What the heck is this green stuff

I turned Matt, Ben, Max & Bobby out today. They had a small turn out pen and it attaches to a couple acre paddock. It just needed a gate fixed up. Got that done this afternoon and turned them out in the paddock, along with the mini mule, Mocha.
Well, other than Matt who has been out in a large field, you'd think none of them had even seen grass or a large area before. First they ran around like a bunch of idiots and then they had their heads down, gobbling the grass up like it was candy.
They still came a running when they heard me shake the grain pail though. They all came into the barn and got their oats. I thought they'd be happy to go back outside, but I had to virtually drag them out. Old habits die hard I suppose. I'd have left them in but it's a pretty warm night and the barn would get fairly humid. They're better off outside where they have totally fresh air and lots of space.