Question: I’ve heard you shouldn’t feed a horse freshly cut hay—is that true?
Horse myths include some long held, but erroneous beliefs about feeding hay. Some people think that freshly baled hay is unsafe to feed to horses.
Answer: This is a long held bit of lore that is untrue. If your hay is baled at an optimum moisture level of 18 to 20% it is ready to feed to your horse. It is dangerous to feed grass that is still drying (this includes lawnmower clippings you may be tempted to throw over the fence).

