Watch the Weather to Avoid Atrazine Runoff

Atrazine, one of farmers’ least expensive and most effective chemicals for weed control, is under the magnifying glass.

Atrazine is up for re-registration review by the Environmental Protection Agency, says Bob Broz, water quality specialist for University of Missouri Extension. Broz recently spoke to certified crop advisers at a meeting in St. Joseph.

Atrazine’s last registration received approval in 2003. EPA reviews products every 15 years after a lengthy process that often involves public opinion as well as science. Closer scrutiny of atrazine use comes when it appears in drinking water supplies at higher rates than allowed by EPA. In 2007 and 2009, EPA determined that atrazine is not likely to cause cancer in humans or affect amphibian gonadal development.

Broz says atrazine is one of the most scrutinized and studied pesticides. More than 500,000 farmers use it to control grass and broadleaf weeds on 50 percent of the country’s cornfields.

EPA monitors 40 Midwestern watershed areas vulnerable to atrazine runoff. The agency measured levels every four days during the growing season. The 40 watershed areas represent a large number of soil types and conditions, including watersheds along Missouri claypan soils.

Two of the three Missouri watersheds originally selected for ecological review exceeded EPA trigger values, causing a closer look at the watersheds and the conditions that might have caused the exceedance. The levels have now been within the approved guidelines for atrazine runoff. Missouri farmers use 1.3 to 1.5 pounds of atrazine per acre, below the maximum amount they could use. Applicators often mix it with other herbicides to have more consistent control. It works well on no-till fields.

Broz says atrazine and herbicide runoff is seasonal, April through June. Some soils are more likely to allow runoff. Weather appears to be the main factor in runoff amounts. “Timing is everything,” Broz says. Intensity and duration of rainfall affect runoff. While farmers cannot control rain, they can manage the land to prevent or reduce runoff, he says.

Farmers can do their part to prevent runoff by applying atrazine when weather conditions are right, using management practices such as filter strips, cover crops, riparian areas and no-till planting. “If we find atrazine in runoff water, it means the farmers are losing money and weed control,” Broz says.

MU Extension specialists held nearly 70 meetings across the state last year focusing on pesticide application and educating farmers about the best practices to avoid atrazine runoff.

To learn more about water quality, go to

Source: University of Missouri

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