A recent post at the Reuters Environment Forum by Physics Professor Gidon Eshel looks at some of the issues. Here are some of his points with more comments:
- Grazing cows are ruminants: they get their energy from the cellulose in the grass they eat, but by a complex symbiosis. Microbes in their guts break down cellulose and generate methane as waste. Other microbes in their guts eat the methane. As the microbes die, the cows digest them. Unfortunately some of the methane escapes, and methane is a powerful greenhouse gas. (See a more detailed and disgusting discussion in this previous post.) Cows belch out about 100 million tons of greenhouse gases (CO2 equivalent basis) as methane in the U.S. annually, about 2% of U.S. greenhouse gas emissions.
- Cows fed grain can extract energy directly from the starch in the grain, just as we do. They can have a much different gut flora and may produce much less methane.
- On the other hand the production of grain (mostly maize) to feed to cattle requires a lot of fertilizer. And use of nitrogen fertilizer causes the release of nitrous oxide, another greenhouse gas. The production of corn to feed cows accounts for about 50 million tons of U.S. greenhouse gas emissions per year (CO2 equivalent).
- Also, most cattle finished on grain spent their early year or two on pasture. They are only fed grain to fatten them up at the end, and to put more fat (marbling) in their flesh to make it grade higher in the USDA system of meat grading. (See related post on cattle on feed.)
- Although this makes it look like grass-fed beef are responsible for twice the emissions of feedlot beef, that doesn't take into account the fact that vastly more beef is raised on feed than on pasture. On a per-pound-of-meat basis grass-fed is much more polluting.
- However, thousands of animals brought together in feedlots produce enormous amounts of manure. If this is left to break down anaerobically in waste lagoons it releases huge quantities of methane.
- But because all this manure is already being collected, the methane it produces can be captured and burned for its energy value, just like natural gas.
- Cattle on pasture naturally leave cow-flops here and there, where they break down naturally, releasing carbon dioxide into the atmosphere without producing any useful energy.
- Grazing beef takes a huge amount of land. More than a quarter of the U.S. is used for grazing and rangeland. It would otherwise be desert or prairie.
- However, little grazing land is irrigated. Production of maize takes large amounts of irrigation water, which is one reason a pound of beef embodies hundreds of gallons of water.
But in any case it seems likely that grass-fed cattle have a larger environmental hoofprint than cattle fed grains and other feed.
[crossposted from the HaraBara blog] [Updated 9 August 2010]