Postpartum endocrine function of cattle, sheep and swine.

Reproductive efficiency can be increased in farm animals by decreasing the interval from parturition to conception. At parturition the placentae are expelled and the corpora lutea of pregnancy regress, resulting in dramatic reductions in plasma concentrations of progesterone and estrogens. The early postpartum period usually is characterized by ovarian inactivity and the absence of estrus in cows, ewes and sows. Plasma concentrations of progesterone and estradiol are minimal before the first postpartum ovulation and estrus. The first ovulation may occur before the first estrus. However, occurrence of a "silent ovulation" varies with species and management. The lack of postpartum ovarian activity in most species probably is caused by reduced gonadotropin secretion, since follicular growth can be induced in anestrous postpartum cows, ewes and sows by gonadotropin treatments. Pituitary gonadotropin content and plasma gonadotropin concentrations are usually low at parturition and increase during the postpartum period. Hormonal treatments have resulted in marginal success in decreasing the postpartum anestrous interval. Therefore, the interval from parturition to conception in cows, ewes and sows may be reduced by methods developed from research to elucidate the factors and mechanisms that regulate postpartum gonadotropin secretion.