You need to look at their condition score and the quantity of poo coming out. If they need more condition or more fibre then add more hay (or other energy / fibre sources). Behaviour, eg resource guarding or acting cold, is another thing to watch out for.
You need to look at their condition. Mine have not had any hay at all this winter. It has been v mild and they grass has been growing almost all of the way through. 2 of the 3 are still fatter than I would like.
Condition aside, I give hay if mine haven’t got their heads down picking.
If they are standing round looking bored I’ll top it up. If they leave it it’s too much.
I think that’s easiest if the horses are not fat. I’d not bother putting hay out just for the sake of it.
We have three on 14 acres along with 30+ sheep. It is rough grazing left to grow into foggage and still a reasonable amount of pull on it. No hay given at all and the ponies haven't lost any weight sadly, best we can say is they haven't gained any.
I'm cutting my girls down. The 2 smaller ones have held their weight far too well. Would like them to drop a bit in the next month or so. Exercise is increasing so hopefully waistlines decrease!
We have 14 on 33 acres of rough grazing, with some proper oldies. We haven't hayed at all this winter. Two of mine are a good weight, one needs to drop more.
Mine have wanted to come in for a bit of hay this last week or so. They come in for about 45 minutes of hay and then out again. Our grass is definitely coming through. Will be time for muzzles soon!