June 8, 2006: The ideal sustainable livestock
farm is a closed system made up of crops grown to feed the
livestock and humans who live there, and manure that is returned
to the soil to ensure fertility. As with everything else in
life, however, the ideal is rarely attained.
Vegetable farmers need manure to feed their crops. Dairy
and meat producers usually have more manure than they can
use. It would seem the perfect arrangement for vegetable growers
to haul away the manure from a nearby farm (in some cases,
your neighbors are happy to deliver it) to apply it to fields.
But modern agricultural inputs—especially those used
in confined, intensive settings—means that growers,
especially those who are certified organic, need to ask a
few questions before accepting that generous offer.
Recent studies have shown that manure can contain residues
of antibiotics and other medications routinely given to livestock,
as well as pesticides and heavy metals such as copper, zinc,
arsenic, cadmium and lead. Appropriate composting may take
care of some of these substances, but some it will not.
Organic certification standards allow raw manure to be spread
on fields at least 90 days before harvesting crops where the
edible portion does not touch the soil (i.e. sweet corn) and
120 days where the edible portion does touch the soil (lettuce,
carrots, onions, etc.). All other animal manure must be composted,
and standards require that manure be shaped into a windrow,
turned a minimum of five times in 15 days and achieve a recorded
temperature between 133°F and 170°F in order to be
considered finished compost. Anything else is termed “raw
manure,” regardless of how long it has been piled up
behind the barn.
Organic standards prohibit the use of sewage sludge, sometimes
called biosolids, because of the possibility of heavy metal
contamination. Municipal yardwaste, such as grass clippings
and leaves, fall under the same category as manure—not
prohibited, but demanding scrutiny as to possible contamination
from pesticides, herbicides and synthetic fertilizers.
||"Residues of antibiotics and
other drugs, such as Ivermectin, a popular worming medication,
have been found in animal manures, and traces of these
drugs can be found in plants grown in soil where residue-laden
manure was applied."
Antibiotics, such as tetracycline, are routinely fed to swine,
poultry and dairy cattle. Residues of antibiotics and other
drugs, such as Ivermectin, a popular worming medication, have
been found in animal manures, and traces of these drugs can
be found in plants grown in soil where residue-laden manure
was applied. In a study published in the Journal of Environmental
Quality in October, 2005, Kumar et.al. found that antibiotic
residues were detected in green onions, corn and cabbages
grown with the addition of manure containing antibiotic residues.
In another article in the same publication, Chandler et.al.
found that antibiotic residues in manure still retained their
bacteria-killing properties and could be contributing to the
formation of new strains of antibiotic-resistant bacteria.
These manures were not composted.
Other substances that can contaminate manure are metals such
as copper and zinc, which are part of the recommended minerals
added to livestock feed, and cadmium and lead, which can enter
the chain through crops grown in contaminated soil or air
pollution in industrial areas (see sidebar). In a paper published
in The Scientific World in 2002, researchers Allan
Barker and Gretchen Bryson found that composting can significantly
reduce pesticide residues and can bind heavy metals and reduce
their uptake by plants.
One substance of particular concern is arsenic. Recent news
stories reported that arsenic was found in several commercial
brands of chicken and samples from ten fast food restaurants.
The study was conducted by Dr. David Wallinga, Director of
Food and Health for the Institute for Agriculture and Trade
a Minnesota-based, nonprofit advocacy group promoting sustainability
and family farms.
Arsenic is included in Roxarsone, a medication sometimes
fed to broilers raised in confinement to protect against outbreaks
from coccidia. The fact that arsenic is detectable in the
meat of chickens begs the question: How much is ending up
in the manure, and where is it going then?
Arsenic: common additive, complex agricultural
Brian Baker, research director of the Organic Materials Review
located in Eugene, Oregon, is trying to answer that question.
“We're looking at ways to reduce the accumulation of
heavy metals, and arsenic is of particular concern,”
Baker said. “Arsenic toxicity is problematic in a number
of ways. The arsenic substitutes for phosphates causing a
deficiency in plants. Beans, peas and other nitrogen fixing
plants are very susceptible to this phosphate deficiency.”
Arsenic was one of the first-generation herbicides and was
used as a pesticide in apple orchards. Soil scientists noticed
that orchards with high arsenic content in the soil couldn’t
grow clover (another nitrogen fixer) as an under crop.
“Organic farmers don’t want arsenic in the soil,”
Baker pointed out. “It will accumulate in crop tissue
and can pose a human health hazard. Everything goes somewhere.
We've been looking at [arsenic] levels in the 30-40s ppm range.
We’re not sure yet what the levels should be, but it
definitely should be a concern.”
||"If a grower has to choose between
manure that may contain antibiotic residue and manure
that may contain arsenic, Baker recommends staying away
from the arsenic. Antibiotics break down quickly and the
composting process should take care of them. Not so with
If a grower has to choose between manure that may contain
antibiotic residue and manure that may contain arsenic, Baker
recommends staying away from the arsenic. Antibiotics break
down quickly, and the composting process should take care
of them. Not so with arsenic. “Heavy metals are more
of a concern,” Baker said. “Organic growers are
better off with dairy or layer manure.”
So perhaps organic growers should avoid importing manure
altogether and stick to buying compost and fertilizers? That
isn’t a fool-proof solution either. Prepared fertilizers
are expensive, and Baker recommends staying away from commercial
compost unless growers can be absolutely sure what is in it.
According to Baker, during the debate on whether to allow
sewage sludge in organic farming, the question of manure from
factory farms was raised. Unlike the European Union, The U.S.
has no definition of Concentrated Animal Feeding Operation
(CAFO) or “factory farms” and does not prohibit
this manure from being used by certified-organic growers.
Many scientists and sustainable farming activists are of
the opinion that manure from factory farms contains as many
heavy metals as sewage sludge and should be prohibited in
organic agriculture. The EPA limits the amount of heavy metals
in sewage sludge, but manure from factory farms is unregulated.
“There are operations that will take sludge, CAFO manure
and urban green waste and make it into commercial compost,”
Always know your source and their
The best insurance with imported manure is to know your
source. An organic grower for almost 20 years, Darrell Frey
Sisters Farm in northwestern Pennsylvania has imported
many tons of manure to his farm. He had a long-standing agreement
with a race-horse stable where he parked his dump truck at
the stables and the owners filled it up for him every week.
The manure was then driven back to the farm and composted.
Some of it was formed into windrows to be used in the vegetable
gardens outside and some was loaded into bins in the bioshelter,
or solar greenhouse, to be used in the indoor planting beds
and in potting soil. The composting process also provides
bottom heat for starting seedlings on top of the compost chambers,
and fans can be used to circulate heat through the indoor
Like many vegetable growers, Frey doesn’t have the
time or space for farm animals. The bioshelter is home to
a flock of laying hens that are fed organic feed from which
they produce enough manure in a year to fill one of the compost
chambers. To meet the rest of his compost needs, Frey must
either import manure or use approved fertilizers. Fertilizer
can be expensive, but manure has its drawbacks, too. “My
biggest problem with imported manure was weeds,” Frey
said. “Every year we seemed to find new varieties.”
||"As with most aspects of sustainable
farming, education is your best bet. Talk to neighbors
about what they are feeding their animals and the possible
effects it could have."
Frey is currently getting his manure from a nearby farmer
who owns horses and other livestock. Before making the agreement,
Frey went to the farm and checked out the agricultural practices
to ensure that he wouldn’t be bringing anything onto
the farm that he didn’t want.
According to Baker, this is a good idea. “No matter
what nutrient source you use, it's not going to be perfectly
clean,” Baker said. “Feed sources need to be monitored
as well. Choose manure from feed sources that are relatively
uncontaminated.” Baker also recommends layering suspect
poultry or dairy manure with high-carbon organic matter during
composting. This should take care of antibiotics and pesticide
Where, what and how
The most important considerations when importing manure
- Where is it coming from?
- What did the animals eat?
- How will it ultimately impact my crops?
A grower’s best bet is horse manure, because antibiotics
are usually not a concern, followed by dairy and layer manure.
Baker recommends staying away from manure from factory farms,
particularly hog and broiler operations which may rely heavily
on drugs. Another consideration is that manure from confined
hogs often contains high levels of copper.
As with most aspects of sustainable farming, education is
your best bet. Talk to neighbors about what they are feeding
their animals and the possible effects it could have. Chances
are they will be just as concerned about what is going into
the soil as you are—especially if they are buying your
potatoes and chard.