Genetic association between temperament and heifer rebreeding in Nellore cattle

The aim of this study was to estimate genetic correlations between three temperament traits and the subsequent rebreeding of Nellore heifers. The temperament of 16,874 animals were evaluated at 18 months of age by using: 1) movement score (MOV), measuring the animal movement inside the crush from 1 (without movements) to 5 (jumping); 2) flight speed (FS), measured the speed when each animal left the crush after weighing and; 3) temperament score (TS), applied after exit the chute, scoring the animal in a pen from 1 (best temperament) to 5 (worse temperament). Heifer rebreeding (HR) was used to evaluate the heifers' ability to become pregnant, given that they had calved once. The HR of 30,431 animals were recorded as a binary trait, considering "0" for the heifers that failed or "1" when they succeeded in get pregnant after their first calving. Bayesian inference and Gibbs sampling was performed to estimate (co)variance components applying an animal model which included random effects of additive genetic and residual effects, fixed effects of contemporary group (CG), and age of the animal at yearling as covariable (linear and quadratic effect). A linear model was used for FS and threshold models for MOV, TS and HR. The CG for MOV, FS and TS included farm and year of birth, sex and management groups at birth, weaning and yearling. For HR, the CGs considered farm, season and year of birth, and the classification of being precocious heifers (defined according to the cow ability to calve the first calf before 30 months of age or not), year, season and management groups from birth to weaning of the first offspring. The pedigree file included 61,292 animals. Heritability estimates (? SD) for MOV, FS, TS and HR were: 0.14 ? 0.04, 0.22 ? 0.02, 0.19 ? 0.04 and 0.23 ? 0.02, respectively. The genetic correlations estimates (? SD) between HR and temperament traits are unreliable due to the fact that the estimated standard errors were greater than or equal to the genetic correlations estimates (-0.22 ? 0.11, -0.06 ? 0.10, -0.12 ? 0.14 for MOV, FS and TS, respectively), indicating that there is not genetic association between these traits studied. Based on these results, we conclude that it will be necessary to consider temperament and the success in rebreeding separately in the selection index for Nellore cattle.