More than any other single animal health factor, bovine respiratory disease (BRD) affects profitability in feedlot cattle. “In the U.S., 1.4% of all feedyard cattle perish before reaching harvest weight and, of those, the majority are due to BRD,” says Alison Van Eenennaam, University of California-Davis Extension beef geneticist. “And the trend is increasing.”
Given the high cost to the industry, and the industry’s embrace of DNA marker technology, the obvious question is: will a DNA test be developed to identify cattle that may be genetically resistant to pathogen-fueled diseases?
The answer is yes, says Mark Enns, Colorado State University (CSU) genetics researcher. When that will happen, however, is a lot more difficult to predict.
Ongoing research at CSU and at South Dakota State University (SDSU) indicates disease resistance in cattle in a heritable trait. In a two-year trial in a commercial feedyard setting involving around 3,000 calves, Enns estimates about a 17% heritability when looking at whether or not a calf was treated for BRD. While that may seem low, Enns points out that it’s a higher heritability estimate than heifer pregnancy and milk production, traits for which most breeds have EPDs.
When Enns looked at whether or not an animal was pulled from its pen and treated for any reason, the heritability estimate goes to 24%. “That tells me there is some genetic difference in that animal’s ability to cope with the environmental, pathogen-associated challenges associated with a feedlot,” he says.
But what’s the value of that? To answer that question, Van Eenennaam and Mike MacNeil, animal genetics researcher at the ARS Fort Keogh Livestock and Range Research Laboratory in Miles City, MT, collaborated to develop a model index to estimate the relative value of incorporating selection pressure for BRD resistance.
The model made a lot of assumptions, including the production system it would evaluate – a terminal sire program with all the calves retained through the feedyard. Traits included in the index, in addition to BRD resistance, were weaning weight, average daily gain (ADG), feed intake, yield grade and marbling score.
The relative economic values estimated in the model suggest that to maximize profitability, BRD incidence should be very heavily emphasized in terminal sire selection, followed by a relatively uniform emphasis on weaning weight, post-weaning ADG and feed intake, and less emphasis should be placed on marbling score and yield grade, Van Eenennaam says.
In a commercial feedyard environment, however, a number of factors gang up to confound scientific inquiry. That makes it difficult to tie that data back to the research and development needed to identify gene markers associated with an animal’s ability to respond to a vaccine, in the case of the SDSU research, or fight off a pathogen challenge, in the case of the CSU work.
In short, while researchers are convinced that disease resistance is a heritable trait, they’re a long way from developing a DNA marker test that cattlemen can use to select for the trait.
A DNA marker test will eventually be developed, researchers predict. “The number of DNA markers found to be associated with economically important traits should increase substantially throughout this decade,” says Michael Gonda, SDSU animal geneticist. “For animal health traits, the limiting factor will now be collection of a sufficient number of phenotypes on animal health that can be used for DNA testing.”
In the meantime, Enns says the effort is concentrated on finding indicators that are more easily measured. An example is ultrasound data collected on breeding cattle to predict the ability of their calves to marble. It’s likely those indicators for animal health genomics can be found, he says, but it will take a level of cross-discipline cooperation.
“We’ve got a lot of carcass progeny tests out there,” he says. “Can we use that phenotypic information that’s flowing back?” In a carcass trial, the animals are being followed all the way to harvest, so health and treatment information in routinely collected. Perhaps, as researchers sort through that data with a new focus, answers will arise.