Many of our herbicide options for weed control in wheat need to be applied during favorable growing conditions in order to achieve satisfactory results.
Often, many herbicide applications applied late fall fail to provide satisfactory results because they were either applied when the weeds were too big or when the weeds were not actively growing.
It is easier to control small actively growing weeds compared to well developed weeds late fall. For example, well-tillered grassy weeds become more difficult to control due to the plant now having multiple growing points (each tiller). Certain herbicides, like Group 1 ACCase Inhibitors, will need better spray coverage to get the product on each tiller, otherwise parts of the plant will survive.
Another disadvantage to spraying late would be the wheat crop itself is bigger, which could cause spray skips from intercepting the spray. As winter approaches, winter annual weeds will start to go dormant and cease growth. This greatly reduces herbicide uptake and ultimately can reduce control. Read herbicide labels for guidance on spraying in cold temperatures. Some labels will even provide statements about growing conditions prior to application, at application or even days after application.
Many wheat producers are familiar with the Clearfield Plus system. The herbicide Beyond used in this system is a great example of needing to be applied to actively growing feral rye to achieve adequate control. To improve control, it is recommended to use sequential applications of Beyond using a methylated seed oil (MSO) adjuvant. The first 4 ounces/a application in the fall and the other 4 ounces/a applied in the spring.
In addition to Clearfield systems, wheat producers now have another technology to utilize to control grass weeds. The new system is called CoAXium Wheat Production Sysytem. The trait for CoAXium is called AXigen. The only labeled herbicide for this technology is Aggressor, which is Quizalofop-P-ethyl a Group 1 ACCase Inhibitor. Variety names ending with an AX designates varieties that have the AXigen trait.
Varieties are bred to have a two-gene tolerance to this herbicide. Since these varieties are technically not fully resistant, application timing is important to reduce crop injury. Applications can be made once the wheat reaches five leaves in the fall and up to jointing in the spring.
Recent field trials at OSU have confirmed crop injury can occur when Aggressor is applied after jointing.
Apply Aggressor at 8-12 fl oz/A for single applications in the fall or spring. Apply 8 fluid ounces/a for sequential fall and spring applications if heavy infestations are present in the fall. To delay onset of herbicide resistance, it is recommended to not use the CoAXium Wheat Production System for two consecutive crop years.
Since the Aggressor herbicide only controls grasses, tank mixing another herbicide will be needed to control broadleaf weeds. Do not tank-mix with dimethylamine salt (Amine) formulations of 2,4-D or MCPA as these herbicides are very antagonistic with Aggressor and will severely reduce grass control. Ester formulations of 2,4-D or MCPE can tank mixed with Aggressor.
The CoAxium system will be a great option for controlling many annual winter annual grass weeds, including feral rye, jointed goatgrass, cheat, bromes, rescuegrass and wild oats. Caution is warranted for use on ryegrass, especially if ACCase resistance is suspected. Use of another ACCase herbicide, Axial XL, has been heavily used for ryegrass and resistance has been confirmed by OSU.
Bushong is Oklahoma Cooperative Extension Service northwest area agronomist.