Introduction
The agricultural sector in India contributes significantly to the emission of greenhouse gases, resulting in climate change. In fact, in the agricultural sector, the major contribution is through enteric fermentation in ruminants, which accounts to the tune of 59%; paddy cultivation 23%; soils 12%; manure management 5% and burning of crop residues 1%.
So, broadly speaking, Animal Husbandry contributes significantly to global warming, through the emission of greenhouse gases, viz. Methane and Nitrous Oxide. Methane produced through rumen/enteric fermentation has a global warming potential which is 28 times higher than that of CO2. However, Nitrous Oxide (N2O) released from manure and urine has a global warming potential of 265 times greater than CO2. Therefore, reducing emission of greenhouse gases from livestock is a crucial component of any comprehensive strategy to combat climate change.
Well, it is a known fact that feeding of good quality and nutritious feeds including feed additives, to dairy animals result in increase in milk production and reduce methane emission. However, India, with such a large bovine population, and with a limited land area, faces an acute shortage of good quality green fodders in the country.
Being one of the largest producers of rice and wheat, India gets enormous stocks of crop residues, as its by-products. In fact, due to lack of good quality fodders, our bovine stock in India mainly depends up on poor quality fibrous crop residues (straws from wheat, rice and jowar etc.) as the roughage part of their ration. This results in less milk production and more methane emission per liter of milk. It can be reversed by propagating and campaigning the practice of feeding well balanced commercial feeds, rich in energy, protein, minerals, trace minerals as well as vitamins, including some feed additives.
Strategies For Reducing Methane Emission from Ruminants
Reducing Methanogenesis through Dietary Manipulations
Structural carbohydrates viz. cellulose and hemicelluloses are fermented at a slower rate in rumen than non-structural carbohydrates viz. starch and sugars, but these cellulosic stuffs yield more methane per unit substrate fermented. The production of methane in rumen is also related to the retention time of roughages in rumen. Cellulosic diets have a higher retention time in rumen. Therefore, more the quantity of cellulosic diets fed to animals, which having more retention time in rumen, shall flourish more population of Methanogenic bacteria Viz. Methanobrevibactor, Methanomicrobium and Methanosarcina in rumen, and thereby resulting in much more methane production in rumen.
By changing the dietary regime of the animals, the methane emission from cattle and buffaloes can be reduced. In case enough of good quality green fodder is not available, the concentrate feed part may be increased to meet the nutrient requirement of the animals for protein and energy (Digestible Organic Matter) as per their requirements. After conducting large number of feeding trials (Bencher, 2001), it has been seen that through dietary manipulations, the reduction in methane production could be:
a) 40% by increasing concentrate proportion in the diet
b) 22% by replacing fibrous concentrates with starchy concentrates
c) 15% by increasing the digestibility of forages
d) 28% by increasing the leguminous forages compared to grassy forages
e) 20% when hay diet is replaced by silage diet
Feeding of long chain unsaturated fatty acids also inhibit methane production accompanied by increase in propionic acid in rumen. This is precisely attributed to the shift of hydrogen atom to cause the biohydrogenation of unsaturated fatty acids to saturated fatty acids in rumen rather than producing methane in rumen.
Other Strategies:
A variety of feed additives, including some live agents have been incorporated in the feeds, which have been found to help reduce methane emissions from ruminants.
1. Plant secondary metabolites: Such compounds are found in tree leaves viz. terpenoids, phenolics, phenolic glycosides, tannins (both hydrolysable as well as condensed), lignins, saponins, alkaloids, polysaccharides (gums), essential oils etc. All these have anti-microbial activities against one or the other types of cellulolytic microbes in rumen. There is no doubt that these are bitter in taste, but the animals in hilly areas have adapted themselves to the consumption of such plants, resulting in lesser methane emission.
2. Direct inhibitors/ chemical feed additives: Halogenated methane analogues (but are not much effective). Apart from propionate, dicarboxylic acid, fumerate and malate, have been suggested as potential hydrogen acceptors in rumen, and thereby, help in reducing methane production in rumen.
3. Defaunation: Since Protozoa are known for their methane producing ability in rumen, reducing the protozoal population in rumen, using defaunating agents can also help as a methane mitigating strategy. Plant secondary metabolites like saponins, present in certain tree leaves have been used as dafaunating agents to decrease protozoal population in rumen, and thereby reducing methanogenesis in rumen.
4. Microbial feed Additives: Probiotics based on (Asperillus oryzae), the microbial feed additives, have been reported to reduce methane production by 50% as a result of reduction in protozoal population. Bactriocin are naturally occurring bacterial products. Since many lactic acid bacteria produce bacteriocins, it may well be part of the reduced methane production, observed at low pH is due to bacteriocin effect rather than a direct pH effect.
Nitrous Oxide emission from urine and dung
Some part of the dietary N given to ruminants is lost through dung and urine as urea and other nitrogenous compounds, the quantities depending up on the type of protein fed to the animal. The highly rumen degradable proteinous oil cakes given to these animals produce lot more ammonia in rumen.
And if due to lack of availability of carbon skelton in rumen, viz., CO2, this ammonia is not trapped back to synthesise, first the amino acids, followed by microbial protein synthesis in rumen. In that case, the excess ammonia after absorption from rumen wall is transported to liver. In liver, ammonia is converted to urea, and excreted out through urine. In this process, the animal undergoes a double loss viz. part of the dietary protein, and part of its own energy, which the animal has to use for the synthesis of urea from ammonia in liver.
There is no emission of N2O gas as such during enteric fermentation, however, the presence of nitrates in rumen on feeding green fodder, having higher HCN (Hydrocyanic Acid), may release small amounts of N2O reduction reaction in the gut.
Animal manure and its management
Animals produce large amounts of dung, which consists of 20% moisture content and a sizable amount of energy and nitrogen of dietary origin. It produces methane during fermentation and Nitrous oxide due to nitrification and denitrification reactions. There is no systematic dung management practice followed in India. While roughly 50% is converted to dung cakes and used for cooking purpose after drying in the rural areas, the rest is utilized as organic manure. Due to the drying process, dung cakes are hardly able to emit methane, may be only 5-10% of its wet form. However, the heaps of wet dung are capable of emitting both Methane as well Nitrous oxide.
Environment friendly aspect of feeding bypass protein to ruminants
Feeding of bypass protein to ruminants (the proteins which are less degradable in rumen), produces less ammonia in rumen, which, apart from enhancing microbial protein synthesis in rumen, results in more absorption of intact amino acids from the intestines.
The higher quantities of amino acids reach liver, where these are used as precursor for glucose synthesis through the process of gluconeogenesis. This results in more supplies of glucose to mammary gland for milk synthesis. On the other hand, it also results in less outgo of urea through urine, which otherwise could pollute the environment through its conversion to Nitrous Oxide.
By Dr T.K. Walli, Think Grain Think Feed
