Maximum Residue Limits (MRLs)

It is important to note that certain products, even if registered, may have market access concerns. To ensure the safety of Canadian food, maximum residue limits (MRLs) set the maximum allowable amount of a pesticide residue on a crop or in a processed crop product (e.g. oil or flour). Residue levels are typically assessed for pesticides registered on crops grown for food. MRLs even exist on imported food for pesticides or pesticide uses not registered in Canada.

Health Canada’s Pest Management Regulatory Agency (PMRA) is responsible for setting MRLs in Canada. Similarly, importing countries set their own MRLs (also referred to as ‘import tolerances’) that Canadian crop exports are subject to. Trade issues between importing and exporting countries can arise due to variability in MRLs or a lack of established MRLs. Crop pesticide uses that may contribute to trade irritations have been flagged on product pages in the Saskatchewan Ministry of Agriculture’s Guide to Crop Protection. Manitoba Agriculture and Saskatchewan Ministry of Agriculture have included such statements in their guides on products uses with known or potential MRL issues.

As of January 1, 2017, products of concern for mustard growers include those containing quinclorac (Clever; Masterline Quinclorac). More information on MRLs and ‘flagged’ products is available at keepingitclean.ca.

Producers can follow these practices to help prevent exceeding MRLs:

  • Read and follow product labels, especially with respect to registered crops, maximum application rates, maximum number of applications per season, crop stage and pre-harvest intervals.
  • Talk to your commodity buyer before applying a pesticide, especially for new pesticide chemistries, new products and products registered on new crops.
Soil active, in-crop herbicides

All soil active herbicides used in mustard must be applied to the soil prior to the seeding of the crop. All but one of these herbicides are very polar (the molecules of the herbicide chemical have strong positive/negative electrical charges) and will bind strongly to organic matter and clay. As a result, the use rate of those polar herbicides will vary depending on the organic matter and clay content of the soil it is used on. The other is less polar and its efficacy is affected by organic matter. This also means that heavy crop residue cover on fields may result in the very polar herbicides becoming bound to that residue, resulting in reduced activity. All but one of these herbicides are also very volatile, which means that some type of incorporation is required to prevent those herbicides from gassing off into the atmosphere.

There are four herbicide options registered for brown, oriental and yellow mustards (Avadex, Fortress, trifluralin and Authority) that can be applied to the soil prior to seeding, either in the fall just prior to freeze-up, or in spring once the field is dry enough to access. Avadex (triallate) is a Group 8 herbicide that is used only for wild oat control. Trifluralin (Treflan, Bonanza, Rival) products are Group 3 herbicides that can be used to control a variety of broadleaf and grassy weeds. Fortress contains the active ingredient from Avadex along with a reduced rate of trifluralin, and can control several grassy weeds and be used for suppression of many broadleaf weeds. Authority (sulfentrazone) is a Group 14 herbicide that may be applied to the soil surface without incorporation, prior to seeding or up to three days after seeding for the control of kochia only.  Edge (ethalfluralin) is a pre-emergent Group 3 herbicide registered to control and suppress many broadleaf and grassy weeds in yellow mustard only.

Granular Avadex and Fortress formulations have an option for application in late fall, prior to freeze up, only without the need for immediate incorporation, whereas Edge and trifluralin require immediate incorporation to prevent losses from gassing off (volatilization) or exposure to sunlight (photodegradation). The application and incorporation instructions of the label should be read and followed carefully.

Authority relies on rainfall to incorporate it into the germination zone of the emerging kochia. Should there be too little rainfall after application then performance may suffer, but if too much rainfall occurs in a short time span, injury may occur to the mustard crop.

Post-emergent (foliar), in-crop herbicides

Post-emergent weed control in mustard is largely comprised of Group 1 herbicides for the control of grassy weeds, with the exception of Muster Toss-N-Go. Muster Toss-N-Go is a Group 2 herbicide, registered for use on brown and oriental mustards only.

Group 1 herbicides with the active ingredients clethodim (Select, Centurion, Arrow, Shadow and Patron), sethoxydim (Poast Ultra) and quizalofop (Assure II for all mustards and Yuma GL for oriental mustard only) are registered to control grassy weeds in all three types of mustard. As with most other broadleaf plants, mustards are tolerant to Group 1 herbicides throughout the entire crop’s life cycle. However, pre-harvest intervals are set on product labels for food residue purposes and weed control benefits are typically greatest when weeds and crops are small. Adequate weed control may still be achieved when grassy weeds are sprayed prior to the six-leaf stage with clethodim and sethoxydim and prior to tillering with quizalofop, but the yield benefit is greatest when the crop is less than four leaves (prior to growth stage 14).

Muster Toss-N-Go is a Group 2 herbicide that is registered for broadleaf weed control in brown and oriental mustards only. Yellow mustard tolerance to Muster Toss-N-Go is poor due to its close relationship with wild mustard. Brown and oriental mustards are tolerant to Muster Toss-N-Go only from the four-leaf stage up until the start of flower bud formation (growth stages 14 to 50). This herbicide targets broadleaf weeds.
In both brown and oriental mustards, Muster Toss-N-Go can be tank mixed with Assure II to provide control of both grassy and broadleaf weeds in one pass.

Pre-harvest herbicides

Glyphosate (Roundup WeatherMax) may be applied prior to harvesting mustard to control perennial weeds for the following season, but is not intended for rapid crop dry down. Glyphosate may provide harvest management benefits by killing off any green annual weed material present in a mature mustard crop. The perennial weeds controlled include quackgrass, Canada thistle, toadflax and dandelion. Roundup WeatherMax is registered for use prior to mustard harvest when grain moisture is less than 30 per cent and crops are largely ripe. For effective perennial weed control, five to seven days must pass to allow the herbicide to translocate to the root systems of the weeds prior to harvest. Premature applications may result in glyphosate residues in harvested seed exceeding the MRL allowed which can cause problems with marketability of the harvested grain. To ensure greatest weed control and herbicide residue levels below regulated MRLs, it is important that application timing and pre-harvest intervals stated on the product label be followed.

Talk to your grain buyer before using a newly registered product or an existing product with new uses to ensure that MRLs will not be an issue.

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