Think dairy farm, and your mind may wander to images of cows grazing dewy green pastures, as glistening silos and red-walled farmhouses slumber in the distance. But something sinister is lurking in the grass: cow feces crawling with disease-causing Escherichia coli bacteria. A new study, however, reveals that these bacteria are much less likely to enter groundwater and cause illness if farmers spray their fields with water rather than flooding them, as is traditional.
In a previous study, chemist and environmental scientist Murray Close of the Institute of Environmental Science and Research in Christchurch, New Zealand, and colleagues investigated a high incidence of digestive illnesses among residents of a rural community in the southeastern region of the country. It turned out that nearby drinking wells were contaminated. When the researchers examined water from wells located near dairy pastures that used flood irrigation, they found E. coli from cow feces in about three-fourths of the samples.
In the new study, Close and colleagues looked at groundwater under New Zealand pastures watered with spray irrigation. During the 6-year study, they sampled groundwater from 10 plots of land every month for a total of roughly 700 samples. They found that only about 3% of the samples contained E. coli, compared with 77% in the previous experiment. Close attributes the large difference to the fact that bacteria escape to the groundwater more easily if the soil is wetter.
These results, reported in the May-June issue of the Journal of Environmental Quality, aren't just important for human health, say experts. They should also help countries conserve more water, as some spray-irrigation systems are up to twice as efficient as flood irrigation. "About 70% of the freshwater use on the planet is related to agriculture, so there is going to be a need to make more investments [in spray irrigation] and use less water," says economist Neilson Conklin, president of the Farm Foundation, a think tank in Oakbrook, Illinois. Plant biologist Molly Jahn, dean of the College of Agriculture and Life Sciences at the University of Wisconsin, Madison, says the new health incentive tips the balance in favor of spray irrigation despite the hefty price tag. (On a field housing 70 hungry cows, the cost of equipment alone can exceed $35,000.) Jahn says the next challenge will be to balance the costs and benefits. "What we're trying to do in sustainable agriculture is make sure we get that calculus right."