They would be mush under a cow herd feeding frenzy.
I also know you will never change your Grandpa's mind, I am assuming he is of my generation and when I grew up on a farm you fed an animal all it wanted to eat. It is hard to do, limiting the food intake seems unnatural, but it really works.
I even watch the feed Bert has access too. I feed him low value food and force him to hunt and scratch for his food. Of course, he is trying to become a woman's man and spending more time wooing the girls than eating.
Cows can out run our dogs, our milk cow caught one of the turkeys once!!! And them suckers can run.
He is from your generation.