206 EFFECTS OF A COMBINATION OF A PRID, PGF_2α AND eCG, ON ESTRUS SYNCHRONIZATION AND PREGNANCY RATE FOLLOWING EMBRYO TRANSFER IN HOLSTEIN HEIFERS

Abstract

Successful bovine embryo transfer requires synchronization of luteolysis, estrus and ovulation. The objective of the present study was to evaluate the effect of a combination of a PRID, PGF_2± and eCG, on estrus synchronization and pregnancy rate in recipient heifers. A PRID^® (ASKA Pharmaceutical Co., Ltd., Tokyo, Japan) was inserted into the vagina at random days of the estrous cycle for 7 (n = 35) or 9 (n = 43) days. Two days before removal of the PRID, the heifers were injected with PGF_2± IM (2 mL Resipron^®-C containing 0.25 mg mL^-1 cloprostenol; ASKA). About half of the heifers in each group received 250 IU eCG IM (Serotropin^®; ASKA) at the time of PRID removal. Blood was collected several times from the start of treatment for 7 (n = 9) or 9 (n = 9) days and on the day of embryo transfer by jugular venipuncture; plasma was immediately separated and stored at -20°C until assayed for plasma concentrations of estradiol-17α (E₂) and progesterone (P₄). The E₂ and P₄ determinations were performed by enzyme immunoassay after extraction by diethyl ether. Pregnancy was determined by ultrasonography on Day 30 (Day 0 = estrus). The rates of successful standing estrus (no. in estrus/PRID inserted), embryo transfer (no. transferred/estrus), and pregnancy (no. pregnancy/transferred) were compared between groups. Data were analyzed by chi-square analysis or Fisher's PLSD test following ANOVA. Injection of eCG at the time of PRID removal had no significant effect on the rates of successful standing estrus, embryo transfer, or pregnancy (P > 0.05). The proportion of heifers treated for 9 days that exhibited standing estrus (93%, 40/43) was significantly higher than the proportion of heifers treated for 7 days that exhibited standing estrus (66%, 23/35, P < 0.01). Of the heifers that were treated for 9 days, the proportion of heifers exhibiting standing estrus within 2 days after the end of treatment was significantly higher (93%, 37/40) than for heifers that were treated for 7 days (65%, 15/23; P < 0.01). Pregnancy rates of heifers treated for 9 days (84%, 32/38) and 7 days (81%, 17/21) were not significantly different. The E₂ : P₄ ratio normally increases during follicle growth and CL regression. The plasma E₂ : P₄ ratio between the time of injection of PGF_2α and the time of PRID removal was significantly higher for heifers that were treated for 9 days than it was for heifers that were treated for 7 days (P < 0.01). These results suggest that a combination of PRID treatment for 9 days and injection of PGF_2α 2 days before PRID removal successfully synchronized estrus in recipient heifers and led to high pregnancy rates following embryo transfer.