The Genetic Principles of Crossbreeding

ferent breed makeups. It is widely used in commercial beef production because of the benefits it has to offer cow-calf producers. Improvements in efficiency can be dramatic if appropriate breed combinations are used. Crossbreeding does not eliminate the need for outstanding purebred cattle since efficient systems require well characterized, superior purebred cattle. Crossbreeding in beef cattle did not receive widespread approval until the last few decades; however, a large percentage of commercial cattle today are crossbred. Crossbreeding is one type of a larger class of mating systems called outbreeding. Outbreeding has the opposite effect of inbreeding and is defined as the mating of relatively unrelated individuals. Other forms of outbreeding include: 1) linecrossing, which is the mating of members of different families within a breed, 2) grading-up, which is mating the sires of a given breed to females of a particular breed and their female progeny each generation, in order to propagate the breed of interest, and 3) hybridization, which is the mating of individuals of different species. Crossbreeding is beneficial for two primary reasons. First, a well-designed crossbreeding system allows the producer to combine the desirable characteristics of the breeds involved in the cross while masking some of the disadvantages of the breeds. This is frequently referred to as breed complementarity. The second benefit arises from heterosis, which often is referred to as hybrid vigor. In addition to these primary benefits, crossbreeding also enables a producer to change a herd rapidly with the introduction of new breeds. Heterosis Defined Heterosis is a measure of the superior performance of the crossbred relative to the average of the purebreds involved in the cross. The probable cause of most heterosis is due to combining genes from different breeds, concealing the effects of inferior genes. Heterosis may result in the crossbred being better than either parental breed or simply better than the average of the two. For example, an Angus x Hereford crossbred calf may generally grow faster than either Angus or Hereford purebreds. However, a Charolais x Angus crossbred calf may not grow as fast as a purebred Charolais or have the ability to marble similar to purebred Angus but will likely be better than the average of the purebreds for both traits. This difference is usually expressed as a percentage of the average performance of the purebreds. The general formula for calculating percent heterosis is given below: