Corn silage harvest is done or wrapping up and soybean harvest has started. The earlier start to our harvest season should permit some timely weed control ahead of any planned winter wheat planting. In a recent OSU Extension CORN newsletter, Mark Loux outlined some herbicide options for weed control prior to no-till wheat planting.
The small marestail plants that have just recently emerged or will emerge yet this fall are a primary target of fall weed control, along with other small, emerged winter annual weeds that can overwinter and have a negative effect on wheat the following spring. This includes chickweed, deadnettle, annual bluegrass, mustards, etc. Herbicide treatments at this time can also have considerable activity on biennials (wild carrot, wild hemlock), dandelion, and Canada thistle, although herbicides are often more effective on these weeds later in the fall. The larger summer annual weeds (ragweeds, marestail larger than rosettes, foxtails, etc.) are going to die after the first hard frost, and soybean harvest decimates these weeds to the point that herbicides won’t be effective on them anyway. Where wheat is planted into a fallow situation, it may be necessary to target the large summer annuals with herbicide in order to ensure that they do not interfere with planting or wheat stand establishment.
Herbicide options for burndown of existing weeds prior to planting of no-till wheat include glyphosate, Gramoxone, Sharpen, and dicamba. Among these, the combination of glyphosate and Sharpen probably provides the best combination of efficacy on marestail, flexibility in application timing and residual control. Dicamba labels have the following restriction on preplant applications – “allow 10 days between application and planting for each 0.25 lb. ai/A used”. A rate of 0.5 lb. ai/A would therefore need to be applied at least 20 days before planting. We do not know of any 2,4-D product labels that support the use of 2,4-D prior to or at the time wheat planting. There is some risk of stand reduction and injury to wheat from applications of 2,4-D too close to the time of planting. Liberty and other glufosinate products are also not labeled for use as a burndown treatment for wheat. Sharpen should provide limited residual control of winter annuals that emerge after herbicide application, and the rate can be increased from 1 to 2 oz./A to improve the length of residual. Gramoxone should also effectively control small seedlings of marestail and other winter annuals. Be sure to use the appropriate adjuvants with any of these, and increase spray volume to 15 to 20 gpa to ensure adequate coverage with Sharpen or Gramoxone.