Postemergence Grass Herbicides (April 2, 2014)

Postemergence grass herbicides have been available for the past 20 to 25 years. These include fluazifop (Fusilade-1985), sethoxydim (Poast-1986, Segment, Vantage and others), and clethodim (Select-1991, Select Max, Arrow, Envoy, Volunteer, and others). They only control grasses and are registered on numerous broadleaf vegetable and field crops as well as trees and vines. These herbicides are all classified as lipid biosynthesis inhibitors. They work by inhibiting the production of an enzyme (ACCase) used to produce fatty acids which are needed in the formation of cell walls and other plant membranes. They are slow acting and have no soil activity. There are some herbicides that use this same mode of action but are used safely on wheat and barley. These include Discover (clodinafop),Dakota, Puma (fenoxaprop), Axial (Pinoxaden) and Achieve (tralkoxydim) which are commonly used to control Canarygrass, wild oat and other grasses.

These herbicides are all fairly broad spectrum and control most grasses although there are differences between them on some grass species. Clethodim will control sprangletop while sethoxydim and fluazifop will not. The same is true for annual bluegrass which is controlled only by clethodim when it is small. All of these are weak on sandbur.

These herbicides have no soil activity and typically need to be applied 2 or 3 times to achieve season long weed control. They all require the use of a crop oil concentrate to help penetrate the leaf surface except for Select Max which requires either a non-ionic surfactant or crop oil.

These herbicides are normally very safe to the crops that they are registered on. There have been only a few instances over the past 25 years where crop injury has occurred. One was to melons where above labeled rates of Select Max was applied in overlaps or at the ends of fields. This was only from Select Max. Another instance was several years ago on onions where liquid fertilizer (AN20) was previously sprayed over the onions for weed control. The third instance was to some leafy vegetables, especially arugula, where the crop oil concentrate caused leaf burn.

Although these herbicides once seemed fool proof, each year more failures are being reported. Last year, for instance, Poast and Select did not control Rabbitfootgrass in a couple fields and Canarygrass was missed by Poast in others. The only documented case of herbicide resistance in this region has been the resistance of Canarygrass to sethoxydim, fluazifop and clethodim in the Imperial Valley. There are several potential causes for herbicide failures and resistance is only one of them and is rare in this region.

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To contact Barry Tickes go to: btickes@ag.arizona.edu.