Why Crossbreds May Be Superior to Purebreds: Breeding A Better Goat

Unfortunately, while a good pool of information on the average performance of Spanish goats and similar breeds exists around the world, much less information on other meat breeds as purebreds in commercial systems in the southern United States exists. There are two major reasons for this. First, since the Boer, Kiko and Tennessee have only recently arrived in the South, their numbers in our herds are small. Second, because they are new (and expensive), owners have managed them in a special manner and provided them with special health care and feed. Therefore, their capabilities under commercial conditions are just coming to light.

However, despite these drawbacks, there are some characteristics each breed brings to a mix. When the first cross is made, both desirable and undesirable characteristics are pooled. Because of the type of gene action taking place, some less desirable characteristics often are unexpressed in the early crosses. Yet the genes responsible for all traits are still included in the offspring produced: the direct breed effect.

The second thing that happens during crossbreeding is a genetic phenomenon called heterosis. Heterosis, once called "hybid vigor," occurs when breeds or inbred lines cross. The greater the genetic distance between breeds, the greater the level of heterosis. Heterosis level is not the same for all traits. Those traits low in heritability, like fitness and reproduction, show more advantage from heterosis.

Heterosis is a genetic phenomenon which causes the average offspring to perform better than the average of its two parents. For example, if purebred Spanish kids gain an average of three–tenths of a pound per day and purebred Boer kids a half–pound per day, a producer should expect halfbreds to gain an average of four–tenths of a pound per day. However, if the halfbreds gain 4.5 tenths of a pound per day, heterosis may be the cause. The real advantage is if halfbreds grow faster than the best breed. If the halfbred kids gain six–tenths of a pound per day, heterosis is responsible for the two–tenths per day bonus. Like inbreeding depression, heterosis occurs when dominance gene effects operate. It is absent when traits are influenced only by additive gene action. The traits related to fitness and reproduction, which are primarily influenced by additive gene action, are impacted most strongly by heterosis.

In a program where the producer is upgrading to Boer, for example, the 3/4–bred kids from a backcross to the Boer won’t be as genetically diverse as the halfbreds. Their performance may be lower in some traits because some of the heterosis in the halfbreds has been lost. As the upgrading process continues, the level of heterosis lessens. In most farm animals, the traits that benefit most from the presence of heterosis are those relatively low in heritability, such reproduction and mothering ability, hardiness and disease resistance and growth and development.

Since early crosses are more heterozygous than purebred or high–grade animals, they are – despite excellent individual performances – much less predictable in their breeding value than purebreds. The point is that as the diversity of the crossbreds lessens, the amount of heterosis also lessens. This means that the "boost" evident in the first crosses also lessens and the characteristics of the contributing breed or breeds take over. So unless one wants to be a purebred breeder of Boers, Kikos or Tennessee Meat Goats by upgrading from a base of common or Spanish goats, it’s better commercially to use a well–organized continuous crossing system. Future articles will explain some of those systems and the advantages of each.

Copyright © Georgia Goat Research & Extension Center
College of Agriculture, Home Economics, and Allied Programs
Fort Valley State University