Dairy cows are used to being hooked up to machines every day and milked. I’ve worked on farms before and have helped with this. One thing never occurred to me, though. What would happen if we quit milking the cows? Do they rupture? Leak? Do the cows find another way to milk themselves? Enquiring minds want to know.
The short answer is that they stop giving milk. Cows (and other mammals) will continue to lactate as long as they are milked.
As for immediate effects I can only judge by my wife’s experience which is that her breasts (udders?) hurt like hell for a few days, very much swollen and tender, and then the pain (and the breasts) gradually subsided.
If not milked for a long enough period of time, today’s dairy cows could very well die. I as well worked in a dairy farm and saw the negative effect of a cow who for some reason missed a few milkings. Cows that are about to be “dryed up” in preparation for giving birth are injected with strong antibiotics to prevent potential problems. But these cows generally are giving less milk than usual anyway.
On a side note, this particular issue is why it is permissable for Jews to milk cows on the Sabbath, when it would usually be forbidden. Since a cow can not miss a day’s worth of milking, and it would cause them severe pain (and perhaps worse), the milking is permitted even on the Sabbath. According to Jewish law it is a divine commandment not to cause unnecessary pain to animals.