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Seasonal infertility can cause breeding herds to exhibit increased weaning-to-estrous intervals, decreased
farrowing rates, delayed puberty in gilts, irregular returns to estrus (as the result of failure to maintain pregnancy),
and decreased boar fertility. Although the significance of seasonal infertility is well documented, there remains
speculation as to its exact cause. Traditionally the domestic pig has been thought of as a nonseasonal breeder,
but there is evidence of its ancestor, the wild pig, being a seasonal breeder. This trait still appears to be present
to some degree in domestic pigs. Some genetic lines of sows exhibit a greater seasonal infertility rate.
Identification of these lines through the examination of producer's records will help in addressing this breeding
problem.
High environmental temperatures also appear to contribute to decreased fertility in the breeding herd. It has been
shown that exposure to high ambient temperatures (>90oF) can cause adult females to exhibit irregular estrous
cycle lengths. Frequency of estrus and the ability of the female to exhibit signs of estrus decrease during times of
elevated environmental temperatures. In addition, females exposed to high temperatures after mating show a
decrease in conception rate and an increase in embryonic mortality.
While numerous studies point to elevated temperatures as being a significant contributing cause to seasonal
infertility, other studies show that lowering the environmental temperature does not totally eliminate its effects. In
some cases, the seasonal decline in reproductive performance starts prior to the occurrence of hot weather.
Increased temperatures can also decrease fertility in boars. When a boar is exposed to high environmental
temperatures (>90oF) for 3 to 7 days, both motility and quantity of sperm produced decrease. Usually there is
also an increase in the production of abnormal sperm. These adverse effects may last for up to 6 weeks following
the time the boar was heat stressed.
Higher temperatures also decrease the consumption of feed and water. Due to this decrease in feed intake, sows
commonly have energy and protein deficits during hot weather. This can cause a significant weight loss in the
lactating sow and result in an increased wean-to-estrus interval or complete anestrus following weaning.
Results of studies conducted to determine the influence photoperiod has on seasonal infertility have been
controversial. Cause-and-effect conclusions have been drawn due to the fact that reproductive performance
decreases during the summer months when day length is at its longest. While it appears day length may play a
role, more research is needed to show whether artificial manipulation of photoperiods can offset the effects of
seasonal infertility.
There is no sure-fire method to overcome the effects of seasonal infertility. Recognizing that this problem exists
and taking steps to prepare for it can decrease the severity on a producer's farm. Below is a list of
recommendations you can provide to producers:
• Examine your breeding records so you can adjust your mating targets. What is your normal farrowing rate?
What happens to that rate for sows serviced during this time of year? Is the problem more severe among certain
genetic lines?
• Minimize heat stress: Use water drippers, sprinklers, misters, or evaporative coolers properly to help keep
sows cool. Make sure that there is adequate air flow with the use of fans and individual snout coolers.
• Feed consumption: Balance your lactation ration on actual intakes. This may require a diet of higher energy and
protein density during the summer. Increase feed intake as rapidly as possible during the first week of lactation.
Research shows that feeding sows three times per day will increase consumption. Keep feed intake high from
weaning until the sow comes in heat.
• Boar usage: Do not overuse boars during this time. Any boar suspected of heat stress should have regular
semen evaluations performed. It is important to provide good boar exposure after weaning to promote return to
estrus.
• Housing: If possible in your operation, house sows in total confinement. They will exhibit less seasonal infertility
than those housed in outside lots.
• PG-600: This product is now approved for inducing estrus in sows. Its use should be considered if your
weaning-to-service interval is increasing, particularly in first parity sows.
• Breed more gilts. In addition to the above recommendations, one of the best approaches to seasonal infertility
is to plan ahead. Be sure you have additional gilts available to breed from July 1 to September 1.
Lawrence. D. Firkins, DVM, MS
Swine Extension Veterinarian
Director, Research Stations
University of Illinois College of Veterinary Medicine
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