In a 200 sow herd, the litter size fell from an average of 10.5 pigs born alive per litter from January to June, to an average of 9.2 for the remainder of the year. Management changes during the first part of the year resulted in half the sows weaning litters at three weeks of age and half the sows weaning at four weeks of age instead of at six weeks as was previously done. The subsequent litter size was 9.3 pigs born alive per litter for three-week weaned sows compared to 10.0 for four-week weaning. The management of gilt breeding was also altered by the necessity to breed groups of 12 gilts in one-week periods of time and therefore a higher proportion of gilts may have been mated on their first estrus instead of their second or third estrus as had been the case. The difference in litter size of first parity sows between the first six months and the second six months was 1.1 pigs. Parvovirus infection may have been a factor contributing to the reduction in litter size amongst first parity sows; two groups of first parity sows experienced an increase in mummified piglets, a reduced far rowing rate, and smaller litter size. However, no attempt was made at diagnosing an infectious agent. Parity distribution remained relatively unchanged during the year and was not associated with the drop in litter size.
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