Fall is a good time to control perennial and winter annual weeds, but spraying immediately after harvest may not provide the best results.
Perennial weeds cut off at harvest need time to accumulate new leaf tissue to absorb herbicides. Four weeks is a minimum recommendation and six weeks is ideal. Even when waiting this long, leaf surface area is still just a fraction of what it was prior to harvest. Therefore fall glyphosate rates may need to increase by 2 to 3 times over pre-harvest rates to get the same concentration of glyphosate into the plant root.
Before spraying, identify the weeds present. Perennials such as thistles and dandelions are best controlled from mid September to early October. As noted, waiting at least a month after cutting will increase the target leaf area, but later dates increase the risk of losing healthy leaf tissue to frost. Without healthy leaf tissue, the herbicide can’t get translocation to the weed’s crown and storage roots where the killing can occur.
If weeds are green and the leaf tissue is still relatively pliable after a frost, growers may still have an opportunity to control perennial weeds with glyphosate or another systemic herbicide. Control can still be obtained on warm sunny days shortly after a frost if no more than 40% of the original leaf tissue is damaged. If most of the weeds are dead, herbicide uptake will be minimal and waiting is recommended.
Warmer temperatures and bright sunshine improve herbicide activity. Apply glyphosate and other systemic herbicides during the heat of day when perennial weeds are actively growing and putting energy into their roots.
October until freeze up is a good time to control winter annuals such as narrow-leaved hawk’s-beard, stork’s-bill, annual sow thistle (common and spiny) and cleavers. That way you get all that have emerged. But check weed staging. Many of the post-harvest product labels have weed staging listed, and winter annuals can hit those stages before October. Apply herbicide when the majority of winter annuals have emerged and are at the right stage for control.
A new consideration: Weeds and clubroot. It can take only three weeks for weeds to form galls, and once galls form, spores form. Letting volunteer canola and other clubroot-hosting weeds (stinkweed, shepherd’s purse, flixweed and mustard) grow for more than three weeks after harvest could increase the clubroot spore load in a field. This goes for all fields — not just this year’s canola fields.
Controlling volunteers and susceptible host weeds must be done to ensure a clean break from canola in non-canola years. Allowing hosts to survive, form galls and produce more spores reduces the effectiveness of a crop rotation to allow spore load reduction in the soil.
Herbicides for fall application on fields planned for canola
|wdt_ID||Herbicides registered for use in the fall on land planned for canola.||Notes and recommendations|
|1||Glyphosate||Apply at least 1,040 GAE (grams of glyphosate acid) per acre to get effective perennial control, given that leaf area will be smaller than pre-harvest. Monsanto does not recommend any tank-mixes with Roundup for use in fall.|
|16||Distinct||Distinct combines Group 4 dicamba and Group 19 diflufenzapyr. BASF recommends it as a tank mix partner with glyphosate for enhanced control of broadleaf weeds. Apply prior to October 1 at 58 g/acre when planning a canola crop the following year.|
|3||Tribenuron (Express SG, Spike, Nuance, MPowerX)||DuPont recommends an application of its Express SG before October 1 on fields planned for canola. Express SG must be tank mixed with at least 0.5 litre/acre glyphosate equivalent for fall application prior to seeding canola in spring.|
|4||Heat LQ||Heat LQ (Group 14 saflufenacil) can be applied in fall to prep land for canola the following spring. Applications can be made any time during the fall on actively growing weeds.|
|19||2,4-D||Use caution. Phenoxies (2,4-D, MCPA) should not be used in the fall on fields planned for canola next spring. The most effective timing is just prior to freeze-up, which does not allow enough breakdown until after the soils warm up in spring.|
|6||MCPA||MCPA has no recropping restrictions listed, but it does present a re-cropping risk for canola – especially when conditions at application are extremely dry. Growers are advised to avoid using MCPA on land intended for canola next spring.|
|7||KoAct||KoAct has both 5 oz of 2,4-D and 4 g of tribenuron. NuFarm is comfortable with an October 15 cutoff with this product.|
|8||Lontrel||You need higher rates and generally get poorer control in the fall than the early in-crop stage.|
|9||Soil-active herbicides approved for fall application on fields planned for canola||These products are applied in the fall but are designed to control weeds emerging early in the spring. The act of incorporation will provide some management of existing fall weeds. Notes and restrictions:|
|10||Triallate (Avadex)||It controls wild oats and wild millet as they emergence through the herbicide layer in the spring. Fall activity is not really needed or desired for these annual weeds anyway.|