Exposure to prenatal maternal heat stress alters genetic parameters for production and fertility traits in Jersey crossbred cattle

This study evaluated the effect of temperature-humidity index (THI) during the last week of dam gestation on the genetic parameters of production and fertility traits in Jersey crossbred cattle using a random regression model (RRM). Heritability estimates and genetic correlations were analyzed across the THI gradient (60–85) to quantify genotype-by-environment (G×E) interactions. Direct heritability estimates for milk yield (MY), fat yield (FY), protein yield (PY), protein percentage (PP), 4% fat-corrected milk (4% FCM), and energy-corrected milk (ECM) were low to moderate (0.14–0.57) and decreased with rising THI from 60 to 70 but gradually increased at extreme values (THI > 80). Maternal heritability remained consistently low (0.03–0.16). For fertility traits, conception rate (CR) and non-return rate at 56 days (NRR56) exhibited low to moderate heritability (0.01–0.28), which declined markedly beyond THI 75. Genetic correlations between the lowest and highest prenatal THI levels declined to approximately 0.60–0.70, indicating genotype-by-environment interaction and the potential for genotype re-ranking under contrasting prenatal heat stress (HS) conditions. The results indicated that genetic parameter estimates varied across prenatal THI gradients, suggesting genotype-by-environment interactions associated with maternal thermal conditions. These findings provide preliminary evidence supporting consideration of prenatal HS in future heat tolerance evaluations and breeding strategies for developing resilient livestock populations.