Breeding is the careful selection and pairing of plants, animals, and other organisms to improve the usefulness of their offspring. Although people have bred plants and animals for thousands of years, breeding became a science only after the early 1800’s. This change occurred as scientists accumulated knowledge about how organisms inherit characteristics. See Heredity ; Genetics .
There are two types of breeding: selection and crossing. Selection consists of identifying the most useful individuals of one kind of organism and letting only the best become parents of the next generation. This process is called artificial selection because people select which organisms can reproduce instead of nature. Crossing involves pairing selected individuals to produce the best offspring possible. Crossing is called hybridization if the individuals are from different species or varieties or are themselves hybrids—that is, offspring of different varieties or species. Many breeders use computers to help model their breeding programs.
Reasons for breeding.
Breeding is normally done for a particular purpose, such as to produce plants and animals that grow faster or are better adapted to the climate in which they live. Each of the different kinds of wheat grown in Texas, Kansas, North Dakota, Canada, and India has been especially bred to grow in its particular climate. Breeding can also increase the size, strength, or resistance to disease of an organism. Breeders crossed imported grapevines with native American vines to produce grapes that resist certain diseases.
Plant breeding.
The goal of plant breeding is to develop more or higher-quality food or natural fiber. All of the plants that people grow for food have resulted from the breeding of wild plants by selection. Almost every plant that has ever been raised has been improved or refined by breeding.
The greatest advances in plant breeding have resulted from combining selection and hybridization. Hybrid corn is a prominent example of a high-yielding crop produced by repeated hybridization of selected parents. The use of selection alone has generally been more successful in wheat, barley, and rice production.
Animal breeding.
Cattle, horses, sheep, hogs, and dogs are bred by selection to produce characteristics desired by the breeder. Breeders seek horses and dogs that are speedy or that have a particular hair color or physical shape. Most cattle, sheep, and hogs are bred to supply the largest amount of high-quality products at the lowest cost to the farmer.
Producing changes through breeding takes longer in animals than in plants. Changes take so long because animals are older than plants when they reproduce, and they have fewer offspring. Therefore, breeders usually try to improve only one quality of the animal at a time. For example, it is extremely difficult to breed a cow that is an excellent producer of both milk and beef. Thus, most breeders concentrate only on milk or only on beef.
The record of an animal’s ancestors is called its pedigree. Breeders and farmers often pay high prices for animals with the best pedigrees, especially if the animal has the qualities that they seek. Breeders hope that animals with good pedigrees will be good parents.
Many breeders use a process called artificial insemination to improve the quality of their stock. The breeder collects semen (sperm-containing fluid) from an outstanding male and places it in the reproductive organs of a female. The semen can also be frozen and stored for future use, or transported to another location for use there. Artificial insemination is used widely in breeding cattle and poultry, and to a limited extent in other animals. It permits the best males to have many more offspring than would be possible by natural mating.
Breeders have also developed a method to increase the number of young from a superior female. This process, called embryo transfer, involves removing a fertilized egg from a high-quality female and transferring it to a healthy, but less-valuable, female. Usually, the superior female is given a fertility drug so that she will produce a larger number of eggs than normal. Less-valuable females carry the transplanted embryos through prenatal (prebirth) development and give birth to the young. Embryo transfer enables a female to have several offspring each year, but it is more complicated and expensive than artificial insemination.
