Agriculture Reference
In-Depth Information
Livestock have a low N retention value,
where less than 30% of the fed N is used by the
animal. Some 50-80% of the unused N is
excreted in urine and the remainder in the faeces
(McCrory and Hobbs, 2001). As a result of this,
livestock manure (urine and faeces) contains a
high fraction of the fed N in the animal's diet.
However, much of this excreted N is not recycled
through application to crop land. About 75% of
the manure N from beef cattle is lost as a combi-
nation of runoff, NH
3
volatilization and denitrifi-
cation, prior to its use as a soil amendment for
crop growth (Eghball and Power, 1994). The
excreted urinary urea is rapidly converted
(hydrolysed by microbial urease in the faeces) to
NH
3
that is emitted into the atmosphere. The
rate of this conversion depends on temperature,
pH and moisture content. The conversion to NH
3
in faecal matter is slower to volatilize during
storage and land applications. Bierman
et al
.
(1999) measured the N content of manure to
infer that between 57% and 67% of the excreted
N in manure in a beef feedlot was lost as NH
3
volatilization (function of diet). Actual NH
3
emission measurements confirm the large emis-
sion of NH
3
from beef feedlots, ranging from
27% in winter to 55% in summer of the fed N as
reported by Todd
et al
. (2005). Flesch
et al
.
(2007) and McGinn
et al
. (2007) reported
63-65% and 63% of the fed N was lost as NH
3
in
a feedlot, respectively.
Ammonia emission continues after manure
is applied to cropland. The proportion of N content
of the manure emitted as NH
3
to the atmosphere
depends on: the type (solid or slurry); manure
additives (McCrory and Hobbs, 2001), e.g. acidi-
fication (Petersen
et al
., 2012); application tech-
nique, e.g. injected or surface applied (Bittman
et al
., 2005); and the pre- and post-surface man-
agement, e.g. tillage, irrigation (McGinn and
Sommer, 2007). In general, reducing the expo-
sure of manure to air reduces the NH
3
emission.
Globally, the release of NH
3
from livestock
manure is recognized as the major source of
atmospheric NH
3
, accounting for 50% of all ter-
restrial sources (National Research Council,
2003). Once released to the atmosphere, a frac-
tion of the NH
3
is deposited back to the local sur-
face while the remainder reacts with atmospheric
acids to form particulate matter (PM) less than
10 μm in diameter. The formation of these fine
aerosols can be an air quality issue (McCubbin
et al
., 2002; Erisman and Schaap, 2004) espe-
cially in confined air sheds where high concen-
trations of PM pose a health risk to people
(Popendorf
et al
., 1985) and animals (MacVean
et al
., 1986). The formation of fine aerosols can
also cause visibility degradation (Barthelmie and
Pryor, 1998). The eventual deposition of NH
3
to
land can also create problems due to acidifica-
tion of soil (van Breeman and van Dijk, 1988)
and elevated soil nitrogen concentrations caus-
ing changes in plant species diversity of natural
ecosystems (Sutton
et al
., 1993).
Source and Impact of Methane
Methane is generated from the fermentation of
carbohydrates that occurs during the digestion
of feeds in ruminant animals (e.g. cattle and
sheep) and during the anaerobic storage of live-
stock (ruminants and non-ruminants) manure.
About 90% of the enteric CH
4
produced by sheep
(ruminant) was reported by Murray
et al
. (1976)
to originate in the rumen while 10% occurred in
the large intestine. However, of this latter 10%
generated in large intestine, approximately 90%
is absorbed into the blood and is released during
respiration. It follows that ruminants eructate
and expire the majority (
c
. 99%) of the enteric
CH
4
through the nostrils and mouth while only
about 1% is lost through the rectum. Although
feed is mostly digested in the rumen, many die-
tary factors increase the proportion of feed
digested post-ruminally. For example, decreas-
ing the particle size of feeds, increasing the level
of intake, use of treatments such as heating to
reduce ruminal degradability, and use of resist-
ant starches, can increase post-ruminal diges-
tion (Galyean and Owens, 1991).
The anaerobic storage of manure in open
lots, lagoons and compost piles is also a source of
atmospheric CH
4
. Leytem
et al
. (2011) investi-
gated the methane emission from dairy wastewa-
ter and compost piles of manure and found
emissions of 103 and 13.5 g m
−2
day
−1
, respec-
tively. Todd
et al
. (2011) reported a mean CH
4
emission from an aerobic dairy lagoon in Texas
to be 40 g m
−2
day
−1
, or 211 g per animal per day.
Methane from livestock significantly con-
tributes to the build-up of GHG in our atmos-
phere. Lassey (2008) estimates enteric CH
4
