The issue with agricultural methane is that it comes from renewables. The cows eat plants; those plants got the carbon out of the air. Left on the ground, bacteria would turn the same biomass into the same amount of methane. In fact, that’s used for fuel in some 3rd world countries as “biogas”. The problem with the cow farts thing is that it’s just looking at outputs without considering the source of the input carbon. If you really care about the climate emergency, you have to ignore the silly noise and focus on the use of fossilized carbon inputs being dug up from the ground.
Now, it is true that for agriculture there are some second-order effects. The fertilizers that enable us to grow the crops to feed the planet are often produced using fossilized carbon. To the extent that protein production (in animals or artificially) requires more feed than simply feeding the grains to people, that matters in terms of excess fertilizer use and some added transport and production costs. Maybe. It’s more true of swine production than beef, for example. It’s just not anywhere near big enough to waste time and political capital trying to overcome enormous cultural hurdles (and resultant backlash).
When you read the statements on agriculture fossil carbon use, most is through power and machinery. The U.S. categorizes it that way because of the way our federal departments are set up (they collect the data). So the addressable part of the agricultural carbon gets back to decarbonizing auto and other machinery, and decarbonizing electrical production.
Worrying about plastic bags from a climate perspective isn’t a second-order effect. It’s just ridiculous. The concern with all plastics is the persistence and chemical nature of the waste. That’s especially true of consumer plastics, where the odds of non-recycled disposal is highest.