for weaning weight and 4,686 for 18 month weight were analyzed. Analyses were processed using two and three-trait animal models (with weaning weight or 18 month weight as third trait). The 18 month weight was included as a covariate for SC in. two analyses, aiming to adjust SC for body weight Heritability coefficients for AFC and SC varied from 0.09 to 0.10 and from 0.33 to 0.34, respectively Genetic correlation between AFC and SC varied from -0.23 to -0.29. The favorable correlation between SC and AFC does not allow to discard the use of SC as a selection criterion for sexual precocity. Multiple-trait analyses including the 18 month weight seem to be more appropriate for age at first calving and for 18 months scrotal circumference evaluations.