Milk BHB Heritability Estimates and Genetic Correlations with Milk Yield in Grazing Dairies in Chile.

This study aimed to estimate the heritability of BHB in milk and evaluate its genetic correlations with other traits in grazing dairy herds from southern Chile. The analysis was based on a comprehensive dataset provided by a DHIA cooperative (Cooprinsem, Osorno, Chile). The dataset included records from 617 grazing farms and 712,629 lactations spanning from 2019 to 2024. The final dataset included 254,979 lactations from 50,794 cows. Milk BHB concentrations were higher in early lactation and remained relatively low and stable thereafter. Heritability estimates for BHB followed a parabolic trend, with the lowest values observed during early and late lactation and a pronounced peak of ~0.20 occurring in month 7. Genetic correlations between BHB and other traits were notably stronger than their phenotypic counterparts, suggesting robust underlying genetic relationships. In particular, the genetic correlation between BHB and milk yield shifted markedly over time, starting at approximately 0.35 ± 0.02 in month 1 and declining to approximately -0.30 ± 0.03 by month 10, whereas the phenotypic correlation remained weakly negative throughout the lactation period.