Steelman: Soybean weed control

No-till soybean weed control 20 years ago was complicated, said Dr. Tom Mueller, a University of Tennessee Institute of Agriculture professor in the Department of Plant Sciences while speaking at the Milan No-Till Crop Production Field Day. "A nonselective burn down herbicide was applied either before or with a pre-emergence mixture containing both grass and broadleaf activity. A sequential application of another mixture with two modes of action was applied after soybean emergence," he said.

Then came the era of Roundup Ready soybeans, and weed control seemed very simplistic. "A farmer sprayed Roundup until there was nothing but soybeans left. These were very good days for no-till soybean weed control, but times have changed," Mueller added. Today, weed control for the state's top row crop is back to being more than just a walk in the park. The problem is the weeds are developing resistance to the control measures.

Mueller said that glyphosate-resistant horseweed, or marestail, made no-till soybean weed control a little more complicated. One simply sprayed 2,4-D/Clarity or Sharpen to kill glyphosate-resistant horseweed. Even if a few horseweed plants survived, yield losses were often small, he said.

But today, Palmer pigweed has developed glyphosate resistance (GR), and it is a much more difficult weed to control. "New strategies and technologies will be needed to control this GR pest," the expert said. He added that other weeds are developing resistance to current weed control systems, too. "New tools are needed to control no-till soybean weeds."

Mueller predicted that in the near future soybeans with new genetic traits will allow for the over-the-top application of 2,4-D or dicamba herbicide. This is good news for growers, but these over-the-top applications will be made at a time when more sensitive non-target vegetation will be present, which concerns many throughout the agricultural community. Mueller said new herbicide formulations are being developed that have a reduced tendency to volatilize under field conditions.

The weed control expert also emphasized that several manufacturers of application nozzles have developed new concepts that maintain an adequate herbicide spray pattern while minimizing the number of small droplets that tend to drift. This should help alleviate concerns over herbicide drift, he said.

Visitors to Mueller's tour stop at the field day also learned that future weed control will depend on choices made by the producer using new application technology. "The operation of these nozzles requires the user to select the appropriate pressure for their system," Mueller said. "The applicator striving to produce no-till soybeans has several decisions to make in the boom setup and sprayer operation. Some are made prior to application, such as choice of adjuvants or nozzles; others are made at the time of application, such as boom height, time of day to spray and whether to stop spraying under windy conditions." Mueller said there is always a potential trade-off between the theoretical factors responsible for reducing drift and the "on the ground" reality of producing soybeans under typical agronomic commercial conditions.

Bruce Steelman

614 Lehman Street

Phone:  615-563-2554

E:mail- tsteelm2@utk.edu

For more information about soybean weed control, contact Bruce Steelman at the Cannon County Extension Office and/or visit the website http://www.utcrops.com/

UT Extension provides a gateway to the University of Tennessee as the outreach unit of the UT Institute of Agriculture. With an office in every Tennessee county, UT Extension delivers educational programs and research-based information to citizens throughout the state. In cooperation with Tennessee State University, UT Extension works with farmers, families, youth and communities to improve lives by addressing problems and issues at the local, state and national levels.