You’ve probably read about media dubbed ‘superweeds’ like glyphosate-resistant palmer amaranth and giant ragweed. Glyphosate-resistant weeds often earn this distinction because they are viewed as a greater management hardship for producers than weeds resistant to other herbicide mechanisms of actions (MOAs). And maybe rightly so. Farmers dealing with glyphosate resistant weeds elsewhere in the world have been reduced to tillage and hand rouging for weed control in some crops.
But, while glyphosate use dominates the Roundup Ready corn, soybean and/or cotton rotation in the US, group 2 herbicides play an (equally?) important role in our more diversified cropping system. For example, group 2 herbicides are used in crops like alfalfa, corn, dry beans, field pea, potato, soybean, sunflower, and in Clearfield and other group 2-tolerant crops. These herbicides are also a popular choice for group 1-resistant grassy weed control in cereals.
The point of this article isn’t to downplay the importance of glyphosate resistance but to elevate consideration of group 2 resistance. In Manitoba, over 10 weed species are known to have biotypes resistant to group 2 herbicides. And herbicide-resistant weed surveys led by AAFC indicate that the prevalence of certain species is increasing (Figure 1).
Figure 1: Prevalence of group 2 resistance in Manitoba in 2003 and 2008 as a percent of the weed species population surveyed (Beckie et al).
The following practices can help reduce the risk of developing herbicide resistant weeds and/or managing existing resistant weed populations:
- Diversifying your crop rotation;
- Using multiple herbicide MOAs effective on target weeds (e.g. herbicide ‘layering’, tank mixing);
- Practising good basic agronomy (variety selection, seeding rates, etc.);
- Judicial use of tillage.
If you suspect group 2 resistance in a weed species on your farm, it’s best to verify this by herbicide-resistance testing. Unfortunately there‘s no quick method – seed from the suspect population needs to be allowed to mature and collected. Samples can be submitted to AgQuest for testing in Manitoba.
In my opinion, knowing if you have group 2 resistance and assessing your risk factors is worth it. Because while glyphosate resistance is grabbing headlines, group 2 resistance may be quietly growing in your fields.