Reaction norm model to estimate genetic correlation between growth and reproductive performance of Nelore females depending on environment

In tropical production systems, beef cattle are raised in highly heterogeneous environments. Heterogeneity is therefore expected to exist in the (co)variance components for traits of economic interest in different production environments. The main objective of the present study was to estimate genetic correlations between growth traits and reproductive performance of beef females depending on the environment. This study was conducted in tropical region of Brazil, applying a multiple-trait linear-threshold reaction norm animal model to field records of heifer pregnancy (HP), hip height (HH), body weight at around 18 months of age (BW18) and postweaning weight gain (PWG) from 20,893 Nelore females. The basic elements used to define a common environmental gradient were the solutions of contemporary groups (CG) for the four traits studied. The four environmental variables (standardized CG solutions) were reduced by principal components analysis to one principal component (PC) by choosing the PC that explained most of the total variance. The overall mean (minimum and maximum) of the genetic correlations across the environmental gradient between HH and BW18, HH and PWG and between BW18 and PWG were 0.64 (0.59 and 0.65), 0.49 (0.33 and 0.49) and 0.70 (0.58 and 0.75), respectively. It was shown that the genetic relationship between growth traits tends to be stronger in favorable environments, a fact that should favor correlated responses under these conditions. In contrast, the overall mean (minimum and maximum) of the genetic correlation estimates across the environmental gradient between HP and HH, HP and BW18 and between HP and PWG were -0.17 (-0.24 and 0.06), 0.08 (-0.02 and 0.32) and 0.14 (-0.11 and 0.36), respectively. Thus, the genetic association between the growth traits and heifer pregnancy shows an antagonism, particularly in favorable environments. However, the genetic antagonism between growth and reproduction is relatively small and selection for growth, especially HH, should therefore not lead to important short- or medium-term losses in heifer pregnancy. Thus, the genotype by environment interaction is an important factor to be considered in genetic evaluations of beef cattle raised in tropical environments.