We can breed them black or red, horned or polled. We can breed them for growth, marbling, tenderness or milk production. But can we breed cattle to resist common diseases, respond to vaccination protocols and stay healthy?
The answer is on the way. Researchers, cattlemen and industry partners from across the U.S. and New Zealand met recently in Kansas City at the National Beef Cattle Evaluation Consortium (NBCEC) Cattle Health Symposium to create a national system for genetic selection for disease resistance in beef cattle.
Why it's important
“Everyone agrees that improving cattle health is desirable,” says Oklahoma State University's Toni Oltenacu. “But we also need to consider how health problems impact animal productivity, zoonotic risks, international trade and consumers' perception of the industry and their subsequent attitude toward animal products.”
And all those reasons point to economics and profitability.
One disease in particular in the research crosshairs is the bovine respiratory disease (BRD) complex.
According to the National Animal Health Monitoring System's (NAHMS) 1999 study, the BRD complex is the major cause of death in pre-weaned calves in the U.S., and accounts for half of all feedlot deaths. In fact, 96% of feedlots report treating BRD, with 14.4% of all feedlot calves treated.
Gary Snowder, U.S. Meat Animal Research Center (USMARC) researcher in Clay Center, NE, has analyzed 20 years of USMARC data involving 43,739 calves — 10.5% of calves were diagnosed with BRD.
Data from Iowa State University's Tri-County Feedlot study on BRD from 2003 to 2006 showed an average incidence of 8%, varying 5% to 12% by year.
Guy Loneragan, West Texas A&M University veterinary epidemiologist, says BRD — also referred to as shipping fever, or Undifferentiated Fever in Canada — is the leading cause of death loss and chronic illness in U.S. feedlots. In fact, it affects more animals than all other diseases combined, and costs the industry more than $600 million annually.
Is resistance heritable?
Chris Morris with AgResearch in Hamilton, New Zealand, shared a comment typical of U.S. bull buyers when he observed that one line of bulls from a herd is usually better able to cope with a high-risk health environment than another line. While it's just hearsay, most cattlemen believe there are differences. But New Zealand data suggest heritability for disease occurrence over all diseases is .28 — easily high enough for selection. Morris also gave these heritability estimates: grass tetany, .36; milk fever, .39; pinkeye, .28; and BRD, .19.
Meanwhile, Snowder's work at USMARC shows statistically significant breed differences in incidence and mortality of pre-weaning and feedlot BRD. He fixes estimated heritability of resistance to BRD in pre-weaned calves at .14, and in feedlot calves at .18 — probably high enough for effective selection.
Resistance or adaptability?
John Pollak, NBCEC director, asked symposium attendees, “When we select for resistance to a specific disease, are we actually selecting for an overall resistance? Does the animal have some innate ability to resist disease challenges? Or is it related to the ability to deal with stress?”
Snowder discussed stress effects and observed, “Maybe, with BRD, we don't want disease resistance — maybe we want stress resistance.”
According to Loneragan, BRD results from the interaction of stessors, pathogens and animal susceptibility, but stressors have the biggest effect. These include transportation, auction market, commingling, weaning, feedlot environment, feed/water deprivation, processing and other factors.
Loneragan's data show that risk of death by feedlot entry weight is a repeatable, predictable trend for health of feedlot cattle. As cattle get heavier, the risk falls, with approximately 600 lbs. as the critical weight.
Mike Engler, Cactus Feeders president and CEO, says Mexican cattle, weighing the same as domestic cattle, but older, have less disease morbidity. Snowder's data agrees. He observed a significant reduction in BRD incidence at 20, 50 and 80 days on feed.
So then, is weight alone the critical factor? Or is weight an indirect measure of an animal's age, giving it more opportunities to adapt to stressors as it ages?
Pollak concludes, “Really, what we're saying is, if stress plays a role, there's an opportunity to affect animals throughout all stages of life production. If we do that, the work has a broader context than just BRD.
“If we could develop a tool where we use the feedlot challenge as the stressor and BRD as the pathogen, we could analyze the animal's reaction, and partition out the part due to stress, and the part due to innate immune ability,” he adds.
What's being done?
NBCEC and USMARC are involved in two projects studying the genetics of BRD resistance in feedlot cattle. The projects are designed to allow expression of the disease to allow complete data collection.
Describing the NBCEC project where cattle are being fed at Colorado Beef in Lamar, CO, Pollak observes, “We're hoping for a fairly high disease incidence in order to have lots of observations to work with. The cattle haven't been mass treated on arrival at the feedlot in order to sustain the integrity of the project.”
Although the cattle were vaccinated and treated according to recommended industry protocols at weaning, more than 40% of the project's 1,551 steers have been pulled and treated for BRD symptoms. The cattle in this project are from a commercial ranch in the Sandhills, and represent a very typical set of ranch-direct feedlot cattle.
Meanwhile, the USMARC project is still in the final planning stages. It will utilize commercial cattle in addition to cattle derived from USMARC projects. The extensive data available will allow correlation of disease resistance results with other USMARC projects, such as feed efficiency and breed interactions.
What are the challenges?
Before a trait can be selected, a measurable and observable phenotype is necessary. In addition, data on the phenotypes must be monitored and collected with reasonable cost and effort. If sufficient phenotypic variation exists in the population, and if the heritability is moderately high, then genetic progress can be made by selecting animals that exhibit the desirable phenotype.
Some traits with simple inheritance are relatively easy to select for when the trait is expressed. For example, if you desire to genetically eliminate horns from your herd, you identify the horned animals and remove them. Easy, right? No, because the polled gene will mask the horned gene when both are present. In this case, available DNA markers allow us to “see” the horned phenotype, and make the selection progress we desire.
Many traditional EPDs were developed and are updated in large part by using data submitted by purebred breeders or collected by breed associations. But unlike weighing calves, can producers correctly identify disease phenotypes?
In the case of BRD, the symptoms can be obvious, but recent feedlot studies indicate that up to a third of animals are misdiagnosed, even by experienced pen riders. The population studies now being conducted on BRD should be useful, along with new DNA technology, in identifying genetic markers to aid in selection.
But questions remain. Who will collect and analyze the information? And, who will pay for all of this and “own” the information? Stay tuned.
Real-world stress test
Steve Radakovich, Radakovich Cattle Co., Earlham, IA, believes stress is directly related to disease susceptibility. “We (the beef industry) manage cattle to remove stress, so they have no tolerance for stress when it hits them,” he says.
Radakovich has been breeding and selecting cattle that can tolerate Missouri tall fescue using a four-way cross of Hereford, Barzona, Red Angus and Senepol. Calving is in April and May so that breeding is in the worst heat of summer on fescue.
Bulls are developed only on forage. After yearling weights are taken, they go to a summer stress test — living only on fescue, heat and water. Bulls are weighed again in September and ranked for fescue stress resistance.
Radakovich reports that his cows are getting bred and the herd is growing. In fact, most of his stress-adapted bulls aren't even going to Missouri; they're thriving and breeding cows in the deserts and temperature extremes of Nevada.
Has Steve successfully selected for fescue disease tolerance? Maybe not. Perhaps it's all about selecting for stress tolerance.