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Schmidt: Growing weed resistance
August 24, 2018

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Two weeks ago, I was alerted by a colleague of two suspicious weed populations growing in Pierce County fields. My heart was filled with dread as I traveled to the fields to have a look. Thankfully, upon my visits to the fields and consulting with one of our weed specialists, the weeds turned out to not be the dreaded waterhemp or Palmer amaranth. They were a pigweed species called Powell amaranth, which is commonly found in ND fields. Even though these weeds turned out not to be waterhemp or Palmer amaranth at this time, we cannot breathe a collective sigh of relief yet because it has been reported that Powell amaranth is beginning to show possible resistance to Group 2 herbicides in a field in a nearby county.

This incident serves as a reminder that regardless of the weed, growing herbicide resistance makes it ever more important to get a handle on weed populations in fields before they get out of control.

The first step in preventing and managing resistant weed populations is to actively look for and identify the weeds so that effective control measures can be taken.

Other ways to prevent and manage herbicide resistant weeds include:

1. Scout fields before and soon after herbicide application to identify escapes. Correctly identify weeds. Use effective herbicides, hand weeding, cultivation/tillage and other methods to kill weeds. Scout fields at the end of the season and draw field maps to denote locations of weed species, weed density, and weed escapes. Save these maps as a field record.

2. Double check chemical application practices recheck calculations for application rate to see if the intended rate is correct, check your nozzles to be sure the correct nozzles are being used, and recheck your calibration to make sure your output matches your intended rate.

3. Crop Rotation results in different planting and harvest times, more herbicide options and decreased risk of herbicide resistant weeds.

4. "Don't forget the PRE". Apply effective PRE herbicides at full rates and include multiple modes of action.

5. Apply effective POST herbicides. Apply herbicides that include multiple mechanisms of action in tank-mix or in sequential applications.

6. Use full herbicide rates and effective adjuvants. Full rates kill weeds with low-level resistance and dead plants cannot produce resistant progeny.

7. Spray small annual weeds. Generally, small weeds (>3 inches) are more susceptible to herbicides than large weeds. Even weeds with low-level herbicide resistance are more susceptible at 1 inch than at larger growth stages.

8. Practice ZERO Tolerance. Scout fields after row closure and kill uncontrolled weeds. Use effective herbicides, hand weeding, cultivation/tillage and mowing to achieve near 100% weed control.

9. Control weeds in field perimeters, drown out, and non-crop areas.

10. Rotate herbicides with different mechanisms of action in consecutive years. Diverse crop rotations allow more herbicide options with different mechanisms of actions to delay weed resistance.

11. Clean tillage and harvest equipment to ensure weed seed will not be transported between fields. This is particularly important in crops that are harvested with a platform header equipped combine.

12. Evaluate weed management at the end of each season and revise to improve weed control the next year.

For more details on these weed management strategies please see the 2018 ND Weed Control Guide at www.ag.ndsu.edu/weeds/weed-control-guides/nd-weed-control-guide-1 .

The bottom line moving forward in preventing and managing weed resistance is practicing ZERO tolerance.

People who see a plant they suspect is waterhemp or Palmer amaranth should contact their local NDSU Extension agent or an Extension specialist as soon as possible.

Schmidt is the NDSU Extension agent for Pierce County.

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