Hello! I see this question pop up every single time a pet owner wants to become a dog breeder.
I say "pet owner" because most seasoned breeders, or those that have done their research in preparations to become a breeder, would know the answer to this question.
I will keep the breed prerequisite for breeding for another post and keep this one strictly related to the age and health of the female dog.
Please note that we are ONLY working with German Shepherd Dogs and our experience is relevant to this breed only. I would consult with a vet in regards to miniature or giant breeds, as I would not have much experience with those.
If you look in canine reproduction book, it will say that "healthy breeding age" starts at two years old. Ideally, by this age a dog has physically matured, completed some kind of health testing and is healthy enough to produce a litter of puppies.
Dogs, however, sexually mature way earlier than that!
Females are expected to have their first heat cycle between 9-12 months. Their second cycle is about 6-8 months later. While a female might not be interested in any sexual activity during her first heat, as she is afraid and does not yet understand the hormonal changes happening to her body. During her first heat, which may even be "silent" - with no or minimal discharge, a dog is typically shy, hides and stays near the owner at all times. Things do change with her second heat, when she already enters her late teenager stage and is much more adventurous. The second heat is expected between 15-20 months and in most cases the most dangerous time for unplanned litters. If you are a pet owner that does not plan to breed, please keep a close eye on your intact female during this time! We do not recommend to spay a dog before 24 months of age. Please read for our reasons for doing so here.
So, let's say that you have a suitable stud and plan to breed your female on her next heat cycle at 24 months, but she comes into heat earlier, like at 19 or 20 months. Can you breed your dog at this age?
There is no definitive answer that will suit every situation and every dog. There are a lot of factors that paly a huge role and I will lay them out here.
HD/ED CERTIFICATION. It is essential that a German Shepherd Dog has completed Hip and Elbow screening before producing a litter. HD and ED screening is a series of X-rays taken by your experienced veterinarian and those radiographs are then forwarded to OFA (in USA) or SV (in Germany) for final evaluation. SV accepts radiographs from dogs as early as 12 months of age and most German breeders have their dogs' hips and elbows screened at 12 months for final certification through SV. In United States, dogs getting radiographs for hip certification BEFORE 2 years of age will receive a preliminary certification from OFA and only those dogs older than 2 years of age will receive a final certificate number provided by the OFA. IMPORTANT: While male dogs can be screened at any time, females can not be taken for Hip and Elbow Screening X-rays while pregnant, while in their heat and about 45 days before and after their heat/weaning puppies, as hormones produced during pregnancy and heat affect the elasticity of the hips, and this can cause undesirable results. If you have not completed HD/ED certification for your female dog prior to 24 months, it is better to skip her heat cycle, complete HD/ED and breed on the next heat.
REGISTRATION AGE. When considering breeding a young female (before 24 months old) it is important to look into legal regulations in your country. Some registries simply will not accept a litter produced by under age dogs. Such, in United States, AKC rules do not allow the registration of a litter out of a dam less than 8 months of age at the time of mating. And thank God for that! Under German SV regulation, the bitch must be at least 20 months old before she can be covered for the first time.
MENTAL STATE. While two year is a recommended age to start breeding, we find that most younger females are not ready to be good mothers at a young age! Dog's mental health matures by the time they are three years old and breeding a female that is 18-20 months old is just like having a human child when you are 16 years old yourself - yes, it is physically possible, but mentally a female has not yet matured to be a good mother. A young GSD female might not care for her puppies, escape from them, refuse to nurse, etc. Sometimes breeders are hand-feeding those puppies simply because a dam is not producing enough milk.
SEASON. The time of the year is important because let's be honest: raising puppies in spring and summer is much easier than it is in winter! Breeding dogs while not in their "ideal" age might also be changed simply because the next heat cycle would fall into the cold months.
AVAILABILITY OF STUD. Not all breeders have a stud readily available, so sometimes they are forced to breed now, and not later.
Summarizing the above said, we find it most beneficial for a female to be between TWO AND THREE YEARS OLD for her first litter. We have seen much better long term results with those females that have matured - both physically and mentally before their first litter.
First litter will create a long lasting effect on the female as a mother and if you plan to have a repeated litter(s) with this dog, it is very important that she is started correctly from the beginning.