One study [1] compared two different types of grasses vs grain feed. The cows that ate grass produced more methane per kJ consumed than those that ate the grain feed. There are a bit stranger numbers preceding these figures, where the cows that ate one grass type actually produced less methane overall than all the other cows, but apparently because they were consuming considerably less (all cows were allowed to eat freely). Mind you, this was a study of 6 cows.
I live on a dairy. Totally not a controlled study, but the cows digestive systems can't handle grazing as well as they can handle a nutritionist-controlled diet. Their manure gets bubbly with gas when eating grass.
That article says the study found that grazed beef release more carbon than the carbon offset by soil sequestration. It doesn't say it releases more carbon then grain-fed, and actually, while I haven't read the source study, the general implication in the article seems to lean toward grazed beef releasing less than grain-fed (just not, according to the researchers, enough less to be carbon-neutral).
