In livestock production, natural selection is not a cost-effective way to improve disease resistance, especially since most sickness occurs in feeder cattle that aren’t going to reproduce.
While beef producers, veterinarians and researchers have worked diligently to protect animal health, most of the focus has been on management factors such as using vaccines, minimizing stress, optimizing nutrition and reducing exposure to pathogens. All those approaches help, but the potential for adding genetic selection to the animal-health toolbox could bring further improvements. And advancements in genomics, linking DNA markers to specific traits related to disease resistance, can make it possible. This potential was high on the agenda at the Beef Improvement Federation’s 2011 conference held in Bozeman, Mont., in early June.
Colorado State University animal scientist Mark Enns, PhD, describes three general classes of animal diseases — genetic defects, non-transmissible conditions and diseases related to vectors such as bacteria or viruses. The industry, he notes, has had considerable success in developing selection tools to fight genetic defects and some success in using selection to reduce non-transmissible conditions such as high-altitude disease. For vector-related diseases, however, we’ve seen little progress in development of genetic prediction tools related to the complex interactions contributing to resistance or susceptibility.
Enns notes, though, that it is common to find cattle that resist heavy pathogen exposure. Genetic differences account for some of this apparent resistance, but measurements of these differences are compounded by variable exposure levels and environmental factors.
With a goal of identifying selection tools for reducing disease incidence, CSU developed a two-year research project in conjunction with Pfizer Animal Genetics and JBS Five Rivers Cattle Feeding.
The researchers fed 1,551 steers in year one and 1,319 in year two of the study. Feedlot crews identified and treated animals showing clinical signs of bovine respiratory disease, and those animals were classed as positive for BRD. The researchers also recorded treatments for other feedlot diseases such as pinkeye and bloat.
Sires of calves in the study were identified with DNA markers, and that parentage information was subsequently used to estimate heritability. Enns notes that BRD treatment rates were 45 percent and 7.1 percent in year one and year two, respectively, illustrating how variability in disease incidence complicates genetic research.
Even with these differences, the researchers estimate the probability that animals were treated for BRD was 17 percent heritable, and the probability an individual was treated for any health-related problem as 24 percent heritable. These estimates are relatively low, but Enns says they are in the same range as heritability of heifer pregnancy and milk production in some beef cattle breeds. Enns notes that collection of treatment data on sire-identified feedlot cattle on a large scale would be impractical, so the researchers are evaluating potential indicator traits and determining if DNA marker tests could be developed to predict susceptibility to BRD.
Genetics and vaccine response
Researchers at South Dakota State University are taking a different approach in exploring the genetics of animal health. SDSU animal scientist Michael Gonda, PhD, notes that while modern vaccines offer clear benefits in reducing cattle disease, sometimes even vaccinated cattle get sick. Reasons for variable vaccine response can include differences in exposure levels, nutrition, other environmental factors and also genetics.
Gonda and his team conducted research based on the hypothesis that vaccine response in cattle is a heritable trait and researchers could identify DNA markers associated with vaccine response. These DNA markers could then be used to develop a DNA test for vaccine response. His study focused on response to a bovine viral diarrhea virus vaccine.
In one of the experiments, researchers sampled 267 Angus and Angus-influenced calves from three herds in different South Dakota locations. The calves were vaccinated for BVDV at 1 to 8 months of age. The researchers collected blood samples from the calves at the time of vaccination and 21 to 28 days post-vaccination. The team found that the sire of the calf was significantly associated with vaccine response in this study, suggesting BVDV vaccine response is heritable in cattle. Because of the small number of calves in the study, the researchers did not estimate a heritability level for BVDV vaccine response.
It should be possible, Gonda says, to identify DNA markers associated with BVDV vaccine response which can be used for genetic selection. He notes, however, that the process of developing and validating such a test, and incorporating it into a practical selection tool, will require considerable time and further research.
The value of BRD resistance
Before seedstock breeders invest in evaluating cattle for disease resistance, and producers pay extra for resistant cattle, a key question to ask is whether the trait has economic value.
Speaking at the BIF conference, University of California–Davis animal scientist Alison Van Eenennaam, PhD, said BRD causes annual losses of more than 1 million animals and $692 million. And that’s just the death loss. BRD also causes enormous economic impact on cattle performance and carcass quality. In spite of advancements in management practices and vaccines, the industry has made little progress in reducing BRD losses.
But as advancements in genomics offer the possibility to evaluate cattle for disease resistance, Van Eenennaam and researchers at UC–Davis developed a model to evaluate whether the high financial impact of BRD outweighs low heritability in the trait’s potential payoff for ranchers or cattle feeders.
The team’s economic model is based on an integrated marketing system in which the cow-calf producer retains ownership through the feedyard, assuming a 10 percent incidence of BRD during finishing and accounting for differences in performance, death loss, carcass quality and feed costs.
In their analysis, the researchers compared the relative economic importance of BRD resistance with that of other traits, using USDA yield grade as the baseline trait with a value of 1. The model estimates the relative value of an EPD for BRD resistance at 37.7, well above that of traits such as weaning weight (5.7), feed intake (4.6), feedlot ADG (3.7) and marbling score (2.6). The huge economic losses associated with BRD account for the difference, in spite of the trait’s relatively low heritability.
Van Eenennaam acknowledges challenges in delivering that potential value across the beef-production chain. Most of the value of BRD resistance would manifest at the finishing phase, where the most losses occur. But before selection pressure can develop to improve resistance, economic signals need to filter back to seedstock producers who would invest in the DNA testing, and to cow-calf producers who would pay premiums for bulls with high ratings. Ideally, she notes, cattle would undergo DNA testing once, early in age, for disease resistance and other traits. Favorable ratings would accompany an animal through the marketing chain, adding value at each production stage.
Value integration of this type will require time and acceptance of the technology. Van Eenennaam compares it with preconditioning documentation programs, which over time evolved from a perceived cost to an investment in the future value of calves.