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Schmidt: Combating resistant weeds

By Staff | Jul 25, 2014

One of the topics during last week’s Field Days discussions both at the Minot Research Extension Center and here at our local Pierce County Small Grains Plot Tour was the continued problem of herbicide resistant weeds.

Herbicide resistance occurs when we repeatedly use a specific herbicide or combination of herbicides to control weed species that have some plants in the population with resistant genes.

North Dakota, like many other states has a growing list of confirmed weeds resistant to a number of chemicals, including some weeds that are resistant to multiple chemical modes of action. A list of confirmed resistant weeds in North Dakota can be found on page 106 and 107 of the ND Weed Guide. As this list continues to grow we need to become more vigilant than ever in our herbicide application strategies to minimize herbicide resistant weeds to ensure that we continue to have adequate herbicide options in our weed control tool boxes for future use.

Among some of the toughest to kill weeds are glyphosate-resistant common ragweed, waterhemp and horseweed, all of which are now well established in North Dakota both in area and in number. Glyphosate resistant kochia and marestail were confirmed in Pierce County in 2011 and in 2012 some kochia populations in the county were found to also be resistant to fluroxypyr (Starane). To make matters worse some kochia populations are suspected to being showing resistance to dicamba as well.

Kochia and horseweed (marestail) should be a top concern for soybean producers because if glyphosate and fluroxypyr resistant kochia become established in a field, there are only a limited number of herbicide options to control these weed populations. This will be especially so if kochia populations with resistance to dicamba are confirmed. It is crucial to start implementing different weed management strategies now before the population of herbicide resistant weeds becomes unmanageable.

If you suspect herbicide resistance, check to see if your suspected weed resistance has already been confirmed in North Dakota. If your suspected weed resistance has already been confirmed in North Dakota, it is best to treat that weed as resistant. However, if the weed resistance has not been confirmed in North Dakota, contact the Extension office to further information about how to test the weed in your field for resistance.

Here are some general weed management guidelines from the ND Weed Guide to keep in mind to help minimize development of herbicide resistant weeds:

Apply an effective POST herbicide for the target weeds (consult the ND Weed Guide for options). When possible, choose a herbicide or tank-mix herbicides together to include multiple modes of action.

Use high herbicide rates and effective adjuvants. Reduced rates allow plants with low-level resistance to survive and produce seed.

Rotate modes of action. Repeated applications of herbicides with the same mode of action allow weeds to form resistance.

Spray small annual weeds. Weeds are much more susceptible to herbicides at younger growth stages.

Have zero tolerance for tough to control weeds. Simply put dead weeds don’t produce seeds.

For more information or to obtain a copy of the 2014 ND Weed Control Guide contact Yolanda at the NDSU Extension Service Pierce County office by calling 776-6234 ext. 5, or by email at yolanda.schmidt@ndsu.edu.

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