[hinkley]

> You exhale carbon dioxide. However, this carbon dioxide comes from the carbon in the food you eat, and the food you eat obtained it from the atmosphere. Thus, it's a cycle. As a system (ignoring food transportation, deforestation, etc.) it's effectively carbon neutral.

All of modern agriculture and food science has been about turning inedible calories into edible ones (think: cooking meat with wood, baking bread). There is almost no food you eat that isn't touched by fossil fuels at some point in the process.

The cows are eating grain raised using anhydrous ammonia (made with natural gas) and processed using diesel/gas/electricity to be edible before the cow ever sees it. That cow is 'eating' all of those fossil fuels. Only silvopasture cows wouldn't be, but have methane emissions from those cows been studied?

[fred_is_fred]

The Haber process which you mentioned is really key here. It uses a lot of natural gas - more interesting than cow methane might be using nitrogen fixing bacteria or crop rotation (legumes) to reduce it's usage.

[analog31]

From what I've read, you can't grow enough legumes to fix the needed amount of nitrogen for high intensity agriculture. The Haber process sustains a sizable fraction of the world's animal population.

[seunosewa]

Human urine would be a sufficient source of nitrogen but it ends up in sewage nitrification-denitrification reactors that eventually turn most of the urea back into nitrogen gas so that the effluent can be safely discharged into water bodies.

[analog31]

I guess that makes sense, the nitrogen has to go somewhere.

[hinkley]

I'm not sure what it does to their methane output, but can't you feed legumes straight to cows? You definitely can to chickens and pigs.