Silage Not Manure, True Cause of High San Joaquin Ozone


A study lays the blame for the unusually high ozone levels in California’s San Joaquin Valley not on dairy cow emissions that took the blame for years, but on millions of tons of fermenting cattle feed. The study was conducted by U.C. Davis researchers and funded by the USDA, California Air Resources Board and the San Joaquin Valley Air Pollution Control District. The research initially was intended to measure the impact of animal manure, urine and flatulence on ozone levels.

The finding further skewers the notion that lagoon emissions and cow farts lead to diminished air quality. Last week, a professor at U.C. Davis published a study challenging livestock’s contribution to global warming.

The previously unrecognized source of fermenting cattle feed is likely the reason why ozone levels have not dropped even as the region has implemented rules for emission control programs, scientists said. When the rules were made several years ago, many experts considered the region’s 2 million cows and their manure as the primary source of these gases. Under the rules the industry spent millions of dollars cleaning up barns, corrals and manure storage areas in the nation’s most productive dairy region. Now those improvements may not have needed to be made.

“What bothers me is the rush to regulate w/o having the full facts or data,” said Ray Prock, a Denair CA dairyman.

Irrigated agriculture in the San Joaquin Valley of California.

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 or lagoon cover systems.

According to Prock, the potential costs of permitting requirements are more worrisome than bagging silage. He said that the new regulations will most likely be based more on best management practices than on implementing new technology like methane digesters.

“We already bag our silage, but I think covering systems may also be a part of the solution,” Prock said.

“We will try to focus on feed handling,” said district executive director Seyed Sadredin. “We might get a lot of pollution reduction by simply having farmers feed the animals out of sacks instead of spreading the feed out on the ground in the open.”

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