Glyphosate resistance has been identified in several weed species, both in the U.S. and in other countries. And as silly as it might sound determining whether a weed is truly resistant is not always straight forward.
“If I sprayed it and it doesn’t die then it’s resistant.” Maybe. Maybe not.
Let’s look at glyphosate resistance first. If you have truly found some resistant weeds there will be discernable patterns. Glyphosate resistant populations generally show up within larger populations of susceptible weeds.
When treated with glyphosate, a mixed population of susceptible and resistant weeds presents a classic pattern of dead weeds side by side with healthy weeds and sick weeds that will eventually recover.
Weeds resistant to ALS inhibitors often show up as completely unscathed in the middle of completely susceptible weeds.
The protocol to follow when you think you have herbicide resistant weeds include: First, look for sprayer patterns, and review the history of the field. If the weeds were treated with another herbicide first, uptake of glyphosate could have been prevented. Other questions include whether or not the weeds were wet or covered in dust.
Once resistance cannot be ruled out the weed or weeds should be removed and potted and sent to an agricultural university for testing. This last step is particularly important. Why? Well one test you don’t want to conduct in the field is resistance to multiple modes of action. Your weed could be resistant to more than one chemistry! That should scare you.
Speaking of resistance to multiple modes of action, want to guess who the worst offenders are? Amaranthus species and Italian ryegrass. Obviously a lot of you guessed that. What might surprise you is that some strains of Italian ryegrass have been listed as resistant to eight different herbicides.
Now other weeds are being examined for glyphosate resistance. Populations of ragweed are being examined in Virginia for glyphosate resistance. This one is particularly scary because populations of ragweed have already been determined to be resistant to ALS inhibitors. Is ragweed the next monster weed? Let’s hope not. We can’t stop resistance development, but identification is our first line of defense.