Endocrine profiles of cows undergoing extended lactation in relation to the control of lactation persistency.

We conducted an experiment in dairy cows investigating the effects of calving season, milking frequency and nutrition on lactation persistency. Cows calved in the Spring (n=12) or Winter (n=12). Commencing in lactation week 9 one udder-half of each cow was milked thrice-daily and half of each calving group received additional concentrate at a fixed rate of 3kg per day above that of the control cows. As reported elsewhere, between lactation weeks 9 and 33 persistency (measured as the slope of decline in milk yield) was significantly improved by frequent milking (P<0.001), by calving in the Winter (P<0.001) and by additional concentrate (P<0.05). The cows were rebred after week 33. When analysis of persistency was extended up to week 20 of the recurring pregnancy only the frequency effect remained significant. Persistency was unaffected by the pregnancy up until pregnancy week 20 but was then greatly reduced (P<0.001). In this paper we report hormone concentrations. GH was unaffected by nutrition but was consistently elevated in the Winter calving group relative to the Spring. IGF1 and prolactin were both unaffected by nutrition and calving season, IGF1 tended to increase as lactation progressed but changes in prolactin were related to time of year more than stage of lactation. Insulin was not affected by nutrition and was lower in Winter calvers, but only during early lactation. Prior to rebreeding, lactation persistency was correlated (slightly) with [GH] but not with [IGF1] or [insulin] and was correlated significantly with changes in GH, IGF1 (both positive) and insulin (negative). In conclusion, whilst bovine lactation persistency is plastic and amenable to beneficial manipulation, the details of its endocrine control remain to be elucidated.