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Pea Weed Control

In a direct seeding system, weed control should start in the fall. Various weed species react differently to the types of tillage systems. Increased tillage favors stinkweed, wild oats and chickweed. Other weeds – such as bluegrass, clover, groundsel, and smartweed – germinate better under reduced tillage.

Winter annuals (stinkweed, flixweed, narrow leaved hawk’s beard and shepherd’s purse) and perennial weeds (quackgrass, Canada thistle and sow thistle) increase and become more visible under direct seeding. Wild oat and green foxtail populations tend to decrease after continuous direct seeding.

  • a pre-harvest application of glyphosate (Roundup®, Touchdown®, Laredo®, Wrangler®, Renegade®, Victor®) effectively controls perennial weeds such as Canada thistle, sow thistle, quackgrass, toadflax and dandelion
  • a fall application of a 2,4-D or MCPA 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
  • do not apply 2,4-D or MCPA in the spring for winter annual weed control prior to seeding pea as this approach can harm pea plants
  • never apply dicamba or dicamba mixtures (Banvel®, Rustler®) as a winter annual weed control measure prior to growing a field pea crop
  • pre-seeding burnoff with glyphosate 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
  • surface-applied fall pre-emergent herbicides are at least as effective as conventional treatments (this may be due to the higher moisture retention in direct seeding systems)
  • apply herbicides early; from the three to four-node pea growth stage
  • follow the growth stage of the crop, rather than spraying by the calendar (see Weed Control under the Post-Seeding Considerations section for more information)
  • control perennial weeds (thistles, quackgrass) the year before seeding pea with a pre-harvest glyphosate product
  • to avoid having residual herbicide damage to the pea crop, keep herbicide records on all fields
  • proper seeding management will help produce a vigorous, uniform pea crop, for better competition with weeds and easier herbicide timing
  • know your weeds – start scouting fields at pea emergence for weed types, and match the herbicide to the weed types
  • know your pea node stages – most herbicides are applied between the two to six-node stage with the best time being the three to five-node stage
  • spray early to remove weed competition
  • to minimize crop stress, use higher water volumes of 15 gallon/acre (70 litre/acre) with broadleaf herbicides
  • watch for other broadleaf herbicide residues in the sprayer tank – thoroughly clean the sprayer before spraying a pea field

To determine which herbicide is best suited for your needs, refer to The Blue Book.

Weed Control the Year Before Pea

Compared to most other crops in the rotation, pea is a very poor competitor with weeds, and weed control is necessary the year before growing pea to produce a high-yielding crop.

  • choose clean fields, free of herbicide residues
  • control perennial weeds (Canada thistle, sow thistle, quack grass, dandelion and toad flax) with a pre-harvest application of a glyphosate product the year before pea, and apply when weeds are actively growing under proper temperature, good moisture and bright light (products include: Roundup®, Laredo®, Renegade®, Touchdown®, Wrangler® and Victor®)
  • under direct seeding systems, 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
  • cases have been seen of serious residue herbicide damage of field pea caused by seeding on rented land with no herbicide records or history – so ensure records on all fields are kept to monitor residual herbicides (even reduced rates of residual herbicides can cause serious injury to the pea crop the following year)

Control of Winter Annuals

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.

  • problem perennials include quackgrass, Canada thistle, perennial sow thistle, toadflax and dandelion
  • 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

Winter annual 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
  • an early spring application of these herbicides has been recommended for winter annual weed control in cereals, but 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
  • under dry, cool soil conditions, these herbicides can persist for much longer

In-Crop Weed Control

  • use good seeding practices to produce a healthy, vigorous and uniform seedling stand
  • know your weeds – what one person calls pigweed may actually be lamb’s-quarters (post-emergent herbicides such as Pursuit® perform well on pigweed but have very poor performance on lamb’s- quarters)
  • the best way to check a weed’s identity is to compare weeds using a weed seedling identification guide – consult the Weed Seedling Guide, Agdex 640-9
  • 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
  • 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 as shown in Figure 20.

Node Staging

  • a pea plant can produce two nodes in seven days under optimum conditions
  • if you are using a post-emergent product, know the correct node stage of the pea plant for safe application – most broadleaf weed control products perform best at the two to five-node stage
  • products such as MCPA Na salt or Tropotox Plus must be applied before the five-node stage, or severe damage can occur to the crop – the earlier these products are used, the safer they are on the crop; the exception is Pursuit, which can be used up to the six-node stage of the field pea plant
  • 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)
  • 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
  • spray early to remove weed competition
  • pea yield potential declined every week spraying was delayed after pea emergence

Herbicide Tank Mixes for Weed Control

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 pea crop – this may result in reduced or no herbicide activity, poor weed control and severe injury to the pea 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

Applications for Weed Control in Pea Split applications – that is, 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