Jackson eats grass ALL the time lol. He loves it. He's just a grazer. He usually does it when he's over excited and exercising a lot though, I almost wonder if he's getting the moisture out of it.
But I can tell when he's eating grass because his stomach is sick. It's only happened 2-3x in his lifetime, but when he races out and is not picky about which grass he eats, and just is non-stop eating violently, I know his stomach hurts and he will puke. I let him. I know I feel better when I am about to throw up and it finally comes out, so I don't want to stop him.
Here is a good video on grass eating: