my understanding of the problem is that methane has a much greater 'greenhouse load' per unit of carbon than other forms of carbon released through the digestive processes of other animals.

does anyone know what the other forms of carbon are?

as indicated above, the methane comes from cow burps, not farts.

in fact cattle don't strictly make the methane, but rather the microbes in their rumen (first stomach compartment). the microbes first break down the ingested plant matter into fatty acids, producing considerable amounts of methane (which is burped). cattle digest the fatty acids and dead microbes. (does this mean that cattle are not strictly herbivores?)