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The objective was to determine whether treatment with 400 IU PMSG and 200 IU hCG (P.G. 600; Intervet America, Inc., Millsboro, DE, USA) at weaning improved rebreeding performance in sows that were limit-fed during lactation. Crossbred sows were allowed ad libitum access to feed or were limited to 3.2 kg of feed/day during an 18-day lactation. At weaning, limit-fed sows received im treatment with P.G. 600 (n = 16) or saline (n = 19) and ad libitum-fed sows received saline (n = 18). The percentage of sows in estrus by day 7 post-weaning was greater (p<0.05), and the weaning-to-estrus interval was shorter (p<0.05), for ad libitum-fed sows compared to limit-fed, saline-treated sows, with limit-fed, P.G. 600-treated sows having intermediate values that were not different from the other two groups. The percentage of sows pregnant and the numbers of corpora lutea and embryos at day 30 post-mating were not different (p>0.1) among groups. In summary, low feed intake during lactation decreased the percentage of sows that displayed estrus within 7 days after weaning and increased the weaning-to-estrus interval. These effects were at least partially remediated by gonadotropin treatment. Pregnancy rate, and litter size at day 30 of gestation, were similar for ad libitum- and limit-fed sows and not affected by P.G. 600 treatment in limit-fed sows.

We previously reported that ovulation rate, but not pregnancy rate or litter size at d 30 after mating, was enhanced by treatment with P.G. 600 (400 IU of PMSG and 200 IU of hCG, Intervet America, Inc., Millsboro, DE) in gilts fed the orally active progestin, altrenogest (Matrix, Intervet America, Inc.) to synchronize estrus. We hypothesized that in addition to increasing ovulation rate, P.G. 600 may have altered the timing of ovulation. Therefore, mating gilts 12 and 24 h after first detection of estrus, as is common in the swine industry, may not have been the optimal breeding regimen, and as a consequence, pregnancy rate and litter size were not altered. The objective of the present study was to determine the effect of P.G. 600 on the timing of ovulation in gilts treated with altrenogest. Randomly cycling, crossbred gilts (5.5 mo old, 117 kg BW, and 14.7 mm of backfat) were fed a diet containing altrenogest (15 mg/d) for 18 d. Twenty-four hours after altrenogest withdrawal, gilts received i.m. injections of P.G. 600 (n = 25) or saline (n = 25). Gilts were checked for estrus at 8-h intervals. After first detection of estrus, transrectal ultrasonography was performed at 8-h intervals to determine the time of ovulation. Gilts were killed 9 to 11 d after the onset of estrus to determine ovulation rate. All gilts displayed estrus by 7 d after treatment with P.G. 600 or saline. Compared with saline, P.G. 600 increased (P = 0.07) ovulation rate (14.8 vs. 17.5, respectively; SE = 1.1). The intervals from injection to estrus (110.9 vs. 98.4; SE = 2.7 h; P < 0.01) and injection to ovulation (141.9 vs. 128.6; SE = 3.2 h; P < 0.01) were greater in gilts treated with saline than in gilts treated with P.G. 600. Duration of estrus (54.4 vs. 53.7; SE = 2.5 h), the estrus-to-ovulation interval (30.2 vs. 31.7; SE = 2.2 h), and the time of ovulation as a percentage of estrus duration (55.8 vs. 57.5; SE = 3.0%) did not differ for the P.G. 600 and saline-injected gilts, respectively. In summary, P.G. 600 advanced the onset of estrus and ovulation following termination of altrenogest treatment and increased ovulation rate; however, treatment of gilts with P.G. 600 had no effect on the timing of ovulation relative to the onset of estrus.

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.