[SIZE=12pt]Shelley,[/SIZE]
It really depends on the horses. If you have them together and have some that will stand and eat all day and eat EVERYONE'S feed, then it's not a good idea. Until recently my yearlings (group of 7) had food out most of the day and night. They would come in (they are on 35 acres of grass!) and eat as they chose. Now though, with 3 new boys from the other farm to argue with, they are getting more competitive and it's gone after mealtime.
If you're just gradually increasing the amount they get and they eat individually (stalls or paddocks), that's one thing and it won't hurt them to leave some and finish it later. That's actually better for them. Free choice feed large amounts could be pretty harmful if they decided to eat all day though.