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Virginia Cooperative Extension -
 Knowledge for the CommonWealth

Keep Boars Cool During Summer

Livestock Update, July 2000

Mark J. Estienne, Swine Research Physiologist, Tidewater AREC

The elevated environmental temperature and humidity associated with summers in Virginia can severely affect swine reproduction. Breeding operations often experience a "summer slump" during which reproductive efficiency is greatly decreased. The adverse effects of "heat stress" are manifested in both the male and female. In this Livestock Update, I'll focus on the deleterious effects of elevated temperature on reproduction in boars and interventional management practices.

Effects of Elevated Temperature on Semen Quality and Libido in
Boars Acute exposure to high environmental temperatures reduces fertility in boars. Boars subjected to heat stress conditions produce ejaculates that have low sperm concentrations, high percentages of abnormal sperm cells (damaged acrosomes, proximal cytoplasmic droplets, etc.) and decreased percentages of progressively motile spermatozoa. Research has indicated that the minimum exposure time and critical air temperature above which production of sperm cells is adversely affected is 85º F and 72 hours, respectively.

The negative effects of acute heat stress on semen quality may be somewhat immediate. A "lag" period of approximately 2 weeks, however, is often observed between the initiation of acute heat stress and the first indications of abnormal sperm production. After the cessation of heat stress conditions, six to seven weeks is necessary before fertility returns to normal. Thus, acutely heat stressed boars can have a protracted, negative influence on reproduction in a breeding operation. For example, boars exposed to 95ºF temperatures for three consecutive days in late-July may be responsible for suppressed conception rates well into September, even in the unlikely situation in which temperatures do not rise above 85ºF after the July "heat wave."

In contrast to a number of experiments that focused on the effects of acute heat stress on fertility, little research has been conducted during which semen quality was assessed in boars chronically exposed to temperatures in the upper range of the thermo-neutral zone (79 to 84ºF). Boars are routinely exposed to these temperatures during the summer in southeast Virginia.

Researchers at North Carolina State University reported data obtained from seven commercial boar studs in southeastern North Carolina from June through October, when average weekly high temperatures at these facilities never exceeded 84ºF. Never the less, during this period there was a significant increase in the number of ejaculates rejected due to poor quality and a decrease in the number of insemination doses per ejaculate. The reduction in the number of insemination doses per ejaculate began 5 to 6 weeks after the weekly high temperature had stabilized at approximately 81ºF. Thus, boars may also be sensitive to chronic periods of moderately elevated temperatures not classically recognized as "heat stress" conditions.

The effects of elevated environmental temperature on various characteristics of libido have not been extensively studied. However, during the summer boars may become lethargic and display a reluctance or refusal to mount a sow in estrus or an artificial sow. Research has shown that boars subjected to heat stress conditions have decreased circulating concentrations of testosterone, which probably contributes to decreased libido.

Management Considerations
Diligent effort should be made to keep boars cool during periods of high environmental temperatures. The following are some key considerations for managers and herdsmen:

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