Q & A WITH PAMELA SHERRATT
Herbicide-resistant Weeds
Q : A :
What advice do you have regarding herbicideresistant weeds ?
Approximately 200 weed species have been found resistant to at least one family of herbicides , and some are important weeds in sports field management . Goosegrass ( Eleusine indica ) was one of the first weeds in which resistance to dinitroaniline herbicides was documented . Annual bluegrass has also shown resistance to several herbicides . For these species , herbicide recommendations have been altered accordingly . For example , oxadiazon is more often recommended for preemergence control of goosegrass .
A common misconception is that weeds acquire resistance to a particular herbicide . There is genetic variability within a population of all living things . A population that is normally susceptible to a particular type of herbicide may have an individual that tolerates the herbicide . If this resistant individual can reproduce , then its offspring will also have the trait that allows it to resist that type of herbicide . If the same herbicide is used repeatedly , the population of resistant weeds will grow and can eventually become the majority . In cases of resistance , application rate quickly becomes a non-factor . Doubling or tripling the rate of the product and reapplying will not be effective . Resistance up to 80x rates has been reported by turfgrass scientists .
The best method to reduce the risk of a weed acquiring resistance is to not use the same product year after year , but rather to rotate among the different herbicidal modes of action ( MOA ) that are available . The reason for this is that if a weed population develops resistance to a particular type of herbicide , there is a good chance that it will resist all other herbicides that work the same way . A summary of the modes of action of herbicides used in cool-season turfgrass can be found online ( https :// hracglobal . com / tools / classification-lookup ).
Proper rotation involves using herbicides with a different MOA for each application . For example , if the objective is to avoid resistance development in crabgrass , then rotating between preemergence herbicides benefin and
prodiamine is not effective because both are dinitroanaline herbicides . It is also not effective to rotate between a dinitroanaline herbicide and the herbicide dithiopyr . Dithiopyr is in a different chemical class ( it is a pyridine ), but both the pyridines and the dinitroanalines have the same MOA . Compared to production agriculture or even warm-season turfgrass , there are not a lot of practical rotation options available for cool-season turfgrass managers . To help with this , you can include postemergence herbicides and also rotate among these chemistries . For crabgrass , use fenoxaprop one year , followed by quinclorac in year 2 , and then either topramazone or mesotrione ( different chemical classes but same MOA ) for year 3 .
For the broadleaf herbicides , if the target is an annual weed , a preemergence herbicide such as isoxaben may be useful . For perennial broadleaf weeds , for a very long time our postemergence herbicides were all synthetic auxins . The registrations ( all since the year 2000 ) of the PPO inhibitors and the ALS inhibitors have provided cool-season turfgrass managers with some important options for herbicide rotation .
So , consider adopting a proper herbicide rotation program . Also , after application , observe whether all the weeds kept growing or if it is just one species that was not controlled . If this is the case , then you may have found an herbicide-resistant population . Report suspected cases to your state extension specialist . They may be able to do tests to verify the resistance , and then offer alter management recommendations for control of the weed . SFM
Dave Gardner , Ph . D ., professor of Turfgrass Science at The Ohio State University , contributed to this article .
Pamela Sherratt Sports turf extension specialist The Ohio State University
Questions ? Send them to Pamela Sherratt at 202D Kottman Hall , 2001 Coffey Road , Columbus , OH 43210 or sherratt . 1 @ osu . edu Or , send your question to Dr . Grady Miller , North Carolina State University , Box 7620 , Raleigh , NC 27695-7620 , or grady _ miller @ ncsu . edu
50 SportsField Management | June 2021 sportsfieldmanagementonline . com