There is no way to produce meat in a sustainable fashion

Cows grazing from http://www.ars.usda.gov/is/graphics/photos/ Image Number K7686-7 Dryland grazing on the Great Plains in Colorado. Each cow on a pasture can emit about 350 liters (230 grams) of methane per day. Photo by Scott Bauer. (Photo credit: Wikipedia)

That's the argument James McWilliams makes in Thursday's NY Times.  He thinks that all the alternatives for producing meat industrially have severe limitations.  Here are the key points in his argument:

• Grass-grazing cows emit considerably more methane than grain-fed cows. Pastured organic chickens have a 20 percent greater impact on global warming. It requires 2 to 20 acres to raise a cow on grass. If we raised all the cows in the United States on grass (all 100 million of them), cattle would require (using the figure of 10 acres per cow) almost half the country’s land (and this figure excludes space needed for pastured chicken and pigs). 
• Many farmers who raise chickens on pasture use industrial breeds that have been bred to do one thing well: fatten quickly in confinement. As a result, they can suffer painful leg injuries after several weeks of living a “natural” life pecking around a large pasture. Free-range pigs are routinely affixed with nose rings to prevent them from rooting, which is one of their most basic instincts. In essence, what we see as natural doesn’t necessarily conform to what is natural from the animals’ perspectives. 
• The economics of alternative animal systems are similarly problematic. Subsidies notwithstanding, the unfortunate reality of commodifying animals is that confinement pays. 
• But rotational grazing works better in theory than in practice. Consider Joel Salatin, the guru of nutrient cycling, who employs chickens to enrich his cows’ grazing lands with nutrients. His plan appears to be impressively eco-correct, until we learn that he feeds his chickens with tens of thousands of pounds a year of imported corn and soy feed. This common practice is an economic necessity.  
• there is no avoiding the fact that the nutrient cycle is interrupted every time a farmer steps in and slaughters a perfectly healthy manure-generating animal, something that is done before animals live a quarter of their natural lives.
• Farmers could avoid this waste by exploiting animals only for their manure, allowing them to live out the entirety of their lives on the farm, all the while doing their own breeding and growing of feed. But they’d better have a trust fund. 

In McWilliams' eyes the only answer today is to stop eating meat.