ASC-224: Crossbreeding Considerations in Sheep

Heterosis Heterosis, or hybrid vigor, refers to the increased vitality or “doing ability” of the crossbreds as compared to the average of the parental breeds. The only way to get heterosis is through crossbreeding. Heterosis is expressed by the crossbred lamb (individual) and by the crossbred ewe (maternal). Crossbred lambs, on the average, are more vigorous and have a higher survivability than purebred lambs. They may also have a faster preweaning growth rate. The F1 (first cross) crossbred ewe exceeds the straightbred ewe in many traits, especially those related to reproductive fitness and longevity. Do all traits respond to crossbreeding? Traits that are lowly heritable (reproductive traits, traits influencing resistance to stress, survivability and longevity) are the ones that respond the most to crossbreeding (that is, have the greatest amounts of heterosis). These are traits that are difficult or slow to improve through selection. In general, as heritability increases, the amount of heterosis decreases. Carcass traits, for example, are highly heritable. Selection results in fairly quick genetic improvement for carcass traits, but crossbreeding has little to no effect. Crossbreeding is often used to improve performance of low heritability traits. Average heterosis effects for various traits are shown in Table 1. The effect of heterosis may seem minor when only one trait is considered. However, when total productivity (for example, lamb survivability and growth rate) is considered, heterotic effects accumulate to provide a rather substantial improvement over straightbred sheep. Also, mating F1 ewes to a ram of a third breed to produce crossbred offspring can maximize heterosis. Table 1. Potential heterosis effects in sheep