Evaluation of Sulfonylurea Herbicides on Cool and Warm Season Turf Species

Michael S. Harrell, Graduate Research Assistant, and David W. Williams, Professor, Department of Plant and Soil Sciences, University of Kentucky, N222 Ag. Sci. North, Lexington 40546-0091; and Barry J. Brecke, Professor, West Florida Research and Education Center, University of Florida, 4253 Experiment Drive, Jay 32565

Corresponding author: Michael S. Harrell. msharr2@uky.edu

Abstract

Turf managers commonly use sulfonylurea herbicides for turfgrass weed control. The objective of this research was to provide herbicide tolerance and efficacy information for seven sulfonylurea herbicides and another herbicide on several turf and grassy weed species. Annual ryegrass (Lolium multiflorum Lam.), Kentucky bluegrass (Poa pratensis L.), creeping bentgrass (Agrostis stolonifera L.), perennial ryegrass (Lolium perenne L.), tall fescue (Festuca arundinacea Schreb.), bermudagrass (Cynodon dactylon L.), and zoysiagrassgrass (Zoysiagrass japonica Steud.) were treated with two rates of eight herbicides. Kentucky bluegrass and creeping bentgrass were the most tolerant of the cool-season species, but injury in some cases was severe. Herbicide applications to bermudagrass resulted in temporary discoloration for some treatments, but never resulted in kill. At label rates, sulfonylurea herbicides may eliminate several grass species from creeping bentgrass, Kentucky bluegrass, bermudagrass, and zoysiagrass.

Introduction

Managing turfgrasses in the transition zone often requires the ability to grow both cool-season and warm-season species simultaneously. If several species are being maintained in adjacent areas, contamination of one species within another is inevitable. Chemical removal of perennial ryegrass overseed from a warm season species has been an option for many years (6). However, a relatively new class of herbicides may allow for several other control options.

Sulfonylurea chemistries are an economically important class of herbicides used for weed control in agricultural crops as well as turf (2,5). The characteristics are very low use rates, excellent selectivity, and low mammalian toxicity (3). These herbicides inhibit the acetolactate synthase (ALS) enzyme that is not present in mammals (7). Sulfonylurea herbicides can be used for grassy weed control as soil and foliar applications (10). This class of herbicides has been studied extensively for use in agricultural crops (4,5), and has been utilized for sedge control in turf (1,8). However, many of the new chemicals have not been fully tested for safety or control of turf species. There have been and are currently many options for control of broadleaf weeds and many grassy weeds in turf. However, the options for chemical removal of one turf species within another are substantially less or unavailable.

The objective of this study was to determine levels of phytotoxicity of several ALS-inhibiting herbicides on common turf and grassy weed species in the transition zone.

Phytotoxicity Evaluations of ALS Inhibiting Herbicides

Trials were conducted in 2002 and 2003 at the Kentucky Agricultural Experiment Station in Lexington, KY. The soil was a Maury silt loam (fine, mixed, mesic Typic Paleudalf) with pH range of 6.3 to 6.9. Plots measured 1.2 by 1.2 m and were arranged in a randomized complete block design with seventeen treatments and three replications of each species. The species tested in this trial were: annual ryegrass (unknown variety), ‘Midnight’ Kentucky bluegrass, ‘Penncross’ creeping bentgrass, perennial ryegrass (mixed varieties), ‘Rembrandt’ tall fescue, ‘Quickstand’ bermudagrass, and ‘Zenith’ zoysiagrass. Annual ryegrass, Kentucky bluegrass, and tall fescue were mown at 4 cm, while all other species were maintained at 1-cm mowing height.

A CO₂-pressurized hand-held boom sprayer with SS8004 even fan spray nozzles was used to apply water at 1016 liter/ha and chemical treatments at two rates (Tables 1 through 8). Rates were selected based on current labels. When application rates were not available for turf species, rates were selected based on labels for other species. All treatments contained 0.5% v/v nonionic surfactant. Treatments were applied 10 May 2002 and 22 May 2003. Test plots were allowed to dry without irrigation or rainfall for at least 24 h following application. Color data were recorded twice between 6 and 18 days following treatment application as a measure of initial phytoxicity. Color was rated visually based on a scale of 1 to 9 (1 = yellow/brown, 5.5 = yellow-green, 9 = dark blue/green). Percent living cover data were collected by visual estimates at approximately 1 month to allow for recovery. Monostands of species were used when available. Tall fescue, perennial ryegrass, bermudagrass, and zoysiagrass untreated plots contained 90 to 100% cover throughout the experiment. Complete, uniform cover of Kentucky bluegrass, annual ryegrass, and annual bluegrass was not available for the duration of the experiment and required mathematical adjustments to determine percentage of control. Percent kill data was calculated as follows: (percent living cover of species / mean of percent cover of untreated test plots) × 100.

Data were analyzed by ANOVA procedure using SAS statistical software (SAS Institute Inc., Cary, NC). Means were separated with Fisher’s F-protected LSD (9). All reported differences were significant at P < 0.05.

Phytotoxicity and Kill of Cool-Season Grasses

Minimal or no discoloration occurred up to and including 7 DAA for all treatments and species. Annual ryegrass, annual bluegrass, and perennial ryegrass were highly susceptible to all of the chemicals tested in at least one year of the study (Tables 1, 4, and 5). Among those three grasses, perennial ryegrass was the most tolerant of bispyrobac-sodium, but kill ranged from 20 to 70% (Table 4). In 2002, sulfosulfuron exhibited less (43 to 63% kill) effect on perennial ryegrass, while complete kill was obtained in 2003. Color ratings of untreated annual bluegrass plots declined from the 7-day rating to the 18-day rating during 2002 due to summer stress. Tall fescue had similar responses to the herbicide treatments with nearly complete kill for all chemicals tested, with the exception of bispyrobac-sodium, which had little or no phytotoxicity the first year, and less than 25% kill the following year. Metsulfuron, chlorsulfuron, and bispyrobac-sodium treatments applied to creeping bentgrass resulted in essentially no kill (0 to 3%), but some discoloration was evident during the first year when compared with the untreated plots (Table 2). Sulfosulfuron resulted in the least kill to creeping bentgrass (< 25%) among the remaining treatments, but would generally not be considered safe due to color ratings between 6.3 and 7.3 for both years. Foramsulfuron and trifloxysulfuron exhibited the greatest kill among treatments of Kentucky bluegrass ranging from 53 to 85%. Other treatments ranged from 0 to 89% kill (Table 3).

Phytotoxicity and Kill of Warm-Season Grasses

As with the cool-season grasses, herbicides resulting in discoloration were generally slow to act with the greatest discoloration occurring at 14 to 19 DAA. Bermudagrass was highly tolerant of all herbicides tested. No kill resulted from any treatment, and only slight color changes occurred indicating some phytotoxicity (Table 8). Significant discoloration resulted from at least one rating in both years for all herbicides to zoysiagrass when compared to the untreated. However, minimal kill resulted from herbicides with the exception of chlorsulfuron and metsulfuron. Metsulfuron applications at either rate resulted in kill ranging from 18 to 39% depending on the year. Chlorsulfuron at the 0.32 kg rate resulted in the greatest kill of any treatment in both years ranging from 37 to 50%. Removal of zoysiagrass from Kentucky bluegrass and creeping bentgrass may be possible with metsulfuron at the rates used in this study. However, complete kill of zoysiagrass was not obtained and may only be successful with repeat applications allowing Kentucky bluegrass to have a competitive advantage.

Results indicate that ALS-inhibiting herbicides may affect several species of turfgrass and grassy weeds. Several options are available for removing annual ryegrass, annual bluegrass, tall fescue, and perennial ryegrass from both cool and warm season species. Phytotoxic effects were generally slow to occur, with little or no significant discoloration during the first 7 days and in some cases, limited response at 14 days. None of the herbicides tested would be effective for killing bermudagrass with a single application.

Conclusions

Results from this study indicate varied responses from ALS-inhibiting herbicides on cool and warm season turf species ranging from 0 to 100 percent kill. Table 9 presents a summary of the effects of eight herbicides on seven common turf species. However, results varied from year to year and the effects of sequential applications were not evaluated. This information, along with varying weather conditions, cultivars, and biotypes of the individual species, should be taken into account before beginning any weed control program.

Acknowledgment

Published with the approval of the Director of the Kentucky Agricultural Experiment Station as publication 05-06-026.

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