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.
• 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.
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:
• Bacteriophages (viruses of bacteria)
• Dietary changes
• 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 Kansas State University reported that feeding distillers grains, a co-product of ethanol production, to feedlot cattle may have a positive association with fecal shedding of E. coli O157. The mechanism is unknown, but they hypothesize that the grains change the ecology of the hindgut where E. coli O157 is most likely to colonize cattle. The authors report that larger studies are underway to investigate this possible link.
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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