Crossbreeding is probably the most misunderstood and underappreciated practice in commercial livestock production. Crossbreeding is the mating of males and females of different breeds or breed types. Purebreeding is the mating of individuals of the same breed or type. Crossbreeding is the recommended breeding strategy for commercial meat sheep and meat goat production.
As a breeding practice, crossbreeding does not denote the indiscriminate mixing of breeds. Rather, it is the systematic use of breed resources to produce offspring of a specific type. For example, a “terminal” crossbreeding program uses a male of superior growth and carcass merit (e.g. Boer) to produce lambs or kids for the slaughter market, while maintaining moderate sized females that excel in fitness and reproductive performance.
All offspring from a terminal cross are sent to the slaughter market (i.e. “terminated”). Another crossbreeding scheme could be aimed at producing crossbred females of a specific type for breeding.
Crossbreeding offers two distinct advantages over purebreeding: heterosis and breed complementarity. Heterosis or “hybrid vigor” is the superiority of crossbred offspring to their purebred parents.
Mathematically, heterosis is the percentage increase in a specific trait (e.g. weaning weight) that progeny have over the average performance of their parents. Heterosis is highest for traits that do not respond well to selection, e.g. fitness and reproductive traits, and lowest for traits that respond well to selection, e.g. carcass and fleece characteristics.
Heterosis occurs in both the crossbred offspring and the crossbred female. Crossbred lambs and kids grow faster and have higher survivability than their purebred counterparts. Crossbred ewes are more fertile and wean 15 percent more pounds of lamb than purebred ewes. Less is known about heterosis in crossbred males, but it is believed that crossbred males are more fertile and aggressive breeders.
Breed complementarity is the other major advantage of crossbreeding. It relates to the fact that there are no perfect breeds, and that each breed possesses certain strengths and weaknesses. In a systematic crossbreeding program, breed resources are combined to balance the positive and negative aspects of each breed in the cross.
Mating Polypay ewes to Suffolk rams is such an example. This cross takes advantage of the reproductive efficiency and moderate maintenance costs of Polypay ewes while producing Suffolk-sired lambs to meet market requirements for fast-growing, heavy muscled lambs.
Crossing a Boer buck onto a Spanish or dairy doe would be another example of breed complementarity. Boer goats are known their outstanding body conformation and carcass quality, while Spanish does are hardier than Boers and dairy does produce more milk.
For crossbreeding to be most effective, it is important to identify the strengths and weaknesses of different breeds and to determine the appropriate role of a breed in a crossbreeding program. For example, you can produce Suffolk x Polypay lambs by either crossing a Suffolk ram onto Polypay ewes or by crossing a Polypay ram onto Suffolk ewes.
If the goal is to produce superior crossbred market lambs, it makes no sense to do the later cross because the breeds are not being utilized to take best advantage of their strengths and weaknesses. On the other hand, Polypay rams could be used to produce crossbred female progeny that would be superior in maternal characteristics to the purebred Suffolk female.
Crossbreeding can also be used to upgrade a flock or herd or to create new breeds. Almost all goat and sheep breeds started out as crossbreds. For example, the Suffolk was originally a “cross” between the Southdown and Norfolk Horn (developed in England). The Kiko is a “cross” between feral (wild) goats and dairy bucks in New Zealand. To upgrade, you breed purebred/fullblood males to crossbred or non-registered females and save each generation of females for breeding to a purebred male. In some breed registries, upgraded individuals can eventually be registered as purebreds.
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