Herbicide Resistance - It's here and needs your management! Start planning for next year


Why so much talk about herbicide resistance?

It's not just because it's topical, but because we have it. In Ontario there are numerous weed species resistant to seven groups of herbicides spread across the province (Table 1). University of Guelph scientists recently confirmed glyphosate resistant giant ragweed in Ontario. In the United States there are 19 weed species resistant to glyphosate, covering up to 11.4 million acres. So, now is the time to understand why herbicide resistance is happening, where it is happening and how to prevent it. Many producers are aware that rotation of crops and herbicides, to different modes of action, is the most effective approach for managing against the development of weed resistance. These approaches are more critical now since there are literally no new herbicides (with new modes of action) coming to the Ontario marketplace anytime soon (Cowbrough, 2009).

How did we get here?

It's tough to say this, but we are human. We tend to repeat successful practices, especially if they reduce our work load and provide us with great results. Unfortunately, this helps to select those few weeds in the population that may have the genes that allow resistance to the herbicide.

Weed Species Resistant to Herbicides in Ontario

To identify alternative control options please see your Publication 75, Guide to Weed Control.

For corn, soybean or wheat visit: www.weedpro75.com

Herbicide Group Site of Action Herbicide Trade Name
(alphabetic order) 		Confirmed resistant weed species in Ontario
(Updated September 2010)		 Locations in Ontario1
1 Inhibitors of acetyl CoA carboxylase (ACCase) Acclaim Super, Achieve, Assure II, Excel Super, Poast, Puma120 Super, Select, Ultra, Venture Currently none confirmed Not applicable
2 Inhibitors of acetolactate synthase (ALS) and also called acetohydroxyacid synthase (AHAS) Accent, Arsenal, Classic, Elim EP, FirstRate, Muster, Option 2.25 OD, Pinnacle, Prism, Pursuit, Refine SG, Telar, Ultim, Upbeet Cocklebur, common ragweed, eastern black nightshade, giant foxtail, green foxtail, green pigweed, lamb's-quarters, redroot pigweed, waterhemp Bruce, Dundas and Glengary, Elgin, Essex, Haldimand/Norfolk, Hamilton-Wentworth, Huron, Kent, Lambton, Middlesex, Oxford, Perth, Simcoe, Stormont, Wellington, Victoria
3 Microtubule assembly inhibitors Bonanza, Dacthal W-75, Dimension, Prowl, Rival, Treflan Currently none confirmed Not applicable
4 Synthetic auxins 2,4-D, Banvel II, Caliber, Cobutox, Compitox, Covitox Plus, Desormone, Diphenoprop, Dycleer, Dyvel, Embutox, Estaprop Plus, Garlon, IPCO Dichlorprop-D, IPCO Premium 2-Way XP Turf Herbicide, Killex, Lontrel, MCPA, Mecoprop, Mecocrop 2,4-D, Meco-D, Par III, IPCO Premium 3-Way XP Turf Herbicide, Release, Sword, Target, Topside, Tordon 101, Trophy,Turf-Rite 2+2, Turboprop, Vanquish Wild carrot Halton, Wellington
5 Inhibitors of photosynthesis at photosystem II, Site A Aatrex Liquid 480, Atrazine, Gesagard, Hyvar X, Princep, Pronone, Pyramin FL, Sencor, Simadex, Simazine, Sinbar, Spin-Aid, Velpar Barnyard grass, common groundsel, common ragweed, green pigweed, goosefoot, redroot pigweed, waterhemp, wild mustard, witch grass, yellow foxtail Brant, Essex, Grenville, Grey, Haldimond/Norfolk, Hamilton-Wentworth, Lambton, Lennox & Addington, Prescott, Niagara, Waterloo, Wellington, York
6 Inhibitors of photosynthesis at photosystem II, Site B Basagran, Basagran Forté, Koril, Pardner, Brotox Green pigweed, redroot pigweed, smooth pigweed Essex, Kent
7 Inhibitors of photosynthesis at photosystem II, Site B (alternate binding site) Diurex 80W, Herbec, Karmex, Lorox Green pigweed, redroot pigweed, smooth pigweed Middlesex, Simcoe
8 Conjugation of acetyl co-enzyme A Betasan, Eradicane, Eptam, Ro-Neet Currently none confirmed Not applicable
9 Inhibitors of 5-enolpyruvylshikimimate-3-phosphate synthase (EPSP) Credit, Credit Plus, EZJect, Factor, Factor 540, Glyfos, Maverick III, Roundup Ultra II, Roundup Weathermax, Sharpshooter, Sharpshooter Plus, Touchdown Total, Vantage, Vantage Plus, Vantage Plus Max, Vantage Plus Max II, Vision, Vision Max Giant ragweed Essex, Kent, Lambton
10 Inhibitors of glutamine synthetase Ignite, Liberty Currently none confirmed Not applicable
11 Inhibitors of carotenoid biosynthesis Amitrol Currently none confirmed Not applicable
13 Diterpene synthesis inhibitor Command 360 Currently none confirmed Not applicable
14 Inhibitors of protoporphyrinogen oxidase (Protox) Aim EC, Blazer, Chateau, Goal, Reflex, Ronstar, Valtera Currently none confirmed Not applicable
15 Conjugation of acetyl co-enyme A Devrinol, Dual Magnum, Dual II Magnum, Frontier Max Currently none confirmed Not applicable
19 Inhibitors of auxin transport system Alanap Currently none confirmed Not applicable
20 Inhibits cell wall synthesis, Site A Casoron Currently none confirmed Not applicable
22 Photo system I - electron diverters Gramoxone, Reglone, Reward Canada fleabane, field peppergrass, Eastern black nightshade Essex, Kent
27 Inhibitors of p-hydroxyphenyl pyruvate dioxygenase (HPPD) Callisto, Converge Flexx, Impact Currently none confirmed Not applicable

Adapted from http://www.plant.uoguelph.ca/resistant-weeds/assets/resistant_weeds_herbicides_2009.pdf

1 Note: not all resistant weed species found in all counties. For resistant weed species by county see: www.plant.uoguelph.ca/resistant-weeds


Now what? Start planning for next year

We need to be aware that these weeds are in horticulture production regions across the province and in the event of control failures herbicide programs will have to be altered.

You likely have a resistant weed population if you have a weed species that should have been controlled but is healthy while other susceptible species have been controlled or a weed control failure even when the correct herbicide rate was used and it was applied at the appropriate weed stage and under favourable environmental conditions.

You can report suspected resistant weeds by contacting the Agriculture Information Contact Centre 1-877-424-1300. By taking advantage of this toll-free number, suspicious weed species will be tested for resistance by the University of Guelph. Any information obtained from this service will allow weed researchers to develop control options for resistant weed populations.

Now is a great time to collect seed from any weed species that you suspect is resistant. You can send samples directly to the University of Guelph. The University of Guelph will test, free of charge, suspected resistant weeds: http://www.plant.uoguelph.ca/resistant-weeds/services/
In order to prevent the development of herbicide resistant weeds producers should take into consideration the following practices:

  • Apply integrated weed management practices. Use multiple herbicide modes-of-action with overlapping weed spectrums in rotation, sequences, or mixtures.
  • Use the full recommended herbicide rate and proper application timing for the hardest to control weed species present in the field.
  • Scout fields after herbicide application to ensure control has been achieved. Avoid allowing weeds to reproduce by seed or to proliferate vegetatively.
  • Monitor site and clean equipment between sites.

For annual cropping situations also consider the following:

  • Start with a clean field and control weeds early by using a burn down treatment or tillage in combination with a pre-emergence residual herbicide as appropriate.
  • Use cultural practices such as cultivation and crop rotation, where appropriate.
  • Use good agronomic principles that enhance crop competitiveness.
    http://www.weedscience.org/In.asp

Don't Let This Be You!

Figure 1. Herbicide resistant pigweed in lettuce field.

Figure 1. Herbicide resistant pigweed in lettuce field.

Figure 2. Herbicide resistant pigweed in carrot field

Figure 2. Herbicide resistant pigweed in carrot field.

Related links:



For more information:
Toll Free: 1-877-424-1300
Local: (519) 826-4047
E-mail: ag.info.omafra@ontario.ca
Author: Kristen Callow - Weed Management Program Lead - Horticulture/OMAFRA
Creation Date: 01 November 2010
Last Reviewed: 01 November 2010