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Clearing Vegetation Around Crops Doesn’t Help Reduce Pathogens on Produce

The effort to improve food safety by clearing wild vegetation surrounding crops is not helping and, in some cases, may even backfire, according to a new study led by researchers at the University of California-Berkeley.

The findings, reported Monday, Aug. 10, in the journal Proceedings of the National Academy of Sciences, call into question the effectiveness of removing non-crop vegetation as a way to reduce field contamination of fresh produce by disease-causing pathogens. This practice led to extensive loss of habitat in a region that is globally important for food production and natural resources.

UC-Berkeley study graphic

Illustration by Mattias Lanas and Joseph Burg.

The practice was implemented largely in response to a 2006 outbreak of pathogenic E. coli in packaged spinach that killed three people and sickened hundreds of others in the United States.

That outbreak was traced to a farm in California’s central coast, a region that supplies more than 70 percent of the country’s salad vegetables. The disease-causing E. coli strain was found throughout the farm environment, including in the feces of nearby cattle and wild pigs, but the cause of the outbreak has never been officially determined.

“Wildlife took much of the blame for that outbreak, even though rates of E. coli in wildlife are generally very low,” said study lead author Daniel Karp, a NatureNet postdoctoral research fellow in UC-Berkeley’s Department of Environmental Science, Policy and Management and The Nature Conservancy. “Now, growers are pressured by buyers to implement practices meant to discourage wildlife from approaching fields of produce. This includes clearing bushes, plants and trees that might serve as habitat or food sources for wild animals. Our study found that this practice has not led to the reductions in E. coli and Salmonella that people were hoping for.”

Instead, the study authors noted that the presence of diverse habitats bordering food crops can actually provide a number of agricultural benefits.

“There is strong evidence that natural habitats surrounding crop fields encourage wild bee populations and help the production of pollinated food crops,” said study senior author Claire Kremen, a UC-Berkeley professor of environmental science, policy and management. “There have also been studies that suggest that a landscape with diverse plant life can filter out agrichemical runoff and even bacteria. Changing this dynamic shouldn’t be taken lightly.”

The researchers analyzed about 250,000 tests of produce, irrigation waters and rodents conducted by industry and academics from 2007-2013. The tests were conducted on samples from 295 farms in the U.S., Mexico and Chile, and targeted the presence of pathogenic E. coli, Salmonella and generic strains of E. coli. The researchers combined the test data with a fine-scale land-use map to identify characteristics of the landscape surrounding the agricultural fields.

The researchers found that the removal of riparian or other vegetation did not result in lower detection of pathogens in produce, water or rodents. Overall, the prevalence of pathogenic E. coli in leafy green vegetables had increased since the outbreak even as growers removed non-crop flora. In fact, the growers who removed the most vegetation experienced the greatest increase in pathogenic E. coli and Salmonella in their vegetables over time.

“Clearing surrounding vegetation is a costly, labor-intensive practice that threatens wildlife habitat,” said Karp. “Since it does not improve food safety, there is no reason to continue this practice.”

The study did find, however, that the likelihood of detecting pathogenic E. coli was greater when fields were within 1.5 kilometers of grazeable land than when they were farther away.

“It is unclear whether it was the cattle or wildlife grazing on those lands that were responsible for the elevated pathogen levels, but there are a number of ways that farming and ranching can co-exist in a diversified system,” said Karp.

Some suggestions include:

  • Leaving strips of vegetation between the grazed areas and fresh produce areas.
  • Fencing off upstream waterways from cattle to prevent waste from going downstream.
  • Planting crops that are usually cooked before being eaten, such as corn, artichokes and wheat, between fresh produce fields and grazeable lands.

After the 2006 E. coli outbreak in spinach, California’s agricultural industry implemented a voluntary certification program called the Leafy Green Products Handler Marketing Agreement. At the federal level, in 2011 President Obama signed the Food Safety Modernization Act, considered one of the most sweeping reforms in farming practices in decades. Both efforts shift the focus to preventing rather than responding to outbreaks.

Notably, neither the federal law nor the state program calls for the removal of wildlife habitat surrounding crops, but private buyers, keen on retaining consumer confidence in their products, may still require growers to take steps that go beyond government regulations.

“The real worry for me is that federal law will be interpreted as the floor rather than the ceiling of what farmers should do,” said Karp. “There is this misguided idea that agricultural fields should be a sanitized, sterilized environment, like a hospital, but nature doesn’t work that way.”

Other co-authors of the study are Sasha Gennet, senior scientist at The Nature Conservancy; Christopher Kilonzo, Melissa Partyka and Edward Atwill at UC-Davis, and Nicolas Chaumont at Stanford University.

The UC Berkeley Center for Diversified Farming Systems, Berkeley Food Institute, The Nature Conservancy NatureNet Fellowship, and The Nature Conservancy of California helped support this research.

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© Food Safety News
  • pawpaw

    Glad the researchers above were willing to test the efficacy of such ‘scorched earth’ policies, whether they actually reduce pathogen load in produce.

    Certified organic producers are required to implement a ‘whole farm plan’ that includes ecological integrity of the landscape. A key part of this is pollinator habitat. Such blooms and shelter also increase the numbers and variety of beneficial insects, which allow many crops to be grown with significant pest pressure. This was and is part of the challenge of FMSA regs, bringing them into line with USDA Organic regs.

    As for the buffers between raw produce fields and any livestock, organic regs also require those now, if any livestock present on the farm. The suggestions from the researchers highlighted above: the organic certifier I am familiar with, they require a written plan for these buffers and strategies, and during farm visits, they want to see them in action.

    There is the concern among small-scale farmers that third party certifiers will indeed see FMSA regs as a floor, implement one-size fits all interpretations, and require these whether they’ve been tested by science or not. Irrigation water will be one area to watch.

  • MaryFinelli

    Wildlife have been convenient scapegoats. If anything, wildlife are being infected by the virulent pathogens that factory farms serve as breeding grounds. All that contaminated manure used as fertilizer and polluting the air and water with pathogens.

    “In conclusion, microbial contamination of produce is influenced by farm management and environmental factors. Specifically, microbial contamination of produce seems strongly influenced by the time since last irrigation, workers’ personal hygiene, and the use of the field prior to planting [i.e., grazing]”: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3697504/

  • Carl Custer

    “The study did find, however, that the likelihood of detecting pathogenic E. coli was greater when fields were within 1.5 kilometers of grazeable land than when they were farther away.” They should have done a better literature search. two they missed:
    Yanamala S., M.F. Miller, G.H. Loneragan, S.E. Gragg and M.M. Brashears. 2011. Potential for Microbial Contamination of Spinach Through Feedyard Air/dust Growing in Close
    Proximity to Cattle Feedyard Operations. Journal of Food Safety 31 (2011) 525–529.
    Berry, Elaine D., James E. Wells, James L. Bono, Bryan L. Woodbury, Norasak Kalchayanand, Keri N. Norman,* Trevor V. Suslow, Gabriela López-Velasco,b Patricia D. Millner. 2015. Effect of Proximity to a Cattle Feedlot on Escherichia coli O157:H7 Contamination of Leafy Greens and Evaluation of the Potential for Airborne Transmission. Applied and Environmental Microbiology 81:1101-1110.
    and our favorite:
    Research: Antibiotic-Resistant Bacteria May Travel Via Feedlot Dust
    By News Desk | January 23, 2015

  • LP

    This one of those times when I am just left scratching my head. I am in Oklahoma so the thought of a farming operation, anywhere, removing a wind-break while destroying natural habitat, especially in a hunting obsessed state, is just astonishing and perplexing. Of course it always comes to what is more profitable, a grazing lease on your un-tilled back forty or the enjoyment of some edge species like quail…which we used to have. One thing we will definitely have again though is a dust bowl if we can’t stick to some sound soil conservation methods. Removal of riparian veg and trees from big bufffer zones is not it.