Beef cattle breeders have heard for years that DNA testing is coming and that it will change the way they breed cattle. At long last, the time is here when DNA testing for economic traits is available, albeit in a very immature form. Breeders must decide whether to use the technology, and if so, how to use it. DNA testing has a number of potential applications in cattle breeding, including parentage testing, tests for genetic diseases or defects, and tests for qualitatively inherited traits such as color or horns. However, most economically important production and end-product traits are influenced by several or many genes and are known as “quantitative traits.”
Several DNA tests for quantitative traits have become commercially available recently and the number of such tests is expected to increase rapidly over the next few years. Considerable infor-mation about a DNA test is required in order to decide whether to use it. The National Beef Cattle Evaluation Consortium is developing a process for the independent validation of DNA tests to help cattle breeders decide which DNA tests will be most effective for them. Several tests have already been through the process.
DNA testing can make evaluations available anytime after birth, which is important for traits that can only be measured late in life or postmortem. Although some continued collection of phenotypes will always be required, DNA testing should allow greater information to be extracted from each phenotype that is measured. This is especially important for traits that are expensive to measure or sex-limited.
The availability of DNA testing will bring, along with all of the advantages, misuse of information, especially in the early years when only a few DNA tests are available. We have heard much discussion of the evils of “single-trait selection.” Breeders must now face the temptation of “single-gene selection,” which may have far greater consequences.
For example, a bull with one of the top (high accuracy) EPDs in his breed for a trait had the least desirable, but most common, genotype (test result) for a DNA test for one of the genes affecting the trait. Semen sales on this bull dropped off sharply following the release of the test result. Apparently, breeders decided that they could not use bulls with the less favorable allele (form) of this gene, a prime example of “single-gene selection.”
This is understandable, but it is not good use of DNA test information because the DNA test provides information about only one of the genes influencing the trait, whereas the EPD provides an estimate of his total genetic merit at all genes that influence the trait. DNA test results should not greatly influence our estimate of the overall genetic merit of individuals with high-accuracy EPDs. However, DNA testing can contribute substantial information about individuals that would otherwise have low-accuracy genetic evaluations, and this is where it is most useful. Education on the effective use of DNA testing is becoming a priority.
In the short run, DNA testing should not be expected to simplify cattle breeding. Selection decisions will be based on more pieces and types of information and breeders will have to decide which tests to run and which animals to test. It is a real challenge to integrate DNA test results with EPDs to make the most effective selection decisions.
In the longer run, the goal is to integrate DNA test results into the existing national cattle-evaluation process so that selection can be based on the resulting DNA-adjusted EPDs, which will weight the information from each DNA test result, the phenotypes and the pedigree appropriately, to provide the best estimate of genetic merit from the information available. The National Beef Cattle Evaluation Consortium and the Beef Improvement Federation are developing the basic framework for this process. Successful implementation will require the joint cooperation of DNA testing companies, breeders and breed associations.
There are challenges in using DNA testing effectively in beef cattle. Nonetheless, cattle breeders are making strides in implementing DNA testing and are making changes in traits, such as tenderness, that have been difficult to select for in the past. Undoubtedly, the way in which DNA testing is used by the beef industry will change over time, but the early adopters of the technology are likely to be in a better position to capitalize on that change.
R. Mark Thallman is a research geneticist, U.S. Meat Animal Research Center ARS-USDA