Effect of lactation upon the postpartum interval.

Several physiological aspects of lactation in the ewe, sow and cow were reviewed relative to possible delaying effects upon postpartum interval to ovulation, estrus and conception. Specifically considered were the possibilities that: high blood flow to the mammary gland and perturbations of that flow may disrupt vascular regulatory mechanisms in the reproductive tract, metabolism of gonadal and adrenal hormones by mammary tissue may disrupt endocrine controls essential for reproduction, suckling or milking-induced release of prolactin and oxytocin may inhibit ovarian or pituitary function and effects of social interactions associated with lactation and suckling may delay resumption of estrous cycles. The postpartum interval appears to be related to suckling intensity. An attractive hypothesis is that pituitary release of gonadotropins is inhibited during lactation as a result of pituitary desensitization to estrogen by suckling-induced surges of prolactin. Evidence for this role of prolactin is, however, circumstantial for all three species and the relationship of prolactin to the postpartum interval in cattle remains nebulous. Therefore, management practices to minimize lactational anestrus should be based upon the generalized concept that "suckling" delays postpartum cycles, rather than upon more specific hypothetical mechanisms.