I'm sorry- I have to disagree, and mabye it's just because of where I live. In Florida, we have grass year round- it gets no richer one season than the other. So having a horse founder or get obese on grass is just ludicrous. And if you are that worried about your horse, and don't want them to be in a stall, then build a seperate paddock filled with dirt, and put another horse in there. There are other ways around sticking a muzzle on a horse. Muzzles are for dogs. Not to mention the fact that having ANYTHING on a horse, even a halter, while they are free in a pasture is very dangerous. Many horses in my area, including one of my own, have gotten injured, sometimes seriously, by getting caught on fencing or whatnot. Sorry, but that's my opinion and the thread did ask 'for or against.' I, clearly, am against.
After-thought: If grass is so bad for horses, why is that their diet naturally???