Missouri corn and sorghum growers who farm in sensitive watersheds should consider a two-pass herbicide program to control weeds effectively while minimizing the amount of atrazine they apply, a University of Missouri (MU) weed scientist says.

Because it is so cheap and effective, atrazine has been used on more than 80% of the corn and grain sorghum grown in Missouri, says Bill Johnson, MU assistant professor of agronomy. "It's extremely important for weed control in the southern Corn Belt. If we lost it, we'd have to spend about $6/acre more on weed control."

Atrazine can contaminate public drinking water supplies through topsoil erosion, Johnson says. Current government standards limit the permissible amount of atrazine in drinking water supplies to 3 parts per billion. "With urban encroachment into these public drinking water supply areas, we in the agricultural community have to be responsible," he says.

The average atrazine use rate in Missouri has increased since the mid-1990s from 1.26 to 1.54 lbs/acre, Johnson says, probably because its low cost appealed to farmers squeezed when commodity prices dropped. At the same time, however, the proportion of Missouri corn farmers using a two-pass herbicide program increased from 25% to 32% since 1998.

"That's a step in the right direction," Johnson says. "Two-pass programs can reduce reliance on atrazine by 30-50%."

Johnson and MU weed scientist Reid Smeda evaluated weed control programs at MU Bradford Research Center near Columbia and at MU Greenley Research Center near Novelty, MO. They used atrazine in combination with other herbicides in both one-pass and two-pass programs.

"Utilizing a soil-applied pre-emergence herbicide and a planned post-emergence herbicide application usually results in better overall weed control and corn yield," Johnson says. "Treatments utilizing only one herbicide application generally did not provide acceptable weed control or corn yield."

Along with erosion control measures such as filter strips, stiff vegetative grass buffers and cover crops, "We need to promote more two-pass corn weed control in sensitive watersheds," Johnson says. "Atrazine rates can be reduced, but not to zero. Cutting it out altogether is not a viable option."