By weight, no cattle do not produce the most methane. Insects do. There are a variety of insects, that also have methanogens in their digestive tracts, that produce methane. (Methanogens are bacteria in the digestive tract that produce methane via fermentation also known as methanogenesis). Cockroaches, termites, centipedes and various arthropods all produce methane as noted in this study from 1994 “Methane production in terrestrial arthropods.” Roughly 200 to 300 hundred cockroaches emit as much methane as a head of cattle.
If you look back at articles from the early 1980’s on termites, based on laboratory (in vitro) experiments, many scientists thought termites could be responsible for up to 30% of global methane emissions. However when the termites were looked at in their natural environments (in situ), the methane produced via methanogenesis in termites digestive tracts was more than offset by methanotrophs in the the soils and degrading materials of the environments that the termites lived in. Methanotrophs are bacteria that digest atmospheric methane (methane oxidation).
Now the problem with so much of the discussion with cattle and methane is that there is no discussion of context. The way enteric emissions (essentially burps) have been measured with cattle is either through masks, SF6 tracers, or chambers. This means the cattle’s emissions are measured out of the context of where the cattle live.
In healthy well managed pastures, cattle help build organic soil matter that stores more carbon via photosynthesis (carbon is pumped by plants roots into the soil in exchange for soil nutrients) and due to the land not being disturbed. When soil is cultivated, that is tilled, soil releases carbon into the atmosphere and thus doesn’t retain as much carbon. Synthetic fertilizers also reduce the soils capacity to retain carbon. When there is more soil carbon, soils retain more water. There are various numbers that show that for every 1% increase in soil carbon, every acre of 6″ deep soil holds any where from 10,000 to 27,000 more gallons of water. Now when soils retain more water, aren’t tilled, and are continuously covered with plants (as is the case with grassland ecosystems used for grazing), soil ecosystems are healthier and contain more soil bacteria including the methanotrophs listed above. Just like with termites, in intact ecosystems, methanotrophs in grasslands being grazed offset the methane created by methanogens via methanogenesis in ruminants (cattle, bison, sheep, goats, yak, wildebeast, etc) rumen. Yes research has indicated that grazing increases methanotrophic activity.
So when looking at methane, and ghg's in general, you can't just look at numbers, you have to look at the entire ecosystem context.
For example, peat bogs emit a lot of CH4, but they also sequester a lot of carbon. Same thing with tropical forests. The trees in the forest are a huge source of methane, but they also sequester a huge amount of carbon; more than enough carbon to offset all the methane these trees produce. No one is suggesting draining peat bogs or tearing down forests because of methane emissions. As previously noted, many different types of insects also produce enteric CH4 via methanogenesis. There are billions of these insects. But they too serve an ecological function, so no point in eradicating all insects. (What would birds eat?).
So same thing with wild ruminants. These animals were in intact functioning ecosystems where all the enteric CH4 was offset via carbon sequestration and methane oxidation. Part of the problem with domestic ruminants are feedlots and poor grazing management. With feedlots, this takes the ruminants out of the ecosystem context. With poor grazing, that also reduces the effectiveness of the ecosystem offsets.
But , as this research from India notes, the biggest problem with messing up the context for where ruminants use to roam is tillage, bare ground and synthetic inputs for Ag production (especially industrial Ag production for commodity crops as well as tilled organic for annual production) since these methods and inputs destroys the capacity of the soils in these ecosystems to function as GHG sinks for both carbon AND methane.
By: Stephen Zwick, Cornell University