Just to preface I plan on discussing this with my vet, but was curious as to other opinions -
My gelding is coming home from boarding. He's always been on dry lot. He's prone to ulcers and has a history of gas colic. My pasture is mostly brown, dormant grass right now, with various patches of green that my other horses keep grazed down. I'd say it's 85% dormant.
I'm wavering on a grazing muzzle. My first thought is to use one, but not for as long as I would for spring/summer. My second thought is that it may cause stress and in turn digestive flare ups. I took about a month to acclimate my other two when they were introduced to pasture.
He will eventually be on turn out 24/7 with my other two, which I hope will help with the ulcers. He's currently been stalled all day and turned out at night.
Here In the frigid NE, putting a drylotted horse on pasture has never been a problem. Even if it looks green, it's not really doing anything. Then as the weather does warm, and the days get longer, it starts on so slowly that the horses make the transition with no problem.
Then come May we pay attention to the too fat crew, and muzzle them.
Some riders change their horse, they change their saddle, they change their teacher; they never change themselves.
I would go with JB's advice, better safe than sorry, and we don't know your location. Mine get dry lotted when the thaws come, field is less torn up by sharp shoes galloping around.
Since learning better, I take the time to slowly let horses get used to green grass in the Spring. Can take about 5-6 weeks. They can't develop the stomach flora to deal with new grass after a winter eating dry hay, in a couple days! That green grass is what throws their system into overload, can easily result in grass founder with sudden diet change. Body can't digest that huge grass intake with wrong stomach flora. Doesn't always matter if animal is fat or thin, stomach is not ready to manage a change to grass diet in only a few days. And you would be AMAZED at how much a horse can graze in a VERY short time, to load up their stomach!
So best to go the safe route, short turnout even on dormant looking grass, probably with the muzzle, and slowly acclimate him to the new routine with time.