There is a belief that the increased acreage planted to genetically engineered (GE) crops has led to an increase in herbicide-resistant weeds. However, results from the University of Guelph’s herbicide-resistant weed-testing laboratory indicate otherwise.
Table 1 (below) summarizes the number of species confirmed as resistant to common herbicides used in Ontario during the past 40 years.
There are currently fewer species resistant to glyphosate than to other herbicide active ingredients that have been used to control weeds in non-GE corn and soybean cultivars. (Known commonly by the trade name “Roundup,” glyphosate is used to control weeds in GE soybean and corn cultivars.)
The one constant regardless of whether the crop is GE or non-GE is how the resistant weed species were selected. When introduced, each of the three herbicides listed in Table 1 were very effective at controlling a wide range of weeds. Because of this, they were used frequently on many acres and over multiple seasons, which imposed a high selection pressure. History repeated itself with glyphosate; some of the first sites with confirmed resistant species had a history of a limited crop rotation and exclusive use of glyphosate to control weeds.
In the short term, if you are affected by herbicide-resistant weeds, you can manage them by tank mixing other herbicides with different modes of action. The Pest Manager app (found at the Grain Farmers of Ontario website) allows you to search for herbicides that will control a resistant weed species (Figure 1 below).
In the long term, we must make the environment unpredictable for weeds by growing diverse crop rotations and cover crops, using tillage and emerging technologies like harvest seed destruction.