What is the best response to give to somebody who doesn't believe that grass-fed cows emit more methane than grain-fed?

[teo123]

I've started a discussion about veganism on forum.hr, and, apparently, a user called mukotrpak thinks that the claim that grass-fed cows emit far more methane than grain-fed cows contradicts chemistry. The argument goes that, since the formula for cellulose (the most abundant carbohydrate in grass) is (C6H10O5)n, we would expect its breakdown to glucose (C6H12O6) to not give any carbon compounds (like methane, CH4) as a side-product, since they both contain 6 atoms of carbon. What is the correct response to that? I am almost certain that argument is incorrect, as it's quite a well-known thing that grass-fed cows emit more methane than grain-fed cows, and there have been many peer-reviewed papers written about it. It's possible to measure how much more methane grass-fed cows emit, and a commonly cited statistic is that it's around 3 times per a litre of milk. If that contradicts chemistry, how come nobody noticed that?

[brimstoneSalad]

First, please provide some links to studies which show the difference is this large. This is not my understanding.

It IS an unfortunate reality that grass feeding is less efficient and probably yields more methane due to that, because grass feeding is probably slightly more ethical than grain feeding since foraging behavior is more likely psychologically beneficial to cows vs. eating out of a trough of food.
In studies, it's hard to control for activity level. Grass feeding requires more walking around. More walking around means less weight gain vs. calorie input.

Regarding the chemistry, the cellulose IS broken down into sugars first by enzymes. It's those sugars that are fermented into methane. That's part of the pathway. They aren't magically absorbed before the bacteria has a chance to act on them. My understanding is that cows principally derive nutrition from the fatty acids which are a product of the fermentation -- this is the case regardless of the source of those sugars.
There are, in my understanding, certain differences in those starches which favor different fatty acid production which result in more or fewer hydrogen which yield methane. This is a matter of current and ongoing study. I recommend that you do not try to understand them without a solid foundation in chemistry, you will likely do so in error and propagate that misunderstanding to the detriment of all.

What boggles my mind is how you still encounter or generate arguments which run counter to fact and that you do not say, "wait, this is obviously wrong. What am I misunderstanding, what are the core assumptions I'm making?"
How do you not, in cases like these, think to check your assumption that cellulose is fermented directly into methane based on a simple stoichiometric equation?

To sum up: You're probably wrong about magnitude of difference. Fermentation doesn't work like you think it does. But yes all indications are that net methane is higher for meat output from grass feeding and grass feeding isn't a solution to the ethical and environmental problems of animal agriculture -- which is arguably a shame, or maybe for the more hardcore vegans arguably a benefit because it stops meat eaters from having a get out of veganism free card. However this is a hard argument to make because the chemistry is complicated and it's a matter of ongoing research. Land use and overall efficiency is an easier argument. Grass feeding prohibits reforestation and just can't feed global demand without massive deforestation.