Crossbreeding is the final tool in the breeding toolbox. The term crossbreeding in its strictest sense refers to mating two purebred animals of different breeds together. This strict use of the word is typically preferred among pet breeders, but in the livestock realm crossbreeding is sometimes used to cover a wide array of breeding systems involving planned crosses between breeds. This includes breeding rotations involving three or more breeds, as well as the use of composite breeds, which are produced through mixing two or more breeds and then maintaining the resulting offspring as a pure breed. An example of a composite breed would be the purebred Braford. (We will use crossbreeding in its broader sense throughout this post.)
Another confusing term related to the practice of crossbreeding is hybrid. Different breeders use this noun very differently. Pearl White Leghorns are sometimes called hybrids even though they are produced by crossing bloodlines of purebred Leghorns—a concept we have already introduced as linecrossing. At the other extreme, in equines a hybrid would be the offspring of a mating between two different species, such as a mule.
Perhaps the best way to explain the term hybrid—at least for the sake of this discussion—would be to define it as any animal that displays hybrid vigor, which would include both the purebred Pearl White Leghorn and the crossbred mule. Thus a hybrid animal could be the product of either linecrossing or crossbreeding.
How It Works
In short, crossbreeding works by pairing unlike genes together to produce the phenomenon of hybrid vigor. Many health, reproductive, and performance traits are inherited in a very complicated manner and appear to be enhanced by hybrid vigor. Some of these traits include:
- Disease resistance.
- Egg production.
- Milk yield.
- Rapid growth.
- Frame size.
However, hybrid vigor does not guarantee that all crossbred animals will display all of these traits all of the time (more on that below).
The mechanism by which crossbreeding pairs unlike genes together was already discussed in the post on linecrossing. However, because in crossbreeding different breeds instead of different bloodlines of the same breed are involved, the effects are more dramatic. The offspring of a crossbreeding system will end up with more pairs of unlike genes than the offspring of a linecrossing system.
The extent to which variability will be introduced through crossbreeding will depend on how similar the two breeds are to one another and how consistent the genetics are within each breed individually. Hybrid vigor is typically maximized when:
- Each parent breed is strongly inbred (and therefore very consistent).
- The two breeds in question are very different from one another in many traits.
These two conditions ensure that the maximum number of unlike genes are paired in the offspring.
Crossbreeding is almost always practiced as a means of capitalizing on hybrid vigor. However, different crossbreeding systems are used to achieve different goals.
In terminal crossing, the crossbred offspring are not expected to reproduce. Two breeds are carefully chosen for their compatibility, each one being expected to bring to the table a specific set of positive traits to offset the negative traits of the other, resulting in offspring that are superior in some way to either parent breed. Those offspring will then be expected to perform in a given manner, such as laying eggs, producing milk, fattening up for slaughter, pulling a plow, or even (in the case of crossbred herding dogs) moving cattle. When the animals have passed their useful life expectancy, they will either be retired or butchered—not bred. This is because hybrid vigor is maximized in the first generation of a cross (more on that below).
Rotational crossbreeding systems involve breeding crossbred offspring back to one of the parent breeds. The offspring of this new mating are then bred to the other parent breed, and so on. Hybrid vigor is significantly reduced but not eliminated after the first generation in this system, since at least a portion of the genes will recombine and form like pairs again, as we will see in a moment. To raise hybrid vigor levels in subsequent generations, some producers introduce new breeds every so often.
In composite breeding, several breeds with desired traits are systematically crossed and recrossed until a stable population achieved. This population is then managed as a pure breed, each animal being mated only to other members of the same composite breed. Obviously, this system is a compromise between consistency and hybrid vigor.
Sometimes crossbreeding is used to improve a pure breed in a practice called upgrading. In this system, the offspring of the initial cross are mated back to animals of the breed being improved. A series of such matings are made until the population reaches a desired percentage of purity. Upgrading is used for three primary purposes:
- Eliminating inbreeding depression in a rare breed, as was done in Ankole Watusi cattle.
- Introducing a new trait into an established breed; for example, many beef cattle breeds from Continental Europe were upgraded with Angus cattle to introduce the black coat color.
- Changing breeds without selling an entire herd or flock and starting over from scratch.
Some crossbreeding systems are extremely complicated and require significant capital and a large land base to maintain, since several breeds have to be kept. Simple crossbreeding systems usually take one of two forms:
- Terminal crossing.
- Breeding programs that involve a sacrifice of hybrid vigor, such as keeping a composite breed or breeding crossbred animals back to each parent breed in alternation.
The major difficulty with crossbreeding is that hybrid vigor is maximized in the first generation and can only decrease thereafter, since the genes will recombine and often form like pairs again. Take the illustration of the black bull with a genetic formula of BB and the red cow with a formula of bb mentioned in the post on linecrossing. If these two animals are bred to each other, all of their offspring produced over their lifespans will be black, but will have the genetic formula Bb. Note what happens when this new generation is bred:
- Bb bred to BB produces 50% BB and 50% Bb offspring.
- Bb bred to bb produces 50% Bb and 50% bb offspring.
- Bb bred to Bb produces 25% BB, 50% Bb, and 25% bb offspring.
No matter what we breed the hybrid Bb cattle to, we will end up with some like genes paired together once again, and therefore a loss of hybrid vigor.
Furthermore, because there are several ways that each gene pair can combine, mating crossbred animals will inevitably produce inconsistent results. Some of the offspring will be superior animals suitable for carrying on the breeding program, many will be merely average, and some will be very far from achieving the breeder’s goals. If the breeder has ample time and resources, as well as a clear plan, he can gradually shape this population into a stable and useful composite breed. For producers who are looking for the quick fix or the surefire solution, terminal crossing with a proven combination of breeds is probably a better choice.
Another problem is the mysterious aura that surrounds the concept of hybrid vigor. Crossbreeding is not a silver bullet. Crossing animals with poor health and temperaments does not magically produce animals with good health and temperaments, unless the genetic strengths of one parent happen to overlap with the genetic weaknesses of the other parent. In rare cases, the genetic weaknesses of one parent can actually mask the genetic strengths of the other parent. This is why there are proven combinations in animal breeding, crosses that are made over and over again because of their consistent results.
However, compromises must often be made when crossbreeding for hybrid vigor. The boost that hybrid vigor can give to a desired performance trait may come with a price. One of the hybrids preferred for commercial production of brown eggs is a cross between a Plymouth Rock hen and a Rhode Island or New Hampshire rooster. This combination, known by a variety of names including Red Star and Golden Comet, is a spectacular layer thanks to hybrid vigor. It lays tremendous eggs, however, and over time this will affect its health in a number of ways, from depleting its calcium reserves to damaging its internal organs. Likewise, commercial Cornish cross broilers put on the weight fast, growing those big, tender chicken breasts you see at the grocery store. Hybrid vigor causes this incredible growth—at the price of high mortality due to a number of leg and heart problems.
In some cases, a poorly chosen or accidental crossbreeding can result in the phenomenon of outbreeding depression. Outbreeding depression occurs when two populations, each suited for a particular environment or purpose, are crossed, resulting in offspring that are suited for neither environment or purpose. In other words, the crossbred offspring are inferior to both parent breeds, at least for any practical use. An example of outbreeding depression would be a cross between a classic llama and a woolly llama. A classic llama is combed out to collect a tough, durable fiber, although it is more valuable for its working ability. A woolly llama is sheared to obtain a fleece that can be spun like fine sheep’s wool. A crossbred llama has a coat with characteristics of both and the benefits of neither—its fleece contains too much coarse fiber to make a garment that anyone will enjoy wearing, but is too dense and woolly for good temperature regulation when at work. Outbreeding depression is one of the reasons pure breeds arose in the first place and that very specific crosses are favored in situations when a little more hybrid vigor is required.
A Final Thought
As we have seen, all four of the tools in the breeding toolbox have advantages. They all have pitfalls, too. There is no such thing as a silver bullet in any area of life, including animal breeding. Each breeder must decide for himself what his goals are and must make sure that those goals are realistic and useful. Then he must chart his own path, using the tool or ideally a combination of tools that will bring him to those goals step by step.