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Pre-harvest Herbicide and Desiccant Application

By Mitchell Japp, MSc, PAg, Provincial Specialist, Cereal Crops; Matthew Bernard, MSc, PAg, Provincial Specialist, Oilseeds; Dale Risula, MSc, PAg, Provincial Specialist, Special Crops

Pre-harvest applications of glyphosate and other products like saflufenacil (Heat) are for perennial weed control. Glyphosate is not a desiccant; any crop dry-down should be considered a secondary "bonus" to the primary reason for pre-harvest glyphosate application, which is to control perennial weeds. Note that neither a desiccant such as diquat (Reglone) nor systemic herbicides will hasten maturity, and can actually lock in green seed. A systemic herbicide requires a plant to be actively growing to be effective; therefore, rate of growth of the plant will influence the rate at which the herbicide takes effect. Plant growth is also influenced by temperature/weather. In contrast, contact herbicides take effect at point of contact and will have a quicker effect while being less influenced by temperature. This is especially important for fields or patches that are very delayed and when harvest is anticipated to happen later in the year during cool temperatures.

Buyers in major markets are closely scrutinizing their grain purchases for residues of pre-harvest products such as glyphosate. Maximum residue limits (MRLs) are established to ensure agricultural products are safe for consumption. MRLs may differ between countries, and the product must meet the specifications of the importing nation. If an MRL has not been established in that country, typically the regulators in that country will default to the MRL that is set by Codex Alimentarius Commission of the FAO/WHO Food Standards Program. When no MRL has been set for a product, there can be discrepancies or inconsistencies between the buyer and seller.

Grain shipments that exceed the MRL could be refused, incurring significant costs throughout the value chain. Additionally, an MRL violation could be detrimental to the Canada brand. Pesticide residues can be found deposited on the exterior of the seed, or inside the seed if the plant has taken it up (translocated) while growing. Different chemistries have different mechanisms of affecting the plant, which is why pre-harvest application timing is important. The timing for pre-harvest glyphosate is at 30 per cent moisture content or less for all heads in cereal crops. The staging is the point after the seed has dehisced from the plant, so there will be no translocation of the herbicide into the seed. A small amount may be directly deposited on the hull, but this would not be enough to exceed MRLs and, generally, the seed is separated from the hull before consumption.

In canola, pre-harvest glyphosate application timing is when the majority of seeds are yellow to brown (at least 30 per cent seed colour change on the least-mature plants, corresponding with less than 30 per cent seed moisture). Saflufenacil (Heat) timing is at 60 to 75 per cent seed colour change, and the pre-harvest interval is three days. Ensure you understand the label, as water volumes and/or adjuvants are important depending on if it is used alone or tank mixed with glyphosate. For diquat, the recommended application is at 90 per cent of the brown seed (after recommended swath timing) on the entire plant, not just the main stem. In years such as this with high variability in maturity, diquat can have the highest impact on yield reduction if applied too early; glyphosate has the earliest timing but slower effect, and saflufenacil has both systemic and contact activity.

Flax must reach physiological maturity prior to pre-harvest herbicide or desiccant application, at 75 to 80 per cent brown bolls. Harvest should begin as soon as samples test at 10 per cent moisture. Glyphosate will be a good option to control weeds and minimize green material from going through the combine, especially since flax straw fibre is already relatively tough. A desiccant would be a good option in fields or patches where plants have matured and bolls are ready to harvest, but the stem is still green. Note that seed being kept for seeding the following year should not have glyphosate applied, as this can reduce germination rates; a contact desiccant will not influence germination rates. As always, ensure the pre-harvest interval (PHI) is respected, not only to avoid MRL concerns, but to maximize effectiveness of harvest management decisions.

Mustard production is heavily dependent on access to export markets, so appropriate timing of pre-harvest products is especially important to avoid MRL-related issues. Because of this, as well as regions where mustard is typically grown, desiccant use is not common but can be helpful for fields with varying maturity. Diquat timing is at 75 per cent seed colour change, and saflufenacil at 60 to 75 per cent seed colour change.

Chemical desiccation in peas provides the opportunity to have faster dry-down and harvest the crop sooner than if left in swaths in the field. There is less exposure to wet weather and less chance for wind to damage swaths. Staging for application of pre-harvest glyphosate is when the majority (75 to 80 per cent) of the pods are brown.

In lentils, similar benefits to using chemical as a pre-harvest desiccation as peas are defined. Lentils can be ready to combine in four to seven days following applications of pre-harvest desiccants, providing there is warm, sunny conditions following application. Reglone (diquat), Heat (saflufenacil) and Good Harvest (glufosinate ammonia) are registered for use as desiccants in lentils. Glyphosate is a pre-harvest weed control product that may supply some dry-down effects with warm and dry weather. Check with your buyer to know aspects of which chemical aids are recommended for various markets.

The challenge in a year like 2019 is that the crop staging for pre-harvest glyphosate application is not likely to occur at a reasonable time for harvest. This is because those late tillers will contain grain that is too immature to receive a glyphosate application, risking accumulation of glyphosate, since the systemic nature of this herbicide is drawn up into the growing parts of a plant.

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