Conventional livestock manure used to fertilize organic crops can contain unwanted antibiotics and chemicals, but time and composting can greatly reduce those levels, experts say.
Several antibiotics found in hog and turkey manure were found to be taken up by vegetable crops and stored in the plant tissue, said Satish Gupta, a soil science professor at the University of Minnesota.
In terms of a health risk, the antibiotics were “not really that big a deal,” as they were detected at a miniscule level of about 10 micrograms per kilogram of plant material, Gupta said.
A person would need to eat more than 100 pounds — 50 to 75 kilograms — of lettuce a day, for example, to hit the daily intake limit for such antibiotics, he said.
“We didn’t find very much concentration in the vegetable,” Gupta said.
Organic crop producers must often rely on manure from conventional livestock operations due to the availability of the material, he said. “There’s not that many big organic farmers.”
Composting the raw manure or distributing in the field long before harvest will further reduce the amount of antibiotics accessible to plants, Gupta said.
“Most of it will degrade, but it doesn’t mean it will have none in there,” he said. “The longer you keep it on the land, it will degrade, and if you compost it, it will degrade.”
The key to reducing antibiotics as well as pesticides is aerobic composting, which ensures proper oxygen flow to beneficial microbes in the manure, said Elaine Ingham, chief scientist at the Rodale Institute, an organization that studies organic practices.
Bacteria and fungi consume unwanted chemicals in the manure, Ingham said.
Aerobic composting is generally accomplished by turning the compost pile regularly to ensure the oxygen isn’t used up and the temperature doesn’t get too high, she said.
The frequency of turning the compost will depend on its composition, with higher levels of nitrogen needing more oxygen, Ingham said.
Eventually, beneficial microbes will establish structures in the compost pile that will allow them to continue their activity as the material cools down, she said.
“It becomes porous,” Ingham said. “Oxygen is going to be able to blow through.”
Beneficial microbes are even able to mitigate the presence of heavy metals in manure by binding them with materials like humic alic or folic acid, she said.
While the heavy metals will still exist, their release will occur at an extremely slow level, Ingham said.
Allowing the compost pile to use up the oxygen and attain excessively high temperatures will prevent such binding and also waste the manure’s nitrogen content, she said.
Apart from conventional manure, organic production faces similar questions about using irrigation water that may contain runoff from conventional fields, said Richard Carr, a technical specialist for Oregon Tilth, a certification agency.
Organic regulations are “practice standards” that require farmers are using natural materials, he said. Farmers are allowed to use raw manure as long as it’s applied 90-120 days before harvest, depending on the crop.
The measure is primarily intended to improve food safety, but if manure were known to contain contaminants, the grower would be expected to find another source or mitigate the issue, he said.
The issue of manure involves a balancing act between avoiding contaminants and giving growers economic fertilizer options, Carr said.
“If you take away manure and compost as soil building blocks, you leave them with nothing,” he said. “The arsenal for organic growers is pretty limited.”
Bryce Purtzer, co-owner of J&D Fertilizers in Canby, Ore., said the potential chemicals in manure will depend on the source.
His company makes composted fertilizer exclusively from chicken manure. The poultry are raised in housing facilities and aren’t exposed to pesticides or herbicides, Purtzer said.
Chickens raised for eggs are rarely treated with antibiotics compared to those raised for meat, he said.
Purtzer said he has tested his composted bulk fertilizer and found beneficial microbes.
“If there were antibiotics present, I don’t think the bacteria would be present,” he said.