Summer Brings A Reduction In Fertility In Breeding Pigs

Bernie Peet is president of Pork Chain Consulting Ltd. of Lacombe, Alberta, and editor of Wes tern Hog Journal. His columns run every second week

It’s difficult to think about seasonal infertility, which is associated with hot weather and declining day length, when we are still in the grip of winter.

However, some things can be done well in advance to mitigate its effects and which require a certain amount of planning ahead. So it’s certainly not too early to be thinking about it.

A recent booklet from Australia’s Pork Cooperative Research Centre (CRC) by Prof. Paul Hughes and Dr. Will van Wettere provides a valuable review of seasonal infertility and details some new research findings.

Seasonal infertility is defined as a reduction in fertility and fecundity in breeding pigs in summer and early fall. It is associated with problems with sows and gilts showing heat, which is seen as delayed puberty attainment in gilts, extended weaning to service intervals in sows, and higher anoestrus rate in both gilts and sows (Table 1). In addition, some herds also see lower litter size in gilts and sows bred during the seasonal infertility period.

Observations by farm staff are not always a good indicator of what is going on in the sow, say Hughes and van Wettere.

“What is interesting is to take a second look at these apparent pregnancy losses, using blood hormone levels that can establish if a pregnancy started and, if it did, when it failed,” they state.

“When this was done, an interesting pattern emerged. Essentially, the vast majority of NIPs (Not-In-Pig), as identified on farm, were actually conception failures (three-week returns), or early pregnancy failures (returns around 25 to 35 days).”

They say that in the case of misdiagnosed three-week returns, they are classified as NIPs due to inaccurate heat checking around three weeks post-insemination. As well, they found early pregnancy failures recorded on farm as NIPs are usually a result of variable ultrasound technique or, more likely, testing too early (less than 28 to 30 days after breeding). So you need to take care, particularly during the seasonal infertility period, to ensure adequate and accurate oestrus detection and ultrasound pregnancy diagnosis procedures, Hughes and van Wettere conclude.

The causes of seasonal infertility are still not fully understood, mainly due to its unpredictability and a variation in its impact from year to year.

“What we do know is that seasonal infertility must be due to either the long daylight hours of summer and early autumn and/or the higher environmental temperatures associated with this time of year,” state Hughes and van Wettere.

According to research at the University of Sydney, sows are at greatest risk of displaying seasonal infertility if they:

1. are parity six or higher

2. take longer than six days to show heat after weaning

3. are weaned early

4. wean fewer than eight piglets

The booklet suggests a number of actions, based on research findings, to reduce the impact of seasonal infertility, including:

maximizing nutrient intake by lactating and weaned sows

providing cooling for lactating and weaned sows

providing additional boar stimulation for oestrus after weaning

group housing sows between weaning and breeding

ensuring that sows and gilts are matched closely for size/weight when grouping them

ensuring gilts and weaned sows are not crowded

breeding during the cooler parts of the day

increasing the frequency of heat checking to twice daily in the seasonal infertility period and breeding sows at first heat detection regardless of when they show oestrus after weaning

including betaine at two kilograms/tonne in the gestation diet if the litter size is relatively low

housing bred gilts and sows individually, or maintaining them in stable groups from before breeding until at least four to five weeks afterwards

individually feeding mated gilts and sows, at least for the first four to five weeks of gestation

feeding gilts a low level (up to 2.3 kg/day) for the first three to four days after breeding

feeding gilts and first parity sows a high level (3+kg/ day) for four to five weeks from day four post-breeding ( This doesn’t seem to have any benefit in older sows and can reduce performance in all sows if applied at other times of the year.)

conducting more frequent and rigorous checks for gilts/ sows returning to oestrus between days 18 and 32 post-breeding

applying a good pregnancy diagnosis procedure at four weeks post-breeding and again three to four weeks later

The recommendations on timing of breeding are based on Pork CRC research that indicated sows appear to ovulate earlier in the heat period during the summer/autumn period. The same research also showed hormonal support of the pregnancy may be reduced in weeks three to four after breeding, increasing the likelihood of pregnancy failure.

Along with these management measures, Hughes and van Wettere emphasize the importance of having enough gilts available to cover for any anticipated reduction in farrowing rate. The number of sows and gilts bred per week can then be increased in order to ensure farrowing targets are met. Depending on the age and weight at which gilts are brought into the herd, this may involve planning several months ahead.

Although some regions of Australia have far higher summer temperatures than most of Canada, we do experience seasonal infertility and it is well worth being aware of how to deal with it.


Table 1:Typical seasonal infertility pattern for pregnancy losses

No. sows

Three-week returns

Negative pregnancy test


Adjusted farrowing rate (%)













From: O’Leary, Final Report to Pork CRC, 2010



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