Effects of P.G. 600 on the onset of estrus and ovulation rate in gilts treated with Regu-mate.

Three experiments assessed the onset of estrus and ovulation rate in gilts treated with gonadotropins after the withdrawal of an orally active progestin. In Exp. 1, all cycling gilts received the progestin (Regu-mate; Intervet America Inc., Millsboro, DE) at a rate of 15 mg/d for 18 d. Twenty-four hours after the last feeding of Regu-mate, 32 gilts received an i.m. injection of 400 I.U. PMSG and 200 I.U. hCG (P.G. 600, Intervet America, Inc.), and 32 gilts received an i.m. injection of deionized water. The percentage of gilts displaying estrus < or = 7 d (P = 0.64) and the injection-to-estrus interval (P = 0.37) were similar for P.G. 600-treated gilts (93.8% and 4.1 +/- 0.1 d) and controls (90.6% and 4.3 +/- 0.1 d). Ovulation rate was greater (P < 0.01) in P.G. 600-treated gilts (28.8 +/- 1.1) compared with controls (17.4 +/- 1.1). In Exp. 2, 58 cycling gilts received Regu-mate (15 mg/d) for 18 d. Twenty-four hours after Regu-mate withdrawal, gilts received i.m. P.G. 600 or water (n = 29/treatment). Gilts were bred via AI 12 and 24 h after first detection of estrus. The percentage of gilts displaying estrus < or = 7 d (P = 0.45) and the injection-to-estrus interval (P = 0.27) were similar for P.G. 600-treated gilts (82.7% and 4.0 +/- 0.1 d) and controls (89.7% and 4.2 +/- 0.1 d). Ovulation rate was greater (P < 0.01) in P.G. 600-treated gilts (26.2 +/- 1.8) compared with controls (18.1 +/- 1.7). Pregnancy rate (P = 0.71) and the number of live embryos at d 30 postmating (P = 0.40) were similar for P.G. 600-treated gilts (91.7% and 15.6 +/- 1.2) and controls (88.5% and 14.1 +/- 1.2). In Exp. 3, prepubertal gilts (142.6 +/- 0.7 d of age) received Regumate (15 mg/d) (n = 20) or a control diet not including Regu-mate (n = 20) for 18 d. Twenty-four hours after Regu-mate withdrawal, all gilts received i.m. P.G. 600. The percentage of gilts displaying estrus < or = 7 d (P = 0.49) and the P.G. 600-to-estrus interval (P = 0.69) were similar for Regu-mate-fed gilts (95% and 4.3 +/- 0.2 d) and controls (88.9% and 4.2 +/- 0.2 d). Ovulation rate was similar (P = 0.38) for Regu-mate fed gilts (16.6 +/-1.6) and controls (14.4 +/- 1.8). In cycling gilts, administration of P.G. 600 after withdrawal of Regu-mate increased ovulation rate, but not litter size at d 30 postmating. There was no beneficial effect of Regu-mate pretreatment on the response to P.G. 600 in prepubertal gilts.