Enteric Methane Estimation using Sulfur Hexafluoride (SF6) Technique

Abstract

A commonly used method of measuring enteric methane (CH₄) emissions from ruminants is the SF₆ tracer technique that measures respired and eructated CH₄. However, within the animal, a small proportion of CH₄ is produced post-ruminally and some of this may escape through the rectum. The comparison of emissions using a chamber technique that measures all enteric CH₄ losses, and the SF₆ tracer technique, could give some insight into the magnitude of post-ruminal emission. The tracer technique enables estimation of methane emissions from large numbers of individual ruminant and pseudo-ruminant animals without the need for their confinement.

Key words: Ruminant, Methane Emission, Sulfur hexafluoride

Introduction

Ruminants are mainly fed on lignocellulosic agricultural by-products like cereal straws, stover, sugarcane bagasse etc. The rumen microbes convert these un-utilizable plant tissues into valuable animal products like meat and milk. The rumen microbial ecosystem is highly diversified which comprises bacteria, protozoa, fungi, and bacteriophages. Unfortunately, fermentation in the rumen is associated with various losses because of the fermentation of different nutrients like carbohydrate (structural and non- structural) and protein. CO₂, methane and ammonia are the metabolic end products which are wasteful as far as the nutrition of animal is concerned but they are unavoidable products of rumen fermentation. During rumen fermentation, the methane production leads to 2-15% of loss in the GE (Moss et al., 2000; Holter and Young, 1992). Domestic livestock since long has been blamed for substantial contribution to global anthropogenic GHG emissions. The world population of domesticated ruminants is thought to be responsible for 29% of total CH₄ emissions due to anaerobic enteric fermentation of feeds. The contribution of India’s livestock to global methane emissions is 10.63%. Also, methane is a potent greenhouse gas having 23 times more global warming potential than CO₂, enteric methane production contributes significantly to global warming. Considering the detrimental effects of these gases, the measurement of their emissions from ruminants is an important aspect from environmental point of view.

The emission of enteric methane from ruminants can be estimated using a calibrated tracer technique developed by Zimmerman (1993) and first deployed by Johnson et al. (1994). The tracer technique enables estimation of methane emissions from large numbers of individual ruminant and pseudo-ruminant animals without the need for their confinement.

The SF₆ tracer technique relies on a known source of a synthetic inert tracer – sulphur hexafluoride (SF₆) – inserted in the rumen, where at least 90% of CH₄ is sourced. ‘Breath’, characterized as respired plus eructed gases, is sampled and analyzed. This technique can be used with untethered, freely-grazing animals. Moreover, the equipment cost per animal under test is much lower than for chamber experiments, enabling dozens or even hundreds of animals to be breath-tested in a single experiment.

Principle

SF₆ gas mixes with CH₄ and other gases present in the rumen and continuously comes out along with them. Gas collection assembly collects fraction of gases emitted by nose and mouth continuously for 24 hr. Total amount of CH₄ produced “during 24 hr is calculated by estimation of CH₄ and SF₆ concentration in the total collected gases.

Requirements

• Permeation tube
• Pure SF₆ gas
• Canister
• Halter
• GLC system and Standards of CH₄ and SF₆

Permeation tube

It is 2-3 inches long tube made up of brass and having a screw cap with teflon disc & frit of 2 µ pore size. 

Canister

It is made up of PV tube having air tight valve which is evacuated with the help of vacuum pump and the pressure is recorded.

Halter

It is made up of strips which can be fitted on the mouth of animal along with gas collection assembly.

Gas collection assembly

Contains a filter connected with very fine capillary. The suction rate can be adjusted by the length of capillary tube. If capillary tube is small, suction rate is high and vice versa. It should be adjusted in such a way that half of canister is filled in 24 hr.

Background

An evacuated PVC canister (background) is placed where animals are kept, to measure ambient CH₄ and SF_6. (Fig: 1)

Procedure

1. Preparation of permeation tube

Pure SF₆ gas is filled in permeation tube by cooling it with the help of liquid nitrogen. About 150 ml (1 g) of SF₆ gas is added and closed tightly (Lassey et al., 2011). Tube is weighed before and after putting the gas to know weight of SF6. Keep at 39±1⁰C water bath. Daily weight of permeation tube is taken to know the release rate of SF₆ gas. It takes 20-25 days to get established release of SF₆.

A requirement of the technique is the release of a tracer gas (sulphur hexafluoride, SF₆) at a known constant rate into the reticulo-rumen of each animal. To achieve this, a slow-release device, known as a permeation tube, is placed in the reticulo-rumen (Zimmerman, 1993). The subsequent pattern of SF₆ emission from a ruminant animal has not been reported. However, it is well known that the rate of methane emission from ruminants can vary considerably throughout the day in relation to feeding (Machmüller and Hegarty, 2006.). Thus, the SF₆ technique requires a representative sample of emitted gases to be collected at a constant rate to ensure that an equal proportion of the sample is collected throughout the typically 24 h sampling period. (Fig: 2)

2. Collection of sample

The permeation tube is inserted into the rumen of animal 2 days before collection period with the help of rubber tube or drenching pipe. Gas collection assemble is put on the animal. Evacuated canister is attached (McGinn et al., 2006). After 24 hr, remove the canister and note down the pressure (upto – 6 or – 7). Then bring it to positive pressure upto +1 to +2 with liquid nitrogen.   

3. Analysis

The concentration of SF6 in the canisters was analyzed by gas chromatography, fitted with an electron capture detector (250°C) to determine SF6 and 3.3 m molecular sieve column with an i.d. of 0.32mm. Another gas chromatograph was fitted with a flame ionization detector (100°C) and stainless steel column packed with Porapak-Q to determine CH₄ concentration (Dlugokencky et al., 2011). The column and injector temperatures were 50 and 40°C in both gas chromatographs. All samples were analyzed in duplicate except standards, which were analyzed in triplicate. Nitrogen was used as the carrier gas at a pressure of 1kg/cm² (Grainger et al., 2007). The standards were run at the beginning and end of each day with the medium standard run every 10 samples throughout the day. Gas concentrations (SF₆ and CH₄) were determined from peak areas and identified from their different retention times relative to the known standards.

The methane output calculated using following formula:

Q CH₄ (mg/d) = Q SF₆ × [(CH₄)_s – (CH₄)_b]/ [(SF₆)_s– (SF₆)_b]

Where

QSF₆ – Release rate of SF₆ (mg/d)
(CH₄)_s and (CH₄)_b – Methane concentrations in sample canister and  background’s canisters (ppm),
(SF₆)_s and (SF₆)_b – Concentrations of SF₆  in sample canister and background’s canisters (ppm).

Advantages

1. It’s highly precise and accurate method
2. This technique eliminates the necessity to restrain or enclose the animal, thus allowing the animal to move about and graze.
3. It is also not necessary to sample directly from the animal’s rumen or throat because the use of the tracer accounts changes in dilution associated with head or air movement.
4. It gives good estimates of the between animal and within animal variation in emissions and is also well suited for comparison of distinct mitigation strategies.
5. The residual variation on measurements is higher than with the chamber method, but it is possible to employ more animals per experiment, which provides a better statistical foundation for testing hypotheses.

Disadvantages

1. SF₆ procedure has been used for large-scale genetic and nutritional evaluations, it remains labour intensive, expensive and dependent on technical specialists for operation and analysis.
2. SF₆ is a greenhouse gases itself, with a GWP 23,900 times that of CO₂ and an atmospheric lifetime of 3,200 years.
3. The residue of SF₆ in meat and milk from farm animals is another issue.
4. It is necessary to train the animal to wear a halter and collection yoke/canister.

References

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