Dairy cows need adequate protein for milk production, so farmers need to supplement their animals' diets with extra protein.

But an Agricultural Research Service scientist has found that while a certain amount of dietary protein is necessary for lactating dairy cows, too much can contribute to nitrogen pollution and, in some cases, decreased milk yield.

Dairy scientist Glen A. Broderick, with the agency's U.S. Dairy Forage Research Center in Madison, Wis., found that the optimal percentage of protein in the diet — which minimizes urinary nitrogen excretion without reducing milk production — is 16.5 percent. This is compared to the 18-19 percent often provided by many in the industry.