Ruminant animals are unique due to their four-compartment stomachs and ability to digest otherwise indigestible, highly fibrous feedstuffs. But in this process of digestion, methane--a potential greenhouse gas--is produced as a product of enteric fermentation, making cattle and other ruminants contributing factors in global warming.
Globally, ruminant livestock produce about 80 million metric tons of methane annually, accounting for about 28 percent of global methane emission from human-related activities. According to the Food and Agriculture Organization (FAO), livestock are responsible for 18 percent of greenhouse gas emission as measured in carbon dioxide equivalent. This includes 9 percent of all CO2 emission, 37 percent of methane and 65 percent of nitrous oxide. Methane has 23 times the global warming potential of CO2 and is expected to cause between 15 to 17 percent of the global warming over the next 50 years (Adam, 2000). Although growth in the agriculture sector has been fairly constant, there is movement toward increased meat animal consumption among developing countries, favoring the growth of livestock populations.
As per FAO statistics, between 1970 and 2002, annual per capita meat consumption in developing countries rose from 11 kgs. to 29 kgs. Annual global production is projected to more than double from 229 million tons at the beginning of the decade to 465 million tons in 2050 and milk output is expect to jump from 580 to 1043 million metric tons. This trend favors the growth of meat industry and animal sectors as a whole. The meat industry is the largest emitter of methane among the various livestock industries as beef animals are often reared on poor quality feed with less-than-optimal management conditions. India, for example, with the largest livestock population at about 464 million (including the highest buffalo population) is greatly contributing to the global warming effects due to methane emission from livestock. The Indian contribution of methane from all sources is around 12 percent of the total world production. Indian livestock produces approximately 8,995 Tg methane per year, or 405.75X108 Kcal per day. The intensive system of animal rearing found there affects not only human lives, but human life globally and net animal productivity through environmental pollution resulting in global warming. Global warming occurring through livestock is of increasing concern.
Methane is normally produced during fermentation of feedstuffs in the rumen (Adam, 2000). Its output depends on the efficiency of fermentation and conversion of feed into animal food products. Among the volatile fatty acids (VFAs) produced during the fermentation, acetate and butyrate are methanogenic and spare hydrogen during formation while propionate is glucogenic in nature and utilizes the hydrogen.
Hydrogen produced is then utilized by the methanogenic microbes (archaebacteria) in the production of methane and later belched out of the animal's mouth and nose, or it can convert the acetic acid into methane. The CO2 produced in the second circumstance can further be used in the first instance for methane production (Morris, 1998). Therefore, the proportion of these VFAs decides the extent of methane (CH4) production. The production of VFAs is basically a function of the type of diet fed. The acetate to propionate ratio is generally lower for cereal grains than for forages. There is an inverse relationship between propionate and methane production. About 8 to 9 percent of gross energy (GE) is lost as a result of methane production. This is a disadvantage for both cows and producers as this energy could otherwise be channeled for production purposes. Similarly, it was found that milk production level also affects the production of methane, as shown in Table 1.
Table 1: Methane production
Methods to reduce the methane emission
Scientists are trying to develop environmentally cleaner cows, often referred to as ‘green' cows, not in color but in the sense of being environmentally friendly. Researchers are trying to alter cattle digestion, either by removing the microorganisms that produce methane from their stomachs or by creating microorganisms that can produce metabolic end products other than methane.
Removal of protozoa from the rumen is known as defaunation and is known to improve the growth, weight gain, feed utilization efficiency and performance of the animal (Coleman,1980), (Chaudhary et al, 1988), (Bird, 1989) and (Chaudhary and Mudgal,1989). Defaunation decreases the methanogensis, the CH4 production, because most of the methanogens remain attached to protozoa pellicle and exhibit an ecto-symbiotic relationship, utilizing hydrogen to covert into methane (Patra,2004), (Santra et al, 1994). Studies (Pal et al, 1994) have revealed that defaunation reduces CH4 production by 20 to 50 percent and improves the feed utilization efficiency
There are several methods to remove the protozoa from the rumen. One method is by dietary manipulation. This can be accomplished in several ways, including increasing the lipid content, since lipids are toxic for the protozoa and thus increases in dietary fat level reduce their number (Mudgal et al, 2003).
Another dietary method is lowering the rumen pH by dietary means, often considered the simplest and most suitable method, and one having no side effects. Increasing the saponin in the diet can also impact methane production. Increases in dietary saponins can bring about marked declines in the protozoal population (Kamra,2005), although this method has more risk of bloat. The use of pelleted concentrates fed adlibitum and of milk, especially of milk fat or cream, has also been found to be effective against protozoa when given after a few hours of fasting (Kreuzer and Kirchgessner, 1987).
A number of plant and plant extracts have also been found to impact protozoal population, including hot water extract of soapnut (Kamra, 1993), Yucca extracts (Wallace et al, 1994), fruit of Sapindus saponaria (Diaz et al, 1999), leaves of Entrobium timoba (Gupta et al, 1993), methanol extracts of Sapindus rarak fruit (Thalib et al,1996), and tree leaves of Enterolobium ciclocarpum (Alberto et al, 1992). Several chemical processes have also been identified as decreasing protozoal population in the rumen gut, but none have been proven safe and fully effective. They include drenching with copper sulfate (Ramaprasad and Raghavan,1981), sodium lauryl diethoxy sulfate (Bird & Leng, 1984), or sodium lauryl sulfate (Kamra et al, 1992).
Direct Inhibition of methane
Direct inhibition of methanogensis is widely reported by using halogenated methane analogues, chloroform, chloral hydrate, amichloral, trichloroactemide, bromochloromethane, 2-bromo ethanesulfonic acid, trichloroethyl pivalate (TCE-P) and trichloroethyl adipate (TCE-A)(Czerkawski and Breckenridge,1975), but none of these is found suitable due to one or more limitations. Amichloral has been identified as a safe methane inhibitor and was shown to increase live weight gain in sheep (Trei et al, 1972); however, its antimethanogenic activity declined with prolonged feeding.
Ionophores, like monensin, lasalocid and silnomycin, can also alter rumen fermentation, resulting in less production of methane. The use of ionophores lead to increases in the production of propionate by favoring the growth of the gram negative bacterial population which results in a shift in fermentation from acetate to propionate. As noted earlier, methane and propionate production are negatively correlated. Table 2 shows the expected methane production at various production levels with an ionophore added to the diet and can be compared to Table 1, which showed the same stages for the same animal without an ionophore added. Another route is to use propionate enhancers in the diet, which will lessen methane formulation due to the inverse relationship between propionic acid and methanogensis.
Acetogens are rumen microbes that convert CO2 and H2 to acetate, which is an energy source for the cow. In vitro studies showed that supplementation of acetogens in rumen fluid decrease the methane production (Lopez et al, 1999). These acetogen microbes are present in the rumen but in very few numbers and attempts to increase the acetogens have not been successful, but methane production can be lowered by using acetogens as a daily feed additive. More research is needed to develop the methods to make them more competitive in the rumen or transferring the acetogenesis genes to already successful ruminal organisms, which could be very helpful in mitigating methane emission.
The greater the animal productivity, the lesser the methane production per unit of food animal product produced. In addition to the methods noted above for decreasing protozoa populations, the type of diet fed to animals can have a major effect on methane production. Studies show that poor nutrition is a major cause of excess methane production, so by dietary manipulation it is possible to mitigate methane production. The forages-to-concentrate ratio affects the acetate-to-propionate ratio, which in turn affects methane production. Concentrates in the diet help by favoring propionic acid production. Secondly, concentrates lower ruminal pH which can eliminate certain protozoa, resulting in lowered methane production. However, very high concentrate feeding is linked with acidosis, laminitis and certain fertility problems. It has been found that a 25 percent increase in the level of non-structural carbohydrate in the diet, can lower methane production by as much as 20 percent.
Feeding of a total mixed ration, high quality leguminous forages, the addition of lipids (especially unsaturated fatty acids), and grinding and pelleting of forages also help reduce methane emission. Table 3 shows the various methods of dietary adjustment for reducing methane emission from ruminants.
Use of probiotics
In general, the feeding of probiotics refers to microorganism which, when fed to the animal, has a positive impact on the host by improving gastrointestinal tract microbial balance. They are also known as direct fed microbes or direct fed microbials (DFM). Commonly used cultures are Aspergillus oryzae, Saccharomyces cerevisiae, lactobacillus spp, Bifidobacterium adolescentis, B. animalis and streptococcus spps. They help in improving the digestibility and overall performance and also provide some B-vitamins and branched chain fatty acids. It was found that supplementation of Aspergillus oryzae reduces the methane emission by up to 50 percent by lowering the population of protozoan (Frumholtz et al, 1989). Similarly, the addition of Saccharomyces cerevisiae can reduce the methane emission by 10 percent (Mutsvangwa et al, 1992). Brevibacillus parabrevis is reported to have the ability to convert methane into CO2.
Biotechnology can play an important role in lowering the methane from livestock by manipulating the rumen bugs to enhance the digestibility of poor quality feed stuff and lower or halt the production of methane. There has recently been development of some vaccines that alter the population of microorganism in the rumen and in particular, discourages the methanogenic archaea (Major, 2000). Baker (1995) revealed the possibility to immunize ruminant animals against their own methanogensis, which decreases the methane output. Similarly Shu et al (1999) revealed an approach which successfully reduced the numbers of streptococci and lactobacilli in the rumen.
Methane and global warming
Methane in the atmosphere breaks down within a decade compared to CO2 which persist for over a century. Therefore, reducing methane emissions will have more immediate impacts on global warming (Adam, 2000). Moreover, reducing methane emission is an easier, more economical and quicker method of reducing global warming (Decorla-Souza, 2001). But this reduction in methane emission cannot be achieved by the efforts of a single person acting on an individual animal. It can be attained by building awareness for dietary impacts combined with strong steps taken by governments.
One step to reduce methane production through reduced livestock numbers could be rationing of the milk quotas in the European Union (EU) and fixing the number of animals to be reared, with proper disposal of the barren, unproductive and stray animals in developing countries like India. Many countries are forthcoming with methane taxes or flatulence taxes to develop some technology to combat the methane emission from livestock. But other alternatives can be tried to reduce the methane emission from ruminants while simultaneously improving animal productivity.