Fall Weed Management

By Marley Voll, Richland County extension agent.

As harvest wraps up it is time to consider your fall weed management plan. There are many factors to consider including what your next crop will be, what weeds you are targeting, and what your crop rotation is.

During the fall, winter annual weeds like cheatgrass, tansy mustard, and marestail germinate and begin growing. A study out of NDSU found that early rosette is when marestail control is most effective. The rosette stage can often be found in fields in late September and early October depending on moisture and temperature. Herbicide options include Xtendimax + Valor, Sharpen, Sharpen + Valor, and Gramoxone + Valor. Glyphosate and 2,4-D can control many small winter annual weeds and Valor can be tank mixed to provide residual control. For kochia control through early spring, fall applications of soil-residuals herbicides including Valor, Spartan, Fierce, or Sonalan and Prowl can reduce kochia by killing seedlings as they emerge in the spring. Apply these herbicides after Oct. 15, and for better efficacy, application can be timed with precipitation, which allows the herbicide to be incorporated into upper 1 inch of the soil especially in no-till. Also, in the fall, mowing down some of the kochia may be an option. Mowing reduces the chance that kochia will blow across the field during Chinook season. Tumbling kochia puts seeds back in the field you have been trying to keep clean.

In pulse crops we have been reliant on herbicides like Sharpen, Spartan, and Valor but recently in North Dakota, kochia resistant to those modes of action were identified. Because herbicide options are limited in pulse crops, minimizing the spread of these resistant weed populations should be a priority. Be on the lookout for populations and contact your extension agent if you suspect resistance. Carefully managing them can reduce the spread of herbicide resistant weeds.

Two other very important weeds to scout for are water hemp and Palmer amaranth. These large pigweeds have been found a couple of times in Montana and continue to spread toward us. Palmer amaranth has been found now in its second county in Montana – Daniels County. Fortunately, the plants that were found were in flower and had not yet produced seed. These pigweed species have characteristics that make them more problematic than other pigweeds including rapid growth, prolific seed production with hundreds of seeds per plant, and resistance to multiple herbicide modes of action. Additionally, these pigweeds cause extreme economic impacts and major yield losses. Main vectors of contamination are seed, especially millet, and farm equipment. Continue to be on the lookout for Palmer amaranth and water hemp and contact the Richland County Extension Office or your agronomist if you suspect you’ve found seeds or plants.