Increased cortisol release and transport stress do not influence semen quality and testosterone release in pony stallions

The use of breeding stallions for equestrian competitions requires that fertility is not negatively affected by competition or transport to the competition site. In this study, effects of cortisol release induced by road transport (600 km), adrenocorticotropic hormone (ACTH) administration (3 × 0.5 mg synthetic ACTH) and placebo treatment on semen quality and testosterone release were investigated in Shetland stallions (N = 13) using a crossover design. Saliva for cortisol and blood for testosterone analysis were collected for 10 weeks after treatments. Semen was collected daily for 5 days directly after treatments and twice weekly for another 9 weeks. Total sperm count, sperm morphology, motility, and membrane integrity were analyzed. We hypothesized that elevated cortisol decreases testosterone concentration and semen quality. Cortisol concentrations increased in response to transport and ACTH (P < 0.001) but not control treatments (peak concentration, transport: 7.6 ± 2.4, ACTH: 13.7 ± 1.5, control: 3.8 ± 0.9 ng/mL). No treatment effects on testosterone existed. Total sperm count decreased with daily semen collections in week 1 (P < 0.01) but did not differ between the treatments. The percentage of motile, progressively motile, membrane-intact, and morphologically defective spermatozoa did not change over time from Days 2 to 6, and there existed no differences between the treatments. In conclusion, road transport evoked a stress response which was mimicked by ACTH treatment. Both treatments had no effect on testosterone release and semen quality. Testicular function in stallions is apparently well protected against transiently elevated cortisol concentrations, and stallions can be transported over longer distances without negatively affecting their fertility.