Green house gases in the atmosphere absorb and emit radiation within the thermal infrared range. This process contributes fundamentally to green house effect. Green house gases (GHG) like carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) produced due to anthropogenic activities are one of the serious concerns now a day.
Non-CO2 GHGs in an abatement strategy can be less expensive and more effective in mitigating climate change.CH4 is the second most important GHG after carbon dioxide, and contributes 15 % to the global warming (IPCC, 2007). CH4 remains in the atmosphere for 915 years and is about 21 times more effective in trapping heat in the atmosphere than CO2 over a 100-year period (FAO, 2006). The global atmospheric concentration of methane has increased from a pre-industrial value of 720 ppb to 1803 ppb in 2011 (IPCC, 2013). Nitrous oxide is another potent greenhouse gas with a long lifetime of 150 years in the atmosphere and large radioactive forcing potential which that of CO2 is 310 times more potent than CO2. The livestock sector represents a significant source of GHG emissions worldwide, generating carbon dioxide, methane (enteric and manure), and nitrous oxide either directly (e.g., from enteric fermentation and manure management) or indirectly (e.g., from feed-production activities and conversion of forest into pasture). About 37 % of anthropogenic methane is attributed to enteric fermentation by ruminants as part of their normal digestive processes. Livestock manure management is also a significant source of methane with global emissions accounting to 9.3 Tg/year.
India ranks first with 13% of the world total livestock population. Contrary to the large population, the productivity of Indian livestock is low as compared to many developing countries. Animals are often fed on crops grown residues and grasses from grazing lands. The use of concentrated feed is low and limited to productive animals only. India produces 12.45% of the total enteric methane emissions. Total methane emissions from India were found to be about 20.56 million tones (INCCA, 2010). CH4 emissions in ruminants also account for a 2 to 12% of gross energy loss of feeds, depending upon the type of diets (Johnson and Johnson, 1995). Therefore, it is very important that CH4 mitigation strategies are need to be followed to improve the utilization of feed energy, rather for productive purposes. CH4 mitigation from livestock can be done using different methods like, diet manipulations including feed supplements, animal manipulations, management systems and rumen manipulations (defaunation, vaccination). Some of the practical nutritional interventions for effective CH4 reduction in ruminants are:
Increasing concentrates in diet
Increasing the level of concentrate in the diet decreases CH4 production. It was found that the relation between Ch4 production and concentrate proportion in the diet are curvilinear. More concentrate in diet increases the feed intake which, in turn, results in improved rumen fermentation and accelerated feed turnover. This which causes large modifications of rumen physico-chemical conditions and consequently, the microbial populations. A shift in the proportion of individual VFA production, leading to a decrease in acetate and a decrease in propionate occurs. This results in lower CH4 production because the proportions of hydrogen sink increases, and thus, partially prevented from going for methanogenesis. Lower rumen pH, which is produced during concentrate feeding, also results in inhibition of methanogens. But very high proportions of concentrates should not be fed as it causes health problems due to acidosis and is also not economically feasible. But decreased enteric CH4 production (per unit of DMI) due to increased inclusion of grain in the diet may be partially offset by increased CH4 emission from manure.
Forage type and quality
Feeding of forage with low fibre and higher soluble carbohydrates, using C3 grasses and less mature pasture, can reduce CH4 production. CH4 emissions can also be lowered by using legume rich forages in diet because of its lower fibre content, higher passage rate and condensed tannin content. Certain plant species like sulla, sainfoin, lotus, lespedeza also found to reduce CH4 emission. Preservation of fodder and forage processing methods like chopping, grinding and pelletting causes decrease in methane production. Chemical treatment (urea, ammonia, calcium hydroxide, sodium hydroxide, urine addition) and biological treatment (fungal treatment like Pleurotus spp, Caprinus spp, Crinipellis spp) of roughages also showed methane reducing effect.
Addition of fat
Addition of fat in the diet inhibits the growth of methanogens and protozoa population and reduces ruminal organic matter (OM) fermentation and hydrogenation of unsaturated fatty acids (acting as an alternative H2 sink) in the rumen. Dietary fat do not undergo fermentation in the rumen that also adds to reduced CH4 production. But higher inclusion of fat also reduced feed intake and digestibility of fibre which leads to lower weight gain and milk yield. Normally medium chain fatty acids like coconut oil, myristic oil, canola oil, plam kernel oil etc shows greater result in reduction of CH4. Appropriate dose and source of fat should be chosen carefully because it may affect VFA production and feed digestibility.
Plant secondary compounds
Tannins, saponins and essential oils are having methane mitigating effect. Tannins can inhibit methanogens, protozoa and hydrogen producing microbes. Tannins are of two types- condensed and hydrolysable tannins, among these condensed ones are having most of the antimethanogenic activity, but hydrolysable tannins are also efficacious. Different sources of tannins have been found to produce varied effects because of difference in chemical structure and level of supplementation. Animal trials with plants or extracts of condensed tannin-containing Lotus corniculatus, Lotus pedunculatus, Terminalia chebula,Ficus bengalensis, Azadiracta indica and Acacia mearnsii have been shown to reduce CH4 underscript) production in ruminants. Saponins are glycosides found in many plants that have a direct effect on rumen microbes. Saponins, decrease protein degradation and favour at the same time microbial protein and biomass synthesis .Saponins can inhibit growth of protozoa in rumen. Effect of saponins on CH4 reduction was found to be more effective in defaunated animals. Antiprotozoal activity of saponins were found to be transient. Use of saponins may also confer nutritional benefits as they might increase microbial protein synthesis due to inhibition of protozoa. Saponins from Sapindus saponaria, Acacia concina, Sesbania sesban, fenugreek and tea saponins have proved effective in this respect. Essential oils and organosulphur compounds also have the ability for CH4 reduction. A limited number of studies are available showing direct effect of EO on rumen archaea. Coriander, garlic, ginger, fennel, clove, eucalyptus are found to be effective in CH4 reduction.
Ionophores
Ionophore antibiotics such as monensin, rumensin and lasalocid also cause decrease in Ch4 production. These ionophores at the doses prescribed do not affect methanogens, however their effect on other microbes, inducing a shift in fermentation towards propionate synthesis, is the most likely mode of action. Ionophores also affect protozoa; the reduction and subsequent recovery in protozoal numbers perfectly matched CH4 abatement.
Organic acids
Organic acids like fumarate, malate or their sodium salts reduce methane emission because of shifting rumen fermentation towards propionate and can act as alternative hydrogen sink. . The addition of fumarate consistently decreased methane production in vitro and increased feed digestibility and VFAs production. Similarly, malate, which is converted to fumarate in the rumen, stimulated propionate formation and also inhibited methanogenesis in some in vitro studies. Inclusion of organic acids in the diet have given varied types of results from in vivo studies. However, at the relatively higher doses required, the dicarboxylic acids are prohibitively expensive as an abatement strategy. Green leguminous fodders are the good source of malic acid.
Alternate electron acceptors
Methanogenesis may be lowered by the addition of electron acceptors such as nitrate and sulfate. Some rumen microorganisms are capable of reduction of nitrate to nitrite and then nitrite to ammonia, use hydrogen or formate or both as the common electron donors. Nitrate can be used as a nitrogen supplement also. However, under some nutritional conditions/feed management, nitrate becomes toxic because of the accumulation of nitrite in the rumen.
Probiotics
Probiotics are directly fed microbials (DFM) that have claimed health benefits when consumed. Limited information is available on the effect of probiotic cultures such as Saccharomyces cerevisiae and Aspergillus oryzae on CH4 production and most of these are in vitro studies.
Defaunation
Defaunation is the process of removal of protozoa from rumen. It causes improved microbial protein supply and animal productivity. As methanogens remain attached to protozoa, removal of protozoa results in reduction in the number of methanogens also which leads to reduced methane production. Large number of techniques were used for defaunation. Plant secondary metabolites like saponin are being used widely now a day for defaunation. But use of certain defaunating agents like lauric acid and coconut oil were found to affect the dry matter intake in cattle.
Conclusions
Lot of advances have occurred in last few years regarding the techniques for mitigation of methane in ruminants. Generally, increasing forage digestibility and digestible forage intake typically decreases emission of CH4. Other effective CH4 mitigation practices include lipid and concentrate feeds supplementation of the diet, plant secondary metabolites, ionophores, organic acids, alternate electron acceptors, probiotics, halogenated compounds and of course, defaunation. The long-term effects of many of these mitigation practices have not been well established. Some additives are toxic or may not be economically feasible to implement. Reduction in methane emissions can only be achieved by improving productivity. So feeding of balanced diet matching the physiological state of the animal and improving the overall nutrient utilization is the most efficient way of decreasing methane emissions.
Dr. Madhu Mohini and Dr. V M Nampoothiri, DCN Division, National Dairy Research Institute (NDRI)
