Effects of strenuous exercise on stallion sperm quality

Some stallions are expected to perform athletically and breed contemporarily. Athletic activity has the potential, especially during the summer months, to induce thermal stress to the testes, resulting in reduced reproductive capability due to decreased sperm quality and libido. Thus, the objective of this study was to determine the effect of strenuous exercise on stallion sperm quality in hot, humid conditions (thermal heat index 38.08 ± 3.77 to 42.88 ± 3.33). Miniature Horse stallions (n = 7) were assigned to treatment group based on age and semen quality. Treatment stallions (n = 3) were exercised 4 d/wk for 90 min for 12 wk, whereas control stallions (n = 4) were tied in the shade. Semen was collected from stallions for 5 consecutive days every 4 wk to evaluate sperm quality. Scrotal and neck temperatures were recorded using implanted subdermal thermosensory devices. In addition, rectal temperature and heart rate were recorded. Exercised stallions had higher mean rectal (39.27 vs. 37.07 °C; P < 0.0001), neck (39.77 vs. 37.44 °C; P < 0.0001), and scrotal (34.90 vs. 33.40 °C, P < 0.0001) temperatures than control stallions during the 90-min exercise and 30-min recovery period. No differences were detected in sperm quality characteristics between treatment groups (P > 0.05). Results suggest that previous reports of reduced sperm quality in stallions in training may have causes other than transiently elevated scrotal and body temperature. Strenuously exercising stallions for up to 90 min under hot and humid ambient conditions may not be harmful to spermatogenesis.