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Pre-Harvest Reminders for Weed Control or Desiccation

By: Brent Flaten, P.Ag. CCA, Integrated Pest Management Specialist

When thinking of pre-harvest management, it is important to understand the difference between spraying a crop desiccant versus using a pre-harvest glyphosate application.

A crop desiccant or harvest aid is used when a crop or other green material is near maturity, but needs to be dried down quickly for harvesting. Under good conditions, sprayed crops, including most weeds, dry down in a matter of four to 10 days. However, there is no long-term control of perennial weeds since desiccants don’t move into perennial root systems. As a result, perennial weeds will regrow from the root.

A pre-harvest glyphosate application is used for controlling weeds, including perennial weeds such as Canada thistle. The registered application crop stage for pre-harvest glyphosate is the same stage as with desiccants, which is at physiological maturity. Product labels will describe that stage. Weed dry down occurs at a much slower rate with pre-harvest glyphosate than with desiccants or harvest aids. It is important to apply at the right stage, since applying sooner can reduce crop yields and run the risk of exceeding maximum residue limits (MRLs).

MRLs are the legally allowed limits of pesticide residue in harvested grain. MRLs are determined during the registration process and are set well below the amounts of residue that could result in health concerns for humans or animals. Most countries will establish their own MRLs for imported commodities; exporters must meet the importers’ MRLs, which is very important for Canada. Canada is a major exporter, and the failure to meet the MRLs of the importing countries can result in the products being denied. To avoid market disruption, MRLs should be taken into consideration when choosing crop protection products and application timing. 

The following strategies will help to ensure pesticide residues remain below MRLs and that there are no associated market disruptions.

  1. Before applying a pesticide, speak with your grain buyer to ensure that the targeted importing country has an established MRL for that product. Just because a product is registered in Canada does not mean that all foreign countries have MRLs in place.
  2. Apply pesticides according to the timing and rates indicated on the product label. The rate of chemical application and the timing will influence the potential residues that are present in the harvested grain.
  3. Follow pre-harvest intervals (PHI) as indicated on the product label. The PHI indicates the number of days that must pass between pesticide application and cutting of the crop, either by swathing or straight combining. 

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