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Lentil – Weed Control

Lentil field with weeds

Summary

One of the biggest challenges for growing lentils is weed control. Weed control is the critical step in preserving yield potential. Unlike cereal and oilseed crops, lentil is a poor competitor against weeds and is highly susceptible to yield loss (20 to 40%). 

Since lentil is a short crop with a thin crop canopy, weed competition can greatly reduce yields. Weeds such as wild tomato or round-leaf mallow, which rarely reduce yield in competitive cereal stands, can be problem weeds in thin lentil crops. Apart from reducing yields, these low-lying weeds will interfere with 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 bur, 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

Wild Mustard, Photo Credit: Nutrien Ag Solutions

KNOWING YOUR FIELD’S WEED HISTORY

 
  • Selecting a field that has weeds that may be controlled culturally or with herbicides registered for use in lentil, are important when planning lentil 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
  • Lentils can be damaged easily by herbicides registered for other crops, or soil residues of herbicides used in previous years. Care should also be taken to avoid drift of herbicides from other fields onto lentil 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 lentil crop up to two years following application.

Cultural Measures

Stinkweed, Photo Credit: Weedinfo.ca

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 lentil crop.
  • Crop rotation goes beyond disease considerations and weed control (including volunteer crop). Crop rotation can influence both the lentil crop and the crop(s) following lentil.
  • Good weed control in lentils requires a long-term strategy involving the entire crop rotation.
  • Lentil production is most successful when grown in rotation with cereals, such as spring or durum wheat. 
  • Crop rotation research conducted by Agriculture and Agri-Food Canada (AAFC) shows yields of cereal crops grown on stubble were best following lentil or pea crops.

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

Seed/

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.
  • Research by Dr. Steve Shirtliffe at the University of Saskatchewan suggests higher seeding rates up to 240 plants per square metre reduced weed populations and increased lentil yields.

TILLAGE
  • Tillage may have a beneficial effect for control of some weeds while having the opposite effect on others.
  • Pre-seeding tillage can be used, but this operation can reduce seedbed moisture.
  • 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.

Harrowing
  • Research completed at Agriculture and Agri-Food Canada showed that post-emergent harrowing with a tine harrow can be used to control weed seedlings when the crop is very short (less than 10 cm), provided that the foliage is dry and the operation is done on a warm, sunny day.
  • An increased lentil seeding rate should be used to offset the plant losses during harrowing.

Post-Emergence Harrowing
  • 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

 
Sprayer on Lentil Field

Herbicides are effective tools for the control of weeds.  These chemicals are capable of killing some kinds of plants (weeds) without injury to other kinds (crops).

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 lentil does not provide a competitive canopy early in the season, weed growth will be greater and more visible in an emerged lentil 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 lentil field.  Lentil 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
  • 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.

KNOW YOUR WEEDS

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

NODE STAGING
  • Follow the growth stage of the crop, rather than spraying by the calendar. Apply herbicides based on the label instructions.
  • For in-crop applications of herbicides the critical weed free period for lentils is between two to five nodes of development.
  • Research on Clearfield® lentils concluded the optimum timing for herbicides was between the five to six node stage and the 10 node stage.
  • Weeds emerging after the 10 node stage were less likely to reduce yield but may lead to harvesting issues.

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 rotation is an important step in slowing the development of resistant weeds.
  • Resistant weeds can have a significant impact in lentil crops simply because lentils are a less competitive crop and there are limited herbicide options.
  • A few examples of herbicide resistant weeds that are particularly troublesome for lentil growers include:
  • Rotating herbicide groups away from Group 1 and 2 products, especially in years where lentils are not grown, may help manage resistant weeds.
  • 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.
  • Herbicide choice should take into account rotation of herbicide modes of action to slow the development of resistant weeds.

 

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 

 
  • A lentil weed control program requires a long-term strategy involving crop rotation, the year before planting, proper field selection and management.
  • The major weeds of concern in lentil include kochia (the vast majority of which are Group 2 resistant), Russian thistle, wild mustard, stinkweed, Canada thistle, dandelion, and quackgrass.
  • Once the competitive weeds have been removed, less competitive weeds such as cow cockle, round-leaf mallow, blue burr, and wild tomato can cause a problem.
  • Remember to consider volunteer crops as weeds as well.
  • Herbicides currently registered in lentil have little to no effect on most of these weeds. It is very important to control perennial weeds in the year(s) prior to seeding lentil. 
  • Control perennial weeds through using fall tillage or a pre-harvest glyphosate product the year before lentil, and apply when weeds are actively growing under proper temperature, good moisture and bright light.
  • Growth habits of winter annual weeds 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. 
  • Late emerging weeds interfere with harvesting, increase dockage, and increase staining and moisture levels in the harvested seed.
  • Even with fall herbicide applications, it’s important that the crop emerge quickly to avoid injury – deep seeding and cold or dry soil conditions can aggravate crop injury problems associated with trifluralin products.

SPRING PRE-PLANT/

PRE-EMERGENT WEED CONTROL 

 
  • Spray early to remove weed competition. 
  • A spring herbicide application, either pre-seed or pre-emergent herbicide (PEH), is recommended as lentils 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. 
  • This can be effective for many winter annual weeds provided environmental conditions are conducive to performance and the weed is at a young growth stage – good soil moisture, high temperatures, bright, sunny days and long day lengths enhance glyphosate activity.
  • A pre-seed or pre-emergence burn-off are important for controlling weeds before the crop emerges. Soil applied products may provide early weed control during early crop growth but usually require moisture for activation.
  • Never apply a spring application of a Group 4 broad spectrum herbicide (for e.g. dicamba or dicamba mixtures, 2, 4-D or MCPA) prior to seeding lentil. These herbicides can be taken up from the soil by lentil and can cause serious injury. Under ideal conditions of warm moist soil, these herbicides are degraded microbially to safe levels in one to four weeks after application. However, under dry, cool soil conditions, these herbicides can persist for much longer.
  • 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 yields.

SAME YEAR – IN CROP

 

POST-EMERGENT WEED CONTROL

 
  • Volunteer wheat and barley are difficult to clean from small-seeded lentil and should be controlled in-crop.
  • For in-crop applications of herbicides the critical weed free period for lentils is between two to five nodes of development.
  • The introduction of Clearfield® technology has given lentil growers additional in-crop herbicide choices. Research on Clearfield® lentils concluded the optimum timing for herbicides was between the five to six node stage and the 10 node stage. Weeds emerging after the 10 node stage were less likely to reduce yield but may lead to harvesting issues.
  • Make sure to follow label directions and apply the herbicide at the correct time of plant development. Some herbicides can move in the soil after heavy rainfall, so if the use of these products is anticipated, lentils must be planted at least 5 cm (2 in.) deep to prevent injury to seedling
  • Follow the growth stage of the crop, rather than spraying by the calendar. Apply herbicides based on the label instructions, most herbicides are applied between the two to six-node 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.

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.
  • Lentils:  Pods on bottom third are brown with hard seeds detached from pod that rattle when shaken and 80% of plant is yellow to brown in colour. Pods in middle third have seeds that are full size and firm showing 100% colour change from light green to tan-brown. Top third of plant will show 50 – 75% colour change and may have slight green colour, but seeds are fully formed and firm.

POST-HARVEST WEED CONTROL

 
  • 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 resource below.

RESOURCES

LINKS