Why is cut grass dangerous for horses?
Grass clippings allowed to accumulate and decay in a pile will provide a suitable substrate for Clostridium botulinum (botulism bug). In the same way, silage allowed to ferment at the wrong pH will also allow growth of the bug, and so incorrectly prepared big-bale silage causes botulism. The bacteria grow in the plant matter and form the botulism toxin, which is then ingested causing botulism. How long it takes the toxin to form will depend on speed of multiplication of the bacteria, which in turn depends on the moisture and the environmental temperature (which affect pH). In spring with nice moist grass I wouldn’t expect it to take long. Botulism in horses is frequently fatal, causing flaccid paralysis (weakness of the muscles) and dysautonomia (a bit like grass sickness but not). Symptoms usually depend on the amount of toxin ingested – large quantities will cause almost sudden death, modest quantities will cause weakness, tremors and gradual paralysis of the jaw and muscles involved
