Heat-load-dependent inbreeding depression for production traits in Italian Holstein cattle

The objective of this study was to test whether the negative effects of inbreeding on production traits varied according to the level of environmental load. Traits analyzed were milk, fat, and protein yields, and somatic cell score (SCS). Environmental conditions were described using temperature (TEMP), relative humidity (RH), and the temperature-humidity index (THI), each divided into five equally sized classes. For each trait, the environmental variable with the largest impact was used to evaluate inbreeding effects across its classes. Inbreeding was measured using pedigree (F_PED), the diagonal of the genomic relationship matrix (F_GRM), and runs of homozygosity (F_ROH).

Genomic-based inbreeding measures resulted in larger estimated inbreeding effects, compared to pedigree measures, with F_GRM and F_ROH showing similar results. RH most affected milk yield, with losses of ~ 600 g/day in the highest RH class. Inbreeding led to losses of ~ 100 g/day per 1% increase, with genomic measures scaled to match F_PED. For milk yield, losses were ~ 20% greater under higher environmental stress. THI was the most impactful variable for fat yield, with losses of ~ 90 g/day in the highest THI category. Inbreeding-related losses increased in more stressful environments, with average reductions of 3 g/day per 1% inbreeding, reaching 4 g/day in the highest THI class. Protein yield and SCS were mainly influenced by TEMP, with losses of ~ 70 g/day of protein and increases of 0.06 SCS units. Inbreeding caused losses of ~ 3.5 g/day in protein yield and increases of ~ 0.006 SCS units per 1% inbreeding. For these two traits, the relationship between inbreeding effects and environmental stress was less clear. For milk and fat yields, there was a significant interaction between environmental and inbreeding effects, suggesting that inbreeding depression influences heat tolerance in dairy cows.

For milk and fat yields, there was a significant interaction between environmental and inbreeding effects, indicating that environmental conditions modulate the expression of inbreeding depression. These findings suggest that inbreeding depression influences heat tolerance in dairy cows.