By Barry Tickes, UA Area Agriculture Agent

Postemergence grass herbicides have been available for the last 20 to 25 years. These products include fluazifop (Fusilade-1985), sethoxydim (Poast-1986, Segment, Vantage, and others), and clethodim (Select-1991, Select Max, Arrow, Envoy, Volunteer, and others).

These only control grasses and are registered on numerous broadleaf vegetable and field crops plus trees and vines. The herbicides are classified as lipid biosynthesis inhibitors.

The products work by inhibiting the production of an enzyme (ACCase) used to produce fatty acids required for the formation of cell walls and other plant membranes. The herbicides are slow acting and have no soil activity.

Some herbicides use this same mode of action but are used safely on wheat and barley including Discover (clodinafop), Puma (fenoxaprop), and Achieve (tralkoxydim) commonly used to control Canarygrass, wild oat, and other grasses. These herbicides are fairly broad spectrum and control most grasses although there are differences between the products on some weed species.

Clethodim  controls sprangletop while sethoxydim and fluazifop do not. The same is true for annual bluegrass which is controlled only by clethodim when small in size. These are weak on sandbur. These herbicides have no soil activity and typically should be applied two to three times to achieve season long weed control.

These herbicides require the use of a crop oil concentrate to help penetrate the leaf surface except for Select Max which requires a non-ionic surfactant or crop oil. These herbicides are normally very safe to the crops for which they are registered.

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 reported. Last year, 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. Resistance is only one and is rare in this region.

Let me know of any failures so we can help determine the cause.

Contact Tickes: (928) 580-9902 or btickes@ag.arizona.edu.