Weed resistance will be the theme of a UNL Weed Research Tour June 26 at the Haskell Agricultural Lab near Concord.

The event will begin with registration at 9 a.m., presentations at 9:15 a.m., and a tour at 10:30 a.m. The Haskell Ag Lab is 2 miles east of Concord (14 miles north of Wayne or 45 miles west of Sioux City).

Field day topics will include:

• Herbicide Mode of Action and Site of Action Groupings,
• How Weed Resistance Develops,
• Weed Resistance in Nebraska and in the Midwest

This field day will highlight the importance of herbicide mode of action, and how to use the Site of Action Numbering System to reduce potential for weed resistance in Nebraska. Also, 20 herbicide studies will be shown during the field tour. Studies will show comparisons of many EPP, PRE and POST herbicides for corn and soybean, as well as control of volunteer Roundup-Ready soybean in Roundup-Ready corn.

Development of Herbicide-Resistant Weeds

Weed resistance to herbicides is not new. It began as soon as man started using chemicals for weed control. Weed scientists predicted in the mid-1950s that repeated use of any herbicide could lead to a shift in weed species, and that herbicide tolerance in weeds can increase with repeated use of the same herbicide.  Soon after that (early 1970s) the first cases of weed resistance occurred in pigweed species showing resistance to atrazine.

Worldwide about 368 herbicide-resistant weed biotypes have been reported to be resistant to 19 herbicide modes of action. Also, at least 50 weed species have been reported to have biotypes resistant to one or more herbicide families. Repeated use of the same herbicide was always the main reason for weed resistance to herbicides worldwide. 

In Nebraska, we have about 20 weed biotypes that are resistant to five main herbicide modes of actions. The list of weed species include:

• common waterhemp,
• redroot pigweed,
• Palmer amaranth,
• kochia,
• shattercane,
• marestail and
• giant ragweed.

The type of resistance includes: triazine, HPPD, ALS, growth regulators and Glycine.  The newest type of resistance is the glycine type (glyphosate-Roundup) confirmed in four Nebraska species (waterhemp, marestail, kochia, giant ragweed), which resulted from repeated use of glyphosate-based products over the last 15 years.           

Field Day Registration

For more information contact Stevan Knezevic (sknezevic2@unl.edu, cell: 402-404-0175), or my technologist (Jon Scott). There is cost for the Field Tour, but courtesy registration is appreciated. Please register by contacting Wendy Winstead (wwinstead2@unl.edu, 402-584-3837).

Stevan Knezevic
UNL Weed Extension Specialist