Making the most of your crossbreeding system

Fewer open cows. Lower death loss. More growth, more milk and more efficiency. In these challenging economic times, the benefits of crossbreeding are more important than ever. While a straight-bred commercial herd should expect increased profitability from implementing any crossbreeding system, matching the right breeds can maximize the benefits of heterosis.

Heterosis is defined as the increased performance of a crossbred animal, compared to the average performance of the pure breeds that make up the cross. Heterosis occurs because the desired level of performance increased fertility, for example is partially the result of dominant genes. Beef producers understand that black coat color is dominant to red; that is, when an animal has one black gene and one red gene, the animal is black in color. Similarly, an animal with one gene for more fertility and one for less might be just as fertile as another animal that is homozygous for high fertility. In theory, there are dominant genes that increase not only fertility but survivability, growth and milk production. Purebred cattle tend to be more homozygous for some favorable genes and some unfavorable ones. While one breed might be homozygous favorable for one fertility gene but homozygous unfavorable for another, a second breed could be just the opposite. When purebred cattle are crossed with other breeds, the crossbred progeny are more heterozygous and benefit from dominance at many gene locations, increasing fertility, growth and other performance traits.

But not all crosses generate equal heterosis. Greater effects of heterosis occur when the breeds crossed are more different in their genetic makeup. Brahman-cross cattle excel for some traits due to the additional effects of heterosis that occur because Brahman breeds are quite different genetically from British and Continental breeds. The amount of benefit from crossbreeding depends partially on the genetic diversity of the breeds being crossed.

In the past, countries of origin have been used to estimate the degree of similarity between breeds. For example, Angus, Hereford and Shorthorn were all developed in the British Isles, while Simmental, Limousin and Charolais originated from continental Europe. From that information, one might conclude, incorrectly, that all British breeds are more similar to each other than they are to Continental breeds, and that British-Continental crosses would yield greater heterosis than British-British or Continental-Continental crosses.

However, with the emergence of tools such as DNA markers, it is now possible to more accurately assess the degree of genetic similarity between breeds. In the last decade, studies have measured genetic diversity of major beef breeds in this manner, using DNA-marker genotype frequencies to measure how similar breeds are genetically and, thus, how much heterosis is gained by crossing those breeds. The studies focused on British and Continental beef breeds, as well as some dairy breeds. Unfortunately, Brahmans and other Bos Indicus breeds were not part of these studies, although their genetic diversity from British and Continental breeds is well-established in other research.

While the non-Brahman beef breeds are typically classified as either British or Continental, the authors of these studies found that each of these categories could be subdivided. Continental breeds were divided into Alpine and French breeds. Alpine would include breeds of primarily German or Swiss origin, such as Simmental, Gelbvieh and Braunvieh, while Charolais, Limousin and Salers were grouped together as French breeds. The researchers found that within those groups the breeds tended to be more similar genetically, but there were greater differences from one group to the other. Similarly, they found that British breeds could be segregated into two groups. Angus, Red Angus and Shorthorn cattle tended to be more similar to each other and were classified as British group A, but Hereford and Highland cattle were more different and were separately classified as British group B. A fifth group, of Spanish origin, would include Texas Longhorns and other native cattle such as the Corriente and Argentine Criollo breeds.

From their results, the authors concluded that heterosis would be less when breeds within the same group, such as Angus and Red Angus or Charolais and Limousin, were crossed. Not surprisingly, the amount of diversity between Angus and Red Angus was the least of any pair of breeds in the studies. However, the genetic diversity between the two British groups was as great or greater than between either of the British groups compared to a Continental group (Alpine or French). This implies that the heterosis expressed in a black baldy (Angus-Hereford cross) would be as great or greater than either an Angus or Hereford crossed with a Continental breed. As long as producers cross breeds from two or more breed groups (Angus/Shorthorn, Hereford, Alpine, French, Spanish and Brahman), the genetic diversity between breeds should be more than adequate to allow high levels of heterosis to occur.

Comments