In the world of newborn puppies, nursing is the name of the game. Up until a puppy is eating exclusively solid foods, his mother will continue to occasionally nurse him.
Nursing starts the day a litter of puppies emerges, often practically instantly after the exit from the womb. For brand new puppies, a mother's breast milk is golden, providing every single nutritional demand, including antibodies, minerals and proteins. No other nourishment is necessary at all, full stop. Until the mother canine decides that the litter is ready to begin weaning, suckling is the way to go.
When a nursing mother dog is in the picture, it's up to her to choose exactly when weaning begins. According to the ASPCA, this process usually starts when a litter is somewhere between 3 to 5 weeks in age. During this time, puppies continue suckling on mommy, but also eat some solid meals, as well. If all goes smoothly, puppies should be weaned completely off of suckling when they reach roughly 7 to 8 weeks in age, although it always depends. Some puppies may cease suckling at six weeks, while it may take a little over eight weeks for others.
If newborn puppies are without their mother, they may either suckle a foster doggie mother who is lactating, or you may feed them using a commercial milk replacer and formula created for puppies. If this is the case, a newborn puppy may never ever "suckle" in the traditional sense of the term. However, with proper puppy formula, bottle-feeding is 100 percent safe and nutritious for newborn pooches, so never fear.
If you're caring for motherless puppies by yourself, it's up to you to decide when to begin the transition to eating "real" foods. The ASPCA encourages starting the procedure around the same time a nursing mama doggie would -- think anywhere between three and four weeks. Aim to complete weaning at around the seven or eight week mark, as well. When puppies are about 2 months old, they should be eating wet and dry puppy food full time.