Abstract / Summary
Abstract Heat stress impairs cattle fertility, but genetic responses to thermal conditions are poorly characterised in local beef breeds under extensive management. We investigated genotype-by-environment interactions for calving interval (CI) in Maremmana cattle using 71,063 records from 17,289 cows and an adjusted temperature-humidity index describing postpartum thermal exposure. Of four windows tested, the 20-day postpartum window showed the strongest association with CI and was used as the environmental gradient in reaction norm models. Heritability at the average environment was 0.05, and additive genetic variance was detected for the slope. Genetic correlations declined as environments diverged, reaching 0.23 between the 10th and 90th percentiles, with sire re-ranking. Single-step GWAS integrating 605 genotyped animals with pedigree and phenotypic information identified regions associated with both reaction norm components, including BTA15 ( DRD2 , NCAM1 , TTC12 ) for the intercept and BTA19 ( SPHK1 , FOXJ1 ) for the slope; several regions also overlapped previously reported fertility QTL. Because thermal exposure is correlated with photoperiod in this seasonal system, the slope reflects genetic response across the observed thermal gradient rather than physiological heat tolerance. Genetic merit for CI therefore varies across postpartum thermal conditions, with implications for reproductive robustness under increasing climatic variability.