Fall weeds: Snow and frost and timing

October 2, 2019 – Issue 27

October 9, 2019 - Issue 28

Snow and fall weeds. When freezing temperatures stop fall weed control plans, snow is likely more of a help than a hindrance with respect to overall weed condition. The snow layer is likely to insulate the weed leaf material from the colder conditions that follow it. That could mean you’re back spraying earlier than you would be with frost alone. Spray decisions — when to spray or whether to spray at all — will depend on leaf condition after the snow is gone. Note that it could take a couple of days to properly assess frost damage, as green leaf material can appear quite healthy immediately after a frost. Read more.

Snow is often less damaging to plants than frost is.

Frost and fall weeds. When scouting to determine the value of post harvest weed control, consider…

  • Are there enough weeds to warrant a spray?
  • Are they winter annuals or perennials? Fall is a good time to hit perennials, biennials and winter annuals. On annuals: Spraying annuals in the fall can be worthwhile if it looks like they’ll produce mature seeds before freeze up. If annuals have already set seed, it may be better to save the burnoff for next spring when those seeds emerge. (NOTE: Research out of southern Alberta suggests that kochia regrowing after a mid-August harvest will not produce seed before a killing frost in most years. If using glyphosate to control those annual weeds, you may simply be adding additional selection pressure for the development of glyphosate resistance.)
  • Are weeds actively growing?
  • Is frost damage less than 40% of the leaf tissue?

If these conditions exist, then a fall herbicide treatment may be of benefit. But this is important: You still want to apply glyphosate and Group 2 herbicides on sunny and warm days for best results. Read more.

Best timing depends on the weeds present. Fall is a good time to control perennial and winter annual weeds, but spraying immediately after harvest may not provide the best results. Before spraying, identify the weeds present. Are they perennials? Winter annuals? Annuals? Clubroot hosts? This article has tips for each.

Herbicides registered for use in the fall on land planned for canola. Notes and recommendations
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. Its reasons are:   –If targeting volunteer glyphosate tolerant canola, Mother Nature should do us the favour by freezing it off. –In the fall, generally the target weed population contains mostly perennial and winter annual weeds.  All academics agree that perennials are low risk for glyphosate resistance so there is not much point in adding another mode of action at this stage. –Depending on the year, the amount of herbicide breakdown between now and freeze-up could be limited, which could result in injury to the canola next spring.
Distinct Distinct combines two actives – Group 4 dicamba and Group 19 diflufenzapyr – in a granular pre-mix. From BASF: “We recommend it as a tank mix partner with glyphosate for enhanced control of broadleaf annual, winter annual and perennial weeds. Distinct must be applied prior to October 1 at a rate of 58 g/acre (80 acres per case or 40 acres per jug) when planning a canola crop the following year.”
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.
Heat LQ The LQ formulation of Heat (Group 14 saflufenacil) can be applied in fall to prep land for canola the following spring at any of the registered rates — 59, 43 or 21.5 ml/ac (80, 40, or 30 acres per case). There is no restriction for when it can be fall applied the season prior to a canola crop; applications can be made any time during the fall on actively growing weeds.
2,4-D Use caution. Clark Brenzil, provincial specialist, weed control, with Saskatchewan Ministry of Agriculture, says: “Phenoxies (2,4-D, MCPA)  should not be used in the fall prior to the seeding of canola. The target of phenoxy herbicides applied in the fall is winter annual broadleaf weeds for the most part. In order to be effectively used for these weeds, research from Ken Kirkland at AAFC Scott, showed that applications of phenoxy were most effective when applied just prior to freeze-up or (Thanksgiving weekend onward to freeze-up or snowfall as long as the spray is not freezing in the boom) so that there is no additional germination of winter annual weeds before freeze-up. When applied at this time, no breakdown will occur until after the soils warm up the following spring. For phenoxy herbicides to have adequate time to break down in the fall they need to be applied in September (and have rain), which the research showed reduced their effectiveness for winter annual control significantly.”
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. MCPA persistence in the soil is slightly longer than 2,4-D. See Clark Brenzil’s comment on phenoxies in general in the 2,4-D notes above.
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.
Lontrel You need higher rates and generally get poorer control in the fall than the early in-crop stage. Lontrel provides its best systemic activity on Canada thistle at the 10-15 cm height when thistles are actively growing and before the bud stage. Clopyralid translocation is most effective during this time frame. As you shift from typical June applications to September/October application, the level of translocation becomes more variable. Weather and growing conditions change,  and movement through to the thistle root system is less effective. To a certain degree this can be compensated by a higher rate structure however the cost/value ratio declines versus a post-harvest application of glyphosate.  
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:
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.
Bonanza, Rival, Treflan (trifluralin) Trifluralin provides pre-emergent control of annual grassy and broadleaf weeds. UAP says the following about Bonanza 10G: Apply between September 1 and soil freeze up using fall rates and follow incorporation instructions. It needs to be incorporated into the soil to provide herbicidal action that will control susceptible weeds in early spring. Trifluralin becomes active at soil temperatures typically above 5°C, so apply it in the fall at cooler temperatures before soil freeze up so it doesn’t dissipate before the spring. 
Edge (ethalfluralin) This grassy and broadleaf herbicide is applied and worked in twice in the fall. It controls a long list of annual grassy and broadleaf weeds. Primary weed control happens in the spring. Edge will be active on annuals that germinate late in the fall, but winterkill will usually take care of these weeds anyway. Winter annuals such as stinkweed for example will not be controlled. Dow AgroSciences recommends Edge for use on canola. It provides pre-emergent weed control for the earliest weed removal, uses a different mode of action (Group 3), provides season long, flushing weed control, and controls a number of hard to kill weeds. It can be a base treatment with herbicide tolerant systems such as Liberty Link or Roundup Ready.
Fortress (triallate, trifluralin) This is applied in the fall but stays dormant all winter. It controls wild oats and wild millet as they emerge through the herbicide layer in the spring. Fall activity is not really needed or desired for these annual weeds.
Canola Watch