The voluntary change would allow plant employees to check carcasses for defects and perform other quality-assurance tasks not related to food safety. That would free up some federal inspectors to focus more on food safety-related tasks, such as oversight and verification, microbiological testing for pathogens, sanitation standards and antimicrobial controls in the plant.

Signing the letter were Senators.: Roy Blunt (R-MO), John Boozman (R-AR), Saxby Chambliss (R-GA), Chris Coons (D-DE), John Cornyn (R-TX), Joe Donnelly (D-IN), Chuck Grassley (R-IA), Kay Hagan (D-NC), Heidi Heitkamp (D-ND), Johnny Isakson (R-GA), Amy Klobachar (D-MN), Mark Pryor (D-AR) and Mark Warner (D-VA).

The letter noted that a risk assessment which accompanied the proposed rule concluded more than 5,000 foodborne illnesses per year would be prevented if the system was changed in this way, saving about $80 million in health care costs annually.

"The effectiveness of this system has been demonstrated through a pilot programme which was run in 25 poultry plants for over 14 years," the letter said. "The proposed rule included data that compared performance measures at the pilot plants and traditional plants, and found that the pilot plants performed as well or better than traditional plants on key standards."

The senators concluded, "We are interested to know when you plan to send the proposed rule to the Office of Management and Budget, and your timeline for implementation of the rule, once finalised."

The Senate letter comes on the heels of a similar letter last month in the House sent from the chairman and ranking member of the House Committee on Agriculture, Rep. Frank Lucas (R-OK) and Rep. Colin Peterson (D-MN), respectively, and the chairman and ranking member of the House Subcommittee on Livestock, Rural Development and Credit, Rep. Rick Crawford (R-AR) and Rep. Jim Costa (D-CA).

In addition, during its consideration of the 2014 Agriculture and Rural Development Appropriations Bill in June, the full House Appropriations Committee adopted an amendment urging USDA to move forward on its proposal to modernise poultry inspection.