Herbicide Resistant Weeds Not Just in Field Crops

With far less herbicide options in horticultural crops as compared to field crops, horticulture producers tend to accept more weed escapes. The reality of this practice is that we could be harbouring and increasing herbicide resistant weeds in our fields and orchards. The following list documents the confirmed herbicide resistant weed species in the province of Ontario by herbicide Group: http://www.plant.uoguelph.ca/resistant-weeds/

Group 2 Herbicides - are acetolactate synthase inhibitors (imidazolinones, sulfonylureas, sulfonylamino-carbonyl-triazolinones and triazolo-pyrimidines), such as Pursuit and Accent.

Group 4 Herbicides - are synthetic auxins (phenoxyacetic acids, benzoic acids and pyridines and quinoline carboxylic acids), such as 2,4-D, Dicamba and Lontrel

Group 5 Herbicides - photosystem II inhibitors (triazines, triazinones and uracils), such as Simazine, Sencor and Sinbar

Group 6 Herbicides - photosystem II inhibitors (benzothiadiazoles and nitriles) with the same site as groups 5 and 7 but different binding behaviour, such as Basagran and Buctril

Group 7 Herbicides - photosystem II inhibitors (ureas) with the same site as groups 5 and 7 but different binding behaviour, such as Lorox

Group 22 Herbicides - photosystem I electron diverters (bipyridilium), such as Gramoxone and Reglone

Glyphosate resistant giant ragweed is suspected and being investigated as well. Growers should 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.

You can also send samples directly to the University of Guelph. The University of Guelph will test, free of charge, suspected resistant weeds. Visit their website for complete submission instructions.

In order to prevent the development of herbicide resistant weeds growers should take into consideration the following practices:

1. Rotate herbicides with different modes of action. For example, do not use simazine (Princep Nine-T) continuously. Consider other pre-emergence broadleaf herbicide options. Avoid making more than two applications of the same herbicide in the same year.
2. Scout orchards and fields to identify weeds. Respond quickly to changes in weed population by controlling weeds before they spread throughout the entire orchard or field.
3. Use non-selective post-emergence herbicides (Round-up, Ignite, Gramoxone) in your weed management program.
4. Use herbicides only as-needed.

Gratitude is expressed to Dr. Peter Sikkema and Mike Cowbrough for their review of this article.