Prefeeding plasma acetoacetate and glucose in healthy, lactating heifers. Variations related to milk yield, metabolic balances and stage of lactation.

When measured before morning feeding plasma acetoacetate (ACAC) increased and glucose (GL) decreased significantly with increasing milk yield (fat corrected, FCM) within 2 groups of half-sister related heifers which did not suffer from clinical ketosis. The groups received respectively A: 7 kg and B: 4 kg/d of concentrates in addition to grass silage ad libitum (each group 88 animals). Correlation coefficients of the same magnitude between the 3 variables mentioned were obtained in the 1st and 3rd month of lactation in spite of a narrowing of the ACAC range from 0.024-0.66 to 0.019-0.16 mmol/l. Reproducibility of differences between animals in milk yield and in the plasma components was evidenced by significant correlations between measurements in the 1st and 3rd month. The B-animals produced 2.4 kg/d less FCM than the better fed A group (A, 1st month of lactation 19.8 +/- 3.1 kg/d) while the mean values for ACAC and GL remained practically unaffected by the difference in feeding. B cows gave consistently 5-6% lower GL means (p < 0.01) than A animals with identical yields. Animals which showed marked ACAC increment during the first 5-6 weeks of lactation produced 3-4 kg more FCM/d than equally fed animals with constantly low ACAC levels. The estimated energy deficit of this ketonaemic, high-producing category corresponded to the production of 4.5-6 kg FCM/d from body reserves. Low ACAC values (< 0.05 mmol/l) were reproduced with a standard deviation of +/- 4% with the automated nitroprusside method described.