Seasonal infertility in swine a perplexing problem

Seasonal infertility in pigs has been a subject of much attention over the years. The problem is perplexing since most species of pigs are not classified as seasonal breeders. However, the genetics of the modern pig used for commercial pork production can be traced back to the European wild pig, which is seasonal, breeds in fall and is anestrus in summer. In this species, changing daylength is considered the primary cue to seasonal reproduction. And while the majority of countries with modern swine genetics report seasonal infertility, most cases link to periods of high environmental temperature rather than changing daylight.

The problem becomes more complicated when considering countries close to the equator experience extended periods of heat stress but with little change in daylength while regions farther from the equator experience relatively short periods of heat stress but dramatic changes in daylight. Yet it is difficult to separate heat stress from daylength since longer days associate with the highest temperatures in summer and even into early fall.

Attributing seasonal infertility in the modern pig to photoperiod is a challenge since most pigs are raised indoors with controlled levels of artificial light. Further, there has not been a clear link of fertility to hours, intensity, or even type of light. However, the pig does perceive changes in light through the release of melatonin. But the melatonin response to light is variable and may depend upon the animals prior conditioning to a lighting regimen. In any event, the effect of light on seasonal fertility in the pig is likely complex, and may involve the perception of changing hours of light and dark over a period ranging from days to months.

Similarly, linking seasonal infertility to heat stress also depends upon the number of hours of heat, the temperature reached, and the number of days of stress. Under heat stress, animals modify their behavior and physiology to minimize heat production and maximize loss. When temperatures exceed normal, animal management and housing systems can aid in maintaining animal body temperature within the normal range. However, there are limits to what either can do to minimize stress. When heat stress occurs, animal body temperature will increase and feed intake will be reduced.

The effects on fertility will depend upon the level of feed reduction, the number of days involved, and the stage of reproduction. Risk for infertility is often compounded for the most susceptible females such as gilts with marginal growth rates and young sows in lactation, where limited body resources, predispose these females to reproductive failure.

In the U.S. each year, fertility failures increase starting in mid-summer and often extend into fall. Industry data shows fewer pigs bred and pregnant in summer and fall, with fewer sows farrowed in fall and winter. The result is that fewer pigs are available for market the following summer. This is bad timing, as peak demand for pork associates with summer and fall outdoor events. During this time of high demand, supplies are lower and prices higher but without targeted numbers of market animals, producers cannot fully capture this period of economic opportunity.

The reality of seasonal infertility is that only a proportion of pigs and herds will show symptoms that may involve delayed puberty, anestrus, pregnancy failure, abortion, lower sperm production, and poor quality semen. The timing and severity of the symptoms can also vary among similar herds in the same region. What this would indicate is that susceptibility to the effects of season can be influenced by how herds manage their at-risk females with respect to age of animal and stage of reproduction.

For example, ensure all animals get 10 to 16 hours of light a day with greater than 100 lux at pig level. Make sure all building fans are set and working properly, and that air movement is occurring at animal level. Check that evaporative coolers are working properly and are effective. Avoiding stress conditions by moving or mixing, feeding, heat check or breeding, vaccination, or weaning in the hotter parts of the day. In the hottest parts of the day, check watering systems to ensure availability throughout the entire building and to each animal. Walk the barns in the hotter parts of the afternoon and look for signs of stress that can include uneaten feed, high respiration rate, lethargy, and direct sunlight. This is the opportunity to make changes before problems arise.

Robert V. Knox is a University of Illinois Extension swine specialist