If your horse(s) are obese, Insulin Resistant, have previously foundered, or have other diagnosed metabolic problems, you keep them off any lush or fresh grass.
ALL other NORMAL horses are just fine with it. We have 50 horses and we have 1 that cannot stand grain or rich grass. So, in other words, 49 of them or 98% of them are just fine on good grass.
We have utilized winter wheat and rye pasture for 40 years. I have never had a horse founder on it in all of these years. I would not turn my one IR horse on wheat pasture.