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Large differences exist among breeds for most bioeconomic traits. These differences are the result of different selection goals in different breeds. Thus, over time, large genetic differences have accumulated among breeds. Results from the Germplasm Evaluation Program at the U.S. Meat Animal Research Center provide evidence that genetic variation between breeds is of similar magnitude to genetic variation within breeds for many bioeconomic traits. However, the heritability of breed differences approaches 100%, whereas, the heritability of differences within breeds for major bioeconomic traits varies from less than 10% to about 50% depending on the trait. Heritability of breed differences approaches 100% because estimates of breed differences are based on the means of a large number of individuals from a representative sample. This tends to average within breed variation. Estimates of heritability of differences within breeds are generally based on a single observation of individuals for a specific trait. Thus, selection among breeds is much more effective than selection within breeds. Breed differences in bioeconomic traits are an important genetic resource and can be used to achieve and maintain performance levels that are optimum for different production-marketing situations for such traits as: (1) growth rate and size, (2) milk production, (3) carcass composition, (4) age at puberty, and (5) climatic and nutritive adaptability. Large breed differences exist for these traits and breed differences may be used to achieve and maintain optimum additive genetic (breed) composition through the formation of composite breeds or through the use of specific crossbreeding systems.