Most horses will founder if let out on pasture year round (Neck breaks, trouble with weight forever, feet problems, death*Look it up*), also unless you live somewhere were it does not snow there will be 2 to 5 months where there will be no hay in the field. You will have to seed the pasture every years which is allot of work. It would be allot better if your made a 200 by 60 pen for your horse with a nice shelter and buy feed. Horses will eat till they flop over they are not like mules whom will conserve food for latter.
Simply not true. Most horses will not founder on pasture. Most horses will do best if out on pasture 24/7. They will have less digestive issues (colic, founder), they will have less behavioral issues, they will be better socialized, they will be better adapted to the environment, they are cheaper to keep... they will be healthier, happier horses. They can be out year round, even with snow on the ground and they will graze somewhat. Yes, you do need to toss additional hay but they will still forage and dig in the snow finding valuable grass to graze on. Horse do regulate what they eat. It does take them some time to adapt when they are used to limited feed but they do figure it out. A horse left to move around is going to have healthier feet, legs and joints. I can also graze my horses for less than $1 a day for 20 of them (not counting my time) or I can lock them up and feed them hay for $50 a day. Why would I? Plus now, I have to pick up poop, they are going to be crabbier, be harder on the fences, not look as healthy, when it rains, they will waste as much hay as they eat plus I have to source, haul, stack and unstack the hay, drive to go get it wasting hours and diesel is still $3.70/gallon so add that in to your hay costs.
If you are having to reseed annually, than you do not have the right type of grass or you aren't managing your pastures correctly. Pasture grass in a perennial grass that you should only need to reseed every 8-10 years. I got 20 years out of my fields. (and my horses are out on it 24/7/365 in the rain, snow and sun) They do need reseeding but that's more because of the drought and prairie dog damage than anything else. A properly reseeded pasture takes one year to establish itself. You can't reseed and graze it at the same time.