Chapter 10a - Weeds

Weeds of Canola

Weeds suppress growth and productivity of canola. Until the registration and release of herbicide-tolerant canola (HTC) varieties, weeds were often the most limiting factor in canola production. Although it is possible to control annual grassy weeds (wild oats and volunteer cereals), perennial weeds (Canada thistle, perennial sow thistle and quackgrass) and certain annual broadleaf weeds in conventional canola varieties, this may require three or more herbicide treatments plus tillage-a considerable cost. In addition to yield losses, hard to control weeds in conventional canola crops, especially those of the cruciferae or mustard family, reduce oil and meal quality by contaminating the canola harvested. The presence of these weeds also limits opportunities for seed production or increases the cost of seed production. The introduction of HTC varieties has given growers many options for herbicide and variety selection.

Integrated Weed Management (IWM)

An Integrated Weed Management (IWM) approach combines different agronomic practices to reduce the reliance on any one-weed control technique. Controlling weeds with only one or two techniques gives the weeds the chance to adapt to those practices. For example, the use of only Group 1 herbicides year after year on a field has resulted in weeds that are resistant to those herbicides. The continuous production of certain crops also gives weeds a chance to adapt (higher wild mustard or stinkweed infestations where conventional canola was grown more frequently). Weeds may also adapt to crops seeded annually on similar dates. One IWM technique might involve altering conventional seeding dates to prevent weed adaptation. Using more than one or two specific weed control techniques means that those techniques or tools will be effective for more years of future use.

The objective of IWM is to maintain weed densities at manageable levels (below the economic threshold) while preventing shifts in weed populations to more difficult-tocontrol weeds. IWM utilizes a variety of control practices to keep weeds "off balance." Weeds are less able to adapt to a constantly changing system that uses many different management practices, unlike a farm system that relies on only one or two cropping systems or weed management tools.

The Weed Problem

Field scouting is important as weeds vary from field to field and also within fields. So each field must be carefully assessed before planting it to canola. Identification, recognition and detection of weeds are important to crop management. The most prevalent weeds based on western Canada provincial weed surveys are:

• wild oats
• wild buckwheat
• lamb's-quarters
• redroot pigweed
• lady's thumb
• Russian thistle
• hemp nettle
• cow cockle
• field horsetail
• shepherd's purse
• chickweed
• wild rose
• green foxtail
• stinkweed
• Canada thistle
• green smartweed
• sow thistle
• wild mustard
• bluebur
• night-flowering catchfly
• barnyard grass
• flixweed
• volunteer canola

The first step in a weed control program is to identify the weeds. Every year, evaluate individual fields for weed types, populations and locations. Keep an up-to-date list of weeds found in each field. Local crop specialists can help identify the weeds. Many good weed identification books are available.

Problem weeds in canola include: the perennials; shadetolerant weeds; and tall-growing, early-germinating weeds such as wild oats. Among the more troublesome weeds that are not controlled in a conventional canola crop with lower cost herbicides are the closely related cruciferous weeds:

• wild mustard
• stinkweed
• shepherd's purse
• ball mustard
• flixweed
• wormseed mustard
• hare's ear mustard
• common peppergrass

These weeds are easily controlled in cereal crops and herbicide-tolerant canola, but dormant seeds and seeds from escapes will germinate and can set more seeds in conventional canola. This can result in a gradual build-up of weed populations when canola becomes more frequent in the rotation. Stinkweed seeds can remain viable and dormant in the soil for up to 10 to 20 years while wild mustard seeds can remain dormant for more than 30 years. Research at the Agriculture and Agri-Food Canada (AAFC) Scott, SK Research Centre has shown that after canola has been in the rotation over a 10-year period, the stinkweed populations increased 15 times over those rotations where only cereals were grown. This emphasizes the need for a proper rotation when growing canola.

Other difficult-to-control or expensive-to-control weeds in canola are:

• Canada thistle
• cleavers
• stork's bill
• lady's thumb
• quackgrass
• sow thistle
• green smartweed

Weed Competition

Weeds can result in reduced yields because weeds compete with the crop for limited resources of light, moisture and nutrients. Weeds also result in:

• increased insect and disease damage through multiplication on weed hosts
• spread of weed seeds
• increased cultivation and chemical control practices
• reduced seedbed soil moisture and structure as a result of increased tillage to kill weeds prior to seeding
• delayed swathing and combining from hard to-pick-up swaths, plugging and higher seed moisture content
• increased heating and spoilage from green weed seeds in storage
• increased dockage with higher cleaning and transportation costs
• contamination resulting in reduced grades and quality from similar inseparable size and shape weed seeds (Many of the hard-to-control weeds listed earlier are in this category.)
• reduced opportunities for seed production

Weed competition reduces canola plant growth and leaf area resulting in increased flower, pod and seed abortion. The yield loss due to weeds can vary widely from year to year depending on:

• time and rate of seeding
• density and spatial arrangement of the crop
• species and density of the weeds
• relative competitiveness of crop and weeds
• moisture availability
• nutrient availability
• relative time of emergence of crop and weeds
• environmental conditions

Weed competition is generally more damaging where one of the following factors exist:

• low canola plant population lacks vigour
• weed density is high and the weeds are vigorous
• moisture and nutrient supply favours weed growth more than crop growth
• weeds significantly reduce moisture supply to the crop
• when weed growth is ahead of the crop
• when environmental conditions have resulted in a shallow rooted crop

Research studies have developed canola yield loss estimates for several weeds at various population levels. Even though most canola fields usually have several different weeds to contend with, these estimates should help in making decisions on which canola system and IWM technique will be economical to manage a particular weed.

Canada Thistle

This weed is three to four times more competitive than wild oats. Estimated yield losses of canola caused by Canada thistle are shown in Figure 1.

Figure 1. Canola Yield Losses Due to Canada Thistle

Yield losses from Canada thistle will vary from year to year due to environmental conditions; the type, stage and vigour of the crop; and when the thistle emerges in relation to the crop. Researchers at the AAFC Lacombe, AB Research Centre developed an equation to predict yield loss through competition from Canada thistle. The equation is:

Per cent yield loss = -3.83 + 1.4X
(x is the number of Canada thistle shoots/m²)

The more samples taken the greater the accuracy in estimating yield loss. A minimum of 10 samples is recommended per quarter section. For example, if a canola field has an average of 15 Canada thistle shoots/m2, the estimated yield loss would be:

Per cent yield loss= -3.83 + 1.4 x 15 = 17

In a canola field with 1,681 kg/ha (30 bu/ac) potential yield, a 17% yield loss from 15 Canada thistle shoots per m2 would be 280 kg/ha (5 bu/ac). The 17% represents total loss. Since no control measure provides 100% weed kill, and since weeds are not killed immediately on emergence, all of this loss can never be recovered. The recovery of at least 75% of the yield loss by spraying with a herbicide would result in an increase of 213 kg/ha (3.8 bu/ac). Decisions must be made on whether the use of a herbicide is economical given current herbicide costs and canola prices.

Sow Thistle

This weed is common in western Canada. It does not usually occur uniformly throughout fields, but where it does, the crop can suffer high yield loss. Soil, rainfall and farming practices influence the competition between canola and sow thistle. Researchers at the AAFC Regina, SK Research Centre developed an equation for determining yield loss in canola:

Percent Yield Loss = -3.81 + 13.76 x (square root of X)
(X being the number of sow thistle rosettes and shoots/0.25 m²)

In a field with 20 sow thistle rosettes/m2 or 5/0.25 m², the yield loss would be:

Per cent yield loss = -3.81 + 13.76 x (square root of 5)
= -3.81 + 13.76 x 2.24
= -3.81 + 30.82
= 27

In general, thistle populations increase in seasons of normal to above normal soil moisture. This can be important if the weeds are not controlled in the year before the canola crop.

Wild Mustard

This weed is more competitive than wild oats. In the early growth stages of canola, it is very competitive resulting in lower yields. Research from the AAFC Lacombe Research Centre has shown high yield losses (Figure 2).

Figure 2. Estimating Westar Canola Yield Losses from Wild Mustard

Wild mustard seed is similar in size and shape to canola seed and is impossible to separate by conventional methods. Wild mustard is a contaminant in canola due to its high erucic acid and glucosinolate content. Contamination of canola with wild mustard or volunteer tame mustard results in serious downgrading. Wild mustard densities of as low as 20 plants/m² (17/yd²) may result in wild mustard seeds being over 5% of the harvested seed. Densities of 48 plants/m² (45/yd²) resulted in 16.7% of the seed harvested being wild mustard. The maximum tolerance of wild mustard in any grade of canola is 5%. Over 5%, the canola is graded "Sample Reject." Over 50%, the canola is graded "Refuse Screenings." Wild mustard in canola is hard to detect visually, but can be detected by magnification. Wild mustard seeds appear smooth and shiny or waxy, while canola seeds have visible seed ridges.

Other Broadleaf Weeds

Researchers at the AAFC Lacombe Research Centre found that canola was more competitive against Tartary buckwheat than were barley, wheat or flax and that a weed population of at least 80/m² (66/yd²) before canola yield losses are greater than 10% (Figure 3).

Figure 3. Effect of Tartary Buckwheat and Hemp Nettle on Canola

Canola yield losses from wild buckwheat are probably similar to those from tartary buckwheat. Canola yield losses due to smartweed, hemp nettle and lamb's-quarters may be higher due to the taller growth habits of these weeds.

Smartweed is very sensitive to crop competition and if the crop emerges ahead of the weed there is very little effect on yield.

Volunteer Wheat and Barley

These volunteers are very competitive in canola and may cause serious yield losses especially where fall harvesting is delayed by poor weather and shattering occurs. Volunteer crops usually are strong competitors at the early growth stages. Volunteer wheat and barley, at seven to eight plants/m² (six to seven/yd²) can reduce canola yield by 10 to 13% as shown in studies at the University of Manitoba in Winnipeg, MB (Figure 4).

Figure 4. Yield Losses in Canola from Volunteer Wheat and Barley

Equal numbers of volunteer cereals were found to be 1.5 times as competitive as wild oats. At higher densities, barley was much more competitive than wheat. Studies on the variety Tobin by Scott, SK AAFC researchers, showed similar yield reductions from volunteer barley but with yield losses of 50% with 90 barley plants/m² (75/yd²). Higher plant populations of barley did not produce further yield reductions. The density of both the canola crop and volunteer cereals will influence yield loss (Figure 5). The following equation can be used to estimate the yield loss from volunteer barley.

Canola yield loss (%) = 100 x 0.032D / (1 + 0.04C + 0.032D)
Where D = barley plants/m²
And C = canola plants/m²

Figure 5. Effect of Volunteer Barley and Crop Density on Canola Yield Loss

Canola yield losses are more severe when volunteer wheat or barley emerges before the canola crop. Base decisions to control cereals on a comparison of present herbicide costs and canola prices.

Wild Oats

Wild oats can significantly reduce canola yields. Estimates of yield losses due to wild oat weed density have been developed for healthy, well-fertilized canola crops with good stand establishment (Figure 6).

Figure 6. Yield Losses in Canola caused by Wild Oats

Crops that are diseased, have insect pressure, or emerge unevenly will not compete well with weeds and have higher yield losses than those indicated.

Researchers at the AAFC Lacombe Research Centre developed an equation to predict canola yield losses from wild oats:

Per cent yield loss = 3.22 x (square root of X).
(X being the number of wild oat plants/m²)

An average infestation of wild oats would range from 60 to 100 plants/m² (50 to 83/yd²). A heavy infestation would range from 300 to 500 plants/m² (249 to 415/yd²).

If a field had 100 wild oat shoots/m² (83/yd²) on average, the per cent yield loss would be:

Per cent Yield Loss= 3.22 x (square root of 100)
= 3.22 x 10
= 32.2

A field with a potential yield of 1,681 kg/ha (30 bu/ac) would, assuming an 85% yield recovery, have a yield loss of 448 kg/ha (8 bu/ac). Even eight wild oats/m² (7/yd²) would result in a 9% yield loss, or at 85% recovery, 129 kg/ha (2.3 bu/ac). Compare the present price of the herbicide with canola prices for that yield recovery. Growers must also consider what level of infestation will occur in the following year if wild oats are not sprayed.

Green Foxtail

Research by the Alberta Research Council (ARC) in Vegreville, AB has shown canola yield losses caused by green foxtail (Figure 7).

Figure 7. Yield Losses in Canola due to Green Foxtail

Green foxtail can result in significant yield losses if it emerges ahead of or with the canola crop. If green foxtail density is less than 100 plants/m² (83/yd²) and the majority of the weeds have emerged more than one week after the crop, yield loss is expected to be insignificant. Green foxtail is a poor competitor in cooler regions unless in dense stands. Canola is an efficient competitor against green foxtail. Under low light intensity green foxtail plants grow very poorly and remain small.

Quackgrass

Research by the Vegreville ARC estimated yield losses in canola due to quack grass density (Figure 8).

Figure 8. Effect of Quackgrass on Canola Yield

The economic threshold in canola is about 20 to 25 quackgrass shoots/m² (17 to 21/yd²), but varies with the price of canola. Quackgrass usually occurs at high densities in localized patches within a field, but can spread rapidly by its underground rhizomes.

Cleavers

Cleavers can be very competitive with canola because it clings to plants when growing toward the light and reduces crop development. One research trial showed that a cleavers density of 100 plants/m² (83/yd²) resulted in a 20% yield loss. The clinging bristles make crop handling and harvesting difficult resulting in swathing difficulties and bunching of the swath. The seeds are similar in size and shape to canola, making them a serious contaminant that will downgrade the crop.

Weed Management Measures

An IWM program that includes several cultural and agronomic factors and their interactions to promote a healthy and competitive crop will reduce yield loss due to weeds. Practices that limit the introduction and spread of weeds include:

• diverse rotations
• higher seeding rates
• varied seeding dates
• shallow seeding depth
• narrow row spacing
• pedigreed seed
• tillage
• adequate fertility
• insect and disease protection
• preventative measures

Rotations

Crop rotations or crop sequences that include forage crops, cereals and oilseeds allow the use of different herbicides and practices that make it difficult for weeds to adapt and build up. Diversified rotations that use a number of different crops allow weeds to be managed at different times over the growing season. Rotations with few crops means there is little variation in seeding dates or herbicide practices and little use of different competitive abilities or life cycles. Crop rotations must take advantage of crop life cycle and weaknesses in the life cycle of the existing weed problem.

Certain crop varieties are more competitive than others. Varieties that cover the ground more rapidly and close their crop canopy earlier can help shade out weeds. More vigorous growing canola hybrids sown at a higher plant population are very competitive and result in reduced weed growth and lower dockage.

Seeding Rates

Higher plant populations allow the crop to shade weeds and make it more difficult to access nutrients and water. Research at the Scott, SK AAFC Research Centre showed that weed biomass decreased and canola yields increased with higher seeding rates (Figure 9).

Figure 9. Effect of Seeding Rate on Weed Biomass and Canola Yield

The additional crop competition may improve herbicide performance. Studies at the Vegreville ARC found that higher canola plant populations reduced the competitive effects of weeds (Figure 10).

Figure 10. Effect of Tobin Canola Seeding Rate on Competition from Tartary Buckwheat [50 weeds/m² (42/yd²)]

Altering seeding rates has several pros and cons. Higher plant populations may be more competitive, however, these crops tend to lodge more readily and may have a higher disease incidence. Since a higher plant population can also have a negative effect on a crop when moisture is in short supply, carefully consider the risks of increasing the seeding rates along with the weed management benefits. In most cases, seeding rates can be moderately increased to obtain weed management benefits without significant negative consequences.

Seeding Date

While most crops in a rotation benefit from early seeding it can be beneficial on individual fields to vary the seeding date from year to year. Fall- or early-seeded canola can result in the crop emerging before the weeds. Some weeds like green foxtail, lamb's-quarters, kochia, redroot pigweed and Russian thistle require more growing degree days to germinate and emerge. An early-seeded canola crop could emerge far enough ahead of these weeds that herbicides may not be required. However, cool season weeds like wild oats, quackgrass, wild mustard and stinkweed may present a problem with early seeding.

In certain situations, later seeding of B. rapa or B. napus varieties may be used to reduce the number of weeds in the crop. Shallow till the soil early in the spring to aerate it and promote weed seed germination. Delay seeding until weed growth has reached a maximum. This growth must be destroyed either by a pre-seed herbicide application or a tillage operation prior to seeding. The disadvantages of this approach include:

• Tillage can cause the loss of surface soil moisture resulting in poor, uneven germination and emergence, and reduced yields.
• Delayed seeding increases the risk of yield loss due to frost or damp harvest weather.
• Most importantly, delayed seeding per se reduces crop yield.

Seeding Depth

Seed as shallow as possible [2.5 cm (1") or less]. Shallow seeding, provided adequate moisture is present, means quicker seedling emergence which helps the crop get ahead of or keep up with the weeds. Deep seeding delays crop emergence and weakens seedlings and they are less competitive.

Row Spacing

Narrow row spacing allows plants to be more competitive. Research at the AAFC Scott, SK Research Centre showed that weed biomass increased and canola yields decreased with wider row spacing (Figure 11).

Figure 11. Effect of Row Spacing on Weed Biomass and Canola Yield

Where wide row spacing is necessary (for example, residue clearance in direct seeding systems), higher seeding rates can help offset the effect of wider row spacing.

Pedigreed Seed

Choose varieties that are very competitive in growth habit. Clean, disease-free, large seed with high germination helps establish a good stand of vigorous seedlings that will compete with weeds. Always ask the seed seller for the types of weed seeds present in the seed lot so that new weeds are not introduced to the farm.

Tillage or Direct Seeding

How a seedbed is prepared can affect crop and weed growth. Ensure the seed is placed in an ideal growing environment to give the crop an advantage. On-row packing provides good seed/soil contact to promote rapid germination and emergence, but leaves the soil loose between rows. Direct seeding systems leave crop residue between rows, which shades the soil and keeps it cool. Weed emergence is slow in early spring, but the majority of weeds generally emerge by mid-season. A "second flush" of weeds is less likely to occur because of higher stubble residue between crop rows. For early-seeded crops, the incrop herbicide application is the most important while for late seeded crops the burn-off herbicide application is the most important. For example, green foxtail problems are reduced in direct seeding systems because weeds are less able to germinate and grow in the direct seeding soil environment.

Research at the AAFC Indian Head, SK Research Centre found that soil disturbances from tillage increased weed densities and that early in the season weed densities were lower than at the end of the seeding season. Direct seeding using a burn-off treatment may be all that is required to control weeds at later seeding dates. This is possible because a tillage operation that encourages a new flush of weeds within the crop is eliminated.

Where reduced or conventional tillage is being used on land to be recropped, start weed control immediately after harvest. Since tillage buries weed seeds and promotes germination, weeds may be controlled by subsequent tillage or frost. The earlier this is done in the fall, the better the weed control the following year. However, early fall tillage after canola increases the dormancy and persistence of volunteer canola in subsequent crops.

Shallow cultivation in the fall, resulting in a light soil covering of weed seeds, is the most effective means of stimulating wild oats germination. Harrowing is relatively ineffective as it does not provide a sufficient soil cover to favour germination.

Deeper tillage penetration may be required to destroy roots of perennial weeds such as Canada thistle and quackgrass. Deeper tillage may also be required to prepare the soil for soil-incorporated herbicides or fertilizers, and to bury some of the straw. Glyphosate can be used pre-harvest or in the fall for more effective perennial weed control without the negative consequences of excessive tillage. Cultivation, herbicides and mowing can be used in combination or separately. Patch spraying is less expensive than field treatment. Canada thistle may be suppressed by fall spraying with a dicamba, 2,4-D or MCPA product right after harvest when weeds are actively growing.

Deep fall tillage buries weed seeds deeper and induces dormancy in wild oats and other smaller weed seeds. Small weed seeds with a long dormancy period (wild mustard and stinkweed) most readily lose dormancy when left on the soil surface. Deep fall tillage can also induce dormancy in volunteer canola.

Winter annuals like stinkweed, flixweed, tansy mustard, groundsel, and winter annual cleavers do not germinate until late September or October. These weeds germinate, establish a root system and form a rosette that is not killed by winter frosts. The plants overwinter and continue to grow as soon as snow disappears in the spring. By the time fields can be worked in the spring, these weeds have often become well established and hard to kill, requiring extra tillage that dries out the seedbed. These weeds can be more effectively controlled by late fall spraying with 2,4-D than with tillage. Tillage often transplants the weeds, allowing them to grow.

Weed seeds may also be encouraged to germinate with shallow early spring cultivation. Spring pre-plant spraying with phenoxy herbicides for winter annual weed control is not recommended, especially in soils with low organic matter. Serious damage to the emerging canola seedlings can occur. Tillage for weed control immediately before or during planting kills weeds and prepares the seedbed. The operation must be shallow, but into moisture to enhance crop germination and emergence. Avoid excessive or deep tillage as this dries out the soil, promotes erosion and may bring up dormant weed seeds. A superior practice involves controlling (burning off) the weeds with glyphosate prior to a direct-seeding operation.

Adequate Fertility

Fertilizer placement affects the crop's ability to compete with weeds. Band nitrogen (N) fertilizer so that seedlings have an advantage over the weeds in accessing the N in the early stages of seedling establishment. Most weeds germinate near the soil surface, therefore, banded fertilizer is not as accessible to the weeds, reducing the number of weeds germinating and their seedling vigour. Broadcast N, especially if it's incorporated, can stimulate weed seed germination (especially wild oats) and is more readily available to weed seedlings. Phosphorous (P) fertilizer with the seed, especially when early seeding, is essential for establishing vigorous seedlings. Apply safe levels of N, P and K (potassium) with the seed to avoid killing or injuring seedlings, making the stand less competitive. Weed competition may reduce the plant's chance of recovery from high fertilizer rate injury.

Insect and Disease Protection

Seed treatments combined with shallow seeding into a firm, moist seedbed may reduce seedling diseases allowing plants to be more competitive with weeds. Protection from insects such as the flea beetle will enhance seedling vigour and growth.

Other preventative measures:

• Ensure that farm equipment is clean so that weeds are not spread from field to field or from one farm to another.
• Cover screenings and grain during transport.
• Restrict domestic animal movement from weedy fields to clean fields.
• Do not allow weeds to set seed.
• Control weeds in ditches, the edges of fields and around sloughs.
• Disrupt or destroy weeds by machine, hand mowing, hand pulling of small patches, early harvest, tillage, grazing, ensiling, burning weeds prior to maturity and mulching small patches with clean straw, manure or plastic.
• Control the spread of weeds through manure by proper rotting or composting of the manure and livestock bedding.
• Control or restrict wind and water movement from weedy to weed-free areas.

Chemical Control Measures

Do not rely on chemical weed control as the only method of protecting crops from weeds. From an IWM standpoint, use herbicides in combination with good agronomic practices. Focus on combinations of optimal agronomic factors that enhance crop health and competitiveness, favour viable weed management systems, and support integrated crop management practises. For example, a recent research study at the AAFC Lacombe, AB Research Centre found that combining the best variety, the highest seeding rate and the earliest weed removal treatment increased yield 41% (compared to the combination of a less vigorous variety, the lowest seeding rate and the latest time of weed removal).

Choose herbicides based on the type of weeds and the tolerance of the crop to various herbicides. Apply the right herbicide in the proper way, at the optimum time and in the recommended concentration. This information can be obtained from the label on the herbicide container, and from public or private crop specialists. Since registrations change from year to year, obtain provincial herbicide guidelines annually. Check the annual provincial publication for current recommendations and always refer to product labels:

• Crop Protection with Chemicals - Alberta
• The Guide to Crop Protection - Saskatchewan and Manitoba
• Guide to Weed Control - Ontario

Weed Control Systems

The four main weed control systems in canola, including the conventional varieties that are also tolerant to some herbicides, are:

• Clearfield - canola resistant to imidazolinone chemistry (Pursuit, Odyssey, Absolute)
• Liberty Link - canola resistant to glufosinate ammonium (Liberty)
• Roundup Ready - canola resistant to glyphosate (Roundup)
• conventional - canola not thought of as "herbicidetolerant" but is resistant to numerous in-crop herbicides

The main features of each weed control system are as follows:

Clearfield

• Clearfield products and registered tank mixes can be applied to Clearfield varieties
• control of various annual grassy and broadleaf weeds using post-emergent applications
• an extended control herbicide-offers control of some second flushes of certain shallow germinating weeds
• systemic mode of action with affected weeds showing stunting, yellowing/reddening within three to 10 days of spraying
• numerous open-pollinated varieties
• products with the same mode of action (Group 2) will not control volunteer Clearfield canola in subsequent years. Products may be co-packaged with a Group 1 or Group 4 product
• allows weed resistance management by switching from a Group 1 to a Group 2 herbicide or using a co-packaged product Group 2 + Group 4 or Group 2 + 1. Broadleaf resistance is a concern, especially on fields with previous Group 2 use. Group 1-resistant wild oats have been identified in all three western provinces
• Some restrictions in recropping in the year following application

Liberty Link

• Liberty herbicide and registered mixes can be applied to Liberty Link varieties
• control of various annual grassy and broadleaf weeds
• suppress some perennial weed species
• applied post-emergent, activity is through contact with green tissue
• affected plants show burning/yellowing within two to 5 days after application
• plants die one to two weeks after treatment
• varieties are widely available, primarily hybrids and some open-pollinated varieties
• unique mode of action (Group 10). Good option for grassy and broadleaf weed resistance management
• no restrictions on recropping in the year following application

Roundup Ready

• Glyphosate and registered mixes can be applied to Roundup Ready varieties. Not all glyphosate products are registered for Roundup Ready canola. Check product labels before using. This system is available to farmers who have purchased a Technology Use Agreement (TUA)
• control of various annual grassy and broadleaf weeds
• control or suppress some perennial weed species
• applied post-emergent, activity is systemic, with affected weeds showing yellowing at the terminal buds within seven to 10 days after application
• varieties are available widely-open-pollinated, synthetic and hybrids
• Roundup will not control volunteer Roundup Ready canola in subsequent years. This is important if following fields where Roundup is used for weed control before crop emergence
• unique mode of action (Group 9), good option for grassy and broadleaf weed resistance management for Groups 1, 2, 3 and 8
• no recropping restrictions in the year after application

Conventional

• various registered herbicides can be applied to conventional canola (conventional canola varieties are not developed for their resistance to a specific herbicide)
• control of various weeds with soil-applied postemergent herbicides
• these herbicides control some of the common weeds within fields-many growers need to apply more than one product to control all the common weeds (one reason why the HTC systems have been widely adopted)
• varieties are available widely, both as hybrid and open-pollinated varieties
• recropping restrictions vary from herbicide to herbicide

How to Choose a System

Every grower will have a different set of management goals to consider when picking a herbicide-tolerant canola. For example, there may be factors that make one weed control system the best choice for one field, but another system might be best for another field.

Use Weed Spectrum to Narrow the Choice

There is no better way to increase the effectiveness of a weed control program than by choosing the product that matches the weeds in a particular field. The best way to choose a weed control system is to narrow down the list of choices by what weeds they control and whether those weeds are present. This is the single most important consideration to use in narrowing the search for the ideal herbicide for a given field. Consider herbicide cost, too. Ordering early or in a company program may reduce costs.

One compelling argument for using weeds as the driver for choosing a canola weed control system is the fact that good varieties are available in every weed control system. For example, the need to compromise weed control choices to get the 'R' blackleg-rated variety is no longer there.

As a starting point, list the most important weeds and make a shortlist that consists of the weed control systems that best fit those weeds. In most cases, this will narrow down the choices to two or three systems.

List the Other "Non-Weed" Factors

Key factors other than weeds are important in the decision process. A variety or hybrid may have performed very well in the area, or blackleg management is critical for a given field and a 'R-rated' variety is essential. However, these factors must not take priority away from the weed control issue. Do not put a great performing variety into a field with a weed infestation that the available herbicide for the system will not control. Choose a competitive variety or hybrid with a herbicide system best matched to the weed spectrum in the field.

Go through the Table 1 list of considerations and rank whether each issue is extremely important (1), somewhat important (2) or not important (3) for each field. Assign a 1 to no more than two considerations per field.

Table 1. Priorities in Choosing Canola Weed Control Systems for a Specific Field

Considerations in Choosing a Herbicide-tolerant Canola	Importance of this Feature in this Field
	#1	#2	#3
Weed spectrum
Herbicide rotation considerations
Recropping issues
Blackleg resistance of the variety
Yield potential of the variety
Standability, harvestability, height of the variety
Days to maturity
Green seed issues

Match Variety Attributes That Suite the Features Marked as High Priority

Numerous information sources are available on harvestability, blackleg resistance or other attributes of varieties. Sources include provincial extension services, the company that distributes the variety and the Canola Council of Canada.

Use these sources to find which varieties have the highest ranking for the attribute considerations checked as #1 priority.

Then narrow that list down further by selecting the varieties that also give satisfactory results of the #2-rated priorities.

Time of Weed Removal

Weeds can be one of the most limiting factors in canola production and deciding when to control weeds is a complex decision. Growers often wonder if they should get rid of weeds early or wait until more weeds emerge to minimize passes. How far along does canola have to be so that any late emerging weeds are too far behind to inflict yield loss?

Time of weed control is a critical question for growing canola because of the competitive nature of canola. In earlier leaf stages, there is a high cost to leaving weeds. Young canola is very non-competitive as it is slow to cover the ground and weeds will likely win the competition for nutrients and soil moisture. But in later stages, the crop is more competitive, letting less than 9% of full sunlight down to the soil surface where late-emerging weeds are in near darkness.

Researchers at Iowa State University in Ames, IA developed a graph that depicts the goal in timing weed control operations (Figure 12). The graph shows that herbicide applications made too early can miss weeds that emerge later, causing yield losses. At the other end of the graph, yield losses are also high because late applications generally leave weeds within the crop too long. With even a few extra days, weeds can use enough moisture and nutrients to leave the crop short of these resources later in the season.

Figure 12. Time of Weed Removal Model

Theoretically, there should be an ideal application timing that is late enough to allow weeds to germinate, but not so late as to allow weeds to cause serious yield loss. The ideal timing is depicted in the graph at the bottom of the curve in Figure 12, where losses are minimal. The ideal timing will likely shift to the left (earlier) or right (later) depending on weed populations, types of weeds, weather and whether crop nutrients are limiting.

When weeds emerge relative to crop emergence can have an impact on the severity of yield loss. In studies at the AAFC Scott, SK Research Centre where 150 barley seeds/m² (124/yd²) were sown into canola prior to and following emergence, the time of barley emergence had significant effects on canola yields (Figure 13).

Figure 13. Effect of Time of Volunteer Barley Emergence on Canola

Yield Losses

The competitiveness of the weed species is also important in determining yield losses. The more competitive the weed, the fewer weeds needed to cause a yield reduction. Research by the Vegreville, AB Alberta Research Council (ARC) on wild oats has shown weeds that emerge before or with the crop cause greater yield loss than weeds that emerge after the crop (Figure 14). Additionally, fewer weeds tend to emerge after the crop has reached the four-leaf stage and those that do are usually weaker and spindly.

Figure 14. Yield Loss versus Weed/Crop Emergence

Farm practices can influence the time of weed emergence. Tillage just prior to seeding can assist in destroying weeds that have germinated, essentially giving the crop a head start. Where tillage has been carried out several days to a week before seeding, a new flush of weeds may already have started germinating. On-row packing provides good seed/soil contact to promote rapid germination and emergence. Timing of spring burn-off can also influence weed emergence. Significant differences in growth rates between weed species further complicate the appropriate time to control weeds. Cleavers, stork's bill and spiny annual sow thistle have rapid growth rates and even under cool conditions advance through their leaf stages while other species are delayed. Timing of control is even more critical when these species are present.

The benefits of early weed removal are supported by numerous research studies across western Canada. During the seedling stage, when the crop is susceptible to competition, weeds like wild oats and volunteer cereals can become serious problems (Figure 15).

Figure 15. Effect of Timing of Wild Oat Removal on Canola Yields (three-year average yield)

Researchers at AAFC Lacombe and Beaverlodge, AB Research Centres summarized nine site-years of spray timings of Roundup Ready canola at Lacombe, Beaverlodge and Ellerslie, AB. Glyphosate was applied at the 1- to 2-leaf stage, 3- to 4-leaf stage and 5- to 6-leaf stage of the crop. Normally the longer weed control is delayed, the greater the damage (Table 2).

Table 2. Crop Staging that Gave the Best Yields in Nine Site-years of Weed Control Trials in Small Plots in Alberta

Year	Lacombe	Edmonton	Beaverlodge
1997	Best yield when crop was sprayed in 1- to 2-leaf	Best yield when crop was sprayed in 1- to 2-leaf	Best yield when crop was sprayed in 1- to 2-leaf
1998	Best yield when crop was sprayed in 3- to 4-leaf	Best yield when crop was sprayed in 3- to 4-leaf	Best yield when crop was sprayed in 1- to 2-leaf
1999	Best yield when crop was sprayed in 5- to 6-leaf	Best yield when crop was sprayed in 3- to 4-leaf. The latest (5- to 6-leaf) application actually yielded better than the earliest	Best yield when crop was sprayed in 1- to 2-leaf

At Lacombe and at Edmonton sites in 1999, the latest applications yielded better because a significant number of weeds emerged after the early and mid glyphosate application. In this situation a second application of glyphosate would have been a good strategy two times out of nine. Dockage in this study was least when weeds were removed at the four-leaf stage. Earlier removal led to higher dockage because of a second flush of weeds, while late removal led to higher dockage due to reduced herbicide efficacy on weeds at an advanced growth stage. Overall early timings provide the best returns because the early emerging weeds do relatively more damage to yield potential than later emerging weeds. In seven out of nine cases, early timings (1- to 4-leaf stage) resulted in the highest canola yields of all treatments. In one case, the latest timing actually yielded the best. Early weed removal may not show a yield response under dry conditions such as at Lacombe in 1998 (Figure 16).

Figure 16. Time of Weed Removal effect on Canola Seed Yield

In trials conducted by the University of Manitoba in Winnipeg, MB at multiple locations over two years, it was necessary to control weeds by the four-leaf stage to prevent greater than 5% yield loss (Table 3).

Table 3. Time Canola can Tolerate Weeds before a 5 or 10% Yield Loss

Location	Year	Length of Time that Weed Infestation could be Tolerated for the Specified Yield Loss
		5% Yield Loss		10% Yield Loss
		Days Weeds could be Tolerated	Crop Leaf Stage	Days Weeds could be Tolerated	Crop Leaf Stage
Carman	98	43	8 to 10	Whole season	Harvest
Winnipeg	98	32	6	34	6-leaf
Carman (early)	99	38	4	38	4-leaf
Carman (late)	99	17	4	19	4-leaf
Winnipeg	99	34	8-10	Whole season	Harvest
Homewood	99	22	6	33	8- to 10-leaf

The length of time weeds could be tolerated until the weeds had caused a 5 or 10% yield loss varied from site to site. A 10% yield loss does not happen until the weeds are allowed to remain in the field to the four-leaf stage. Weeds that emerged after the four- to six-leaf stage seldom impacted actual canola yields to a 10% yield loss level. Concentrate on early emerging weeds and worry less about the later emerging weeds that come up after the crop has hit the four- to six-leaf stage. In two out of six fields, weed control was not needed at all to avoid a 10% yield loss. Therefore, growers can maximize returns by knowing whether to spray as well as when to spray.

Research by applied research associations in Alberta with seven farm field trials across the province found that waiting for late weeds to germinate did not pay. Their results showed a 224 kg/ha (4 bu/ac) yield increase by spraying at the 1-leaf versus the 5-leaf stage (Figure 17).

Figure 17. Effect of Time of Weed Removal and Yield of Canola in Alberta Fields (average of all trials)

Trials by the Canola Council of Canada at Crop Production Centres across western Canada show the impact of early weed removal in canola. Large plots were sprayed for weeds at the 1- to 2-leaf stage of the crop (early), the 3- to 5-leaf stage (mid) and at the 6-plus leaf stage (late). In some years, the mid-timing was missed or not included in the trials. In some years, two broader timings were used as opposed to three narrower timings. The results show a consistent trend toward better yields when application is aimed at the earliest timing (Table 4).

Table 4. Summary of Canola Production Centre Trials on Time of Weed Removal

Year	Location	Seeding Date	Yield at Stage Removed					Best Timing	Early Yield Advantage
			1- to 2-Leaf	1- to 3-Leaf	3- to 5-Leaf	4- to 6-Leaf	6- to 7-Leaf
1998	Russell, MB	May 13	25.4				18.7	Early	35.8%
1998	Whitewood	Apr 23	33.8				31.7	Early	6.6%
1998	Whitewood	May 08	28.2				25.6	Early	10.2%
1998	Naicam, SK	May 04	36.4				36.2	Early	0.6%
1998	Naicam	May 21	32.9				32.5	Early	1.2%
1998	Lethbridge, AB	Apr 21	28.1				28	Early	0.4%
1998	Lethbridge	May 01	25.4				26.4	Late
1998	Andrew, AB	Apr 28	33.8				33.3	Early	1.5%
1998	Andrew	May 08	34.7				34.3	Early	1.2%
1999	Carman, MB	Jun 07	35.8		31.2		28.1	Early	27.4%
1999	Naicam	May 09	40.6		38.5		36.8	Early	10.3%
1999	Russell	Apr 30		43		34.7		Early	23.9%
1999	Russell	May 27		40		39.2		Early	2.0%
1999	Naicam	May 08		38.4		36		Early	6.7%
1999	Naicam	May 21		35.3		34.2		Early	3.2%
1999	Delmas, SK	May 20	37.7		35.6		35.7	Early	5.6%
1999	Russell	NR	33.3		27.4		21.2	Early	57.1%
1999	Vegreville, AB	May 23	37.6		38.9		37.7	Mid
1999	Vegreville	May 06		46.1		45.4		Early	1.5%
1999	Vegreville	May 22		50.3		48.1		Early	4.6%
1999	Lethbridge	Apr 22		27.9		27.7		Early	0.7%
1999	Lethbridge	May 04		46.6		45.9		Early	1.5%
1999	Wanham, AB	May 06	38.2		37.2		29.2	Early	30.8%
1999	Rolla, B.C.	May 17	32.4				20.8	Early	55.8%
2000	Selkirk, MB	May 06	33.4				17.8	Early	87.6%
2000	Naicam	May 05	40.4		38.1		37	Early	9.2%
2000	Vegreville	May 12	40		40.8		29.8	Mid

While there are site-to-site and year-to-year differences, the ideal herbicide application timing on canola is consistently about the same time. In an overwhelming 24 seeding date/locations out of 27, the earliest timing gave the best yield. The early applications showed a 16% yield advantage compared to the latest timing.

The mid-timings (3- to 5-leaf stage) did not have better yields compared to the early timings. Waiting a few extra days means that more late germinating weeds may be up and controllable. But this did not translate into yield. The mid-timings were best in only three cases out of eight trials where three timings were included. Also note that early weed control holds true for earlyseeded or late-seeded crops.

These studies clearly show that control of weeds at the early stage of crop development is one of the most important requirements for growing a high yielding canola crop.

Herbicide Performance

The effectiveness of herbicides depends on many factors including:

• Apply the herbicide at the growth stage most susceptible to target weeds.
• Use properly calibrated application machinery.
• Apply the recommended rate of herbicide (read and follow the label carefully). Take every precaution to avoid harmful side effects. Take great care to avoid contaminating the water sources when filling sprayers.
• Apply the herbicide accurately and uniformly.
• Use the water volume stated on the label.
• Crop competition assists in good weed control. Adequate fertility will increase crop competition. Good crop growth will also suppress later emerging weeds.
• Higher temperatures and low relative humidity result in rapid drying of droplets and greater herbicide loss from evaporation. Crop injury may be increased with some products when both temperature and humidity are very high. Better weed control usually results when herbicides are applied during the cooler parts of the day.
• Soil moisture, organic matter and texture may affect soil-incorporated herbicides. Weed control is best when soil moisture conditions are optimum. Excessive moisture will prevent adequate mixing of the products. Soil high in organic matter and/or clay will require higher rates of soil-incorporated herbicide to obtain adequate control.
• A rain-free period after application of post emergence herbicides is required to allow time for the weeds to absorb the chemical.
• Water containing large amounts of organic matter, clay, and other particles will result in nozzle plugging and reduced weed control.
• Do not spray when wind velocity exceeds 15 km/hr (9 m/hr), or use a wind shroud on the sprayer.
• Most herbicides contain sufficient wetting agent in the formulation product. Do not add extra wetting agent unless indicated on the product label.
• If possible spray weeds that are actively growing. Herbicide activity is usually reduced when applied to weeds subjected to cold and/or moisture stress.

Herbicide Drift, Sprayer Contamination and Herbicide Residues

Spray drift from most herbicides not registered for the canola system being grown can injure plants. For some Group 4 herbicides, the least damage occurs at the 2- to 4-leaf stage of the canola plant, the most at flowering. Spray drift, soil residues or sprayer contamination of some Group 2 herbicides may severely damage canola plants. It is essential that sprayer tanks, booms and nozzles are cleaned out thoroughly, particularly after Group 2 herbicides, to prevent sensitive crop injury from subsequent spray applications. Follow label recommendations for all equipment cleanout. Carefully plan rotations and herbicides.

Incorporation of Soil-Applied Herbicides

The recommended procedures for mixing or incorporating pre-emergent herbicides into the soil are found on the herbicide label. Such procedures will usually give the desired results. Exceptions occur due to the wide variety of soil and climatic conditions. Tillage and incorporation procedures that are necessary for these herbicide applications are not favourable to soil moisture conservation and soil erosion prevention.

Herbicide

The depth of incorporation for maximum effectiveness varies with the herbicide and the weed to be controlled. Course textured soils and soils with low organic matter may require different herbicide rates than other soils.

Soil Conditions

Uniform application and proper incorporation is essential to receive full benefit from a soil-incorporated herbicide. Loose, mellow soils are ideal for mixing the herbicide uniformly to the desired depth. Hard and lumpy soils, and those that are wet and sticky make it difficult to get uniform and proper mixing.

Dry soils will, in some cases, limit herbicide activity. An incorporation procedure that dries out the soil just before or even after seeding can result in poor emergence of the crop. Timing and method of incorporation are important where weather conditions will dry out the soil. A loose or lumpy soil surface will lose moisture in fall and spring. Harrow and pack cultivated soils that are likely to dry out after every tillage operation. Granules placed on hard, lumpy soil before incorporation can fall between the lumps and go too deep, whereas liquid attaches to the soil. Therefore, if granules are to be used, it may be necessary to pre-work the soil to get it into a condition where the granules may be mixed within the desired soil layer. Lumpy or sticky soils may require several tillage operations to get them into a good condition for proper incorporation.

Where soil erosion is not a problem, fall application is usually preferable to conserve spring seedbed moisture. On some soils, the recommended incorporation procedure for either fall or spring may leave the soil so loose, soft and fine that it will be subject to both wind and water erosion. On these soils, consider a minimum incorporation if alternate methods of weed control are available.

Incorporation Equipment

Tandem disc harrows, discers and any type of disc type implement will provide deeper incorporation, more soil breakdown and faster and more uniform mixing to the depth of tillage than most other implements. Use these where deep incorporation is needed, where the soil is lumpy or sticky and where soil erosion is not a problem. The percentages of herbicide distributed at various depths by different tillage machines are shown in Table 5.

Table 5. Incorporation of Herbicides by Tillage

Machine Set at a Depth of 75 mm (3") and Speed of 8 km/hr (5 m/hr)	% of Herbicide Found at the Following Depths
	0 to 25 mm (0 to 1")	25 to 50 mm (1 to 2")	50 to 75 mm (2 to 3")
Tandem and offset disc	40.2	46.4	13.4
Discer	41.4	41.0	17.6
Field cultivator (vibrashank) with harrows	60.8	36.6	3.6
Chisel plow (H.D. cultivator) with harrows	57.3	35.4	7.2
Harrows (diamond dragtype) (tine harrows probably similar)	Most in the top 0 to 33	-	-

Tilling before applying a herbicide will produce a loose but smooth soil surface. This increases the depth and uniformity of herbicide incorporation.

Cultivators tend to give shallow incorporation and uneven mixing. Deep tillage of 10 cm (4") or more will give less soil disturbance. For good mixing, a higher travel speed of 10 to 13 km/hr (8 m/hr) is necessary. This higher speed throws soil upward and sideways to give more even incorporation. Higher speeds also help to break up lumps. Any brand of cultivator can be used, but shank spacing, speed, depth and kind of sweeps must be adjusted.

Harrows are very useful for shallow incorporation. Tine harrows behind cultivators contribute to better mixing due to the lively action of the tines. Drag harrows on stubble fields may not work well for incorporation if they are constantly loaded with a lot of trash.

Rod weeders and wide-blade machines are not capable of mixing herbicide in the soil. Do not use these implements.

In stubble, the most effective incorporation of liquid herbicide is obtained if the field is first worked until about two-thirds of the surface is black. Granules can be applied directly to stubble before tillage, but spread the straw bunches. Weed or volunteer crop growth on fields will also absorb liquid herbicide.

Choose the incorporation procedure after a careful assessment of the field, climatic conditions and herbicide requirements.

Canola Volunteer Control

Volunteer canola can be as easily managed in cereal and other crops as any broadleaf weed. In provincial weed surveys, volunteer canola ranks in the top 20 weeds found in crops across western Canada. Volunteer canola plants, as weeds, are quite competitive to cereal crops (Figure 18) and need to be controlled early (Table 6).

Figure 18. Estimated Yield Loss in Wheat and Barley from Volunteer Canola

Table 6. Wheat Yield Increases once Volunteer Canola is Removed

Canola Removed (days after wheat emergence)	Wheat Leaf Stage	Yield % Increase
7 days	2-leaf	212
14 days	3-leaf	189
21 days	5-leaf	176
30 days	-	120

Research at the AAFC Saskatoon, SK Research Centre found that the average loss of seed during canola harvest operations was over 101 kg/ha (90 lb/ac). This could result in extremely large numbers of volunteer canola plants the year following the canola crop.

Herbicide-Tolerant Volunteer Canola

The large-scale adoption of herbicide-tolerant canola (HTC) varieties in western Canada requires greater attention to volunteer management, especially in direct seeding operations. Volunteer HTC canola means that growers may need to adopt a new weed management strategy in their cropping systems. Products previously used to control volunteer conventional canola may not be an option with volunteer HTC canola. For example, after growing Roundup Ready canola the pre-seeding glyphosate burn-off the next year must now be used with additional products (for example, 2,4-D for cereals, bromoxynil plus MCPA for peas). In addition, although not common, some of the volunteers may be multiple-resistant due to cross-pollination between the various canola systems (conventional canola, Roundup Ready, Liberty Link, and Clearfield).

Check provincial weed control or crop protection guides for current recommendations for controlling herbicide-tolerant canola volunteers.

Pollen Flow and Outcrossing

B. napus is mainly self-pollinating with outcrossing rates of 20 to 30%. The vast majority of canola pollen, which is heavy and sticky, falls to plant surfaces and the ground within a few metres of its source. Canola pollen is also very small-one-third the size of corn pollen and one-half the size of grass pollen. A small percentage (5 to 10%) becomes airborne and floats like dust particles on the wind. Wind is the main pollen mover. Most pollen remains viable for a minimum of 24 to 48 hours and for longer periods under ideal conditions. Bees only do a small amount of outcrossing as each bee normally feeds on about four flowers before being fully covered with pollen. Bees, however, do forage about 2 to 10 km (1.2 to 6.2 miles). Volunteers are another source of pollen both in the field and beside the field. Pollen may also be dispersed by other insects, on clothes and in admixtures of seed.

Pollen from plants of any of the herbicide-tolerant systems can outcross to any nearby canola plants whether or not they are the same or of a different system. In other words, the pollen of herbicide-tolerant canola plants can outcross to nearby non-herbicide-tolerant canola or canola with other herbicide tolerances.

Research studies at the AAFC Saskatoon Research Centre in the 1970's showed that outcrossing levels or pollen flow from large commercial fields to small plots tended to be at low levels (46 m (151') = 2.1%, 137 m (449') = 1.1% and 366 m (1201') = 0.6%). A 1998 research study found substantially lower outcrossing rates between large commercial fields ranging from 0.1 to 1.5% at 20 m (66') to 0.1 to 0.4% at 100 m (328'). A University of Manitoba trial using small fields also reported low levels of outcrossing (Figure 19).

Figure 19. Per cent Outcrossing by Distance from Source in Small Fields

Research studies at the AAFC Saskatoon Research Centre have shown that isolation distances of 100 m (328') are normally sufficient for purity of pedigreed seed. Regulations for production of hybrid seed specify isolation distances of at least 800 m (2,625') to ensure that no stray pollen fertilizes the female plants.

Despite the low probability of outcrossing, the large number of canola flowers and small seed produced ensures a substantial number of outcrossed seed could still be produced. Some seed may shatter onto the ground before or at harvest and germinate the following season with the succeeding crop. At a 0.2% outcrossing rate in a field yielding 1401 kg/ha (25 bu/ac) a shatter and harvest loss of 3% would result in about 4 seeds/m² (10,000 seeds/ac). Although nearly all the plants originating from such seed would normally be killed by frost, herbicide treatment and/or tillage, some could survive to compete with the succeeding crop and warrant further chemical or mechanical control.

Multiple Resistance Canola

In 1998, outcrossing between canola varieties was documented in a grower's chem-fallow field in northern Alberta. Pollen flow from an adjacent field in 1997 conferred herbicide tolerance to volunteers. As the grower did not anticipate this, the same herbicide was applied to control the volunteers, and it was ineffective. Multiple herbicidetolerant canola volunteers were identified, as would be expected through outcrossing. Volunteer canola plants tolerant to Liberty, Roundup and ALS inhibitors and combinations of these are likely present in many fields across western Canada. The volunteer plants most likely to be noticed by growers will be Roundup-tolerant because Roundup is used for chem-fallow and pre-seeding burn-off, while Liberty, Navigator and ALS inhibitors like Pursuit are not. Similarly, ALS inhibitor-tolerant volunteers may remain in cereal or pea fields that used only Group 2 herbicides.

Control of Volunteer Canola

Control of volunteer canola, whether herbicide-tolerant or not, can be achieved by a variety of methods currently used by growers. For effective control of volunteers:

• Keep good records. Effective weed control strategies depend on long-term records for each field regarding crop rotations and herbicides applied.
• After harvest leave canola seeds on or near the soil surface as long as possible. A high percentage of seeds left on the soil surface will germinate in the fall and be killed by frosts while remaining seeds will germinate in the early spring and can be controlled either by frost, through tillage or by herbicides in the succeeding crop. Canola seeds can only become dormant when hydrated but prevented from germinating by water stress or oxygen deficiency. Induction of dormancy can occur only in the dark and is prevented by light. The closer seeds are to the soil surface the less likely they are to persist by developing dormancy. Seeds of B. napus varieties that are incorporated into the soil develop induced secondary dormancy and can persist for up to four years in the soil.
• Avoid tillage in the spring to allow for maximum volunteer emergence. Use tillage immediately prior to seeding to control volunteer canola. Tillage can play an important role in weed control in conventional systems, and growers can maximize weed control benefits by shortening the interval between tillage and seeding operations.
• Rotate canola with cereal, pea and forage crops. Diversifying a rotation permits use of a wider selection of herbicides. Lengthening the rotation depletes the volunteers from the soil weed seed bank.
• For pre-seeding or chem-fallow weed control on canola stubble, add a phenoxy like 2,4-D to glyphosate or use a herbicide like Gramoxone PDQ. Control of volunteers on or adjacent to Roundup Ready canola stubble will not be effective with glyphosate alone.
• Use herbicide mixtures when using Groups 2, 6, or 9 products. All canola volunteers regardless of system are controlled by inexpensive Group 4 herbicides such as 2,4-D and MCPA.
• Rotate to herbicides not in Groups 2, 6, 9 or 10. Rotation of herbicides is important for weed resistance management and for the control of herbicide-resistant volunteer canola.
• Include silaging and green manuring to control volunteers.
• Isolate fields with different herbicide systems. Do not grow them side-by-side in the same field or across the fence line. To minimize the amount of field-to-field crossing, the current research data suggests a minimum of 175 m (575') isolation between fields.
• Scout fields for volunteer canola that are not controlled by herbicide application. Early detection allows time for control before seed set.
• Grow competitive crops. Increase seeding rates, choose competitive varieties, seed early, and place a balanced fertilizer close to the seed.
• Reduce the loss of seed during canola harvest. Swathing at between 20 and 35% seed colour change on the main stem reduces shatter loss. Properly adjust the combine so that seed losses are minimized. Higher combining speeds increase seed loss.
• Use certified seed. Pedigreed seed use reduces the probability of seeding multiple herbicidetolerant canola that arose due to outcrossing in common seed source fields without adequate isolation or separation.

Weed Resistance to Herbicides

Herbicide resistance is of increasing concern in crop production. A 1999 AAFC survey estimated that 2% of Saskatchewan fields and 27% of Manitoba fields had wild oats resistant to herbicides from more than one group. Repeated application of the same herbicide group on a field will promote herbicide-resistant weeds. Studies have shown that Group 1 resistance can develop after just five to 10 applications. Resistance may appear more quickly in areas of heavy wild oat infestation due to a larger genetic variability and more total mutation possibilities in those areas. When planning cropping decisions and annual weed control decisions, use strategies to reduce the risk of developing herbicide-resistant weeds.

Saskatchewan fields and 27% of Manitoba fields had wild oats resistant to herbicides from more than one group. Repeated application of the same herbicide group on a field will promote herbicide-resistant weeds. Studies have shown that Group 1 resistance can develop after just five to 10 applications. Resistance may appear more quickly in areas of heavy wild oat infestation due to a larger genetic variability and more total mutation possibilities in those areas. When planning cropping decisions and annual weed control decisions, use strategies to reduce the risk of developing herbicide-resistant weeds.

In western Canada, a number of weeds have already been identified as resistant to a variety of herbicides including those listed in Table 7.

Table 7. Occurrence of Herbicide-resistant Weeds in Western Canada

Weed Type	Species	Herbicide Groups	Province
Grasses	Wild oat	1	MB, SK, AB
	Wild oat	2	MB, SK, AB
	Wild oat	8	MB, SK, AB
	Wild oat	1,2,25	MB, AB
	Wild oat	1,2,8,25	MB
	Wild oat	1,16	AB
	Green foxtail	1	MB, SK
	Green foxtail	2	MB, SK
	Green foxtail	1, 3	MB, SK
Broadleaf	Chickweed	2	AB
	Cleavers	2	AB
	Cleavers	2, 4	AB
	Hemp nettle	2	MB, AB
	Kochia	2	MB, AB
	Spiny annual sow thistle	2	AB
	Russian thistle	2	AB, SK
	Wild mustard	2	MB, AB
	Wild mustard	4	MB
	Wild mustard	5	MB

Herbicide products are grouped according to the mode of action or way in which they act upon a weed (Table 8). Some herbicide products contain more than one active ingredient that may appear in more than one herbicide group. One important way to help reduce the potential for herbicide resistance is to rotate herbicides. However, it is not enough to simply rotate herbicide products. Rotation must be between herbicide groups.

For current information check provincial guides to weed control or crop protection or check the Canola Council Web site at www.canola-council.org.

At one time it was thought that giving weed populations a "break" from a herbicide would allow the susceptible weed populations to increase in number at the expense of resistant populations. It was thought that resistant weed populations were universally poorer competitors than susceptible populations and, therefore, would eventually decline in numbers when faced with competition from susceptible populations. However, research work conducted in the late 1980's and early 1990's began to identify weed species where resistant individuals were not poorer competitors or less fit than susceptible ones. It is for this reason that researchers are rethinking herbicide resistance control strategies and many are suggesting that, while herbicide rotation is still essential, other management practices must be used together with herbicide rotation.

Weed	Herbicide Group	Herbicides
Wild oat and green oxtail	Group 1	Assure II, Freedom Gold, Fusion, and green Hoe-Grass 284, Muster Gold, foxtail Muster Gold II, Poast Ultra, Pursuit Ultra, Select, Venture
	Group 2	Absolute, Odyssey, Pursuit,
	Group 3	Edge, Trifluralin
	Group 8	Avadex, Fortress
	Group 9	Glyphosate, Eclipse
	Group 10	Liberty
Broadleaf weeds	Group 2	Absolute, Muster, Odyssey, Pursuit, Freedom Gold
	Group 3	Edge, Trifluralin
	Group 4	Absolute, Eclipse, Lontrel
	Group 9	Glyphosate, Eclipse
	Group 10	Liberty

With conventional canola it is difficult to manage herbicides in rotations that include canola because of the need to use a Group 1 herbicide for grassy weed control in canola. The current HTC varieties are tolerant to non-Group 1 herbicides so now canola can be grown without using a Group 1 product. This also provides herbicide options for other crops. HTC varieties offer greater flexibility in field selection because of the broad weed spectrum controlled. HTC canola will allow growers to produce a cleaner crop more efficiently, while effectively managing herbicide resistance. The use of HTC varieties allows growers to not use products for grassy weed control from the same group any more frequently than "one year in three."

There are herbicides in Groups 1, 2, 3, 8, 9 and 10 for grassy weed control in canola and herbicides in Groups 2, 3, 4, 9 and 10 for broadleaf weed control.

Some of the HTC herbicides provide good control of resistant weeds, including Group 1-resistant wild oats. For canola growers that already have weed resistance, the use of HTC varieties may be the only way to grow canola.

In the past, direct seeding operations were unable to use a soil-incorporated Group 3 herbicide and were limited to Group 1 herbicides. HTC herbicides offer increased options for herbicide rotation, an extremely important tool for resistance management in direct seeding systems.

However, HTC is only one tool in the weed control toolbox. Used properly HTC canola can be an important tool in a weed management program. Use an integrated weed management (IWM) strategy that involves a variety of cultural and chemical weed control techniques.

IWM strategies for weed resistance include:

• Do not rely solely on herbicides for weed control.
• Keep accurate records of crop rotation and herbicide use for each field.
• Develop a field-specific long-term weed management plan.
• Scout fields after herbicide application and note weed escapes or species shifts.
• Use non-selective herbicides. Where possible use herbicides like paraquat or glyphosate as alternatives from time to time as part of an overall weed control program.
• Avoid repeated use of one or more similar herbicide.
• Use herbicide-resistant varieties where possible.
• Avoid herbicides with long residual activity.
• Reduce use of herbicides. If possible skip a year, particularly in crops where weed control is not as critical. At the very least, don't try to control every weed in every field.
• Use cultural weed controls, including tillage where practical. Where tillage is not an option, consider delaying seeding to allow for maximum weed seed germination. Then spray with a non-selective herbicide.
• Use clean seed.
• Rotate both crops and herbicides. When rotating herbicides, use products with different modes of action. Plan rotations in such a way as to maximize the ability to use alternate herbicides or weed control practices.
• Rotate field operation timings. Vary the crops in rotation to change time of seeding from year to year.
• Follow label directions regarding management practices and restrictions.
• Use good sanitation practices. Avoid spreading crop seed, weed seed, crop residues or manure from suspicious fields.
• Use mixtures or split applications of herbicides with different modes of action.

A good set of field records is key to minimizing the potential for developing resistant weeds. Records allow accurate determination of the weed control history of every field and will make planning effective weed control strategies much easier. In addition, good records assist in determining the probable causes for a weed control failure should it occur.

For the most up-to-date information on herbicide resistance, contact the provincial weed specialist or government and university weed researchers.