Rotational crossbreeding systems facilitate capture of a sizeable fraction of the approximately 26% increase in weaning weight per cow exposed resulting from hetero-sis. This increase in productivity may be realized with only about a 1% increase in energy consumed by cow-calf pairs. A two-breed rotation system is shown in Fig. 2. All females sired by bulls of breed A are bred to bulls of breed B, and vice versa. This system can be effectively approximated by using bulls of breed A for two or three years, switching to bulls of breed B for two or three years, then back to bulls of breed A, and so on. The rotation systems can also be expanded to include a third or fourth breed, if desired. Breeds used in rotation systems should combine both desirable maternal qualities and desirable growth and carcass characteristics.
Use of a terminal sire breed may increase the amount of retail product produced per cow in the breeding herd by 8%. However, using a terminal sire breed adds an additional level of complexity to rotational crossbreeding systems. A terminal sire system is shown in Fig. 3. The base cow herd is produced as a two-breed rotation. All females less than four years of age (about 50% of the cow herd) are bred in the two-breed rotation, as described above. Breeding young cows to bulls of compatible size should keep calving difficulty at a manageable level. Replacement females all come from this phase of the system. Older cows, with their greater potential for milk production and reduced likelihood of calving difficulty, are bred to a terminal sire breed of bull. All calves sired by the terminal sire breed are sold for ultimate harvest. Terminal sire systems also give commercial producers an opportunity to change sires rapidly, so calves can be quickly changed in response to market demands.
Breeds are used in more specialized roles in a terminal sire system. Therefore, greater attention should be given to maternal qualities in choosing breeds for the rotation part of the system. In choosing the terminal sire breed, more attention should be given to growth rate and carcass composition.
Using composite breeds whose ancestry traces back to several straightbreds is another viable crossbreeding system. Using composites in place of a straightbred provides an opportunity to take some advantage of heterosis, even in very small herds. For very large herds, composites can simplify management relative to rotational crossbreeding systems. Use of composites also facilitates fixing the breed composition, thus holding the influence of each breed constant. Net effects on income can be illustrated comparing generic straightbred, rotation, multi-breed composite, and terminal sire systems (Fig. 1). Heterosis effects are particularly important for cow-calf producers who market their produce at weaning. Use of specialized sire and dam lines appears to be more advantageous when ownership is retained through harvest.
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