This article is from the rec.pets.dogs: Breeding Your Dog, posted to rec.pets.dogs newsgroup. Maintained by Cindy Tittle Moore with numerous contributions by others.
Ideally, a bitch should only be bred every other year and she should not be bred much before two years of age. The season closest to the second birthday is a good one to start with; certainly no earlier than this. In some breeds, you may need to wait one more season before beginning. By this time, she is better prepared mentally for having puppies than she would have been with her first few seasons. Her physical growth is complete and pregnancy at this point won't endanger her health, provided that she is healthy to begin with.
In breeds with Hip Dysplasia, many people wait until after two years of age so that the parents can be certified; however if you have sent in xrays to OFA for preliminary evaluation and they came back as fine, many breeders consider it safe enough to then breed on the season closest to the second year, which can wind up being before the bitch is actually old enough to be certified. (And when the bitch is old enough, she is, of course, duly certified.) But the preliminary xrays _must_ be examined by OFA, not by a local veterinarian. There are many dysplastic dogs out there that had vets look at their xrays and pronounce them "wonderful."
It's important, however, to keep the frequency of breeding low. Even at maximum, you want to allow at least one unbred season between breedings. This allows your bitch to rest and regain her strength. A bitch that whelps too often will produce weaker puppies more likely to die, and the repeated pregnancies are pretty rough on her, too.
For dogs, they should definitely have all their certifications necessary. For many breeds this means that they should be over two years old. Since a dog can be bred at any time, unlike bitches, waiting for two years is not a problem, whereas a bitch often has a season just before two years of age and then has to wait until 2.5 or three which sometimes presents problems in trying to time her litters. But this does not apply to a stud dog, so he should definitely have all of his checks and certifications before being bred. Frequency is not generally a problem although some dogs have problems with sperm production if they breed once a day for several days. They need top-quality feeding and care if they are going to be bred often.