Though the idea behind composite breeding systems has been around for decades, only recently has the practice attracted interest within the beef industry. The reason for this interest is simple. Composite crossbreeding is a functional, low management alternative to traditional crossbreeding techniques.
Composite crossbreeding is accomplished by developing a new breed composed of other existing breeds. Once constructed, the composite breed can be mated like a conventional (straight) breed. There are two major advantages to composite breeding systems.
First, composite breeds can be developed to take advantage of the relative strengths of existing breeds. All breeds have strengths and weaknesses. It is very unlikely that any single breed is optimal for a specific production environment. However, if we can offset a weakness of one breed with strengths from another, then it becomes possible to create a new composite breed that is targeted to specific natural and managerial environments.
The second advantage of a composite breed is that it maintains hybrid vigor over time. Hybrid vigor is the added advantage realized by crossing breeds or lines. When genes from one breed are paired with genes from another breed, bad (recessive) genes tend to be masked by good (dominant) genes.
This effect (hybrid vigor) results in substantial improvement in production efficiency. Hybrid vigor is maximized when two traditional breeds are crossed. Progeny from this mating is called an F1. In the F1, all gene pairs contain one gene from one parent (breed 1) and one gene from the other parent (breed 2).
In most situations, it is not feasible to maintain an F1 herd; so, we usually lose some of the advantages of hybrid vigor over time. Traditional methods used to maintain hybrid vigor require multiple breeding pastures and sire breeds.
On the other hand, a properly designed composite breed will maintain most of the hybrid vigor present in the F1 even though the composite is mated like a purebred breed. The amount of hybrid vigor lost (relative to the F1) is a function of the probability of genes from the same parental breed pairing at any location on any chromosome.
Conversely, the amount of hybrid vigor maintained is a function of the probability of genes from different breeds pairing. In a composite breed, the probability that genes at any location come from different breeds is determined by the number and proportion of breeds used in the development process.
Four breed composite (25% A, 25% B, 25% C, 25% D)
At any location: The probability of...
2 genes from breed A pairing is .252 = .0625
2 genes from breed B pairing is .252 = .0625
2 genes from breed C pairing is .252 = .0625
2 genes from breed D pairing is .252 = .0625
any genes from same breeds pairing = .25 (25%)
Thus, the probability of genes at any location coming from different breeds is:
1 - .25 or .75 (75%)
So, in a 4-breed composite of this type, we expect to maintain 75% of F1 hybrid vigor indefinitely.
Since only one breeding pasture is needed, composite breeding systems are ideal for small to medium-sized herds. Also, in a composite herd, cows, calves and bulls all benefit from the effect of hybrid vigor. The only serious down side to composite breeding systems is lack of availability of quality composite cattle in many parts of the US. However, this is being quickly remedied as new and established seedstock producers have entered the market. Composite breeding systems have had a large effect on the swine industry. Many project similar impact in the beef industry as well.