Managing Manure Nitrogen To Curb Odors
Preliminary experiments show that weekly applications of a urease inhibitor on
feedlot surfaces such as this one blocks ammonia production in manure, thus
preserving nitrogen and reducing emissions.
People often think of livestock manure as waste, but scientists believe such
animal byproducts should be considered production resources.
They are trying to find new ways to use these resources from cattle feedlots
and hog farms in farming operations to address environmental concerns and
residential complaints about odors, says Vincent H. Varel. He is an
Agricultural Research Service
microbiologist at the Roman L. Hruska U.S. Meat Animal Research Center in Clay
Manure from beef cattle feedlots could be valuable for its nitrogen
fertilizer, says Varel. But unfortunately, half to three-fourths of that
nitrogen never reaches the field.
Most of the loss occurs through a process called hydrolysis. Microbes in
animal manure and soil produce the enzyme urease that converts the urea in
urine into ammonia that escapes into the air.
This same process happens in urea-based commercial fertilizers. To prevent
ammonia loss, fertilizer manufacturers routinely add urease
inhibitors--actually, chemical relatives of urea--to block hydrolysis, thus
preserving nitrogen until it's taken up by plants.
In laboratory experiments, Varel also blocked this nitrogen loss from manure
by adding urease-inhibiting compounds. He mixed either feedlot cattle manure or
swine manure with cattle urine into a slurry. Like modern waste-handling
systems, the slurry contained naturally occurring urease-producing microbes.
Varel added as little as 10 milligrams (35 millionths of an ounce) of
urease-inhibiting cyclohexylphosphoric triamide (CHPT) per liter of manure
slurry mixture. In untreated samples, the urease hydrolyzed nearly all the urea
within 1 day. But in treated samples, hydrolysis was completely prevented for
at least 4 days.
Another urease inhibitor, phenyl phosphorodiamidate (PPDA), produced similar
Varel found that adding more urease inhibitor each week provided longer term
control. For example, 100 milligrams of PPDA added weekly to cattle waste
preserved 70 percent of the urea for 28 days. Adding just 10 milligrams weekly
preserved 38 percent.
The researchers are trying to determine why the larger amount of inhibitor
didn't preserve even more urea. An understanding may lead to scaled-up
farm-size applications. "Our laboratory studies with CHPT and PPDA just
started us on the learning curve," Varel says.
He tested a third urease inhibitor, n-(n-butyl) thiophosphoric triamide
(NBPT). This compound is currently being used as a nitrogen preservative in
no-till cropping systems.
Preliminary experiments showed NBPT works even better in the feedlot than in
the laboratory. Under feedlot conditions, Varel explains, NBPT is exposed to
air that converts the compound into a more effective urease inhibitor. The
researchers spread NBPT over a feedlot surface once each week. As cattle
urinate, the chemical binds to urea and blocks ammonia production, thus
Varel says urease inhibitors will reduce ammonia emissions, which contribute
to odors. However, other odor-reducing compounds will be needed to more fully
control a variety of unpleasant-smelling volatile compounds from manure.
Encapsulating these mixtures in starch or other protective materials could
ensure slow release of the active compounds and require fewer applications to
cattle feedlots, manure slurry tanks, and covered lagoons used on livestock
farms.--By Ben Hardin,
Agricultural Research Service Information Staff.
Vincent H. Varel is at the
USDA-ARS Roman L. Hruska U.S. Meat Animal
Research Center, P.O. Box 166, State Spur 18D, Clay Center, NE 68933; phone
(402) 762-4207, fax (402) 762-4209.
"Managing Manure Nitrogen To Curb Odors " was published in
the October 1998 issue of Agricultural Research magazine. Click here to see this
issue's table of contents.