I know for horses, the ideal situation would be to plant in the fall and let it overwinter before putting the horses out on it. If you're going to plant in the spring, do what Celtic said and make sure the roots are well established.
My geese rip out mature hay field grasses and sometimes even have the roots out as well . . . I'd second the other posters and say that the longer you can keep them off of it, the better it will do . . . although it is hard not to give in and just let them munch on things they would like so much . . .