From the Canola Council of Canada:
Post harvest weed control can be a valuable step in preparing fields for canola next year, especially if winter annuals and perennials are present in high numbers. Dandelions and thistles, for example, move a lot of energy into their roots this time of year, so fall is an effective time to control these weeds.
Here are some tips to improve the success of fall weed control in fields planned for canola in 2012:
1. Make sure the plants are actively growing with new supple leaf area to target. Weeds cut off at harvest need time to accumulate new leaf tissue. This is essential for herbicide uptake and efficacy.
2. Frost damaged leaves may look green but they’re not healthy and will not take up herbicide. If frost has occurred, avoid application until leaf condition of the target weeds can be evaluated.
3. Spray in the afternoons when temperatures are warmer and dew is off the plant. Ideally, you want to apply when temperatures are above 10 C and rising. Check product labels for specific recommendations as this may vary.
4. Use registered products. Glyphosate is effective and can be economically applied at the higher rates needed for some larger weeds. Bromoxynil, Heat and tribenuron (Express SG) are potential tank mix options with glyphosate if needed to effectively control the spectrum and size (or stage) of weeds present. Check the guide to crop protection for the list of weeds these tank mixes control. Click your province for a link to your guide: Alberta Saskatchewan Manitoba. With tribenuron in particular, the label says to wait 2 months before seeding canola, and these should be non-frozen months. DuPont’s recommendation is to spray before Oct. 15 on fields planned for canola.
5. Are they perennials, winter annuals or annuals? It helps to know whether the weeds present in the fall will last the winter. Canola volunteers, for example, that emerge in the fall are unlikely to last the winter and do not need to be sprayed.