Genetic associations among blood β-hydroxybutyrate measured by 2 methods, milk composition, and reproductive traits in early-lactation Holstein cows.

This study aimed to estimate genetic parameters for blood β BHB concentrations and to characterize the epidemiology of hyperketonemia (HYK) using both laboratory and strip-based measurement methods at the onset of lactation in Holstein cattle. A second objective was to investigate the associations between BHB levels and milk components and reproductive traits. Understanding these genetic relationships will enable an assessment of the impact of selection for high milk yield on the occurrence of metabolic disorders and reproductive performance. The dataset encompassed BHB records from a total of 5,037 Holstein cows. These animals were born between 2000 and 2020 and were part of 18 experiments involving transition cows conducted at the Federal University of Paraná between 2009 and 2024, across 34 herds in the state of Paraná, Southern Brazil. Productive, reproductive and pedigree data were sourced from the milk recording service of the Holstein Cattle Breeders Association of Paraná State. Blood BHB levels were measured between 3 and 14 d postpartum using 2 distinct methods: β-ketone test strips (BHBstrip) and laboratory testing (BHBlab) employing enzymatic colorimetric methodology. Prevalence of HYK was estimated using BHB levels ≥1.2 mmol/L in the interval from 3 to 14 d postpartum. For genetic analyses, the BHB records were logarithmically transformed and treated as repeated measurements. Genetic parameters for daily milk yield, protein (PP), fat, and lactose percentages, fat:protein ratio (F:P), calving interval, days open, interval between first and last insemination (IFL), and interval between calving and first insemination were estimated using a linear animal model, applying the restricted maximum likelihood method and BLUPF90+ software. The prevalence for HYK was 22.97%. The heritability of BHB ranged between 0.10 ± 0.03 and 0.15 ± 0.03, and repeatability ranged from 0.32 ± 0.02 to 0.37 ± 0.02. Genetic correlations ranged from low to high magnitude, with the highest correlations being identified between BHBstrip and PP (-0.43 ± 0.03 to -0.83 ± 0.003) and BHBstrip and F:P (0.33 ± 0.04) for milk components, and between BHBlab and IFL (0.26 ± 0.03). These values evidence the existence of genetic variability for BHB and their genetic associations with milk components and reproductive traits. Therefore, indirect selection for animals less susceptible to HYK, using indicators such as BHB, PP, and F:P, can yield favorable correlated responses to reduce the occurrence of HYK in Holstein herds. Moreover, the genetic association between BHB and reproductive traits highlights the unfavorable relationship between HYK and reproductive performance.