Genetic analysis of the impact of heat stress on fertility traits in dairy cows in the Netherlands.

This study aimed to use temperature-humidity index (THI) as an indicator for assessing heat stress conditions for fertility traits in the Holstein dairy cattle breed in the Netherlands. Data from AI and calving events of 416,814 first-parity cows from the Netherlands were used, considering different THI definitions based on different numbers of days before and after artificial insemination events. To achieve our aim, we investigated first, at population level, the relationship between different THI definitions and 4 fertility traits: conception rate, interval calving to first insemination, interval first to last insemination, and calving interval. Second, to investigate individual variation in the relationship between THI and fertility, variance components were estimated for each trait using the so-called broken stick model. This model assumes that breeding values are dependent on THI above but not below a THI threshold identified at the population level and explores the presence of genetic variation associated with fertility decline during heat stress. This study revealed considerable changes in fertility traits during periods of heat stress, with a THI threshold of 60 for conception rate and interval first to last insemination and 50 for interval calving to first insemination and calving interval. Interestingly, as THI levels increased, genetic variance and heritability also increased, indicating that at higher THI levels associated with reduced fertility, the genetic variation of fertility traits is greater. Furthermore, significant genotype-by-environment interactions were observed for all 4 fertility traits, suggesting changes in sire rankings between THI levels below and above the threshold. This study provides insights that may help breeding programs and farmers breed animals resilient to heat stress conditions.