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Post-harvest weed control

By Shannon Chant, PAg, Crops Extension Specialist

September 2018

As harvest progresses, it is a good time to determine if any fall weed control is needed. Both winter annuals and perennial weeds can be effectively controlled with post-harvest herbicide applications. Winter annuals, such as stinkweed, Canada thistle and flixweed, can be effectively controlled late in the fall months. Perennial weeds, such as Canada thistle, perennial sow thistle and dandelion can also be controlled after harvest; however, these weeds require a longer period of good growing conditions to absorb and translocate herbicides to the plant roots. As a result, herbicides applied for effective perennial weed control need to be applied earlier in the fall compared to products applied for winter annuals.

Either way, post-harvest weed control is an opportunity to control seedlings that have emerged later in the growing season before they are able to establish, overwinter and become more difficult to control the following year. Whether choosing to spray because of newly-emerged seedlings or to control more established patches of perennial or winter annuals it is important that the plants are actively growing to ensure the best results.

Before spraying it is important to look for new green growth or regrowth over a few days to indicate that the plants are still actively growing and able to take up herbicides. Yellow or sickly plants will not likely take up as much of the herbicide being sprayed, resulting in reduced weed control. Often frosts can occur in the fall that may damage or kill plants. Following a frost, weeds may still look green the day after but it can take up to four days for plants to show any signs of frost damage. If choosing to spray following a significant frost, make sure that new green regrowth is present before using a herbicide that requires plant uptake to work. Glyphosate is a good example of a systemic herbicide that needs good growing conditions (i.e. warm and sunny days) to promote the uptake and movement of the herbicide throughout the plant. If a killing frost does occur and no regrowth is present, then an early-spring burn-off option may be a better strategy for control of target perennial and winter annual weeds.

Pre-harvest herbicide application is becoming a popular tool for controlling perennial weeds in-crop. However, warmer and longer harvest seasons can allow for volunteers, perennials, winter annuals and other nuisance weeds to emerge in fields that were sprayed with glyphosate prior to harvest. Late season fall moisture during harvest can also encourage additional flushes and/or regrowth of weeds within fields. So, it is important to scout all fields to determine if they require post-harvest weed control even if they were sprayed with a pre-harvest product previously.

With the development of herbicide resistant weeds, including Group 2 resistant stinkweed, it is critical that herbicide groups are rotated when spraying. Sometimes multiple applications of the same herbicide need to be used on the same field within the same growing season for weed control. However, if possible, using registered herbicides containing multiple groups or registered tank mixes composed of different herbicide groups will help reduce the risk of developing unwanted resistant weeds in fields.

When controlling weeds in the fall, it is always important to consider herbicide residues as some products may have recropping restrictions.

For more information, please contact your local agronomist, the Crops Extension Specialist in your area or the Agriculture Knowledge Centre at 1- 866-457-2377.

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