Grass-Fed vs. Grain-Fed Beef and the Holy Grail: A
Literature Review
Source of Article: http://www.marlerblog.com/2008/08/articles/lawyer-oped/grassfed-vs-grainfed-beef-and-the-holy-grail-a-literature-review/
Several people have commented that switching from grain to grass feeding
could be one of the solutions to the problem with foodborne
pathogens in cattle and other livestock. Quotes like these are becoming more
common on the Internet and in recent media reports:
“Products from grass-fed
animals are safer than food from conventionally-raised animals.” Eatwild, 2008
“Research has shown that
the strains of E. coli most devastating to humans are the product of feedlots,
not cows. This is due to the animals being forced to eat an unnatural diet, and
not their natural choice, grass.” Grass-Fed Beef: Safer and Healthier,
Animal Welfare Approved, June 15, 2008
If true, changing the cow’s diet would be such a
simple and cheap management practice to implement. Have we found the Holy Grail
for food safety? Below is some research I did on the topic.
OVERVIEW
• Identification of on-farm management practices that would reduce or eliminate
foodborne pathogens in cattle and other livestock
(including diet changes) is an active area of research, but many study results
are inconclusive. E. coli O157:H7, Campylobacter, Salmonella, and other
dangerous pathogens have been repeatedly isolated from both grass and grain fed
livestock, and the studies show conflicting results regarding whether the
levels of pathogens are higher, lower, or the same when animals are fed grass-
or grain-based diets.
• There is no clear and consistent definition in the literature of “grass-fed,”
but the majority of papers describe animals that are on pasture or confined,
but receiving only hay-based diets. Last year, the USDA Agricultural Marketing
Service issued a standard for grass (forage) fed marketing claims. More research
on this topic is needed that compares rates of foodborne
pathogens among grain and grass fed animals using a specific definition such as
the USDA standard or other accepted definition.
• The original study by Diez-Gonzalez published in
Science in 1998, and since cited numerous times in the literature and media,
suggested that cattle could be fed hay for a brief period before slaughter to
significantly reduce the risk of foodborne E. coli
infection. They based this conclusion on a hypothesis that grain feeding
increases acid resistance of E. coli in cattle. Although they showed increased
acid resistance in E. coli from grain-fed cattle, but the sample size was
small, and they used “generic” E. coli stains, not E. coli O157:H7.
• Studies by other researchers worldwide have since found little difference in
acid resistant E. coli O157:H7 among grain- verses grass-fed cattle, and some
even found more E. coli O157:H7 shed by grass-fed animals.
• It has been discovered that E. coli O157:H7 and Salmonella can rapidly switch
from being “acid sensitive” to “acid resistant” within minutes after entering
an acidic environment (such as the human stomach). Thus, even if the
grass-fed/E. coli acid-resistance hypothesis were true, manipulating the diet
may not have any effect since pathogens can adapt quickly to new environments
like the human stomach.
• Outbreaks have traced back to grass-fed and pastured animals, as well as
animals in feedlots. Notably, the E. coli O157:H7 spinach outbreak strain in
2006 was isolated from grass-fed cattle. Another outbreak of E. coli O157:H7
was linked recently to raw milk and colostrum from
cattle raised organically on grass.
• In summary, the scientific evidence at this time does not support a broad
conclusion that grass feeding significantly and consistently reduces the risk
of E. coli O157:H7 or other dangerous foodborne
pathogens entering the food chain. However, more research is needed into the
influence of food animal diets. For example, preliminary experimental data
shows a possible association between feeding dried distiller’s grains and
shedding of E. coli O157:H7 in cattle feces.
INTRODUCTION
A systematic approach is necessary to combat the
emerging challenges in food safety such as the unexplained “uptick” of E. coli
O157:H7 outbreaks and recalls linked to beef products. Interventions to protect
the food supply should ideally occur across the continuum from “farm to fork.”
The “Holy Grail” of pre-harvest (farm-level) food safety would be to find an
effective, affordable, and practical means to prevent or reduce food animals
from shedding foodborne pathogens in the first place
so the dangerous bacteria never enter the human food chain. Since cattle or
other livestock may be located near drinking water sources or vegetable crops,
a farm-level intervention could also help to protect nearby water and crops
from contamination by manure via runoff, transport by wildlife/insects, or
other mechanisms.
Oliver et al (2008) published a comprehensive review of developments and future
outlooks for pre-harvest food safety this month. Examples of potential
farm-level management practices that have been studied for E. coli O157:H7 and
other foodborne pathogens in livestock include:
• Antibiotics
• Bacteriophages (viruses of bacteria)
• Dietary changes
• Immunization
• Probiotics or prebiotics
in animal rations
• Sanitation/hygiene (feed, water, environment)
• Wildlife and insect control
Unfortunately, the best approaches for on-farm control of foodborne
pathogens in livestock remain elusive. No single management practice, or even a
combination of methods, has proven to be very effective or reliable in
preventing foodborne pathogen colonization in
livestock. Clearly, sanitation including clean feed/water sources and insect
control are important, but difficult to maintain in a farm environment.
Livestock immunizations are not available for most foodborne
pathogens with the exception of an E. coli O157:H7 vaccine under development
(and some ask “who would pay for such a program?” since cattle do not become
ill from E. coli O157). Use of antibiotics is problematic because it can lead
to resistance.
GRASS VERSUS GRAIN FEEDING
Definition of “Grass-Fed”
The majority of cattle are fed grass or other forage
at some time during their lives. For the purpose of marketing, the USDA
Agricultural Marketing Service issued a voluntary standard for grass (forage)
fed marketing claims last year that states: “grass fed standard states that
grass and/or forage shall be the feed source consumed for the lifetime of the
ruminant animal, with the exception of milk consumed prior to weaning. The diet
shall be derived solely from forage and animals cannot be fed grain or grain
by-products and must have continuous access to pasture during the growing
season.”
Note that most papers in the literature do not specifically define grass-fed
using this new standard or any other specific definition, but differentiate, in
general, between animals on forage (grass) only verses diets containing grain.
The Study that Started the Controversy
The original study that launched the controversy over grain feeding was
published in Science in 1998 by researchers from Cornell (Diez-Gonzalez
et al). They described potential dietary effects on the acid resistance of E.
coli in cattle fed grain- versus hay-based diets. This study has since been
cited numerous times in the literature and media, but later studies have not
been able to reproduce the findings. This may be due, in part, to several
limitations in the original study design including: 1) small sample size and 2)
“Generic” E. coli levels were measured, not E. coli O157:H7.
In 2006, Hancock and Besser wrote a summary of the
evidence surrounding the hypothesis that feeding hay instead of grain would
reduce the problem with E. coli O157:H7, purportedly because the stomachs of
grain-fed cattle are more acidic. They concluded: “while one cannot rule out a
role of cattle diet on affecting exposure and infectivity of E. coli O157:H7 to
humans, the data available at present demonstrate that cattle on a wide variety
of diets (including 100% forage diets) are regularly and similarly colonized
with this pathogen.”
Another interesting study from a research group in The Netherlands discovered
that E. coli O157:H7 and Salmonella can rapidly switch from being “acid
sensitive” to “acid resistant” within minutes after entering an environment
with reduced pH (such as the human stomach). Thus, even if the grass-fed
hypothesis were true, manipulating the diet may not have any effect since E.
coli O157:H7 can adapt quickly to new environments like the human stomach.
Recent Findings in the Literature
In searching through the literature since Hancock and Besser’s review, several new papers relevant to the
discussion were found.
1. Nutritional aspects of grass-fed beef.
Leheska, J. M., L. D. Thompson, J. C. Howe, E. Hentges, J. Boyce, J. C. Brooks, B. Shriver, L. Hoover, and
M. F. Miller. 2008. Effects of conventional and grass feeding systems on the
nutrient composition of beef. J Anim
Sci.
• This paper explores the question about whether there are differences in
nutrient composition of grass-fed beef compared with conventional (grain)-fed
beef. Researchers have previously found higher omega-3 fatty acids and CLA
(conjugated linoleic acid) in forage-fed beef, and
lower fat content overall. Some consumers prefer eating grass-fed meat because
they believe it is “healthier,” and/or tastes better than conventional beef.
• The authors of this study enrolled only producers that were marketing
grass-fed beef and confirmed that “100% of the diets were made up of native
grasses, forages, or cut grasses or forages.”
• Fatty acid composition of grass-fed and conventional-fed beef was found to be
different, but the authors conclude “the effects of the lipid differences between
grass-fed and conventional raised beef, on human health, remains to be
investigated.”
2. Papers continue to be published about possible effects of diet on E.
coli O157:H7 prevalence and concentration.
For example, a research team from
CONCLUSIONS
In summary, the scientific evidence at this time does not support a broad
conclusion that grass feeding significantly reduces the risk of E. coli O157:H7
or other dangerous foodborne pathogens from entering
the food chain. However, more research is needed to better understand the
influence of diet, especially the use of different types of grains in animal
feed.
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