Experimental design and treatments.

Treatments were applied perpendicular to turf strips in a split-plot design where whole plots were species and subplots were herbicides. Treatments included fenoxaprop-p-ethyl (delineated fenoxaprop) at 50 g a.i./ha and metamifop at 200, 400, or 800 g a.i./ha. Applications were made at 1, 2, 3, or 4 weeks after seeding. Herbicide formulation, application dates, and turfgrass mowing heights during establishment are presented in tables. A nontreated control was included. Fenoxaprop was chosen as a standard comparison with metamifop at a recommended rate for annual weed control in most cool-season grasses (Anonymous, 2004). Applications were made with CO₂-pressured sprayers calibrated to deliver 374 L·ha⁻¹ with a single flat-fan nozzle (9504E flat-fan; TeeJet Spraying Systems) at 222 kPa.

Measurements and data analysis.

Turfgrass cover was visually measured on a percent scale where 0 equals no turf cover and 100 equals complete plot cover. Cover was rated weekly from 2 to 8 WAS and at 21 WAS. Grid counts were made for each species by counting the number of 161-cm² squares within a 0.58-m² grid (36 total squares per grid) with shoots present (yes/no) of the desirable turf species at 20 WAS in the 2012–13 experiment. Data were subjected to analysis of variance at the 0.05 P level. Species were analyzed separately as randomized complete blocks. Means were separated with Fisher’s protected least significant difference test at α = 0.05. Year-by-treatment interactions were not detected, and thus, years were combined.

Creeping bentgrass.

Groundcover of nontreated creeping bentgrass increased from 42% at 2 WAS to 83% at 8 WAS (Table 1). All treatments applied 1, 2, or 3 WAS reduced creeping bentgrass cover from the nontreated from 4 to 8 WAS. Although metamifop at 200 g·ha⁻¹ reduced cover at these timings, reductions from the nontreated ranged from 10% to 18% at 8 WAS, whereas 400 to 800 g·ha⁻¹ reduced cover 35% to 71%.

Table 1.

Groundcover of ‘Penn A-4’ creeping bentgrass treated with herbicides after seeding in two combined field experiments, 2011–13, Griffin, GA.

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Metamifop treatments were generally less injurious to creeping bentgrass than fenoxaprop when applied 4 WAS (Table 1). Metamifop at 400 and 800 g·ha⁻¹ and fenoxaprop applied 4 WAS reduced creeping bentgrass cover on three dates and measured 33% to 45% by 8 WAS. Creeping bentgrass cover was similar to the nontreated on all dates from metamifop at 200 g·ha⁻¹ applied 4 WAS.

At 21 WAS, creeping bentgrass cover was similar to the nontreated from metamifop at 200 g·ha⁻¹ applied at all timings after seeding, averaging 83% cover (Table 1). However, all other treatments reduced measurements from the nontreated, ranging from 23% to 64% cover. Reductions in creeping bentgrass groundcover were exacerbated by increased metamifop rate from 400 to 800 g·ha⁻¹ at all timings, whereas fenoxaprop reduced cover more when applied 2, 3, and 4 WAS compared with 1 WAS.

Perennial ryegrass.

Groundcover of nontreated perennial ryegrass increased from 2 to 8 WAS, ranging from 58% to 85% (Table 2). Fenoxaprop applied 1 WAS reduced perennial ryegrass cover at 2 and 3 WAS but was similar on other evaluation dates. Perennial ryegrass treated with metamifop at 800 g·ha⁻¹ at 1 WAS reduced cover from the nontreated at 2 WAS but 200 and 400 g·ha⁻¹ were similar on all dates. Fenoxaprop and metamifop at all rates applied 2, 3, and 4 WAS did not reduce perennial ryegrass cover from the nontreated on any date. Perennial ryegrass cover was similar to the nontreated at 21 WAS, averaging 88% across all treatments.

Table 2.

Groundcover of ‘Manhattan V’ perennial ryegrass treated with herbicides after seeding in two combined field experiments, 2011–13, Griffin, GA.

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Tall fescue.

Groundcover of nontreated tall fescue increased from 2 to 8 WAS ranging 39% to 84% (Table 3). Fenoxaprop and metamifop at 400 and 800 g·ha⁻¹ applied 1 WAS reduced turf cover from the nontreated on one date but tall fescue recovered and was similar to the nontreated from 5 to 8 WAS. Treatments applied 2 WAS did not reduce turf cover from the nontreated except metamifop at 800 g·ha⁻¹, which reduced cover ≈5% from the nontreated at 5 and 6 WAS. Tall fescue cover was similar to the nontreated at all evaluations from metamifop at all rates when applied 3 and 4 WAS. Fenoxaprop applied 3 WAS reduced tall fescue cover 5% from the nontreated at 5 WAS. Cover was also reduced on all evaluation dates from metamifop applied at 4 WAS. However, reductions from the nontreated never exceeded 7% and would likely be considered acceptable in weed control programs during tall fescue establishment. There were no meaningful differences detected among treatments for tall fescue cover at 21 WAS, because all plots had 8% or less difference from the nontreated.

Table 3.

Groundcover of ‘Titan’ tall fescue treated with herbicides after seeding in two combined field experiments, 2011–13, Griffin, GA.

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Grid counts.

Treatment-by-application-timing interaction was not detected for grid count measurements for any species, and thus, main effects are presented separately. For nontreated creeping bentgrass, 100% of grids had turf present at 20 WAS, whereas plots treated with metamifop at 200 and 400 g·ha⁻¹ were similar (Table 4). However, fenoxaprop and metamifop at 800 g·ha⁻¹ reduced the presence of creeping bentgrass in grid counts to 68% and 71%, respectively. Treatments applied 2, 3, and 4 WAS reduced the presence of creeping bentgrass to 72% to 86% cover in grid counts but treatments 1 WAS were similar to the nontreated. Perennial ryegrass and tall fescue were present in all sampling grids across all treatments and application timings at 20 WAS.

Table 4.

Grid counts for turfgrass presence at 20 weeks after seeding ‘Penn A-4’ creeping bentgrass, ‘Manhattan V’ perennial ryegrass, and ‘Titan’ tall fescue treated with herbicides in a field experiment, Griffin, GA.

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Metamifop may have potential for use on perennial ryegrass and tall fescue seedlings, but reductions in creeping bentgrass cover may limit use to established turf only at rates tested. Although turf seedling tolerance to metamifop has received limited investigation, researchers have reported tolerance to other AOPP herbicides. For example, Dernoeden (1987) noted perennial ryegrass and tall fescue seedlings were tolerant to fenoxaprop from 90 to 280 g·ha⁻¹, and treatments effectively controlled smooth crabgrass (Digitaria ischaemum Schreb. Muhl.). In Georgia, Johnson and Carrow (1993) reported fenoxaprop temporarily injured tall fescue, but treatments did not reduce turfgrass density from the nontreated. In Virginia, Beam et al. (2005) noted fluazifop plus fenoxaprop treatments injured newly seeded tall fescue less than 5% at 5 weeks after initial treatment, suggesting applications have potential for grassy weed control during turf establishment.

Although AOPP herbicides are efficacious for grassy weed control, turf managers may also apply other chemistries in newly seeded cool-season grasses. McElroy and Breeden (2007) reported mesotrione at 280 g·ha⁻¹ applied sequentially at 28 and 42 d after emergence reduced tall fescue cover less than 5% from the nontreated. Hart et al. (2004) noted quinclorac at 400, 600, and 800 g·ha⁻¹ caused chlorosis to creeping bentgrass, but applications did not reduce establishment from the nontreated. Despite injury from these herbicides to seedling turfgrass, practitioners should consider the benefits of controlling summer annual grasses with herbicides and the potential for enhancing turfgrass competition with other weeds in subsequent growing seasons.

Perennial ryegrass and tall fescue generally have greater tolerance to fenoxaprop than creeping bentgrass, but metamifop may have comparable safety across these species. Flessner and McElroy (2011) reported singly or sequentially applied metamifop from 100 to 800 g·ha⁻¹ caused minimal injury to established creeping bentgrass, kentucky bluegrass (Poa pratensis L.), and tall fescue. Fenoxaprop may be safely applied to mature perennial ryegrass and tall fescue at rates up to 200 g·ha⁻¹, but researchers have reported significant injury (greater than 20%) to established creeping bentgrass at lower rates (Carroll et al., 1992; Dernoeden, 1987, 1989; Johnson and Carrow, 1995; Neal et al., 1990). Shim and Johnson (1992) noted sequential applications of fenoxaprop at 70 to 200 kg·ha⁻¹ caused unacceptable injury to creeping bentgrass, and turf did not fully recover after 3 months compared with nontreated. Henry and Hart (2004) reported creeping bentgrass density was reduced up to 33% from the nontreated after sequential applications of fenoxaprop at 40 or 70 g·ha⁻¹, respectively.

Growth stage of grasses influences efficacy of AOPP herbicides, and treatments are often more phytotoxic when applied before tillering (Anderson, 1976; Derr et al., 1985; Wu and Santelman, 1976). Derr et al. (1985) reported goosegrass, large crabgrass (Digitaria sanguinalis L.), and giant foxtail (Setaria faberi Herrm.) control from fluazifop was substantially reduced when treatments were made at late tillering stages. Neal et al. (1990) noted fenoxaprop was less effective on tillered smooth crabgrass and large crabgrass compared with two to five leaf growth stages. Efficacy of AOPP herbicides may have important implications for cool-season turfgrass seedling tolerance, and further investigation is warranted to determine metamifop rates that are comparable to mesotrione, quinclorac, and other chemistries during establishment.

Overall, results suggest applications to creeping bentgrass should be delayed greater than 4 WAS for fenoxaprop at 50 g·ha⁻¹, greater than 4 WAS for metamifop at 400 and 800 g·ha⁻¹, and 3 WAS for metamifop at 200 g·ha⁻¹. Additionally, fenoxaprop applications should be delayed 2 WAS for perennial ryegrass and tall fescue, whereas metamifop could be safely applied at all rates at 1 WAS. These recommendations are based on turfgrass maintained at mowing heights evaluated. Physiological effects of lower mowing height could potentially exacerbate turfgrass injury to these herbicides during establishment. Contrarily, higher mowing heights for grasses such as perennial ryegrass could increase seedling turfgrass tolerance to herbicides and warrant further investigation. Research is also needed to evaluate cool-season turfgrass seedling tolerance to multiple applications of metamifop alone and with other herbicides during establishment.