The "correct" way to acclimate horses to grass is to start with 15 minutes and increase by 15 min intervals everyday until you hit 4 hrs. Then you are supposed to be fine for letting them stay out all day.
For my horses I kept them on a lead and just brushed them when it was 15 minutes. 30 minutes I turned them out for a bit while I walked around the pasture, and got there breakfast/dinner ready. I'm lucky though and I don't have problems catching my horses. There was another thread about this last year...
I think it would help to feed them a bunch of hay before they went out too. That way they already have some gut fill and are less likely to gorge themselves.
Edit - I give grass clippings to my horses, but as MaggiStar said we feed them right away and I spread them out so they aren't stacked up enough to ferment as quickly. They never get enough to eat for more than a couple hours.
Last edited by MN Tigerstripes; 04-10-2010 at 12:13 PM.