Seasoned horse breeders know the potentially dangerous effects of grazing broodmares on pasture containing endophyte-infected tall fescue: agalactia (poor milk let-down), dystocia (difficulty foaling), thickened placenta ("red bag" foal), and even foals that are born weak or dead.

But veterinarians know less about another potentially toxic substance that could be lurking in broodmares’ feed: a mycotoxin called zearalenone. Fortunately, researchers are working to better understand the effects of this substance.

At the 2013 Society for Theriogenology Conference, held Aug. 7-10 in Louisville, Ky., Heath King, DVM, Dipl. ACT, an assistant clinical professor at the Mississippi State University College of Veterinary Medicine, presented a poster chronicling recent research on zearalenone’s effects on mares’ reproductive performance.

Mycotoxins are harmful secondary compounds produced by molds that are found in the soil and vegetable matter including grains, forages, and feed. They can form in the field both before and during harvest and can continue to form under suboptimal storage conditions after harvest. In horses these toxins can cause a wide range of clinical signs, including respiratory, gastrointestinal, neurologic, and reproductive problems—even death