Climatic instability with recurrent drought and heat stress imposes significant constraints on beef production in Namibia. Elucidating animal genetic responses to these environmental pressures is essential for enhancing herd resilience. This study assesses genotype-by-environment interactions (GxE) under prenatal drought and heat stress on birth weight (BW) and weaning weight (WW) in Namibian Simmentaler (SM) and Simbra (SB) cattle. Four environmental conditions (EC) were defined to characterize cumulative precipitations over 365, 280 and 90 days before birth, and average temperature humidity index (THI) over 90 days before birth. The genetic parameters of the traits in each breed were modeled as functions of the EC using a bivariate reaction norm model. Estimated direct heritabilities for BW (SM: 0.22-0.43; SB: 0.35-0.52) increased gradually from drought or heat stress conditions to more favorable EC. Conversely, direct heritabilities for WW (SM: 0.10-0.36; SB: 0.25-0.47) were low under moderate conditions and high at both extreme of the EC gradients. Maternal heritabilities for BW (SM: 0.05-0.22; SB: 0.08-0.19) and WW (SM: 0.04-0.21; SB: 0.07-0.12) were consistently lower and mainly increased with improved EC. These variations in heritabilities underline reduced selection response and genetic gain under drought and heat stress. Negative direct (SM: -0.42-0.66; SB: -0.07-0.88) and maternal (SM: -0.27-0.32; SB: 0.31-0.94) genetic correlations (rgEC) between same traits from extreme EC gradients confirm the presence of strong GxE effects due to precipitation and THI. Direct rgEC were larger for BW and lower for WW, relative to the maternal rgEC, reflecting that GxE mainly influences maternal genetic effects for prenatal growth, and direct genetic effects for postnatal growth. Cumulative precipitations over 365 days and THI over 90 days implied the largest GxE influences on BW and WW, respectively. Simbra showed reduced environmental sensitivity relative to SM, in line with its Brahman background. Comparisons of estimated breeding values along EC gradients evidenced the occurrence of sire re-rankings in both breeds. About 50% robust, 30% plastic and 20% extremely plastic genotypes among the elite bulls in each breed indicate selection opportunities for robustness against time-lagged drought and heat stress. The findings highlight the significance of G × E and the potential to mitigate animal sensitivity, thereby optimizing breeding strategies in Namibian SM and SB cattle.
Vanvanhossou et al. (2026) studied this question.