My main question is: if my dog wants to eat grass, should I prevent him? I have heard that dogs do this to throw up, and seen this reason alluded to here: Why do dogs eat grass?, but am not 100% sure if this is correct. He usually does throw up if he eats a lot of grass. Here are the thoughts that are going through my mind, and why I am not completely decided on which path to take:
Reasons for preventing eating grass
He could eat a poisonous plant, which brings up another question: Do dogs avoid eating poisonous plants?
He might just think it is "free extra food" and not realize it will make him sick (this assumes he is not sick to start with, which I am not sure if this is what is going on)
I don't want to have to clean up the vomit afterward
Reasons for allowing eating grass
It might be that he is already sick, and needs to throw up for some reason (e.g. he might feel better after throwing up, and could be eating it on purpose to throw up)
Maybe there is some nutrient in the grass that he does not get from dog food.