Field Experiments. Experiments were conducted on an established ‘Titan' tall fescue lawn at the University of Georgia Griffin Campus from July to September in both 2014 and 2015. Soil was a Cecil sandy clay loam (Fine, kaolinitic, thermic typic kanhapludults) with 6.2 pH and 2.5% organic matter. The field was mowed weekly at a 7.5-cm height with a rotary mower, and clippings were returned. Irrigation was applied as needed to prevent turfgrass wilting. The majority of the smooth crabgrass population in this tall fescue lawn had three or more tillers on the day of initial treatments in both years.

Mesotrione was applied once at 280 g ha⁻¹ or twice at 140 g ha⁻¹. Both mesotrione treatments were applied with triclopyr (Turflon Ester 4L, Dow AgroSciences, Indianapolis, IN 46268) at 0, 560, or 1,120 g ae ha⁻¹. Fenoxaprop (Acclaim Extra 0.57EC, Bayer Environmental Sciences, Research Triangle Park, NC 27709) was applied at 195 g ha⁻¹ as a standard comparison treatment. Application rates were chosen from labeled use rates of these herbicides in tall fescue (Anonymous 2008a,b, 2011). Treatments were applied with a CO₂-pressured sprayer equipped with a single 9504E (TeeJet Spraying Systems Co., Roswell, GA 30075) flat-fan nozzle that delivered 374 L ha⁻¹ at 222 kPa. All treatments, except fenoxaprop, included a nonionic surfactant (Activator 90, 90% alkylphenol ethoxylate and tall oil fatty acid, Loveland Products, Loveland, CO 80538) at 0.25% v/v. The initial and sequential treatments were applied on July 1 and July 21 in 2014, respectively, and July 10 and July 31 in 2015, respectively.

Smooth crabgrass ground cover was visually evaluated on a percent scale where 0 equaled no cover and 100 equaled complete cover. Smooth crabgrass control was measured visually on a percent scale where 0 equaled no control and 100 equaled complete control. Foliar bleaching of smooth crabgrass and tall fescue were visually evaluated on a percent scale where 0 equaled no bleaching (whitening of leaves) and 100 equaled complete bleaching. Tall fescue injury was rated on a percent scale where 0 equaled no stunted growth and 100 equaled complete desiccation. Ratings were made every 7 d until 10 wk after initial treatment (WAIT).

Laboratory Experiments. Experiments were conducted at the University of Georgia (Griffin, GA). Smooth crabgrass seed was collected by hand from indigenous plants in Griffin, GA and seeded into pots (3.8-cm diameter and 20-cm depth) in a greenhouse set for day/night air temperatures of 32/25 C. Soil in these pots was a mixture of sand and peat moss (85 : 15 v/v). Grasses received fertigation biweekly (MacroN 28-7-14 Sprayable Fertilizer, LESCO Inc., Cleveland, OH) and were watered as needed to promote growth. Plants selected for inclusion in laboratory experiments were at a 3- to 5-tiller growth stage.

Smooth crabgrass was acclimated for 72 h before treatments in a growth chamber set for 32/25 C (day/night) with a 12-h photoperiod. Plants were prepared for radiolabeled treatments by covering the third, fully expanded leaf with flexible film (Parafilm, Bemis Company Inc., Neenah, WI 54956). A broadcast treatment of mesotrione at 280 g ha⁻¹ with triclopyr at 0 or 1,120 g ha⁻¹ was applied with the aforementioned CO₂-pressured sprayer calibrated to deliver 374 L ha⁻¹. The formulated herbicides and surfactant used in field experiments were applied for these treatments. The film was removed from the targeted leaf after broadcast treatment, and two 1-μL droplets of ¹⁴C-mesotrione containing 150 Bq each were applied with a 10-μL syringe. The spotting solution contained 0.75 μg μL⁻¹ of mesotrione plus 0 or 3 μg μL⁻¹ of triclopyr to simulate droplets of spray solution. The nonionic surfactant was added to radiolabeled solutions at 0.25% v/v.

Plants (roots plus shoots) were harvested at 1, 6, 24, 72, and 168 h after treatment (HAT). The treated leaf was excised and rinsed with 10 mL of methanol inside a 20-mL scintillation vial. The base of the leaf was held with forceps, and rinsate was applied towards the leaf tip with a 5-mL pipette on the leaf surface. This methodology was chosen from pilot experiments that completely removed adsorbed ¹⁴C immediately after treatment (with and without triclopyr), and previous research with ¹⁴C-mesotrione on grain sorghum [Sorghum bicolor (L.) Moench.] (Abit and Al-Khatib 2009). Plants were then divided into four sections with shears, including: the treated leaf, all aboveground biomass except the treated leaf and tillers, tillers, and roots. The samples were then oven-dried for 14 d at 40 C.

Samples were combusted in a biological oxidizer and radioactivity was quantified with liquid scintillation spectroscopy (LSC; Beckman LS 6500®, Beckman Coulter Inc., Fall River, MA 02720). The entire plant was oxidized as one sample to calculate absorption over time from harvests at 1, 6, and 24 HAT. Plant parts were oxidized separately from harvests at 72 and 168 HAT to quantify radioactivity distribution. Methanol from rinsate was evaporated from vials in a forced-air hood, and radioactivity was quantified with LSC. Foliar absorption was quantified by dividing the total radioactivity recovered in plants by the total ¹⁴C applied. Radioactivity distribution was determined by dividing the total radioactivity recovered in plant parts from the total ¹⁴C recovered per plant. Radioactivity recovery was calculated by dividing the ¹⁴C in rinsate and plant samples by the amount applied.

Experimental Design and Data Analysis. The design for the field experiments was a randomized complete block with four replications of 0.9- by 3-m plots. The plots in 2015 were adjacent to the plots in 2014. The design for the laboratory experiment was completely randomized with four replications, and the experiment was repeated.

Data were subjected to analysis of variance using the General Linear Model Procedure in SAS (SAS v. 9.3, SAS Institute Inc., Cary, NC 27513) and means were separated with Fisher's LSD test at α = 0.05. For absorption data, regression analysis was performed after plotting means on graphs and selecting equations that characterized the relationship of foliar absorption over time. The equations used included the following linear (Equation 1) and two-parameter exponential (Equation 2) regression models:

Smooth Crabgrass and Tall Fescue Bleaching. The initial ground cover of smooth crabgrass measured 45% (± 0.9 standard error) and 35% (± 0.1) on the day of initial treatments in 2014 and 2015, respectively. The single application of mesotrione at 280 g ha−1 alone bleached smooth crabgrass 42 and 53% at 1 and 2 WAIT, respectively (Table 1). Bleaching declined to 20% at 3 WAIT, and measured <5% from 4 to 10 WAIT. The sequential application of mesotrione at 140 g ha−1 bleached smooth crabgrass 16 to 39% from 1 to 5 WAIT.

Table 1.

Smooth crabgrass foliar bleaching in tall fescue from herbicide treatments in field experiments, 2014 to 2015, in Griffin, GA. Results were pooled over years.

Splitting the mesotrione rate in two applications of 140 g ha⁻¹ reduced the initial degree of bleaching on two dates compared with the single treatment at 280 g ha⁻¹. However, foliar bleaching was extended for a longer period. By 4 WAIT, bleaching from the single and sequential treatment measured 4 and 22%, respectively (Table 1). Smooth crabgrass bleaching declined to < 5% at 6 WAIT from mesotrione applied sequentially. Triclopyr tank mixtures with all single and sequential mesotrione applications reduced bleaching to 2% or less on all assessment dates. Differences in triclopyr rates at 560 and 1,120 g ha⁻¹ were not detected for smooth crabgrass bleaching. Tall fescue bleaching and injury was not detected from any treatment (data not shown).

Smooth Crabgrass Control. The single application of mesotrione at 280 g ha−1 provided 20% greater control of smooth crabgrass than the 140 g ha−1 treatment (68 vs. 48% control) at 3 WAIT (Table 2). The sequential application of the low rate improved control similar to the single application of 280 g ha−1 from 6 to 10 WAIT. Sole use of mesotrione provided poor control (46 to 65%) of smooth crabgrass on these dates regardless of application regimen.

Table 2.

Smooth crabgrass control in tall fescue from herbicide treatments in field experiments, 2014 to 2015, in Griffin, GA. Results were pooled over years.

The mesotrione plus triclopyr tank mixtures controlled smooth crabgrass ≥ 85% from 3 to 10 WAIT, and were more effective than mesotrione alone (Table 2). Smooth crabgrass was controlled by 91% from triclopyr plus mesotrione at 280 g ha⁻¹ at 2 WAIT, and was significantly greater than the low mesotrione rate applied with triclopyr (80% control). However, the sequential treatments of mesotrione plus triclopyr improved smooth crabgrass control to > 90% from 4 to 9 WAIT. The tank mixtures of mesotrione plus triclopyr provided better smooth crabgrass control than fenoxaprop at 195 g ha⁻¹ (80 to 84%) from 4 to 10 WAIT.

The reductions in foliar bleaching of smooth crabgrass by combining HPPD-inhibiting herbicide with triclopyr are consistent with previous research. Brosnan et al. (2013) found tank mixtures of topramezone and triclopyr resulted in less smooth crabgrass bleaching than topramezone alone. Smooth crabgrass control was not improved by using triclopyr with topramezone, but there were also no antagonistic effects on topramezone efficacy. Perhaps topramezone is more efficacious on multitiller smooth crabgrass than mesotrione, and therefore there is less potential for improving control with triclopyr. Further research is needed to investigate the effect of application timing, rate, and growth stage on mesotrione efficacy for controlling smooth crabgrass with triclopyr.

Mesotrione is a potent inhibitor of HPPD enzymes, and the depletion of carotenoids results in cellular membrane peroxidation and chlorophyll degradation (Havaux 1998; Lee et al. 1997; Siefermann-Harms 1987; Trebst et al. 2002). Consequently, susceptible species exhibit foliar bleaching in the presence of light. We postulated that the addition of triclopyr could enhance damage to thylakoids and cellular membranes by mesotrione prior to chlorophyll degradation. Further research is needed to determine if the presence of triclopyr induces phytotoxicity from mesotrione that is independent of chlorophyll degradation and subsequent bleaching. This supposition, along with the effects of triclopyr alone on the carotenoid biosynthesis pathway, warrants further investigation with and without HPPD inhibitors.

¹⁴C-Mesotrione Absorption and Translocation in Smooth Crabgrass. Harvest-by-triclopyr interaction was detected for foliar absorption of 14C-mesotrione. Recovery of the applied radioactivity averaged 79% (± 1.2). Smooth crabgrass absorption of mesotrione when applied alone increased linearly over time, and reached 15% of the applied radioactivity at 168 HAT (Figure 1, Equation 1). The absorption rate was significantly increased when triclopyr was applied as a tank mixture with mesotrione. Peak foliar absorption of 14C-mesotrione was reached at 24 HAT when tank-mixed with triclopyr, and the asymptote measured 23% (± 1.8) of the applied radioactivity (Figure 1, Equation 2). Smooth crabgrass absorbed about four-fold more mesotrione in tank mixtures with triclopyr at 24 HAT, as compared to mesotrione alone. Mesotrione absorption levels plateaued at 24 h when applied with triclopyr, suggesting further uptake was inhibited due to rapid injury of smooth crabgrass.

Figure 1.

Foliar absorption of ¹⁴C-mesotrione in smooth crabgrass as influenced by triclopyr in two laboratory experiments. Mesotrione was applied at 280 g ha⁻¹ and triclopyr was applied at 0 or 1,120 g ae ha⁻¹. Results were pooled over experimental runs. Standard errors of the mean are represented with vertical bars. Data for the mesotrione treatment were regressed with a linear model, y = β₀ + β₁(x), where y is absorption, β₀ is the intercept, β₁ is the slope, and x is hours. Regression equation for mesotrione: y = 6.3 + 0.05x, r² = 0.49; standard errors for β₀ and β₁ were 0.7 and 0.01, respectively; 95% confidence limits for β₀ and β₁ were 8.5 to 11.6 and 0.08 to 1.3, respectively. Data for mesotrione + triclopyr were regressed with the following equation, y = β₀ * (1 – exp (−β₁ * x)) where β₀ is the asymptote, β₁ is the slope estimate, and x is hours. Equation for mesotrione + triclopyr: 23.2 * (1 – exp (−0.17 * x)), r² = 0.41; standard errors for β₀ and β₁ were 1.8 and 0.2, respectively; 95% confidence limits for β₀ and β₁ were 20 to 27 and 0.05 to 0.28, respectively.

Harvest-by-triclopyr interaction was not detected for radioactivity distribution; and thus, results were pooled over harvests (Table 3). Smooth crabgrass retained 72% of the absorbed radioactivity in the treated leaf when mesotrione was applied alone. The triclopyr and mesotrione tank mixture reduced translocation of radioactivity, compared to the exclusive mesotrione treatment, as 84% of the absorbed ¹⁴C was retained in the treated leaf. The majority of the translocated radioactivity was recovered in nontreated shoots of the main plant and tillers. About 13% of the absorbed radioactivity was translocated to nontreated tillers when mesotrione was applied alone. However, translocation was reduced to 7% when triclopyr was applied with mesotrione. Triclopyr did not reduce radioactivity translocation to nontreated shoots (9%) of the main plant or roots (3%).

Table 3.

Distribution of the absorbed radioactivity for smooth crabgrass treated with ¹⁴C-mesotrione and triclopyr in laboratory experiments. Results were pooled over experimental runs.

Uptake and translocation of ¹⁴C-mesotrione have been studied in corn (Zea mays L.), Canada thistle [Cirsium arvense (L.) Scop.], common lambsquarters (Chenopodium album L.), giant foxtail (Setaria faberi Herrm.), ivyleaf morningglory (Ipomoea hederacea Jacq.), and soybean [Glycine max (L.) Merr.] (Armel et al. 2004, 2005; Mitchell et al. 2001). In corn, common lambsquarters, giant foxtail, and ivyleaf morningglory, foliar uptake was rapid, with 55 to 90% of applied radioactivity being absorbed within 24 h (Mitchell et al. 2001). However, the susceptible weeds had faster foliar absorption than the tolerant species, corn (Mitchell et al. 2001). Armel et al. (2005) found the greater susceptibility of Canada thistle to mesotrione at the rosette stage was associated with higher foliar absorption levels than plants at the bolting stage.

Alterations in absorption and translocation have been shown to improve the selectivity of other herbicides when applied with tank-mix partners. For example, Shaw and Wesley (1993) reported that acifluorfen and lactofen enhanced the absorption of ¹⁴C-chlorimuron and ¹⁴C-imazaquin, respectively, in common cocklebur (Xanthium strumarium L.), pitted morningglory (Ipomoea lacunosa L.), and prickly sida (Sida spinosa L.). Delrosario and Putnam (1973) reported that an insecticide, carbaryl, enhanced ¹⁴C-linuron penetration into carrot (Daucus carota L.) and parsnip (Pastinaca sativa L.) leaves compared to the herbicide alone.

Mesotrione exhibits acropetal and basipetal translocation after foliar absorption, but susceptible species have shown greater translocation of mesotrione than tolerant species (Armel et al. 2004; Mitchell et al. 2001). Tank mixtures that reduce this translocation pattern could be antagonistic with mesotrione or other HPPD inhibitors. In other species, Wehtje et al. (1993) found that 4-(2,4-dichlorophenoxy) butyric acid (2,4-DB) significantly reduced translocation of radioactivity from ¹⁴C-chlorimuron in peanut (Arachis hypogaea L.), but enhanced ¹⁴C translocation in Florida beggarweed [Desmodium tortuosum (Sw.) DC.]. Reductions in translocation of ¹⁴C-haloxyfop-methyl when tank mixed with bentazon, chlorimuron, or imazaquin in sorghum have also been reported (Croon et al. 1989).

Although triclopyr reduced the percentage of foliar-absorbed ¹⁴C-mesotrione translocation, the tank mixture enhanced the speed of mesotrione absorption by over six-fold. Perhaps any potential antagonistic effects that result from altered translocation by triclopyr are overcome by enhancing the speed of foliar absorption in smooth crabgrass. Additionally, enhanced foliar uptake of ¹⁴C-mesotrione when applied with triclopyr might result in greater herbicide concentrations in target tissues. Compounds associated with radioactivity translocation were not characterized, and it is unknown if translocation of the parent herbicide to tillers is affected by triclopyr. The degradation rate of mesotrione contributes to the differential tolerance levels of many species to applications (Mitchell et al. 2001). The addition of triclopyr could reduce mesotrione metabolism in smooth crabgrass, resulting in greater efficacy for control. Moreover, the formulated triclopyr ester herbicide was used to evaluate absorption for consistency with field experiments. Perhaps other triclopyr formulations might have different effects on mesotrione physiological behavior and efficacy. Therefore, further research is needed to investigate mesotrione efficacy, absorption, and metabolism as influenced by various formulations, rates, and salts of triclopyr that are available to end users.

Implications for Enhancing Smooth Crabgrass Control. The addition of triclopyr enhances mesotrione efficacy for controlling multitiller smooth crabgrass in tall fescue. Smooth crabgrass bleaching was almost completely eliminated when triclopyr was included with mesotrione. These effects would be beneficial for end users concerned over reductions in turf aesthetics from the bleaching effects of mesotrione. Smooth crabgrass control was also improved from ∼ 65% by mesotrione alone, to greater than 90% when triclopyr was applied in tank mixtures. Enhanced control from the tank mixture is associated with faster foliar uptake of mesotrione when applied with triclopyr. Bleaching of crabgrass also can be minimized by reductions in translocation of mesotrione or derivatives by triclopyr. Overall, triclopyr alters the absorption and translocation of mesotrione in multitiller smooth crabgrass that results in better control and less foliar bleaching as compared to mesotrione alone.