I prefer the grain and sledge hammer method,you just have to make sure you hit it square,or they get pretty upset.
Seriously I would not shoot a cow with a bow and arrow,if it isn't a game animal I am not using hunting equipment to kill it.
A buddy of mine several years ago was practically begging a friend of his who was going to be butchering a steer to allow him to shoot it with a broad head.He was going on an elk hunt and he wanted to check penetration.