Inside the "sniffing corral" at Watkinsville,
Georgia, soil scientist Lowry Harper checks equipment for an experiment
measuring methane emissions by cattle.
This is the first noninvasive outdoor measurement of methane emissions
It's relatively easy to measure emissions of greenhouse gases from the
exhaust pipe of a car or the smokestack of a factory.
But how about methane from rice paddies, landfills, termite mounds,
livestock waste, or cows' breath? And what of airborne carbon dioxide or
nitrous oxide from soil?
Lowry A. Harper, an ARS agricultural microclimatologist, and ARS soil
chemist Ronald R. Sharpeboth at the Southern Piedmont Conservation
Research Center in Watkinsville, Georgiameasure the gases with an array
of outdoor sampling tubes.
These connect to a laser spectrometer for trace gas analysis made in a
nearby trailer. The spectrometer is one of about 10 such instruments being used
in the world.
The first of these outdoor measurements came far from Georgianear
Canberra, Australia. There Harper and scientists from Australia's Commonwealth
Scientific and Industrial Research Organization were able to sample methane
emissions from grazing cattle.
Harper is currently repeating the Australian experiment at Watkinsville,
Georgia, with four Angus beef cows and four calves in a fenced pasture. He and
colleagues will do the same experiment at College Station, Texas, with four
crossbred beef cows and calves.
Soil chemist Ronald Sharpe seals connections on a sniffing
In these experiments, a perforated "sniffing" tube is placed along
each side of a fence enclosing four cows in a 24- by 24-meter pasture.
"The fence is 3 ½ meters high, which our experience shows is the
appropriate height to sample the gases emitted in this size pasture,"
Hoses draw air continuously into the trailer, where the laser spectrometer
measures the methane content of air entering and leaving the pasture area,
allowing calculation of the amount emitted by the cows.
"This and the Australian tests are the first noninvasive outdoor tests
of methane production by cattle," Harper says. "Prior tests were done
in indoor chambers or outdoors with sampling devices attached above the
From the outdoor test in Australia, Harper and colleague Tom Denmead found
that a cow grazing can emit more than 350 liters of methane per day. "That
more than estimates from tests of confined animals," he says.
The Australian studies showed that cows fed grain rather than pasture grass
emitted about 100 liters per day, about half as much as previous tests
Scientists found a 3-hour cycle of peaks and dips in methane production,
with the greatest emissions at dawn and dusk, presumably when the cows eat the
most grass. The cycle continues throughout the night as cattle awaken during
the night for food. Cows and other cud-chewers, such as sheep and goats,
release methane almost solely through belching, says Harper.
"The methane comes from fermentation in the cow's rumen, the first of
four stomachsand the one nearest the cow's mouth. The longer food stays
in this rumen, the more methane is produced. Oats and other grains are more
digestible than grass or hay, so they spend a briefer time in the rumen."
Support specialist Jeff Scarborough (left) and soil scientist
Lowry Harper check a part of the methane sampling system.
Harper says these studies show that "current climate change models used
for predicting animal methane emissions do not account properly for the large
effects of feed quality on methane production.
"So we can hope that total methane emissions from cattle throughout the
world can be significantly reduced through diet. This could be accomplished
either by feeding cattle grain in a feedlot or planting more digestible pasture
According to the indoor estimates of the Intergovernmental Panel on Climate
Change, cud-chewing animals account for about a sixth of the world's total
emissions of methane. Cattle account for about three-quarters of those
Notes Harper, "Each molecule of methane has about 20 times the global
warming potential of a carbon dioxide moleculepartly because it lasts
longer in the atmosphere, and partly because of the way it reacts with
Increasing atmospheric concentrations of methane are believed to be
responsible for about a quarter of the predicted global warming associated with
the so-called greenhouse effect.
Grazing beef cattle are the biggest contributors, since they make up about
half the 100 million or so head of cattle in this country. The rest are about
evenly split between dairy and beef cattle on feedlots.
Harper is also working with Sharpe to adapt the equipment to measure methane
emissions from landfills and nitrous oxide from farm fields.
"Nitrous oxide is produced as microbes eat nutrients in the soil. This
gas gradually filters upward through soil pores and cracks until it is released
into the air," Harper says. "We'd also like to measure carbon dioxide
released as the soil is plowed or tilled.
"If this equipment works, we could provide more appropriate information
to help make climate change models more accurate at predicting the emissions of
several greenhouse gases," he says. "We could also find ways to
reduce the emissions."
While this equipment should work on small animal waste lagoons, Harper and
Sharpe have developed a different system to measure methane, carbon dioxide,
and ammonia emissions from large dairy and swine waste lagoons on private
Harper explains that lagoons are large, shallow pits, from 0.25 to 5
hectares in size, where all water used in dairy or swine barns is stored. They
look like lakes, except for the color of some of them and for the continuous
methane bubbling. "Sometimes, huge bubbles bring large mats of organic
matter from the bottom mud up to the surface," Harper says.
Most of the wastewater comes from hosing down the barns to flush out manure.
As an environmental measure, farmers store the liquid manure in the lagoon and
apply the wastewater to their farmland as needed for fertilizer.
Harper and his colleagues mounted the equipment on a metal barge 2 meters
wide by 6 meters long, and floated the barge on pontoons out to the middle of
the lagoon. They let legs attached to the barge sink down for support. Then
they let air out of the pontoons until the barge sank a couple of inches below
the water's surface, thus keeping the barge's frame from interfering with the
Monitoring devices project from 20 centimeters to 270 centimeters above the
surface of the water. These instruments collect gases from the air and pump
them through tubes back to a trailer on the lagoon's banks. There the gases are
sampled and analyzed by one of the two laser spectrometers used by Lowry and
Lowry hopes the devices on the barge will not only help modelers better
evaluate the gases emitted from animal waste lagoons, but also establish
whether there's enough methane emitted to make it worthwhile for a farmer to
use the methane as fuel for an electrical generator. By Don
R. Sharpe are at the USDA-ARS
Phil Campbell, Senior, Natural Resource Conservation Center, 1420
Experiment Station Road, Watkinsville, GA, 30677; phone (706) 769-5631, fax
"Monitoring Methane" was published in the
June 1995 issue of
Agricultural Research magazine.