Breeding for thermotolerance in dairy cattle: Production versus fertility traits.

The decline in production of milk and its components has been extensively studied as an indicator of heat tolerance for genetic evaluations. However, the antagonistic relationship between high production and functionality raises questions about the suitability of using productive traits as indicators of heat tolerance. This study aimed to estimate changes in the relationship between production and fertility under thermoneutral (TN) and heat stress (HS) conditions, to define breeding strategies that enhance adaptation to high heat loads while maintaining both productivity and functionality. The analyzed dataset included records on 100,467 Holstein cows of first-lactation milk, fat, and protein yields (703,574 records for each yield) and conception rate (CR) at first insemination in first lactation of 247,378 cows in Spain. Temperature-humidity indices averaged over the day of milk recording or day of artificial insemination and the 2 previous days for milk traits, or the subsequent 7 d for fertility, were used to measure the heat load associated with each record. Bicharacter sire models were employed, incorporating one of the yield traits and the fertility trait. Models included random regressions with Legendre polynomials for production traits and a broken-line function for fertility to describe the trait responses to increasing heat loads. This approach allowed for the estimation of trait levels under TN and HS conditions, the slope of response under HS as heat tolerance indicators, and the correlations among these variables. The 3 yield traits exhibited estimated negative genetic correlations between their level under TN conditions and their slopes of response under HS, ranging from -0.38 for fat yield to -0.59 for milk yield. For CR, this correlation was close to zero. Estimated genetic correlations between yield traits under TN conditions and the decline in CR under HS were nearly null, ranging from -0.06 for fat yield to 0.07 for protein yield. This suggests that cows with higher production potential under TN conditions are not necessarily more susceptible to fertility decline under HS. Conversely, the correlations between fertility potential under TN conditions and the slopes of production decline under HS were positive, ranging from 0.34 for protein yield to 0.51 for fat yield. This indicates that cows with lower production losses under HS tend to have better fertility performance under TN conditions. Furthermore, the correlations between heat tolerance based on production and fertility declines under HS were positive, ranging from 0.22 for fat yield to 0.65 for milk yield. This suggests that a significant proportion of animals have the potential to maintain both productive and fertility levels under HS. Finally, the genetic correlation between fertility and production traits improved as heat load increased. For milk yield, this correlation shifted from -0.30 under TN to nearly null under extreme heat conditions. Reaction to heat load in functional traits such as fertility should help in selecting animals that show high levels of production under HS due to a better adaptation to hot conditions driven by functional reasons.