Can We Select for Cattle that are Less Prone to Disease?
by Alison Van Eenennaam, PhD
B
ovine Respiratory Disease (BRD), also known
as shipping fever or pneumonia, is the
leading cause of illness and death for the
backgrounding and feedlot cattle industries
and is the most important cause of diseaserelated economic losses. Of the cattle
that become ill in the feedlot, 67 to 82% suffer from
respiratory disease. In the United States, 1.4% of all
feedlot cattle perish before reaching harvest weight and
of those, the majority are due to respiratory disease.
Indeed, more feedlot cattle die from BRD than all other
diseases combined and this trend is increasing. This is
despite the fact that more than 25% of large feedlots
use a respiratory vaccine to combat the disease, and
almost all (99.8%) feedlot animals diagnosed with
BRD are treated with an antibiotic as part of the
treatment regimen. The costs to treat an animal for BRD
(excluding labor, veterinary fees or indirect costs) have
been reported to be $37.90 per animal. In addition to
death-loss and treatment costs, detrimental effects of
BRD include reduced carcass weights, daily gains, yield
grade, carcass fat measurements, and muscle shearforce measurements.
If you consider a disease like BRD, it is well known that
stressors like weaning, transportation, comingling at
the sale yard, and dust can all increase its incidence.
There is a reason that buyers offer a backgrounding
premium for cattle that have been weaned prior to
sale, are enrolled in a known vaccination program, and
are shipped as an entire load that can be fed as a pen
thereby avoiding the stress of comingling necessitated
by smaller lots. These management factors help to
minimize the probability that an animal will get BRD.
Evidence that BRD susceptibility also has a genetic
component is demonstrated by breed differences in
BRD morbidity and mortality, and the fact that BRD
prevalence in unweaned calves and feedlot cattle
has been reported to be heritable. Heritability (h2)
is defined as the proportion of observed variation in
a particular trait that can be attributed to inherited
genetic factors in contrast to environmental ones. It is
the relative importance of genetics versus environment
in determining phenotype. A trait that has high
heritability (e.g., height) is largely determined by
genetics, and a trait that has low heritability is greatly
influenced by the environment. Susceptibility to disease
is typically found to have a low heritability (h2 ~ 0.10.2).
One study found that the heritability of BRD incidence
in feedlot animals was 0.18. There are some technical
reasons why BRD heritabilities tend to be low under
field conditions. These include suboptimal diagnosis
(e.g., not all sick animals are identified and healthy
animals may be incorrectly diagnosed as ill), and some
susceptible animals will appear resistant to a disease
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when in fact they have not been exposed to the disease
agent (viruses and opportunistic bacteria in the case
of BRD). These confounding factors add environmental
noise to field data which decreases heritability.
Field studies therefore likely underestimate the true
importance of genetics in BRD incidence, and thus also
undervalue the potential gains that could be made by
breeding for disease resistance.
Trying to understand the genetic potential of animals
to remain healthy and free of BRD was the focus
of a large field trial carried out by researchers at
the University of California, Davis this past year.
Faculty from the UCD Department of Animal
Science joined with colleagues from the School of
Veterinary Medicine to identify 1,000 BRD case
and 1,000 co-located control animals on a calfraising ranch in the Central Valley. This trial was led
by Dr. Terry Lehenbauer, Associate Professor and
Director, Veterinary Medicine Teaching and Research
Center (VMTRC) in Tulare. In addition to carefully
diagnosing sick versus healthy animals using an
objective scoring system to minimize subjectivity
in classifying cases and controls, deep pharyngeal
swabs were taken for bacteriological and viral
workups at California Animal Health and Food Safety
(CAHFS) diagnostic laboratories, and blood was taken
for DNA extraction. The objective of this study was
to standardize both the environment experienced
by these calves and diagnosis, thereby increasing
the probability that differences in BRD disease state
resulted from genetics, rather than differential
exposure to BRD disease agents, incorrect diagnoses,
and/or other environmental stressors.
DNA from each of these 2000 animals is now being
run on a “high-density” DNA panel that interrogates
770,000 genetic markers (aka the “770K SNP chip”)
at locations situated throughout the genome. The
expectation is that some markers will always (or at
least more frequently) be associated with the animals
that are healthy. Such genetic markers are akin to
signposts, indicating the location of regions of the
genome harboring genes that help animals fight off
BRD. Such markers will be the genesis of new DNA
tests that can help identify animals that are less likely
to succumb to BRD.
This research is being funded by a five-year USDA
grant entitled “Integrated Program for Reducing Bovine
Respiratory Disease Complex (BRDC) in Beef and
Dairy Cattle.” The goal of this program is to reduce the
incidence of BRD in beef and dairy cattle by capitalizing
on recent advances in genomics to enable novel genetic
approaches to select for cattle that are less susceptible
to disease. This effort, known as the BRD CAP
(Coordinated Agricultural Project), involves a multiinstitutional team led by Dr. James Womack at Texas
A&M University, and involves research groups from
Washington State University, the University of Missouri,
Colorado State University, New Mexico State University,
and the University of California, Davis. For more
information on the BRD CAP see www.BRDComplex.org.
The USDA, through the Agricultural and Food
Research Initiative (AFRI) competitive grants
program, is investing in several other similar grants
focused on using DNA-based technologies to make
genetic progress in traits that have proven difficult
to improve using traditional selection on expected
progeny differences (EPDs). This includes projects
focused on the development of genomic approaches
to improve “feed efficiency” and “fertility” of beef
cattle. These traits are not the “low hanging fruit” of
genetic improvement. They are typically traits that
are measured late in life, are expensive to measure or
are not routinely measured at all, and frequently have
low heritabilities making it difficult to differentiate
the genetic component of phenotype from the
environmental influences. However, they are the most
valuable traits in terms of the beef cattle industry. A
1% improvement in feed efficiency, fertility or reduced
BRD disease incidence in the feedlot would all be of
tremendous value to the U.S. beef cattle industry.
Dr. Van Eenennaam is a Cooperative Extension Specialist in the field of Animal Genomics and
Biotechnology in the Department of Animal Science at UC Davis. She received a PhD in Genetics from
UC Davis. The mission of her extension program is “to provide research and education on the use
of animal genomics and biotechnology in livestock production systems.” Dr. Van Eenennaam has a
multifaceted research program and she works closely with the beef cattle industry.
On Sunday, July 1, 2012, at PacVet, Dr. Van Eenennaam is conducting a session, “An Integrated Program for
Reducing Bovine Respiratory Disease Complex in Beef and Dairy Cattle.” This talk introduces the attendee to this
coordinated project, whose goal is to integrate research, education, and extension activities to develop costeffective genomic and management approaches to reduce the incidence of pneumonia in beef and dairy cattle.
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