Zearalenone is a mycotoxin that mimics the reproductive hormone estrogen. This mycotoxin is produced primarily by the fungus Fusarium graminearum, the same fungus that produces deoxynivalenol in maize and small grains. Swine are the most commonly affected domestic animals, but cattle and poultry may also be affected. The clinical effects of zearalenone may include an enlarged uterus, swelling of the vulva and vagina (known as vulvovaginitis) [Figure 16], enlarged mammary glands, anestrus (periods of infertility), and abortion. Zearalenone may be passed to nursing piglets through the mother’s milk. A commercially available derivative of zearalenone (zeranol) has been used as a growth hormone to increase weight gain in beef cattle. Zearalenone contamination is economically important in maize and hay. High humidity and low temperatures favor the production of zearalenone by F. graminearum in maize. Zearalenone contamination has been reported in moldy hay fed to cattle, swine, and sheep, but conditions favorable for fungal growth and toxin production in hay have not been investigated in detail.