Two Canadian weed scientists recently published a paper ranking their top 10 herbicide-resistant weed management strategies. Here is a paraphrased summary of their list:

10. accurate recordkeeping
  9. strategic tillage
  8. site-specific weed management
  7. weed sanitation
  6. rotation of herbicide selectivity mechanisms in wheat
  5. herbicide group rotation (site of action)
  4. multiple herbicide groups
  3. scouting fields before and after herbicide applications
  2. enhanced competitiveness of the crop
  1. crop diversity

How would these strategies work in Iowa? Weeds developing resistance to herbicides continue to be an increasing problem for corn and soybean growers. Bob Hartzler, Iowa State University Extension weed management specialist, offers the following observations.

“Aside from Strategy 6, which deals with the selectivity mechanisms of herbicides used in wheat production, all of the tactics recommended for the Canadian Great Plains are appropriate for Iowa’s production system,” says Hartzler. “You could argue over the ranking of the different strategies, but the critical point is to incorporate as many of the techniques as feasible into your weed control strategy.”

Diversify your weed management
ISU Extension weed control specialists and field agronomists have for many years been emphasizing the need for farmers to diversify their weed management with a focus on using multiple, effective herbicide groups. If used alone, multiple herbicide groups will not prevent further evolution of resistance. However, this tactic of diversifying by using multiple groups is easily implemented on all acres. 

“We have focused on this tactic since many or most of the commonly used herbicide programs fail to use herbicides in a way that places significant selection pressure on weeds of concern,” says Hartzler. “Programs often use reduced application rates due to economic concerns, thus eliminating much of the benefit of using multiple herbicide groups to reduce selection of resistant weeds.”

The two Canadian researchers, Beckie and Harker, list as their No. 1 tactic: crop diversity. This is the most effective tool for slowing herbicide resistance, says Hartzler. As in the U.S. Corn Belt, the majority of farmers in the Canada prairies use a crop rotation (canola and spring wheat) that provides marginal benefits in terms of managing weeds and herbicide resistance. 

Record weed problems
The authors recognized the economic factors that lead to adoption of crop rotations that contribute to weed resistance. “Since most growers are unable or unwilling to adopt the most effective strategy for managing herbicide resistance, it is imperative that the other tactics are used,” says Hartzler.

An important point to consider is that most of the strategies on this list can be implemented with very little change to a farmer’s current production system, he notes. “Accurate recordkeeping of weed problems and historic weed management strategies, and scouting fields prior to and following herbicide applications are all tactics that any farmer can and should do for each field,” Hartzler adds. “Incorporation of any of these strategies into an existing weed management program will improve weed control and slow the evolution of weed resistance to herbicides.”

The paper, “Our top 10 herbicide-resistant weed management practices,” by scientists J.H. Beckie and K. Neil Harker, appeared in Pest Management Science.