I've never let cows wean their own calves, or known anyone that did that... so I'm curious. How old is a calf when a cow weans it naturally? How big? Are there calves that refuse to wean?
Edit: To put an age on your question...8ish to 11 months of age they're all weaned I feel like.
I'm sure there are Mommas that won't wean their calf. If one was going this route, she should make the cull/beef list. The calf is going to want milk, it should be the mommas job to wean it.
So far for me, they've all been weaned by Momma a month or two before calving, or earlier.
Had four last year that I sold Momma and kept their calves, they were 500 ish lbs at that time. The ones that stayed on Momma longer were a bit larger than their siblings and they all were born close together timewise.
I make my animals survive on grass and mineral though and probably take an approach that most do not. I don't like unnecessary inputs as I have plenty of things to spend my money on already. I don't like unnecessary management as I have plenty of things to do already.
Was really an act of laziness I suppose the first time. But it worked, and has worked since. So for me, in my operation, we have stopped weaning for now. If a Momma has trouble with that, I will cull her.
So far I have observed first calf heifers do seem to wean their calves considerably earlier than a mature cow. Something I was glad to see. Not saying that's always going to be the case. I want to think these mommas have an idea of what they're doing. I might treat 1st timers differently though if the need presents itself. Makes me wonder if there is a mechanism in their brain that tells their body to dry up if their condition is too low or if they are getting close to calving.
Again, not a shoe that fits all. But I think it's a doable concept if your operation doesn't really need to wean. We don't sell at salebarns and don't have a bull. We steer bull calves also before they're active.
Plenty of reasons to wean though.