A fight is brewing over the practice of feeding chicken feces and other poultry farm waste to cattle.

A coalition of food and consumer groups that includes Consumers Union and the Center for Science in the Public Interest has asked the Food and Drug Administration to ban the practice. McDonald’s Corp., the nation’s largest restaurant user of beef, also wants the FDA to prohibit the feeding of so-called poultry litter to cattle.

Members of the coalition are threatening to file a lawsuit or to push for federal legislation establishing such a ban if the FDA doesn’t act to do so in the coming months.

Farmers feed 1 million to 2 million tons of poultry litter to their cattle annually, according to FDA estimates.

Using the litter — which includes feces, spilled chicken feed, feathers and poultry farm detritus — increases the risk of cows becoming infected with bovine spongiform encephalopathy, or mad cow disease, said Michael Hansen, a senior scientist at Consumers Union.

That’s because the spilled chicken feed and the feces contain tissue from ruminants — cows and sheep, among other mammals. The disease is transmitted through feeding ruminant remains to cattle.

“It takes a very small quantity of ruminant protein, even just 1 milligram, to cause an infection,” said Steve Roach, public health program director with Food Animal Concerns Trust, a Chicago-based animal welfare group that is part of the coalition.

Although it is rare, people can contract a fatal form of the disease by eating meat from cows with BSE.

The National Cattlemen’s Beef Assn., the beef industry’s main trade group, said the ban was not needed and that several FDA reviews had determined that the chance of cattle becoming infected with mad cow disease from eating poultry litter was remote.

“Science does not justify the ban, and the FDA has looked at this now many times,” said Elizabeth Parker, chief veterinarian for the trade group.

Parker noted that the FDA this year banned the use of certain types of tissue from any form of animal feed, even that eaten by chickens. Those tissues include brain, spinal cord material and other high-risk tissues where the pathogens believed to cause mad cow disease typically are found. The tissue ban greatly reduces chances that prions, implicated in mad cow, can find their way into the food chain, Parker said. She also said the disease was not a threat to public health.

“We have tested 800,000 cattle in recent years and have not found any evidence of BSE circulating in the herd,” Parker said.

But others remain concerned.

“I still think you need to totally restrict using any ruminant protein in feed that gets back to ruminants,” said Linda Detwiler, a food safety consultant and former U.S. Department of Agriculture veterinarian.

Prohibiting high-risk tissues as a feed source makes the chances of transmitting mad cow disease through poultry litter low but does not remove all risk, Detwiler said.