LAUSR

The age at first calving from Brown Swiss heifers raised in the Semiarid of Brazil was analyzed as well as, the time until the event by the non-parametric method of Kaplan-Meier and the gamma shared frailty model, under the survival analysis methodology. Survival and hazard rate curves associated to this event were estimated and identified the influence of the considered covariates over time. The mean and median ages at the first calving were 987.77 days and 1003 days, respectively, and the significant covariates through the Log-Rank test, in the Kaplan-Meier analysis, were season of birth, calving year, sire (heifer’s father) and calving season. In the analysis by frailty model, the genetic values and the frailties of the sires (fathers) for the first calving of their daughters were predicted modeling the hazard function of each heifer as a function of the birth season as a fixed covariate and sire as a random covariate. The frailty followed the gamma distribution. Sires with high and positive genetic values possessed high frailties, which meant a shorter survival time for their daughters at the event, i.e., reduction in their age at first calving.