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Fall Weed Control Considerations (09/23/21)

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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 calendar approaches October.

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 statewide dry conditions last fall. However, many across the state, particularly the eastern half of North Dakota, have received adequate rainfall since August 1 to stimulate germination of winter annual weeds. Those of greatest concern are horseweed (marestail), narrowleaf hawksbeard, and brome grasses. 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 our broadleaf weeds. Be careful to pay attention to crop rotations if dicamba is utilized. For winter annual grasses like the bromes, glyphosate is usually sufficient. In general, an application of glyphosate + 2,4-D over the next month will control most winter annual weeds and set us up for success next spring. For more detail on different herbicides and weeds, please see page 7 of the 2021 Weed Control Guide for a table on weed control efficacy with fall applications.

Perennial Weed Control

Fall is also one of the best times to control our perennial weeds. Canada thistle, dandelion, and foxtail barley are the perennial weeds of most concern in rows crops. The questions we receive most often for control of perennial weeds is “when does our spray window close in the fall”. Like everything else in agronomy, there is no black and white answer. However, there are a few key points to consider:

  1. In general, herbicide applications will work best as long as daytime highs are in the 50’s or above, and we do not receive a frost or freeze at night.
  2. Control of Canada thistle and dandelion can often be better after we receive our first frost. The first frost is often a signal for plants to send more carbohydrates to the roots to survive the winter, and our systemic herbicides will follow this translocation pathway.
  3. It is best to avoid a herbicide application if there is a frost or freeze in the morning. Systemic herbicides like glyphosate or 2,4-D work best when plants are actively growing. A frost or freeze in the morning will slow down growth of these plants on a given day. So if warm weather is set to follow a frost event, it is best to wait for a better day.
  4. Once a hard freeze occurs (28 degrees F or below), plants will need to be evaluated. If above-ground plant tissue is no longer green, then herbicides will not be absorbed into the plants. However, if plants remain green, and if there is a series of warm days following a hard freeze, applications may still continue as the plants will still be growing and susceptible to herbicide applications.

Fall applications for Control of Kochia Next Spring

Dr. Brian Jenks at the North Central REC has conducted a series of trials over the years for fall applications of herbicides to control kochia the following spring. In no-till situations, he has observed 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) that we use for kochia control. This helps explain why we see good residual control where we solely have snowfall and early season rain as the precipitation for activation. The best case scenario is near complete kochia control into mid spring, though we at least consistently see a reduction of kochia pressure (i.e. fewer plants to form a kochia ‘mat’) that allows for better coverage of spring applied herbicides. 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) + Valor.

Herbicide Carryover Concerns for 2022

The dry conditions throughout the summer have led to many concerns about herbicide carryover to certain crops next year. It will be beneficial to most everyone this winter to take a look at the herbicides applied this summer, and the rainfall requirements to allow for rotation to certain crops next year. The products of most concern are anything containing clopyralid, or many of our Group 27 herbicides that we use in corn. One note about clopyralid is that we get several questions each winter about a soil test that reports a certain PPM or PPB of clopyralid in a soil sample. As far as I’m aware, nobody has any clue what clopyralid reported in PPM or PPB means biologically for crops planted into that field next 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 the field of concern (i.e. 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.

 

Joe Ikley

Extension Weed Specialist