Did he suddenly get turned out on it, or had he been there awhile? Suddenly getting turned out on rich spring grass would be the same as getting into the feed bin. If a horse is on pasture as it grows I don't "think" it would be high risk to founder.
Becky
Sudden - was purchased and turned out on it. The horses that had been there didn't founder.
I feel like our grass can shoot up in a matter of days in spring... bout of rain and you can watch it grow ;~D Then a week without rain and it stops growing. I am having to watch it all the time to see if there is enough out there to eat, or too much.