This is from Horse and Hound magazine:
"British Horse Society appeals to gardeners to dispose of grass cuttings with care after three donkeys die
The BHS has issued a warning about the dangers to equines of ingesting grass cuttings.
Three donkeys from Surrey are already reported to have died this year after eating cuttings.
Spokesperson for the BHS, Nichola Gregory, said: "A pile of grass clippings is very attractive to horses, ponies and donkeys, but once eaten can prove fatal.
"Eating grass clippings can cause colic and, if the grass begins to ferment, it can cause a lethal build up of gasses within the gut.
"This causes the animal severe pain and damage to the digestive system, resulting in a horrible death."
The society is appealing to all gardeners to dispose of waste in a "responsible manner" and not to dump it where grazing animals may be tempted."
Just a caution, our pastureland is rich though, maybe this doesn't apply i drier climates, after all we cut grass and dry it for hay, it is when it is in its wilting state it is dangerous I believe.