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Summer Infertility

Summer Infertility in Free Range Pastured Pigs

By Lee McCosker


Historically, the sow only produced one litter per year in spring as many livestock species do.  Spring usually offers the best conditions for young animals by providing a more pleasant climate and good feed availability and all contribute to the survival of these offspring.   Selective breeding and controlled climatic conditions inside commercial pig sheds has meant that the pig is now expected to have at least two litters per year.  Nature often intervenes however and even under commercial conditions summer infertility can wreak havoc on the number of sows that fall pregnant and the number of litters born. Heritage breed pigs such as the English Large Black or Wessex Saddleback are more prone to mimicking their ancestors and often only produce one litter in a year under sub optimal conditions.

Day length and sows returning to heat

Summer, or seasonal, infertility is a natural reaction to day length and exposure to intense ultra violet light.  High temperatures also have an effect and combined with long hours of daylight may cause the sow to abort a pregnancy very early, or temporarily halt the heat or oestrous cycle. When abortion occurs, it is often so early that the sow simply appears to have returned to heat, sometimes a bit later than expected, or seems to have missed a cycle. The heat cycle of a sow is approximately 21 days. So, long hot summers will have an impact on the number of sows that farrow in the following months and can mean that there will not be as many pigs available for sale as anticipated.  Some years are worse than others especially if there has been an extended number of continuous hot days.

Its not just the temperature affecting sow productivity

It is important to know that it is not just high temperatures that cause summer infertility.  The length of day is always going to trigger this very natural occurrence in some sows. Hormonal changes maintain a pregnancy and progesterone is produced by the corpus luteum (located in the ovary) and without it the pregnancy will fail. When days are long and ultra violet light exposure is high, the corpus luteum may respond by disappearing which will result in the termination of the pregnancy. Summer infertility is said to effect white breeds more than coloured pigs so maintaining a cross breeding program infused with a coloured breed may be of benefit.

There are many things you can do to lessen the effect of summer infertility but you will most likely experience it with some sows each year and you need to manage for that so the interruptions to the breeding program are minimal and you are not left with a hole in your production and unable to supply standing orders or have the pork available that you had anticipated. The type of grazing available to sows can have on impact the amount of heat generated in the sows body and therefore her overall temperature.  If the sow has access to long, dry and fibrous pasture she will attempt to ferment a lot of it in the hind gut (caecum, colon) and this process produces a lot of heat.  During summer give your sows access to green and more digestible pastures or if this is not possible, keep dry pastures slashed or mown so that over consumption of these dry older grasses cannot occur. When pigs are hot they tend to eat a lot less so provide a highly nutritious and easily digestible feed to encourage the sow to eat and maintain her condition during the three weeks after mating.

Tips to minimize the effects of summer infertility

Following are steps you can take to minimize the effects of summer infertility:

  • Provide plenty of shade for all pigs and make sure it is available throughout the entire day
  • Provide wallows or a sprinkler system that will allow the pigs to cool themselves
  • Site hutches or shelters to catch any breezes and cools the inside of the structure
  • Manage pastures so that they are not just old and rank grasses
  • Make feed and water easily accessible so the sows do not have to travel far to reach it
  • Only handle and move pigs in the cool of the morning or evening
  • Maintain stress free conditions and keep sows in ‘friendly’ groups to minimize aggressive behaviour
  • Look after the boar as well as his libido may fall in hot conditions and semen is also affected by heat
  • Use crossbred sows that have been infused with coloured breeds
  • Increase the number of matings in summer by approximately 15% to allow for those that do not produce a litter at this time

This article is an exert from  Pigs, Pasture and Profit by Lee McCosker and will be available from the   PROOF website 

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The opinions, advice and information contained in this website have not been provided at the request of any person but are offered by  PROOF and Australian Pig Farmers solely for informational purposes. While the information provided has been formulated in good faith, it should not be relied on as a substitute for professional advice. Australian Pig Farmers, PROOF or do not accept liability in respect of any action taken by any person in reliance on the content of this publication.


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