When it comes to weed control, we’ve heard the warning many times before, but it bears repeating: farmers need a strong herbicide program, with pre-emerge products plus post-emerge products, featuring multiple modes of action.
Bob Hartzler, weed scientist at Iowa State University, says the challenges farmers face with herbicide resistance are growing
“Things are changing, so we need to look at things differently than we have in the past,” says Hartzler, speaking during the National No-Till Conference in mid-January.
In all, the U.S. has 165 weed species that contain herbicide resistance. Some of the Corn Belt’s most troublesome weeds have developed resistance to multiple modes of action, according to the Weed Science Society of America. Waterhemp – the most troublesome weed corn and soybean growers face, according to the WSSA – has developed resistance to a whopping seven herbicide groups, including: 2, 4, 5, 9, 14, 15 and 27.
“Herbicide groups 2, 5 and 9 are of little use to manage waterhemp anymore. The resistance is fairly low in Groups 14 and 27, but is spreading quickly,” Hartzler says.
Other key weeds, and the number of herbicide groups they are resistant to include: and the number of resistant herbicide groups
- Marestail: 5 herbicide groups
- Common ragweed: 5 herbicide groups
- Foxtails: 4 herbicide groups
- Giant ragweed: 2 herbicide groups
- Cocklebur: 2 herbicide groups.
HOW RESISTANCE OCCURS
Even more troubling? The speed and methods by which weeds are gaining this resistance are troubling. Weeds are evolving quickly through one of two mechanisms; the weed specialist says:
- The most common form of resistance is target site resistance, where a protein within the weed has been altered by a mutation that prevents the herbicide from binding. Since the herbicide cannot bind to the protein, the protein functions properly and the plant acts as if it was never sprayed.
- In non-target site, or metabolic resistance:, something within the plant prevents the herbicide from reaching its target. The plant metabolizes, or withstands the herbicide, acting as if the target does not know the herbicide is present and the plant survives.
Non-target site resistance is occurring more frequently, says Hartzler, who warns that some plants are capable of developing metabolic resistance to herbicide chemistries the plants have never been exposed to.
Australian weed scientists found that rigid ryegrass populations that had developed resistance to Zidua (Group 15 herbicide) had also developed built-in resistance to Group 8 herbicides – even though they’d never been to those chemistries. Enzyme selection through repeated exposure to Group 15 herbicide was providing resistance to a completely different herbicide group.
The scary thing is, that same type of weed resistance is occurring in waterhemp in the U.S., Hartzler says.
“That’s the concern we have with the type of resistance we’re seeing in the countryside now, and I think that’s occurring with waterhemp, and I think we see that occurring with waterhemp,” Hartzler says. There are waterhemp populations already resistant to Group 4 herbicides; in 2019, researchers found waterhemp and palmer amaranth populations resistant to Group 15 herbicides. However, the populations reported in 2019 to have new resistance to Group 15 herbicides were already resistant to Group 27 herbicides, which are metabolism based.“It appears the enzymes in the Group 27 resistant waterhemp are also able to metabolize these other, completely unrelated herbicides,” Hartzler says. “That is scary, some say.”
The good news is that not all Group 27 resistant populations contain the cross resistance to other herbicide groups. “But some of them can, and we know that as Group 27 resistance becomes more common across the landscape, it’s more likely we’ll see more cross resistance,” he adds.
MOVING FORWARD
Herbicide use is still important, but farmers must be thoughtful in how they’re deployed, Hartzler says.
Research shows that using multipe sites of action are more effective than rotating herbicides; however, multiple sites of action aren’t foolproof. Continuous use of the same herbicides will still result in herbicide resistance eventually.
“You’ve got to make sure that individual herbicides you’re using are providieng effective control,” he explains. “One herbicide that provides 95% control, combined with another herbicide providing good control is the approach needed if we want to protect these herbicides.”
Note the resistances that occur within your field and throw out the herbicide groups that aren’t effective against the target species. Glyphosate, for example, provides little if any control over many broadleaf weeds. It is still effective on most grasses, so it’s a good tankmix partner. But because it cannot be counted on to provide braodleaf control, it doesn’t fit the “effective mode of action” piece of the puzzle.
Hartzler adds that farmers need to use the full labeled rate of herbicide – especially with pre-emerge products. Also, apply herbicides in a timely fashion. Waiting too long to apply herbicides will speed up metabolic resistance.
The weed scientist also recommends farmers:
- Focus on the weed seed bank, striving for 100% weed seed control.
- Incorporate alternative weed control tactics. Cover crops can help thwart small seeded weeds, which tend not to succeed when the ground is covered. Also, manufacturers are developing a host of weed seed control products that fit on combines. These may be worth a look.
