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Addressing questions about pre-harvest herbicide use

By Clark Brenzil, PAg., Provincial Specialist, Weed Control, Regina

August 2018

In my role as Provincial Specialist in Weed Control, I sometimes get asked about how, when and why herbicides are used prior to harvest. Below is some basic information on herbicide use patterns that I’ve provided to answer these questions.

When used to stop crop growth

As a rule, desiccants are used on indeterminate crops, which will continue to grow after the first seeds are ripe and ready to harvest. Desiccants remove moisture from the small amount of remaining green tissue at the top of the crop plant, as well as green weed material, so that the crop harvest can be completed. This process is better than swathing to get the crop in condition to combine because:

  1. Swathed crops laying on the ground can develop mould that can reduce the nutritional quality and safety of the grain;
  2. Direct combining reduces the risk of the ripest bottom pods shattering and spilling their contents onto the ground, thereby losing yield; and
  3. It results in fewer passes over the field with heavy equipment (sprayers are typically twice as wide as swathers and have a smaller “footprint”), thereby reducing compaction and improving soil health.

Herbicides used for desiccation do not move into the seed, as they are bound quickly to the parts of the plant they touch. They leave little if any residue in/on the seed.

When used to control weeds

Another reason for applying a herbicide prior to harvest is to control perennial weeds, particularly in more northerly areas where the interval between harvest and killing frosts is very short. The only herbicide registered for this type of use is glyphosate because it has a very low health risk profile and it is very effective for controlling perennial weeds when used in this manner. In this case, applications are made when the crop is physiologically mature and no longer actively moving sugars and other nutrients into the grain.

This application requires lower herbicide rates than similar applications following harvest to control the target perennial weed, resulting in lower environmental exposure. In addition, fallow that might otherwise be needed to manage perennial weeds is eliminated, along with the requirement for tillage and the negative environmental impacts of that practice.

The safety of pre-harvest herbicide use

All crop protection product uses, including the use of herbicides prior to harvest, are approved by the Pest Management Regulatory Agency (PMRA) of Health Canada. Pesticide uses are evaluated, often with equal or greater vigour than prescription and non-prescription drugs, for safety to both the user and the consumer of the treated commodity and protection of the environment. In addition, Canada requires that the manufacturer provide some type of value assessment for the use of a particular pesticide; if it provides no value, the use will not be approved.

The review of crop protection products, including herbicides, involves extremely large data packages. Review of this material by many Health Canada experts in toxicology, health and environmental effects can take as long as 18 months for each individual pesticide or use pattern. Residues in food are an important part of this evaluation, and residue trials are conducted to measure the amount of pesticide remaining in the grain after application of the proposed use. This ensures that a particular use pattern does not result in levels of the pesticide in food commodities that exceed the established “Maximum Residue Limit,” or MRL, for the pesticide.

Determining which products and uses are approved

The value assessment is used to weigh the benefit of a particular pesticide use to the risks presented by it, both to human health and the environment. Benefit is a broad term and could apply to, but is not limited to:

  • Improved pest control;
  • Improved yield;
  • Better harvest ability, which leads to less fuel use;
  • Improved grain quality;
  • Reductions in overall pesticide use;
  • Reduced risk of off-target movement/exposure; and
  • Reduced tillage and the negative environmental impacts of that practice.

A crop protection product of low risk and high benefit is likely approved with little concern, whereas a product with little benefit and high risk is not going to be approved. Both of these are relatively easy decisions, whereas the tough decisions are the products that fall in the middle. Luckily the herbicides being used prior to harvest are in the low-risk category and provide a range of benefits, depending on the products.

Sometimes these concepts are difficult to visualize, so I recommend Risk Bites for explanations of some of the sometimes-complex issues surrounding the field of toxicology and risk in a very straightforward manner.

Ensuring herbicides are used correctly

We at the Saskatchewan Ministry of Agriculture, along with the PMRA, are diligent in ensuring that pesticides are used correctly and in responding to concerns that pesticides are being misapplied. We have a Pesticide Investigator that responds to complaints of misapplication by licensed applicators, and the PMRA responds to reports of pesticide misuse by both licensed applicators and farmers. In addition, the Canadian Food Inspection Agency (CFIA) conducts random testing on food items produced in Canada, as well as those imported from other countries, to check for pesticide residues that exceed the MRLs established for Canadian consumers. The Canadian Grain Commission also conducts random tests on grain samples for pesticide residues.

Many foreign countries also test shipments arriving from outside of their borders, including Canada, on a routine basis to look for pesticide residues that exceed their MRLs. Because of this, the grain merchant industry is very sensitive to pesticide residues above allowable limits. They routinely hold back samples of each load of grain delivered to their elevators as a means of tracking back to any point the source of contamination of individual loads of grain, should there be a problem detected by the buyer. With large bulk loads of grain moving both domestically and overseas, the liability for contamination is huge and would bankrupt a farm when caught.

If you have any further questions, please contact the Agriculture Knowledge Centre at 1-866-457-2377.

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