Can heat stress during performance affect stallion reproduction?

This article was written by SDSU student Kirby Javers under the direction and review of Dr. Rosie Nold, Dr. Sara Mastellar and Dr. Rebecca Bott as part of the AS 489: Current Issues in Animal Science course.

As many performance horse owners know, a desire to perform well in competition is always the goal. Along with great performance in the show ring, comes the desire to pass on our champion’s genetics, whether breeding a broodmare or promoting a stallion. However, to attain this prowess and be worthy of breeding large numbers of mares, some stallions are put through training and showing programs that may raise their body temperature. Increases in scrotal temperature have been associated with reduced fertility. For example, high fevers have been associated with temporary reductions in semen quality. A recent study took place at Texas A & M to determine whether or not the heat stress induced by heavy exercise effects a stallion’s scrotal temperature, and thus, quality of sperm.

In 2013, nine miniature stallions were studied in south-central Texas during 12 weeks over the hot summer months. These stallions ranged in age from five years to seventeen years of age, and were sorted into a control group and a test group, balancing for age and known semen quality. To maintain uniformity, stallions were housed in stalls of identical criteria, beginning thirty days prior to the initiation of the twelve-week testing period. Stallions received similar base diets and each stallion received supplements needed to maintain body condition scores of 5 or 6 based on the Henneke scale. During the first week ejaculate was collected from each stud to provide a baseline for comparison during the trial.

The control group of stallions was tied outside in the shade and allowed to rest while the test group was put through a 90-minute exercise regimen interspersed with 30 minutes of temperature and heart rate sampling. Exercise was performed with intensity for five minutes, closely followed by five minutes for rest and data collection, for a total 120 minutes each day. This workout program consisted of strenuous exercise, once a day, four days a week, for three-week intervals. Before and after each three-week exercise stent, the stallions were given the week off and those weeks were utilized for semen collection and evaluation. Semen was collected and studied. Sperm was evaluated for total motility (sperm movement), as well as other abnormalities that could impair fertility such as, but not limited to, bent tails, detached heads and DNA quality.

The temperature testing of each horse was made possible by subcutaneous (below the skin) heat receptors placed in the neck and scrotal regions. Rectal temperatures were also taken to obtain further measures of core body temperature and to ensure that no horses were at risk for sickness due to being overheated. Heart rates were monitored as well. At set times throughout each day, environmental temperature, as well as humidity levels, were recorded.

The scrotal temperature of the test group during exercise was higher than the control group that was being rested. As expected, as body temperature rose, so did the rectal and scrotal temperature. Even though the scrotal temperature increased with the intense exercise, the temperature never rose above the temperature known to hamper sperm or sperm production. Therefore, it was no surprise that the sperm from the exercised stallions showed no, or only insubstantial differences from the control, or rested group. It was observed, however, that the sperm numbers and quality dropped for both test and control groups throughout the study. This elucidates the possibility that the environmental heat and humidity that increased throughout the study played a larger role in hindering sperm production than the short intense exercises. However, more tests would need to be performed to either support or refute this conclusively.

Based on the results from this study, increases in core and scrotal temperature induced by exercise did not reduce semen quality. It should be noted that this was a small group of stallions and that careful monitoring was in place to ensure stallions did not overheat, or approach hyperthermia, and that the stallions received frequent short period of rest during the intense exercise. In conclusion, stallions that received carefully planned bouts of intense exercise and that were fed to maintain an ideal body condition did not have reduced semen quality compared to non-exercised stallions.