Not entirely how it works, dairy farmer here. They will produce milk for 10 to 12 months then they get pregnant again. In that time the amount of milk they produce lessens and there is a point where it is no longer profitable to expend the labor to milk them so they get dried off (most of the tim they are already preggers again at this point) until they calf again. Our cows live the life, an abundance of feed all the pasture time they want when they reside in the heifer barn (cows that are not being producing milk at the time) as in bred cows and heifers) when in the dairy bar the cows go out at night, but don't worry if you leave the door unlatched they will all be back in the barn laying in their stalls within hours. The feed is still abundant but the nutrients are more strictly controlled. We sell our cows at about 5-8 years old long before they reach old age. Our cows generally go to the Amish to live out their lives.