Glycemia, glucocorticoids and adrenocortical reserve in postpartal dairy cows

The useful life of dairy cows on many modern farms is short With the aim of detecting possible differences in metabolism in relation to parity, blood glucose, serum cortisol and adrenocortical reserve were measured in five gr oups of ten healthy Holstein-Friesian cows during the second week after cal ving. The groups of cows represented the populations in the first five lact ations on the farm. Mean daily milk production from day 7 to day 14 showed steady increases in each group. The average for the whole week was signific antly higher for lactation 2 or 3 than for lactaction 5 (P < 0.05), while t he heifers gave the lowest average yield. Conversely, the average blood glu cose concentration was higher-in the groups of heifers and cows in lactatio n 5 than for those in lactations 2 or 3. Thus, daily milk yield was inverse ly correlated with blood glucose concentration (r = - 0.482; P < 0.0001; n = 400). The overall temporal profile of changes in blood glucose level show ed a minimum on day 10, although there were marked differences in the patte rn between different lactations. Moreover, individual variability between c ows within a group decreased with parity. Basal serum cortisol concentratio ns were significantly lower in the group of heifers than for cows in lactat ion 3 or 4. The mean response to stimulation by adrenocorticotropin tended to diminish with increasing parity as determined by the cortisol increment at 60 min after intravenous injection. This may reflect a lower secretion r ate or an increased rare of utilisation of cortisol. Further work is necess ary to show whether the slightly lower milk yield and serum cortisol parame ters, together with higher blood glucose concentrations found for cows in l actation 5 are inherent characteristics of the result of adaption.