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New strategy to reduce livestock ammonia emissions

May 12, 2011 – As concerns
about air pollution from large dairies and other concentrated animal feeding
operations (CAFOs) continue to mount, scientists are reporting a practice that
could cut emissions of an exceptionally abundant agricultural gas – ammonia –
by up to 30 percent.
May 12, 2011 – As concerns
about air pollution from large dairies and other concentrated animal feeding
operations (CAFOs) continue to mount, scientists are reporting a practice that
could cut emissions of an exceptionally abundant agricultural gas – ammonia –
by up to 30 percent.

In the May-June 2011 issue
of the Journal of Environmental Quality, a team led by Mark Powell, a soil
scientist with the U.S. Department of Agriculture’s (USDA) Agricultural
Research Service’s U.S. Dairy Forage Research Center in Madison, WI, describes
how natural plant compounds known as tannins can reduce both the amount of
nitrogen cows excrete in urine, and the action of a microbial enzyme in manure
that converts the nitrogen into ammonia on the barn floor.

The U.S. EPA already
monitors ammonia emissions from large animal operations under the “Superfund”
act, and in April a coalition of citizen groups petitioned the agency to begin
regulating ammonia under the Clean Air Act, as well. Besides its pungent smell,
ammonia that volatilizes from cattle manure is highly reactive in the
atmosphere, forming particulates that travel long distances and contribute to
environmental problems such as acid rain, nutrient pollution, and smog.

Feeding tannins to cattle
could not only help dairy farmers reduce these impacts and meet regulatory
standards, Powell says, but tannins could also boost nitrogen use efficiency in
cows, thereby decreasing the need for expensive protein supplements. Only 20 to
35 percent of feed nitrogen ends up in milk on commercial dairy farms, with the
remainder excreted about equally in manure and urine as the compound, urea.

Urea is produced when
nitrogen-rich proteins break down mainly in the cow rumen, forming ammonia gas
that’s eventually converted to urea before being excreted. Tannins are thought
to cut urea production by somehow allowing more protein to escape digestion in
the rumen and enter the cow intestine, where it’s used more efficiently to
produce milk protein.

Tannins are perhaps best
known for their role in leather tanning, but Powell began investigating them in
ruminant feed more than two decades ago in West Africa. In the communities
where he worked, tannin-rich shrubs were grown as windbreaks, and to amend the
soil and feed livestock. Tannins in the diets of cattle, sheep and goats are in
fact well-studied in the tropics, where the vegetation tends to be naturally
higher in the astringent plant chemicals, Powell explains. “But tannin
research, in terms of ruminant nutrition, is relatively new in temperate
environments.”

In the new study, Powell
and dairy scientists from the University of Wisconsin-Madison fed tannin
extracts from red quebracho and chestnut trees to dairy cows that also received
two concentrations of crude protein: a low level of 15.5 percent protein, and a
higher one of 16.8 percent. What they found is that dietary tannin cut ammonia
emissions from the cows’ manure by an average of 30 percent at the low protein
level, 16 percent at the high level, or 23 percent overall. In other words,
cows that consumed tannin expelled significantly less urea, thus making less
available for conversion to ammonia.

But a drop in urea
production wasn’t the only effect. To his surprise, Powell discovered that
tannins also appear to inhibit urease, the enzyme that converts urea to
ammonia. Urease activity in the feces of tannin-fed cows was significantly
lower than in the feces of control animals, resulting in an 11 percent drop in
emitted ammonia – or one-third of tannin’s total impact on emissions at the
high protein level. And when the researchers applied tannin directly to manure
on the barn floor (rather than feeding it to cows), the effect was even greater:
Ammonia emissions declined by nearly 20 percent.

The tannin sources
investigated in the study are already approved for animal feed, and “the levels
we used amount to pennies per cow per day,” Powell says, suggesting they could
offer a cost-effective means to cut ammonia losses from the barn floor, as well
as from manure that’s applied to farm fields as fertilizer. Powell is now
working with chemists to determine exactly which compounds in the tannin
mixtures produce the effect, with an eye toward manufacturing a synthetic
substitute later on.

In the meantime, he has
set his sights on another important air pollutant that is prodigiously produced
by cows. “We have another experiment looking at higher doses of tannin in dairy
cattle,” he says. “We want to see if it can reduce methane emissions.”

View the article abstract
at https://www.agronomy.org/publications/jeq/abstracts/40/3/907.