The HOS test and its relationship to fertility in the stallion

The hypo-osmotic test has been used successfully on equine semen and was considered to be a simple and accessible method which could be a useful addition to routine equine semen analysis. It was therefore of interest to determine whether the hypo-osmotic test is significantly correlated to proposed criteria of fertility. The stallions were divided into two groups on the basis of threshold levels of fertility. A significant difference (P<0.05) was found between the two groups for the following parameters: progressive motility, morphologically normal spermatozoa, percentage of swelling with the hypo-osmotic test, percentage of pregnant mares and number of services per pregnancy. The hypo-osmotic test provided a simple evaluation of membrane function and the results obtained show stallions with low swelling scores (<40%) to be of doubtful fertility. The hypo-osmotic test was not correlated with percentage of pregnant mares but showed a tendency to correlate with the number of services per pregnancy, therefore it could be an additional method for evaluating stallion fertility. Further studies are needed to confirm this observation.