Washington ? What makes milk or meat organic? After a drawn-out debate, the Agriculture Department has significantly narrowed the definition to livestock that spend a third of the year grazing on pasture.

New rules announced Friday say organic milk and meat must come from livestock grazing on pasture for at least four months of the year, and that 30 percent of their feed must come from grazing. The old rules said only that animals must have “access to pasture.”

It took years to craft the new regulations, which offer clarity for ranchers, food companies and consumers, who have forked over billions of dollars for organic food without a crystalline standard for livestock.