Agriculture Reference
In-Depth Information
crop by-products and stubble (Masters and
Wright, 2005).
Grazing livestock are significant contribu-
tors to anthropogenic greenhouse gas produc-
tion, particularly through the production and
release of methane gas. Methane acts as a potent
greenhouse gas with 25 times more global
warming potential than carbon dioxide.
Methane is an end-product of enteric fermenta-
tion of plant material by the complex microbial
ecosystem that resides in the rumen and is
released into the environment, mostly through
eructation (Murray
et al
., 1976). Enteric meth-
ane is produced in the rumen when hydrogen is
released by other microorganisms (e.g. fungi,
rumen protozoa) during fermentation and used
by methanogenic archaea, also called methano-
gens, to reduce carbon dioxide. Thus, methane is
produced by the microorganisms living inside
the rumen rather than by the animal itself.
Domesticated ruminants (cattle, goats, sheep
and water buffalo) produce as much as 86 million
metric tons of methane per year (McMichael
et al
.,
2007), of which, approximately 55.9 million met-
ric tons of enteric methane are from non-dairy
cattle, 18.9 million metric tons are from dairy cattle,
9.5 million metric tons are from sheep and goats,
and 6.2-8.1 million metric tons are from water
buffalo (Johnson and Ward, 1996; McMichael
et al
., 2007). In comparison, the global yearly
enteric methane contribution from non-ruminant
livestock has been estimated to be 0.9-1.1 million
metric tons from camels and pigs and 1.7 million
metric tons from horses (Johnson and Ward,
1996). In many countries, ruminant livestock is
the single largest source of methane emissions
from the agricultural sector. In Australia and the
USA, enteric methane accounts for approximately
70% and 73%, respectively, of agricultural meth-
ane emissions (Environmental Protection Agency,
2010; National Greenhouse Gas Inventory, 2010).
This equates to nearly 55.6 million metric tons
carbon dioxide (CO
2
) equivalents of methane from
Australian livestock and nearly 141 million met-
ric tons CO
2
equivalents of methane from US live-
stock. This represents a significant loss of feed
energy from animal agriculture and an economic
loss to farmers because the feed is converted to
methane instead of production outputs, such as
wool, beef or milk.
Because the continuous growth of the
human population is expected to result in
an increase in the number of domesticated
ruminants, high methane production from
domesticated ruminant livestock is an impor-
tant undesirable trait due to its negative
impact on animal production and its contribu-
tion to climate change. Thus, reducing meth-
ane emissions by livestock has become an
integral part of sustainable agriculture and
climate change abatement strategies (Thorpe,
2008). Opportunities to manipulate gas pro-
duction, either per animal or per unit of prod-
uct, through feeding strategies and rumen
manipulation, are now being identified.
Rumen Microbiome
The microbial ecosystem is very complex and
involves thousands of species of bacteria (10
10
-
10
11
cells ml
−1
), archaea (10
7
-10
9
cells ml
−1
),
protozoa (10
4
-10
6
cells ml
−1
), fungi (10
3
-10
6
cells ml
−1
) and viruses (10
9
-10
10
cells ml
−1
)
(Klieve and Swain, 1993; Koike and Kobayashi,
2009), which play an important role in the
digestion of feed, and interact with their host
and each other. The rumen microorganisms
also supply energy and protein to the host in
the form of volatile fatty acids and microbial
protein (Hungate, 1966). Not only is the eco-
system complex, but also relatively poorly
understood, particularly inter-species interac-
tions and interactions with the host. For that
reason, when considering methane mitigation
strategies, the relationship that rumen metha-
nogens have with other microorganisms is
especially important. For the context of this
chapter, discussion will focus on targeting the
methane-producing methanogens and or
decreasing methanogenesis.
Methanogenic archaea
There are over 120 species of methanogens rep-
resenting seven orders, which include 13 fami-
lies and 35 genera (Euzéby, 2012). For a recent
review of the diversity of gut methanogens in
herbi vorous animals, see St-Pierre and Wright
(2012). Most methanogens (e.g.
Methanobrevibacter,
Methanobacterium, Methanomicrobium
) are able
to use hydrogen, carbon dioxide and formate as
