Duration of estrus in relation to reproduction results in pigs on commercial farms.

This research was conducted to determine factors that influence duration of estrus, AI strategy, and reproduction results between and within commercial swine farms that use AI. Data from 15,186 sows and gilts on 55 farms for a period of 6.1+/-4.2 mo per farm were used in this study. The average duration of estrus was 48.4+/-1.0 h, ranging from 31 to 64 h, and was consistent from month to month within a farm (repeatability of 86%). Differences in duration of estrus between farms accounted for 23% of the total variation in duration of estrus. On most farms (n = 45), gilts showed a shorter (P < .05) duration of estrus than sows (40.8+/-1.1 h vs 48.5+/-1.0 h). The duration of first estrus after weaning was longer (P < .0001) compared with that of repeat-breeder sows (50.2+/-1.0 h vs 46.8+/-1.0 h). Duration of estrus decreased (P < .05) when interval from weaning to estrus increased from 4 to 6 d (56.0 +/- 1.2 h vs 45.8 +/-1.2 h). The regression of interval from onset to estrus to first AI and interval from weaning to estrus varied between farms and ranged from -7.4 to +1.3 h/d; four farms had a positive relationship. Farrowing rate decreased (P < .05) from 89.7+/-2.7% to 78.2+/-5.74 when the interval from weaning to estrus increased from 4 to 10 d. The litter size decreased (P < .05) from 11.7 to 10.6 pigs when the interval from weaning to estrus increased from 4 to 7 d. Compared with a single AI, double AI in sows and gilts resulted in a 4.3 and 7.0% higher (P < .05) farrowing rate, respectively. When the first AI was performed after expected ovulation, reproduction results were lower than when AI was performed before or at expected ovulation in sows. Duration of estrus was not related to farrowing rate or litter size in individual pigs. Number of inseminations per estrus, time of AI, and duration of estrus were correlated, which made it difficult to assess which of these factors was primarily related to the farrowing rate or litter size. Knowledge of average duration of estrus on farms and of factors that influence the duration of estrus on commercial farms can help to improve the efficiency of the AI strategy specific for each farm.