Cows have something of a bad reputation among environmentalist organizations; they've been accused of being a major source of methane, and the fields they require to graze in takes up a large amount of space. These factors make beef an apparently unsustainable source of protein--but can it be made more sustainable?
In this situation you'd expect scientists to be able to provide an answer, but they seem to be conflicted. On one hand, Discovery News and Science News both agree that grass-fed cows emit more emissions and are thus less sustainable than corn fed cows (although corn-fed beef is a less nutritious source of protein for the consumer). Those articles were published in 2010 and 2009, respectively.
But on the other hand, Time Magazine reported in 2010 that grass-fed cows could "save the planet" due to the fact that, though grass-fed cows may produce more methane, their net emissions are lower as a result of carbon sequestered by the soil.
A 2012 article from the Portland Tribune reports both sides of that story, but adds that regardless of what cows are fed, the amount at which Americans are consuming them makes it impossible for all cows to be grass-fed. No matter their methane production, such a high rate of beef consumption appears to be absolutely unsustainable.
Lisa Abend. (Jan. 25, 2010). How Cows (Grass-Fed Only) Could Save the Planet. Time Magazine. Retrieved 2012-02-07 from http://www.time.com/time/magazine/article/0,9171,1953692,00.html
Reed Jackson. (Jan. 19 2012). Are grass-fed cows better for the earth? Portland Tribune. Retrieved 2012-02-07 from http://www.portlandtribune.com/sustainable/story.php?story_id=132691751796459300
Jessica Marshall. (Jan. 27, 2010). Grass-fed Beef has Bigger Carbon Footprint. Discovery News. Retrieved 2012-02-07 from http://news.discovery.com/earth/grass-fed-beef-grain.html
Janet Raloff. (Feb. 15, 2009). AAAS: Climate-friendly dining ... meats. ScienceNews. Retrieved 2012-02-07 from http://www.sciencenews.org/view/generic/id/40934/title/Science_%2B_the_Public__AAAS_Climate-friendly_dining_%E2%80%A6_meats