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Dry Bean Weed Control

Summary

One of the biggest challenges for growing dry beans 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. Dry bean can experience severe yield loss with even low weed pressure. Good weed control for dry bean is also important, as not only can weeds impact yields, but green weedy material can harbour diseases and cause quality issues by staining beans during harvest. 

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

Canada Thistle

Knowing your field’s weed history

 
  • An integrated weed management strategy is important to reduce weed control issues, and a weed management plan should start a year or two prior to planting dry beans on a field.
  • Selecting a field that has weeds that may be controlled culturally or with herbicides registered for use in dry bean is important when planning dry bean production. 
  • Ensure that bean fields are free of perennial weeds such as Canada thistle, perennial sow thistle and quackgrass.
  • Know which annual weeds are present, and use the field only if these weeds can be controlled.

Choose clean fields, free of herbicide residues
  • Dry beans 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 dry bean fields.

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 dry bean crop the following year.

Cultural Measures

Stinkweed, Photo Credit: Weedinfo.ca

CROP ROTATION
  • The most important rule of weed management for dry bean is to plant into a clean field.
  • 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 dry bean crop.

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

SEEDS/

SEEDING RATES
  • 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.

TILLAGE
  • If dry beans are grown in a row crop system, inter-row tillage is often used to help manage in-crop weed issues. Certain non-selective herbicides can also be sprayed inter-row as well. Inter-row cultivation should be done on warmer days when beans are limp and less likely to break if contacted, and when weeds will tend to wilt and die more quickly.
  • Tillage may have a beneficial effect for control of some weeds while having the opposite effect on others.
  • Increased tillage favours 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
  • Rod-weeding five to seven days after seeding provides excellent weed control without herbicide use, and good tolerance to dry bean, however dry beans 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
  • 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

Wild Oats, Photo Credit: Producer

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 dry bean does not provide a competitive canopy early in the season, weed growth will be greater and more visible in an emerged dry bean crop.   

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.

SPRAYER
  • Thoroughly clean the sprayer before spraying a dry bean field.  Dry beans are also very sensitive to many herbicide residues and herbicides used in rotation should be checked for cropping restrictions.
  • 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
  • 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 an 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.

KNOW YOUR WEEDS

ALLOW FOR CROP STRESS
  • During periods of crop stress (heat, drought, frost, or after land rolling) the ability of the dry bean 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 

NODE STAGING
  • Follow the growth stage of the crop, rather than spraying by the calendar. Apply herbicides based on the label instructions.

HERBICIDE TANK MIXES
  • 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 
  • 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.

RISKS OF SPLIT APPLICATIONS

 
  • 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.

MANAGING HERBICIDE RESISTANT WEEDS 

 
  • 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

Dandelion

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

 
  • Herbicide options are limited in dry beans and choosing clean fields and controlling problematic weeds in the crop rotation prior to growing dry beans is important.
  • Effective options for perennial weed control of weeds such as Canada thistle, perennial sow thistle, dandelions, and quackgrass for dry beans are not available, so care should be taken to control these weeds in the rotation prior to growing dry beans.
  • Weed management for dry bean crops should be considered in the fall prior to growing dry beans.
  • 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. If a Group 4 herbicide (such as 2,4-D, MCPA) is being used, apply in the fall to protect against residues from spring applications.
  • Control perennial weeds through using fall tillage or a pre-harvest glyphosate product the year before dry bean and apply when weeds are actively growing under proper temperature, good moisture and bright light. 

SPRING PRE-SEED/

PRE-EMERGENT WEED CONTROL 
  • Spray early to remove weed competition. 
  • Dry beans are typically seeded after May 25 to avoid the risk of late spring frosts and this presents a good opportunity to control weeds prior to seeding or prior to crop emergence in the spring.
  • Herbicide selection should be done carefully and when choosing in-crop herbicides, growers should check labels before application, as not all herbicides have been tested on all dry bean types, and certain varieties will have different tolerance to herbicides.
  • Pre- and post-emergent herbicides for annual weed control are limited for dry bean, and effective perennial weed control herbicides are not available.
  • There are available pre-seed or pre-emergent herbicide (PEH) for broadleaf and grassy weeds. This provides early season weed control and may provide control of weeds for which no in-crop control is available.

SAME YEAR – IN CROP

POST-EMERGENT WEED CONTROL

 
  • Follow the growth stage of the crop, rather than spraying by the calendar. Apply herbicides based on the label instructions.  Application can vary after the first trifoliate leaf; up to, and including the second foliate (prior to flower), to fully expanded trifoliate to second fully expanded trifoliate; as well as any 65-day, and 80-day pre-harvest interval.
  • 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 dry bean plant but for the weeds as well – in general, the smaller and younger the weed, the better the control achieved.

PRE-HARVEST WEED CONTROL

 
  • 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.
  • Dry beans: Crop has lost 80% – 90% of leaves and 80% of pods are yellow.

POST-HARVEST WEED CONTROL  
  • See resource below.

RESOURCES

LINKS

Special thanks to Saskatchewan Pulse Growers.