I do not top my dog from grass eating , as I have to assume she is doing it for a reason, , yes I have the same butt problem at the other end sometimes, but most healthy dogs will eat only a small amount, if your dog is so bad I would recommend a check up at the vet, he/she may be suffering from IBS or other stomache intestinal problem, some people say don't let them eat grass it makes them sick, the answer to that is, that is probably the reason they re doing it, because they feel unwell nd being sick relieves the pressure