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Pre-Harvest Herbicide Timing in Soybeans

A pre-harvest herbicide treatment can make harvest more efficient when there are an unacceptable number of weeds in a soybean crop. Both conventional and glyphosate tolerant fields that are weedy can benefit from a pre-harvest herbicide. Drier plant matter will pass more easily through the combine, lowering seed losses and reducing seed staining. Appropriate product selection for the target weed species and application timing are important for the success of any pre-harvest application.
Pre-harvest herbicides do not speed up how fast soybeans mature, nor make soybean seed dry down faster.  But they will speed up harvest timing by 1-3 days because remaining leaves will drop more quickly, and green material will dry up. Green tissue will be killed such as leaves, stems, or pods on soybeans, but green seed does not dry down more quickly. Crop quality often increases because the harvest timing after the field has been sprayed is more predictable, and therefore harvest schedules can be better managed. Most importantly a pre-harvest burndown will dry down weeds, making combining easier and cleaning up the field for winter wheat planting.
What are the target weed species?
If perennial weeds are the target, glyphosate is the preferred product because the shorter days and cooler temperatures that occur in the fall act as a trigger for perennial weeds to reallocate sugars to their roots for over-winter storage. Applying glyphosate in the fall promotes movement of the active ingredient down to the roots, providing more effective control of Canada thistle, perennial sow thistle, field Bindweed and dandelion.
IMPORTANT NOTE: Contact your buyer to verify what pre-harvest products are acceptable to use. Glyphosate is not allowed in some food grade soybean (e.g. non-GMO) contracts.
If annual weeds are the target, glyphosate will still be effective but the speed of activity is slow compared to other pre-harvest options such as Aim (carfentrazone), Eragon LQ (saflufenacil) and Reglone (diquat). Therefore glyphosate is often tank-mixed to provide broad-spectrum activity but with increased speed of activity on broadleaf weeds in particular. It is generally not recommended to mix one of the fast acting pre-harvest options with glyphosate if perennial weeds are the primary target. This is because their ability to quickly burn leaf tissue can sometimes impede translocation of glyphosate within a perennial plant.
Application Timing
Application timing is critical.  Do not spray too early.  If application occurs early yield losses can occur, and chemical residues will be taken into the seed.  Herbicide residues in the harvested seed will result in marketing problems. If large patches of soybeans in the field remain green, it will be necessary to wait for those areas to mature before spraying. It is safe to spray when seed fill is complete. This can be determined by a colour change of the seed from green to yellow. A change in colour of the pods is an excellent indicator that the seed has also changed colour. Since the colour change of the pods is gradual, it may be difficult to determine when the pods have truly “changed” from green to yellow or brown. A change in colour is defined by the absence of any green colour left on the pod. At least 90% of the pods need to have changed colour and no longer contain any green. This stage of the crop typically occurs when there is at least 80% leaf drop, although a field should not be assessed by leaf drop alone. See Figure #1. Pod colour change is the best indicator for when to spray.
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