Effect of exogenous gonadotropins on the weaning-to-estrus interval in sows

The efficacy of PG 600 (400 IU PMSG and 200 IU hCG) for accelerating the onset of estrus was determined for sows weaned during the summer. Yorkshire sows (average parity = 4.6), nursing 8.6 +/- 0.2 pigs (mean +/- SEM) were weaned after 27.7 +/- 0.4 d of lactation and were treated intramuscularly with either PG 600 (n = 35) or with 0.9% saline (n = 35). Sows were checked for estrus once daily in the presence of a mature boar. Treatment with PG 600 increased (P < 0.05) the percentage of sows in estrus within 7 d after weaning (97.1 vs 82.9%). Relative to controls, sows given PG 600 expressed estrus sooner (3.8 +/- 0.1 d vs 4.5 +/- 0.1 d; P < 0.01). Sows exhibiting estrus within 7 d after treatments were artificially inseminated 0 and 24 h after first exhibiting estrus. The percentage of inseminated sows that farrowed tended to be higher (P < 0.07) for control than for PG 600-treated sows (96.6 vs 82.3%). The number of pigs born live was similar (P > 0.1) for sows treated with PG 600 and with saline, and was 12.7 +/- 0.6 and 11.7 +/- 0.7, respectively. Pigs farrowed by saline-treated sows, however, tended to be heavier (P < 0.09) than pigs farrowed by sows treated with PG 600 (1.49 +/- 0.06 kg vs 1.34 +/- 0.06 kg). In summary, PG 600 accelerated the onset of estrus in sows weaned during the summer. Sows mated during the induced estrus, however, tended to have a lower farrowing rate and farrowed lighter pigs than control sows inseminated during a natural estrus occurring within 7 d after weaning.