Verdure: It’s resistant

Annual sedge has become resistant to ALS-inhibiting herbicides. Researchers examined the level of resistance and how it developed.


The recognition of herbicide resistance in some of turfgrass’ most troublesome weed species is a research topic of great interest to many turfgrass weed scientists. Annual sedge (Cyperus compressus) is certainly one of those troublesome weeds.

To control annual sedge, turfgrass managers often apply halosulfuron, a herbicide that is an acetolactate synthase (ALS) inhibitor. ALS inhibitors are the most widely used herbicides for post-emergent control of sedges (Cyperus species) in turfgrass, so anecdotes about possible resistance were concerning. Therefore, researchers at the University of Georgia and Auburn University collected sedges from a hybrid bermudagrass field to determine the level of resistance to ALS herbicides and how this resistance was developing.

First, annual sedges were collected from a hybrid bermudagrass field, a location where plants were uninjured by a previous application of halosulfuron at the labeled rate (1 ounce a.i./acre, or 70 grams a.i./hectare). Another sample was collected from a location where the herbicide still controlled the sedge (the susceptible population).

Seed was produced from potentially resistant and susceptible plants, planted, and allowed to grow. Next, all plants were sprayed with halosulfuron at rates of 0, 0.06, 0.12, 0.25, 0.5, 1, 2, 4, 8 or 16 ounces a.i./acre (0, 4.4, 8.8, 17.5, 35, 70, 140, 280, 560 or 1,120 grams a.i./hectare). After eight weeks of growth, the plants were harvested and their dry weight was determined. An additional experiment was conducted to determine the mechanism of the resistance. To do that, the researchers measured ALS enzyme activity, and the ALS genes were sequenced.

So, was the collected population of annual sedge truly resistant to halosulfuron? Yes, it was.

In the susceptible population, only 0.12 ounce a.i./acre (8.8 grams a.i./hectare) of halosulfuron was required to reduce the plant biomass by 50%. In the resistant population, even the highest rate (16 ounces a.i./acre, or 16 times the labeled rate) of halosulfuron did not reduce plant growth by 50%. In fact, across all application rates, the biomass of the resistant population was reduced by about 12%. In comparison, biomass in the susceptible population was reduced by 80%, but only when halosulfuron was applied at a rate above the label rate of 1 ounce/acre.

So, how did this resistance develop? Gene sequencing revealed that a substitution resulted in resistance to ALS inhibitors. This mutation in the resistant population was identified, resulting in a Pro-197 to Ser amino acid substitution codon. What this means is that repeated use of halosulfuron could increase the spread of resistant sedges, especially if they are species in which the seed will outcross.

Suggestions for limiting the potential of increased herbicide resistance included hand weeding or the use of pre-emergent herbicides such as dimethenamid, oxadiazon, sulfentrazone or S-metolachlor. Those herbicidal options were provided with the caveat that applicators must always follow all product labels and consider all limitations for use, such as turfgrass injury.

Source: McCullough, P.E., J. Yu, J.S. McElroy, S. Chen., H. Zhang, T.L. Grey and M.A. Czarnota. 2016. ALS-resistant annual sedge (Cyperus compressus) confirmed in turfgrass. Weed Science 64:33-41.

Editor’s note: Read all of Beth Guertal’s recent Verdure columns.

Beth Guertal, Ph.D., is a professor in the Department of Crop, Soil and Environmental Sciences at Auburn University in Auburn, Ala., and the 2019 president of the Crop Science Society of America. She is a 21-year member of GCSAA.