Definition of a seasonal infertility problem in pigs.

A summer infertility problem was investigated on a large intensive piggery in a warm temperate climatic zone in Eastern Australia. The period of infertility correlates with the period of summer heat stress. The infertility problem was manifest as a delayed return of sows to oestrus after mating and an increase in the number of sows found non-pregnant when due to farrow. Litter size was apparently not adversely affected. Gilts and first parity sows were most frequently involved. The subsequent breeding performance of sows affected by this infertility problem was quite normal. There was no apparent effect of heat stress on the boars. It is proposed that the infertility is caused by heat stress imposed more than seven days after mating causing whole litter loss and the return of the sow to oestrus.