Last Part
There are many CH4 abatement possibilities, but it is crucial to develop new technologies. However, no single method by now is available by which enteric CH4 can 100% be abated. Thus, many strategies are required to apply simultaneously to mitigate enteric CH4 to get either per kg beef or per liter milk production. Some of them include first calving at younger age, improved genetic merit of animals, shorter calving interval, better quality fodders, disease control, strategic use feed supplements such as probiotics, ionophores, NPN, minerals, fatty acids, organic acids or extracts etc. and better feed efficiency. Ruminants fed tropical forages produce more CH4 than those fed temperate ones. The temperate fodders are of good quality because they contain less fiber, more crude protein and nonstructural carbohydrates than those of tropical ones. Animals fed blend of grasses and leguminous fodders produce less CH4 than those fed grasses only. Tannins are secondary compounds which negatively influence fiber digestion thus affecting enteric CH4 production in ruminants. Use of high quality animal feed, (no doubt an expensive option for third world or developing countries) generally improves productivity and lowers the amount of GHS emitted per Kg of the product.
In ruminants productive system, nutritionists challenge is to devise ways and means by which daily weight gain can be increased by mitigating CH4. In this way quantitatively less amount of feed is used for maintenance and total lifetime enteric CH4 production are reduced, resulting in less CH4 per Kg live weight or per Kg meat produced. Considerable efforts are being devoted to develop strategies through maneuvering ruminal microorganisms to produce less CH4. These basic mitigating strategies include direct inhibiting methanogenesis by redirecting H2 into alternative products, decrease H2 production in the rumen and redirecting H2 utilization. Defaunation (removal of protozoa) in lambs produced 26% less enteric CH4 per Kg DM intake. However, much contradiction exists in the results that have been reported between in-vivo and in-vitro data and between long and short term defaunation trials.
As mentioned earlier, type of feed offered to animal can have a major effect on CH4 production. The forage to concentrate ratio of the diet has an impact on ruminal fermentation. Acetate to propionate ratio increases with increasing forage to concentrate ratio and vice versa. The propionate might be the next major alternative H2 sink after CH4. It would therefore be deduced that enteric CH4 production would be mitigated when animals are fed high concentrate ration. Garlic (Allium sativum) also inhibits ruminal methanogens. Plant extracts like allicin, diallyl disulfide, allyl mercaptan and garlic oil are also considered to inhibit rumen methanogens. Use of ionophores is presently banned in some of the countries. But their inclusion in the ruminant diet reduces enteric CH4 production even up to 25%. Monensin one of the common ionophore inhibits cellulolytic bacteria and protozoa; hence fermentations shift from acetate to propionate production. As discussed earlier, propionate acts as H2 sink. Contrarily, some studies have shown that enteric CH4 production was not mitigated with prolonged use of ionophores. Various potential propionate precursors can reduce enteric CH4. Acrylate and fumarate are commonly used to reduce CH4 production. As compared to acrylate, fumarate was more effective in artificial rumens. Fumarate has been shown to lower 38% CH4 production in continuous fermenters, when forage was used as substrate. But in some studies, supplementation of organic acids like fumarate or malate showed non-significant reductions in CH4 when added to the diets of growing beef cattle or dairy cows. Fatty acids are the next good option to mitigate CH4 as they provide an alternate H2 sink and also possess ability to retard protozoal growth. But they can be supplemented to a certain limit. Methane production was 22% reduced when fed sunflower oil. Feeding canola oil or linseed oil negatively affects animal performance. Oil supplementation lowers feed intake and reduces fiber digestibility. Nitrates had previously been considered toxic. Recently some studies have shown that they can be amicably used as feed supplement in roughage based diets. Nitrates in rumen environment are reduced to NH3 (Capturing H2), promoting microbial cell synthesis. Nitrates if supplemented in roughage based diet improve its nitrogen content and at the same time mitigate significant amount of CH4. If ruminants are slowly adapted to nitrates, the incidence of nitrate poisoning can substantially be minimized. Use of nitrates as a CH4 mitigant still requires more investigation.
Recently, some efforts have been made to develop certain vaccines against methanogens. Use of bacteriophages as a biological control strategy could also prove effective for inhibiting methanogens. This redirects H2 to other reductive rumen bacteria such as acetogens or propionate producers. The only limitation is that to date no phage specific to rumen methanogens have been reported. Siphophages or siphoviridae phages infect methanogens such as Methanococcus, Methanobrevibacter and Methanobacterium spp. but siphoviridae phages have yet to be isolated from the rumen.
According to FAO, GHS emissions from livestock sector can be reduced to 30% through usage of better available practices or technologies. The CH4 emission reductions can be achieved by enabling all livestock producers to follow the practices already being practiced by most efficient operators. Greenhouse gas emissions from livestock add up to 7.1 Gigatons of CO2 equivalent (CO2-eq) per year. The main sources of GHS emissions are ruminant digestive process (39%), feed production and processing (45%) and manure decomposition (10%). The remainder is attributable to the transportation and processing of animal products.
In conclusion, cutting GHS emissions especially CH4 from ruminants is indeed a doable task and can be achieved by opting existing best practices and technologies in manure management, feeding, health and husbandry of livestock. Involving all stakeholders i.e. public and private sector, international organizations, civil society, research and academia may help resolve this important problem speedily by implementing solutions addressing the livestock sectors complexity and diversity.
The authors (a) Tamgha-i-Imtiaz is the OSU Distinguished Alumnus, distinguished national professor and (b) is Izaz-i-Fazeelat, Institute of Animal Nutrition and Feed Technology, University of Agriculture, Faisalabad.