Some mums naturally start to hold off once they reach this age because this is the age where you can start to wean the pups. I know mine used to start deliberately holding off once the pups reached a certain age because of this but I have also known dogs that don't and puppies from those litters can be a pain to wean to the point where you have to keep mum away from them some of the time in order for them to get hungry.

Start with something soft but not pure liquid which can be harder for pups this age to lap. Porridge, chicken soup with the larger chunks of chicken taken out, weetabix etc....are all popular things that breeders use with weaning pups first off.