Pre-harvest products and how to use them

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Canola growers have three primary reasons for a pre-harvest spray application:

1. Weed control

2. Desiccation

3. Manage uneven crop

Use scenarios for pre-harvest spray

Weed control

Pre-harvest can be a good time to manage weeds in and above the canopy, especially when straight combining. The act of swathing and curing can also provide weed control.

If weed control is the goal, assess the weed situation before spraying. Moderate to heavy infestations of annual or grassy weeds should be sprayed out pre-harvest. Good control can be provided pre-harvest for perennial weeds as well. 

Effective control is challenging on big weeds: apply product at the recommended rate with high water volumes to penetrate the crop canopy and ensure thorough coverage.

Waiting for fall weed control may not be the best timing for these weeds. Growers should wait at least 10 days after harvest to apply herbicide. You want the weed to start actively growing again after harvest because herbicide uptake and translocation happen more-efficiently in actively growing weeds.

Note: Set reasonable expectations on preventing seed set. Charles Geddes, weed management scientist with Agriculture and Agri-Food Canada in Lethbridge, Alberta, says: “There is no easy way to tell if weeds will have mature seed, but in my experience, it happens earlier than one might think. I suggest that the latest a weed should be left is until it is flowering. Once it starts to flower, then some of the seed can mature on the plant after it is terminated. From a weed management perspective, some of the contact-type desiccants are hit and miss. They are highly dependent on the environment (humidity, sunlight and temperature), and when you use them on large plants, this can result in incomplete control.”

Desiccation

If desiccation (crop and weed dry-down) is the goal for straight combining, this decision should wait until just before harvest – for two reasons:

  • (1) With enough time, leaving a pod-shatter tolerant variety to dry-down naturally can be a suitable (and lower cost) option to swathing and pre-harvest spraying. Preliminary results from a project assessing pre-harvest aids in canola suggests these products are less necessary under warm and dry conditions in the late summer/early fall.
  • (2) Diquat desiccant can’t be applied early anyway. Timing is 90 per cent brown seed throughout the whole crop.

Manage uneven crop

When crop establishment is uneven, often due to dry conditions at and after seeding, this “staginess” can continue to harvest. Rainfall pre-harvest can also inspire green regrowth, often on canola crops that experienced a generally dry season up to that point. 

Swathing is one option to dry-down green patches in an uneven field. Swathing also cuts off large weeds, allowing them to cure before harvest.

For canola standing for straight combining, leave the crop as long as possible to see if the later plants can catch up. Take advantage of the pod shatter trait and hope the earliest pods hang on. 

Here are tips and risks for each product when used on un-even crop:

  • Diquat (Reglone Ion) stops maturity, so any seeds green when sprayed will stay green. The risk with an uneven crop is that chlorophyll levels will be locked in, which is why Diquat should not be used on crops with extreme variation in maturity.
  • Saflufenacil (Heat LQ). With a slightly earlier application timing and both contact and systemic activity, this product is better suited than diquat to manage some variation in maturity.
  • Glyphosate has the earliest application timing of all the products, but it is critical to wait until seed moisture content is less than 30 per cent in the least-mature areas. By waiting until 50 to 60 per cent seed colour change in the least-mature areas of the field, growers can be confident seed moisture will be at less than 30 per cent. See the Keep It Clean staging guide for glyphosate.

More specifics on each pre-harvest product for canola

Glyphosate

Glyphosate is registered for pre-harvest perennial weed control in canola. The glyphosate label indicates application at less than 30 per cent seed moisture, when the majority of the seeds are yellow to brown in colour. At 50 to 60 per cent seed colour change in the least mature areas of the field, seed moisture will be less than 30 per cent – so you can use that as a guide. Applications made before this stage increase the risk of excessive glyphosate residue in the seed. Canola is very efficient at moving glyphosate or other systemic compounds into the seed prior to physiological maturity. Excessive pesticide residues in the seed can result in export problems.

When using glyphosate in uneven fields, make sure the least mature areas are at 50-60 per cent seed colour change before application.

The rate of drydown with preharvest glyphosate is heavily dependent on environmental conditions, so weed control should still be the main goal. Glyphosate will not hasten crop maturity.

Note, not all glyphosate brands are registered for aerial application. Many aerial applicators are reluctant to apply glyphosate by air.

Heat LQ

Heat LQ (Group 14 saflufenacil) fits with any canola herbicide system, including glyphosate-tolerant varieties. Crop is usually ready seven to 21 days after application, but it could be longer in cool, cloudy and rainy conditions.

Heat LQ has a labelled application timing of when “60-75 per cent of seeds have changed colour,” but as of 2019 the intention for this was to be a “seed colour change” rating, meaning 60-75 per cent seed colour change on the main stem. BASF finds greater performance and consistency when applied later, at around 80 per cent SCC. The pre-harvest interval is three days.

When applied pre-harvest in canola, BASF only supports Heat LQ when tank mixed with glyphosate. The benefit over straight glyphosate is more rapid dry down of crop and weeds. Merge is required with the tank mix. The tank mix provides crop dry down in any herbicide-tolerance system as well as broadleaf weed control, including perennials. Use a minimum water volume of 40 L/ac (10 gal/ac). BASF recommends five gal/ac for aerial applications.

A tip for Heat LQ use: Heat is activated by sunlight, so application on a sunny day maximizes its performance.

Reglone and other diquat brands

If desiccation alone is the goal, Reglone (diquat) is a contact herbicide registered in canola to dry immature green material to facilitate harvest. Reglone does not hasten crop maturity. It shuts the plant down quickly and basically STOPS it from maturing, which can lock in high green seed levels if applied prematurely.

Diquat application timing is 90 per cent brown seeds. At this stage, all but 10 per cent of the seeds on the very top and outer most branches haven’t completely turned black or brown. This is very different from a seed colour change (SCC) measurement that only refers to the main stem and includes any seeds with a degree of speckling or mottling.

Syngenta provides the following tips on how to stage the crop:

  • Survey the field. Look for brown colouring in the upper pods and stems. You should not see any yellow or green.
  • Listen for a rattle in your pods. Mature seeds are loose in the pod and rattle when the plant is shaken.
  • Look for brown seeds. Strip out seeds from several areas of the field. The crop is ready when 90 per cent of each plant has seeds that have turned completely brown.

Applying Reglone earlier may result in higher green seed. Efficacy will be maximized with the highest water volumes feasible (minimum 91 up to 222 L per acre ground (24-58 US gallons) or 18 L (4.75 US gallons) per acre aerial).

Combine as soon as the crop dries down. Reglone can significantly increase both pod shatter and pod drop if harvesting is delayed, so be prepared to combine as soon as green seed and seed moisture have reached suitable levels. The Reglone label says to combine no later than 14 days after application. Syngenta only supports Reglone sprayed on labelled shatter tolerant varieties, largely for this reason.

A tip for Reglone use: Reglone is activated immediately in day light, so application in the evening will allow the herbicide to spread slightly from the droplet contact point before activation the next morning and result in more complete uptake by plant tissues.

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