Endocrine patterns of the post-partum cow.

Milked dairy cows generally have a shorter post-partum interval to ovarian cyclicity than suckling dairy or beef cows. In milked and suckling cows, there is a strong seasonal influence with spring-calving cows remaining anoestrous longer. Increasing the suckling intensity further delays the onset of ovarian cyclicity, probably by increasing the frequency or strength of its inhibitory influence on hypothalamic activity. Plasma FSH levels rise in most cows 5-10 days after calving and thereafter the random changes observed have little relationship to the onset of cycles. Recovery of FSH release therefore occurs earlier post partum than recovery of LH release. Hyperprolactinaemia is not a cause of reproductive failure in milked or suckling cows because there is no correlation between plasma prolactin levels and the onset of ovarian cycles. Plasma LH concentrations undergo significant changes directly related to the initiation of ovarian cycles, with low plasma levels immediately post partum, followed by an increase in basal secretion and the development of clear LH episodes. This pulsatile pattern appears earlier in dairy than in beef cows and is further delayed by suckling compared to milking. Before the first ovulation there is an increased frequency and peak height of LH episodes leading to a rise in plasma LH levels and eventually to a preovulatory-type LH surge which results in the first ovulation. These changes in the pattern of LH release appear definitive in the initiation of ovarian activity in post-partum cows.