Postpartum acyclicity in suckled beef cows: A review

Prolonged postpartum acyclicity in suckled beef cows is a source of economi c loss to beef cattle producers. Duration of postpartum acyclicity is influ enced by suckling status, nutritional status, calving season, age, and seve ral other factors. Although uterine involution begins and ovarian follicula r waves resume soon after parturition, dominant follicles of these waves fa il to ovulate, due to a failure to undergo terminal maturation. As a result , postpartum anovulatory dominant follicles are smaller than the ovulatory follicles in cyclic cows. Failure of postpartum dominant follicles to under go terminal maturation is due to absence of appropriate LH pulses, a prereq uisite for follicular terminal maturation prior to ovulation. Absence of LH pulses early post partum is primarily due to depletion of anterior pituita ry LH stores, although GnRH pulses are also absent during this period due t o suckling. Following replenishment of LH stores between Days 15 and 30 pos t partum, absence of LH pulses is due to continued sensitivity of the hypot halamic GnRH pulse-generator to the negative feedback effect of ovarian est radiol-17 beta, which results in absence of GnRH pulses. This negative feed back effect of estradiol-17 beta is modulated by suckling which stimulates release of endogenous opioid peptides from the hypothalamus. As the postpar tum interval increases, sensitivity of the GnRH pulse-generator to the nega tive feedback effect of ovarian estradiol-17 beta decreases. This is follow ed by an increasing frequency of GnRH discharges and LH pulses, terminal fo llicular maturation, ovulation, and continued cyclicity. The first ovulatio n post partum is usually followed by a short cycle due to premature luteoly sis because of premature release of PGF(2 alpha) from the uterine endometri um, which is possibly intensified by the suckling-induced oxytocin release from the posterior pituitary. A model for the postpartum ovulatory acyclici ty and for the resumption of cyclicity is presented. (C) 2000 by Elsevier S cience Inc.