Two Canadian weed scientists recently published a paper ranking their top 10 herbicide-resistant weed management strategies. The following is my paraphrasing of their list:
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
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. One could argue over the ranking of the different strategies, but the critical point is to incorporate as many of the techniques as feasible.
The ISU Crops Team has emphasized the need for diversifying 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 is easily implemented on all acres.
We have focused on this tactic since many/most commonly used herbicide programs fail to use herbicides in a way that places significant selection pressure on weeds of concern. Programs often use reduced rates due to economic concerns, thus eliminating much of the benefit of multiple herbicide groups in reducing selection of resistant weeds.
Beckie and Harker's number one tactic, crop diversity, is the most effective toolfor slowing herbicide resistance. As in the U.S. Cornbelt, the majority of farmers in the Canada prairies utilize a rotation (canola - spring wheat) that provides marginal benefits in terms of managing weeds and herbicide resistance.
The authors recognized the economic factors that lead to adoption of crop rotations that contribute to weed resistance. Since most growers are unable/unwilling to adopt the most effective strategy for managing herbicide resistance, it is imperative that the other tactics are utilized.
An important point is that most of the strategies on the list can be implemented with very little change to a farmer's current production system. 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.
Incorporation of any of these strategies into an existing weed management program will improve weed control and slow the evolution of resistance.