Crossbreeding in Beef Cattle: Evaluation of Systems1

The basic objective of beef cattle crossbreeding systems is to optimize simultaneously the use of both nonadditive (heterosis) and additive (breed differences) effects of genes. Experimental results evaluating rotational crossbreeding systems indicate that high levels of heterosis are sustained in successive generations and that the relationship between loss of heterosis and loss of heterozygosity approaches linearity. Major differences among breeds have been demonstrated for most characters that contribute to production efficiency. Results based on both experimentation and computer simulation indicate that differences in additive genetic merit of breeds for specific characters can be used in specific crossbreeding systems to synchronize genetic resources with other production resources and to provide for complementarity through terminal sire breeds. Rotational crossbreeding systems have the advantage of using heterosis in all females and progeny in a self-contained commercial herd; however, fluctuation between generations in additive genetic composition rewjkes use of breeds that are generally compatible. This requirement restricts the use that can be made of breed differences to synchronize germ plasm resources with other production resources and eliminates the use of complementarity other than in a combined breed-rotation, terminal-sire system. A static terminal-sire crossbreeding system provides opportunity to synchronize germ plasm resources with other production resources in about 50% of the cow herd, to use maximum (F1) heterosis in about 67% of the calves marketed and to use complementarity in more than 50% of the calves marketed. A breed-rotation system involving young cows to meet replacement requirements combined with a terminal-sire system on mature cows can use individual and maternal heterosis from rotation crossing plus complementarity and individual heterosis from terminal crossing. Maximum (F1) individual heterosis and complementarity can be used in the terminal sire component of the herd, which contributes about 67% of the calves marketed. Composite breeds have potential as an alternative or a supplement to continuous crossbreeding systems. The high percentage of the national beef breeding herd represented by units that are too small to use effective crossbreeding systems on a self-contained basis and the wide fluctuation between generations in additive genetic composition in breed rotation crossbreeding systems are the primary reasons to determine the feasibility of composite breed development. Retention of initial heterozygosity after crossing and subsequent random mating within the crosses (inter se) is proportional to 1 , where Pi is the fraction of each of n breeds in the pedigree of a composite breed. Thus, retention of heterozygosity favors the inclusion of an optimum number of breeds, balancing the increased heterosis retention against possible loss of average additive merit from the inclusion of additional breeds. There is a need to determine linearity of association of loss of heterosis with loss of heterozygosity in composite populations. There is also a need to determine the additive genetic variation, particularly for fitness-related characters, in composite populations relative to parental breeds that contribute to them. Furthermore, there is a need to characterize breeds (Bos taurus and Bos indicus)in a range of production environments (climatic and nutritive) to provide a basis for effective selection between breeds to achieve the most optimum additive genetic composition in composite breeds for general adaptability to the production situation consistent with the role intended for the composite breed. An important consideration in the development of composite breeds is to maintain population size large enough that initial advantage of increased heterozygosity is not dissipated by early re-inbreeding of composite populations.