Written by: Joe Lyman, DVM, MS It’s that time of year when as soon as you’re done mowing, it looks like it’s time to mow again. Inevitably the grass gets just a little bit longer than we’d like before we get the chance to mow again, especially in paddocks and pastures. If you want one more reason to mow frequently and not leave long clippings around the pasture, how about a little-known fact about those clippings? They can be deadly. Grass clippings can contain spores from the Clostridium botulinum bacteria. These spores can become active when grass clippings begin to heat up from the beginning of the fermentation process in the grass clippings. Wet and dense clippings are most at risk for this. Once the bacteria start to grow, they produce botulinum neurotoxin, one of the deadliest toxins known to man. A microscopic amount of this toxin is a lethal dose to a horse. The toxin paralyzes the horse’s muscles, ultimately paralyzing the diaphragm and leaving a horse unable to breathe. Don’t leave large clumps of grass clippings on fields, and never gather them into piles for later feeding to horses. Even short clippings can be problematic, especially if close mowing has mixed soil in with them. Since you can’t see botulinum neurotoxin, apparently normal clippings can still be deadly. Better safe than sorry. Never feed grass clippings to your horse, and avoid leaving them in pastures where horses will graze.