Breeding stock is a group of animals used for the purpose of planned breeding. When individuals are looking to breed animals, they look for certain valuable traits in purebred animals, or may intend to use some type of crossbreeding to produce a new type of stock with different, and presumably superior abilities in a given area of endeavor. For example, when breeding swine the "breeding stock should be sound, fast growing, muscular, lean, and reproductively efficient." The "subjective selection of breeding stock" in horses has led to many horse breeds with particular performance traits.
Mating animals of the same breed for maintaining such breed is referred to as purebred breeding. Opposite to the practice of mating animals of different breeds, purebred breeding aims to establish and maintain stable traits, that animals will pass to the next generation. By "breeding the best to the best," employing a certain degree of inbreeding, considerable culling, and selection for "superior" qualities, one could develop a bloodline or "breed" superior in certain respects to the original base stock.
In the United States, a backyard breeder is someone who breeds animals, often without registration and with a focus on profit. In some cases the animals are inbred narrowly for looks with little regard to health. The term is considered derogatory. If a backyard dog breeder has a significant number of breeding animals, they become associated with puppy mills. Most puppy mills are licensed with the USDA.
Hammond K. Gianola, D (1990), Advances in Statistical Methods for Genetic Improvement of Livestock (Advanced Series in Agricultural Sciences), Springer-Verlag Berlin and Heidelberg GmbH & Co. K, ISBN3-540-50809-0