So what did they do with the calf?
In a small dairy situation the calves are kept off of the cows for either the morning or the evening milking and so the cows' milk production remains high the entire time. The calf weans, but from the cow's udder's perspective the calf is still suckling like mad and so the cow keeps producing milk. The calf grows up and is bred themselves, doubling the milk output over the course of two to three years.
Now this isn't sustainable in a paperclip-optimizer kind of way, so only the best milkers are kept and the ones that either have a hard time getting pregnant, or are ornery, or produce less milk, or get sick regularly wind up getting harvested for their meat. But there's no need to kill the calves (at the local dairy scale).
Side note: on the other side of the calves killing scale, some farmers (in Swissland at least) started to kill male calves directly at birth because they found it more profitable at scale. Yummy Toblerone!
Then what happens to the calf? They raise it to the end of it's natural lifespan as a pet? Are male and female calves treated differently?
> Most Dairy cows don't need to have a calf to produce milk.
I can't find anything to corroborate this. Every source I find on the topic says dairy cows produce milk for up to about one year after giving birth, then they need to be re-impregnated to begin producing milk again. Do you have a source for this claim?