I have a super easy-keeper Paso gelding. He is insulin resistant. Is your mare just sort of “fat all over” or does she have a cresty neck, “lumpy” fat and fat around her tail head? My boy’s neck gets cresty and he will get lumpy fat above his tail if I don’t manage him properly. Those can indicate IR not “just” overweight. You can ask your vet about it.
You can try a grazing muzzle if you must leave her on pasture.
I keep my Paso on a dry lot – NO grass – while my pasture is green. So he’s on that dry lot eating hay only from spring when the grass gets green until it goes dormant in the fall (I have Bermuda – luckily we have a winter and it goes dormant). I can’t even leave my boy out with a grazing muzzle though – he just can’t have lush green grass.
You can try keeping her up during the day and out on the pasture at night too (and maybe even with a muzzle then). VERY generally speaking – most grass is safer at night than during the day. Grass sugar levels change greatly all day long – trigged by temperature, rain, no rain, amount of sun/clouds – it’s fickle stuff!
Good for you for starting to notice her weight and look for some answers – I think we are far to used to seeing horses that are too fat – and it’s just not healthy for them.
Good luck with her – easy-keepers sometimes are not ‘easy’ for us human caretakers!