Heat stress is an important environmental challenge for pig production, with negative effects on reproductive performance. This study evaluated the impact of heat stress on farrowing rate (FR) and weaning–to–insemination interval (WII) in Iberian sows, estimated their genetic parameters, and assessed their relationship with heat tolerance (HT), defined as the individual rate of fertility decline under heat stress. Data from a closed commercial Iberian pig population were analysed, including 2 388 sows having 9 583 WII records, and 3 009 sows having 13 290 first insemination records collected between 2011 and 2020. Meteorological information was used to calculate average daily temperature (TAVG) and temperature–humidity index (THI) as thermal parameters, and piecewise regression models identified thresholds above which fertility traits were affected. Both FR and WII were significantly influenced by these thermal parameters: FR decreased by 0.158 (0.019) log‑odds per unit increase in THI above 18.63, whereas WII increased by 0.732 (0.077) days per unit increase in THI above 15.47. Based on the estimated THI thresholds, a heat load index (HLI) was defined as the number of THI units exceeding the threshold at which fertility traits begin to decline. Reaction norm models were used to fit individual responses to HLI, characterised by the intercept (baseline trait level, HLI=0) and the slope beyond the threshold (interpreted as the HT of the individual). Bayesian animal models revealed low heritability for both traits at baseline level, but heritability increased with HLI, suggesting genetic variability becomes more relevant under challenging environments. Genetic correlations between the baseline level and HT showed antagonistic trends. Genetic correlations between fertility traits at baseline level and under increasing HLI showed a declining trend: from 0.98 to 0.07 for FR and from 0.99 to 0.69 for WII. Although with wide 95% posterior density intervals, these estimates suggest the potential existence of genotype-by-environment interactions. The joint analysis of estimated breeding values (EBVs) for intercepts and slopes identified four groups of animals for each trait, ranging from heat–tolerant animals that maintained or improved EBVs under increasing HLI, to heat–sensitive animals whose EBVs declined. These results provide the first genetic evaluation of fertility traits in Iberian pigs across thermal conditions, demonstrating existence of genetic variability for heat tolerance in this breed, traditionally considered as heat-adapted, and underline the relevance of considering genotype-by-environment interactions in breeding programs, suggesting that jointly selecting for fertility and heat‑tolerance could improve reproductive efficiency of Iberian pig under climate change.
