Heterosis, or hybrid vigor, is the performance advantage crossbreds wield over the average of their straightbred parents. Maximum heterosis is realized in the first cross of distinctly different parents.
Heterosis and complementarity are powerful forces that combine to produce the total advantage of beef cattle crossbreeding. It's an advantage that can amount to as much as 25% greater lifetime productivity — pounds of calf weaned per cow exposed — for crossbred cows as compared to straightbred cows, says Jim Gosey, recently retired University of Nebraska (UN) Extension beef specialist.
Traditional crossbreeding systems — rotations and rotation-terminals — are very efficient in maximizing heterosis. The problem is, they're more complex than many producers would like.
“Composite seedstock can be used in a commercial crossbreeding program with greater management ease than traditional crossbreeding systems,” Gosey says.
In fact, the management challenge inherent in some traditional crossbreeding systems is what drives some commercial cow herds to become straightbred herds. It's an attempt to achieve management simplicity along with greater uniformity in their cattle in pursuit of a premium, non-commodity product.
Gosey cautions, however, that composites must be carefully formed. The same attention to breed choices and sire selection must be applied in building composites as in straightbreeding.
Formation of the composite
While hybrids and composites are both crossbreds, hybrids are generally considered to be F1, or first crosses, of purebred parents. Composites are the result of matings among crossbred parents.
“The composite seedstock breeder must take special care to plan the formation of the composite to avoid inbreeding,” Gosey warns. Thus, a “closed composite” requires a large herd size, estimated at 25 sires/generation to hold inbreeding to less than 0.5%/generation.
|Crossbreeding system||Minimum % |
|Maximum % |
|Percent F1 |
|Rotate purebred A & B bulls||33||67||67|
|Rotate purebred A, B & C bulls||14||57||86|
|Rotate F1 A×B & F1 C×D bulls||17||33||83|
|Composite A×B×C×D bulls||25||25||75|
|Composite A×B bulls||50||50||50|
|Composite A × (B×C) bulls||50||50||63|
|Rotate F1 A×B & F1 A×C bulls||50||50||67|
|Table Sources: Proceedings from the Range Beef Cow Symposium XIX|
A composite seedstock breeder using an “open composite” approach has a much lower requirement for herd size, since he continually evaluates and introduces new sires (and perhaps breeds), probably via artificial insemination (AI). This holds inbreeding to a minimum.
“Existing breeds of cattle are mildly inbred lines,” Gosey explains. “Heterosis is the recovery of accumulated inbreeding depression. Thus, managing inbreeding in composite breed formation is critical to success.”
Composite breed types don't sustain as high a level of heterosis as do traditional rotation crossbreeding systems (Table 1), but do allow for more complementarity between breeds. Several examples are shown in Table 1 that level the contribution of a given breed (Breed A in this example) or several breeds.
Table 2 demonstrates the impact of the number of breeds and the impact of equal contribution of each breed to the foundation generation. The number of breeds used in the foundation of a composite accounts for most of the heterosis retained. However, the heterosis level falls as the contribution of each breed to the foundation is less than equal.
“Heterosis retained is proportional to the heterozygosity retained,” Gosey explains. “So a four-breed composite would produce 75% heterosis, and that level would be maintained over time.”
The initial loss of heterosis is due to the loss of heterozygosity that occurs between the Fl and F2 generations, but is maintained in subsequent generations of crosses in a composite.
Some breeders have assumed that variation in composite populations is greater than that found in purebred populations. But Gosey says a definitive study of three composite lines and their parental purebreds, conducted by USDA's Meat Animal Research Center (MARC) in Clay Center, NE, found no significant difference in the coefficient of variation for reproduction, production or carcass traits measured (Table 3).
Another criticism is composites lack the EPD accuracy found in many purebred breed evaluations. It illustrates the need for an expanded multiple-breed evaluation between the most widely-used breeds in commercial production.
That need was recently addressed when the Brangus, Limousin, Salers and Red Angus associations created a new company, Performance Registry Services (see “Better Odds,” November 2005 BEEF). The program's goal is to provide commercial producers with National Cattle Evaluations by delivering a single suite of EPDs for all the participating breeds on a single base. The joint venture will make Total Herd Reporting of all cattle feasible for participating breed members, regardless of breed combination and data processing, while allowing the individual breeds to maintain their singular identities.
|Number of |
|% Heterosis||% Crossbred |
|Retail product %||.04||.06|
|Item||British||Continental||Marc I||Marc II||Marc III|
|% YG 1&2||38||89||83||56||53|
|% CH +||70||30||43||55||66|
|% Non-conformance to 70 - 70 - 0 target|
“Along with multi-breed EPDs, there will be decision-support software, search engines and a centralized data warehouse for commercial producers to use these tools,” Gosey adds. “It will make it easier for producers to evaluate breed inputs into crossbreeding programs.”
He says the commercial user of composite breed types has to worry about few of the constraints the composite-seedstock breeder encounters. That's because the cattle can be managed as straightbreds in a one-pasture system.
“Composite breeds offer the opportunity to use genetic differences among breeds to achieve and maintain the performance level for a variety of economically important traits that are optimum for a wide range of production environments and market scenarios,” Gosey says. “These traits include climatic adaptability, growth rate and mature size, carcass composition, milk production and fertility. Further, composite breeds may provide herds of any size an opportunity to use heterosis and breed differences simultaneously.”
The beef industry's oft-stated goal is to produce finished cattle at least 70% USDA Choice or better, 70% Yield Grade 1 & 2, and with zero defects and “out” cattle. Composites offer an opportunity to counter the antagonism between USDA Quality Grade and YG (see Table 4).
“This 70-70-0 target is difficult to achieve with British or Continental breeds alone,” Gosey says. “But a blend of these two types, as found in the MARC II (Ω Continental: ½ British), does a much more acceptable job of meeting the 70-70-0 target. A composite can lower the risk of non-compliance to a market target.”
Careful selection of foundation sires used in developing a composite can further move a herd toward meeting market targets. Table 5 shows six lots of steers born at the Gudmundsen Sandhills Laboratory near Whitman, NE, sired by bulls produced in the UN teaching herd.
Unlike the MARC Germ Plasm Utilization project, where bulls were sampled across a broad spectrum of each breed, the foundation sires in the teaching herd were selected using EPDs to be:
- Above average in calving ease,
- Average in milk production,
- Average or below in mature size, and
- Above average in marbling and other carcass traits.
The result, Gosey says, is steers that average 87% USDA Choice or better, and 66% YG 1 & 2. Several of the individual lots of cattle easily surpassed the 70-70-0 market target.
What concerns Gosey about the shifting genetics in much of the nation's commercial cow herds is the loss of most of the heterosis that once existed. That loss of heterosis shows up in the same lowly heritable traits that would be associated with inbreeding depression, namely reproductive, fitness and longevity traits, he says.
“Thus, the price paid for loss of heterosis occurs as a number of very small losses that can amount to a substantial sacrifice in lifetime productivity — 25%,” he adds.
Perhaps the availability of estrus synchronization protocols for timed AI will assist some commercial producers in using some of the traditional crossbreeding programs in the future, he says.
Existing one-pasture crossbreeding programs can deliver adequate — though not maximum — heterosis. They're also simple to manage, utilize complementarity, and can be designed to produce uniform calf crops while helping avoid important genetic antagonisms.
“These programs offer commercial producers a practical tool to enhance management effectiveness and increase profitability,” Gosey concludes.