Controlling weeds is a big part and an important part of soybean production, as they can cause significant yield losses.
An effective integrated weed management (IWM) strategy includes field scouting, crop rotation, cover crops and weed control.
When is the best time to remove weeds for optimal soybean yields?
As with many things, timing is everything, and that includes early weed control. You’ll need to consider this crucial factor when planning your IWM strategy for soybeans.
In a study of 13 fields in Ontario and Nebraska, it was successfully shown that crops should remain weedfree before the V3 (third node) growth stage to maintain maximum soybean yield potential.
The study also showed that the period during which soybeans should be absolutely free of weed competition corresponds to the VC to V3 growth stages, respectively, because weeds that emerge after the V3 growth stage can affect soybean yield, but to a much lesser extent.
However, keep in mind that weeds that emerge after the V3 growth stage can potentially increase the weed seed bank in the soil and interfere with harvest operations even if soybean yield is only slightly affected.
An overview of the most destructive soybean weeds
Not all weeds affect soybean yields in the same way. The table below shows soybean crop yield losses due to weeds in two different known population densities.
| Yield losses |
| Weeds | 1 plant/m2 | 5 plants/m2 |
| Annual broadleaves | ||
| Cocklebur | 15% | 41% |
| Eastern black nightshade | 14% | 40% |
| Giant ragweed | 14% | 40% |
| Lamb’s quarters | 13% | 38% |
| Pigweed | 12% | 36% |
| Ragweed | 10% | 33% |
| Velvetleaf | 6% | 23% |
| Wild mustard | 5% | 20% |
| Lady’s-thumb | 4% | 15% |
| Wild buckwheat | 4% | 15% |
| Annual grasses | ||
| Volunteer corn | 4% | 15% |
| Giant foxtail | 3% | 12% |
| Proso millet | 3% | 12% |
| Barnyard grass | 3% | 12% |
| Fall panicum | 2% | 10% |
| Green foxtail | 2% | 8% |
| Yellow foxtail | 1% | 5% |
| Old witchgrass | 1% | 4% |
| Crabgrass | 1% | 4% |
Reference: Agronomy Guide for Field Crops by OMAFRA
Herbicides vs. mechanical weed control
You will need to use effective methods during the critical period to keep a handle on weeds and minimize yield losses. They may involve using herbicides and/or mechanical weed control.
The timing of these operations during the critical period differs from year to year and region to region due to variations in climate, soil type, weed species and population density.
Using herbicides pre- or post-harvest to control perennial weeds is one of the most effective methods.
Mechanical weeding provides some control of young annual weeds when done with a set of light harrows that work the soil shallowly before crops emerge (5 days after seeding) like a comb. You can also do inter-row cultivation with a light to medium-heavy weeder if the soil is dry and can support the equipment without leaving tire marks.
Weeds and soybeans: a bad match
Weeds can cause significant soybean yield losses, so farmers need the right strategy to control them, whether through weed control or mechanical methods, or a combination of both.
Good luck!