There are certain questions within our culture for which there are simply no good answers. For example, how many times have we heard the classic question, “If a tree falls in a woods and there’s no one to hear it, does it still make a sound?” One question I’ve been asked recently, for which the answer is equally elusive, is “How long must I wait to grow vegetables after applying manure to the field?” With the 2015 season quickly winding down, it will soon be time to start making plans for next year’s crops. Part of those plans will undoubtedly include the question of manure use.
While manure is a good source of plant nutrients and organic matter, it may also contain human pathogens that can be transferred onto fresh fruits and vegetables. After manure is applied to a field, the bacterial community in the manure changes as it adapts to conditions in the soil. Given all the variables involved, exactly what happens to the bacteria is anyone’s guess. Soil type, manure type, soil moisture levels, and temperature all play a role in how quickly manure degrades and the bacterial community changes. While research is starting to shed some light on the issue, the fate of human pathogens in manure applied to the soil is still poorly understood. This was noted by FDA in their latest draft of the Proposed Rule for Produce Safety. Publications supplemental to the proposed rule stated that:
“The agency is deferring its decision on an appropriate time interval [between manure application and crop harvest] until it pursues certain actions. These include conducting a risk assessment and extensive research to strengthen scientific support for any future proposal, working with the U.S. Department of Agriculture and other stakeholders.” (FSMA Proposed Rule for Produce Safety: http://www.fda.gov/Food/GuidanceRegulation/FSMA/ucm334114.htm)
The National Organic Program specifies an interval of 120 days between the application of raw manure and harvest for crops that come in contact with the soil and an interval of 90 days for crops that do not come in contact with the soil. These serve as good general guidelines, but the intervals were not developed to assure food safety.
Remember, in addition to any general guidelines, always follow the guidance given by your particular audit protocol or food safety plan.
Those using properly composted manure may apply it at any time, taking care to avoid application to the edible parts of the plants. Those producing crops with edible parts underground may also want to avoid applying composted manure while crops are in the field. Proper composting in this context involves a high-temperature process that kills most human pathogens. The high temperatures must be documented, along with other details of the composting process. A summary of what is involved in producing compost may be found at: http://content.ces.ncsu.edu/compost-production-and-use-in-sustainable-farming-systems.pdf.
Unless you have documentation that manure was composted properly, it should be treated as nothing more than aged raw manure, and there should be an interval of several months between application and harvest of a fruit or vegetable crop.
In addition to composting, other heat and chemical treatments can reduce pathogens in manure. Often manure treated with these methods is sold commercially as a bagged or bulk product. Growers using such products should get information about the manure treatment from the supplier, and ask specifically whether the treatment is documented to kill human pathogens.
One of the best ways to utilize manure is to apply it in the fall. A fall application, followed by incorporation and a cover crop, is a good way to insure a lengthy application-to-harvest window. One additional advantage is that fall-applying manure cuts one more job from what is, for most people, an increasingly hectic spring season. For those wishing to use a fall manure application, Purdue’s Midwest Cover Crops Field Guide (ID-433, https://mdc.itap.purdue.edu/item.asp?Item_Number=ID-433) can provide valuable cover crop information. Purdue’s Manure Management Planner, although designed for field crops, can help with estimating nutrient content from various manures (http://www.purdue.edu/agsoftware/mmp/).
An additional option may be to simply use manure only on agronomic crops. As a whole, the Midwest is a very strong agronomic region. Corn and soybeans are still the main crops. Where possible, growers can use this to their advantage by applying manure to agronomic crops in the year before fields are rotated into vegetables. This gives an interval of a year or more and growers can still reap some benefits (increased organic matter, plant nutrients, etc.) from the manure application, with reduced risk to their vegetable crops.
If you must apply manure in the spring, avoid planting short-season crops in fields that have received an application. Long season vegetables, such as staked tomatoes, whose edible parts do not contact the ground, carry the least risk of being contaminated with a foodborne pathogen. Other options would be to grow late-season crops that normally are not consumed, such as pumpkins or gourds. Yet another option would be crops that are cooked prior to eating, such as sweet corn.
Across the state, growers have worked hard to incorporate food safety into the culture of their farms. Paying attention to how and when manure is applied is a good way to reduce the risk of a future outbreak.