I understood that studies had shown that if you keep horses off the grass and reduce turnout to keep their weight down, they can learn to up the rate at which they do eat, so when they are turned back out they just make up for lost time. A horse or pony that always has access to very little grass just has to keep moving to find it and doesn't get into the starve-stuff cycle. I would have a track system if I owned my own land, but as I don't I manage their weight by using the principle of the biggest area possible that still leaves them slightly hungry, and give them hay to fill the gap and for roughage. So I have a selection of fields and just open up and close off as needed. At the moment, they have access to two small fields that have very poor grass (one field is a wood and the other is used as a sheep pen, and a taped off section of a bigger field that I adjust. I love the way that they have the choice of grazing and have to move between the various areas. It is a pain to keep poo picked though!