co-written by Horst Bohner
When poor herbicide performance or late seed germination results in unacceptable amounts of weeds in a soybean crop, a pre-harvest herbicide treatment will aid in a quicker, more efficient harvest. Drier plant matter will pass more easily through the combine and will result in lower seed losses and reduced seed staining. Appropriate product selection for the target weed species and application timing are important for the success of any pre-harvest desiccant.
Pre-harvest herbicides do not speed up maturity, nor make soybean seed dry down faster, they only serve to drop remaining leaves and dry out green material. They will kill green tissue such as leaves, stems, or pods on soybeans but will not help speed up maturity or dry down green seed. Crop quality can be increased because the harvest timing after the field has been sprayed is more predictable, and therefore harvest schedules can be better managed.
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.
If annual weeds are the target glyphosate will still be effective, but the speed of activity is slow compared to the other pre-harvest options of Aim (carfentrazone), Eragon (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 is critical. Do not spray too early. If application occurs too early yield losses can occur and chemical residues may 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. The occurrence of yellow 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. Another way to assess if the field is ready is to determine if the seed has separated from the pod. Once the seeds turn yellow, the next visual change is that the seeds start to shrink inside the pod and separates from the white membrane inside the pod. This indicates that the seed is at full maturity. This test is most accurately done by collecting random pods from the top one third of the plant. If all the seed has separated from the pod than the crop has reached physiological maturity and can be sprayed.
The following products can be used for pre-harvest applications in soybeans. Use products based on the label rates and with the recommended water volumes.
IMPORTANT NOTE: If you are harvesting food grade soybeans (e.g. non-GMO) for the export market, there may be restrictions on what pre-harvest herbicides you can use. Contact your buyer to verify what pre-harvest products are acceptable to use.
1. AIM EC at 47 mL/ac + non-ionic surfactant at 0.25% v/v
- The pre-harvest interval for this product is 3 days.
2. REGLONE at 0.68 L/ac + AGRAL 90 at 0.1% v/v
- Apply within 7 days of variety maturity and harvest 5-7 days after application.
3. GLYPHOSATE at 0.67 L/ac (540 g/L)) to 1 L/ac (360 g/L)
- Apply when the crop is 30% grain moisture or less.
- Do not apply to crops being grown for seed, as glyphosate can negatively effect seed germination.
- Apply at least 7 days prior to harvest.
4. ERAGON at 28.5 g/ac + Merge at 0.5% v/v
- Apply when stems are green to brown in colour and pods are mature (yellow-brown) and 80-90% of the original leaves have dropped.
- The pre-harvest interval for this product is 3 days.
Comparative studies in Ontario
There is limited public research comparing performance of pre-harvest treatments on different weed species. Dr. Peter Sikkema has conducted four trials from 2010 to 2011 on edible beans and his results are summarized below. Regardless of treatment used, the expectation should be that the pre-harvest treatment will improve harvest efficiency but it will not result in a complete “dry down” of target weeds.
Table 1. Visual Control of lamb’s-quarters, ragweed, pigweed and foxtail 8 days after application of various dessicant treatments.
|Visual Control (%) 8 days after application|
|glyphosate (1 L/ac)*||32||20||38||62|
|Reglone (0.92 L/ac) + Agral 90 (0.1% v/v)||74||80||78||48|
|glyphosate (1L/ac)* + Aim EC (47 mL/ac) + NIS1 (0.25% v.v)||39||25||50||64|
|glyphosate (1L/ac)* + Eragon (28.5 g/ac) + Merge (0.5% v.v)||49||72||66||65|
|glyphosate (1L/ac)* + Reglone (0.92 L/ac) + Agral 90 (0.1% v.v)||70||77||76||49|
* glyphosate rate per acre is based on a product concentration of 360 g/L (e.g. Roundup Original)
1 NIS = Non ionic surfactant (numerous products/trade names are available)
Source: Dr. P. Sikkema, 4 Trials: DB10D1A, DB10D1B, DB11D1A and DB11D1B. Weed Control Trials Research Report, 2010- & 2011. University of Guelph, Ridgetown Campus.