Herbicide resistance is the inherited ability of a plant to survive and reproduce following exposure to a normally lethal dose of herbicide. Resistance has been growing exponentially in production agriculture over the last two decades and is becoming increasingly common in turfgrass management.
1. Can herbicide resistance be predicted? – No, because of the wide variety of plant and management factors that contribute. However, common turfgrass weeds like annual bluegrass (ABG) and goosegrass have the characteristics to develop resistance in all situations if managed incorrectly.
2. How widespread is resistance in turf? – Weed resistance is most commonly found in weed species in the south, but isolates of common cool-season weeds like ABG, goosegrass, crabgrass, spurge, nutsedge, and plantain have been found with herbicide resistance.
3. What factors lead to herbicide resistance?
- High genetic diversity –species like ABG and goosegrass are highly variable
- High reproductive ability – ABG and goosegrass have tremendous seed production potential
- Limited initial susceptibility to a herbicide – many POST herbicides have only limited efficacy on ABG or goosegrass
- Using herbicides at the lowest or even below label rates to save expenses
- Repeated use of a single herbicide or herbicides with the same site of action (SOA)
- Cultural practices favoring survival, reproduction, and spread of target weed(s)
4. What can you do to limit resistance? Cultural practices to maximize turf competition over weed completion are essential. These include:
- Mowing heights – as high as feasible for the given area on the course;
- Fertility- use judicious rates and effective timings to maximize turf competitiveness;
- Irrigation/drainage – maintain turf as dry as feasible to limit relatively shallow-rooted weeds;
- Aerification timing – avoid aerification during periods of weed germination and aerify during peak growth of desired turf’
- Overseeding – if bermudagrass fairways are annually overseeded with perennial ryegrass, consider skipping this practice every two to three years to enable using non-selective herbicides to control annual bluegrass; and
- Disease and nematode control – limit openings in the canopy and maintain maximum health for competitiveness.
Besides cultural practices, herbicides should be used prudently to maximize control. Measure include:
- Use appropriate rates and adjuvants to maximize control of individual applications.
- Apply when weeds are most susceptible, usually younger weeds or annuals prior to emergence.
- Use tank mixtures of herbicides with different Site of Action (SOA) (shown below), which research has shown to be more effective than rotation to delay resistance. (An example of this is Bayer’s PRE3 recommendation for ABG where a tank mix of Specticle, Tribute Total, and simazine are applied in late fall instead of only a preemergence application of Specticle in September.)
- Rotate herbicides with different SOA as often as practical. Most turf weed scientists agree everyone to three seasons.
- Monitor for escaped weeds and control immediately with spot applications with a different SOA.
Common Herbicides Used In Turf & Their Site of Action (formerly mode of action)
|Common Name||Example trade name||Type||Site of action (WSSA)|
|Thiencarbazone + Foramsulfuron+ Halosufluron||Tribute® Total||POST selective||2+2+2|
|Simazine||Princep® 4L||PRE/POST selective||5|
|Indaziflam||Specticle® Flo||PRE selective||29|
Mudge is a Green Solutions Team Manager and Dr. Reicher is a Green Solutions Team Specialist with the Environmental Science Division of Bayer. For the original article, visit here.