Sows are usually weaned at a time of day most convenient for processing the weanlings, with little thought as to any effect it may have on the sow’s return to heat. However, with a goal of optimizing pregnancy rate and litter size, having better control over the weaning to estrus or heat interval (WEI) and time of breeding becomes extremely important. Research with the University of Manitoba sow herd at the Glenlea Research Station has been focusing on determining whether the hour of weaning, as well as the photoperiod the sows are in, can impact the WEI and the time of ovulation. Improving the predictability of WEI and ovulation will help optimize breeding management and reproductive performance of sows. In the first phase of the research, conducted as part of the Master of Science (M.Sc.) degree of Kelly Bowen, multiparous Cotswold sows were housed under a short photoperiod (9 hours of light) from entry into the farrowing barn at 105 days of pregnancy, throughout their 17-day lactation and during the rebreeding period. Sows were weaned either as soon as the lights came on at 8:00 AM or at 4:45 PM, just before lights went off, with the idea that any differences would show up by weaning at these two extremes of the day. Following from these initial studies, the recent M.Sc. research of Jennifer Burt has investigated the actual timing of ovulation, with real-time ultrasonography, following 8:00 AM weaning, as well as the influence of longer day-lengths (photoperiod) during lactation. Sows were kept under either long days (14 hours light) or short days (9 hours light) throughout their time in the farrowing barn until weaning at 17 days. Breeding barn photoperiod was at 9 hours of light as is routine in our Glenlea Swine Research Unit. Combined, these investigations clearly demonstrate that sows kept under short photoperiod during 17 day lactation and weaned as soon as lights come on, have less variable WEI than do those sows weaned later in the day or kept under longer photoperiod during lactation. In effect more sows under short day-lengths and weaned early in the day, began their standing heat period between 8:00 AM and noon when heat checks are routinely conducted. With time of ovulation being fairly consistent from actual estrus onset in these sows, being able to accurately determine the onset of heat ensures more reliable time of insemination to optimize conception and subsequent litter size.