The following article is from the K-State Agronomy e-Update 10/24/11.
Weed Control in Wheat
1. Timing of cheatgrass herbicides on wheat
Producers who want to treat their fields of continuous wheat 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, Olympus Flex, and Maverick – 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 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 Flex.
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.
None of the herbicides previously discussed controls feral rye. To suppress feral rye in wheat, producers would need to have planted a Clearfield wheat variety and use Beyond herbicide. Beyond can also provide control of cheat, Japanese brome, downy brome, and Italian ryegrass. Like the other herbicides, fall applications of Beyond generally are more effective for weed control than spring treatments.
Producers need to realize that rye and ryegrass are not the same plant. Feral rye and Italian ryegrass are two different grassy weeds. PowerFlex and Olympus Flex can give very good ryegrass control, but will not control rye. It is important that producers make that distinction when they hear or read advertisements about ryegrass control with PowerFlex or Olympus Flex.
-- Dallas Peterson, Weed Management Specialist
2. Controlling winter annual broadleaf weeds in wheat
The main winter annual broadleaf weeds in wheat in Kansas are the mustard species and henbit. These species germinate in the fall or early winter, overwinter, then bolt in the spring.
With a couple of exceptions, the winter annual broadleaf weeds can be controlled relatively easily with the array of herbicide options available for wheat. The exceptions are:
* ALS-resistant populations of bushy wallflower (treacle mustard) and flixweed (tansy mustard)
* Late applications
ALS-resistant populations of bushy wallflower and flixweed
Bushy wallflower (treacle mustard) and flixweed (tansy mustard) populations resistant to ALS-inhibiting herbicides (such as Finesse, Glean, Ally, Express, Affinity, Amber, Olympus, PowerFlex, or Beyond) have been confirmed in central Kansas in recent years. This class of herbicides has been around for many years and has historically provided excellent control of mustard species in general. A complete failure to control one of the mustard species with an ALS-inhibiting herbicide is probably a pretty good indication that an ALS-resistant population has developed.
ALS-resistant bushy wallflower was first confirmed in Marion county in 2005, and based on field reports, seems to be fairly common in Marion, Dickinson, and Saline counties, but also may be present elsewhere. It was highly resistant to all ALS-inhibiting herbicides evaluated.
ALS-resistant flixweed was first confirmed from Saline County in 2007. ALS-resistant flixweed still seems to be confined to that general area. Some ALS-inhibiting herbicides still had some activity on the resistant biotype, but history tells us that if we switched to another ALS-herbicide, it wouldn’t be long until we had resistance to those products as well.
ALS-resistant mustard populations likely developed in continuous wheat fields where either Glean or Finesse was routinely applied in combination with a topdress fertilizer application and in the absence of a tank-mix partner such as 2,4-D or MCPA.
Fortunately, the ALS-resistant mustards can still be controlled with a timely application of 2,4-D, MCPA, or the new herbicide Huskie. These products can be tank-mixed with ALS herbicides or used as an alternative to ALS herbicides. MCPA and Huskie can be safely applied to wheat in the fall through spring after the wheat has 2 leaves. However, 2,4-D should not be applied until wheat is fully tillered, which generally does not occur until sometime in spring.
These herbicides are active primarily through foliar uptake and have limited soil activity, so ideally they should be applied to plants with viable foliage and when temperatures are 50 degrees or higher to achieve optimum performance. If applying MCPA or 2,4-D with topdress nitrogen fertilizer, the ester formulation needs to be used as amine formulations are not compatible with liquid N fertilizers.
Starane or dicamba products generally do not provide very good mustard control, so wouldn’t be very good choices to help control ALS-resistant mustards.
Many producers like to wait to apply broadleaf herbicides until winter or early spring, for a variety of reasons. This normally works well for control of mustard species, but is less effective for henbit control. Winter annual broadleaves do not generally cause much yield loss if left uncontrolled in the fall. However, these weeds should be sprayed in early spring when they are actively growing, but before they begin to bolt.
Producers who wait too long before making a herbicide application can run into a problem if weather conditions such as high winds or wet weather prevent timely application. If the herbicide application is delayed too long, weeds can become too large to control effectively and yield loss may occur.
-- Dallas Peterson, Extension Weed Management