Nice Farm Smells. Hay, particularly alfalfa hay, has such a sweet smell it’s like perfume — seriously. Good hay smells so good that you can tell without too much trouble when hay is bad. Hay can mold if it stays damp for long periods. And you don’t want to feed moldy hay to animals. While most fungus on hay is harmless, a few are trouble-makers. Some molds produce toxins called mycotoxins, which can cause problems such as intestinal upset and abortions in pregnant livestock like horses and cows. Other molds can instigate respiratory issues.