• Heat stress effects on growth were evaluated in a local dual-purpose cattle breed. • Thermal sensitivity changed across early, intermediate, and late growth stages. • Heat stress reduced growth rate, mainly in early adaptation and under lasting heat. • Little variation occurred in genetic merit rankings across thermal conditions. • Findings support breed resilience but suggest complementing evaluations with thermal sensitivity. Climate change and higher temperatures are increasingly challenging livestock production systems, raising interest in resilience and adaptability. This study evaluated genotype-by-environment interactions (G×E) for average daily gain (ADG) and body weight (BW) in Rendena young bulls, using performance test data from 1 792 individuals collected between 1985 and 2023, within an 11-month testing period. Reaction norm models were fitted using both average temperature-humidity index (THIₘ) and average maximum temperature-humidity index (THIMAXₘ) as environmental covariates. Since both indices showed comparable model fit, THIMAXₘ was selected to characterize thermal conditions. Analyses were performed on ten consecutive subperiods (P1-P10), corresponding to the time intervals between successive weighings, and on adaptation, growing, and total phases to investigate how environmental sensitivity varies across physiological stages. Results showed that G×E patterns were detectable but strongly phase dependent. For ADG, the genetic correlation between intercept (baseline performance) and slope (environmental sensitivity) was negative in the initial phase (P1: −0. 74), likely reflecting stress associated with transport, arrival, and acclimatization. Positive correlations were observed in the following recovery period (P2: +0. 71) and in the broader adaptation phase (+0. 63), indicating improved expression of genetic potential as conditions shifted from cold and stressful to mild and more favourable. Under sustained heat load, however, the correlation resulted negative, particularly in the growing phase (−0. 79), suggesting that animals with higher genetic merit for growth may be more sensitive to thermal stress. Heritability estimates from reaction norm models also varied along the environmental gradient. For ADG, heritability in the growing phase declined from approximately 0. 75 under cooler conditions to 0. 20 at high THIMAXₘ levels, consistent with reduced additive genetic expression under heat stress. Despite these changes in variance components, impact of G×E was limited. Spearman rank correlations between estimated breeding values at low (10th percentile) and high (90th percentile) THIMAXₘ levels remained consistently high, especially in growing and total phases (>0. 99 for both traits). Overall, the findings indicate that while genetic variation in heat stress sensitivity exists in the Rendena breed, it leads mainly to changes in scale rather than substantial re-ranking. Current selection strategies for growth appear robust across the range of thermal conditions represented, although monitoring environmental sensitivity may aid future breeding under warmer climatic scenarios.
Rulli et al. (Wed,) studied this question.
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