What a good question! Again, you could write a whole paper on this, or a thesis, but let me try to hit the major points. I’m  going to have to break up the answer into two bits here:

1) Grass-fed animals, on an individual animal level, produce more methane per day than grain-fed ones. But why is that?

Let’s start with a view of what’s going on inside the animal: 

Herbivores like cattle and sheep have a very complex ecosystem of microbes in their gut, particularly in the part of the stomach called the rumen. 

The rumen is like a giant fermentation vat - it’s anaerobic (no oxygen), warm, and has a pH ranging from neutral-ish to slightly acidic. 

Feed goes in, gets regurgitated and chewed to break it down into smaller pieces, and then the rumen microbes break it down.  

While some nutrients exit the rumen into the acid part of the stomach without microbes getting a hold of them, the majority of nutrients in feed go to keep the microbes healthy and happy. 

The byproducts of the microbes’ actions on these feeds help feed the animal

The basic equation is this: 

Feed + microbes -> VFAs + CO2 + methane +microbial protein

VFAs, volatile fatty acids, are short-chain fatty acids that get absorbed by the gut and used for energy - in fact, these account for >70% of a cow or sheep’s energy!  

Rumen microbes use nitrogen in feed to grow and make more microbes, and when they get washed out of the rumen into the acid stomach, become a major source of protein to the animal, especially on low-protein diets. 

Waste products like carbon dioxide and methane get burped out and become greenhouse gases.

Methane is what the rumen does with excess hydrogen. 

There’s been research that shows that the level of hydrogen in the rumen affects the rate of certain chemical reactions, especially ones needed for microbial function, and too much hydrogen can make it harder for some microbes to function.  

So methane production by specific methanogenic microbes reduces hydrogen in the rumen, allowing microbes to go on their merry way. 

What you feed cows alters how much hydrogen microbes produce as a byproduct of fermenting feed.  

The major VFAs, acetate, propionate, and butyrate, are always going to be produced, but the ratios differ depending on diet. 

When acetate or butyrate is produced, so is hydrogen, and hydrogen levels rise in the rumen.  

When propionate is produced, the reaction uses up hydrogen, and hydrogen in the rumen decreases.

Pasture-based diets contain lots of cellulose, which produces mostly acetate when fermented.  

This is good, because cellulose is one of the things that humans definitely can’t digest, so cows are turning human inedible food into tasty meat and milk

 But it also means that there’s more hydrogen in the rumen because of the higher acetate levels.  

Mostly-grain diets, which have more starch, favor propionate, so less hydrogen and therefore less methane gets produced by the animal itself

There are other more complex effects involving different microbial groups, plant compounds, and pH effects, but let’s stick with this for now. 

There’s also the factor that methane production is driven by how much feed enters the rumen, which is driven by how much feed the animal needs to meet its energy requirements.  Forages usually have lower energy per pound of feed and are less digestible, so an animal needs to eat more. This, combined with acetate being the major VFA, means that on a per day basis, a grass-fed animal will in general produce more methane than a grain-fed one. 

However, the nice thing about grass-fed beef is that the inputs to the system are lower.  On native pasture, the only inputs are often rain and manure.  On managed pasture, there may be irrigation, seeding, fertilizer, etc.  

For grain-based diets, you have to add on the energy (and greenhouse gases) from producing the feed, processing the feed, and transporting the feed, versus the greenhouse gases from managing pasture.  But grain-fed cattle eat a lot of byproducts from other industries that would otherwise go to waste (beet pulp, distiller’s grains, barley hulls) so you need to consider that. Emissions from feed can make up a good chunk of the overall emissions associated with animal production, so the answer gets even more complex fast.  

This specific kind of analysis, of assigning greenhouse gas emissions and summing them up for a product, is part of a technique called Life Cycle Assesment - that is, looking at the life cycle of a product to determine the inputs and outputs and the emissions associated with them.  I’m doing one right now on sheep production in California and it’s utterly fascinating, but it shows that in these situations, there often isn’t an easy answer, and it depends a lot on where you set the boundaries and what you define as an impact. The debate is ongoing, and there really isn’t one clear-cut answer right now. 

So, moving on to part 2 of your question:

Is it bad to let cattle graze land unsuitable for crops because the animals themselves produce more methane than the same cow on a grain-based diet? DEFINITELY NOT.  

Cattle grazing on rangelands is definitely sustainable if managed right.

 I discussed this on my previous post here http://animalsustainability.tumblr.com/post/159885334236/hey-there-ive-been-really-enjoying-reading-your but grasslands need large herbivores to survive, and given how much land is grassland, not producing livestock on grasslands wastes a lot of land that could feed people. Removing herbivores also changes ecosystem balance for many other species that rely on herbivores to clear out excess brush, provide manure, or alter habitats.

If we don’t graze these native rangelands with something, then we risk habitat degradation and impacts on the other species that live there.  Large herbivores are an important part of the grasslands’ circle of life, and help promote ecosystem health if managed sustainably.  Grass-fed systems are also important for using land responsibly to feed everyone. 

Methane is just one part of the big picture. We need to look at ecosystem health, and the methane and other GHGs needed to produce what we’d feed these cattle if we didn’t feed them pasture.

So to answer your question, Both grain-fed and pasture-based systems have their place in modern agriculture, and neither is strictly better than the other.  And the fact is: all systems have the potential to be sustainable!

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