I still wonder if cows have to have a calf every year to keep producing milk. With horses anyway they will keep making milk, if they are health, as long as something is taking the milk out of them. There are people who breast feed their children 'til they are 5 years, some times older, even without having more kids. Here in America most women wean their children years earlier, but the women don't go on producing milk for x number of years until the kid is 5. And every once in a while there is that story where a domestic animal that has given birth before starts nursing orphaned baby animals and starts producing milk. Like the cat who nursed puppies or one dog taking in another's pups and nursing them even though she hasn't given birth in years.
\tAll that being said I don't know if cows have to give birth every year to keep producing milk. BUT COWS HAVE TO GIVE BIRTH EVERY YEAR TO ENSURE THAT THERE ARE ENOUGH REPLACEMENT HEIFERS. Most dairy don't send cow eggs and bull sperm off to some lab, so scientists can find the X sperms to put into the eggs to make them female, then have the eggs sent back to the cows to be implanted. The farmers need to make sure that even if the calves are mostly males, they will still have plenty of females.
A plastic hutch is no substitute for a mother. Replace dairy, not mothers.