Agriculture Reference
In-Depth Information
were found to be highest in the summer months
and lowest in the winter and spring, while barn
emissions were found to be the highest in the
spring and autumn and lowest in the winter and
spring (Rumsey et al ., 2012). The study was not
exhaustive enough to determine the mecha-
nisms behind the seasonal variation. Chung
et al . (2010) measured VOC emissions from six
dairies in the San Joaquin Valley of California
and found VOC fluxes from alleyways in free stall
barns were significantly lower after flushing
(a method of cleaning and removing the manure
from the barns) than before flushing. Wind
velocity over the surface of manure is another
important factor contributing to the emission
rate of VOCs, with higher wind velocities corre-
sponding to higher emission rates (Parker et al .,
2010). In summary, VOC emissions from animal
manure seem to be influenced by environmental
factors and manure management techniques,
though further research is needed to better
understand the drivers of fresh and stored VOC
emission variation.
enteric CH 4 emission mitigation strategies will be
discussed: fats, forage quality/forage-to-concen-
trate ration, ionophores, alternative H 2 sinks
and plant compounds.
Fats (specifically unsaturated fatty acids)
can reduce enteric CH 4 emissions in two ways:
acting as a sink for H 2 during biohydrogenation
and causing toxicity to H 2 -producing organisms.
When unsaturated fats (those that contain dou-
ble carbon-to-carbon bonds) enter the rumen,
rumen bacteria perform a process called biohy-
drogenation, during which they saturate the
double bonds by using H 2 . Therefore, biohydro-
genation processes can act a competitor for H 2
with methanogens and have the potential to
reduce CH 4 production in the rumen (Ellis et al .,
2008). Additionally, unsaturated fats can be
toxic to H 2 -producing protozoa that often have
endo- and ectosymbiotic relationships with
methanogens (Hegarty, 1999). More unsatu-
rated fatty acids have been found to have a
greater toxic effect on protozoa and some Gram-
positive bacteria, which is proposed to be the
result of their ability to disrupt plasma mem-
branes (Maia et al ., 2007). Research feeding
ruminants fats has shown variable effects on
CH 4 emissions that is believed to be due in part to
the fatty acid profile of the fat source and
the concentration at which the fats are fed
(Beauchemin et al ., 2008, 2009). The concen-
tration of fat in the diet is an important consid-
eration because if too much fat is fed (over 5-6%
of dry matter intake (DMI) for dairy cattle)
rumen fermentation processes can be negatively
impacted and DMI and animal performance can
be deleteriously affected (Martin et al ., 2008).
Additionally, the cost and contribution of sourc-
ing some of the fats fed in experimental diets
(e.g. coconut oil) should be considered in the
total GHG emissions from livestock systems.
Forage quality and the forage-to-concentrate
ratio can greatly influence CH 4 emissions from
ruminants as diets that contain more structural
carbohydrates tend to produce greater amounts
of acetic acid as an end-product, and subse-
quently more H 2 and CH 4 than those diets that
contain more non-structural carbohydrates
(Ellis et al ., 2008). Consequently, the high con-
centrate diets fed to cattle in feedlots tend to
result in fewer CH 4 emissions per unit of feed
compared with higher fibre diets such as those
consumed by dairy cattle and ruminants grazing
Mitigation Options
GHG emission mitigation opportunities
Emissions of GHG across all farms within each
sector of animal agriculture (e.g. beef cattle pro-
duction, dairy production, swine production)
vary widely due to differences in genetics, feed-
ing regimes and management across farms. As a
result, using generalized emission factors per
animal do not accurately reflect the emissions
from individual animal farming operations and
mitigation strategies should seek to reduce
CO 2 -eq emissions per unit of output (kg of milk,
kg or pork) rather than CO 2 -eq emissions per
animal. Thus, the overview of GHG mitigation
strategies outlined below will be framed in the
context of their ability to reduce CO 2 -eq emis-
sions per unit of output.
Mitigating CH 4 emissions from enteric fer-
mentation directly requires reducing the output
of methanogens residing in the gastrointestinal
tract, which can be achieved by reducing the
supply of CH 4 precursors, inhibiting the metha-
nogenesis process and/or reducing or eliminat-
ing methanogen populations. The following
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