Cover Story

Taking Grass Weeds out of Non-mowed Fine Fescues with ACCase-inhibiting Herbicides

By Peter Landschoot, Ph.D., Professor of Turfgrass Science

One of the toughest tasks for a turfgrass manager is to control grass weeds without injuring desirable grasses. One group of postemergence herbicides, the acetyl-CoA carboxylase-inhibiting herbicides (or ACCase-inhibiting herbicides), is particularly important for selective suppression or control of stubborn weeds like quackgrass, wirestem muhly, tall fescue, bentgrass, and annual grasses in non-mowed fine fescue stands (Figure 1). Achieving successful weed control with ACCaseinhibiting herbicides requires some experience and careful examination of herbicide labels for rates, timing of applications, and use of surfactants.

FIG. 1. Tall fescue infesting a stand of fine fescue.

General characteristics of ACCase-inhibiting herbicides

The ACCase-inhibiting herbicides are foliar absorbed and move through phloem to the growing points of target weeds. Hence, they are classified as systemic herbicides. Generally, herbicides in this group act slowly on susceptible grass weeds, taking two to three weeks to express complete suppression or control following applications. The mode of action of ACCase-inhibiting herbicides involves inhibition of the acetyl-CoA carboxylase enzyme (abbreviated ACCase) in plants. ACCase is an essential enzyme catalyst in fatty acid and cell membrane synthesis. The ACCase inhibitors are important for controlling weed grasses in broadleaf crops such as soybeans, in ornamental beds, and in stands containing some wildflower species because of the high degree of tolerance of broadleaf plants to these herbicides. Fine fescue species are known to have a high degree of tolerance to most ACCase-inhibiting herbicides, perhaps because of a less sensitive ACCase enzyme or the ability to quickly degrade the herbicide (Al-Khatib, 2019). Sedges are also tolerant of ACCase-inhibitors, thus none of these herbicides are labeled for control of nutsedge or false green kyllinga.

The PROPs, FOPs, and DIMS

The ACCase-inhibiting herbicides include two major subgroups, the aryloxyphenoxypropionates and the cyclohexanediones. To simplify terminology, aryloxyphenoxypropionates are often referred to as PROPs and FOPs, and include three important grass herbicides used in non-mowed fine fescues: fenoxaprop-P-ethyl (Acclaim Extra), fluazifop-P-butyl (Fusilade II Turf & Ornamental Herbicide and Ornamec 170), and quizalofop-P-ethyl (Assure II and Targa). The cyclohexanediones, frequently called DIMs, include sethoxydim (Segment II and Sethoxydim SPC) and clethodim (Tapout, Envoy Plus, and other products).

Acclaim Extra (fenoxaprop-P-ethyl) is known mostly as a postemergence summer annual grass herbicide but can also suppress common bermudagrass in some cool-season turfgrasses. Acclaim Extra is labeled for fine fescues provided application rates do not exceed 28 fl oz/acre for a single application. A trial conducted at Penn State in June of 2020 showed no injury to non-mowed creeping red fescue following applications of Acclaim Extra at 13, 20, 28, and 39 fl oz/acre with a nonionic surfactant added to the mixtures. Acclaim Extra has only postemergence activity, and plants should not be under drought stress when applications are made. As with all postemergence grass herbicides, avoid mowing operations 24 hours before and after application to allow better contact with the leaf surface and translocation of the active ingredient. Acclaim Extra should not be mixed with 2,4-D or MCPP as these herbicides may reduce efficacy.

Fusilade II and Ornamec 170 (fluazifop-P-butyl) are postemergence annual and perennial grass herbicides. These products can be sprayed over the top of many ornamental broadleaf plants and suppress or control summer annual and perennial grasses such as bermudagrass, quackgrass, and wirestem muhly in non-mowed stands of fine fescue. Fusilade II and Ornamec 170 can also be used to control weeds in stands of tall fescue when used at low rates. In trials conducted at Penn State, Fusilade II performed better than other ACCase-inhibiting herbicides with respect to suppression of quackgrass when applied at 16 fl oz/acre and mixed with a nonionic surfactant in June and again in early September. Fusilade II also showed good control of escaped creeping bentgrass in fine fescue stands following a 16 fl oz/acre application in September. Creeping bentgrass is not listed as a weed species on the Fusilade II label. For optimum herbicide efficacy, a nonionic surfactant should be mixed with Fusilade II. The product label states that a crop oil concentrate should not be used when spraying over turf and ornamentals.

Caution should be used when spraying fluazifop-P-butyl products in fine fescue stands containing native warm-season grasses. A preliminary trial on a local variety of little bluestem (a warm-season native grass species sometimes used in fine fescue stands) revealed stunting and foliar injury following an application of Fusilade at 16 fl oz/acre with a nonionic surfactant (Figure 2).

FIG.2. Plot of severely injured quackgrass following a June application of Fusilade II.

FIG.2. Herbicide-induced stunting of little bluestem after a summer application Fusilade II.

Assure II and Targa (quizalofopP-ethyl) are similar to Fusilade II and Ornamec 170 in that they are very safe on fine fescues and are labeled for control of summer annual and perennial grasses, including bermudagrass, quackgrass, and wirestem muhly. Recent trials with Assure II at Penn State have shown excellent safety on fine fescues at the 12 fl oz/ acre rate when mixed with a nonionic surfactant. Although we have not tested quizalofop-P-ethyl herbicides on other species, the Assure II and Targa labels contain information on safe establishment of wildflowers in non-crop areas. Preliminary trials with Assure II for quackgrass control in fine fescue stands have shown good suppression, but more studies are needed to determine how this herbicide compares to other ACCase-inhibiting herbicides. Assure II does not appear to have inhibitory effects on escaped bentgrass that contaminates fine fescue stands. Quizalofop-P-ethyl herbicides have been used for postemergence control of Japanese stiltgrass in wooded areas, but this invasive weed does not appear on the Assure II or Targa labels (Jackson et al., 2020).

Segment II and Sethoxydim SPC (sethoxydim) are DIM ACCase-inhibiting herbicides, and like the PROPs and FOPs, provide post-emergence suppression or control of numerous annual and perennial grass weeds, including Japanese stiltgrass, bermudagrass, quackgrass, wirestem muhly, perennial ryegrass, and seedling tall fescue. The Segment II label has specific information for use on fine fescues managed as naturalized areas on golf courses. Trials at Penn State have demonstrated excellent safety on fine fescue species at rates up to 24 fl oz/acre when combined with 1.5 pt/acre methylated seed oil (this rate is slightly higher rate than the label recommended rate for fine fescues). However, sethoxydim herbicides can injure most other cool-season turf grasses, as well as little bluestem, when applied at maximum label rates.

Trials at Penn State have shown good efficacy on wirestem muhly and suppression of quackgrass when Segment II is applied twice (June and September) at 24 fl oz/acre and mixed with methylated seed oil. Our trials have also demonstrated very good suppression of 1-year-old Ky-31 tall fescue in fine fescue stands when Segment II was applied once in early September at 24 fl oz/ acre with methylated seed oil as an additive. Based on the results of this trial, sethoxydim appears to be the most effective ACCaseinhibiting herbicide at controlling tall fescue (Figure 3) in naturalized stands of fine fescues. Methylated seed oil and crop oil concentrate products are recommended as additives for sethoxydim products for enhanced weed control. According the to the Segment II label, non-ionic surfactants are not recommended as additives.

FIG.3. Non-treated control plot of 1-yr-old Ky-31 tall fescue mixed with hard fescue.

FIG.3. A plot of the same Ky-31 tall fescue/hard fescue mixture following a September application of Segment II (right). Note the reduction of tall fescue in the Segment II-treated plot compared to the non-treated control.

Tapout and Envoy Plus (clethodim) are not recommended for use on naturalized fine fescue stands at current label rates due to phytotoxicity concerns. Penn State trials with Tapout at 12, 24, and 32 fl oz/acre mixed with a nonionic surfactant have demonstrated various degrees of injury to fine fescues, with the 12 fl oz rate showing the least amount of injury and 32 fl oz causing severe foliar browning (Figure 4). On the positive side, none of these treatments killed fine fescues, and treated plots recovered several weeks after application. A noticeable effect of Tapout treatments included seedhead stunting, thinning, and/or complete inhibition, which could be a desirable outcome in some situations. Clethodim-containing herbicides can control annual bluegrass (along with many other annual and perennial grasses), which makes these products somewhat unique. We plan to conduct more studies with clethodim products at lower application rates to determine if these herbicides have a beneficial role in naturalized fine fescue stands.

FIG.4. Plots of fine fescue treated with Tapout (clethodim) herbicide. The plot within the yellow flags on the left was treated with 24 fl oz Tapout/acre plus a nonionic surfactant, and the plot on the right was treated with 12 fl oz/acre plus a nonionic surfactant. Both treatments reduced seedheads and the 12 fl oz rate showed only minor foliar injury.

Resistance issues

Herbicide resistance is becoming a significant problem in areas where products with similar active ingredients are used repeatedly to control certain types of weeds. At least three weed species (goosegrass, annual ryegrass, and annual bluegrass) have developed resistance to ACCase-inhibiting herbicides.

In the case of goosegrass, affected plants were resistant to diclofop-methyl, sethoxydim, clethodim, fenoxaprop-Pethyl, and fluazifop-P-butyl (McCullough et al., 2016). This occurrence was detected in Georgia after 30 years of exclusive use of sethoxydim for goosegrass control in centipedegrass. A recent report from New Zealand revealed annual bluegrass resistance to an ACCase-inhibiting herbicide (haloxyfop) following applications to fine fescue turf on a golf course (Ghanizadeh et al. 2020). Although turfgrass herbicide resistance in the northeast U.S. is rare, these examples serve as a reminder to alternate among different mode-of-action classes of herbicides when making repeat applications over multiple seasons. Fortunately, ACCase-inhibiting herbicides are used infrequently in non-mowed stands of fine fescue in Pennsylvania, and no resistance problems have been reported.

Label information

Some ACCase-inhibiting herbicide product labels do not provide specific information on treating naturalized areas of fine fescues on golf courses and other large properties, and instead refer to use in non-crop areas. If in doubt about proper use rates for certain weeds in fine fescues, turfgrass managers may want to check with manufacturers for more specific information concerning product uses. Always refer to product labels for mixing with other herbicides, especially broadleaf herbicides.

References

Al-Khatib, K. 2019. Acetyl CoA Carboxylase (ACCase) inhibitors. University of California Division of Agriculture and Natural Resources. http://herbicidesymptoms.ipm.ucanr.edu/ MOA/ACCase_inhibitors/

Jackson, D.R., S. Wurzbacher, A. Gover, and S. Templeton. 2020. Japanese stiltgrass. Penn State Extension. https://extension. psu.edu/japanese-stiltgrass

Ghanizadeh, H., C. H. Mesarich, and K. C. Harrington. 2020. Molecular characteristics of the first case of haloxyfop-resistant Poa annua. Scientific Reports 10,4231. https:// doi.org/10.1038/s41598-020-61104-0

McCullough, P.E., Y. Jialin Yu, P.L. Raymer, and Z. Chen. 2016. First report of ACCaseresistant goosegrass (Eleusine indica) in the United States. Weed Science. 64(3):399-408. https://doi.org/10.1614/WS-D-15-00203.1