What is the goal with a pre-harvest application? 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. Perennial weeds could be left alone for a late fall application, which will likely provide better control. 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 a lot of canola crops moving quickly toward maturity, harvest will not be as rushed and crop should have lots of time to dry down naturally for straight combining. Desiccant spray may not be needed. (2) Diquat desiccant can’t be applied early anyway. Timing is after 90% brown seed throughout the whole crop.
Here are the pre-harvest options for canola:
Glyphosate is registered for pre-harvest perennial weed control in canola. Glyphosate is to be applied when the majority of seeds are yellow to brown in colour and seed moisture is less than 30% (which typically occurs 7 to 14 days before harvest). Applications made before the 30% moisture stage (physiological maturity) 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.
Some growers who straight combine canola that is not a glyphosate-tolerant variety will apply glyphosate to even out the crop for easier harvest. When using glyphosate in this fashion, make sure a very minor portion of the field is immature for the reasons indicated above.
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.
Heat LQ (Group 14 saflufenacil) fits with any canola herbicide system, including glyphosate-tolerant varieties. Crop is usually ready 7 to 21 days after application, but it could be longer in cool, cloudy and rainy conditions.
Application timing is 60-75% seed colour change (and BASF recommends 75%). 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 HT system as well as broadleaf weed control, including perennials. Use a minimum water volume of 40 L/ac (10 gal/ac). BASF recommends 5 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.
Apply at 90+% brown seed. This means the entire plant has 90% or more of its seed totally brown in colour. This is unlike a seed colour change rating, which refers to the main stem only and includes anything with any degree of colour change.
Applying Reglone earlier may result in higher green seed. The label indicates that it should be only applied to napus (Argentine) canola to facilitate dry down of lodged canola crops. Efficacy will be maximized with the highest water volumes feasible (minimum 91 up to 222 L per acre ground or 18 L 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.
This Country Guide article includes a summary of recent PAMI research on these products.
Read the PAMI report on its study, “Straight cutting canola in Manitoba: Comparison of pre-harvest aids“