Agriculture Reference
In-Depth Information
rumen cannula can contribute to elevated SF
6
concentrations (Beauchemin
et al
., 2012). In
this case, the assumption that all the SF
6
exits
the rumen via the same path and in the same
proportion as the CH
4
cannot be assumed.
Another limitation of this technique is
related to its application for evaluating treat-
ment differences. Since the technique uses the
animal as the measuring unit, the technique
error as well as the within- and between-animal
variability must be well understood before con-
sidering using the SF
6
tracer technique for treat-
ment differences. Despite these limitations,
earlier work by Ulyatt
et al
. (1999) reported that
with sufficient animal numbers, the SF
6
tracer
technique gave similar emissions to other emis-
sions techniques. They reported that the tech-
nique is especially useful for grazing CH
4
measurements, but that there is a need to know
the dry matter intake accompanying the CH
4
emissions, since intake affects CH
4
emissions
and confounds treatment differences.
Enteric tracer technique
The most widely utilized tracer technique for
determining enteric CH
4
emissions in ruminants
was developed by Johnson
et al
. (1994) where
the tracer gas is SF
6
. The SF
6
is released at a con-
stant rate (1-2 mg day
−1
for sheep; 2-7 mg day
−1
for cattle) from a permeation tube placed in
the rumen (reticulo-rumen) of the animal.
Prior to placement in the rumen, the release
rate of SF
6
is determined by weighing the tubes
(stored at 39°C) over several weeks. In select-
ing permeation tubes for an animal trial, care
is needed to ensure the release rates are similar
since it has been reported that the rate of
release of SF
6
can influence the calculated CH
4
emission (Vlaming
et al
., 2005), thus increasing
the between-animal variability.
A continuous sample of expired and eruc-
tated air collected over typically 24 h near the
nostrils and mouth is stored in an evacuated
canister mounted on the animal. After removal
of the canister from the animal, it is pressurized
with nitrogen and then analysed for CH
4
and
SF
6
concentrations using gas chromatography
(GC). Background concentration of CH
4
and SF
6
tracer are subtracted to obtain the rumen-
derived concentrations. A detailed protocol for
the SF
6
tracer technique is reported by Johnson
et al
. (2007).
There has been concern expressed about
the assumption of a constant SF
6
release rate
from the permeation tube located in the rumen
of the animal (Ulyatt
et al
., 1999). The root of
the issue is that the SF
6
tracer technique cannot
be calibrated since the permeation tube cannot
be recovered except where a rumen cannula is
used. Lassey
et al
. (2001) reported an error of up
to 15% in CH
4
emission associated with the
assumption of a constant release rate, and
offered a means of correcting this assumption.
The use of the SF
6
tracer technique inside
barns with low ventilation may also be problem-
atic since this can lead to a high background
concentration of SF
6
that approaches the sam-
pled SF
6
concentration at the nostrils/mouth.
Very small differences (measured in parts per
trillion, ppt) between the background and canis-
ter SF
6
concentrations can approach the resolu-
tion of detection, and cause serious error in the
CH
4
emission calculation. The use of the SF
6
tracer technique on animals with a poorly fitted
Atmospheric tracer technique
A tracer gas can be released into the atmosphere
external to the animal to measure CH
4
emissions
(Eqn 15.3). The assumption in this case is that
the tracer and targeted gas (CH
4
) are mixed in the
air by turbulence diffusion, in the same fashion.
Johnson
et al
. (2002) released SF
6
as a tracer
(at 50.2 μg h
−1
) in a room housing cattle and the
concentrations of CH
4
and SF
6
were measured
once the concentrations reached steady-state.
The concentrations inside the room were meas-
ured from collected air samples using GC.
Kaharabata
et al
. (2000) released SF
6
at 16 loca-
tions (
c
.20-32 g h
−1
per location) inside a barn
containing 90 dairy cows, and measured CH
4
and SF
6
concentrations downwind (of the barn)
by analysing air samples using GC. Griffith
et al
.
(2008) used a controlled release of nitrous oxide
(N
2
O) as a tracer gas from points along the
upwind side of a field containing grazing cattle.
The average CH
4
emission from the small con-
fined herd was accomplished by measuring N
2
O
and CH
4
concentrations simultaneously down-
wind of the confined herd using a FTIR (Fourier
Transform InfraRed) open-path sensor. In this
latter study, there is a clear advantage to using a
tracer gas that is compatible with an analyser
that allows continuous monitoring.
