Association of β-hydroxybutyrate concentrations in blood with body condition score and milk yield of Holstein cows

The objective was to investigate the association between blood β-hydroxybutyrate (BHB) concentrations at 5 (±1) DIM, and the dynamics of BCS (1-150 DIM) and milk yield (1-305 DIM) in Holstein cows.As a secondary objective, we examined how BCS at dry-off and its change during the dry period were associated with subsequent blood BHB concentrations and the probability of hyperketonemia, defined as BHB ≥ 1.2 mmol/L.This retrospective observational study included 9,763 lactations (1,814 primiparous; 7,949 multiparous) from 8,408 Holstein cows calving between January 2022 and February 2025 in a commercial dairy in Colorado, USA.Blood BHB concentrations were measured using a cow-side test at 5 (±1) DIM, as cows exited the morning milking.Two video cameras mounted on the sorting gate at each milking parlor exit generated BCS after each milking.Analyses included categorization of cows by BHB quartiles, followed by binary classification of hyperketonemia using 4 BHB thresholds (0.8, 1.0, 1.2, and 1.4 mmol/L).Primiparous and multiparous cows were analyzed separately using ANOVA and logistic regression.Least squares means (LSM) for daily BCS and milk yield were calculated by BHB quartile and binary categories and were used to construct BCS and lactation curves that were evaluated using repeated-measures analysis.The ANOVA showed that the interaction BHB quartile x DIM had a significant effect on BCS only in multiparous cows, indicating that BCS dynamics over time depended on BHB quartile.Nonetheless, comparisons at specific time points showed that, except at calving, primiparous cows in the highest BHB quartile had lower BCS than those in BHB Q1 and Q2.In multiparous, BHB Q4 was associated with higher BCS at dry-off and calving but lower BCS at 60 and 90 DIM compared with BHB Q1 and Q2.Using binary BHB classifications, associations with BCS were more pronounced in primiparous cows at the 0.8 and 1.0 mmol/L thresholds, whereas results in multiparous cows were consistent across all thresholds.Associations between BHB and milk yield varied by parity and DIM.The effect of the interaction BHB quartile × DIM was significant only in multiparous cows, where the highest BHB quartile had the greatest milk yield from 30 DIM onwards.However, differences in milk yield by BHB quartile existed at specific time points in both primiparous and multiparous cows.When cows were classified by HYK status, a significant HYK × DIM interaction was observed only in multiparous cows, with HYK-positive cows producing more milk across all 4 thresholds.The analysis considering the dry period indicated that greater BCS at dry-off and BCS loss during the dry period were positively associated with higher BHB concentrations in early lactation.Differences in BHB across quartiles of BCS at calving were only evident between the lower quartiles (BCS Q1 and Q2) and BCS Q4, which had the greatest BHB concentrations.Overall, greater blood BHB concentrations at 5 DIM were associated with lower BCS and greater milk yield in multiparous cows.BCS at dry-off and BCS change from dry-off to calving were associated with BHB concentrations in the subsequent lactation.