I have a 5yo mini donkey that I got from a rescue in MD. she was in a huge lush pasture with 20 other donkeys with no muzzle. When I brought her home to my 5 acres my farrier suggested I muzzle her because she was showing signs of founder.
I got her a mini muzzle and she wore it all spring/summer/fall with no hoof or weight issues.
Now that it's winter and there's barely any grass should I keep the muzzle on? There is a round bale in the shed she shares with my 3 horses but she obviously not first in line to get it!
Mine don't have access to grass in the winter unless the snow is at least a foot deep, at which point I figure they have to work so hard to get at it that it offsets the calories. So no, I don't muzzle my Shetland in the winter but I do use small-hole hay nets so she doesn't just stand and stuff herself. I also partially clip her to make her not QUITE so energy-efficient. Not to the point where she's uncomfortably cold, just removing a little bit of her natural R-value.