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RESEARCH

Seashore paspalum (Paspalum vaginatum Sw.) and St. Augustinegrass [Stenotaphrum secundatum (Walt) Kuntze] are

warm-season turfgrasses planted in tropical, subtropical, and warm-humid regions. Seashore paspalum is a fine-textured species that is grown for lawn, golf, and sports turf. It is a popular selection in coastal areas for its superior tolerances to salinity, poor soils, and irrigation with salt-laden water (Dudeck and Peacock, 1985; Trenholm et al., 1999; Lee et al., 2004). St. Augustinegrass is planted for lawns, parks, and commercial turfgrass areas in the Southern United States. The species is maintained at 5- to 7.5-cm height and has excellent tolerance to heat, drought, and traffic stress ( Johns et al., 1983; Peacock and Dudeck, 1984).

Bermudagrass [Cynodon dacytolon (L.) Pers.] is the most popular warm-season species planted for lawns and recreational turf in the world. Bermudagrass has aggressive lateral growth that enhances recovery from traffic, disease, and environmental stresses (Carrow, 1996; Trappe et al., 2011). Consequently, bermudagrass is also a prolific weed in polyculture with other turfgrasses. Bermudagrass may be controlled with ACCase inhibitors, such as fenoxaprop, or siduron, in certain turfgrasses ( Johnson and Carrow, 1989, 1993, 1995). However, these herbicides are not labeled for use in seashore paspalum or St. Augustinegrass due to excessive injury potential ( Johnson and Duncan, 2003). Practitioners have traditionally relied on spot applications of glyphosate or imazapyr for bermudagrass control, but these treatments cause unacceptable injury to desirable turfgrasses ( Johnson, 1988; Griffin et al., 1994).

Physiological Basis for Bermudagrass Control with Ethofumesate in Seashore Paspalum

and St. Augustinegrass

Patrick E. McCullough,* Jialin Yu, and Christopher R. Johnston

ABSTRACTBermudagrass is a problematic weed in seashore

paspalum and St. Augustinegrass with limited options

for control. Ethofumesate is the only herbicide labeled

for bermudagrass control in these turfgrasses, but the

physiological basis for selectivity is not well understood.

The objectives of this research were to evaluate the

efficacy and fate of ethofumesate in bermudagrass,

seashore paspalum, and St. Augustinegrass. In the

greenhouse, ethofumesate rate that reduced shoot

biomass 50% from the nontreated plants was 3.8, >27, and

>27 kg a.i. ha-1 for bermudagrass, seashore paspalum,

and St. Augustinegrass, respectively. In the laboratory,

absorption of 14C-ethofumesate ranked bermudagrass

> seashore paspalum > St. Augustinegrass from root

uptake, and bermudagrass > seashore paspalum = St.

Augustinegrass from foliar uptake. St. Augustinegrass

translocated ~20% less of the total absorbed

radioactivity to shoots than the other grasses after

root uptake. Metabolism of 14C-ethofumesate ranked

bermudagrass > St. Augustinegrass > seashore

paspalum. All grasses metabolized the herbicide

to ethofumesate-2-keto, methanesulfonic acid, and

another polar metabolite. Of all grasses, seashore

paspalum produced the highest and lowest levels

of ethofumeaste-2-keto and methanesulfonic acid,

respectively. Radioactivity recovery linearly decreased

by 81 to 35% of the applied 14C, from 1 to 7 days

after treatment, suggesting volatilization is a major

contributor to ethofumesate losses. The susceptibility

of bermudagrass to ethofumesate results from

greater absorption than seashore paspalum and St.

Augustinegrass. The intermediate tolerance of seashore

paspalum to ethofumesate results from root absorption

rather than foliar uptake. Metabolism rate does not

appear to be correlated with tolerance levels of these

species to ethofumesate.

P.E. McCullough, J. Yu, and C.R. Johnston, Dep. of Crop and Soil Sciences, Univ. of Georgia, 1109 Experiment Street, Griffin, GA 30233. Received 11 June 2015. Accepted 23 Nov. 2015. *Corresponding author (pmccull@uga.edu).

Abbreviations: DAT, d after treatment; LSC, liquid scintillation spectroscopy; Rf, retention factor.

Published in Crop Sci. 56:1306–1313 (2016). doi: 10.2135/cropsci2015.06.0359 © Crop Science Society of America | 5585 Guilford Rd., Madison, WI 53711 USA This is an open access article distributed under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Published March 28, 2016

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Ethofumesate is a benzofuran herbicide that inhibits lipid synthesis, respiration, and photosynthesis in susceptible species (Duncan et al., 1981). Bermudagrass is highly susceptible to injury from ethofumesate, and sequential applications may be used for suppression in tolerant species, such as tall fescue (Festuca arundinacea Schreb.) or Kentucky bluegrass (Poa pratensis L.) ( Johnson and Carrow, 1995; Meyer and Branham, 2006). McCarty (1996) found sequential applications of ethofumesate alone or tank-mixed with atrazine provided good (80–89%) and excellent control (95%) control of bermudagrass in St. Augustinegrass, respectively. Johnson and Carrow (1993) reported that five sequential ethofumesate applications suppressed bermudagrass by 52 to 73% in seashore paspalum. The addition of flurprimidol with ethofumesate treatments enhances bermudagrass control from ethofumesate alone, and may reduce the number of sequential applications required for suppression (Johnson and Carrow, 1993; Johnson and Duncan, 2000).

St. Augustinegrass has excellent tolerance to ethofumesate and may be treated with up to 3.36 kg a.i. ha-1 (McCarty, 1996; Grichar and Havlak, 2010; Anonymous, 2012). Although seashore paspalum is labeled for use, applications are restricted to £0.56 kg ha-1 due to greater susceptibility to injury than St. Augustinegrass ( Johnson and Duncan, 2003; Unruh et al., 2006; Grichar and Havlak, 2010; Anonymous, 2012). Ethofumesate is the only herbicide currently labeled for selective bermudagrass control in these turfgrasses. However, the physiological basis for bermudagrass control in seashore paspalum and St. Augustinegrass has received limited investigation. The objectives of this research were to evaluate efficacy, absorption, and metabolism of ethofumesate in bermudagrass, seashore paspalum, and St. Augustinegrass.

MATERIALS AND METHODS

Growth Responses to EthofumesateGreenhouse experiments were conducted at the University of Georgia in Griffin, GA. ‘Princess-77’ hybrid bermudagrass [Cynodon dactylon (L.) Pers. × C. transvaalensis Burtt-Davy], ‘Sea Spray’ seashore paspalum, and ‘Palmetto’ St. Augustinegrass were established in pots of 3.8-cm diameter and 20-cm depth. Single plants of bermudagrass and seashore paspalum were established from seed (Pennington Seed Inc., Madison, GA). St. Augustinegrass was planted from single tillers in pots from stolons harvested in a local field. Soil was a mixture of sand and peat moss (80:20 v/v), and pots were watered as needed to promote germination and subsequent growth. The greenhouse temperatures were set for 32 and 25°C (day/night) and irriga-tion was provided to prevent turfgrass wilt. Plants selected for treatment were at a 4- to 7-tiller growth stage.

Ethofumesate (Prograss 1.5EC, Bayer Environmental Sci-ence, Research Triangle Park, NC) was applied to grasses at nine rates ranging 0.105 to 27 kg a.i. ha-1. Treatments were applied in a spray chamber at 187 L ha-1 with a single flat-fan nozzle (Model

8002E, Tee Jet Spraying Systems Co., Roswell, GA). Grasses were not irrigated for 24 h after treatment, but were then irri-gated as needed to prevent wilt. Shoots were harvested at 4 wk after treatment, oven-dried at 60°C for 72 h, and then weighed.

Root Absorption and TranslocationGrasses were established in the greenhouse as previously described. Five plants of each species were removed from pots, roots were rinsed to remove soil, and the plants were then grown hydroponically in a 9.5-L plastic tank filled with half-strength Hoagland solution (Hoagland and Arnon, 1950). Grasses were placed through holes in the plastic lid that facilitated root sub-mergence in the solution. The tank was covered with aluminum foil to shield roots from light, and an aquarium pump provided oxygen to the solution. The tank was then placed in a growth chamber (Percival Scientific Inc., Perry, IA) set for 32 and 25°C (day/night) with 12 h photoperiods of 350 μmol m-2 s-1.

Plants were acclimated to hydroponic culture for 1 wk in the growth chamber and tap water was added to compensate for losses. The solution of the tank was then spiked with 1 µM of technical grade ethofumesate (99% chemical purity, Chem Ser-vice Inc., West Chester, PA) plus 85 kBq of 14C-ethofumesate (4.3 MBq mg-1, U-benzene ring labeled, 98.7% chemical purity). Plants were harvested 3 DAT. Roots were rinsed under a stream of tap water for 20 s, and then blotted dry with paper towels. Roots were separated from shoots with shears and samples were oven-dried for 5 d at 40°C. Samples were then combusted in a biological oxidizer (OX-500, R.J. Harvey Instrument Corp., Tappan, NY), and radioactivity was quantified with liquid scintillation spectroscopy (LSC) (Beckman LS 6500, Beckman Coulter Inc., Fall River, MA). Absorption was determined by dividing the total radioactivity recovered by sample dry weight. Translocation was determined by dividing the 14C recovered in shoots by the total radioactivity in the plant (roots + shoots).

Foliar Absorption and MetabolismGrasses were established with aforementioned materials and methods but were thinned to one plant per pot during estab-lishment. Grasses selected for treatments were at a 4- to 7-tiller growth stage and placed in the aforementioned growth cham-ber. Grasses were acclimated in the growth chamber for 1 wk and irrigated as needed to prevent wilting.

A single leaf on each plant designated for radiolabeled treatments was covered with flexible film (Parafilm, Pechiney Plastic Packaging, Menasha, WI). A broadcast treatment of ethofumesate at 1.68 kg ha-1 was applied to grasses with a CO2–pressurized sprayer calibrated to deliver 187 L ha-1 with a single 9504E flat-fan nozzle. Immediately after the broadcast applica-tion, the film was removed from leaves and two 1-µL droplets of 14C-ethofumesate containing 1.7 kBq each were applied to the second fully expanded leaf with a 5-µL syringe (Eppen-dorf North America, Hauppauge, NY). The spotting solution contained 9 µg µL-1 of formulated ethofumesate to simulate droplets of spray solution. A nonionic surfactant (Activator 90, Loveland Products, Inc., Greeley, CO) at 0.25% v/v was included in the broadcast and radiolabeled treatments.

Plants (roots + shoots) were harvested at 1, 3, or 7 DAT. The treated leaf was excised from shoots and rinsed in a 20 mL glass scintillation vial with 5 mL of acetonitrile/water (1:1). The

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results for radioactivity recovery were regressed against the following equation using the Regression Procedure in SAS:

0 1( )y x=b - b ´

where y is percent of the applied radioactivity recovered, b0 is the intercept and b1 is the slope. These equations were chosen after plotting means on graphs and selecting regression equa-tions that provided the best fit for the data. Experiment × treatment interactions were not detected, and thus, results were pooled over experimental runs.

RESULTS AND DISCUSSION

Growth Responses to EthofumesateEthofumesate rate × species interaction was detected for shoot biomass reductions, and thus results are presented by species. Shoot biomass for the nontreated plants at 4 wk after treatment measured 0.52 (0.1 SE), 0.53 (0.03), and 0.59 (0.03) g plant-1 for bermudagrass, seashore paspalum, and St. Augustinegrass, respectively (Fig. 1). The application rates required to reduce shoot biomass 50% from the nontreated measured 3.8, >27, and >27 kg ha-1, respectively. These rates are all above the highest labeled rate of ethofumesate in turfgrass (3.4 kg ha-1), but provide a relative comparison of growth responses among species under greenhouse conditions. Results are consistent with previous reports of bermudagrass susceptibility to ethofumesate injury compared to other turfgrasses (McCarty, 1996; Johnson and Duncan, 2000, 2003).

St. Augustinegrass biomass was reduced more than seashore paspalum at rates greater than 3.4 kg ha-1. Generally, seashore paspalum is more susceptible to injury in field conditions than St. Augustinegrass ( Johnson and Duncan, 2003). This is likely attributed to differences in maintenance levels of these grasses. Seashore paspalum is typically mown from 1- to 3-cm height for fine turfgrass, while St. Augustinegrass is maintained at 5- to 7-cm height. Grasses were not clipped during the 4-wk experiment, and the relative differences from the nontreated plants may have reduced the separation for growth inhibition between species. Maintenance regimens including mowing height, mowing frequency, irrigation, and fertility could influence the susceptibility of seashore paspalum to ethofumesate injury in the field, and warrants further investigation.

Root Absorption and TranslocationGrasses exhibited significant differences in root absorption of 14C-ethofumesate. The hierarchical rank for root absorption (Bq g-1 based on whole plant weight) of 14C-ethofumesate from high to low was: bermudagrass > seashore paspalum > St. Augustinegrass (Table 1). Bermudagrass and seashore paspalum absorbed 111 and 77% more 14C-ethofumesate than St. Augustinegrass, respectively. All grasses translocated the majority (61–83%) of the total root-absorbed radioactivity to shoots, but

leaf was held with forceps at the base and rinsate was applied downward toward the leaf tip. Roots were then separated from shoots with shears, and samples were stored at −20°C until analysis. Rinsate was evaporated in a fume hood and radioac-tivity was quantified with LSC.

Shoots were minced and homogenized (FSH 125, Fisher Sci-entific LLC, 300 Industry Drive, Pittsburg, PA 15275) in 20 mL of acetone/water (9:1) for 30 s. Samples were then sonicated for 1 h (Branson CPX8800H, Branson Ultrasonic Corporation, Dan-bury, CT), centrifuged at 4800 g for 10 m, and the supernatant was transferred to separate tubes. A 2-mL aliquot was sampled from the supernatant, and radioactivity was quantified with LSC.

The supernatant was then transferred to glass vials (Thermo Scientific, Bellefonte, PA), and evaporated to dryness on a heating block set for 40°C in a forced-air hood. Samples were resuspended in 75 µL of acetone and spotted on 20 × 20-cm silica gel plates with 250 μm thickness. The plates were developed to 16 cm in a glass chamber with dichloromethane. The plates were air-dried and metabolites were detected with a radiochromatogram scanner (BioScan System 200 Imaging Scanner, BioScan, Washington DC) connected to a com-puter equipped with Laura Chromatography Data Collection and Analysis Software (LabLogic System Inc., Brandon, FL). Metabolite standards of ethofumesate 2-keto and methanesul-fonic acid (Santa Cruz Biotechnology Inc., Dallas, TX) were spotted on silica gel plates and developed in dichloromethane as previously described. The plates were air-dried and the reten-tion factor (Rf) values were identified using a fluorescence indicator to measure the distance from the origin.

Residue from the shoots and roots were combusted sepa-rately in a biological oxidizer (OX-500, R.J. Harvey Instrument Corp., Tappan, NY) and radioactivity was quantified with liquid scintillation spectroscopy (LSC, Beckman LS 6500, Beckman Coulter Inc., Fall River, MA). Foliar absorption was quantified by dividing the total radioactivity recovered in supernatant and residue by the total 14C applied. Radioactivity recovery was quantified by dividing the total 14C in the leaf rinse, supernatant, and residue by the amount applied.

Experimental Design and Data AnalysisThe experimental design for greenhouse experiments was a randomized complete block with five replications. A block design was used in the greenhouse to minimize potential vari-ability of location on plant responses to treatments. Laboratory experiments were completely randomized with five replica-tions. All experiments were repeated once.

Data were subjected to analysis of variance with the Gen-eral Linear Model procedure in SAS (SAS Institute, 2011) to test for the interaction of treatment with experiment repetition. Shoot biomass data were analyzed with the Nonlinear Regres-sion Procedure in SAS and regressed against ethofumesate rate using the following equation:

{ }0 11 [exp( )]y x=b ´ - -b ´

where y is percent shoot biomass reductions from dose x, b0 the asymptote, and b1 the slope. In laboratory experiments, means were separated with Fisher’s LSD test at α = 0.05. Orthogonal polynomial contrasts were used to analyze the relationship of harvest timing with 14C absorption and total recovery. The

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Fig. 1. Shoot biomass reductions from the nontreated for ‘Princess-77’ bermudagrass, ‘Palmetto’ St. Augustinegrass, and ‘Sea Spray’ seashore paspalum at 4 wk after treatment with ethofumesate in two greenhouse experiments. Results were pooled over experimental runs. Vertical bars represent standard errors of the mean (n = 8). Data were regressed against ethofumesate rate using the following equation, y = b0

× {1 – [exp(−b1 × x)]}, where y is percent shoot biomass reductions from dose x, b0 the asymptote, and b1 the slope.

Equations: Bermudagrass, y = 50.82 × {1 – [exp(−0.99 × x)]}, standard errors for b0 and b1 were 6.7 and 0.47, respectively; 95% confidence intervals for b0 and b1 were 38 to 64 and 0.1 to 1.9, respectively; St. Augustinegrass, y = 43.31 × {1 – [exp(−0.37 × x)]}, standard errors for b0 and b1 were 5.1 and 0.14, respectively; 95% confidence intervals for b0 and b1 were 33 to 53 and 0.1 to 0.6, respectively; Seashore paspalum, y = 22.21 × {1 – [exp(−0.30 × x)]}, standard errors for b0 and b1 were 5.9 and 0.24, respectively; 95% confidence intervals for b0 and b1 were 10 to 34 and −0.2 to 0.8, respectively. Shoot biomass for the nontreated plants measured 0.52 (0.1), 0.53 (0.03), and 0.59 (0.03) g for bermudagrass, seashore paspalum, and St. Augustinegrass, respectively.

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Foliar Absorption and TranslocationSpecies × harvest interaction was not detected for foliar absorption of 14C-ethofumesate (Table 2). Differences among harvests (1, 3, and 7 DAT) were not detected, suggesting peak foliar absorption was reached at 1 DAT. Bermudagrass absorbed 39% of the applied radioactiv-ity, and was significantly greater than seashore paspalum and St. Augustinegrass that averaged 27%. The differen-tial levels of foliar uptake in bermudagrass from tolerant warm-season turfgrasses are similar to previous reports in other grasses. Kohler and Branham (2002) found a suscep-tible species, annual bluegrass (Poa annua L.), had ~20% greater foliar uptake of 14C-ethofumesate than tolerant cool-season turfgrasses, creeping bentgrass and perennial ryegrass (Lolium perenne L.). Duncan et al. (1981) reported that sugarbeet had less foliar absorption of 14C-ethofume-sate than three species with greater susceptibility to injury. All grasses translocated 0.9% (0.3 SE) of the absorbed radioactivity to roots at 7 DAT, but meaningful differ-ences among species were not detected (data not shown). Researchers have reported limited (<3%) translocation from the treated leaf for foliar-applied 14C-ethofumesate in turfgrasses, sugarbeet, and various weeds (Eshel et al., 1976; Kohler and Branham, 2002).

Species × harvest interaction was not detected for radioactivity recovery, and the main effect of species was not significant. Radioactivity recovery linearly decreased over time and averaged 81, 59, and 33% at 1, 3, and 7 DAT, respectively (Fig. 2). There were no meaning-ful differences detected among species for radioactivity recovery (data not shown). Losses in radioactivity over time are consistent with previous research with 14C-etho-fumesate. In three grasses, Kohler and Branham (2002) reported the total radioactivity recovery declined from 1 to 14 DAT, and averaged 81 to 48% of the foliar-applied

St. Augustinegrass had ~20% less than other grasses (Table 1). The hierarchical rank for specific radioactivity translocated (Bq g-1, based on shoot weight) from high to low was: bermudagrass > seashore paspalum > St. Augustinegrass.

Differences in root absorption of ethofumesate among tolerant and susceptible species have been previously reported in other crops. Duncan et al. (1982) reported that the susceptible species, redroot pigweed (Amaran-thus retroflexus L.) and lambsquarters (Chenopodium album L.), translocated more radioactivity of root-absorbed 14C-ethofumesate than a tolerant species, sugarbeet (Beta vulgaris L.). Ethofumesate is moderately mobile in the upper 7.5 cm of the soil profile, and turfgrasses may read-ily absorb the herbicide from the soil solution (Schweizer, 1976; Haggar and Passman, 1981). However, the bioavail-ability of ethofumesate increases when organic matter is 2% or less, which could influence bermudagrass control and turfgrass injury potential (Gardner and Branham, 2001; Satrallah et al., 2002).

The susceptibility of turfgrasses to herbicide injury has also been correlated with levels of root absorption. Singh et al. (2015) reported seashore paspalum had greater root uptake of 14C-atrazine and 14C-simazine than turf-grasses with greater tolerance levels, bermudagrass and zoysiagrass (Zoysia japonica L.). Yu et al. (2013) reported annual bluegrass (Poa annua L.) had greater root absorp-tion of 14C-amicarbazone than creeping bentgrass (Agrostis stolonifera L.) and tall fescue (Festuca arundinacea Shreb.). In other experiments, annual bluegrass had similar levels of root absorption of 14C-primisulfuron-methyl to Ken-tucky bluegrass (McCullough et al., 2015). However, annual bluegrass translocated more primisulfuron acid to shoots, than Kentucky bluegrass, due to less root metabo-lism. Perhaps, the reduced levels of 14C-translocation to shoots in St. Augustinegrass, compared to bermudagrass and seashore paspalum, is related to root metabolism of ethofumesate to immobile metabolites, and warrants fur-ther investigation.

Table 1. Root absorption of 14C-ethofumesate and radioac-tivity translocation to shoots at 3 d after treatment in hydro-ponically grown ‘Princess-77’ bermudagrass, ‘Sea Spray’ seashore paspalum, and ‘Palmetto’ St. Augustinegrass. Results were pooled over two experimental runs.

Species Absorption Translocation

Bq g plant-1 % 14C absorbed Bq g shoot-1

Bermudagrass 724 76 677

Seashore paspalum 608 83 568

St. Augustinegrass 343 61 306

LSD0.05 83 6 88

Table 2. Foliar absorption percentage of 14C-ethofumesate in ‘Princess-77’ bermudagrass, ‘Sea Spray’ seashore pas-palum, and ‘Palmetto’ St. Augustinegrass in two laboratory experiments. Results were pooled over experimental runs.

Foliar absorption

% 14C applied

Species

Bermudagrass 39

Seashore paspalum 26

St. Augustinegrass 28

LSD0.05 7

Harvest (DAT)†

1 32

3 34

7 28

LSD0.05 NS‡

† DAT = day after treatment.

‡ NS, nonsignificant.

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14C-ethofumesate. The researchers also reported that losses after 48 h were minimal (£12%) at 5 and 15°C, but increased to 75% at 35°C.

Volatilization may be a key contributor to ethofumesate losses and reductions in efficacy. Researchers have reported that initial ethofumesate applications in spring effectively controlled bermudagrass in seashore paspalum and St. Augustinegrass (McCarty, 1996; Johnson and Duncan, 2003). The potential losses of ethofumesate in sequential application programs in summer could warrant higher application rates or irrigation within 24 h to help minimize losses. Grasses were irrigated after 24 h of ethofume-sate treatments in greenhouse experiments, which could have minimized volatilization and increased the amount of herbicide available for absorption. This practice could have mitigated potential differences in tolerance between seashore paspalum and St. Augustinegrass by minimiz-ing volatilization under greenhouse conditions. Further

research is needed to evaluate the influence of adjuvants or irrigation on bermudagrass control from ethofumesate at various seasonal application timings.

MetabolismSpecies × harvest interaction was not detected for etho-fumesate metabolism, and thus results are presented by main effect. The Rf of ethofumesate was 0.8 in the dichlo-romethane solvent and is consistent with previous research (Duncan et al., 1981). Three metabolites were detected in all species at Rf 0, 0.2, and 0.7 (Fig. 3). Metabolites at Rf 0 and 0.7 were identified from reference standards as meth-anesulfonic acid and ethofumesate-2-keto, respectively. The degradation of ethofumesate in all grasses linearly increased from 1 to 7 DAT (Table 3). Conversely, meth-anesulfonic acid levels linearly increased from 1 to 7 DAT. The metabolite at Rf 0.2 and ethofumesaste-2-keto aver-aged 4 and 12% of the total metabolites recovered, but levels were similar across harvest timings.

Grasses exhibited significantly different levels of etho-fumesate degradation and metabolite formation (Table 3). The hierarchical rank of 14C-ethofumesate metabolism from high to low was: bermudagrass > St. Augustine-grass > seashore paspalum. The same trend was detected for levels of methanesulfonic acid, as bermudagrass had the highest levels and seashore paspalum had the lowest. Degradation rate does not appear correlated with species tolerance levels to ethofumesate.

Bermudagrass was the most susceptible species to ethofumesate, but had the fastest metabolism. Conversely, bermudagrass produced the least amount of the 2-keto derivative and was two- to threefold less than seashore paspalum and St. Augustinegrass, respectively. The pat-terns of metabolite formation suggest that the nature of detoxification differs in bermudagrass from turfgrasses with greater tolerances. Duncan et al. (1981) reported that a tolerant species, sugarbeet, had similar metabolism of ethofumesate to three susceptible species when treated at the two-leaf stage. However, at the six-leaf stage, sugarbeet

Fig. 3. Radiochromatogram scan of ‘Princess-77’ bermudagrass metabolites at 24 h after treatment.

Fig. 2. Recovery of applied radioactivity following 14C-ethofumesate treatments to ‘Princess-77’ bermudagrass, ‘Palmetto’ St. Augustinegrass, and ‘Sea Spray’ seashore paspalum in two experi-ments. Results were pooled over species and experimental runs. Vertical bars represent standard errors of the mean (n = 30). Data were regressed against a linear equation where y is radioactivity recovered and x is day after treatment. r2 = 0.44; y = 85.12 – 7.32x.

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had ~12 to 20% less metabolism of ethofumesate than the susceptible species, common ragweed (Ambrosia artemisiifo-lia L.) and lambsquarters.

The radioactivity extraction levels, or acetone-water soluble 14C, decreased linearly from 1 to 7 DAT, and ranged 83 to 75% of the total radioactivity recovered (data not shown). Kohler and Branham (2002) reported a linear reduction in extractable radioactivity from 14C-etho-fumesate in cool-season grasses, ranging 94 to 78% from 1 to 14 DAT. In sugarbeet, researchers reported 62 to 81% of recovered radioactivity was as water-soluble metabolites at the two-leaf growth stage, but these levels increased to 90% at the six-leaf stage (Duncan et al., 1981). There were no meaningful differences detected among species for levels of extractable radioactivity (data not shown).

Ethofumesate Selectivity for Bermudagrass ControlEthofumesate is the only herbicide labeled for bermudagrass control in St. Augustinegrass and seashore paspalum. The susceptibility of bermudagrass to ethofumesate may result from greater absorption than seashore paspalum and St. Augustinegrass. Peak foliar absorption of ethofumesate was reached after 24 h in all grasses. Irrigation after this period could help promote root absorption and minimize ethofumesate volatilization in the field. Seashore paspalum may only be treated with a third of the standard ethofumesate rate for St. Augustinegrass due to greater injury potential. The greater susceptibility of seashore paspalum to ethofumesate injury, as compared to St.

Augustinegrass, was not detected in biomass data collected in greenhouse experiments. However, differences in mowing height and management practices may contribute to ethofumesate tolerance in St. Augustinegrass compared to seashore paspalum in the field. The intermediate tolerance of seashore paspalum to ethofumesate results from greater root absorption than St. Augustinegrass. All grasses metabolized ethofumesate to the 2-keto derivate, methanesulfonic acid, and another polar conjugate, but the rate of degradation does not appear correlated with species susceptibility. Overall, differential levels of root and shoot uptake is the physiological basis of ethofumesate selectivity for bermudagrass control in St. Augustinegrass and seashore paspalum.

AcknowledgmentsThe authors would like to thank Vijaya Mantripagada for tech-nical assistance with this research.

ReferencesAnonymous. 2012. Prograss herbicide label. Bayer CropScience,

Environmental Science Division, Research Triangle Park, NC.

Carrow, R.N. 1996. Drought resistance aspects of turfgrasses in the southeast: Root-shoot responses. Crop Sci. 36:687–694. doi:10.2135/cropsci1996.0011183X003600030028x

Dudeck, A.E., and C.H. Peacock. 1985. Effects of salinity on sea-shore paspalum turfgrasses. Agron. J. 77:47–50. doi:10.2134/agronj1985.00021962007700010012x

Duncan, D.N., W.F. Meggitt, and D. Penner. 1981. Physiological basis of sugarbeet (Beta vulgaris) tolerance to foliar applications of ethofumesate. Weed Sci. 29:648–654.

Table 3. Ethofumesate metabolism in ‘Princess-77’ bermudagrass, ‘Sea Spray’ seashore paspalum, and ‘Palmetto’ St. Augustinegrass in two laboratory experiments. Results were pooled over experimental runs.

Metabolites

Methanesulfonic acid(Rf 0)†

Metabolite(Rf 0.2)

ethofumesate-2-keto(Rf 0.7)

ethofumeste(Rf 0.8)

--------------------------------------------------------------------------- % of 14C extracted ---------------------------------------------------------------------------

Species

Bermudagrass 61 5 8 26

Seashore paspalum 24 3 13 60

St. Augustinegrass 44 3 18 35

LSD0.05 8 NS‡ 5 9

Harvest (DAT)§

1 31 5 16 48

3 43 3 11 43

7 55 3 13 29

LSD0.05 8 NS NS 9

Linear * NS NS *

Quadratic NS * NS NS

Species * * * *

Harvest * NS NS *

Species × harvest NS NS NS NS

* Significant at the 0.05 probability level.

† Rf = retention factor.

‡ NS, nonsignificant.

§ DAT = day after treatment.

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