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.
A retrospective study of a seasonal infertility problem in a large intensive piggery showed that during the periods of infertility there was a significant increase in the numbers of sows returning to oestrus in the periods 22 to 37 and 44 to 53 days after mating. A prospective study showed that the majority of sows affected by the seasonal infertility problem returned to oestrus during the fourth of fifth week after mating. However, some sows did not exhibit a normal behavioural oestrus at this time and so the oestrus was undetected until and sow completed another oestrous cycle accounting for the second period of increased returns at 44 to 53 days. Some sows failed to exhibit behavioural evidence of oestrus at this second to exhibit behavioural evidence of oestrus at this second oestrus and so had very prolonged mating to detectable oestrus intervals.
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