By Mark Loux, Weed Scientist

This summer’s weather caused problems with weed control in some areas of the state, and this certainly includes our two major weeds — giant ragweed and marestail. As we move through harvest and into the season of wheat planting and fall herbicide application, be sure that strategies effectively address marestail since there is an abundance of marestail seed blowing around. 

The larger plants evident now in wheat stubble or above the soybean canopy may be producing seed, but these are not the plants that will overwinter and cause problems next spring. 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 pre-plant applications — “allow 10 days between application and planting for each 0.25 pounds of active ingredient per acre (ai/A) used.”  A rate of 0.5 pound 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 ounces per acre 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 15-20 gallons per acre to ensure adequate coverage with Sharpen or Gramoxone.

There are several effective post-emergence herbicide treatments for wheat that can be applied in November to control these weeds, in fields where pre-plant burndown treatments are not used. The most effective post-emergence treatments include Huskie or mixtures of dicamba with Peak, tribenuron (Express, etc.), or a tribenuron/thifensulfuron premix (Harmony Xtra, etc.). We discourage application of 2,4-D to emerged wheat in the fall due to the risk of injury and yield reduction.