Most of the time, Bonnie's calf runs free with her and takes all her milk. When I want some milk, I go out in the evening and put Little Sir in the barn; next morning Bonnie has a gallon of milk for me in the right two teats, and a gallon for Little Sir in the left two. This all works out very well: I don't have to ever milk the cow if I don't want to, but when I need milk, I can get it from her.
Except... I had better be ready to wake up pretty early the next morning after I've separated her from her calf. Along about six A.M., she starts bawling, and doesn't stop until I go out and remedy the situation.
She is not mooing for me to milk her; all she's concerned with is her calf. Although if her calf weren't around, she'd soon learn to bawl at me at milking time, simply because she knows she gets some sweet feed when I milk her.
One thing that has always irritated me is the way movies and TV shows depict horses and cows that are sick or in pain as being very vocal. Actually, a sick animal is usually a quiet animal. Cows moo because they want to be with their cow friends, or because they've lost track of their calves. Horses nicker and whinny if they're away from friends, or if they see a new horse friend they'd like to greet, or possibly because they think their human is going to feed them something.
When I was in junior high school, I recall reading a paperback book written by a veterinarian who had worked for Barnum and Bailey circus. He told about a fire that injured many of the animals; he said one thing that struck him was how quiet it was; the animals hardly made a sound.