Selecting For Disease Resistance: A Consumer-Friendly GMO

If we can select for cattle that are resistant to disease or deselect cattle that are prone to disease, we can improve profitability and reduce use of antibiotics, a win for the farmer and the consumer.

Use of genomics has transformed agriculture as much as human medicine. It has certainly caused heartburn for farmers in the direct sales business as consumers have concerns about GMO produced foods. But what I’d like to focus on in this article is a form of the genetically modified organism. This article is based on determining what genes are associated with disease and selecting for or against this set of genes. If we can select for cattle that are resistant to disease or deselect cattle that are prone to disease, we can improve profitability and reduce use of antibiotics, a win for the farmer and the consumer.

I’ll start by saying I am not a geneticist. Even if I were, there is not enough space in this entire magazine to explain how genetic selection works. But I will give you the “genomics for dummies” version in a paragraph or less.

First, note that the terms genetics and genomics are used interchangeably. That is not entirely correct. Genetics is the study of a single gene, whereas genomics is the study of all genes and their interaction with the environment. For example there is one gene that affects genetic defects, while there are groups of genes that affect the expected progeny differences (EPDs) for performance, carcass and survivability traits.

It would be convenient if there were one gene that results in easy calving, high marbling, high fertility or disease resistance. But that is not the case; it is a group of genes, and furthermore the trait is also impacted by the environment. This is why you need to understand that when a company tells you their test accounts for 15 percent of the variation in a given trait, they are referring to genetic variation only. These traits also have an environmental component.

For example, you can have a positive test for marbling, but if you don’t feed the animals correctly, then the cattle won’t produce the expected marbling, regardless of genetics. The final point to remember is that genetic tests should not be used as a stand-alone evaluation. EPDs are still the gold standard in selection. Genetic testing should be used to enhance EPDs, not replace them.

The greatest promise and use of genomics is for traits that are difficult to measure (disease resistance) are lowly heritable (fertility) or can only be measured late in life (stayability) or after the animal is dead (tenderness). I recently attended a session on selection for resistance to bovine respiratory disease (BRD) at the National Cattlemen’s Beef Association Cattlemen’s College in Nashville. The presenter was Dr. Alison Van Eenennaam, a faculty member at University of California-Davis. The following is what I learned.

Even with a greater understanding of animal nutrition, health and stress management and greatly improved vaccines, BRD is still the No. 1 cause of death in finishing cattle. In fact, 1.4 percent of all feedlot cattle perish before reaching harvest. This has not changed in the last 30 years. One reason for this is that a bovine has 30 percent of the lung capacity of a similar-sized horse, but requires 250 percent more oxygen. I remember being at a processing facility where they were harvesting bison. The owner showed me the trachea of the bison, which must have been three to four times the size of one from a beef breed. Domesticated cattle just don’t have the lung capacity and associated organs to support anything but a completely low-stress environment.

Our production system that involves commingling cattle from small farms that typify the U.S. beef industry creates a disease challenge. It is estimated that the economic cost of BRD is more than $250/head in the finishing phase only. We also know that BRD affects calves before they get to the finishing phase, which increases the financial losses from the whole production system. Discovering the gene complex that controls response to BRD would be a huge benefit to producers and consumers alike.

The good news is researchers have determined the heritability of BRD susceptibility is considered moderate at 18 to 29 percent. However, this is the heritability of the genome, that is, the group of genes that are involved in BRD. The bad news is that BRD is similar to the common cold in that it is a complex of viral and bacterial organisms. Current research is focused on identifying individual genes that regulate specific pathogens involved in the BRD complex. This work has been completed with beef and dairy calves and is being analyzed. If specific genes can be identified in the genome, heritability estimates should increase. Additionally, if they can find the specific gene that results in susceptibility, this test would be useful across breeds, which is currently not the case with most genetic tests.

The final step is to incorporate the results into an index that producers can use to select against sires that produce progeny susceptible to BRD. An index is built assigning a dollar value to the trait relative to other traits. Van Eenennaam and a colleague have estimated that the relative economic importance of selection to decrease incidence of BRD should be weighted six times more heavily than selection for growth traits (e.g., weaning weight) and 16 times more heavily than carcass traits (e.g., marbling). They have suggested a BRD index with a 1 percent increase in BRD death loss that would be associated with the loss of $2.08/head in the Angus index $Feedlot ($F). This is equivalent to a 1-pound increase in hot carcass weight EPD. Though close, researchers still estimate that we are about five years from having a reliable disease susceptibility index.

In our segregated beef industry the question becomes “Who pays for the genetic testing?” In a perfect world, the feedlot would pass value for receiving healthy cattle back to the calf supplier, who would then pass some of that value to the bull supplier. For the astute cow/calf producer, they will develop a marketing channel that rewards them for buying bulls with more information that can be used to extract value from the feedlot. While it’s exciting to see where this technology can lead, everyone along the production and market channel through the consumer has to receive value, or itwill not be useful.