PGF2alfa: more than just a luteolytic agent?

Abstract

It has been conclusively proven that prostaglandin F2a (PGF) is the hormone responsible for inducing luteolysis in cattle. Since this discovery, PGF has been intensively used in reproductive management strategies for synchronization of estrus. Recent data, coupled with revisitation of some older reports, suggests that PGF may have the potential to serve a broader role in estrous synchronization protocols than just the induction of luteal regression. The first objective of this proposal is to determine if administration of PGF 2 d prior to the onset of the 5-d CO-Synch + CIDR in anestrous beef cows and prepubertal heifers affects: 1) ovulatory response to the initial GnRH administration, 2) magnitude of the GnRH-induced LH surge at the initiation of estrous synchronization, 3) ovarian follicle dynamics during synchronization, 4) ovulation to the second GnRH administration, and 5) induction of estrous cycles in anestrous/prepubertal females. It is hypothesized that administering PGF prior to onset of the 5-d CO-Synch + CIDR program in anestrous females will result in a greater GnRH-induced LH surge and increase the ovulatory response to GnRH. Furthermore, it is hypothesized that such a response will result in a more synchronous growth of ovarian follicles during synchronization, a more homogeneous population of ovulatory follicles at the second GnRH administration, and result in the induction of estrous cycles in a greater proportion of previously anestrous females.The primary goal of objective #2 is to determine if a 25 mg injection of PGF administered 2 days prior to the initiation of a 5-d CO-Synch + CIDR increases timed AI pregnancy rates in lactating beef cows. Additionally, two subsidiary goals are to first determine if PGF administration 2 d prior to the 5-d CIDR protocol increases the proportion of cows that ovulate to the GnRH administration and confirm the efficacy of the CoPGF (2, 25-mg doses of PGF given coincidently) treatment to induce regression of spontaneous and accessory CL. Will be accomplished in 110 multiparous cows and 50 nulliparous heifers, reproductive status will be determined using color Doppler transrectal ultrasonography to detect the presence or absence of functional luteal tissue. Ultrasound diagnosis will be conducted twice, 10 d apart concurrent with blood sampling for subsequent progesterone analysis to confirm reproductive designation. All animals will be assigned to either receive 25 mg PGF or saline two days before initiation of a 5-d CO-Synch + CIDR program (PG-5d and 5d treatments, respectively; d of CIDR insertion = d -5). Ovarian ultrasonography will be conducted on d -7, -5 and -3 to determine ovulatory response to GnRH-1, d -1 to d 3 in a subset of females (15/treatment/age group) to evaluate ovarian follicular dynamics during synchronization, and on d 3, 5, 9, and 13, concurrent with blood samples to evaluate progesterone concentrations, diameter of dominant follicle at GnRH-2, the proportion of females induced to ovulate following GnRH-2 and the proportion of females induced to develop a functional CL, respectively. The magnitude of the GnRH-induced LH surge following GnRH-1 administration will be determined in a subset of anestrous cows (10/treatment) and heifers (10/treatment) with blood samples taken at h 0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 4.0 5.0, 6.0, 8.0, 12.0, and 24 following the GnRH administered on d -5. Spring-calving crossbred beef cows (n = 400) will be assigned, within parity (primi- or multiparous) and by days postpartum, to treatments. Blood samples for P4 analysis will be collected on d 17 and 7 of the experiment to determine cyclicity status. 2 d prior (d - 7) to the initiation of the 5-d CO-Synch + CIDR protocol half of cows will receive 25 mg of PGF (PG-5dCIDR) and the remaining cows will receive 5 ml of saline (5dCIDR). All cows will receive 100 ¼g of GnRH at CIDR insertion on d 5 and a blood sample will be collected on d -7 in cyclic cows. On d 0 (h 0), the (AU)