Short communication: concentrations of nonesterified fatty acids and β-hydroxybutyrate in dairy cows are not well correlated during the transition period.

The objective was to use longitudinal data of blood nonesterified fatty acids (NEFA) and β-hydroxybutyrate (BHBA) concentrations to describe the relationship between NEFA and BHBA in dairy cows during the periparturient period. Blood NEFA and BHBA concentration data collected from d 21 prepartum to 21 postpartum for 269 multiparous Holstein cows were selected from 4 different studies carried out within our research groups. Overall, NEFA concentrations were increased beginning near parturition with a relatively steady elevation of NEFA through d 9, after which concentrations gradually decreased. Prepartum BHBA concentrations began to increase beginning several days before parturition, continued to increase during the first week after parturition, and remained elevated through d 21 postpartum. Of the 269 cows included in the data set, 117 cows (43.5%) had at least one postpartum hyperketonemic event (BHBA ≥1.2mmol/L), and 202 cows (75.1%) had at least one event of elevated postpartum NEFA concentrations (≥0.70mmol/L) between 3 and 21 d in milk. Area under the curve (AUC) was used to investigate relationships between metabolites over time. Overall, the correlations between transition period NEFA and BHBA AUC were weak. We detected a negative correlation between prepartum BHBA AUC and postpartum NEFA AUC (r=-0.26). A positive correlation existed between postpartum NEFA AUC and postpartum BHBA AUC; however, the correlation coefficient was low (r=0.26). Large variation was found between the day of maximum NEFA concentration within the first 21 d in milk and day of maximum BHBA concentration for the same period. The mean and median times of maximum NEFA concentration were 6.8 and 6 d, respectively, whereas the mean and median times of maximum BHBA were 9.6 and 8 d, respectively; however, the range in days for both the mean and median day of maximum concentrations was very large. Overall, our data set indicates a weak relationship between blood concentrations of NEFA and BHBA during the periparturient period of dairy cows, suggesting that elevated concentrations of one should not be extrapolated to suggest elevated concentrations of the other metabolite.