Although it has a relatively low population density, California's San Joaquin Valley has some of the worst air pollution in the nation, especially when it comes to ozone (O3), a gas that can cause respiratory and cardiac problems. To counteract the air pollution, California and San Joaquin Valley regulators have taken significant actions, but results to date have been disappointing. It seems that the people who estimate the pollutant emissions from transportation, industry and agriculture missed something. The San Joaquin Valley is home to much of California's dairy industry, so it has been suggested that the masses of manure or perhaps the cows themselves were a big source of air pollution.
A new study comes to a somewhat surprising conclusion: cattle feed is a previously unnoticed source of ozone precursors, possibly being responsible for more ozone formation than light-duty cars and trucks in the valley. Basically, as the cattle feed ferments in huge covered piles, all sorts of alcohols, aldehydes and other reactive gases are formed and can react with oxides of nitrogen to form ozone after being released. The biggest contributor is corn silage because of its popularity among dairy farmers (10 million metric tons feeding 1.9 million cows in a recent year), possibly leading one to remark something like "Corn, is there anything you can't screw up?" However, other types of feed also emit ozone precursors, so it's conceivable that switching from corn silage to alfalfa or oat silage could have a similar impact (the articles, however, are not clear on how switching feeds would change things). To reduce this source of pollution, farmers will need to change the way they feed their cows so that or the way that they store silage — the articles are short on specifics. Dairy farmers in other concentrated dairy areas probably don't need to rush out and change their practices, as the geography of the San Joaquin Valley makes it a place that is uniquely susceptible to air pollution. (Fresno Bee; Green at the New York Times; Science News; the full article is in Environmental Science and Technology, subscription only, but might be available for free via EurekAlert!)