What Is Systematic Crossbreeding?

To remain competitive with alternative meat products, particularly pork and poultry, the beef industry must reduce cost of production and fat while maintaining tenderness and palatability of its products. Producers have two powerful breeding tools - systematic crossbreeding programs and composite populations - to assist in this mission.Both tools offer the benefits of heterosis, breed differences

To remain competitive with alternative meat products, particularly pork and poultry, the beef industry must reduce cost of production and fat while maintaining tenderness and palatability of its products. Producers have two powerful breeding tools - systematic crossbreeding programs and composite populations - to assist in this mission.

Both tools offer the benefits of heterosis, breed differences and complementarity to help producers match genetic potential with market preferences, the climatic environment and available feed resources.

Heterosis Heterosis can have substantial effect on profitability. Defined as the difference between the average of reciprocal F1 crosses (A x B and B x A) and the average of the two parental breeds (A and B) mated to produce the reciprocal crosses, heterosis was found in one study to increase weaning weight per cow exposed 23%.

The increase came from the favorable effects heterosis has on survival and growth of crossbred calves, and also on reproduction rate and weaning weight of calves from crossbred cows (Figure 1). More than half the advantage depended on the use of crossbred cows.

The performance of each cross usually exceeds that of either parent breed, especially for comprehensive traits like lifetime production and herd life. For example, lifetime production and longevity of Hereford x Angus cows (3,258 lbs. of calf weaned over a herd life of 11 years) and Angus x Hereford cows (3,514 lbs. weaned over 10.6 years) was significantly greater than that of either straightbred Angus (2,837 lbs. weaned over 9.4 years) or Herefords (2,405 lbs. weaned over 8.4 years) in the Fort Robinson heterosis experiment.

Bos indicus x Bos taurus crosses (i.e., Brahman x Hereford) yield even higher levels of heterosis, averaging double the pounds of calf weaned as those reported for corresponding traits among straightbred Bos taurus breeds.

Over a number of generations, about 68% of F1 heterosis is maintained in two-breed rotations, 86% in three-breed rotations, 50% in two-breed composite populations and 75% in four-breed composite populations.

Breed Differences For most traits, the breeding value range of differences between breeds is comparable to the breeding value range of individuals within breeds (Figures 2 and 3). No single breed excels in all important beef production traits. When carcass and meat traits are considered, breeds that excel in retail product percentage produce carcasses with marbling levels below optimum and carcass weights above optimum. Moderately sized breeds with higher genetic potential for marbling produce carcasses frequently discounted for unacceptably high numbers of Yield Grade 4 carcasses.

Our research shows that 50:50 Continental and British crosses perform well. In fact, if discounts for yield grade differences are similar to those for USDA quality grade, in temperate environments, cattle that are half-Continental and half-British have a much better chance of hitting profitable targets for retail product percentage, marbling and carcass weight.

Matching Genetics To Environment To optimize reproductive rate in the cow herd, genetic potential for environmental stress, mature size and milk production should be matched with both actual environment and economical, available feed resources. Because preferred feed resources vary by area, breeds chosen for the cowherd should be well adapted to feed resources within a given area. Remember, reproduction potential of cows with large size and high milk declines if environment and feed can't meet the higher requirements for maintenance and lactation.

Producers in the subtropical regions of the U.S. favor Bos indicus x Bos taurus crosses. In one study, weaning weight per cow exposed was significantly greater for the Bos indicus x Bos taurus F1 crosses (Brahman x Hereford, Brahman x Angus, Sahiwal x Hereford, Sahiwal x Angus) than for the Bos taurus x Bos taurus F1 crosses (Hereford x Angus, Angus x Hereford, Pinzgauer x Hereford, Pinzgauer x Angus) in both Florida and Nebraska.

The advantage was especially large in Florida (Figure 4). In the hot, humid Gulf Coast, 50:50 ratios of Bos indicus to Bos taurus inheritance may be optimal. A little further north (i.e., Southeast Oklahoma, central Arkansas, Tennessee and parts of North Carolina), 25:75 ratios of Bos indicus:Bos taurus inheritance may better suit needs.

Complementarity Complementarity is defined as crossing breeds to combine direct and maternal breed and heterosis effects to optimize performance levels. Complementarity also helps match genetic potential for growth rate, mature size, reproduction and maternal ability, and carcass and meat characteristics with the climatic environment, feed resources and market preferences. Crossing specialized male breeds with crossbred females maximizes the impact of desired characteristics and minimizes the impact of undesired characteristics of each breed.

Alternative Crossbreeding Systems Alternative crossbreeding systems use genetic differences among breeds, heterosis and complementarity, with differing degrees of effectiveness (Figure 5). No single system is suited for all herds. In choosing a system, it's important to consider herd size, labor, facilities and breeds that match genetic potential to the market target, climate, feed and other production resources.

Static-terminal sire crossing systems. In a static terminal sire crossing system (Figure 5), straightbred females of breed A are mated to straightbred males of breed A to produce straightbred replacement females. Straightbred females of breed A are also mated to bulls of breed B to produce F1 crossbred females (BA).

All crossbred BA females are mated to breed C, a terminal sire breed. Static crossing systems work well in species with high reproductive rates (poultry, swine) but less well in species with lower reproductive rates (cattle).

A high percentage of straightbreds are needed to produce straightbred and F1 replacement females, sacrificing the benefits of individual and maternal heterosis.

Rotational crossing systems. In a two-breed rotation, females sired by breed A are always mated to males of breed B. Females sired by breed B are always mated to breed A (Figure 5). In a three-breed rotation, a third breed is added to the sequence.

In rotational crossbreeding systems, heterosis is retained at high levels. On the other hand, intergenerational variation can be quite large in rotational crossing systems, especially if breeds that differ greatly are used.

In a three-breed rotation, 57% of the cows' genes are of the breed of their sire, 29% are of the breed of their maternal grandsire and 14% are of the breed of their maternal great-grandsire (which is the same as the breed to which the females are to be mated).

Because of this variation, rotational systems using comparable breeds work best. In cow herds, producers need to keep an eye on breed compatibility for traits such as birth weight to minimize calving difficulty, size and milk production to stabilize feed requirements. In market animals, breed compatibility for production traits is most important.

Using F1 bulls or composite bulls in rotational crossing systems can significantly reduce intergenerational variance, especially if breeds chosen to produce F1 bulls optimize performance levels in their crosses (i.e., 50:50 Continental/British inheritance, or 50:50 Bos indicus/ Bos taurus inheritance). For the commercial producer, there's little difference between use of F1 bull rotational crossing systems and use of bulls from composite populations.

Seedstock producers have only recently begun to produce F1 bulls in significant numbers for use in commercial production.

Composite populations. Composite populations developed by mating like animals resulting from two or more breed crosses provide an alternative to more complex crossbreeding systems. Management requirements in these composite herds are similar to straightbred herds (see Figure 5), yet substantial heterosis can be maintained in composite populations, so long as adequate numbers of sires are used in each generation to avoid re-inbreeding. Heterosis increases as number of foundation breeds increases.

Producers can take better advantage of genetic differences among breeds in composite populations than with alternative crossbreeding systems by keeping breed percentages at optimum levels. For example, if the optimum level of Bos indicus germplasm is 25% for a specific environment, the contribution of Bos indicus can be maintained at 25% in a composite population. Intergenerational variation is not a problem in composite populations, after the initial population formation.

Cattle breeders already have developed a significant number of composite populations in diverse geographic regions around the U.S.

Terminal crossing. In terminal crossing systems, crossbred females excelling in maternal performance are mated to sires of a different breed that excels in growth traits, ensuring excellence in carcass and meat characteristics in the resulting progeny. All progeny, both male and female, are produced for slaughter. Regardless of whether females are produced in a static crossing system, rotational crossing systems or composite populations, breeders can take advantage of complementarity among breeds (Figure 5) by terminal crossing.

General Considerations * Rotational systems generally make more effective use of heterosis.

* Genetic potential for USDA quality and yield grades can be optimized more precisely in cattle with 50:50 ratios of Continental to British inheritance than in cattle with higher or lower ratios of Continental to British inheritance.

* Composite populations maintain significant levels of heterosis, but less than rotational crossing of any specific number of contributing breeds.

In general, EPDs available for bulls from purebreds used in rotational systems tend to be more accurate than EPDs for bulls used in a composite population because they're based on a larger number of records.