Your overview of pre- and post-harvest herbicide applications

Q & A with an expert

Fall herbicide applications can be an important tool to kill weeds like Canada thistle.

Q: What can I do to control perennial weeds on my farm? 

A: Late or end-of-season herbicide applications present growers with an opportunity to gain long-term control over tough-to-kill perennial weed populations. 

The main goal of this application should be weed control and not hastening crop maturity. In fields with weed populations composed of quackgrass, Canada thistle, dandelion, perennial sow thistle and foxtail barley, fall herbicide applications can be a very important tool. Many of these weed species can be suppressed with in-crop applications to minimize their effects on yield, but the best long-term control will occur when herbicide applications coincide with the plants’ shift to building root reserves before winter. 

Over the years, there has been a shift in the timing of fall herbicide applications from post-harvest to pre-harvest, especially since high- clearance sprayers have become more common. Glyphosate, while not being a crop desiccant, is a very common pre-harvest herbicide to apply as it provides strong, broad- spectrum control of many perennial weeds. There are also desiccants available that work specifically to dry down crop and weed material for an earlier and easier harvest. 

Tank mixing with every application is important, and this timing is no different. By applying tank mixes with multiple, effective modes of action, we can reduce and delay potential resistance issues in our fields. 

When looking into pre-harvest applications, crop staging is of utmost importance. Guidelines are set by government agencies to prevent maximum residue limits (MRLs) from being exceeded. If these limits are exceeded, there can be issues selling and marketing the crop. Correct staging at pre-harvest timing is vital, and if help is needed to correctly identify proper timing, seek out the help of local agrologists. 

Post-harvest timing of weed control provides an opportunity to target perennial weeds after harvest has taken place without potential MRL issues. However, there can be some limitations as weeds need to be actively growing to take up herbicide, and there are products with potential soil residual issues that could affect next year’s seeding intentions. 

Jordan Peterson, PAg, CCA, is a manager of agronomic services for Nutrien Ag Solutions in northern Alberta. 

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