Herbicide resistance is a major concern for the modern day turfgrass manager. Herbicide resistance is the inherited ability of a plant to survive and reproduce following exposure to a dose of herbicide that is normally lethal. Over time and after repeated herbicide treatments tolerant or resistant biotypes and their offspring thrive, leading to resistant weed populations and herbicide failure, resulting in not only the loss of an effective herbicide, but decreased turfgrass quality and higher costs for weed management.
Causes of Herbicide Resistance
Herbicide resistance occurs because of continual use of herbicides in the same HRAC (Herbicide Resistance Action Committee) group that contain the same Mechanism of Action (MOA) and implementing weed management programs with little to no herbicide rotation or integrated management practices. Applying herbicides below recommended label rate also makes resistance more likely. Over time, these contributing factors naturally select out biotypes, allowing resistant populations to rapidly multiply and spread. As the resistant population begins to spread, herbicide applications become less effective.
Why Resistance Cannot Be Overlooked
Herbicide resistance increases weed control costs affecting an operation’s bottom line. Not only is there an increase in cost of control, but options for control begin to dwindle as herbicide resistance grows. New MOAs are becoming increasingly harder to discover and bring to market. As a result, new MOAs are not introduced too often in our industry. The last new MOA that was introduced to our industry occurred in 2019 when methiozolin, a thioesterase inhibitor, was launched for poa control. It is crucial that turfgrass managers implement an effective weed management strategy to maximize currently available technology.
Developing an Effective Resistance Management Program
Effective resistance management includes integrated pest management strategies utilizing the following tools.
1. Sound Agronomics. Herbicide resistance management starts with sound agronomic practices. Implementing proper nutrient management, irrigation and cultural practices for the desirable turf can greatly reduce weed competition. These practices should be the basic building block for any resistance management program.
2. Herbicide Rotation. Singular MOA herbicides are no longer enough to stem resistance development in hard to control weeds. Diversification must become part of the program. Multiple MOAs should be at the core of a 'starts clean, stays clean' approach to effective weed control.
3. Product Stewardship. Always read and follow label directions. Applying an herbicide according to label directions and at label rates is a vital part of resistance management. Following label directions provides optimal weed control when properly applied. Disregarding label directions can speed up resistance, reduce control and cause undesirable turfgrass responses.
4. Scouting. Every square foot must be evaluated to craft a diversified weed control plan. Scouting can identify potentially herbicide resistant weeds and gauge the effectiveness of your herbicide applications.
Today's turfgrass manager must incorporate an effective resistance weed management program. Failure to do so will not only cost the applicator time and money, but it can also greatly reduce options for future effective weed control.