In SW MN/E SD -- dock is fairly common. I have one grazing paddock that frequently floods out and the first thing to regrow is curly dock. Cattle tend to ignore it in my experience. In hay, I'd say the same thing, cattle pick around it.
One site said cattle would need to eat 10-20 lbs of green leaves to get sick.
I've been around cattle and curly dock my entire life and I can't think of an instance that an animal got sick and it was linked to that plant.