I would like to know what “slightly overweight” actually is. What is his body score? Is he just getting a hay belly or is he actually packing on fat? Has he always been on this pasture? Is he suddenly getting more grain/better hay if he’s stalled at night?
These are all good questions to ask before putting a grazing muzzle on a horse. My gelding and mare will cycle through a “slightly overweight” size down to a “slightly underweight” size and I would never think to put a grazing muzzle on either of them. I usually cut grain down if I think they are starting to get heavier than preferred.
Now, if this horse has never been on grass pasture before and is just now packing on pounds, then that is a problem. There could be an easy keeper under there who needs preventative attention to stop too much weight from being gained. In that case, a grazing muzzle may be a good solution.
I would personally call in a vet’s opinion if you think he is only slightly overweight and see what he/she has to say, and he/she could offer specific advice to changing your horse’s diet and exercise if need be. Sometimes weight is more of an opinion on horses, and if your horse is a different breed than what the barn owner is used to, you may be facing an uphill battle. A vet will provide more unbiased remarks about weight and the horses well-being. My Quarter Horse looks like a tank compared to the skinny, tall Thoroughbreds at the barn, but that doesn’t mean he’s overweight in his own right. You can still easily feel his ribs on his stomach and he’s been like that all summer.