There are many different sheep breeds. Breed selection is based upon personal preference, regional marketing opportunities, and sheep production goals. The most important traits for commercial production in areas with adequate rainfall for pasture growth throughout the growing season are ability to lamb year-round and prolificacy. Color, fineness, and staple length are important to many farmers with specialized wool markets. Some producers utilize milking breeds for specialty cheese and yogurt production.

Growth rate, feed efficiency, and carcass traits at a given market weight are most closely related to degree of maturity (weight in relation to mature weight). Sheep with large potential mature size gain more muscle and less fat to a given weight than sheep with small potential mature size. Therefore, larger mature size sheep are more efficient, gain faster, and produce leaner carcasses. A disadvantage of large mature size sheep is the higher cost of maintaining a breeding flock. Thus, for selecting within breed for growth rate, feed efficiency, or carcass leanness should only be done while keeping mature size from increasing. This is a slow and daunting task.

Sheep producers can take advantage of breed differences in mature size by using a terminal sire program. Moderate-size, aseasonal-breeding, prolific ewes are bred to rams of large mature size breeds to produce market, or terminal, lambs.

Most of the sheep breeds of the world are documented at the Oklahoma State Sheep Breeds Site.