This is due to exposure to the sun, as most of this hay is stored outsid e in their drier climate. Hays should be clean, free from mold and as dustless as possible. If hay has been rained on or improperly cured and stored, it may mold. Mold, which has a white or bluish appearance, causes respiratory problems. Weeds in hay are more likely to mold than forage plants.
Weeds are coarser, and some are toxic to horses.
Avoid musty-smelling hay. But don't go on smell alone to determine if hay is bad. The reason that hay should also be free of foreign materials is that rocks can crack or break a horse's tooth. Other materials, such as wire or nails, can also cause injuries. Also avoid blister beetles in alfalfa hay.