Tolerance of little bluestem to ACCase-inhibiting herbicides in Pennsylvania

Little bluestem [Schizachyrium scoparium (Michx.) Nash.] is a warm-season perennial grass that is sometimes planted with fine fescues (Festuca spp.) in infrequently mowed rough areas on golf courses, commonly referred to as naturalized or native grass areas and minimal-mow rough. This native species is often used for its aesthetically pleasing reddish-gold culms and inflorescences during late summer and fall.

In Pennsylvania, perennial and annual grass weeds invade fine fescue/little bluestem rough, and golf course managers occasionally use postemergence herbicides as control options (Landschoot, 2018). ACCase-inhibiting herbicides control a variety of grass weeds, but relatively few studies have examined the effects of these herbicides on little bluestem (Patton et al., 2021).

The objective of this study was to evaluate the tolerance of little bluestem to four ACCase-inhibiting herbicides: fenoxaprop, fluazifop, quizalofop, and sethoxydim. Experiments were conducted in 2021 and 2022 in adjacent areas at the Landscape Management Research Center in University Park, PA. Both experiments were performed in an eight-year-old non-irrigated and non-fertilized stand of strong creeping red fescue (Festuca rubra ssp. rubra Gaudin) ‘Garnet’ and little bluestem ‘Ft. Indiantown Gap-PA Ecotype’ (Ernst Conservation Seed). Little bluestem visual cover in the experiment areas at the time of treatment applications was approximately 50-60%. The stand was mowed once per year in October at 5 inches. Soil at the experiment site is a Hagerstown silt loam (fine, mixed, mesic, Typic Hapludalf), with a pH of 6.4, 38 mg/kg Mehlich-3 P, and 186 mg/kg Mehlich-3 K.

Herbicide treatments included fenoxaprop (Acclaim Extra, 0.57 lb fenoxaprop/gal; Bayer Environmental Science) at 28 fl oz product/acre with 0.25% v/v non-ionic surfactant (Lesco 90/10 Nonionic Surfactant; Lesco Inc.); sethoxydim (Segment II, 1.5 lb sethoxydim/gal; BASF) at 16 fl oz product/acre with methylated seed oil at 1.5 pints/acre (Lesco Methylated Seed Oil; Lesco Inc.); fluazifop (Fusilade II T/O, 2 lb fluazifop/gal; Syngenta Crop Protection LLC) at 16 fl oz product/acre with 0.25% v/v non-ionic surfactant; and quizalofop (Assure II, 0.88 lb quizalofop/gal; Amvac Chemical Corp.) at 12 fl oz product/acre with 0.25% v/v non-ionic surfactant. A non-treated control was included in each experiment. Herbicide treatment rates were based on maximum product label rates for control of grass weeds in fine fescue. All treatments were applied once on June 17, 2021, and June 8, 2022. Application dates coincide with preferred timing for control of grass weeds in central Pennsylvania. Total precipitation during the 2021 and 2022 evaluation periods was 19.7 and 9.7 inches, respectively.

All herbicide treatments were applied using a backpack sprayer equipped with a boom fitted with a 9504E flat fan nozzle (TeeJet Technologies) at 40 psi with a water volume of 87 gal/acre. The experimental design was a randomized complete block, and each treatment was replicated three times. Plot size was 30 ft². Criteria for evaluating herbicide effects on little bluestem included visual ratings of foliar injury, as well as inflorescence with supporting culm height and yield measurements. Foliar injury ratings were made on June 30 and July 27, 2021, and June 21 and July 22, 2022 with 0 indicating no injury and 10 representing complete desiccation of foliage. Culm/inflorescence height was assessed on September 15, 2021 and September 1, 2022 by measuring the tallest inflorescence with supporting culm in the centers of 10 plants in each plot. Yields were determined by harvesting all culms/inflorescences above the leaf canopy in each plot on September 25, 2021 and September 15, 2022 and weighing immediately after harvest. All data were subjected to analysis of variance using PROC MIXED in SAS version 9.3, and means were separated using Fisher's protected least significant difference test at α = 0.05.

Sethoxydim, fluazifop, and quizalofop treatments caused noticeable injury on little bluestem foliage on all rating dates in 2021 and 2022 (Tables 1 & 2). Fenoxaprop produced higher foliar injury ratings than the control only on June 30, 2021, and injury symptoms were minor (reddening of leaf tips). Injury ratings for the sethoxydim treatment were higher than all other treatments on June 30, 2021 and June 21, 2022. However, by July 27, 2021, no differences in foliar injury were observed among sethoxydim, fluazifop, and quizalofop treatments, and injury appeared as moderate to severe foliar desiccation. On July 22, 2022, the sethoxydim treatment produced a higher injury rating than fluazifop, and the quizalofop injury rating did not differ from sethoxydim and fluazifop treatments.

All herbicide treatments reduced culm/inflorescence heights in 2021 and 2022 when compared to controls (Tables 1 and 2). Sethoxydim, fluazifop, and quizalofop treatments had similar effects on culms/inflorescences in 2021, reducing heights by >50% relative to the control. The fenoxaprop treatment produced height reductions intermediate between the control and the other herbicide treatments in 2021.

In 2022, sethoxydim, fluazifop, and quizalofop treatments reduced culm/inflorescence heights by >50% compared with the control; however, the fluazifop treatment had a stronger effect on height reduction than quizalofop. As in 2021, the 2022 fenoxaprop treatment resulted in height reductions intermediate between the control and the other herbicide treatments.

All herbicide treatments produced lower culm/inflorescence yields compared to the controls in 2021 and 2022 (Tables 1 and 2). In 2021, yields were reduced by >50% for all herbicide treatments relative to the control, with sethoxydim, fluazifop, and quizalofop treatments showing the greatest reductions. The fenoxaprop treatment resulted in yields that were intermediate between the control and sethoxydim and fluazifop treatments, whereas quizalofop treatment yields were not different from those of sethoxydim, fluazifop, and fenoxaprop.

In 2022, sethoxydim and fluazifop treatments showed the greatest yield reductions, followed by quizalofop. Fenoxaprop produced higher yields than all other herbicide treatments.

Results of this study demonstrate that the ACCase-inhibiting herbicides sethoxydim, fluazifop, and quizalofop, and to a lesser degree fenoxaprop, cause foliar injury and inhibit culm/inflorescence development of little bluestem. Differences in foliar injury, culm/inflorescence heights, and yield means between years could have been due to higher rainfall amounts in 2021 compared with 2022, as well as subtle variation in soil conditions and other factors between experiment sites. In most cases, sethoxydim, fluazifop, and quizalofop injured and stunted little bluestem plants to a greater degree than fenoxaprop. These findings are similar to those of Patton et al. (2021) in which sethoxydim caused >30% injury to little bluestem plants. Practitioners should be aware that applications of the ACCase-inhibiting herbicides sethoxydim, fluazifop, and quizalofop can be detrimental to little bluestem when used to control grass weeds in mixed little bluestem/fine fescue rough.

Peter Landschoot: Conceptualization, data curation, formal analysis, funding acquisition, investigation, methodology, project administration, writing—original draft.

The author declares no conflict of interest.