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Weed Control

SU Herbicides Control Weeds, Poa in Advance of Overseeding Program

October 1, 2005 By: Sowmya (Shoumo) Mitra TurfGrass Trends


Several sulfonylurea herbicides (SU) are being introduced into the turf market for selective control of broadleaf weeds; difficult-to-control grasses like annual bluegrass (Poa annua), clumpy ryegrass (Lolium perenne), creeping bentgrass (Agrostis palustris Huds.); and sedges like yellow nutsedge (Cyperus esculentus), purple nutsedge (Cyprus rotundus), green kyllinga (Kyllinga brevifolia) and false green kyllinga (Kyllinga gracillima).

The SU herbicides provide superintendents with tools to control weeds before overseeding warm-season turfgrasses and controlling cool-season turfgrasses during spring transition.

Single application and sequential applications of Monument 75 WG, applied at 7.06 g/acre and 9.33 g/acre, were very effective in controlling annual bluegrass on GN-1 bermudagrass maintained under fairway management conditions. The photograph was taken 50 days after treatment, and the plots in the back are the untreated plants, with a dense stand of annual bluegrass.
Single application and sequential applications of Monument 75 WG, applied at 7.06 g/acre and 9.33 g/acre, were very effective in controlling annual bluegrass on GN-1 bermudagrass maintained under fairway management conditions. The photograph was taken 50 days after treatment, and the plots in the back are the untreated plants, with a dense stand of annual bluegrass.

Some SU herbicides, such as Manage (halosulfuron), can be used to selectively control sedges in cool-season turfgrasses while some, such as Monument (trifloxysulfuron), can control sedges in only warm-season turfgrasses (Murphy et al., 2004). Trifloxysulfuron (Monument) has been reported to control various sedges and should be applied as a late-spring transition aid in removing perennial ryegrass since it controls perennial ryegrass rapidly (Yelverton, 2004).

SU herbicides tend to move laterally, so application of SU's in saturated soils should be avoided. To reduce lateral movement, a short irrigation (0.25 inch or 0.6 centimeter) can be applied after herbicide application (Yelverton, 2004). Hydrolysis of SU herbicides, which leads to degradation of the parent herbicide molecule, is favored under acidic soil pH conditions compared to neutral and basic soil pH conditions (Sarmah et al, 2000).

SU herbicides have been used in agricultural crops for more than 25 years, and numerous weed species have been reported to have developed resistance to these acetolactate synthase (ALS) inhibiting herbicides (SU's and imidazolinone). Annual ryegrass (Lolium rigidum) has been reported to be resistant to ALS-inhibiting herbicides, which might involve two mechanisms: increased metabolism of the herbicides and/or an herbicide-insensitive ALS enzyme (Christopher et al., 1992). Hence, care should be taken in rotating SU herbicides with other herbicides with different modes of action.

Poa control before overseeding

Poa annua infestation in an overseeded stand of perennial ryegrass is a major problem for superintendents. Introduction of various sulfonylurea herbicides has given superintendents new tools in managing Poa annua.

The best strategy to control Poa is to apply sulfonylurea herbicides before overseeding, but care should be taken not to apply the herbicides too close to overseeding. Monument, Revolver, TranXit, and Certainty are very effective in controlling annual bluegrass. Since sulfonylurea herbicides are systemic in nature, the absorption, translocation and inhibition of the ALS enzyme takes at least 14 days to provide optimum control of Poa.

Monument application at 7.06 grams (g)/acre (0.24 ounces (oz)/acre) can control over 90 percent of the Poa annua population within 28 days after application. In our experiments, a lower rate of application of Monument (7.06 g/acre) was as effective as a higher rate of application (9.33 g/acre) for controlling Poa.

Revolver applied at 6 milliliter (ml) per 1,000 square feet (0.2 fluid [fl] ounces/1,000 square feet) controlled over 90 percent of Poa within 28 days after treatment (DAT). Optimum control was achieved between 30 DAT and 60 DAT. Efficacy of a single application of Revolver reduced after 90 DAT. Sequential application of sulfonylurea herbicides are more effective in controlling Poa compared to a single application. Sequential application of Revolver within four weeks to six weeks after the first application would increase the efficacy of Revolver in controlling Poa over a longer period. 

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