Lowering methane emissions
Enteric fermentation in ruminants is one of the biggest anthropogenic sources of methane – a potent greenhouse gas which contributes significantly to global warming. So what are the possibile solutions for reducing methane emissions from livestock?
Globally livestock is responsible for about 14.5% of human-induced GHG emission, with cattle (beef and dairy) responsible for about 61% of these emissions – pig and poultry each contribute less than 10%.
Reducing the microorganisms producing methane in the digestive system of the cow is clearly of interest in climate smart agriculture as this would not only mitigate, in theory, it would also increase the productivity of the cow (remember, 6-12% of the energy in the cows feed is lost by methanogenesis). A number of different possibilities have been explored:
Options to reduce enteric methanogenesis
Reducing methanogens or other microbes involved in methanogenesis
As you will see below, there are a variety of approaches that aim to reduce the archaea that produce methane or other microbes involved in methanogenesis. So if there are so many options, why is there no preferred method? The answer is that the microbial community of the rumen is highly complex.
A whole ecosystem within the cow, including hundreds of different species of microbes, has evolved over thousands of years. Removing one group of microbes affects others and can negatively impact the whole digestive process. For example, removing archaea that produce methane by combining hydrogen and CO2, can result in an excess of hydrogen in the cow’s rumen which may impair digestion. The protozoa that produce the hydrogen, are important for the breakdown of plant material and digestive efficiency, and ultimately the productivity of the cow.
So while exploring the options of methane mitigation, we must keep in mind that removing specific players, responsible for methanogenesis, may affect the delicate balance of the microbial ecosystem within the digestive system and have a negative effect on the cow’s ability to digest food and ultimately to thrive.
Immunisation against methanogens
This is basically a vaccination against the microbes that produce methane. So far results are not conclusive as vaccines work very specifically on certain microbe strains and there is variation in the efficiency.
Probiotics can shift the microbial community in the rumen towards a lower proportion of methanogens and a higher proportion of microbes producing more volatile fatty acids, used as a source of energy by the ruminant. Ionophore antibiotics such as Monesin have been shown to reduce methanogenesis, presumably by the shift in the fermentation processes and the reduction of certain microorganisms.
In some cases it has also been shown that organic acids like malate, fumarate or acrylate, drastically reduce methane emission, but the results of these studies vary greatly and remain inconclusive. Many plant secondary compounds such as tannins, saponins or essential oils have been shown to directly reduce methanogens and hydrogen production in the rumen.
Some oils such as linseed, coconut, garlic and cotton oil are considered to be amongst the most effective additives for methane mitigation. Adding fats to the cow’s diet offers a promising solution for reducing methanogenesis, without having a significant negative impact on other functions of the rumen.
Other substances like Nitrates, 3-nitrooxypropanol, or Bromochloromethane have also proved to be effective in reducing methane emission.
When using feed additives, factors like costs, palatability, a possible build-up of resistance in the microbial community, negative impacts on the environment, traces in milk or meat or even change in taste must be considered. Although many feed additives have the potential to reduce methane emissions, more research is required to determine whether or not they are effective in the long term (or if the rumen microorganisms adapt to them) and if there are potential risks.
Changes in a cow’s diet
Figure 2: Different plants used for cattle feed. ©University of Reading. (Click to expand)
University of Reading research suggests that replacing a pure grass diet with forage mixtures can be beneficial, as some plants like flowers could have the potential to lower methane emission due to secondary plant compounds inhibiting methanogenesis. Also tannin rich legumes like sainfoin have been reported to reduce methane emission. In general, cows fed on maize silage, emit less methane than cows fed on grass silage. You will explore the benefits of different forages later this week.
The diet of cows is generally balanced between forage – grass, hay, or silage rich in structural carbohydrates such as cellulose – and energy rich concentrates containing more sugar and starches. The more concentrates the cow feeds on, the lower the production of methane, in relation to the cow’s productivity, for example, per litre of milk. This is because the main substrate for methanogenesis are fibrous carbohydrates. However, feeding large amounts of concentrates without the addition forage can lead to an acidic environment in the rumen which is bad for the cow.
There is variation in the production of methane between individual animals, regardless of their diet. Depending on the feed efficiency of an animal, the relative amount of methane produced can be lower. There is some evidence that the level of methane production is a heritable trait in cows, which suggests this trait may be selected for breeding. However, the fermentative processes in the digestive system depend on the community of microorganisms, so the extent to which these microorganisms depend on the genetics of the cow, is highly debated.
As you see there are a vast number of possibilities for reducing methane emission from our dairy cows. Adapting the cow’s diet to increase their digestive efficiency appears to be one of the most promising approaches for reducing methane emissions. One option for lowering the carbon footprint of dairy farming would be to offset the CH4 produced by the cows by maintaining enough plants to take up an equivalent amount of CO2. However, in order to balance out the CH4 emissions of one dairy cow, you would need roughly 0.5 hectares of mature pine forest. To balance the emissions of a dairy herd of 500 cows, a pine forest the size of 250 football pitches is required.
References and further reading:
- Hammond, K. J., Humphries, D. J., Westbury, D. B., Thompson, A., Crompton, L. A., Kirton, P., Green, C. & Reynolds, C. K. (2014). The inclusion of forage mixtures in the diet of growing dairy heifers: Impacts on digestion, energy utilisation, and methane emissions. Agriculture, Ecosystems & Environment, 197, 88-95. Please note: this is a reference and only the abstract is free to view.
- Hammer, T.J, Fierer, N., Hardwick, B., Simojoki, A., Slade, E., Taponen, J., Viljanen H., & Roslin, T. (2016). Treating cattle with antibiotics affects greenhouse gas emissions, and microbiota in dung and dung beetles. Proceedings of the Royal Society B, 283(1831). Please note: this is a reference and only the abstract is free to view.
- Hristov, A. N., Oh, J., Firkins, J. L., Dijkstra, J., Kebreab, E., Waghorn, G., & Gerber, P. J. (2013). Special topics —Mitigation of methane and nitrous oxide emissions from animal operations: I. A review of enteric methane mitigation options. Journal of Animal Science, 91(11), 5045-5069. Please note: this is a download.
- Martin, C., Morgavi, D. P., & Doreau, M. (2010). Methane mitigation in ruminants: from microbe to the farm scale. Animal 4(03), 351-365.
© University of Reading