Annalisa Hultberg, Extension Educator, food safety
Many produce farms raise livestock such as chickens, goats, sheep and cattle alongside their fruit and vegetable crops. Some graze the livestock in the fields or in high tunnels during the winter or in the spring before planting the vegetables. While animal-based soil amendments such as manure and poultry litter can build the health, tilth, fertility and water holding capacity of your soil, they also can pose microbial risks and should be used safely to reduce the potential for causing illness. Here are some guidelines to help you minimize any potential risk of contamination and foodborne illness as you use these soil amendments this fall and into next spring.
My animals aren't sick, how dangerous can their manure be?
The answer is that all manure can carry a risk of microbial contamination and can contain pathogens that can make people sick. While many of these pathogens are normal residents in the animals’ digestive system and do not make them sick, there’s often no way to know whether or not they carry human pathogens. For example, chickens can harbor Campylobacter or Salmonella in their guts and rarely show signs of illness. The same is true for cows and goats that may carry E. Coli O157:H7.
These otherwise healthy animals shed pathogens in their manure, which can spread to feathers, fur, saliva, or other body parts. Therefore, farmers need to assume that anything coming from an animal or having had contact with an animal (e.g. manure, bedding, feathers, saliva) could contain those same pathogens.
How long should I wait between applying manure or grazing my animals and harvesting produce?
The best practice when using compost that has not been fully treated is to follow the National Organic Program guidelines for "day to harvest" intervals. Wait at least 90 days between application and harvest if the produce is not grown in contact with the soil, like tomatoes. Wait at least 120 days between application and harvest if the product is grown in contact with the soil, like melons or lettuce. This will reduce the potential for contamination significantly, because if there are pathogenic microbes present in the compost the time interval will allow time for many to die off. Some might persist even after this time period though, so still continue to use caution and apply the compost so that it doesn't directly touch the edible portion of the crop.
So when should I apply raw manure or graze animals before harvest?
On many farms the easiest way to follow the "days to harvest" intervals is to apply raw product or graze animals in the fall months, or on fallow fields. This ensures adequate time in between application and harvest. If you want to apply raw, untreated product in this spring from your animals, or graze them on land you plan to plant, ensure that you keep good records and follow the "days to harvest" intervals as outlined above.
What about chickens in my high tunnel through the winter?
If you have chickens in the high tunnel over winter to protect them, make sure to take them out of the tunnel early enough to follow the 90/120 day guidelines, which likely means it will still be winter for early season crops that might take only 30 days to mature, like lettuces. Make sure to wash all surfaces in the tunnel plastic well before planting. Putting down plastic mulch over the ground after incorporating the litter well into the soil will also reduce the potential for product contamination. Salmonella can persist in the soil depending on the concentration and number of birds in the tunnel, so proceed with caution if you are using your tunnel for poultry. Finally, consider growing lower risk products that are grown off the ground, like tomatoes, in the tunnels that have had poultry in them.
Can I make my own fully composted manure from my animals?
Yes. You can follow NOP guidelines and fully compost the manure from your animals. Fully composted manure that has been made according to the NOP guidelines of turning, temperature and with records does not need to be applied with the 90/120 day application to harvest days guidelines. You can also purchase this product, like Sustane.
For more information on compost and fall application, see previous post here from the manure management team at the University of Minnesota.
This article is written in part with material by Anne Sawyer, University of Minnesota Extension, Water Resources
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