DES MOINES - The days of simple weed control are over, and it's time to change the game plan, especially as more weeds figure out how to beat the system and develop resistance to herbicides.
"Resistance is here to stay, and I think it might start spreading fairly quickly," said Bob Hartzler, an Extension weed specialist and professor of agronomy at Iowa State University.
In years past, the weeds that farmers battled had evolved in the tillage era and could be managed effectively with herbicides. Today, farmers are fighting weeds that have evolved in the chemical era, Hartzler noted, and these weeds have gained a level playing field by developing resistance to some of the most effective herbicides available.
-Messenger photo by Darcy Dougherty Maulsby
Bob Hartzler, an ISU Extension weed specialist and professor of agronomy, stressed the urgency of herbicide resistance management with agronomists who attended a recent Agribusiness Association of Iowa meeting in Des Moines.
Why are weeds like waterhemp are especially adept at herbicide resistance? It comes down to seed production, Hartzler said. "Resistance is a numbers game. Under ideal conditions, one waterhemp plant can produce five million seeds. All that seed increases the probability that resistance will show up," he said.
That doesn't explain why giant ragweed is developing resistance, however, since it produces far fewer seeds that other agronomically-important weeds. "Ragweed has a completely different resistance strategy than waterhemp," Hartzler said. "Our best guess is that ragweed grows so rapidly that the rate of herbicide we're using is too low. This allows individual weeds to survive with low levels of resistance."
To fight resistance, it will not be enough to follow the basics of starting with clean fields, using weed-free seed, scouting fields and controlling weed escapes, said Hartzler, who acknowledged that not all growers respect and follow these fundamental agronomic practices. Reducing the evolution of herbicide resistance will also require more sophisticated herbicide management. Hartzler encourages growers to use full rates of herbicides, include multiple herbicides in a weed control system and rotate crops to allow alternate herbicides to work.
In addition, he advises growers to:
Pay attention to the new herbicide labels. The key to managing resistance is to rely on multiple mechanisms of action.
Herbicide labels include a standardized system to explain each product's MOA, which eliminates the need for farmers, crop consultants and ag suppliers to learn the MOA of all the active ingredients used in agriculture today.
Herbicide labels include a box labeled "herbicide group" near the top of the label. The number in the box represents MOA of the active ingredient, based on a system developed by the Weed Science Society of America.
Pre-mixes containing more than one mode of action will have multiple numbers listed.
"To use the information properly, you must still know the activity of the individual herbicides on the weeds present in the field to ensure that the target weeds are being affected by multiple MOAs," Hartzler said.
Use a pre-emerge on corn and soybeans.
Pre-emerge herbicides should be viewed as foundation products in a weed control program for soybeans, because 80 percent to 90 percent of the time a grower will need to use a post-emerge product to achieve season-long weed control.
When selecting any pre-emerge product, select a product that's active on the weeds in your fields. Also consider any existing resistance problems, crop safety, carryover risk and cost.
"Since this isn't a one-pass weed control system, you may not need to use a 'Cadillac' pre-emerge product," Hartzler said.
Select the right herbicides to control specific weeds.
Generally, the greater number of MOAs used, the less selection pressure is placed on weeds, Hartzler said.
However, designing an integrated program is not as simple as randomly adding MOAs, said Hartzler, who recommends hitting weeds with a variety of MOAs over a three- to four-year period.
The different MOAs used in the program must have good activity on the important weeds in the field to successfully reduce selection pressure.
"You have to know what weeds individual herbicides control," Hartzler said.
Above all, avoid the worst-case scenario of planting Roundup Ready corn, Roundup Ready soybeans and only using glyphosate to control weeds.
Winning the weed war
Unfortunately, all these practices probably won't be good enough to control herbicide resistance issues, said Hartzler, who offers additional keys to success in the war against herbicide-resistant weeds:
Know the enemy.
In the days before glyphosate, farmers had to pay close attention to the weeds they were fighting and understand the weeds' biology. In the era of herbicide resistance, this knowledge has become vital once again.
Optimize herbicide effectiveness.
Knowledge is power, Hartzler said. "By understanding a weed's biology, you know its weak points, and you'll know when you can use a herbicide in the most effective manner.
"Timely application will be important in this new era of weed control."
Look at cultural practices. Hartzler said cropping conditions can be manipulated to reduce the density and competitiveness of weeds through cultural practices ranging from row spacing to planting rates.
By themselves, cultural practices may only provide 25 percent to 30 percent of a farmer's total weed control, but they can be part of the solution by enhancing the competitiveness of the crop.
"Now's the time to get on the bandwagon and start thinking about how you can diversify your weed management program," Hartzler said. "We need to use herbicides more wisely than we have in the past to be sure we have valuable weed control tools available to us in the years to come."
Contact Darcy Dougherty Maulsby by e-mail at firstname.lastname@example.org.