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Horses are seasonal breeders with a natural breeding season beginning in spring and extending through midsummer. In this study, quantitative and qualitative parameters of chilled stallion semen were compared between fertile and subfertile stallions and between the breeding and the non-breeding season. Semen quality parameters compared included ejaculate volume, sperm concentration, total sperm number, sperm morphology, and computer-assisted semen analysis (CASA)-derived sperm movement characteristics obtained from two groups of warmblood stallions (   n  =  8   ; four fertile stallions and four subfertile stallions), which differ in the seasonal pregnancy rate 80 %-90 % (fertile) vs. 40 %-60 % (subfertile). A total of 64 ejaculates were collected from the stallions (   n  =  8   ; four in the breeding season and four in the non-breeding season of each stallion). No significant differences in the semen quality parameters between the fertile and the subfertile stallions in the non-breeding season were observed. However, in the breeding season the proportion of morphologically normal sperm, total motility, progressive motility, average path velocity (VAP), and curvilinear velocity (VCL) were significantly higher in the fertile group (   P  &lt;  0.05   ) when compared with the subfertile group. In addition, a significant seasonal variation in the proportion of morphological normal sperm was found in the fertile group between the breeding and the non-breeding season (   P  &lt;  0.05   ). Moreover, significant seasonal variations (   P  &lt;  0.05   ) in CASA parameters of mean VAP, straight line velocity (VSL), and beat-cross frequency (BCF) were observed in the fertile and the subfertile stallions, which tended to be lower in the non-breeding season. In conclusion, differences between the fertile and the subfertile stallions were observed only in the breeding season, and a few of CASA-derived parameters seemed to be significantly lower during the non-breeding season in both the fertile and the subfertile stallions.

Seasonal variations in quantitative and qualitative sperm characteristics in fertile and subfertile stallions