Various supplements are commonly given to breeding stallions in hopes of improving semen quality, but conflicting reports exist on their effectiveness. Researchers have found, however, that the supplement ingredient DHA, an omega-3 fatty acid found in fish oil, does have a positive affect on semen quality.

In one study, when stallions supplemented with DHA were compared with a control group that did not receive the supplement, semen from both groups showed good sperm motility (the ability of the sperm to move in a forward motion) after 24 hours of cooling, but the sperm from supplemented horses swam straighter and faster. After 48 hours, changes were more obvious, with sperm from treated horses showing better total motility, progressive motility, and rapid motility. Supplemented stallions whose cooled semen doesn't generally sustain a good quality showed significantly improved semen quality.

Lipids (a group of substances that include fatty acids) are highly concentrated in sperm and sperm membranes. The different amounts and concentrations account for the differences in how well semen from a particular species will react to cooling and freezing. The supplement used in the study was designed for boars, whose semen has some of the same characteristics as stallion semen, namely a similar distribution of polyunsaturated fatty acids and a low tolerance to cold shock.

Studies in humans and other animal species have shown that supplementation with vitamins C and E and a combination of L-carnitine and L-acetyl-carnitine also have improved sperm output, concentration, and motility. However, it is not known whether these supplements would have any effect (eit