
I think it depends on the bitch and the litter.
If there are a lot of pups and it is draining mum's reserves start now. But if there is only a few fat puppies and the bitch has lots of milk I wouldn't even try until the 4 week mark and then not be too stressed if they are not interested. In my last litter, the pups were huge and didn't really get going on other food until 5 weeks. My bitch was in great shape so I wasn't concerned.