[twic]

There is no input of methane. The cows eat cellulose, and bacteria in their gut break the cellulose down into small molecules, including methane. Reduction in methane production can come from a reduction in the amount of cellulose broken down, or an increase in the production of other small molecules.

The other small molecules in question seem to be what are called "volatile fatty acids" (VFAs), which i think are more or less the same as "short-chain fatty acids" [0], and they're good for the animal.

Acetate and propionate are two main VFAs. A study on sheep [1] found that "Sheep fed Asparagopsis had a significantly lower concentration of total volatile fatty acids and acetate, but a higher propionate concentration.". If there's less methane and also less VFAs, then i assume less cellulose is being broken down. But more of the carbon from it is going into VFAs.

It would be good to have a study like this in cows.

[0] https://en.wikipedia.org/wiki/Short-chain_fatty_acid

[1] https://www.publish.csiro.au/an/an15883