Glyphosate-resistant marestail, discovered in 2000 in Delaware soybean fields, was the first glyphosate-resistant broadleaf weed in the U.S. Today, 14 species of glyphosate-resistant weeds have been identified across 36 states in the U.S.(1), with more than 60 million U.S. acres affected by at least one glyphosate-resistant species.(2)
Using glyphosate as an example, resistance to the chemistry built up in weeds because growers became too dependent on it as a single year-after-year solution.
What's more concerning, this problem goes far beyond glyphosate. Different biotypes of weeds are also showing resistance to other chemistries, and several weed populations are also now showing resistance to multiple herbicide chemistries, to further complicate the matter. Currently, there are 54 resistant weed biotypes in the U.S. that are resistant to more than one herbicide site of action.
Today, nearly every state in the U.S. is impacted by at least one kind of resistant weed.(3)
The key to a herbicide's effectiveness is its site of action. This specifically refers to how a herbicide binds to a specific protein, preventing the protein from completing whatever process a plant needs it to carry out. Proteins are catalysts for reactions. If they're not available, a weed will not survive. Glyphosate, for example, disables the synthesis of aromatic amino acids in a plant by binding to the enzyme EPSP synthase.(4)
There are believed to be thousands of sites of action in plants capable of being targeted by herbicides. Creating herbicides to target the many sites, however, is not so simple. Several factors weigh into the process, which is why there are only about two dozen sites of action registered today for commercial products.
This limited number of active ingredients, combined with the rampant spread of resistant weeds, makes it more important than ever that growers target multiple sites of action – known as overlapping sites of action – to reduce the selection pressure on any single site of action.
Options for overlapping
The notion of a single-pass glyphosate system is becoming more and more obsolete for growers. Remember that in a field containing millions of weeds, only one weed needs to survive to become a problem. This is why it's critical that zero weeds be left standing at the growing season's end to add to the seed bank.
Most herbicides, like glyphosate, only target one herbicide site of action. Other chemistries – such as those which target very long chain fatty acid biosynthesis, microtubule assembly, auxin mimicry and cellulose biosynthesis to name a few – have more complicated sites of action, which have appeared to lead to a slower development of resistance in weeds.
In addition to the complexity of herbicide sites of action, it is important to consider the benefit of herbicides that provide contact control or residual control. Contact herbicides will only control those susceptible weeds that are emerged at the time of application. Glyphosate is an example of a contact-only herbicide. Residual herbicides, when properly activated by rainfall, provide a barrier in the soil – killing weeds as they try to emerge from the seed bank. Some herbicides have both contact and residual herbicide activity and thereby offer maximum flexibility in controlling weeds at multiple stages of growth.
Combining the effects of these or other effective herbicides in overlapping sites of action programs is the best way to prevent resistant plants from surviving and setting seed. This can be done using three different application methods: sequential applications, rotating chemistries and tank mixing.
As new products come to market, herbicides formulated with different overlapping sites of action are also becoming more prevalent for growers. These herbicides are important additions to a weed management program, as they add convenience to the grower, help prevent errors that may occur during tank mixing and hit multiple weeds with multiple sites of action all at once, decreasing the likelihood of developing resistance.
The way forward
It's critical that growers not forget the lessons of the recent past. Being lulled back into the idea of “one is enough” with herbicide programs is a dangerous mindset.
A zero-tolerance approach toward weeds using a herbicide program with overlapping sites and other best management practices will give growers the best odds in combating their weed-resistance issues.
Want to know more? Here are some weblinks to sources that offer further insight into the issue of resistant weeds: