Skip to content

Soybean – Weed Control


One of the biggest challenges for growing pulses is weed control. Unlike cereal and oilseed crops, pulse crops are generally not competitive with weeds and are highly susceptible to yield loss (20 to 40%) as a result of weed competition. 

Challenges include:

  • the development of herbicide resistance in weed populations
  • limited options for herbicides on the market
  • chemistries not registered solely for pulses due to its small global market.

For all of these reasons, it is important to take an integrated approach to weed control which combines: cultural measures, preventative measures, and effective use of herbicides.

Common winter annual weeds include flixweed, downy brome, shepherd’s-purse, stinkweed, narrow-leaved hawk’s-beard, blue burr, dog mustard, ball mustard, common groundsel, yellow whitlow grass and common pepper grass.

Perennial weeds include quack grass, Canada thistle, perennial sow thistle, toadflax and dandelion.

Preventative Measures

Shepherd’s Purse

Knowing your field’s weed history


  • Selecting a field that has weeds that may be controlled culturally or with herbicides registered for use in soybean, is important when planning soybean production. 
  • Avoid fields with known infestations of perennial weeds such as Canada thistle or sow thistle, biennial and/ or Group 2 resistant weeds such as cleavers, kochia, wild mustard and smartweed.

Choose clean fields, free of herbicide residues

  • Soybeans can be damaged easily by herbicides registered for other crops, or soil residues of some herbicides used in previous years.  Care should also be taken to avoid drift of herbicides from other fields onto soybean fields.
  • Growers should also be aware of past herbicide applications on fields planted to soybeans, as residual herbicides such as chlopyralid, metasulfuron, and dicamba (if not a dicamba tolerant variety) may cause injury to soybeans.

Maintain Accurate records of herbicide use

  • Keep accurate up-to-date records monitoring residual herbicides on all fields as even reduced rates of residual herbicides can cause serious injury to the soybean crop the following year.

Cultural Measures

Kochia, Photo Credit: Wikipedia

Crop Rotation

  • Grow a rotational crop that provides good competition to weeds, allows for a wide range of herbicide options, and is easy to control as a volunteer in the following soybean crop.


  • Good sanitation practices, such as cleaning harvest and seeding equipment to avoid spreading weed seeds between fields.



  • Choose a clean, healthy seed.
  • Proper seeding rates/management will help produce a healthy, vigorous, uniform crop, for better competition with weeds and easier herbicide timing.
  • Soybeans require seeding into warm soil temperatures and are typically seeded a bit later in the spring allowing time for control of early emerging weeds prior to planting.


  • Tillage may have a beneficial effect for control of some weeds while having the opposite effect on others.
  • Increased tillage favors stinkweed, wild oats and chickweed. Other weeds – such as bluegrass, clover, groundsel, and smartweed – germinate better under reduced tillage
  • Tillage may be a tool to reduce kochia populations. Kochia appears well adapted to no-till with germination beginning at 50 cumulative growing degree days (well before other common weed species). Burial of kochia seed to at least 1 cm or deeper can result in reduced germination or death of the germinated seed prior to emergence.
  • Tillage to bury kochia seed should not be overlooked as a part of an integrated weed strategy for kochia control. However, this has limited value where minimum or no-till is practiced.


  • Rod-weeding five to seven days after seeding provides excellent weed control without herbicide use, and good tolerance to soybean, however soybean must be seeded 2.75 inches to 3 inches deep.
  • Tillage 10 to 12 days prior to seeding helps stimulate weed growth for control with the rod-weeder.


  • Harrowing between seeding and emergence of the crop can control newly emerged weed seedlings and remove weeds that escaped previous tillage operations.
  • Harrowing should be avoided during crop emergence and for several days afterwards, to permit effective rooting and stand establishment.

Post-Emergence Harrowing

  • Post-emergence harrowing has been researched at Agriculture and Agri-Food Canada. A higher seeding rate should be used to offset the plant losses due to harrowing.
  • Post-emergent harrowing should be done under warm, dry conditions to improve weed control and to prevent the spread of diseases.

Effective Use of Herbicides

Common Chickweed, Photo Credit Karan A. Rawlins, University of Georgia, Bugwood. org

To determine which herbicide is best suited for your needs, refer to Alberta Blue Book (Crop Protection Manual). This manual provides a comprehensive and up-to-date guide for the selection and application of chemicals to protect your crop. 

Because soybean does not provide a competitive canopy early in the season, weed growth will be greater and more visible in an emerged soybean. After the soybeans form a canopy, they are better able to compete with weeds.

The decision to spray or not to spray should be based on economics. If the potential yield loss is greater than the cost of the chemical and application, then you should spray. Prior to spraying, producers should evaluate the sensitivity of the surrounding environment and avoid spray drift.


  • Thoroughly clean the sprayer before spraying a soybean field.  Soybean can be easily damaged by herbicides registered for other crops.
  • Ensure your sprayer is functioning properly (leaks, plugs, pressure gauge, etc.) and choose appropriate nozzles for the situation.
  • Maximize sprayer efficiency – ensure the herbicide hits the target, water volume is adequate, nozzles provide good coverage, and travel speed is reasonable enough to ensure a good spray pattern.
  • Resource:  Sprayers 101 


  • Crop scouting and anticipating in-crop weeds are key components of a successful weed control program. 
  • Perennial weeds are best controlled pre-harvest in the previous crop. Fields should be inspected again before freeze-up and first thing in the spring as this is an ideal time to control winter annuals.
  • The next weed inspection should be just prior to planting to time pre-seed burn-off.
  • Fields should be inspected again upon crop emergence to establish the frequency and distribution of weed species and to determine appropriate herbicide products for post-emergent weed control.
  • One to two weeks after applying a herbicide, scout for both weed control symptoms and crop injury symptoms. If the weeds are not completely dead, look for symptoms of herbicide activity such as yellowing, purpling, twisting, cupping, or bleaching. Timely post-spray audits may leave enough time to perform a rescue treatment if necessary.



  • During periods of crop stress (heat, drought, frost, or after land rolling) the ability of the crop to tolerate herbicide application may be reduced.
  • Crop injury can be reduced by waiting approximately four days after the crop stress occurs before applying herbicide, by maintaining water volumes at label recommendations, and by applying the product in the evening.
  • Correct application of herbicides: To minimize crop stress, use higher water volumes of 15 gallon/acre (70 litre/acre) with broadleaf herbicides.

Resource:  Application of Herbicides Under Stressful Conditions pdf (466 KB)


  • Follow the growth stage of the crop, rather than spraying by the calendar. Apply herbicides based on the label instructions.
  • Critical weed free period in soybeans is generally recognized as lasting from emergence to the second or third trifoliate leaf stages (V2-V3), but can last well into the reproductive stages.
  • Ontario Ministry of Agriculture indicates that yield loss caused by weeds can be reduced to less than 5% by maintaining good weed control during the critical period.
  • For post-emergent weed control in soybeans, most herbicide recommendations include emergence to third trifoliate, as well as any 75-day or 80-day pre-harvest interval.


  • Tank mixes, or herbicide products offering both broadleaf and grassy weed control, should be applied when either weed group is nearing its maximum growth stage for good control.
  • Never use unregistered mixes or ‘cocktails’ in a crop – this may result in reduced or no herbicide activity, poor weed control and severe injury to the crop.
  • Surfactants can affect both weed control and crop safety – the use of an incorrect surfactant is very risky.
  • All adjuvants are not equal – producers changing adjuvants, or even altering adjuvant rates in the herbicide or tank mix, should expect variable results in weed control.


  • Separate applications of herbicides on the same field may give better weed control at a lower cost under the following conditions:
  • Grassy weeds are well established but broadleaf weeds have not emerged: in some years, cold spring conditions and low soil temperatures result in rapid growth of grassy weeds (like wild oats) but slower growth of broadleaf weeds.
  • Grassy weeds occur mostly in patches: patch spraying with a grassy weed control chemical in a second pass will be more economical than using a tank mix over the entire field
  • Weed populations vary throughout the field: more economical weed control can be achieved by varying the rates of either the grassy or broadleaf herbicide – this result would not be possible with a tank mix.



  • Most post-emergent herbicides are applied in the two to six-node stage, which only allows for a two-week window to complete all herbicide applications.
  • In years with rainy or windy weather, the second herbicide may be applied too late and increase the risk of crop injury and reduce weed control.
  • Split applications may cost more.
  • Delayed herbicide applications are usually less effective, and a late application may be after substantial yield losses from weeds have already occurred.



  • Herbicides have different modes of action. Some modes of action are easy for weeds to develop resistance to as it only requires variation in a few genes (high risk of resistance), while others may require changes in multiple genes (low risk of resistance).
  • Herbicide-resistant weeds are more likely to occur under the following conditions:
    • High weed number
    • Too frequent use of a single herbicide group or mode of action
    • Not using recommended rates
    • Allowing surviving weeds to set seed.
  • Herbicide choice should take into account rotation of herbicide modes of action to slow the development of resistant weeds.
  • Applying mixtures of herbicides with multiple modes of action at the same time, where products are active on the same weeds of concern, is one of the most effective means to slow the development of herbicide resistance.
  • Research indicates that alternating between two modes of action for wild oat control will double the number of years for resistance build-up, and alternating with a third mode of action will increase the time of resistance build-up to four times as long as for a single mode of action for wild oat control.
  • Use integrated control methods through the rotation, such as higher seeding rates, promoting quick crop emergence, and using herbicides only when economic thresholds are reached.

Timing of Herbicide Application


Timing of herbicide application is very important. Earlier herbicide application means weeds are well-exposed, are smaller (generally weeds are easier to control at a younger stage), and the crop is less susceptible to injury.

Weed Control the Year Before


  • Weed management for soybean crops should also be considered in the fall prior to growing soybeans.
  • Controlling winter annual weeds – their growth habits make them difficult to control. Winter annuals germinate in the fall and overwinter as rosettes, producing seed the following year. If these weeds are allowed to bolt the following year, prior to herbicide application, control becomes nearly impossible. Therefore, timing of the herbicide application for control of winter annuals is critical. 
  • Control perennial weeds through using fall tillage or a pre-harvest glyphosate product the year before soybean, and apply when weeds are actively growing under proper temperature, good moisture and bright light.





  • Spray early to remove weed competition. 
  • Soybean seedlings are non-competitive and yield potential is maximized when weeds are controlled in a critical weed free period that lasts until at least the third trifoliate.
  • Soybeans are typically seeded a bit later in the spring to allow soil to warm up. This presents a good opportunity to control weeds prior to seeding or prior to crop emergence in the spring.
  • Since some herbicide systems developed in soybeans are the same as those developed in canola, volunteer herbicide tolerant canola is often a primary weed of concern in soybean production.
  • The action threshold (5% yield loss) for volunteer canola control is 2.5–3.2 plants/m2.
  • A wide range of commercial herbicides are registered for use in soybeans, and there are many different herbicide options to tank mix to address canola in soybean crop.
  • A spring herbicide application, either pre-seed or pre-emergent herbicide (PEH), is recommended as soybeans are relatively poor competitors, especially early in the growing season. This provides early season weed control and may provide control of weeds for which no in-crop control is available. 
  • In a direct seeding system, a spring burn-off application of glyphosate may provide effective weed control – delayed seeding to allow spring weed growth may result in high flower blast and lower soybean yields.





  • Follow the growth stage of the crop, rather than spraying by the calendar.  Apply herbicides based on the label instructions – most are applied up to, and including, the third trifoliate leaf stage.
  • With more use of direct seeding, farmers have seen shifts in weed communities. Weeds traditionally controlled by cultivation – such as winter annuals and perennials – are increasing. Both winter annuals and perennials are poorly controlled by in-crop herbicides.
  • Perennial weeds increase and become more visible under direct seeding while wild oat and green foxtail populations tend to decrease after continuous direct seeding.
  • Timing for effective herbicide application is critical, not only with respect to the growth stage of the plant but for the weeds as well – in general, the smaller and younger the weed, the better the control achieved.



  • A pre-harvest application of glyphosate effectively controls perennial weeds.
  • Appropriate application stage is when the crop is at physiological maturity (30% seed moisture or less). Know the proper staging for harvest aid products and ensure the entire area being sprayed is at the recommended stage.
  • Soybeans: Crop has lost 80% – 90% of leaves and 80% of pods are yellow.



  • A fall application of an herbicide from mid-October to freeze-up is critical to control winter annuals – these can be tough to control in the spring, especially if allowed to grow past bolting stage.
  • See Resources below.