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.
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.
Selecting a field that has weeds that may be controlled culturally or with herbicides registered for use in peas, is important when planning pea production.
Avoid fields with known infestations of perennial weeds such as Canada thistle or sow thistle (no herbicides are registered for complete thistle control in field pea), biennial and/ or Group 2 resistant weeds such as cleavers, kochia, wild mustard and smartweed.
Choose clean fields, free of herbicide residues
Peas 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 pea fields.
Maintain Accurate records of herbicide use
Serious residue herbicide damage of field pea has been caused by seeding on rented land with no herbicide records or history.
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 pea crop up to two years after application.
Diverse crop rotations provide varied competition to weeds, and allow for a wide range of herbicide options reducing the risk of resistance.
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 pea crop, for better competition with weeds and easier herbicide timing.
Tillage may have a beneficial effect for control of some weeds while having the opposite effect on others.
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 peas seeded 7.5 cm (3 in) 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 pea 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 has been researched at Agriculture and Agri-Food Canada. Peas were found to be somewhat tolerant to 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
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 field pea does not provide a competitive canopy early in the season, weed growth will be greater and more visible in an emerged pea crop – it takes a full two weeks longer than other crops for the pea crop to develop a canopy to decrease light penetration for weed growth.
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 pea field. Peas 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.
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, 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
The best way to check a weed’s identity is to compare weeds using a weed seedling identification guide. Various references include:
During periods of crop stress (heat, drought, frost, or after land rolling) the ability of the pea 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 with broadleaf herbicides.
Follow the growth stage of the crop, rather than spraying by the calendar. Apply herbicides based on the label instructions:
A pea plant can produce two nodes in seven days under optimum conditions.
Node staging – not the height of the pea plant – determines time of spraying (under drought conditions, a pea plant can reach five nodes and still be only 3 in. or 7.5 cm tall).
If you are using a post-emergent product, know the correct node stage of the pea plant for safe application.
When counting node stages on a pea plant, the first leaves (called scale leaves) are very small and close to the stem – these are not counted.
The point where the first true leaf joins the stem is counted as the first node; the second node occurs where the second leaf joins the stem and so on.
Research in northeast Alberta and the Peace River region showed pea yields were higher and more consistent with application at the second node stage compared to the eighth node stage. Yield was increased (22% to 125%) seven times out of 10 with early applications.
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 herbicide rotation to slow the development of resistant weeds. Resistant weeds are troublesome in peas simply because they are a less competitive crop and there are limited herbicide options.
A few examples of herbicide-resistant weeds that are particularly troublesome for pulse growers include:
Rotating herbicide groups away from Group 1 and 2 products, especially in rotational years where field peas are not grown, can help prevent or manage resistant weeds.
Resistance can build with each application and applications do not have to be consecutive year after year. With high risk herbicides in Groups 1 and 2, the longer you can rotate away from these chemistries, the better the resistance.
On average if a grower has applied Group 1 or Group 2 herbicides more than 10 times in a field, there is a high risk of resistance developing among one or more weed species.
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.
Preventing kochia from setting viable seed for one or two years greatly reduces kochia populations in a field because the seed is short lived in the soil.
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 field pea crops should also be considered in the fall prior to growing field peas.
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, and apply when weeds are actively growing under proper temperature, good moisture and bright light.
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 peas 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.
Most broadleaf in-crop products perform best at the 2nd to 6th above-ground node stage and late application may result in crop injury and yield can be reduced up to 25% by delaying weed control until four weeks after emergence.
Research conducted at Agriculture and Agri-Food Canada (AAFC) demonstrated the importance of early weed removal in pea production. Annual weeds were removed from pea crops at intervals of one, two, three, and four weeks after crop emergence. Pea yields did not decline after one week, declined 7% after two weeks, 12% after three weeks, and 26% after four weeks.
This approach 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.
Pea yield potential declined every week spraying was delayed after pea emergence.
Peas may be slower to emerge, allowing more time if a post-seed/pre-emergent herbicide application is planned. However, with good growing conditions and shallower seeding, emergence can be more rapid, so timing must be watched closely.
Winter annual weed control – 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 field peas. These herbicides can be taken up from the soil by pea 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 pea yields.
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 as the application stage vary for the different herbicides (for e.g. 5th node to early bud, 3rd to 8th nodes; 4 to 7 inches in height; up to 6 inches in height; as well as any 60-day, 65-day, and 75-day pre-harvest schedule).
Timing for effective herbicide application is critical, not only with respect to the growth stage of the field pea plant but for the weeds as well – in general, the smaller and younger the weed, the better the control achieved.
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.
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.
Field peas: Bottom pods are ripe and dry with seeds detached from the pods. 80% of the plant is yellow to brown in colour. Top pods wrinkled and seed firm. Top of plant may have slight green colour.
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.