Hormone Concentrations and Semen Parameters in Criollo Breed Stallions Under Training

The stress associated with training may reduce reproductive efficiency in Criollo stallions. The objective of this study was to compare semen quality and hormone concentrations in Criollo stallions under training or under regular field conditions. Criollo breed stallions (n = 18) were evaluated during the spring. The exercise group (n = 9) performed 1 hour of exercise per day and participated in competitions during the experimental period. The control group (n = 9) neither performed exercise nor participated in competitions. Serum and semen samples were obtained every 15 days (two separate ejaculates an hour apart). Sperm motility, velocity, and morphology were evaluated with a phase-contrast microscope and concentration by a hemocytometer. Diff-Quik stain was used to identify polymorphonuclear cells, and the degree of chromatin condensation was evaluated with the toluidine blue stain. The sperm survival test was performed at a room temperature of 22°C. Semen evaluation was performed in raw samples and in samples diluted in a skim milk and glucose-based extender. Cortisol, testosterone, and estradiol were measured using radioimmunoassay. There was no effect of exercise on testosterone and estradiol concentrations (P = .28 and P = .97, respectively). However, in the exercise group, cortisol concentration was higher after exercise (P = .004). There was an effect of exercise on the following semen parameters: gel-free volume (P < .001), sperm motility (P < .0001), total number of sperm (P = .0001), normal sperm morphology (P < .0001), and total number of morphologically normal and motile sperm (P < .001). No effect of exercise was found in the following semen parameters: color, pH, and sperm concentration. This study showed that exercise had a negative impact on seminal quality; nevertheless, semen parameters were within the normal ranges established for the equine species.