“..produce more milk than their calves need.”
Yep, and if they’re not milked, the udders become very painfully engorged, and can bleed and burst if not stanched by an infusion of certain hormones to force the pressure-releasing “let down” reflex, which allows the milk to ooze out.
This is not only true of cows but also of human females.
Please don’t ask me how I know. :-)
Yes I know that lactating cows can become very sick if not regularly milked. And I know women can leak for the same reason. You can ask me how I know this, but you probably don’t need to.