Fighting Herbicide Resistance

It is critical that as we make decisions regarding our herbicide programs that we consider the need to manage to prevent herbicide resistance. Herbicide-resistant weeds are now common in the Pacific Northwest:

• Kochia, prickly lettuce, downy brome and Russian thistle resistant to "SU" type   herbicides (Glean,   Amber, Ally, and other ALS inhibitors (Group 2).
• Wild oat and Italian ryegrass resistant to Hoelon and other ACCase inhibitors   (Group 1).
• Powell amaranth resistant to triazines and other photosystem II inhibitors (Group   5).
• Yellow starthistle resistant to Tordon and other synthetic auxins (Group 4).
• Wild oat resistant to Fargo and Avenge.

The appearance of herbicide-resistant weed is strongly linked to repeated use of the same herbicide or herbicides with the same site of action in a monoculture cropping system such as wheat fallow or in noncrop areas. My first personal experience with herbicide resistance was in the mid-1980's with the use of Oust along roadsides. Oust, a "SU" type herbicide, came out as a new product for roadside use. In three short years, kochia resistance started developing in our program. Our response to the situation was to tank mix and increase rates. It is interesting to note that tank-mixing herbicides as a resistance management strategy is not a recommended practice according to "Herbicide-Resistant Weeds and Their Management," a PNW Extension publication #437. The publication gives the example that when a long-residual (Glean) and short-residual (2,4-D) herbicides are tank-mixed, both herbicides may control emerged broadleaf weeds. However, Glean will continue to control weeds throughout the growing season and could continue to select for resistant plants. So tank-mixing to control the weed spectrum or if it will reduce herbicide use rates, but not for resistance management.

Preventing herbicide-resistant weeds

• Herbicide rotation - avoid year-after-year use of herbicides that have the same   site of action.
• Short-residual herbicides - use herbicides that do not persist in soil for long time periods and arenot applied repeatedly within a growing season
• Crop rotation
• Cultivation can be effective. Do not use the same site-of-action herbicide in   fallow as was used to control weeds in the crop.
• Accurate record keeping to insure that you have an effect herbicide rotation
• Clean seed to prevent the introduction of herbicide-resistant weed seeds.

Recognizing herbicide-resistant weeds

Irregular patches of a single weed species such as kochia in the field are an indicator of herbicide resistance, especially when

1. There are no other apparent application problems.
2. Other weeds are controlled.
3. There are no or minimal herbicide symptoms on the single weed species not controlled.
4. There was a previous failure to control the same species in the same field with the same herbicide. Do you have the vague feeling this has happened before?
5. Your records show repeated use of one herbicide or herbicides with same site of    action.

What to do if you suspect herbicide resistance?

• Do not respray the field with the same herbicide.
• Report your suspicion to university research or extension personnel in your county.
• Collect plants or seed that can be used to confirm resistance has developed.

For your color copy of "Herbicide-Resistant Weeds and Their Management" complete with an updated chart showing the site of action of each herbicide contact your local Extension Service office.

Did You Know?

Individual Russian thistle plants used 20 gallons of soil water while growing with the crop. From wheat harvest in early August until killing frost in late October, each Russian thistle used an additional 27 gallons of soil water. Water use occurred within a 5-ft radius of the Russian thistle. (William Schillinger, research agronomist, WSU)