Composite Breeding

Composite, synthetic, and hybrid are used interchangeably to represent cattle with multiple breed genetic makeup, packaging desirable traits of two or more breeds into one breed type. Composite breeding by definition is cattle of two or more breeds designed to retain heterosis (hybrid vigor) without further crossbreeding and managed as a purebred population. Advantages associated with the use of composite breeding include simplicity, retained heterosis (avoid inbreeding), breed complementarity matched to environmental resources and marketing specifications, management of genetic antagonisms, the uniformity from generation to generation, and minimal or no difference in variation in quantitative traits compared to contributing purebred generation. Disadvantages include the potential loss of retained heterosis if inbreeding is not avoided, availability of performance-tested composite bulls (this is changing with developments in computing and molecular technologies) matching environmental resources and marketing specifications, and poor composite development, lacking foresight in industry direction and environmental fit. Composite breeding exploits breed differences, making use of breed complementarity while balancing genetic antagonisms and management/environmental resources. Composite breeding is crossbreeding made simple. For commercial cow-calf producers using single sire breeding pastures, the use of a composite bull adds pounds of calf weaned per cow exposed without adding complicated pasture management. Composite bull selection is like choosing any other bull to breed to the cow herd. The primary challenge is finding the performance-tested composite bull that fits the producer’s natural and marketing environments, meeting specific cow herd needs.

Establishment

Planning composite breeding takes time. The selection of the appropriate breeds is the critical step. Composite breeding takes advantage of breed complementarity, managing genetic antagonisms while taking advantage of retained heterosis. Environmental resources and market specifications require consideration in the development of composite breeding. The development of a composite breed requires a large female foundation (500 or more females) and 25 or more sires per generation to establish an effective population size once the composite stabilizes (three generations of inter se mating, within closed population mating). An effective population size is paramount to avoid the loss of heterosis in future generations due to inbreeding. Composite breeding will fail without careful selection of breed types but also sires and replacement heifers within subsequent inter se matings. Many of the first composite breeds developed were British cattle crossed with Brahman (Bos indicus) breeding to match subtropical production environments. Examples include Beefmaster, Brangus, Braford, Charbray, and Santa Gertrudis. Other composite lines of cattle have focused on British cattle crossed with Continental breeds to address genetic antagonisms for carcass traits such as yield versus marbling. Where feed resources may be limiting, a higher percentage British breed composite may excel, but where feed is not a limiting factor, a higher percentage Continental composite may fit. Examples of composite lines of cattle of British:Continental breeding include SimAngus, MARC II, Stabilizer, and Balancer. Breed associations with performance programs, including EPDs, exist for many composite breeds. The future is bright for composites in the genetic prediction arena with new advances in genome technology.

Retained Heterosis

The main advantage of composite breeding is retained heterosis (hybrid vigor). The more breeds in the composite then the more retained heterosis possible (two-breed composite, 50% versus eight-breed composite, 87.5%; Table 1; Gregory et al., 1993). Assuming equal breed contribution, the percent retained heterosis is a simple function of the number of breeds in the composite [(n-1)/n, where n = # of breeds used]. Thus, a four-breed composite has the potential to retain 75% of the maximum possible heterosis [(4-1)/4 = .75 or 75%]. Assuming equal breed contribution, using a four-breed composite translates to an estimated 17.5% increase in pounds of calf weaned per cow exposed. Retained heterosis can only be realized by avoiding inbreeding. This relates back to the effective population size when the composite was created.