Characterization of factors influencing estrus and ovulation in sows may facilitate development of procedures for improving reproductive performance. The experiment was conducted in confinement during 1997 to 1999 using 174 Large White x Landrace sows. After weaning, sows were checked for estrus twice daily. In the 1st yr, transrectal ultrasound was performed once daily and in the 2nd yr twice daily at estrus and on every day until ovulation. The effects of lactation length (less than or equal to 16 d, 17 to 24 d, 25 to 31 d or greater than or equal to32 d), parity (1, 2, or greater than or equal to 3), season (winter, spring, summer, or fall) and weaning-to-estrus interval (3, 4, 5, or 6 to 8 d) and their interactions on estrual and ovulatory responses were studied. There was no effect of frequency of ultrasound on any response variable, so data across years were pooled. Percentage of sows expressing estrus within 3 d of weaning was influenced by lactation length (P < 0.001), with sows lactating less than or equal to 16 d (35.2%) less likely to express estrus than sows lactating greater than or equal to 17 d (94 %). A parity x season interaction was observed (P < 0.001) for estrus, with the lowest expression in parity 1 (73.0%) and parity 2 sows in fall (67.2%), compared with >= parity 3 sows (98.1%). No explanatory variable had a significant effect on weaning-to-estrus interval (4.4 d) or on follicle size at estrus (8.1 mm). Ovulation hour after onset of estrus was affected by weaning-to-estrus interval (P < 0.01), with sows returning in 3 d ovulating at 46.2 h and between 6 and 8 d at 30.2 h. For sows that expressed estrus within 8 d of weaning, the percentage of sows ovulating was influenced by lactation length (P < 0.001) and weaning-to-estrus interval (P < 0.001). Sows that lactated:! 16 d were less likely to ovulate (78.0%) than those lactating greater than or equal to 17 d (> 92%). Sows that returned to estrus in 3 d were also less likely to ovulate (79.5%) than sows returning greater than or equal to 4 d after weaning (> 92%). A parity x season interaction was also observed on ovulation (P < 0.001), with parity 1 and 2 sows less likely to ovulate after expressing estrus in fall and spring compared with parity 3 and greater sows. The data suggest lactation length, early return to estrus, and parity by season effects are associated with risk of failure to express estrus and ovulate.
