"Keeping cows outdoors reduced fuel use, as well as the carbon dioxide emissions from farm equipment. And one very important point: “When farmland is transitioned from rotated crops to perennial grassland, you can build up lots of carbon in the soil and substantially reduce your carbon footprint for 20 to 30 years,” said Rotz.
"Raising cows in pasture is also good for water quality because of a huge drop in sediment erosion. The runoff of phosphorous also drops significantly.
"Grass-fed cows aren’t as efficient in producing milk, at least in terms of volume. Each confined cow produces 22,000 pounds of milk a year, while the pasture-raised cow produces only 13,000 pounds. However, the total amount of milk protein and fat in the milk produced was essentially the same, because “the foraging cows produced milk with far more fat and protein,” explains Rotz. Even more important in the argument, Rotz says, is that 130 grass-fed cows can produce the same amount of milk as 80 confined cows on the same amount of land. That's because amount of land needed to raise feed (grain) for the 80 confined cows is the same as the amount of land needed for grazing 130 cows."
Via Mark Sisson. P.S. Original link no longer at the source, here's the archive.org version.