I have read this article of 2019 and still don't understand which one is better (climate-wise). Grass-fed beef involves more belches and produces less meat but seqesters carbon in grass. But what's the bottom line? Does grass-fed beef have lower carbon footprint than feedlot-finished beef or not? It's been a while since 2019. Do they have any recent life-cycle analyses published in peer-reviewed journals?
It is complicated, on the one hand you have longer growing times and lower food conversion efficiency of pasture raised cattle, on the other pastures are a way of carbon sequestration.
There is a report prepared for Whole Foods Market, Inc. which may not be considered impartial, that concluded that pasture raised cattle were marginally (~10%) better than feedlot.
|Final Live Weight (kg)
|Feed Production (kg CO2 eq/kg live weight)
|Transportation (kg CO2 eq/kg live weight)
|Enteric Fermentation (kg CO2 eq/kg live weight)
|Manure Management (kg CO2 eq/kg live weight)
|Total (kg CO2 eq/kg live weight)
|Carbon Sequestration (kg CO2 eq/kg live weight);
|Total with Carbon Sequestration Factor (kg CO2 eq/kg live weight)
The the difference in enteric fermentation methane emissions are discussed by this paper:
When the cattle were grazed on pasture, they produced .23 kg CH4·animal−1·d−1, which corresponded to the conversion of 7.7 to 8.4% of gross energy into CH4. When the same cattle were fed a highly digestible, high-grain diet, they produced .07 kg CH4·animal−1·d−1, corresponding to a conversion of only 1.9 to 2.2% of the feed energy to CH4. These measurements clearly document higher CH4 production (about four times) for cattle receiving low-quality, high-fiber diets than for cattle fed high-grain diets.
It is worth noting the difference in magnitude between these two sources. I read this as significant uncertainty about the values. What is certain is that beef of one of the most environmentally damaging foods and we should be doing all we can to reduce its production.
You seem to think that the majority of footprint of beef (and similarly, milk) is caused by the carbon footprint of the stuff we feed to cows.
The carbon footprint of beef is almost completely caused by methane emitted by cows. This obviously depends on the timescale, if you consider for example a 10000 year timescale then that would be different but the timescales of interest for climate change are 100 years (used to be the most important timescale) but today more like 20 years (because climate change is happening way too fast).
In the long term, one methane molecule converts to one carbon dioxide molecule. Its weight increases from 16.042 grams/mole to 44.01 grams/mole. Thus, 16.042 grams of methane has the same 10000 year carbon footprint as 44.01 grams of carbon dioxide, or stated in another way: global warming potential is 44.01/16.042 = 2.7434.
But in short term, methane is a terrible greenhouse gas. In 100 years, GWP is 25-30 and in 20 years, it's 86.
An in those timescales, the problem with beef is the methane. It isn't what you feed to cows.
Any small increase of that methane more than offsets differences caused by the stuff you feed to cows. A cow produces ~100 kg of methane per year and gives ~200 kg of beef. Beef cattle lives for 2-3 years so it's 200-300 kg of methane per 200 kg of beef, or 1-1.5 kg of methane per 1 kg of beef. In 20 year time scale, that would be 86-129 kg of CO2 equivalent per 1 kg of beef. For pork it's about 5-10 kg of CO2 per 1 kg of pork, and that's almost entirely CO2 and not methane. So beef is more than 10x as bad as pork in 20-year timescale, the most important for preventing climate change from becoming dangerous.
About the only positive features of beef and milk are that their production is independent of production of cereal. If we have multiple poor years in cereal farming, we don't have much opportunities for feeding the world population since so large percentage of our foodchains is dependent on cereal. Beef and milk produced from grass offers another way to produce food from stuff humans can't eat. Pigs and chickens eat what humans could eat, but cows eat stuff humans can never eat (or you can eat grass but don't expect to get any nutrients out of it).
But I personally think that it would be far better to just start gradually increasing stored amounts of cereal (only gradually to not cause a global famine) to prepare for multiple consecutive bad years in farming cereal, and entirely phase out beef and milk. That would do wonders to slow down climate change, and at the same time removing the usefulness of main benefit of cows: food security.
Lab-grown beef is an entirely different matter, though. We will have lab-grown beef, and it will phase out using cows to produce beef. But how quickly lab-grown beef becomes cheaper than real cows, we can't be sure. It could be too slowly to prevent dangerous climate change.