A Study of Cloprostenol-Induced Oestrus and Spontaneous Oestrus by Means of the Milk Progesterone Assay

Milk samples from 366 cows in six commercial dairy herds were taken daily for 30 days (one herd) or at two to four-day intervals for 50 to 70 days (five herds) and assayed for progesterone content to study oestrous cyclic patterns. 267 treated animals received one or more cloprostenol (Estrumate; ICI Ltd) injections in order to control oestrus and to compare their oestrous patterns with 99 untreated animals. Eighteen per cent (inter-herd range 7 to 33%) of 155 cows treated with two cloprostenol injections 11 days apart failed to synchronize and this appeared to be mainly due to an extended period of eight or more days low progesterone following the first injection. This rendered the second injection ineffective and meant that fixed-time insemination was carried out when progesterone levels were too high to permit follicular maturation or ovulation. However, 18% of 337 natural oestrous periods in the trial also showed such an unusually long period of basal progesterone though it was not possible to attribute any cause to this irregularity which only appears in certain herds. Further detailed work is in progress to investigate the endocrine basis of this observation though it is suggested that adoption of suitable regimes can reduce incidence of asynchrony to 4.5% or less even where the herd incidence of long low progesterone periods is high.