5 Good Reasons
COLIC: A horse may gorge itself on piles of clippings. The large amount of grass can cause excessive fermentation in the hind gut. This can lead to colic or laminitis.
CHOKE: Horses tend to gobble down grass clippings quickly. Unlike grazing on grass out of the pasture and slowly chewing it, clippings require little effort or chewing to swallow. There is a potential risk that the clippings can get lodged in the esophagus.
MOLD: Piled up grass clippings can quickly grow mold, especially in hot and humid environments. Mold can also lead to colic.
BOTULISM: Dense piles of grass do not allow for air to circulate through grass clippings. This in an invitation for the deadly botulism toxin to develop.
POISONOUS PLANTS: Horses unknowingly eat toxic weeds and plants they normally would avoid when they get mixed in with the mower clippings.