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Enteric methane missions is the product of enteric fermentation which occurs during the digestive process whereby carbohydrates are broken down by micro-organisms that are found in special stomach compartments in ruminants called the "rumen". Most of the methane (CH4) that is produced in the rumen is belched by the animal and there is also a small amount that is produced in the large intestine that is excreted as flatulence (Gibbs and Leng 1993).
Gibbs and Leng (1993) also described a number of factor that affect the production of CH4 in ruminants and these are as follows:
The feed characteristics (both chemical and physical)
The level of feeding and schedule
The use of feed additive
The activity carried out by the animal
The health of the animal
They then concluded that the feed characterisitics and level of feeding was show to have the most influence on enteric methane emissions while the other factors play a minor role and didn't have much of an influence.
In Guyana, estimates of enteric methane emissions done for the purpose of preparing a National Communication to the United Nations Framework Converntion on Climate Change (UNFCCC) using the Guidelines for National Greenhouse Gas Inventories Tier 1 method (IPCC 1996), so there were never any previous studies conducted on a detailed basis to get accurate emissions factors for the various categories of livestock.
However Gibbs and Leng (1993) did estimate some emission factors for geographical location that have similar condition namely Latin America and the data for this region is based on data that was gather from Brazil.
There were detailed enteric methane emissions studies that were done in other regions such as Canada estimates show a 10.6 % increase from 1990 to 2000 in ruminant animals.Data collected from Canadian research indicate that energy losses associated with enteric CH4 emissions range from 2 to 11.3 % of gross energy intake. Numerous mitigation strategies have been suggested in the literature, including: manipulation of rumen microfloral populations, diet manipulation to provide alternate hydrogen acceptors, diet manipulation to shift the fermentation pathway, management for improved productivity, and genetic selection for low methane emitting animals. Results from Canadian trials show that there are opportunities to reduce enteric methane emissions in commercial production systems (Wittenberg 2008).
methods used for measuring enteric methane emissions
Experiment Based Methods
There are different methods that can be used to measure enteric methane emissions from cattle which were used in previous studies and these are as followed:
Meausring the enteric methane emissions by means of chamber
This method is regarded as the standard methods in measuring methane emission that come from ruminants because of the fact that the environment can be fully controlled and the instruments that are being used can be calibrated to suit so this can make the instruments that are being used very reliable (Storm, et al. 2012). This method has also been in use for the last century mainly for studying animals' energy metabolism (Johnson, Ferrell and Jenkins 2003).
The chambers capture the exhaled breath and measure the gas composition of the exhaled breath by pumping it through flow meters and different gas sensors.
Sulphur hexafluoride (SF6) Tracer Technique
This technique was pioneered and developed in the mid 90s by Johnson et al (Johnson, et al. 1994) for measuring methane emissions from ruminants so this is a relatively new technique and it has been adapted by a number of researchers in different countries that are counducting similar studies that involves measuring methane emissions from ruminants (Pinares-Patino, et al. 2008).
This method can be used and it is ideal for situations in which emission from large groups of animals need to be taken simultaneously. It works by dosing a SF6 charge permeation tube into the reticulo-rumen of the animal and also involes the collection of breath samples which are analised for analysis of CH4 and SF6 concentrations (Lassey 2007).
The calculation that s used to determine the daily methane emission is based on theCH4/SF6 ratio of concentrations and the permeation rate of SF6.Adjustments are made to compensate for background concentrations (Pinares-Patino, et al. 2008).
In Vitro Gas Production Technique
This method is traditionally used to simulate fermentation of feed and feedstuff in the rumen of rumenants.It has lately been modified to compensate for and measure CH4 production due to the spark of interests in greenhouse gases by researchers. This method involves the feed being fermented under laboratory conditions using the microbes that are found in the rumen and the gas that is produced is captured and analised to obtain the in vitro production of CH4 (Storm, et al. 2012). The time frame per experiment ranges from 1 to 4 weeks which allows a wide range of feeds to be tested fast and economically.
Other techniques that are used are as follows:
The CO2 technique described by Madsen, et al. (2010) which is done on the basis of using CO2 as tracer gas which is naturally emitted to be able to estimate the CH4 produced.
There are techniques that involve whole buildings or areas that can be divided into micrometeorological and non-micrometeorological (Storm, et al. 2012). It measure atmospheric gas fluxes then relates them to the gas fluxes from the animals or focuses on systems intead of an individual in which a tracer is used (Harper, Denmead and Flesch 2011).
Combine feeder and CH4 analyzer techniques are also used to measure enteric CH4 emissions from animals but it is a newly patented system so it is not a frequently used method (Storm, et al. 2012).
Proxy methods: certain biological samples are taken from the animal e.g milk or feaces and certain characteristics are correlated with methane production from the animal (Chilliard, et al. 2009, Vlaeminck, et al. 2006, Dijkstra, et al. 2011, Montoya, et al. 2011).
These models involve a complexity of equations that mathematically predict methane emissions based on dry matter intake (DMI), intake of carbohydrates, digestibility and intake of dietary energy, animal size, milk components, digestibility of dietary components, et cetera.Thes methane prediction models can be classified as:
Empirical (statistical) models: these involve descriptions of the animals' responses to condition changes; most likely changes in diets which doesn't requires little or no scientific understanding (Kebreab 2012). Despite most of these models having successfully predicted methane emissions some of the variable that have to be inputed into these model cause many difficulties in predicting CH4 prodution so this was addressed by developing model that relies on the least amount of input or that use common input variables for which data could be easily obtained (Sejian, et al. 2010).
"The limitation of using some of the extant models, such as the equation of Moe and Tyrrell (1979), is the difficulty of obtaining reliable model input variables, which might have compromised the predictive ability of the model in the study" (Sejian, et al. 2010). The most accurate equations that can be used to predict enteric CH4 emissions was developed by Ellis, et al. (2007) and this equation requires minimum inputs.
There are many differenent types of empirical equations that are used in fore enteric methane emission which area regression based and there are those that were developed as early as the 1930s and 1940s.they are as follows:
Methane (McaI/day) = (18â€‰+â€‰22.5 Ã- DM1 (kg/day)) Ã- .013184 (Mcal/g of methane) (Kriss 1930)
Methane (Mcal/day) = (17.68 + .04012 Ã- digested carbohydrate (g/day)) Ã- .013184 (Mcal/g of methane) (Bratzler and Forbes 1940)
Methane (Mcal/day)â€‰=â€‰âˆ’0.494â€‰+â€‰0.629 Ã- DMI (kg/day) -0.025 Ã- DMI2(kg/day) (Axelsson 1949)
In developing all of these equations methane production in the rumen were plotted statistically against the digestion of certain feed characteristics e.g MEI, starch, carbohydrates, et cetera in which quantity was also considered. The methane production variables that were used in the development of the equaions were measured mainly by means of a calorimeter.
It was also observed that these equations developed during this perioad had its greatest accuracy when applied to a group of animals rather than an individual to which it was determined to be inappropriate.
There are more recent equations of this type that were developed by Kirchgessner et al (1995) on the nutrient compositon of the diest the animal receive each day, Yates, et al. (2000) which is based mostly on dry matter intake and the Mits3 equation (Mills, et al. 2003) which is based on three easily obtainable variable i.e starch, non detergent fiber (NDF) and Metabolisable energy intake (MEI) so this can be used under conditions where there has been no previous studies were done.
Mechanistic models: These are dynamic models which are based on mathematical descriptions of the biochemistry of the fermentation patterns that occur in the rumen of ruminants (Kebreab 2012). Some of the most common models of this type that are used today are as follows:
Statistical/Emperical models which were developed by (Moe and Tyrrell 1979)