In September, McDonald’s Corporation’s vice president of sustainability, Bob Langert, talked to Bloomberg News about his company’s efforts to pursue sustainability, a subject that he said “is everybody’s business now.”

Langert spoke of beef specifically and the difficulty of knowing what sustainable beef really means, saying, “Can we say we’re buying any sustainable beef today? No, we can’t. Could we be buying sustainable beef? We might be. What I mean by that is that there are no standards, measures, accountability and traceability to make those claims today.”

When companies like McDonald’s and Wal-Mart are talking about sustainability and taking part in the Global Conference on Sustainable Beef, it’s a sure sign that there is consumer interest in the subject. But the problem of a definition remains. “Organic” has parameters outlined by the USDA; “sustainable” does not.

Sustainability is a hot topic in the world of seafood, too, and that industry has made extensive efforts to address the subject itself. The widely used Monterey Bay Aquarium Seafood Watch offers its own color-coded ranking of seafood varieties that follows a simple rating system: Best Choice, Good Alternative, Avoid. On its website, consumers can click on different fish and learn more about where and how they’re raised and caught . The group offers regional pocket guides, mobile guides and a chart that provides sustainable alternatives to the least sustainable seafood. Shoppers standing at the fish counter can go to their app before deciding what to buy.

Other guides do exist — Target is using seafood sustainability ratings from the Marine Stewardship Council or the Global Aquaculture Alliance as it moves to selling only sustainable seafood by 2015 — but there appears to be at least a broad base of agreement among them about what sustainability means in the sea.

On land, things certainly seem more complex. Grass-fed beef is often mentioned in connection with sustainability, but there’s little agreement on it. A recent op-ed in the New York Times declared, “Grass-grazing cows emit considerably more methane than grain-fed cows...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...” That author concluded avoiding animal products was the true sustainable choice. The beef industry itself also disputes the notion that grass-fed beef is always more sustainable, pointing out, for example, that it takes grass-fed cattle longer to reach market weight, meaning their production requires more land and water and produces more manure.

Others point to organic production as part of the definition of more sustainable beef. That may not be right either, according to the beef industry. The Sustainable Beef Resource Center points to new research demonstrating that if some of the inputs that organic beef production eschews (including ionophores and steroid implants) were withdrawn, 17 million more acres of land and 138 billion more gallons of water would be required to produce the same total amount of beef.

In its own search for increased sustainability, McDonald’s has its initial efforts focused on five areas, which its analysis suggested held the greatest potential for positive change: poultry, coffee, palm oil, packaging and beef. The last may prove to be the toughest.