Stress and summer infertility in pigs.

A serious year-round fertility problem on a commercial piggery was investigated. The problem was characterised by a high proportion of non-pregnant sows and gilts which showed a delayed return or failure to return to oestrus after a normal mating to a fertile boar. Several factors were identified that we considered to be placing undue stress on the breeding stock from wearning through mating and early pregnancy. When steps were taken to reduce the influence of these stressors there was a marked improvement in the herd farrowing rate. Across all months this improvement was largely due to a reduction in the number of sows showing delayed return or failure to return to oestrus after mating. There were no concomitant changes in other indices of performance. In conclusion this study has shown that stressful factors other than summer heat stress can cause a syndrome of delayed or failure to return to oestrus, this syndrome is commonly and perhaps wrongly called 'summer or seasonal infertility'. When given the appropriate combination and/or intensity of stressful stimuli, it can be manifest at any time of the year.