Wednesday, August 30, 2017

Using Genomic Tests to Detect Genetic Abnormalities in Beef Cattle

Christian P. Lewis
South Dakota State University undergraduate student

Rapid advances in science and technology are appearing throughout agriculture. One of the newest technologies that has worked its way into the cattle industry is DNA testing and the use of genomic data.

Practical Uses of Genomic Data

Genetic abnormalities are not a common problem in beef cattle production, but they do appear if precautions are not taken. Most frequently, a genetic abnormality appeared because both the cow and bull were carriers of a recessive allele that causes the abnormality. An animal is termed a “carrier” when they have a dominant allele that is masking the recessive allele. Figure 1 illustrates how an abnormality can appear by mating two carriers. Genetic abnormalities will appear when a calf has two copies of the recessive allele that it got from its sire and dam.

A
a
A
AA
Aa
50% chance the calf will be a carrier
a
Aa
aa
25% chance the calf will have the abnormality
Figure 1: Mating two carriers (Aa) of a recessive allele (a) that is completely masked by the dominant allele (A)

Without a DNA test for an abnormality, the only way you will know an animal is a carrier or not is when you mate the suspected carrier to a known carrier and offspring with the abnormality are born. If you want to test for a genetic abnormality, the first thing that you should do is contact your breed association to see how they want DNA collected for a test and where to send DNA samples.

DNA Collecting Basics

There are three common ways to obtain DNA samples from cattle: blood samples on FTA cards, tissue samples, and hair samples. If testing young calves, a blood sample is often preferred. There are several videos online that demonstrate how to collect DNA samples if you are new to DNA testing.

Collecting blood samples


Collecting tissue samples


Collecting hair samples




Managing Known Defects

After you receive the results, there are three possible ways to keep the tested abnormality from appearing again:

  1. Cull the carriers.
  2. Make sure not to mate two known carriers.
  3. Utilize crossbreeding.

Culling the known carriers and not using carrier bulls will eliminate the abnormality from appearing in your herd again. By always using non-carrier bulls, none of your calves will ever present the abnormality.  If the genetics from the known carriers are too valuable to cull, you must plan your mating decisions so that two carriers are not allowed to mate. If you use a carrier bull, all calves sired by this bull should either be sold after weaning or tested for carrier status before they are bred. Crossbreeding may be the easiest way to avoid genetic abnormalities. It’s very rare for one abnormality to segregate within two breeds, but it’s not unheard of. For example, Tibial Hemimelia (TH) segregates in both Shorthorn and Maine Anjou, as some of the same sires were used in both breeds. So, mating Shorthorn with Maine Anjou presents a risk of the TH abnormality appearing in calves.  Further, if both the sire and dam share a breed (e.g., both sire and dam are Angus-influenced), mating these individuals could still result in the appearance of a genetic abnormality.

DNA Test Available

Producers can often order tests for genetic abnormalities along with other DNA tests. These DNA tests are priced based on how many tests you want them to perform. Most DNA tests for genetic abnormalities cost approximately $25 per head, but the cost per head may be lower if you test for more than one genetic abnormality or purchase another DNA testing product (e.g., Igenity Profile, PredicGEN).  If you wish to learn more about genetic abnormalities and available DNA tests for these abnormalities, you can visit websites maintained by Zoetis Animal Genetics https://www.zoetisus.com/animal-genetics/beef/index.aspx or Neogen Corporation http://genomics.neogen.com/en/genetic-health-and-conditions.  Both of these companies provide DNA testing services for the beef industry.  A more comprehensive list of DNA test providers can be found in the article titled “Managing Genetic Defects” authored by Dr. Alison Van Eenennaam (see references list). 

Conclusion

In today’s production environment, it is often easier to take precautionary steps than fix something after it becomes a problem. Genetic abnormalities may not appear in your calf crop this year, but they could down the road. If your cowherd is at risk (e.g. you know you have used carrier bulls in the past), testing your cowherd for abnormalities will at the very least give you peace of mind. It might even save a calf from developing an abnormality, which in turn, will lead to more pounds to sell at weaning time.

References
Van Eenennaam, A. 2015. Managing Genetic Defects. http://articles.extension.org/pages/72662/managing-genetic-defects

As part of our USDA-NIFA local adaptation grant, Michael G. Gonda at South Dakota State University has developed a course titled "Applied Beef Cattle Breeding." Christian wrote this article while participating in that course.

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