Phenotypic and genetic analysis of somatic cell score response to heat stress in first lactation Holstein cattle.

Previous studies have examined the effects of heat stress on dairy cattle and have estimated genetic parameters of heat tolerance for milk production traits. To further investigate heat stress in Canadian dairy cattle and to identify potential mitigation strategies through genetic selection, this study evaluated the relationship between somatic cell score (SCS) and the temperature-humidity index (THI) in different regions across Canada. The association between THI and SCS was assessed for the whole lactation (5-305 DIM) and for early lactation (5-100 DIM). After determining THI thresholds, a genetic evaluation was performed using a univariate reaction-norm model with a heat stress function to estimate genetic parameters and breeding values for heat tolerance of SCS from 5 to 100 DIM (SCS100). Additionally, bivariate analyses were performed with SCS100 paired with milk, fat, and protein yields. Across Canadian regions, the association between whole lactation SCS and THI was weak and not uniform, with marginal increases in SCS as THI rose. In contrast, a more pronounced association was observed in early lactation (DIM < 100). The THI threshold at which SCS100 began to increase ranged from 52 to 60, with the increase ranging from 0.005 to 0.012 SCS per unit of THI. The heritability estimates ranged from 0.08 (±0.01) to 0.10 (±0.01) and were higher at high THI values. The genetic correlations between heat tolerance component of SCS100 and heat tolerance of milk production traits ranged from -0.13 (±0.16) to -0.55 (±0.14), suggesting that animals that are genetically more susceptible to heat stress for milk production also tend to have higher SCS under heat stress. Finally, a high Spearman rank correlation between the relative breeding values for SCS100 under thermal comfort and heat stress conditions indicated limited genotype by environment interaction. Overall, these results contribute to better overall understanding the effects of heat stress and show that, for SCS, little additional benefit is achieved by explicitly incorporating a heat-tolerance component into the selection model. Selecting animals that maintain production under heat stress would also lower SCS under heat stress helping producers breed cows that are more productive and resilient.