Depending on your horse and your location, you need 2-5 acres of well managed pasture, with pasture rotation techniques used to feed a horse without supplementing additional hay. Cavets being if you're in So-Cal of parts of Arizona or Texas with less dense & less nutritious grasses its not going to matter how much land you have if your grass sucks.
If you want to feed your horse with only the grass that grows on your property, you have to think of yourself as a "Grass Farmer" and care for the pasture as much as you do the horse that lives on it.
You have to implement rotational grazing areas, measure grass heights, harrow, aerate, dethatch, overseed and add compost and run hay reports.
It really isn't as easy as "I have 5 acres, I guess the horse is fed. " Without proper pasture management techniques horses can strip a pasture in months, making it uninhabitable for grass growths for years upon years.
We've all seen the farms that that do this. Pasture if shirt to no grass, mostly dirt and the only thing growing are islands of weeds. It takes at least 3 years to completely rehab a pasture once its been "desert-ified" by horses, to establish a healthy pasture, and that's 3 years of maintaining (all that work I listed above ) PLUS keeping the horses off of it (or less than 5 hours a week) durring those 3 years.
Be envious when you see lush green pastures, they are a TON of work.
You can find A TON of really really good resourses on horses, grazing and pasture rehabing resources here: King Conservation District Pastures Posted via Mobile Device