Whatcom County is in the northwest corner of Washington State and has a rich history of agricultural production. This region is the top producer of processed red raspberries in the U.S., with product shipped globally. The area also has an active dairy industry, with over 45,000 dairy cows (USDA NASS, 2017). There has been recent interest in integration of these systems through the use of fertilizers derived from dairy manure on berry crops to provide soil fertility and improve soil health while turning a waste product into a resource for the dairy industry. Could a fertilizer product from dairy manure feasibly be used on nearby red raspberry fields? With food safety scares making regular appearances in national news, it’s understandable that farmers may be hesitant to apply fertilizer derived from manure to produce fields – particularly when that produce is consumed raw. For this reason, food safety was a critical component of a project by Washington State University researchers looking at application of fertilizers derived from dairy manure on berry crops in Northwestern Washington.
I was interested to read the article published recently in Frontiers in Microbiology (Sheng et al. 2019) detailing the results of some of the food safety work which could have implications for future connections between dairies and berries in Whatcom County.
Manure and raspberries – What’s the risk?
The source of foodborne pathogens for fresh produce outbreaks traced back to farms can include fecal material from birds and other wildlife, contaminated irrigation water, poor sanitation, or livestock manure (Beuchat, 2002). Produce, like individually quick frozen (IQF) raspberries, that are not subject to a microbial killing step prior to consumption, are particularly susceptible to risk of foodborne pathogens. An outbreak of a foodborne pathogen originating from their products can be potentially catastrophic for produce farmers, and may put them out of business. For this reason, any discussion of using fertilizers derived from livestock manure for crops bound for human consumption must include consideration of food safety.
Regulations and standards are in place to prevent outbreaks of foodborne pathogens due to manure application. The Food Safety Modernization Act outlines specific requirements around use of “biological soil amendments of animal origin,” including manure-based amendments. Good agricultural practices (GAPs) are standards for the use of manure-derived fertilizers include treatments to reduce pathogens (e.g., composting, or anaerobic digestion) and maximizing the time between application to production areas and harvest of the crops. In addition to regulations and standards, results from field research conducted under realistic production scenarios are important for evaluation of food safety risks.
Lina Sheng, who was at the time a Food Science Ph.D. student in the lab of Meijun Zhu at WSU, and colleagues conducted a field study during the 2017 and 2018 growing seasons to evaluate the impacts of dairy manure-derived fertilizer application on the microbial safety of red raspberry production. Manure-derived fertilizers (anaerobically digested liquid effluent, aerobically composted dairy manure, and more concentrated refined fertilizers such as ammonium sulfate, and phosphorus solids) were applied to raspberry plots on a 4.79 acre field in Whatcom County along with standard synthetic fertilizer (for comparison).
Dairy manure-derived fertilizers potentially carry different foodborne pathogens (Hutchison et al., 2005), which can be persistent in soil for a long time. For this reason, it was important for the researchers to take fertilizer, soil, foliar and fruit samples in order to understand whether the pathogens originated from the fertilizer, how long they persisted in the system, and whether they moved to other parts of the plant. Samples were collected by researchers clad in latex gloves and plastic boot covers and shipped to Pullman where Sheng led the laboratory analysis (Figures 1 & 2). Particularly problematic strains of E. coli producing Shiga toxin, Salmonella spp., and Listeria monocytogenes were quantified in the laboratory because they are organisms potentially present in manure and are frequently involved in fresh produce outbreaks.
What the research showed
No Shiga toxin-producing E. coli or Listeria monocytogenes was detected in any of the samples in this study. Though Salmonella spp. were detected in some of the fertilizers, Salmonella was not detected in soil samples taken 2 or 4 months after fertilizer application, nor was it detected in foliar or fruit samples, indicating that by 2 months after application, Salmonella from the fertilizers was no longer present at detectable levels (detection limit of 3 MPN/g).
Sheng and colleagues concluded that application of manure-derived fertilizers to the raspberry plots once per growing season 4 months prior to harvest under GAP, did not introduce microbiological safety risk.
Implications for use of manure derived fertilizers in raspberry production
This study is good news, from a food safety perspective, with no significant additional food safety risk from the application of manure derivatives to raspberries under GAP. However, the study authors emphasize that caution should be exercised when interpreting the results of this type of study. Because foodborne pathogens do not have uniform distributions, the fact that they weren’t detected in this study does not mean that they do not exist in manure-derived fertilizers of the type used in this study. There’s more work to be done before manure-derived fertilizer application in berries becomes a regular occurrence for a number of reasons, including the fact that for producers who are set up for using conventional fertilizers, specialized equipment is needed to apply manure-derived fertilizers to established crops like raspberries. Many producers are strongly averse to anything that smells of risk in terms of food safety. This is understandable given the fact that one outbreak could put them out of business.
The bottom line is that farming is ripe with risks. Minimizing the risks associated with using dairy manure-derived fertilizers as a sustainable source of fertility for a variety of crops could have great benefits for dairy and crop producers alike. This study is one step towards overcoming that hurdle in raspberry systems in Northwestern Washington.
Beuchat, L.R. 2002. Ecological factors influencing survival and growth of human pathogens on raw fruits and vegetables. Microbes Infect. 4: 413-423.
Hutchison, M. L., Walters, L. D., Avery, S. M., Munro, F., and Moore, A. (2005). Analyses of livestock production, waste storage, and pathogen levels and prevalences in farm manures. Appl. Environ. Microbiol. 71, 1231–1236.