“The answer, my friend, is blowin’ in the wind.” — Bob Dylan

The question in the case of food safety is: “What’s blowing in the wind?”

Turns out it can be dangerous pathogens such as E. coli, Campylobacter and Salmonella when manure is spread on the land, which is a common agricultural practice. At least that’s what a team of scientists found during a three-year research project.

For some farmers, such an offer is as good as money in the bank. But research continues to pile up, showing that one grower’s fertilizer can be another’s pathogen-spurred recall.
The goal of the study was to estimate infection risks associated with eating leafy greens contaminated by pathogens emitted from nearby manure application sites and to use the results to provide “science-based recommendations for the protection of public health.” To get more specific, the project involved applying manure onto land and measuring how far bacteria from the manure drifted. The overall goal was to find a safe distance between manure applications and crops. The manure was from dairy farms and the research was done in northern New York state. According to the results of the study, “Bioaerosol Deposition to Food Crops near Manure Application” published in the Journal of Environmental Quality, pathogen drift does indeed occur, but at varying degrees at varying distances. For the research project, manure was applied by a conventional broadcast system using a mobile spreader, not with a manure gun which “shoots” liquid manure onto the fields. The scientists collected air samples downwind of the manure application sites for an eight-hour period beginning immediately after application. Additional background samples were collected near other fields that had not had any manure applied to them during the previous six months. Sampling was done on days when there was no rain. Measurement results were used to model the transport of pathogens to crops that could be grown at various downwind distances.
Undergraduate research students Lisa D'Auria (left) and Jessica Castilleja (center) unpack and prepare bioaerosols and particulate sampling equipment with graduate student Seungo Kim (right). Photo credit: Christopher Lenney.
Undergraduate research students Lisa D’Auria (left) and Jessica Castilleja (center) unpack and prepare bioaerosols and particulate sampling equipment with graduate student Seungo Kim (right). Photo by Christopher Lenney
Not surprisingly, the farther from the manure application, the lower the probable risk that someone would get sick from eating leafy greens contaminated by pathogens emitted from the manure. For example, overall median one-time infection risks decreased markedly based on distance: from one in 1,300 chances at 0 meters directly downwind from the field to one in 6,700 chances at 100 meters (328 feet) and one in 92,000 chances at 1,000 meters (3,280 feet). A median infection risk of one in 10,000 means that one out of every 10,000 people eating a typical serving of the greens would be likely to become infected — a level generally considered by federal agencies to be an acceptable risk level, said project researcher Michael A. Jahne, who at the time the research was being done was with the Institute for a Sustainable Environment at Clarkson University in Potsdam, NY. With overall risks ranging from 0 percent to 100 percent, median risks have a 50 percent chance of occurring. As part of the project, researchers also looked at “peak risks,” which would occur if the manure came from a herd with an E. coli disease outbreak, for example. While cows don’t become ill when infected with the form of E. coli that can be fatal to people, they do shed it in their manure, which means there would be more E. coli in the manure from such a herd. In a case like that, the peak risk would be one in 18 chances at 0 meters, 1 in 89 chances at 100 meters, and one in 1,200 chances at 1,000 meters. As can be expected, the pathogen load on the leafy greens diminished with time, which means the timing of harvest after manure applications at nearby fields would be an important food-safety measure for growers to take into consideration. The study recommended that a 160-meter (524.9 feet) distance be provided between fields receiving manure applications and nearby leafy green crop production. It also pointed out that additional distance or delay before harvest will provide further protection of public health. In other words, in addition to following food-safety practices on their own farms, growers need to take nearby manure applications at other farms into consideration when making plans for harvesting. The research project was supported by the National Research Initiative Competitive Grant and the Agricultural Food and Research Initiative from the National Institute of Food and Agriculture Air Quality. Researcher Jahne said that other crops, such as fresh fruits, that will be eaten raw would also have associated risks. “Absolutely,” he said, pointing out that this issue is not limited to leafy greens. For Jahne, the “take-home message” of this research is that land applications of animal manure can present public-health risks from airborne transport of bacteria to nearby produce. “Such risks may be reduced by providing sufficient setback distance between the two areas and should be considered when planning and managing agricultural activities,” he said. Another researcher with the project, Shane Rogers, also of Clarkson University, said in an article from the American Society of Agronomy that additional distance and delay between manure application and harvest would provide further protection.
Undergraduate research student Jessica Castilleja take samples from bioaerosols and particulate deposition samplers during a field sampling event. Photo credit Christopher Lenney
Undergraduate research student Jessica Castilleja take samples from bioaerosols and particulate deposition samplers during a field sampling event. Photo credit Christopher Lenney
Does it really matter? The report provides sobering realities about some of the public health risks associated with foodborne pathogens and manure. An estimated two-thirds of deaths from foodborne disease are due to zoonotic bacterial pathogens also found in manure. Zoonotic refers to infections transferred from animals to humans. Studies have demonstrated survival on field lettuce of up to 77 days for E. coli O157:67 and 63 days for Salmonella spp. Jahne said that these survival rates demonstrate that once crops become contaminated with pathogens, the pathogens can survive for extended periods of time to eventually cause human illness. “Physical setback distance may thus be a better barrier for protection of food quality than delay before harvest,” he said. Previous research has shown that E. coli O157:H7 from livestock feedlots, where large numbers of animals are confined and fed, can become airborne and drift onto nearby leafy green crops. Although the Food Safety Modernization Act has safeguards to protect produce from microbial contaminants in soil amendments such as compost and irrigation water, these protections would not prevent contamination of crop fields by airborne pathogens from nearby manure application sites. “It’s definitely been overlooked,” said Jahne, referring to airborne contamination. “We hope that this research, and related research, will provide the data necessary to support these safeguards in the future. Right now, farmers don’t have guidance for how to do it. This is an important issue for consumers and for farmers.” Trevor Suslow, food-safety guru at the University of California, Davis, agreed. He emphasized that growers always need to pay attention to research like this. Complimenting the researchers on the project for doing a through, well-designed study, Suslow also said that it’s important to balance the implication of the study with real-world conditions. It will always be influenced by local conditions, the methods of manure applications, and the type of crops near the manure applications. “The survival of the pathogens depends on so many things,” said Suslow. Nevertheless, he said, the importance of this published research is that it can be used in the process of evaluating current standards in the Food Safety Management Act and possible amendments to it. His advice to growers is to document anything in their decision path relevant to nearby manure applications so they can show why and how they arrived at any decisions they made, which would include type of crop being grown, downwind and upwind conditions, and time of year. Even so, he said he’s seen or heard of cases where growers factor in all of their pre-planting assessments and risks, when “all of a sudden, a neighboring farmer starts spreading manure nearby.” That can happen because even if a neighboring farm doesn’t have any livestock, it can source it from a feedlot or dairy farm. And, yes, there’s a lot of manure available. According to USDA’s Natural Resources Conservation Service a typical milking cow daily produces about 80 pounds of manure per 1,000-pound weight unit (a dairy cow weights about 1,400 pounds), while a beef cow produces about 59.1 pounds of manure per 1,000-pound unit each day, and a pig produces about 80 pounds of manure per 1,000-pound unit a day. While manure provides valuable nutrients such as nitrogen to the soil, it also comes with the probability of harboring foodborne pathogens that pose a risk to public health.
Undergraduate research students Lisa D'Auria (left) and Jessica Castilleja (center) unpack and set up bioaerosols sampling equipment under the direction of graduate student Michael Jahne (right).
Undergraduate research students Lisa D’Auria (left) and Jessica Castilleja (center) unpack and set up bioaerosols sampling equipment under the direction of graduate student Michael Jahne (right). Photo by Christopher Lenney
Why leafy greens? According to the report, leafy green crops, which are often minimally processed and consumed raw, typically in salads, were chosen for the research project because they have frequently been implicated in recent food-borne disease outbreaks. One of the most publicized, an E. Coli outbreak in fresh spinach in 2006, sickened 199 people and killed three people. Leafy greens are an important crop for farmers because in many cases they can be grown year round. Then, too, they are very popular with consumers. A recent USDA report revealed that the availability of romaine and leaf lettuce has almost doubled, rising from 6.6 pounds per person in 1997 to 11.4 pounds in 2013. The growing popularity of prepackaged, ready-to-eat salad greens contributed to the rise in availability of these two lettuces. Availability of other fresh greens — collard greens, escarole and endive, kale, mustard greens, and turnip greens — came in at 2.5 pounds per person in 2013, while fresh spinach availability was 1.6 pounds per person. However, head iceberg lettuce availability was only 14.1 pounds per person in 2013, a drop of 41 percent from 24 pounds in 1997. Jahne said that this information is relevant to the research project, especially since these types of greens have more exposed surface area than head lettuce. Scott Horsfall, CEO of the Leafy Greens Marketing Agreement said “this is the kind of research we need in the food safety community.” Ten years ago, the LGMA set the recommended distance between manure applications and leafy greens at 120 meters (393.7 feet) — 40 meters less than the recent research project’s recommendation. But, Horsfall said, back then everyone knew there wasn’t enough information about the necessary distance. “We knew there were gaps,” he said, “and our standards were designed to be updated if necessary. It’s an area that not a lot is known about. Everyone agreed that a setback was needed. This sort of research helps.” The LGMA’s recommended setback was not set in stone. Growers were urged to take varying conditions into account when deciding on what size of a setback to use and when. FDA extends comment period on manure use In response to multiple requests from stakeholders, the U.S. Food and Drug Administration is extending the comment period for public input to help the agency develop a risk assessment for evaluating the risk of human illness associated with the consumption of produce grown in fields on which raw manure or other untreated biological soil amendments of animal origin are used as fertilizer. The agency is extending the comment period by 60 days, to July 5, 2016. It is hoped that the additional comments will help the agency make a decision on the minimum time interval between the application of raw manure and the crop harvest when certain application methods are used. The use of raw manure (or other untreated biological soil amendments of animal origin) as fertilizer in growing crops is covered by the final produce safety rule mandated by the FDA Food Safety Modernization Act. To submit comments electronically to the docket, visit Docket No. FDA-2016-N-0321. For more information: •Federal Register notice for the manure comment period extensionRisk Assessment of Foodborne Illness Associated With Pathogens From Produce Grown in Fields Amended With Untreated Biological Soil Amendments of Animal OriginRaw Manure under the FSMA Final Rule on Produce SafetyFDA to Extend Comment Period on Request for Scientific Data, Information, and Public Comments on Assessing the Risk of Human Illness Associated with Raw Manure as Fertilizer (To sign up for a free subscription to Food Safety News, click here.)