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Seasonal Infertility in Pigs

by 5m Editor
15 April 2011, at 12:00am

Questions on what seasonal infertility is and how to minimise it are answered by Professor Paul Hughes and Dr Will van Wettere of Australia's Pork CRC.


  • What is seasonal infertility?
  • What are the major risk factors?
  • What’s new?
  • How to minimise it?

What is Seasonal Infertility?

Seasonal infertility is a reduction in fertility and fecundity in breeding pigs at a particular period of the year – usually summer and early autumn. It shows up mainly as two problems:

  • more difficulty coming on heat – seen as delayed puberty attainment in gilts, extended weaning-to-oestrus intervals in sows and higher anoestrus rates (stales) in both gilts and sows

  • higher rates of early pregnancy failure (see table 1) – most usually detected as more irregular returns to service 25 to 35 days after breeding, although some herds may not detect these failures until later in pregnancy (see Table 2).

In addition, a few herds also see lower litter size in gilts and sows bred during the seasonal infertility period.

Table 1. Typical seasonal infertility pattern for pregnancy losses
Spring Summer/Autumn
No. of sows 135 175
3-week returns 5 14
Negative Pregnancy Test 5 22
Abortions 1 4
Adjusted farrowing rate 91.9% 77.1%
O’Leary, Final Report to Pork CRC, 2010

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. When this was done, an interesting pattern emerged (Table 2). 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). In the case of misdiagnosed three-week returns, they are classified as NIPs due to inaccurate heat checking around three weeks post-insemination. 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 35 days after breeding).

Thus care should be taken, particularly during the seasonal infertility period, to ensure adequate and accurate oestrus detection and ultrasound pregnancy diagnosis procedures.

Table 2. What really happens to summer pregnancies – an analysis of 25 summer pregnancy failures
On-farm observations What the hormones tell us
3-week returns 6 12
Early pregnancy failure 9 12
NIPs 10 1
van Wettere et al, Report to Pork CRC 2008

Despite more than 40 years of research on this topic around the world, we still do not fully understand what causes seasonal infertility, mainly due to its unpredictability. In some years, it is hardly seen while in other years, it has a major impact on farrowing rates and herd fertility (see Figure 1). Worryingly, it can be occurring on some farms in a region, or even in some sheds within the same farm, but not others.


Figure 1. Proportion of sows in the same herd losing their pregnancies in the summer/autumn period in 2007 and 2008
(van Wettere et al. Report to Pork CRC, 2008)

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. In fact, it seems to be due to both these factors as it is experienced in Scotland (extremely long daylight hours in summer but low temperatures) and in Australia (relatively short summer days but much higher temperatures).

What are the Major Risk Factors?

Pork CRC-funded research at the University of Sydney by Drs Michael Bertoldo, Chris Grupen and Trish Holyoake has been identifying the key risk factors for seasonal infertility in sows.

The take-home messages from their studies are that sows are at greatest risk of displaying seasonal infertility if they:

  1. are at parity 6 or more
  2. take longer than five days to return to heat after weaning
  3. are early weaned
  4. wean fewer than eight piglets.

Interestingly, these are also factors that contribute to reduced sow fertility and fecundity for the rest of the year, suggesting that seasonal infertility is most likely to be shown by sows that are of questionable fertility, or have been subjected to sub-optimal management, regardless of season.

This is almost certainly true in gilts as well. Those gilts that are most resistant to puberty stimulation, are also the most prone to delayed puberty in the summer and early autumn. What is more, increasing a gilt’s stimulation by providing regular boar contact from around 25 weeks of age will reduce, but not eliminate, the seasonal delay in puberty attainment.

It seems clear that individual gilts and sows appearing to be at greatest risk of showing a seasonal infertility problem, are those that are low ranking and group-housed, where competition for feed is high.

What’s New?

Recent Pork CRC research at Roseworthy by Dr William van Wettere and Professor Paul Hughes suggests that two key elements of seasonal infertility are that:

  1. sows may ovulate earlier in the heat period during the summer/early autumn
  2. hormonal support of the pregnancy may be reduced in weeks three to four after breeding.

In other Pork CRC-funded research at the University of Sydney (by Drs Bertoldo, Grupen & Holyoake), evidence has emerged that the eggs shed in the seasonal infertility period are of poorer quality than those ovulated over the rest of the year. This could result in poorer fertilisation, failure of fertilised eggs to develop through the embryonic growth stages and/or poorer corpus luteum formation, causing reduced/inadequate release of progesterone, which is the key hormone required for pregnancy support.

How to Minimise Seasonal Infertility

Realistically, we do not yet adequately understand the causes of seasonal infertility to eliminate it – the best we can do is to take those steps that research and practical experience tell us will lessen its impact. Thus, for example, we should attend to the key risk factors outlined earlier.

We should also consider:

  • maximising nutrient intake by lactating and weaned sows

  • provisioning cooling for lactating and weaned sows

  • providing additional boar stimulation for oestrus after weaning

  • group housing sows between weaning and mating/insemination

  • when grouping gilts or sows, ensuring you match them closely for size/weight

  • ensuring gilts and weaned sows are not overcrowded

  • mating/inseminating during the cooler parts of the day

  • increasing the frequency of heat checking to twice daily in the seasonal infertility period, and mating/inseminating sows at first heat detection regardless of when they return after weaning, given the recent finding that sows appear to ovulate earlier in the heat period during the summer/autumn period. Once the first mating/insemination has occurred, further matings/inseminations can be provided at 24-hour intervals, as normal.

  • including betaine at 2kg per tonne in the gestation diet if the litter size is relatively low

  • housing mated/inseminated gilts and sows individually, or maintaining them in stable groups from before mating/insemination until at least four to five weeks post mating/insemination (Mixing gilts in early pregnancy is risky, especially if it occurs after day four post-mating/insemination)

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

  • low (up to 2.3kg per day) feeding gilts for the first three to four days after mating/insemination

  • high (more than 3kg per day) feeding gilts and parity 1 sows for four to five weeks from day four post mating\insemination (This does not 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-mating/inseminating

  • applying a good pregnancy diagnosis procedure at four weeks post mating/insemination, and again three to four weeks later.

Alongside these changes, improve planning to help anticipate the seasonal infertility period and have enough additional gilts on hand to cover for the anticipated drop in farrowing rate. However, this must be achieved without crowding the gilts, i.e. they must have at least 1.8 square metres of space each, regardless of the number involved.

Lastly, as the period of seasonal infertility tends to coincide with the peak in staff holiday absences, it is worth carefully organising staffing schedules through this period to ensure some of the staff more experienced in this area, are on hand each week.

Further Reading

- Go to our news item on this story by clicking here.


April 2011