Seasonal boar infertility occurs worldwide and contributes to economic loss to the pork industry. Summer infertility of boars has been correlated to elevated ambient temperatures and the resulting increase in scrotal temperature.
However, a mechanism by which temperature damages sperm production in boars is not clearly understood, and the length of the impact to sperm production is variable. To investigate the impact of heat stress on boars, researchers at the University of Wisconsin-Madison took two approaches. The first was to induce heat stress to the boar scrotum and testes via scrotal insulation. The second was to utilize changes in ambient temperature and the impact on boars in commercial boar studs.
Scrotal insulation involved the attachment of insulating material to the boar’s scrotum for 48 hours. The insulation was then removed, changes in testicular morphometry (germ cell development) were evaluated, or collection of sperm was continued for up to 45 days to capture production of abnormal sperm in boar ejaculates.
Scrotal insulation in the studies has produced increases in scrotal temperatures from 3.4 to 5.0 degrees F (1.9 to 2.8 degrees C). When testicular sections are evaluated following scrotal insulation (five control and five insulated boars), there is a significant (p<0.05) loss of developing germ cells in meiosis and increases in cellular debris in the lumen of seminiferous tubules. Further histological changes suggest scrotal insulation damages junctional complexes between Sertoli cells needed for germ cells to progress from mitosis to meiosis. Many germ cells do survive the heat stress and continue on in development and are eventually ejaculated.
Sperm with decreased motility, abnormal morphology or changes to sperm head/nuclear shape appeared 19 to 37 days after scrotal insulation (five boars). This corresponds to the same stages of germ cell development for which researchers observed significant losses in the testicular histology evaluation. The studies found heat stress induces germ cell death, or if the germ cells survive, abnormal or damaged sperm are ejaculated, but with a delay of two to five weeks in their appearance relative to the date of heat stress.
Researchers looked at the timing of their investigation as fortunate as the summer of 2012 had record-breaking heat throughout the Midwest and most of the United States. The study was conducted in a boar stud equipped with cool-cell technology and scrotal drips. A total of 1,181 boar ejaculates, with an average of 49.2±1.7 per week, were collected between June and November of that year. An upper pig thermo-neutral zone temperature was selected of 73 degrees F (22.2 degrees C), based on the work of Brown-Brandl et al, 2013 for pigs with modern lean genetics. The external barn average weekly maximum temperature went above the thermal neutral zone from the third week of May through the third week of September 2012. Even with cool-cell technology, interior average weekly maximum temperatures went above the pig thermo-neutral zone the second week of June and remained elevated until the second week of September. Sperm characteristics were examined as averages for all boars collected within a week, with control values from the first three weeks of June. Sperm head/nuclear shape began to change relative to the control values (p<0.05) in the first week of July but remained changed seven weeks (49 days) beyond the return to the thermo-neutral zone, or until the last week of October.
A second experiment was conducted at the same commercial boar stud in 2014, a summer with more moderate temperatures. However, interior barn temperatures would have reached above the thermal neutral zone from the third week in June through the last week of August. The sperm head/nuclear shape remained changed at three weeks (p<0.05) following the return to the thermo-neutral zone, but then later returned to normal (p>0.05).Results of the studies indicate boars of modern genetics are sensitive to both an experimentally induced short-term (48 hours) heat stress, as well as longer environmental changes. However, the temperature increases required to impact sperm production are minor. The temperature at which a boar’s sperm production appears to be impacted by environmental temperatures is below the previously accepted 84 degrees F and may be closer to the upper end of the pig thermo-neutral zone. The magnitude of the heat exposure does affect how long sperm production remains impacted following return to the pig thermo-neutral zone.
Researchers suggest that boar studs need to maintain boar housing at temperatures below 73 degrees F in the summer to prevent summer heat impacts on semen quality and quantity. In the face of increasing summer temperatures, even for two to three days at a time, it is critical to install infrastructure to keep barns in the pigs’ thermo-neutral zone and below 73 degrees F.