Summary - We conducted two retrospective studies, one involving 2800 sows in a single herd inAustralia over 3 years,
the other involving22,715 litters from 112 herds in Ontario,Canada over 5 years. Uttersize and farrowingrate were
studied individually and combined as an index - pigs produced per mated female (PPMF)- to investigate how they
relate to length of weaning-to-estrus interval. Litter size, farrowing rate, and PPMFall decreased among sows mated on
days 7-10 postweaning compared to sows mated either 3-6 days postweaning or 11-14 days postweaning. PPMFwas
reduced by approximately 30% in that less-efficient period. Approximately II % of sows were detected in estrus during
the less-efficient period. Forty percent of parity-one sows with a weaning-to-estrus interval of 7-10 days also had weaning-to-estrus
intervals of 7-10 days in parity two. If sows that came into estrus 7-10 days postweaning were not mated
until their next estrus (between days 28 and 31), their efficiency at least equalled that of sows that came into estrus
during the more-efficient interval.Almost 30% of the delay- mated sows came into estrus 7-10 days postweaning in
their next parity. We discuss the dip in efficiency associated with the occurrence of estrus 7-10 days postweaning and
compare it to the second-litter productivity drop described by other investigators