Genomic prediction via reaction norm models for pregnancy loss using temperature humidity index in brahman cattle

Brahman cattle are widely used in tropical production systems due to their adaptability; however, reproductive efficiency remains constrained by pregnancy loss (PL), particularly under heat stress. This study evaluated genotype-by-environment (G × E) interactions for PL using reaction norm models incorporating the temperature-humidity index (THI) as an environmental descriptor. Records from Brahman heifers in two Bolivian herds were combined with weather data to calculate accumulated THI for the 30 d before and after rectal palpation. Pregnancy loss, defined from diagnoses performed between 60 and 150 d of gestation, was analyzed as a binary trait using threshold RNM fitted with either a pedigree-based relationship matrix ( $A$ ) or a combined pedigree-genomic matrix ( $H$ ). Genetic variation was detected for both the intercept and slope of the reaction norm. Heritability of the intercept increased from 0.23 ( $A$ ) to 0.37 ( $H$ ), whereas the slope showed moderate heritability (0.32 for $A$ ; 0.33 for $H$ ). Mean heritability of PL across accumulated THI levels was 0.10 ( $A$ ) and 0.13 ( $H$ ). Negative genetic correlations between intercept and slope (-0.41 for $A$ ; -0.34 for $H$ ) and between extreme THI levels (-0.65 for $A$ ; -0.49 for $H$ ) indicate substantial reranking of animals under contrasting thermal conditions. Although variance component estimates were broadly similar between models, incorporating genomic information improved prediction accuracy and reduced dispersion of breeding values. These results demonstrate that accumulated THI captures meaningful environmental sensitivity for PL and that integrating genomic information with reaction norm models enhances selection for reproductive resilience in Brahman cattle under tropical conditions.