Mark Rosenberg
Herbicides should be applied soon to minimize the impact of winter annual weeds on wheat growth and development. A select number of weeds are a problem in wheat including field pennycress, dandelion, marestail, and downy brome or “cheatgrass”. Canada thistle and ragweed will also begin to emerge soon.
Downy brome has become one of the most challenging weeds in winter wheat. It is a winter annual that emerges in the fall, often late September or early October. It begins growing again in the spring along with the winter wheat and competes with wheat for light, moisture, and nutrients. Downy brome is best controlled with fall (mid Oct.) applications of ALS-inhibiting herbicides such as PowerFlex (pyroxsulam), Olympus (propoxycarbazone), or Maverick (sulfosulfuron). Downy brome is often much more difficult to control in the spring, but if you missed the fall application, it is important to make spring applications as early after emergence as possible. Some effective spring herbicides include PowerFlex or Rimfire Max (propoxycarbazone + mesosulfuron). Unlike some other downy brome herbicides, these do not have restrictive crop rotation limitations.
There are a number of herbicides available for control of winter annual broadleaf weeds in wheat. Consider using products such as Huskie, Express, or Affinity products, or Harmony Extra. Express, Affinity products, and Harmony Extra are often mixed with other broadleaf herbicides to control additional weed species such as prickly lettuce and ALS resistant kochia.
If Canada thistle is an especially big problem make sure the rosette is present for the best control possible. WideMatch (clopyralid + fluroxypyr) is one of the best herbicides for Canada thistle control, but additional herbicides may be needed to control mustard species or common lambsquarters.
When selecting herbicides, also consider future crops and avoid carryover injury. One common question we get is the risk of herbicide carryover to cover crops planted in wheat stubble. Most labels do not define this risk. The SD Wheat Commission funded some SDSU research to evaluate carryover risk. The results indicated several common cover crop species, such as oilseed radish, turnips, canola, pulse crops (peas, lentils, etc.), are surprising tolerant to many typical wheat herbicides. The highest risk herbicides may be those used for downy brome control (mentioned above), Ally products (metsulfuron), and Beyond (imazamox for ClearField wheat). WideMatch and Huskie (bromoxynil + pyrasulfotole) may also have a slight risk of carryover. Carryover risk often increases on low organic matter and high pH (7.5 or greater) soils.
Be sure to follow label guidelines to minimize risk of crop injury, yield loss, and carryover. Labels for some products specify the number of tillers or leaves that wheat should have before treatment is allowed. All labeled wheat herbicides can be applied prior to jointing, but the number of herbicide options decrease as the crop progresses through jointing and more advanced stages.
Take time to dissect plants in the field to help determine growth stage. Do not use wheat height to determine growth stage. Also consider the weather and soil conditions when choosing the herbicide to avoid drift and potential crop injury, which may occur during cold conditions.
Source : SDSU