ISSUE #360



Herbicide resistance ( R ) occurs from repeated use of a herbicide that eliminates susceptible (S) weed species and allow R weed species to increase in the absence of competition from the S plant species. Genetically diverse weed species may contain a small percentage of plants that are R to herbicides having the same mode of action. Risk of weed resistance increases by using herbicides that provide near 100% weed control (ALS inhibitors). Growers should not rely on one herbicide class in a crop rotation system, like using trifluralin in small grains and broadleaf crops.

Types of Resistance

Altered Site of Action

ALS inhibitors and other herbicides act on one specific site in a plant which may allow diverse plant species to be resistant to the herbicide or mutate to express resistance. Herbicides that affect one enzyme in a plant usually are prone to altered site of action resistance.

Altered Herbicide Metabolism

Plants prevent herbicide toxicity by rapid degradation. Corn degrades atrazine by this mechanism. This type of a resistance is more serious than altered site of action type resistance because it involves several plant processes.

Cross and Multiple Resistance

A plant resistant to different herbicide chemistries having the same mode of action is cross-resistant. A plant resistant to different herbicide chemistries having different modes of action is multiple resistant.

ALS resistant kochia can be found across ND and developed originally where Glean and SU herbicides were used extensively. Kochia resistance has spread in the RRV and has escaped control from those using Raptor for the first time. Use of UpBeet in sugarbeet and SUÕs in small grain have caused selection pressure resulting in this development.

Recently, common cocklebur, sunflower, yellow foxtail, giant foxtail, and pigweed/waterhemp species have developed resistance to ALS inhibitor herbicides due to repeated use. Resistant kochia can be expressed after 3 to 5 herbicide applications.

Plant growth regulator (PGR) resistant kochia was discovered in a survey conducted in 1993. Resistance was evaluated primarily against dicamba. (Banvel)

Trifluralin (DNA) resistant green foxtail is found in areas of ND where trifluralin is used consecutively in small grain crops, row crops, and fallow. DNA resistant foxtail has been documented in the RRV of ND and MN. Resistant green foxtail can be expressed after 8 to 12 herbicide applications.

ACCase resistant wild oat is found within ND and MN from over 2,600 lines tested. Hoelon and fenoxaprop (Puma, Dakota, Tiller, and Cheyenne) resistance has been documented in every county sampled in ND. Resistance has varied from complete resistance to recovery from near death and all possible responses in-between. Wild oat resistance to Assure II has been documented in the RRV of ND and MN. Resistant wild oat can be expressed after 8 to 12 herbicide applications. Wild oat resistance to Poast has not been documented in ND.

Assert (ALS) resistant wild oat has been documented from approximately 200 wild oat lines tested from ND/MN. No wild oat plants were unaffected but several lines exhibited less than 20% control when evaluated 4 weeks after application.

ALS resistant waterhemp has been documented near the RRV of MN after commonly infesting mid-west and plains states. Continued use of accent in corn, Raptor in soybean and dry bean and SUÕs in small grains and sugarbeets may cause waterhemp resistance problems.

Weeds species most likely to develop resistance are genetically variable and have a rapid like cycle with short seed dormancy such as kochia and Russian thistle.

Strategies to Minimize Herbicide Resistance Weeds

  1. Use herbicides only when necessary Herbicide use should be based on economic thresholds.
  2. Rotate herbicides with different modes of action in consecutive years.
  3. Apply herbicides in tank-mix, prepackage or sequential mixtures that include multiple modes of action.
  4. Rotate crops, particularly those with different life cycles,
  5. Combine mechanical weed control practices like rotary hoe and cultivation with herbicide treatments. Hand removal of surviving seeds will prevent seed production of resistant plants.,
  6. Use pre-crop, in-crop, and post-crop tillage. No weed has become resistant to steel!
  7. Scout fields regularly and identify weeds that escape herbicide treatments. Monitor changes in weed populations and restrict spread of potentially resistant weeds that match the field history and a herbicide use pattern.
  8. Clean tillage and harvest equipment before moving from fields infested with resistant weeds to those that are not.

Use the color-coded brochure to assist in herbicide management and selection to reduce resistant weed development. This information is from the 1999 ND Weed Control Guide. Refer to it for more information.