There's a cautionary tale told among dryland graziers of animals starving to death standing belly deep in dry grasses. They are stuffed full from having eaten as much as they could, and somewhat bloated due to having plenty of drinking water to slake their thirst. The problem is that the dry grasses are low in protein and without sufficient protein they can't digest the cellulose of the dry grasses and get the "energy" (as cattlemen call it) value of the complex carbohydrates contained in the dry grasses.

The digestive system of ruminants can be understood by analogy to a compost pile. If you don't have enough nitrogen then the bacteria can't work and the pile just sits there, a cold and soggy mess rather than a rapidly decomposing proto-soil verging on being hot due to the feverish activity of bacteria which release great quantities of heat as well as gasses as they munch their way through the carbon in the pile. The rule of thumb is that you need a 30:1 carbon:nitrogen ratio for optimal results.

It is the nitrogen in protein that enables rumen bacteria in ruminants to thrive and digest the cellulose, the carbon, in dry grasses. They are largely the same as many of the bacteria in soil and compost piles, though it is only the anaerobic bacteria, the methanogens, that live and work in the airless rumen without the oxygen breathing aerobic bacteria that live with the anaerobic bacteria in soil and compost . . . at least near the surface and in well aerated conditions without too much moisture.

There are two interesting and contentious points in this: 1) enteric fermentation in a ruminant's gut is much like the ubiquitous bacterial decomposition of organic matter in the soil - many of the bacteria are the same species; 2) those bacteria are methanogens, meaning that they generate methane as a byproduct of digestion, and this has climate nutter knickers in a twist.

It's a false concern, a misunderstanding of the natural carbon cycle that has gone on forever and so is not sensibly viewed as an anthropogenic source of GHG emissions, but AGW advocacy is not about good science or sensible understandings, it's about politics and a warped sort of nativist religion.

A more interesting and sensible issue is that there are management actions that can reduce the production of methane from enteric fermentation, and while this is not a true AGW issue it is a good management issue since more of the energy embodied in forage can be extracted by ruminants for their own benefit. They can get more food energy out of the same forage and so need less forage to thrive.

Pasture management, including forage species selection, stocking rate and continuous vs. rotational grazing strategies have all been shown to influence enteric methane emissions in Canadian production systems. Perhaps the most promising pasture management strategy identified to date for mitigation of enteric emissions is the inclusion of legumes in the forage species mix. A cow-calf study at Brandon, Manitoba compared performance and enteric emissions of alfalfa-grass and grass only pastures over the course of a grazing season (McCaughey et al., 1999). Dry matter intake was greater for cows grazing alfalfa-grass pastures than for grass-only pastures (11.4 vs. 9.7 kg d-1), however, methane production, adjusted for differences in body weight, was the opposite (0.53 vs 0.58 g kg BW d-1, respectively). Energy lost as enteric methane emissions were 7.1 % of GEI for alfalfa-grass vs. 9.5 % of GEI for grass-only pastures. An 11 % increase for calf growth rates on the legume-grass pasture would serve as further incentive to consider legume incorporation as a mitigation strategy. The lowered methane loss observed with legumes is attributed to the lower proportion of structural carbohydrates and faster rate of passage of legumes, which will shift the fermentation pathway towards higher propionate production.

The extent to which forage species can influence enteric methane emissions of pastured ruminants is not known under Canadian conditions. In New Zealand, Waghorn et al. (2002) fed sheep a wide range of fresh cut, good quality forages and observed a two-fold range in methane emissions, from 11.5 g CH4 kg-1 DMI with birdsfoot trefoil to 25.7 g CH4 kg-1 DMI with a ryegrass, white clover pasture. All forages were delivered to the animal daily and had a DM digestibility of 70 % or greater. Animals grazing on pasture have the ability to be more selective than animals in this feeding trial, therefore the possibility exists that differences between forage species is even greater for pastured animals.

An 11% increase in calf growth is huge. But I suspect that the issue isn't legumes per se, it is protein, or nitrogen at the chemical level. Legumes are often higher protein than many grasses since they produce their own nitrogen in symbiosis with soil bacteria that fix atmospheric nitrogen. Increasing the nitrogen ratio in the rumen in many instances accomplishes what the researchers observed. Digestion is faster and more complete, which reduces methane emissions while increasing the benefit to the animal, especially growing calves that require more protein.

More can be done. Nitrogen levels aren't the only consideration.

All methanogen species can utilize hydrogen ions (H2) to reduce CO2 in the production of CH4 as this reaction is thermodynamically favorable to the organisms. Availability of H2 in the rumen is determined by the proportion of end products resulting from fermentation of the ingested feed. Processes that yield propionate and cell dry matter act as net proton-using reactions, whereas a reaction that yields acetate results in a net proton increase (Hegarty, 1999). Other substrates available to methanogens include formate, acetate, methanol, methylamines, dimethyl sulfide and some alcohols, however, only formate has been documented as an alternative methane precursor in the rumen (Jones, 1991).

Symbiotic relationships are known to exist between methanogens and rumen microflora. The maintenance of a low ruminal partial pressure for H2 can increase yield of acetate and increase the energy yield for methanogens as well as other microbes such as Ruminococcus albus (Wolin and Miller, 1988). Some methanogens are ingested by, and live within protozoa as metabolically active endosymbiots. Findlay et al. (1994) suggests that these endosymbiotic methanogens may generate 37 % of rumen CH4 emissions. Further, Stumm et al. (1982) has estimated that 10 - 20 % of rumen methanogens may be attached to the outer surface of protozoa, with attachment increasing 10 to 100 fold after feeding as compared to before feeding. Thus, a wide range of factors can influence the enteric CH4 emissions due to a direct or indirect impact on microbial populations and activity levels.

There is speculation and some supporting data for the use of char as a dietary supplement. Much of it is shrouded in fuzzy nativist religion and mostly advocated as a method to produce manure doped with char. Char that is well wetted and steeped in urine or "manure teas" more quickly benefits soil by providing substrate for microorganisms. But the similarity of enteric fermentation to decomposition of organic matter in soil suggests that the true benefit may be in providing substrate and attachment points for rumen microflora. If so there could be further reduction of methane production and emission, and further increase in energy yield for ruminant growth.