Every so often, an old timer will offer the advice, "If it was easy, everyone would do it."
Managing against herbicide resistance is not easy, but everyone should do it. In our recent issue, we revisited the Weed Science Society of America's mechanism of action numbering system for herbicides.
The WSSA's system classifies herbicides according to mechanism of action, which combines mode of action (how a chemical kills the weed) with site of action (which part of the weed the herbicide targets). University of Illinois weed scientist Aaron Hager says this is a concise, yet precise, manner for describing how a herbicide works without a serious chemistry lesson.
"If you look at the numbers and they're different, the mechanisms of action are different," he notes.
But wait. According to the WSSA's chart, groups 3, 15 and 23 are all classified as mitosis inhibitors. If a plant is resistant to a group 3 mitosis inhibitor, does that mean it's also resistant to a group 15 mitosis inhibitor?
This is where things get a little more complicated. "The simple answer is that it depends on the particular plant," Hager responds.
A plant resistant to one mitotic inhibitor could be resistant to all. However, Hager says cross-resistance is uncommon when dealing with mitotic inhibitors. This isn't the case for all herbicides.
Hager says cross resistance can be common with ALS inhibitors. "Just because a plant is resistant to one or more imidazolinone herbicides does not necessarily mean it is also resistant to sulfonylurea herbicides," Hager says. "The pattern of cross resistance among ALS inhibitors is dependent on the specific amino acid substitution in the ALS enzyme."
He goes on to say that some amino acid substitutions in the ALS enzyme confer resistance to only one family of ALS inhibitors, while another substitution could confer resistance to all families of ALS inhibitors.
Most importantly, Hager reminds growers to know and understand what sort of resistance they're dealing with in their fields. Not all resistances are created equal. Sometimes a "hotter" mixture can control a weed population, when in reality a different chemistry should be used.
When gauging resistance, Hager says scientists treat a population of weeds that are known to be resistant to the chemistry being tested. They also treat a population of susceptible weeds. From here, they divide the herbicide rate needed to kill 50% of the resistant weeds by the rate used to kill 50% of the susceptible weeds. Using this number, they can gauge "how resistant" weeds are to certain chemistries.
"Some herbicides could have a number around 5,000," Hager notes. "It means this weed is much more resistant to that chemistry than say one that comes in at 300."
Resistance levels can also vary depending on the method of application. Hager says it's common for a herbicide to perform well when soil applied. The same herbicide, foliar-applied, may fail dismally on tough weeds.
Again, the key is knowing what's going on in the field. And, remember, if it was easy, everyone would do it.