U.S. Sen. Mark Udall wants to make sure new federal rules don't create too much of a burden for craft brewers and ranchers in Colorado.

Udall, D-Colo., on Monday urged the Food and Drug Administration in a letter to swiftly complete a "risk assessment" of the reuse of brewers' spent grains - the byproducts of the mashing and brewing process - as livestock feed. The spent grains, whose sugars are fermented to make craft beer, are an economical and high-protein food source for beef and dairy cattle.

According to a 2013 Brewers Association survey of its members, brewers currently re-sell or give away nearly 90 percent of their spent grain as livestock feed. The FDA's rulemaking could require brewers to dispose of these spent grains at landfills, and a media statement from Udall's office said that could force small breweries to incur an average cost of nearly $43 million per year, which could be passed onto consumers in higher prices or hinder job creation.

"Colorado's craft brewers are leading the way forward for their industry, creating some of the world's most innovative beers and sustainability practices," Udall said in the statement. "The FDA needs to ensure our food supply remains safe, but its new proposed rule may unjustifiably hurt Colorado's brewers and farmers,

"That's why I am urging the FDA to swiftly complete a risk assessment of brewers' uses of spent grains as a cost-effective and safe livestock feed. When brewers succeed, so do countless other businesses and sectors of our economy."

John Carlson, executive director of the Colorado Brewers Guild, echoed Udall's concern about the impact of the proposed federal rules.

"Colorado is the state of craft beer," he said. "We strive to innovate, adding that the use brewers' spent grain is "thoughtful, elegant and efficient. The costs of complying with the proposed federal regulations will be higher than the cost of sending spent grain to a landfill. Increased costs for Colorado brewers and Colorado ranchers and wasted food for livestock is not a good way to move forward."

Udall is a member of the Senate Small Brewers Caucus.