Agriculture Reference
In-Depth Information
on rangelands and pastures. Aguerre
et al
. (2011)
fed dairy cows increasing amounts of forage
relative to concentrate and found CH
4
emissions
increased from 538 to 648 g per cow per day for
cows fed a 47:53 versus a 68:32 forage-to-
concentrate diet. While feeding higher ferment-
able carbohydrate (e.g. maize grain) diets can
produce lower CH
4
emissions, they are typically
more expensive and can lead to rumen acidosis
and a reduction of the animal's productive life,
which makes those diets more typical for 'fin-
ishing' cattle and sheep (Owens
et al
., 1998).
Furthermore, feeding all ruminants high concen-
trate diets minimizes the unique attribute these
animals have when compared with monogastrics:
they convert the most abundant organic mole-
cule on Earth, cellulose, into high-quality food
and fibre products (Oltjen and Beckett, 1996).
Ionophores are antimicrobials fed to live-
stock that can transport ions across plasma
membranes and include the compounds monen-
sin and lasalocid. Both of these compounds were
originally used as coccidiostats (to inhibit coc-
cidiosis) in poultry and later cattle, but they have
also been found to increase feed efficiency
(Tedeschi
et al
., 2003; Chapman
et al
., 2010).
The mode of action of ionophores for improved
feed efficiency is they can transport cations
(such as Na
+
) across the plasma membranes of
Gram-positive bacteria (which tend to be fibre-
fermenting bacteria that produce acetic acid),
thereby causing those bacteria to expend energy
on pumping out the cations rather than on
growth (Russell and Houlihan, 2003). Thus,
there is a net shift from Gram-positive to Gram-
negative bacteria within the rumen resulting in
a lower acetic-to-propionic acid ratio and
decreased CH
4
emissions, as less H
2
is available
for methanogenesis (Tedeschi
et al
., 2003).
However, ionophore research with dairy and
beef cattle has not always shown consistent
reductions in CH
4
emissions, and possible expla-
nations for the variable results include the iono-
phore dose, the time it is included in the diet
(e.g. rumen microbes adapt to the ionophore
over time) and the diet composition (Guan
et al
.,
2006; Hamilton
et al
., 2010). Further explora-
tion of the inconsistencies of ionophores needs
to be completed before these compounds can
definitively be proposed as a viable enteric CH
4
emission mitigation technique for ruminants.
Additionally, past research has found that up to
50% of monensin can pass through the digestive
tract intact in steers (Donoho
et al
., 1978), and
Varel and Hashimoto (1981) found that CH
4
production from anaerobically stored manure
was inhibited from cattle fed monensin. This
could be a beneficial, or in the case of manure
CH
4
biodigesters where CH
4
production is
desired, an unbeneficial side effect of feeding
cattle ionophores.
Alternative H
2
sinks (e.g. sulfates and
nitrates) are another enteric CH
4
emission miti-
gation strategy that has received more research
attention in recent years. Sulfate-reducing
bacteria utilize H
2
to reduce sulfate and other
oxidized sulfur compounds, and in an environ-
ment where sulfate is not limiting can out-
compete methanogens for H
2
(Ellis
et al
., 2008).
However, feeding diets too high in sulfate can
depress DMI and cause negative health effects,
so its use in reducing CH
4
emissions has been
limited so far, though the increased prevalence
of sulfur-containing distiller's grains from
ethanol production may contribute to decreased
CH
4
emissions from ruminants (Ellis
et al
.,
2008). Nitrates in the feed are first rapidly con-
verted to nitrite, which can be detrimental to
the animal if it accumulates at too high of a level
(it can convert haemoglobin to methaemoglobin,
preventing O
2
transportation throughout the
body) (van Zijderveld
et al
., 2010). However,
research has shown that slowing 'stepping up'
the amount of NO
3
−
in the diet can successfully
reduce CH
4
emissions in sheep and cattle with-
out causing any negative health effects (van
Zijderveld
et al
., 2010, 2011). It remains to be
seen if this mitigation strategy would be viable
on-farm, as the economic cost and risk of health
effects may be too high for some farmers to
accept. Acetogens (organisms that reduce CO
2
with H
2
to acetic acid) can be found in rumi-
nants and represent an ideal H
2
competitor
because acetic acid can be used by the animal as
an energy source; however, acetogens cannot
compete as well as methanogens for H
2
, which
makes establishing dominant populations in the
rumen difficult (McAllister and Newbold, 2008).
Plant-derived compounds such as tannins,
essential oils and saponins are being increas-
ingly researched as 'natural' alternatives to
antimicrobials like ionophores. Essential oils
(e.g. cinnamaldehyde and garlic oil) are so-
named in reference to their odour, not that
