By Dallas Peterson, Extension weed management specialist

Producers who want to treat their wheat fields with a cheatgrass herbicide have to decide when to apply it. Should they spend the money and apply it this fall or wait until spring to see if the wheat is going to yield enough to pay for it?

Each of the most commonly used cheatgrass herbicides — PowerFlex, Olympus, and Beyond (Clearfield wheat only) — is most effective on cheatgrass when applied in the fall, especially for control of downy brome. They can also be effective when applied in winter if the cheat is actively growing, or in the early spring. But control is most consistent when applied in the fall. These products generally should be applied when the cheatgrass is small and actively growing, and when the wheat has at least three leaves but prior to jointing.

Another benefit of fall application compared to spring application is that a fall application helps minimize rotational restrictions because of the extra time between application and planting the next crop. Fall application may even open the door for double-cropping or planting failed acres to soybeans in the spring following PowerFlex or Olympus.

The cheatgrass species present is a very important factor in the level of control to expect. All the listed herbicides can provide very good control of true cheat and Japanese brome (unless the weed population has developed resistance to the ALS class of herbicides), but are less effective on downy brome. ALS-resistant cheat and Japanese brome were first documented in Kansas in 2007. ALS-resistant cheatgrass first appeared in fields with a long history of Olympus and Maverick use. These populations were cross-resistant to all of the cheatgrass herbicides used in wheat. Since that time, other fields with ALS-resistant cheatgrass have also been confirmed.

The only herbicide previously mentioned that can provide control of jointed goatgrass or feral rye is Beyond, but producers would need to have planted a Clearfield wheat variety in order to use Beyond herbicide. Growers now have the option of planting one- or two-gene Clearfield wheat varieties. Two-gene varieties are usually designated as CL+ or CL2. Two-gene varieties have a higher degree of resistance to Beyond, and thus have an expanded window of application and can be used in combination with methylated seed oil for enhanced weed control. Beyond is only labelled for suppression of rye, but once again, fall applications generally provide the best control.

PowerFlex and Beyond can also provide good control of ryegrass, while Olympus will only provide suppression. Producers need to realize ryegrass is different than cereal rye. Thus, it is important that producers make that distinction when they hear or read advertisements about ryegrass control. Beyond can provide control of both ryegrass and rye, while PowerFlex can give very good ryegrass control, but will not control rye. Olympus can provide suppression of ryegrass, but will not control rye.