Among the complexities inherent in the bovine respiratory disease syndrome (BRDS), a genetic component leaves some animals more susceptible, or more resistant, than average. Identifying those animals for selection purposes however, only now is becoming feasible through the use of genomic technology. During the recent BRDS conference in Denver, University of California-Davis animal scientist Alison Van Eenennaam, PhD, outlined progress in the nationwide BRD CAP project, a 5-year USDA-funded Coordinated Agricultural Project to address the problem of BRD in dairy and beef cattle.
The project involves a multi-institutional team led by Dr. James Womack at Texas A&M University, with research groups from Washington State University, University of Missouri, Colorado State University, New Mexico State University, USDA ARS, and the University of California, Davis. The primary objective of the project is to use the tools of modern genomics to identify cattle that are less susceptible to BRD.
Van Eenennaam says genomic evaluations and genomic-enhanced selection are most useful for genetic traits that are difficult to measure, can only be measured late in life or have low heritability. Disease resistance fits that definition, but the ability to select against disease susceptibility could provide significant economic benefits. She notes, for example, that the dairy industry currently applies genetic selection for mastitis resistance in spite of low heritability and some adverse relationships with other economically important traits.
The BRD CAP project has included two large genome wide association studies (GWAS), which were conducted using a case/control design on pre-weaned Holstein dairy heifers in one study and Bos taurus beef feedlot cattle in anther. Researchers used a calf-health scoring system to identify BRD cases and controls, and used genomic technology such as the Illumina BovineHD 770K Bead Chip for DNA analysis. Through these studies, the researchers estimate heritability for BRD susceptibility ranging from 19 to 21 percent for dairy calves up to 29.2 percent for beef cattle when using numerical scores as a semi-quantitative definition of BRD.
Heritability at those levels could allow significant genetic progress through genomic-enhanced selection. Ultimately, researchers hope to develop genomic breeding values for disease susceptibility to incorporate into selection indices. Van Eenennaam says this process likely will be easier in dairy cattle because of the predominance of the Holstein breed, as accuracy of genomic predictions declines when applied across breeds. Over time, more detailed whole-genome evaluations could provide more accurate across-breed predictions.
Asked whether selection for resistance to BRD could have adverse effects such as increasing susceptibility to other diseases, Van Eenennaam said there probably will be some optimum level of resistance, which could be achieved by removing cattle with the most genetic BRD susceptibility from herds, while continuing to include traits for overall health and performance among selection priorities.
Find more information about the BRD CAP project on the project website.