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© 2010 Plant Management Network.
Accepted for publication 1 December 2009. Published 12 March 2010.

Early and Late Postemergence Control of Dallisgrass in Tall Fescue

J. T. Brosnan, G. K. Breeden, and M. T. Elmore, Department of Plant Sciences, University of Tennessee, Knoxville, TN 37996; and J. M. Zidek, Senior Research Scientist, ZedX Inc., Bellefonte, PA 16823

Corresponding author: J. T. Brosnan. jbrosnan@utk.edu

Brosnan, J. T., Breeden, G. K., and Elmore, M. T., and Zidek, J. M. 2010. Early and late postemergence control of Dallisgrass in tall fescue. Online. Applied Turfgrass Science doi:10.1094/ATS-2010-0312-02-RS.

Abstract

Dallisgrass (Paspalum dilatatum Poir.) is a problematic turfgrass weed throughout the southern United States. A two-year study was conducted evaluating applications of fluazifop at 105 g/ha, mesotrione at 280 g/ha, and fluazifop + mesotrione at 105 + 280 g/ha, respectively, for control of dallisgrass in tall fescue (Festuca arundinacea). Treatments were applied in early spring shortly after dallisgrass broke dormancy [< 160 growing degree days (GDD_10C)] and early summer well after dallisgrass broke dormancy (> 500 GDD_10C). Yearly accumulated GDD_10C values were calculated using a base temperature of 10°C beginning on 1 January. Applied at < 160 GDD_10C in 2008, a single application of fluazifop provided 73% control of dallisgrass at 28 days after treatment (DAT) and 90% control at 76 DAT. When applied at > 500 GDD_10C in 2008, a single application of fluazifop only provided 46% and 0% control of dallisgrass at 28 and 76 DAT, respectively. Similar trends were also observed with sequential applications each year. Dallisgrass control with fluazifop + mesotrione was not greater than fluazifop alone at either timing. These data suggest dallisgrass is more susceptible to fluazifop when emerging out of dormancy in early spring.

Growing Degree Day Modeling

Associations between heat accumulation and plant maturity date back to the early 1700s (40). Growing degree day (GDD) models are a specific type of heat accumulation model used to monitor changes in crop phenology in response to the rate at which heat units (measured as GDD) are accumulated during the growing season. GDD models have been shown to accurately predict the growth of various cereals and grasses (6,10,16,20,23,25,26,28,38). Additionally, GDD models have been used to maximize the efficacy of insecticide (36) and plant growth regulator (7,13) applications to managed turfgrass systems.

Research investigating the use of GDD models to maximize the efficacy of herbicide applications has been limited to synthetic auxin herbicides used for broadleaf weed control (31). Using a GDD model with a base temperature of 50°F (GDD_50F), Schleicher et al. (31) reported that ester formulations of 2,4-D + 2,4-D,P made prior to approximately 130 to 145 GDD_50F provided only 57% control of dandelion (Taraxacum officinale Weber), while applications after 130 to 145 GDD_50F provided greater than 80% control. A similar response was reported with amine formulations applied before and after a range of approximately 150 to 180 GDD_50F. Schleicher and Throssell (30) noted that applications of ester formulations of 2,4-D + 2,4-D,P at 110 GDD_50F only resulted in 43% control of dandelion, while those made at 160 GDD_50F and after 210 GDD_50F resulted in 87 and 100% control, respectively. The researchers suggested that this response was likely due to poor translocation of weak acid herbicides prior to 160 GDD_50F.

Research investigating the use of GDD application timings for the postemergence control of grassy weed species, like dallisgrass (Paspalum dilatatum Poir.), is limited. This warm-season perennial grass, originating from Uruguay, was introduced to the United States from South America and first collected in Louisiana in 1840 (8,9). Named after A. T. Dallis, dallisgrass was originally promoted for use in forage and pasture areas since the beginning of the 20th century (19). Dallisgrass persists in a wide range of soil types and is adaptable to various environmental factors including moisture, fertility, and mowing. Henry et al. (17) suggested that this adaptability may have increased the spread of dallisgrass as a turfgrass weed, while McCarty et al. (21) suggest that rhizomatous growth and seedhead production may be factors as well.

Dallisgrass Control

Options for selective postemergence control of dallisgrass are extremely limited. Sequential applications of monosodium methanearsonate (MSMA) have been shown to provide dallisgrass control (37) in turf. However, application rates required to provide complete control are often injurious to desirable warm- and cool-season turfgrasses (17,37). Furthermore, a United States Environmental Protection Agency (EPA) ruling determined that applications of MSMA for turfgrass weed control will not be legal after 2013 (39). Sulfonylurea herbicides such as foramsulfuron, trifloxysulfuron-sodium, and sulfosulfuron are all labeled for dallisgrass suppression (2,3,4); however, none of these herbicides are safe for use on tall fescue (Festuca arundinacea) turf, a widely used species in the United States transition zone (37).

Fluazifop-p-butyl (hereafter denoted as fluazifop) is an aryloxyphenoxypropionic herbicide that disrupts fatty acid synthesis in susceptible species by inhibiting acetyl-CoA carboxylase (32). Evers (15) reported that applications of fluazifop at 140 and 280 g/ha effectively desiccated a dallisgrass-bermudagrass [Cynodon spp. (L.) Rich.] sod prior to overseeding with annual ryegrass (Lolium multiflorum Lam.) While fluazifop is labeled for control of various warm-season grasses in tall fescue turf (1), the efficacy of fluazifop for selective control of dallisgrass in tall fescue has not been reported.

Mesotrione was recently registered for control of several weeds in cool-season turf (5). Mesotrione inhibits the enzyme 4-hydroxyphenylpyruvate dioxygenase (HPPD; EC 1.13.11.27), which is important in the production of plant carotenoids (32). Reicher et al. (27) reported that single and sequential applications of mesotrione at 280 g/ha did not provide acceptable dallisgrass control. However, control may be increased beyond levels observed by Reicher et al. (27) with mesotrione applications to dallisgrass plants breaking dormancy in spring. Furthermore, the efficacy of mixtures of fluazifop + mesotrione has not been evaluated.

Minimal data has been published describing optimal timings for selective dallisgrass control. Furthermore, the efficacy of fluazifop and mixtures of fluazifop + mesotrione have not been evaluated. The objective of this research was to generate preliminary data describing the efficacy of fluazifop and mesotrione applied alone, and in combination with one another, for selective dallisgrass control in tall fescue at two postemergence timings.

Evaluating Dallisgrass Control

Research was conducted in 2008 and 2009 on a mature stand of tall fescue turf (cultivar unknown) naturally infested with dallisgrass at the East Tennessee Research and Education Center-Plant Sciences Unit (Knoxville, TN). The research site had been infested with mature dallisgrass since 1998. Dallisgrass cover averaged 60% prior to initiating the study. Plots were established on a Sequatchie loam soil [Fine-loamy, siliceous, semiactive, thermic humic Hapludult], measuring 6.2 in soil pH and 2.1% in organic matter content. Field trials were conducted in an area of full sunlight and maintained as a golf course rough with respect to irrigation, fertility, and mowing during both years. Irrigation was applied on an as-needed basis to prevent the onset of moisture stress. Plots were maintained at a 10-cm height of cut during the study.

Treatments were as follows: (i) fluazifop at 105 g/ha; (ii) fluazifop at 105 g/ha + mesotrione at 280 g/ha; (iii) fluazifop at 105 g/ha followed by (fb) fluazifop at 105 g/ha 3 weeks later; (iv) mesotrione at 280 g/ha fb mesotrione at 280 g/ha 3 weeks later; (v) fluazifop at 105 g/ha + mesotrione at 280 g/ha fb fluazifop at 105 g/ha + mesotrione at 280 g/ha 3 weeks later; and (vi) untreated check. Rates selected represent the maximum labeled use rates for fluazifop and mesotrione in desirable tall fescue turf. All herbicide treatments were mixed with a nonionic surfactant (Activator 90; Loveland Products Inc., Greeley, CO) at a 0.25% v/v ratio and applied with a CO₂-pressured sprayer calibrated to deliver 280 liter/ha (30 gal/acre). The spray boom contained four flat-fan nozzles (8002; TeeJet, Spraying Systems Co., Roswell, GA) spaced 25 cm apart. A wheeled aluminum frame maintained the boom 25 cm above the surface while spraying. The boom applied a 1.5-m spray swath, leaving a 0.15-m strip of untreated turf along border of each plot. Plot size was 1.5 by 3.0 m.

Two application timings were evaluated in this study, early- and late postemergence. Timings were quantified using accumulated growing degree day values (GDD_10C). Yearly accumulated GDD_10C were calculated on a Celsius scale using the equation:

                                    GDD_10C = [(T_max – T_min) / 2] – T_base                  

where T_max represents the daily maximum air temperature, T_min represents the daily minimum air temperature, and T_base represents the minimum temperature required for the growth of a particular plant species (24). In this research, a value of 10°C was used for T_base to mirror that used by other turfgrass researchers investigating the use of GDD models for broadleaf weed control applications (30,31). Air temperature (2.0 m above ground level) data were collected from the National Weather Service Station located at McGhee Tyson Airport (KTYS), approximately 11 km from our research site. Daily maximum and minimum temperature data (T_max and T_min) are valid for a given date from midnight to midnight local time.

Early postemergence treatments were applied at < 160 GDD_10C, while late postemergence treatments were applied at > 500 GDD_10C in both years. In 2008, early and late postemergence treatments were initiated on 7 April and 3 June, respectively. Accumulated GDD_10C values on these dates were 86 and 505 GDD_10C, respectively. In 2009, early and late postemergence treatments were initiated on 16 April and 27 May. Accumulated GDD_10C values on these dates were 159 and 509 GDD_10C, respectively. Environmental conditions on these dates are described in Table 1.

Table 1. Environmental conditions when herbicide treatments were initially applied in Knoxville, TN, in 2008 and 2009

 ^x Accumulated growing degree day values from 1 January using the formula GDD_10C = [(T_max – T_min) / 2] – T_base, where T_max is the daily maximum air temperature, T_min is the daily minimum air temperature, and T_base measured 10°C.

 ^y Air temperature and relative humidity measured using a hand held weather meter (Kestrel 3000; Nielsen Kellerman Inc., Boothwyn, PA).

 ^z Measured at 2.54-cm depth using a hand held digital soil thermometer.

Dallisgrass control was evaluated visually on a percent scale, where zero equaled no control and 100 equaled complete control (relative to the untreated check) at 14, 21, 28, 41, 55, 62, and 76 days after treatment (DAT); however, dallisgrass control data collected at 14, 21, and 62 DAT are not presented. Tall fescue injury was also evaluated using a percent scale where 0 equaled no injury and 100 equaled dead turf. Tall fescue injury was evaluated at 14, 21, 28, 41, 55, 62, and 76 DAT as well; however, data collected after 41 DAT are not presented. Weed control and turf injury were assessed visually, as Yelverton et al. (41) reported that visual ratings of herbicide responses in turf were highly correlated with data collected using the line intersect method or digital image analysis.

Separate studies were conducted for each GDD_10C timing at adjacent locations each year, with each arranged as a randomized complete block design with three replications. This design was selected as a second randomized complete block design trial was initiated in early summer 2008 (the > 500 GDD_10C timing) after we observed significant treatment responses when initially conducting a single randomized complete block design trial that spring (the < 160 GDD_10C timing).

Data from each timing were subjected to arcsine square-root transformations. Interpretations were not different from non-transformed data; therefore, non-transformed data are presented here for clarity. Data from the untreated check were excluded from statistical analysis to stabilize variance as well. Data were subjected to ANOVA using the general linear model procedure in SAS (29), with main effects and all possible interactions tested using the appropriate expected mean square values described by McIntosh (22). GDD_10C timing and herbicide treatment were treated as fixed effects in the model, while replication (nested within GDD_10C timing) was treated as a random variable. Fisher’s least significant difference (LSD) values were calculated when the F-ratio was significant at the 0.05 level. These procedures have been used by other researchers evaluating herbicide efficacy (11,14,33,35). Significant year-by-treatment and timing-by-treatment interactions were detected in dallisgrass control and tall fescue injury data; thus, data from each are presented individually.

Control for Treatments Applied at < 160 GDD_10C

Applied at < 160 GDD_10C, a single application of fluazifop at 105 g/ha provided 73% control of dallisgrass at 28 DAT and 90% control at 76 DAT in 2008 (Table 2). Sequential applications of fluazifop responded similarly, as dallisgrass control at the < 160 GDD_10C timing measured 76% at 28 DAT and 97% at 76 DAT in 2008 (Table 2).

Table 2. Dallisgrass control with fluazifop and mesotrione applied in early spring (< 160 GDD_10C) and early summer (> 500 GDD_10C) in Knoxville, TN, in 2008.

 ^x Accumulated growing degree day values from 1 January using the formula GDD_10C = [(T_max – T_min) / 2] – T_base, where T_max is the daily maximum air temperature, T_min is the daily minimum air temperature, and T_base measured 10°C.

 ^y DAT = days after treatment.

 ^z fb = followed by.

Similar responses were observed for both single and sequential applications of fluazifop in 2009 (Table 3). While a direct statistical comparison was not made, dallisgrass control with fluazifop applications at the < 160 GDD_10C timing tended to be greater at the end of the trial 2008 and than 2009. Differences in rainfall may have allowed for more re-growth in 2009, as the site received approximately 32 cm of rainfall during the 76 days after applying treatments at the < 160 GDD_10C timing in 2009, compared to only 20 cm in 2008. Temperature may also be a factor; a total of 850 GDD_10C accumulated during the 76 days after applying treatments at the < 160 GDD_10C timing in 2009, compared to only 708 GDD_10C in 2008. Despite these differences, all fluazifop treatments applied at < 160 GDD_10C provided at least 60% dallisgrass control at the end of the study each year. This is greater than the control reported by Henry et al. (17) following sequential summer applications of foramsulfuron and a summer application of MSMA followed by foramsulfuron (1 or 2 weeks later) in bermudagrass.

Table 3. Dallisgrass control with fluazifop and mesotrione applied in early spring (< 160 GDD_10C) and early summer (> 500 GDD_10C) in Knoxville, TN, in 2009.

 ^x Accumulated growing degree day values from 1 January using the formula GDD_10C = [(T_max – T_min) / 2] – T_base, where T_max is the daily maximum air temperature, T_min is the daily minimum air temperature, and T_base measured 10°C.

 ^y DAT = days after treatment.

 ^z fb = followed by.

Mixtures of fluazifop + mesotrione did not increase dallisgrass control compared to applications of fluazifop alone at the < 160 GDD_10C timing either year. This is likely due to the fact that sequential applications of mesotrione alone failed to provide acceptable dallisgrass control at the < 160 GDD_10C timing. Control measured ≤ 10% on all but one evaluation date in 2008, and all evaluation dates in 2009 (Tables 2 and 3). These results are similar to those reported by Reicher et al. (27) for single applications of mesotrione.

Control for Treatments Applied at > 500 GDD_10C

When applied at > 500 GDD_10C in 2008, a single application of fluazifop only provided 46% control of dallisgrass at 28 DAT; this dissipated to 0% at 76 DAT. Similarly, sequential applications only provided 56 and 30% control on the same evaluation dates in 2008 (Table 2). In 2009, dallisgrass control following single and sequential applications of fluazifop at the > 500 GDD_10C timing measured 0 and 33%, respectively, at the end of the trial (Table 3). Similar to what was observed at the < 160 GDD_10C timing, mixtures of fluazifop + mesotrione did not improve dallisgrass control compared to applications of fluazifop alone.

Tall Fescue Injury

In both years, tall fescue injury for treatments applied at < 160 GDD_10C measured ≤ 13% (Tables 4 and 5). While statistically significant differences were detected between fluazifop and mesotrione treatments at 21 and 28 DAT each year, they are likely of no practical significance as injury measured 0% for all treatments after 41 DAT each year (Tables 2 and 3).

Similarly, injury for all treatments applied at > 500 GDD_10C measured ≤ 12% each year with the exception of 41 DAT in 2009, when injury was as high as 32% from sequential applications of fluazifop and fluazifop + mesotrione (Table 4). This increased level of injury was likely because these treatments were applied when air temperatures measured 31°C.

Table 4. Tall fescue injury with fluazifop and mesotrione applied in early spring (< 160 GDD_10C) and early summer (> 500 GDD_10C) in Knoxville, TN, in 2008.

 ^x Accumulated growing degree day values from 1 January using the formula GDD_10C = [(T_max – T_min) / 2] – T_base, where T_max is the daily maximum air temperature, T_min is the daily minimum air temperature, and T_base measured 10°C.

 ^y DAT = days after treatment.

 ^z fb = followed by.

Table 5. Tall fescue injury with fluazifop and mesotrione applied in early spring (< 160 GDD_10C) and early summer (> 500 GDD_10C) in Knoxville, TN, in 2009.

 ^x Accumulated growing degree day values from 1 January using the formula GDD_10C = [(T_max – T_min) / 2] – T_base, where T_max is the daily maximum air temperature, T_min is the daily minimum air temperature, and T_base measured 10°C.

 ^y DAT = days after treatment.

 ^z fb = followed by.

Differences in Application Timing

While direct statistical comparisons could not be made, these data suggest that dallisgrass is more susceptible to treatment with fluazifop at < 160 GDD_10C compared to > 500 GDD_10C. When applied at > 500 GDD_10C in 2008, dallisgrass control at 55 DAT with a single application of fluazifop measured 65% less than that measured for the same treatment applied at < 160 GDD_10C. The same response was observed in 2009, as a single application of fluazifop at > 500 GDD_10C provided 53 and 80% less control at 28 and 55 DAT, respectively, compared to a single application at < 160 GDD_10C. Trends in dallisgrass control observed for single and sequential applications of fluazifop at the < 160 GDD_10C and > 500 GDD_10C timings were also observed for mixtures of fluazifop + mesotrione as well (Tables 2 and 3).

Results suggest that dallisgrass plants may be more susceptible to fluazifop treatments applied while emerging from dormancy (< 160 GDD_10C). Reasons explaining this difference in susceptibility cannot be identified from these data. As temperatures increase during the growing season, dallisgrass plants may undergo phenological changes, reducing the ability of fluazifop to inhibit acetyl-CoA carboxylase, which could consequently reduce the level of control observed following treatment with fluazifop alone or in mixtures with mesotrione.

While smaller weeds are generally more susceptible to herbicide applications, the exact mechanism affecting seasonal variability in dallisgrass susceptibility to fluazifop is not clear. Differences in susceptibility may potentially be related to differences in the metabolism of fluazifop at < 160 GDD_10C and > 500 GDD_10C. Hidyat and Preston (18) reported that differences in fluazifop metabolism were responsible for the resistance of a large crabgrass [Digitaria sanguinalis (L.) Scop.] population to fluazifop at rates of 0 to 1600 g/ha. Similarly, Cummins et al. (12) reported that differences in blackgrass (Alopecurus myosuroides Huds) susceptibility to fenoxaprop, another aryloxyphenoxyproponiate herbicide similar to fluazifop, were related to metabolism. Biotypes less susceptible to treatment with fenoxaprop were reported to express elevated levels of glutathione transferase (EC 2.5.1.18), an enzyme responsible for detoxifying herbicides in various members of the Poaceae family. Tal et al. (34) reported that the phenyl group of fenoxaprop applied to wheat (Triticum spp.) and barley (Hordeum vulgare) was rapidly displaced by glutathione; however, this response did not occur in two species susceptible to fenoxaprop, crabgrass (Digitaria spp.) and oat (Avena sativa). Dallisgrass could potentially express elevated levels of glutathione transferase at > 500 GDD_10C, that consequently reduce the activity of fluazifop applications for dallisgrass control.

Turfgrass managers should avoid initiating applications of fluazifop for dallisgrass control in tall fescue after 500 GDD_10C. Future research is needed to evaluate the efficacy of fluazifop at additional GDD_10C timings, as well as the effects of application timing on the efficacy of other 4-hydroxyphenylpyruvate dioxygenase inhibiting herbicides (applied alone and in combination with fluazifop) for selective dallisgrass control in tall fescue turf. Furthermore, assays of dallisgrass enzymatic activity are needed to determine the mechanism(s) responsible for the differences in control observed at various GDD_10C application timings.

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