Timely Weed Control and Herbicide Interaction
Pre-plant or Pre-emergent Herbicide Activity
- Authority, Avadex and trifluralin are less affected by in-crop environmental conditions than foliar applied herbicides, but efficacy can be greatly affected by soil organic matter levels.
- Authority can be applied pre-plant or post-plant/pre-emergent in flax;
- Herbicide effectiveness decreases as organic matter approaches 6%.
- Rain shortly after application is required for activation.
- If there is no rain following application, then weed seedlings that have germinated below the treated layer may grow through the layer of treated soil. While there are exceptions to every rule, in general, if soils are dry enough for the herbicide not to activate, they will be too dry for weeds to germinate. Emerged weeds will not be controlled.
- For emerged weeds, use Authority Charge (Authority + Aim). Authority Charge may be tank mixed with glyphosate for larger or perennial weeds.
- Trifluralin is registered for fall application of granular formulations only and trifluralin must be incorporated. Trifluralin may not be applied to the surface without incorporation. Trifluralin breaks down rapidly under anaerobic conditions so may lose some effectiveness under saturated soil conditions, i.e. may see ring around pot holes with weed escapes.
- Avadex can be applied in fall or spring, prior to planting the crop and may be surface applied in the fall prior to seeding flax.
- In zero-till situations remember weed seedlings are emerging from ¼ to ½ inch soil depth. In conventional tillage, weed seedlings can emerge from greater soil depths; up to 8 inches for wild oat, although most do not emerge from deeper than 4 inches.
Sprayer Clean Out to Reduce Crop Damage
- Injury to flax from herbicide residues in the sprayer tank can be a problem resulting from improper clean-out. For example, Group 2 – sulfonylurea herbicides, may deposit in the sprayer plumbing and then are pulled into solution with the use of other herbicides with strongly ‘cleansing’ adjuvants.
- Use the proper nozzle and main line screen sizes recommended by the manufacturers of dry (DG, WDG, WSG) or liquid suspension (L, SC) herbicides. Many sprayers are equipped from the manufacturer with screens that are much smaller than recommended for dry or suspension herbicides.
- Ideally the sprayer should be cleaned out and main screens removed and inspected at the end of each spray day. Do not allow unused herbicide solution to sit in the sprayer overnight.
- A detergent (i.e. ClearOut, All Clear) could be added to the rinse water along with ammonia to break down any greasy deposits and allow the ammonia to clear out sulfonylurea residues.
- For ‘imi’ Group 2 herbicides, water alone is sufficient as a rinse.
- For more information, see http://sprayers101.com/an-easier-way-to-clean-your-sprayer/
- When spraying fields adjacent to flax fields, remember that flax is sensitive to herbicide drift of broadleaf herbicides not registered for use in flax, as well as Roundup (glyphosate) and Liberty drift.
Interaction of Environment on Herbicide Performance – Crop Safety & Weed Efficacy
- Under cool temperatures, herbicide performance and crop safety can be impacted;
- Crops and weeds need to be actively growing to metabolize the herbicide in order to have adequate crop safety and weed control. Plants stop growing below 5 o C and are under stress above 28 o C.
- Group 1 herbicides – graminicides;
- Weed efficacy is reduced when grassy weeds are not growing well (i.e. environmental stress).
- As flax is very tolerant to Group 1 herbicides, application usually results in minor to no crop injury.
- Group 6 herbicides (Bromoxynil and Basagran) – contact herbicides;
- Better efficacy on weeds under warm/sunny conditions.
- If conditions are too cool and weeds too large, weeds can out-grow the herbicide injury and as such only be suppressed.
- If conditions are too warm (> 28 o C), herbicide application can injure flax seedlings and in the worse-case scenario, can kill the main growing point. The plant often regrows from lateral buds resulting in two or more stems.
- Flax growth staging is important as well. Do not apply when flax is < 2 inches in height as this may result in damage to the growing point.
- Group 4 herbicides (MCPA, Curtail M) – growth regulators;
- Addition of MCPA to bromoxynil (Buctril M) provides better control of larger weeds in cooler conditions.
- Although flax has good tolerance to Group 4 herbicides, it is not immune and damage may occur.
- Under warm and humid conditions, flax may rapidly uptake Group 4 herbicides and this may result in plant damage. The damage may resemble ‘simulated drought’ symptoms from restricted water transport to the top of the plant (including bolls and seeds).
- Curtail M, which is a mixture of two Group 4 herbicides, is typically safe on flax, but if used under hot conditions or late (flax > 6 inches), may result in wilting, stunting and flower abortions.
Lontrel (one of the components of Curtail M) is very safe on flax, but has a limited spectrum of weeds controlled.
Post-Application Scouting for Performance and Herbicide Resistant Weeds
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Flax fields should be inspected 2 – 4 weeks after herbicide application for weed control assessment;
- If patches of weeds are growing in the field, determine if it was due to a miss in application (all labelled weeds survived), or herbicide resistance (most of the labelled weeds were controlled by the herbicide but one species survived).
- Some control issues may be due to environmental and/or staging issues as indicated above. However, weed resistance to the herbicide could also be the cause. For more information, please check out these websites;
- http://www.agriculture.gov.sk.ca/Guide_to_Crop_Protection
- http://www.gov.mb.ca/agriculture/crops/guides-and-publications/pubs/crop-protection-guide-herbicide.pdf
- http://www1.agric.gov.ab.ca/$Department/deptdocs.nsf/all/agdex32
- Saskatchewan/Manitoba Field Guide to Crop Protection and Alberta Blue Book on page 41 has more information how to identify weed resistance
