Multiple manifestations of season on reproductive performance of commercial swine.

Seasonal variation of reproductive performance of female swine was analyzed by use of a data base that included farrowing records from 42 commercial swine herds. Farrowing rates of sows bred during December or January were higher (P < 0.001) than those bred in July or August (79.2 vs 74.1%). The percentage of sows with irregular (prolonged) return-to-estrus intervals after mating in December or January (2.63%) was lower (P < 0.01) than those mated in July or August (4.65%). Sows mated in November through January had greater total number of pigs born per litter (11.66 vs 11.19 pigs per litter), more pigs born alive per litter (10.39 vs 9.84 pigs per litter), and greater litter birth weights (15.20 vs 14.28 kg) than sows mated in June through August (P < 0.001). Adjusted 21-day litter weights were lower (P < 0.001) for litters from sows farrowing in July or August (52.04 kg) than sows farrowing in all other months (55.13 kg) except June and September. Sows that had their litters weaned in June through August had longer (P < 0.001) weaning-to-estrus intervals (7.95 days) than sows that had their litters weaned in November through January (6.84 days). The effects of season varied across parties. Primiparous sows had the greatest seasonal variability in weaning-to-estrus interval, whereas multiparous sows had the greatest seasonal variability in total number of pigs born per litter and pigs born alive per litter.