When pigs are weaned and lactation ceases, the process is again activated and it continues through the complete cycle. This is true when weaning occurs at four weeks of age or later. The process still proceeds reasonable well with three-week weaning. However, as weaning age goes below three weeks of age, the normal course of events are increasingly disrupted by physiological responses that reduce reproductive efficiency. Although sows can show normal heats by at least two weeks after farrowing, many of these heats are infertile because no ovulation occurs.
Uterine involution is required for fertility to be restored.
In general, two weeks are required for the uterus to involute substantially and a 3-week period is required for involution to be complete. Not only is there the direct effect on litter size, but also the effect on extending the interval from weaning to rebreeding plus the effect of an increase in culling on the average parity of the herd. If the question being asked in the effect on annual net profit, then parameters such as post-weaning mortality, cost of nursery feed, cost of lactation feed, facility cost and perhaps others need to be included. Much of this information must come from retrogressive analysis of the production records of commercial production units . These records came from 1985 to 1995 and represented a mix of environment and genetics.