INTERRELATIONSHIPS BETWEEN BLOOD METABOLITES AND AN EVALUATION OF THEIR USE AS CRITERIA OF ENERGY STATUS OF COWS IN EARLY LACTATION

Abstract

Correlation and regression coefficients were calculated for blood metabolites, milk components, energy balance and efficiency of energy utilization for a total of 120 cows. The animals were on propylene glycol and glycerol feeding experiments, and correlation coefficients were calculated for combinations of parameters with the objective of assessing the effect of additive and level of additive on the constancy of these relationships. In order to attempt an explanation of the variation in relationships between treatment groups, two groups each comprising 19 animals were chosen, based on blood ketone body levels. These groups provided a comparison between stressed (ketotic) and non-stressed (control) animals. Blood glucose was more highly correlated with acetoacetate and β-hydroxybutyrate than with free fatty acids when calculated for treatment groups. When calculated for the two selected groups, significant correlations (P < 0.01) for glucose vs. acetoacetate, β-hydroxybutyrate and free fatty acids were −0.56, −0.61 and −0.28, respectively, for the ketotic group. No significant correlations (P > 0.05) were found between these parameters in the control group. The correlations between blood glucose, acetoacetate and β-hydroxybutyrate and energy balance were very low. Plasma-free fatty acid levels were more closely associated with energy balance than were the other blood metabolites. When energy balance and free fatty acid correlations were compared for ketotic and control groups, only the control group showed a significant (P < 0.01) coefficient (r = − 0.42). The correlation between mean plasma-free fatty acids and mean body weight change over the experimental period for all animals, with time and treatment effects removed, was significant (r = − 0.54, P < 0.01). The estimated free fatty acid level at zero body weight change was 0.53 μ eq per ml plasma. It is concluded that although free fatty acid levels are more sensitive than glucose as an indicator of energy status of the lactating cow, the excessive variability of this relationship during early lactation drastically limits its usefulness.