Mares might not be the only ones with clear breeding seasons. Swiss researchers recently confirmed that stallion semen integrity also appears to be dependent on the season.

While seasonal variations of routine semen parameters are already well-documented, the amount of DNA fragmentation (DNA separating or breaking strands into pieces, which can cause sub- or infertility) also varies in stallions over the course of a year, with prominent differences between spring/summer and fall/winter, said Fredi Janett, DVM, and Dominik Burger. Janett works at the Clinic of Reproduction of the University of Zürich, while Burger is the head of equine reproduction research at the Swiss Institute of Equine Medicine of the University of Berne and Agroscope, in Avenches.

The month of the year appears to have a significant effect on several parameters of collected semen in breeding stallions, the pair relayed at the 10th annual Swiss Equine Research Day, held last year in Avenches.

In their study, they investigated ejaculates from 15 Swiss National Stud breeding stallions (the Franches-Montagnes breed) once a week for one year. They analyzed volume, sperm concentration, total sperm count, sperm motility, and susceptibility of the sperm DNA to denaturation (the destruction of the usual nature of a substance), which is associated with decreased fertility.

Despite spring and summer being the main breeding seasons, sperm concentration in ejaculates was actually lower during these months than in the fall and winter, they said. Total sperm count actually reached its highest peak in mid-summer—often as the breeding season ends. Sperm motility—the sperm’s ability to move productively through the mare’ reproductive system—was found to be higher in spring and summer than in winter and fall, Janett added.

The researchers assessed DNA fragmentation using a sperm chromatin structure assay (SCSA), which detects a parameter known as the DNA fragmentation index (DFI). The DFI measures the percent of damaged sperm in the total ejaculate. Higher DFI values are generally associated with lower foaling rates, Janett said.

The pair found a seasonal pattern in equine sperm DNA fragmentation. The DFI in this study was lower, on average, from December to February and in October compared to June and July—meaning the winter sperm DNA was not as fragmented (potentially improving fertility) as the summer sperm DNA, Janett said. Further exploration is needed to determine whether there’s a relationship between climatic factors and seasonality in sperm DNA fragmentation and what that relationship is.

“These findings will help up better assess how we breed stallions,” the pair explained. Their next steps are to use the data to develop practical applications in breeding centers.