Air officials for years have blamed dairy cow emissions for the unusually high ozone levels in California's San Joaquin Valley, but a new study points more to what goes into the animals than what comes out.
The study — funded by the U.S. Department of Agriculture, California Air Resources Board and the San Joaquin Valley Air Pollution Control District — initially was intended to measure the impact of animal manure, urine and flatulence on ozone levels.
University of California, Davis researchers, however, found that the bigger ozone culprit appears to be millions of tons of fermenting cattle feed. This previously unrecognized source is likely the reason why ozone levels have not dropped even as the region has implemented control programs, scientists said.
"The take-home is that feed sources might be more important than all of the things we've been caring about in the past," said Michael J. Kleeman, a professor in UC Davis' department of civil and environmental engineering who was the study's lead investigator.
When tests on animal waste failed to find as much ozone as expected, researchers turned their attention to the silage — giant mixes of corn, alfalfa, almond shells and corn stalks that's piled to ferment under black plastic. The alcohol-drenched concoction is scooped with tractors and dumped into dairy cow feed troughs.
Researchers found that the gases emitted during the fermentation react in the atmosphere to turn oxygen into ozone.
"What was surprising was the scale of the issue," Kleeman said. "We looked at the census of agriculture and realized how much is being used."
The valley, which is under federal mandate to reduce its ozone levels, is the No. 1 dairy production region in the country with nearly 1.5 million dairy animals. Repeated exposure to ozone can scar lung tissue and worsen asthma, emphysema and bronchitis.
With air officials focusing for years on animal waste as the reason why smog is highest here despite having less vehicle traffic than large cities, many dairies invested in expensive methane digesters to deal with the problem.
Dairy operators long suspected that something other than the manure lagoons could be to blame and requested more studies.
"We felt more likely it was coming from the fermentation process and likely the feed," said Michael Marsh, executive director of Western United Dairymen.
The focus on controlling emissions at dairies and other confined animal operations to improve air quality has grown in recent years — along with the size of the operations. In 1992, only 10 percent of all dairy cows lived on farms with more than 1,000 others. By 2007, that increased to 36 percent, most in the San Joaquin Valley.
Researchers working on the four-year study developed a portable smog chamber to test onsite the waste from dairies and feedlots.
The study was published this month in the American Chemical Society's journal of Environmental Science & Technology. More studies are required to ensure that the team's findings are correct, Kleeman said.
"There are still other reasons to care about those waste emissions, but for the urban regional ozone problem, it looks like the waste isn't as important as feed," Kleeman said.
Already the San Joaquin Valley Air Pollution Control District is working to amend emissions regulations to focus on new requirements for handling silage, said Executive Director Sayed Sadredin. In June, he plans ask the board to require that dairies bag their silage, a move that could cut ozone emissions by 90 percent.
While bagging silage adds expense, dairy operators say the cost is far less than a $2 million methane digester.