Fall is often one of the best, and most overlooked, times for weed control. Here are a few considerations for different weed control scenarios as the fall continues:

Winter annual weeds. Those who are experienced with no-till production are familiar with the challenges that winter annual weeds present. In general, winter annual weeds were not of great concern in 2021 due to the dry conditions. However, many across North Dakota, particularly the eastern half, have received adequate rainfall since Aug. 1 to stimulate germination of winter annual weeds.

Those of greatest concern are horseweed (marestail), narrowleaf hawksbeard and bromegrass. Horseweed and hawksbeard are particularly difficult to control in the spring, so fall can be the best time for their control. Fall applications of 2,4-D or dicamba are usually sufficient to control broadleaf weeds. Pay attention to crop rotations if dicamba is used. For winter annual grasses like the bromes, glyphosate is usually sufficient. In general, an application of glyphosate plus 2,4-D will control most winter annual weeds and set up success next spring.

Perennial weed control. Fall is also one of the best times to control perennial weeds. Canada thistle, dandelion and foxtail barley are the most concern in rows crops. When determining a window to spray,  consider these points:

Kochia control. Brian Jenks at the North Central Research Extension Center has conducted trials over the years for fall applications of herbicides to control kochia the following spring. In no-till situations, he found fair-to-excellent control of kochia following an October application of Valor (flumioxazin).

Flumioxazin is less water-soluble compared to other herbicides like sulfentrazone (Spartan). This is why good residual control is seen where snowfall and early-season rain cause activation. The best-case scenario is near complete kochia control into midspring. Note that a fall application of Valor will not control emerged winter annual or perennial weeds. If you have a mix of winter annual weeds, and a desire to control kochia next spring, you will need a mix of glyphosate, 2,4-D (or dicamba) and Valor.

Herbicide carryover. The dry conditions throughout the summer have led to many concerns about herbicide carryover to certain crops next year. This winter, look at the herbicides applied this summer, and check the rainfall requirements to allow for rotation to certain crops next year. The products of most concern are anything containing clopyralid, or many of the Group 27 herbicides used in corn.

As for past use of clopyralid, some farmers are concerned about soil tests reporting the chemical in parts per million or billion in a soil sample. The effect of clopyralid biologically on plants is not known for crops planted into a field the following year.

The best thing to do in fields of concern would be to take a soil sample this fall and conduct a bioassay over the winter. A thorough bioassay would take soil from a field treated with the herbicide and compare it to a field with a similar soil type without that residual herbicide.

The desired crop would be planted into flats or pots containing soil from both fields, and the growth of crops from both soil sources would be compared. If no herbicide injury is detected in the field treated with the residual herbicide, it would generally be safe to plant that crop into that field next spring.

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