Pre-Harvest Herbicide use in Soybeans

August 2, 2021

Pre-harvest herbicide use can contribute to a better harvest. It can also help soybeans achieve food grade when the fields have a certain weed density that could potentially stain the soybeans and in turn affect its grading for human consumption.

The effectiveness of this solution is determined by: (1) the choice of the right herbicide for the targeted type of weeds, (2) when the product is applied.

Pre-harvest herbicide use in soybeans

Pre-harvest herbicide use in soybeans will accelerate the time until harvest by 3 to 10 days as the products will kill the green tissues of the soybean plants (leaves, stems and pods) and the weeds without desiccating the beans.

This technique generally improves the harvest quality as it: (1) makes the harvest time after spraying more predictable, (2) dries out the weeds, which facilitates the work of the combine harvester and the possible cleanup of perennial weeds in the fields.

Pre-harvest herbicides to use on soybeans

Glyphosate is the herbicide of choice when targeting perennial weeds. Applying glyphosate in the fall helps the active ingredient translocate to the roots. This makes it possible to fight certain weeds more easily, including Canada thistle, Perennial sow-thistle and field bindweed, to name only a few.

Glyphosate is also recommended for targeting annual weeds. However, since it is slower-acting that other products, it is suggested to use one of the following fast-action herbicides instead:

  1. Carfentrazone (Aim EC)
  2. Saflufenacil (Eragon)
  3. Diquat (Reglone)

Tip: don’t mix glyphosate with a fast-action herbicide if targeting perennial weeds. 

When to apply pre-harvest herbicide for soybean crops

Once the soybean seed has detached from the pod, the bean is considered mature and the field can be sprayed. To check their maturity, randomly pick some pods from the upper level of the soybean plant and split them open. If the skin separates from the seed, they’re ready!

Here are four other ways to determine if it’s time to apply a pre-harvest herbicide:

  • 80-90% of leaves have fallen 
  • Green to brown stems
  • Pale yellow to brown pods
  • Bean moisture level of 30% or less

Table 1: Pre-harvest burn-down program

Glyphosate (540 g/L) Reglone 240 Diquat 240 Eragon LQ Eragon LQ + Glyphosate AIM EC 
Dose 0.67 L/acre 0.5 to 0.7 L/acre 30 to 60 ml/acre 30 to 60 ml/acre +
0.67 L/acre 
47 ml/acre 
Volume of water 10 to 12 gal/acre 20 to 55 gal/acre 20 gal/acre 10 to 20 gal/acre 20 gal/acre 
Adjuvant None (if the correct water volume is used)Agral
1 L/1000 L of water 
Merge 400 ml/acre Merge 400 ml/acre Non-ionic surfactant at 0.25% v/v 
Effect on weeds Slow-acting
10 to 15 days 
Fast-acting 4 to 7 days Intermediate-acting 7 to 10 days Intermediate-acting 7 to 10 days Intermediate-acting 7 to 10 days 
Required wait time before harvest 7 days None 3 days 7 days 3 days 
Application stage 80-90% defoliated brownish pods 90% defoliated
80% yellow pods 
90% yellow pods 90% yellow pods 90% yellow pods 
Use on seed production No Yes Yes No Yes 

Note: The grower must always use products approved for pre-harvest burn-down.

Questions to ask to choose the right herbicide

Here are some examples of questions to help you choose the right herbicide for pre-harvest use on soybeans:

  • What type of weeds are being targeted: perennials or annuals (broadleaf or grassy)?
  • Is it for commercial production or not? There may be restrictions on the use of pre-harvest herbicides for food-grade soybeans or seed production. 
  • What is the desired time period before harvest (determined based on the grower’s situation)?
  • Is there the presence of horsetail, nightshade or other tough weeds?

This concludes our overview. With that, we wish you a great harvest!