Its possible, I have my two horses on 1 1/2 acres. (Not ideal, working on getting my entire 20 acres fenced in) But I do it, and I keep the pasture healthy and fairly decent length, until winter that is.
Every spring I cut down any weeds that they won't eat and every season I pick up all the poo and put it in a compost.
I often will close the gate to one side of the pasture and let it grow back a bit, for a week or so, and then do the same for the other side.
It is more work than just putting them in the pasture, but if you take the time and work I'm confident you can do it. (:
Granted, my horses get a lot of exercise. I frequently go on 15+ mile rides so you want to make sure they have room to burn off energy if you wont' be messing with them.
Falling Maples Homestead www.fallingmaples.wix.com