Hybrid Bermudagrass Control with Glyphosate and Fluazifop

Hybrid Bermudagrass (Cynodon dactylon (L.) Pers. × C. transvaalensis Burtt-Davy) Control with Glyphosate and Fluazifop

Travis C. Teuton, Graduate Research Assistant, Department of Plant Sciences, University of Tennessee, Knoxville 37996; J. Bryan Unruh and Barry J. Brecke, Associate Professor and Professor, West Florida Research and Education Center, University of Florida, Milton 32583; Grady L. Miller, Associate Professor, Department of Environmental Horticulture, University of Florida, Gainesville 32611; and Thomas C. Mueller, Professor, Department of Plant Sciences, University of Tennessee, Knoxville 37996

Corresponding author: Travis C. Teuton. tteuton@utk.edu

Abstract

During bermudagrass golf course renovations, complete removal of existing bermudagrass is imperative for establishing a pure, contaminant-free stand of a new bermudagrass cultivar. Field studies were conducted at the West Florida Research and Education Center near Jay, FL in 2001 and 2002 to evaluate hybrid bermudagrass control. Glyphosate (4.5 kg ai/ha), glyphosate (4.5 kg ai/ha) + ammonium sulfate (1 kg of N per ha), glyphosate (4.5 kg ai/ha) + fluazifop-p-butyl (0.4 kg ai/ha), and glyphosate (4.5 kg ai/ha) + fluazifop-p-butyl (0.4 kg ai/ha) + ammonium sulfate (1 kg of N per ha) were applied sequentially either two or three times at 4-week intervals. At 12 weeks after initial treatment (WAIT) (4 weeks after final herbicide application), glyphosate + fluazifop-p-butyl and glyphosate + fluazifop-p-butyl + ammonium sulfate applied three times provided 99% bermudagrass control. Single-degree-of-freedom contrast analysis indicated that there was a 12% increase in control for three versus two applications at 12 WAIT (P < 0.01). Contrast analysis also indicated a 7% (P < 0.01) increase in bermudagrass control 12 WAIT with fluazifop-p-butyl + glyphosate relative to glyphosate alone. The addition of ammonium sulfate increased control at 4 WAIT (P < 0.05) by 4%, however, no differences in control were observed at 8 and 12 WAIT. Turfgrass managers considering bermudagrass renovation should use three applications of glyphosate + fluazifop-p-butyl at 4.5 + 0.4 kg ai/ha with repeat applications scheduled 4 weeks apart to obtain complete control of the existing bermudagrass.

Introduction

Bermudagrass (Cynodon dactylon (L.) Pers.) is common throughout the tropical and subtropical regions of the world and is found along ditch banks, in lawns, sod farms, and high maintenance turfgrass areas such as golf courses and sports fields (1). Bermudagrass, like many other plants, can be a weed or a desirable species. As a weed, bermudagrass is difficult to control due to the aggressive growth of stolons and rhizomes, and the lack of selective herbicides (16). One obstacle to successful bermudagrass renovation is complete removal of existing bermudagrass and reestablishment of a new variety free from contamination from the old variety (Fig. 1)

Fig. 1. Common bermudagrass contamination in newly sprigged Tifsport bermudagrass following a single application of glyphosate at 2.25 kg ai/ha three weeks before sprigging (photo courtesy of Jan Weinbrecht).

Herbicides used in the past for controlling common bermudagrass included TCA (trichloroacetic acid) and dalapon, but these treatments provided inconsistent control and are no longer registered for use (5). Glyphosate provides better control than TCA or dalapon and has replaced these older herbicides. However, at least three applications of glyphosate at 2.2 to 5.6 kg ai/ha are needed to obtain satisfactory control of common bermudagrass (7). Boyd (3) reported that two applications of fluazifop-p-butyl (0.9 kg ai/ha) tank mixed with glyphosate (4.9 kg ai/ha) provided 98% bermudagrass control, equivalent to control observed with three applications of glyphosate alone at 4.9 kg ai/ha. Main et al. (8) reported 99% bermudagrass control with either glyphosate (1.7 kg ai/ha) or glyphosate + clethodim (0.3 kg ai/ha) followed by either glyphosate or glyphosate + clethodim 2 weeks after initial treatment. Waltz et al. (13) reported season long control of bermudagrass in centipedegrass (Eremochloa ophiuroides (Munro) Hack.) with four applications of clethodim + crop oil concentrate applied 4 weeks apart. Griffin et al. (6) and Boyd and Rodgers (2) reported imazapyr applied at 1 kg ai/ha provided acceptable control of common bermudagrass. Although effective, treatments of imazapyr required up to 1 year of fallow prior to reestablishment.

Webster et al. (15) reported that hybrid bermudagrass (Cynodon dactylon L. Pers × C. transvaalensis Burtt-Davy) cultivars have a less aggressive growth habit and are less difficult to control than common bermudagrass cultivars with glyphosate, clethodim, fluazifop-p-butyl, glufosinate, and quizalofop.

Ammonium sulfate (AMS) is a spray additive used to enhance weed control when tank-mixed with glyphosate and other weak acid herbicides (4,10,17). AMS is thought to increase glyphosate efficacy by increasing translocation across the plasma membrane (17). Also in areas where hard water is present, AMS will also compete with calcium, magnesium, and other metal ions that bind with glyphosate and make it unavailable (10). However, little is know about the effects of AMS when tank mixed with glyphosate and fluazifop-p-butyl for control of bermudagrass.

Even though previous research indicated bermudagrass could be controlled with glyphosate or glyphosate tank mixed with fluazifop-p-butyl or clethodim, turfgrass managers continue to have difficulty eliminating bermudagrass during golf course renovation (B. Unruh, personal communication). Therefore the objectives of these studies were to determine the effectiveness of glyphosate and fluazifop-p-butyl for bermudagrass control at two and three applications, and to elucidate if the AMS tank-mixed glyphosate and fluazifop-p-butyl increases bermudagrass control.

Evaluating Hybrid Bermudagrass Control

Field experiments were conducted in 2001 and 2002 at the West Florida Research and Education Center near Jay, FL on a 4-year-old stand of ‘Tifway’ bermudagrass. The soil type was an Orangeburg sandy loam (fine-loamy, kaolinitic, thermic Typic Kandiudults) with pH 5.5 and 2% organic matter. Irrigation was applied twice weekly at 1.3 cm per application for the duration of the study to insure adequate moisture. The experimental design was a randomized complete block with four replications. Herbicides were applied to 1.5-×-3-m plots using a CO₂-pressurized back-pack sprayer calibrated to deliver 187 liter/ha.

Bermudagrass was maintained at 1.3 cm height typical of a golf course fairway. Initial treatments were applied on June 6, 2001 and May 24, 2002. Treatments included: glyphosate (4.5 kg ai/ha or 4.0 lbs ai/acre); glyphosate (4.5 kg ai/ha) + AMS (1 kg of N per ha or 0.9 lbs of N per acre); glyphosate (4.5 kg ai/ha) + fluazifop-p-butyl (0.4 kg ai/ha or 0.36 lbs ai/acre); glyphosate (4.5 kg ai/ha) + fluazifop-p-butyl (0.4 kg ai/ha) + AMS (1 kg of N per ha). All glyphosate treatments were made with the isopropylamine salt of glyphosate (0.5 kg ai/liter or 0.45 bs ai/acre).

Each herbicide treatment was applied sequentially either two or three times with a 4-week interval between applications. The 4-week interval allowed surviving bermudagrass time to regrow prior to application of the sequential treatment. Herbicide rates were the maximum labeled rates of both glyphosate and fluazifop-p-butyl. All fluazifop-p-butyl treatments included a nonionic surfactant at 0.25% (v/v) [Induce nonionic low foam wetter/spreader adjuvant contains 90% nonionic surfactant (alkylarylpolyoxyalkane ether and isopropanol), free fatty acids, and 10% water, manufactured by Helena Chemical Company, Memphis, TN].

Nearly complete bermudagrass control is required due to the aggressive nature of bermudagrass and the low level of contamination tolerated by turfgrass managers. Visual estimations of bermudagrass control were recorded 4, 8, and 12 weeks after initial treatment (WAIT) on a scale of 0 to 100% where 0 = no control and 100 = bermudagrass death. Acceptable bermudagrass control was set at > 98% (no more than a few bermudagrass plants per 1.5-×-3-m plot).

SAS (11) Proc Mixed was used to perform analysis of variance. Shapiro-Wilk test was > 0.90 indicating normal distribution. Means were separated using Fisher’s Protected LSD at the 0.05 level of probability. There were no year by treatment interactions, therefore data were pooled over years. Single-degree-of-freedom contrasts were preformed and estimates (difference in the average of treatment means) calculated to determine any differences between two versus three applications, fluazifop versus no fluazifop, and AMS versus no AMS.

Bermudagrass Renovation and Herbicide Application

Bermudagrass control 4 WAIT was > 84% for all plots receiving a herbicide application (Table 1). However, none of the treatments provided acceptable control at this evaluation when only the initial herbicide treatment had been applied. By 8 WAIT after bermudagrass had been treated twice, control increased for all treatments and most provided acceptable control (Table 1).

Table 1. ‘Tifway’ bermudagrass control with glyphosate, fluazifop, and ammonium sulfate at Jay, Florida in 2001 and 2002.

Treatment	Rate (kg ai/ha)	No. of applications	Bermudagrass control^a (%)
Treatment	Rate (kg ai/ha)	No. of applications	4 WAIT^b	8 WAIT	12 WAIT
Glyphosate	4.5	2	86 ab	98 bc	79 c
Glyphosate	4.5	3	85 ab	100 a	97 ab
Glyphosate + AMS^c	4.5 + 1	2	93 a	97 c	80 c
Glyphosate + AMS^c	4.5 + 1	3	92 ab	100 a	96 ab
Glyphosate + fluazifop^d	4.5 + 0.4	2	84 b	99 ab	91 b
Glyphosate + fluazifop^d	4.5 + 0.4	3	85 ab	100 a	99 a
Glyphosate + fluazifop + AMS	4.5 + 0.4 + 1	2	88 ab	98 bc	92 b
Glyphosate + fluazifop + AMS	4.5 + 0.4 + 1	3	85 ab	100 a	99 a
LSD (0.05)			8	1	6

^a Data were pooled over 2001 and 2002 and means within a column followed by the same letter are equivalent according to Fisher’s Protected LSD test, P = 0.05

^b WAIT = weeks after initial treatment.

^c AMS = Ammonium sulfate.

^d All treatments with fluazifop-p-butyl were applied with a non-ionic surfactant at 0.25% (v/v).

At 12 WAIT, control with two applications of glyphosate + fluazifop-p-butyl combinations were better than with two applications of glyphosate alone (Table 1). However, bermudagrass control for all treatments with only two herbicide applications was unacceptable (79 to 92%). Bermudagrass control for treatments with three applications was better than for two applications (Table 1). However, bermudagrass control was still unacceptable for glyphosate and glyphosate + AMS treatments. Three applications of glyphosate + fluazifop-p-butyl or glyphosate + fluazifop-p-butyl + AMS were the only treatments considered acceptable 12 WAIT. Although these treatments were not statistically different from glyphosate alone, notes from visual observations indicated that bermudagrass treated with the glyphosate alone had more bermudagrass plants remaining and were deemed unacceptable. These results were similar to those of Waltz et al. (14) who reported > 80% control of common bermudagrass with sequential applications of either glyphosate or glyphosate + fluazifop-p-butyl.

Single-degree-of-freedom contrasts indicated that there were no differences (P > 0.05) in bermudagrass control between two and three applications when evaluated 4 and 8 WAIT (Table 2). This was expected since all treatments at this point had equal numbers of applications. However when evaluated 12 WAIT, three herbicide applications provided 12% better bermudagrass control than two applications. This indicates that at least three sequential applications are needed to control bermudagrass at a level > 98%.

Table 2. Contrast and estimates (%) of two versus three applications, fluazifop versus no fluazifop, and the effects of the addition of AMS for increasing bermudagrass control.

Contrast	4 WAIT^a		8 WAIT		12 WAIT
Contrast	Control^b	Estimate^c	Control	Estimate	Control	Estimate
3 vs. 2 applications	NS	--	NS	--	**	12
Fluazifop vs. no fluazifop	NS	--	*	1	**	7
AMS vs. no AMS^d	*	4	NS	--	NS	--

^a WAIT = weeks after initial treatment.

^b NS = nonsignificant, * and ** indicate significant at the 0.05 and 0.01 probability levels, respectively.

^c Estimates represent percent increase in bermudagrass control.

^d AMS = ammonium sulfate.

Single-degree-of-freedom contrasts and estimates indicated that the addition of fluazifop-p-butyl did not enhance bermudagrass control at 4 WAIT (P > 0.05) (Table 2). By 8 WAIT (after two sequential treatments) the addition of fluazifop-p-butyl was estimated to increased bermudagrass control 1% over glyphosate alone (P < 0.05). By 12 WAIT bermudagrass control increased an estimated 7% with the addition of fluazifop-p-butyl (P < 0.01). These results indicate that the addition of fluazifop-p-butyl to glyphosate significantly enhances bermudagrass control, especially later in the season.

Single-degree-of-freedom contrasts and estimates also indicated that there was an estimated 4% increase in bermudagrass control with the addition of AMS at 4 WAIT (P < 0.05) (Table 2). However, there was no difference in control with AMS 8 or 12 WAIT (P > 0.05). This indicates that AMS will provide added control for single applications. However, AMS is not needed when sequential applications are utilized. In areas where hard water is present, the addition of AMS may increase bermudagrass control and should be utilized (9).

Bermudagrass will remain a problem when renovating golf courses due to its deep rhizomes and lack of complete herbicide control (3,7,16). These deep rhizomes remain dormant for long periods of time and allow bermudagrass to regenerate when growing conditions are favorable. This regeneration and subsequent regrowth becomes a problem when renovating turfgrass since deep rhizomes are difficult to kill with herbicides and multiple herbicide applications will be necessary (3,7,8,13). From this research, a golf course superintendent wishing to renovate a bermudagrass golf course should use three applications of glyphosate (4.5 kg ai/ha) + fluazifop-p-butyl (0.4 kg ai/ha) with sequential applications at 4-week intervals to achieve acceptable control (> 98%). The addition of AMS is needed only if a single application of glyphosate will be utilized. If complete control is desired, an application of a soil fumigant such as methyl bromide would be required, however these applications are expensive and may not give complete bermudagrass control (12).

Acknowledgments

Appreciation is extended to Vernon Tedder and Jan Weinbrecht for technical assistance. Also, appreciation is extended to Arnold Saxton, statistics professor in the Department of Animal Sciences at the University of Tennessee, for his help with statistical analysis.

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