Herbicide Resistance

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We currently have 19 weed biotypes that are resistant to herbicides in North Carolina.

Herbicide resistance can be defined as the ability of a plant to survive after an application of a herbicide that is normally lethal to that plant. Herbicide tolerance is different from herbicide resistance. Herbicide tolerance means the weed species in question has never been controlled by a particular herbicide product. With herbicide resistance, the weed species in question used to be controlled by a particular herbicide but now is unaffected by that herbicide. Herbicide resistance can increase the costs of managing these difficult weeds. The number of spray applications may increase or the types of herbicide products that will need to be used are often at a higher cost.

Some of the more common weed in North Carolina that have developed resistance to herbicides are:

Weed Name  Resistant to Trade Name
Palmer Amaranth

Herbicide Group 2 – Chlorimuron‐ethyl

Herbicide Group 9 – Glyphosate

Herbicide Group 27 – Mesotrione

Classic

Roundup

Callisto

Common Ragweed

Herbicide Group 2 – Cloransulam‐methyl

Herbicide Group 14 – Fomesafen

First Rate

Flexstar

Italian Ryegrass

Herbicide Group 1 – Diclofop‐methyl

Herbicide Group 2 – Mesosulfuron‐methyl

Hoelon

Osprey

Common Lambsquarters Herbicide Group 5 – Atrazine AAtrex
Goosegrass

Herbicide Group 3 – Trifluralin

Herbicide Group 14 – Oxadiazon

Treflan

Ronstar

Horseweed Herbicide Group 9 – Glyphosate Roundup
Common Cocklebur

Herbicide Group 2 – Imazapyr 

Herbicide Group 17 – DSMA/MSMA

Arsenal

Ansar

The following practices can cause evolved resistance to develop more quickly in cropping systems:

  1. Relying exclusively on herbicides for weed control.
  2. Using herbicide programs with limited diversity relative to MOA.
  3. Monoculture production that leads to diminished crop health and entrenchment of pest problems.
  4. Failure to establish adequate stands that limit competition of the crop with weeds.
  5. Allowing pests other than weeds to impact crop health and ability of the crop to compete with weeds.
  6. Applying herbicides at rates below those recommended by the manufacturer.
  7. Applying herbicides at the manufacturer’s suggested use rate but to weeds that are larger than the suggested stage of growth.
  8. Applying herbicides in ways that minimize efficacy, such as using the incorrect adjuvant systems, applying the herbicide in combination with other agrochemicals that could be antagonistic, applying herbicides to weeds that are stressed, or applying herbicides in water that contains salts that limit herbicide absorption.
  9. Applying herbicides in spray volumes, at spray pressures, and at ground speeds that do not allow adequate weed coverage for complete control.
  10. Allowing weeds that are not controlled or are partially controlled to produce viable seed and contribute to the soil seedbank.
  11. Allowing weeds emerging after crop harvest to reproduce and contribute to the soil seedbank. 

Conversely, the following approaches to herbicide use can decrease the occurrence of evolved resistance:

  1. Using a diversity of weed control tactics other than herbicides.
  2. Establishing adequate stands of healthy crop plants that can out-compete weeds and close the crop canopy as rapidly as possible.
  3. Minimizing the impact of pests other than weeds on crop growth and development.
  4. Applying a diversity of herbicides representing multiple MOAs.
  5. Applying herbicides at the manufacturer’s suggested rate when weeds are within label specifications.
  6. Avoiding application of herbicides when weeds are stressed.
  7. Using the appropriate adjuvant system and avoiding co-applying agrochemicals that together can reduce herbicide efficacy.
  8. Applying herbicides using equipment and ground speeds that allow adequate coverage of weeds.
  9. Preventing escaped weeds from reproducing and contributing to the soil seedbank.
  10. Considering the long-term consequences of decisions made in the short term relative to the soil seedbank and evolved herbicide resistance.

As always if you have any questions please do not hesitate to contact me at 828-652-8104 or at tom_devine@ncsu.edu