Canola growers have three primary reasons for a pre-harvest spray application:
- Weed control
- Desiccation
- Manage uneven crop
Use scenarios for pre-harvest spray
Weed control
Pre-harvest can be a good time to manage weeds, especially when straight combining. The act of swathing and curing can also provide weed control.
Effective control is challenging on big weeds: apply product at the recommended rate with high water volumes to penetrate the crop canopy and achieve thorough coverage.
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.” Critical period for weed seed 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) Application timing for diquat desiccant is 90 per cent brown seed throughout the whole crop.
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