Genetic variation in Nelore heifer pregnancy due to heat stress during the breeding season

Abstract The objective of the present study was to identify genetic variation in heifer pregnancy due to heat stress in Nelore cattle during the breeding season. The Nelore heifers were born between 2003 and 2013 on six farms located in three Brazilian States. Heifer pregnancy (HP) was analyzed as a binary trait, with a value of 1 (success) assigned to females that were diagnosed pregnant and a value of 0 (failure) assigned to those that were not pregnant after breeding season. A threshold-reaction norm model was adopted to estimate (co)variance components and genetic parameters over temperature-humidity index (THI) values for HP. On the phenotypic scale, HP of Nelore cattle was slightly reduced by the increase in THI values (-0.85%/THI). The posterior means of the heritability estimates ranged from 0.17 at THI 82.5 to 0.01 at THI 88. The mean genetic correlations for HP between different THI values were below the unity. The genetic correlation between extreme THI values (82.5 and 88) was -0.47 (0.10). Genetic variation in the response to heat stress and heterogeneity in the (co)variance components and genetic parameters were observed across the THI scale. The breeding values of animals for HP were sensitive to changes in THI, thus demonstrating an important effect of the genotype-by-environment interaction due to heat stress.