Pasture question

Hey there, anyone around on a holiday Monday? My horses have their pasture pretty much stripped. I have fenced a new spot, which was hayed about a month ago and has regrown very nicely. Grass about 4inches. Should I just turn them out on it all day, or just give them a few hours at a time? They have been on grass all summer but their pasture was getting pretty lean.

I'm always worried about founder or colic...

Thanks

I always left them out for the day after haying...but I brought them in at night...I had one piggy that never stopped munching...