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Condensed Tannins, Belching Ruminants and Atmospheric CO2:
A Novel Natural Way of Reducing Methane Emissions

Volume 5, Number 32: 7 August 2002

In a press release dated 01 May 2002, the New Zealand agricultural research institute AgResearch Grasslands announced a discovery that the organization says "could have a major impact on New Zealand's future methane emissions," which would, of course, have major implications for the war on global warming.  So what's the good news?

The New Zealand scientists, according to the press release, have recently proven that condensed tannins found in certain pasture plants can reduce methane emissions from grazing ruminants, such as sheep and cattle, and thereby reduce the global warming potential provided by this powerful greenhouse gas.  This finding is of special importance to New Zealand, because the methane expelled in the breath of sheep and cattle - which is a by-product of the fermentation of feed in the rumen of these animals - accounts for approximately 90% of the country's methane emissions.  Wedded to the Kyoto Protocol, as New Zealanders appear to be, a significant reduction in such a large national source of one of the atmosphere's most potent greenhouse gases would go a long way towards helping them meet their Protocol-mandated reductions in these (supposedly) climate-altering substances, which would be, as the press release states, "very welcome."

So what are condensed tannins?  And what do they have to do with atmospheric CO2?

Condensed tannins are naturally-occurring compounds found in a number of different plants, including some pasture species.  They are secondary carbon compounds produced in leaves that sometimes act to deter herbivorous insects.  In New Zealand, the "legume lotus" is one of the primary sources of these substances; and the AgResearch scientists determined that sheep and cattle feeding on it reduce their methane emissions by as much as 16%.  So thrilled are they by this finding, they are now talking, not only of using more tannin-producing species as animal forage, but of genetically introducing tannins into other pasture species as well.

The role of the ongoing rise in the air's CO2 content in this rapidly developing scenario may be deduced from a 1999 study of its effects on condensed tannin production in four genotypes of Lotus corniculatus (Fabaceae), specimens of which were collected half a world away in meadows south of Paris, France.  In that study, Goverde et al. (1999) determined that a 350-ppm increase in the atmosphere's CO2 concentration increased tannin production in one lotus genotype by approximately 17%, in a second genotype by 33%, in a third by 61%, and in a fourth by 140%.

It is interesting to note, in this regard, that whereas the world's scientists are just now discovering this significant means of combating one of the atmosphere's most powerful greenhouse gases, i.e., methane, nature has been employing the technique since the dawn of the Industrial Revolution, steadily boosting tannin production in plants that are eaten by ruminants as the air's CO2 content has gradually risen.

These findings are truly welcome, yet they are only a part of the good news reported by the New Zealand scientists, who note that tannins "have a variety of other animal related benefits, such as improved milk yields, increased liveweight gain, decreased internal parasite burden and reduced occurrence of bloat, dags and fly strike."  And, again, all of these tannin-induced benefits would be expected to be significantly enhanced by increases in the air's CO2 content that increase forage tannin concentrations.

It is also important to note that ruminants comprise a great group of animals (four-footed, hoofed, even-toed, cud-chewing mammals that have a stomach consisting of four divisions or chambers) in addition to sheep and cattle, including antelope, bison, buffalo, camel, deer, giraffe, goat, llama, etc., and that these animals eat a number of other types of plants, which may also experience increases in leaf tannin production as the air's CO2 content rises, as has in fact been found to be true for a number of different species, including both deciduous and evergreen trees (Lindroth et al., 1993, 1995; Traw et al., 1996; Hattenschwiler and Schafellner, 1999) and grasses (Goverde et al., 2002).

In light of these observations, it can be appreciated that many wild and domesticated animals the world over may be participating in this important natural "program" for reducing methane emissions to the atmosphere.  Could it be they are partially responsible for the reduced rate-of-rise of the atmosphere's methane concentration that has been observed over the past few decades?  If so, we can expect to see more of the same as the air's CO2 content continues to climb; for the biosphere, it seems, takes care of its own, as demonstrated by this unique negative feedback phenomenon that tempers greenhouse gas-induced global warming.

Dr. Sherwood B. Idso
Dr. Keith E. Idso
Vice President

Goverde, M., Bazin, A., Shykoff, J.A. and Erhardt, A.  1999.  Influence of leaf chemistry of Lotus corniculatus (Fabaceae) on larval development of Polyommatus icarus (Lepidoptera, Lycaenidae): effects of elevated CO2 and plant genotype.  Functional Ecology 13: 801-810.

Goverde, M., Erhardt, A. and Niklaus, P.A.  2002.  In situ development of a satyrid butterfly on calcareous grassland exposed to elevated carbon dioxide.  Ecology 83: 1399-1411.

Hattenschwiler, S. and Schafellner, C.  1999.  Opposing effects of elevated CO2 and N deposition on Lymantria monacha larvae feeding on spruce trees.  Oecologia 118: 210-217.

Lindroth, R.L., Kinney, K.K. and Platz, C.L.  1993.  Responses of deciduous trees to elevated atmospheric CO2: Productivity, phytochemistry, and insect performance.  Ecology 74: 763-777.

Lindroth, R.L., Arteel, G.E. and Kinney, K.K.  1995.  Responses of three saturniid species to paper birch grown under enriched CO2 atmospheres.  Functional Ecology 9: 306-311.

Traw, M.B., Lindroth, R.L. and Bazzaz, F.A.  (1996) Decline in gypsy moth (Lymantria dispar) performance on an elevated CO2 atmosphere depends upon host plant species.  Oecologia 108: 113-120.