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Wesley Kline, PhD
Introduction: The United States Department of Agriculture and the Food and Drug Administration consider animal manure as a significant source of potential contamination for produce. This does not mean manure should not be used to improve soil structure or fertility only that it must be handled properly. Untreated, improperly treated or recontaminated manure that enters surface or ground water through runoff, may contain pathogens that taint produce. Crops in or near the soil (i.e., carrots, lettuce, radishes, herbs, etc.) are most vulnerable to pathogens which may survive in the soil. Low growing crops that may be splashed with soil during irrigation or heavy rainfall are also at risk. Produce where the edible portion of the crop does not contact soil is less at risk of contamination provided the produce that does contact the ground (e.g., windfalls) is not harvested.
Growers using manure need to follow good agricultural practices to minimize microbial hazards. Growers also need to examine their specific growing environment to identify obvious sources of fecal matter that could be a source of contamination. The foodborne illness outbreak that occurred in Colorado during 2011 resulted in 147 illnesses and 33 deaths. The pathogen identified was listeria monocytogenes which normally is associated with farm soils and animal feces. How did the cantaloupe become contaminated with listeria? The Food and Drug Administration identified several possibilities including contamination from a cattle feedlot. The packinghouse delivered their cull fruit to a feedlot and the truck driver may have transported listeria back to the packinghouse. Growers who raise animals, used manure or have their packing operations close to animals must assess possible sources of contamination.
Apply manure in the fall or at the end of the season to all planned vegetable ground or fruit acreage, when soils are warm, non-saturated, and cover cropped preferably to grain or forage crops. If applying manure in the spring spread the manure two weeks before planting and at least 120 days prior to harvest. Incorporate manure immediately after application by either injecting or plowing.
At present there is no standard for the application of manure under the Food Safety Modernization Act/Produce Safety Rule except the National Organic Standards can be followed which is 120 days prior to harvest if it may touch the harvestable part of the crop or 90 if it is not likely. In both cases the manure must be incorporated or injected into the soil.
Treatments: There are various methods used to treat manure, so it is safer as a fertilizer than raw manure. The following information is from various sources.
Active: Active treatments involve a greater level of intentional management and a greater input of resources. Active treatments include pasteurization, heat drying, anaerobic digestion, alkali stabilization, aerobic digestion, or combinations of these. Composting is an active treatment commonly used aerobically or anaerobically, by microbial action in a controlled and managed process. When composting is carefully controlled and managed, and the appropriate conditions are achieved, the high temperature generated can kill most pathogens in several days. Thus, the risk of microbial contamination from composted manure is reduced compared to untreated manure.
Temperatures above 131ºF for a minimum of three days in an aerated windrow followed by curing with checking temperatures as the pile heats up and cools down during curing is required. The other option is to turn the pile at least five times over a period of 15 days allowing it to reach a temperature of least 131ºF to destroy most pathogens and weed seeds. Compost pile turning or covering with a tarp is critical since the outside of the pile does not reach adequate temperatures to control pathogens.
Under organic standards, “Processed manure must be treated so that all portions of the product, without causing combustion, reach a minimum temperature of either 150ºF for at least one hour or 165ºF and are dried to a maximum moisture level of 12%; or an equivalent heating and drying process could be used. In determining the acceptability of an equivalent process, processed manure products should not contain more than 1×10³ (1,000) MPN (Most Probable Number) fecal coliform per gram of processed manure sampled and not contain more than 3 MPN Salmonella per 4-gram sample of processed manure.”
Protecting manure and compost: When compost is delivered to the production site and stored for future application, it must be properly stored to reduce recontamination and the likelihood of contaminating the production area or adjacent fields. Growers may use the following methods to reduce potential contamination.
Consider barriers or physical containment to secure manure storage or treatment areas where contamination from runoff, leaching, or wind spread is a concern. Physical containment may include concrete block or soil berms, pits or lagoons. Practices such as storage on concrete slabs or in clay-lined area may reduce the potential source of leachate entering groundwater. Such storage must be away from irrigation sources, spray water sources or any other water source.
Consider practices that will minimize leachate from manure storage or treatment areas contaminating produce. Rainfall onto a manure pile can result in leachate, potentially containing pathogens. Growers may want to consider covering manure piles, such as storing manure under a roof or covering piles with an appropriate impervious covering.
Alternatively, growers may consider collecting water that leaches through manure that is being stored or treated. Collecting leachate allows the grower to control its disposal (e.g., on vegetative grass ways) or use (e.g., to control moisture during composting). Leachate may pose a microbial hazard similar to the manure from which it originates.
Manure leachate and teas: Growers using manure leachate or manure tea in fresh produce production areas should follow good agricultural practices, such as maximizing time between applications and harvest to minimize microbial hazards. Manure teas are different from compost teas. If using manure teas they should be applied in the same way as fresh manure (two weeks prior to planting and 120 days from harvest.). Compost teas can be used if prepared properly. Use only properly composted manure, make the tea with water that has been tested to show that there is no detectable generic E. coli present and do not add produce such as molasses to stimulate bacterial activity. Some additives may be contaminated with pathogens or if pathogens are present at low levels in the tea, the additive may stimulate growth. If these are added, the tea should be considered raw manure and handled the same manner as manure.
Conclusion: Natural fertilizers, such as composted manure, and fertilizers containing natural components, should be processed and handled in a manner to reduce the likelihood of pathogen introduction into produce production areas. Growers and manure suppliers should apply good agricultural practices that ensure all materials receive an adequate treatment, such as thorough mixing and turning outside edges into the center of a compost pile. Cold spots that do not receive an adequate treatment can cause recontamination of the rest of the batch. Grower treating or composting their own manure should have a documented procedure to follow. Growers purchasing manure should obtain a specification sheet from the manure supplier for each shipment of manure containing information about the methods of treatment. Remember, as with any other good agricultural practice record keeping is required.
References
. 2011. National Organic Program. Guidance Compost and Vermicompost in Organic Crop Production. . 2011. National Organic Program. Guidance Processed Animal Manures in Organic Crop Production.
. 2011. Commodity Specific Food Safety Guidelines for the Production and Harvest of Lettuce and Leafy Greens. California Leafy Green Agreement Anusuya Rangarajan, etal. 2000. Food Safety Begins on the Farm a Grower’s Guide. Cornell Good Agricultural Practices Program. Ithaca, NY. . 1986. United States Department of Health and Human Services – FDA and CFSAN. The Guide to Minimize Microbial Food Safety Hazards for Fresh Fruits and Vegetables.
Darbie Granberry. Handling Organic Fertilizers: Manures, Biosolids and Compost. USDA-CSRESS Proj. No. 00-51110-9722.