Agriculture Reference
In-Depth Information
Eructation
CH
4
Feed carbohydrates (cellulose, sugars, etc.)
Methanogenesis
Fermentation
H
2
CO
2
H
2
CO
2
CO
2
Acetic acid
Butyric acid
Propionic acid
VFA absorbed and used by
ruminant
Fig 9.1.
Simplified representation of the fermentation and methanogenic processes that occur in the rumen.
fermentation processes and the major resulting
fermentation products are volatile fatty acids
(VFAs). The main VFAs produced are propionic,
acetic and butyric acid, which are absorbed by
the animal through its rumen wall and can be
oxidized for energy production, used for fat pro-
duction and/or in the case of propionate, used
for gluconeogenesis (production of glucose)
(Bergman, 1990). Non-structural carbohydrates
from plants (e.g. starch) tend to be fermented to
propionic acid (though some dietary sugars are
fermented to butyric acid), while structural car-
bohydrates (e.g. cellulose) tend to result in the
production of acetic and butyric acid (Ellis
et al
.,
2008). When propionic acid is the end-product
of fermentation there is a net use of hydrogen
(H
2
), while there is a net production of H
2
when
acetic and butyric acid are the end-products of
fermentation (Russell, 2002).
The microorganisms responsible for enteric
CH
4
are called methanogens (classified within
the Domain Archaea) and are strictly anaerobic
(they require an oxygen-free environment to
live) (Janssen and Kirs, 2008). Methanogens
create CH
4
as an end-product of CO
2
reduction
with H
2
in their electron transport chains to pro-
duce energy required for their life processes,
though some methanogens may also reduce
formate (Russell, 2002). The removal of H
2
by
methanogens keeps the partial pressure of H
2
in
the rumen low enough to prevent the inhibition
of NADH-linked hydrogenases that would
reduce the overall efficiency of rumen fermenta-
tion (Russell, 2002). Often methanogens are
closely associated with the organisms that pro-
duce H
2
, and ecto- and endosymbiotic relation-
ships between methanogens and protozoa have
been observed (Finlay
et al
., 1994). Therefore,
reducing the amount of H
2
available to metha-
nogens or directly inhibiting methanogens or
methanogenesis (methane formation) processes
are the major ways to influence the total amount
of CH
4
produced from enteric sources.
Nitrous oxide emissions from enteric fer-
mentation are likely negligible and are often not
included in estimates for national GHG invento-
ries or life cycle assessments of emissions from
each sector of animal agriculture (Monteny
et al
.,
2001; Casey and Holden, 2006). Kaspar and
Tiedje (1981) found a small amount of N
2
O orig-
inating from rumen fluid supplemented with
nitrite and attributed it to dissimilatory nitrate
reduction processes; however, the rumen fluid
was incubated at temperatures below normal
livestock body temperatures. Some studies meas-
uring N
2
O emissions from cattle in chambers
have detected or quantified small amounts of
N
2
O, but those studies cannot separate emissions
