The trace element selenium appears to reinforce sperm quality and thus maintain a stallion's fertility. When selenium is lacking, motility drops, sperm membranes break down, and the sperm's acrosome (which covers the head) can deform. But according to a new study by German scientists, feeding your stallion more selenium won’t necessarily improve his sperm quality. The problem, they say, is not how much selenium is in the horse's system, but how much gets into the sperm themselves.

Researchers studied a group of stallions all supplemented with the same amount of selenium (1.3 mg daily) and determined that the concentration of selenium in the horses' blood had nothing to do with how much selenium was in the sperm, according to Holger Bertelsmann, PhD, research scientist at Helmholtz-Zentrum Berlin für Materialien und Energie in Berlin. Horses with high levels of selenium in the blood could have very low levels in the sperm, and even horses (from previous studies) with moderate selenium deficiencies could have high levels in the sperm, he said.

A horse’s sperm apparently has preferred status for selenium supply within the horse’s system, he reported. And stallions with low levels of selenium in the sperm most likely suffer from a malfunction of the biological mechanism that feeds the sperm with the element, and not a lack of intake.

But that's no reason to hold off on the selenium, according to Bertelsmann. Selenium also helps fight certain infections and supports cardiovascular function. "Low dietary selenium supply can still indirectly lead to poor reproductive performance in stallions," he said, adding that the recommended daily intake is about 0.5 mg for an 1100-pound horse