Zero tillage relies on herbicides to control weeds. With numerous herbicides available and with new ones being developed, they are and will be, an effective management tool. The trick to managing weeds, however, is to keep their numbers low and to limit their spread. This is done by rotating many things, particularly herbicides and crops. Integrated Weed Management (IWM) means doing things that give the crop an advantage over weeds.

Our ability to continue zero tilling will depend on our skill in successfully managing weeds. Zero tillage can permit cost effective and efficient weed management which makes it an attractive system. However, weeds that are difficult or costly to control might reduce zero tillÌs attractiveness. Integrated weed management1 will reduce our reliance on herbicides.

Good weed management is preventing a build up of hard to control weeds and reducing yield losses from them. Trying to eliminate weeds is futile and expensive. Attempting it may even encourage herbicide resistance. We need to avoid the over-use of herbicides in zero tillage by integrating a balance of management and technology tools. In short, we need to use integrated weed management.

PREVENT WEED PROBLEMS

By using clean seed and equipment and controlling weeds in field margins, new weeds will be kept out. Controlling weeds in field margins is important for zero tillers as this is the source of problem weeds like dandelion, goatÌs beard and scentless chamomile.

Field weed seeds can be removed by collecting chaff. In many cases, over 200,000 weed seeds per acre have been removed by chaff collection. Most of them can be killed by turning them into silage or compost with livestock manure.

Chaff collection helps reduce weed spread.2 About 85% of weed seeds entering a combine end up in the grain tank. The remaining 15% are expelled from the combine, up to 140 m (430 ft) from where they were harvested (Figure 1). Collecting the chaff reduces this spread.

This tool along with precision agriculture, where weed patches are mapped and dealt with on a map unit basis, should prove effective when used together.

KEEP WEED NUMBERS LOW

There are six ways to do this:

• vary seeding dates in a field from year to year
• use pre-emergence herbicides in the rotations
• encourage quick crop emergence before weeds
• spray in-crop weeds early rather than waiting for all weeds to emerge,
• seed soon after spraying burn-off treatments
• minimize soil disturbance between rows at seeding

If these techniques are used effectively then occasionally in-crop spraying can be avoided. However, if this option is used, the weeds will need to be watched to ensure that their densities do not build up to difficult levels in subsequent years. A risky technique some use is to spray a "pre-emergent" burn-off after seeding. When it works, it gives good weed control, as newly emerged weeds are killed and the crop can get a jump on later emerging weeds. However, if rain or wind after seeding prevent spraying, weeds will escape control and affect crop establishment.

HELP CROPS CHOKE WEEDS

Give crops the advantage over weeds by using high quality seeds that produce vigourous seedlings and by using competitive crop types and varieties.

Other agronomic tools like high seeding rates and shallow and uniform seeding depth will help the crop emerge quickly and compete with weeds. If a crop has an advantage over weeds then spraying may not be necessary.

KEEP WEEDS "OFF-BALANCE"

Changing weed management techniques in a field year after year makes it hard for weeds to adapt and is a powerful tool. Using different techniques does not allow one type of weed to dominate and will delay the development of herbicide resistance.3

Cropping plans should include rotating competitive and non-competitive crops, seeding dates within a field from year to year, herbicides groups, pre and post-emergence herbicides, pre and post-harvest herbicide windows and crops with different life cycles, including perennial and winter annual crops.

Herbicides can be applied in annual crops during four windows: before seeding, in the crop, pre-harvest and post-harvest (Figure 1).

Using all four options may not be necessary if crop sequencing is planned (Figure 2).

If weeds are controlled the previous fall, then pre-seeding herbicides may not be needed the next spring. Delaying seeding in some fields allows most weeds to be killed with pre-seeding herbicides. However, with early seeding, in-crop herbicide use is more likely as weeds will emerge with, or shortly after, the crop (Figure 3).

With delayed seeding, in-crop herbicides may not be required, especially in zero tillage where weeds have not been stimulated by tillage. It can be hard to reduce herbicide use during mid-season seeding as both burn-off and in-crop treatments are needed.

A multi-year weed management planner can be used to see how crop rotations and herbicides effect weed communities. For example, a four year pea-wheat-canola-wheat sequence where only pre-seeding and in-crop treatments are applied, will select for Canada thistle, dandelion, or winter annuals that germinate in the crop (Figure 4). Many weeds become problems with this approach.

Perennial weeds like dandelion and Canada thistle, as well as new winter annuals, like cleavers and night flowering catchfly will germinate under a maturing crop and are difficult to control with herbicides the next season.

A more balanced approach is to use all of the windows over several years within a field (Figure 5 above). In this example, pre-harvest herbicides are applied one year in four and post-harvest herbicides are used every year to reduce the build up of perennial and winter annual weeds. Where perennial and winter annual weeds are increasing problems these non-traditional herbicide windows of more than one year in four may be necessary.

Examining your three or four year crop rotation and herbicide use patterns may explain why new weeds appear. Crop rotations which give balanced herbicide windows will suppress weed changes and reduce weed densities.

"With my no-till program, my goal is to use less chemical. I don't spray much in-crop for broadleaves - some, but not much. I'm not paranoid about weeds, I can accept a few out there. The expense trade-off is important. We probably shouldn't be looking at totally weed free fields."

Darrel Oech, Beach, North Dakota

OTHER BASIC ISSUES

Many weed management issues with zero tillage systems have been discussed in the first manual produced by the Manitoba-North Dakota Zero Tillage Farmers Association. New zero tillers would find this information very useful. It covers issues like the need for a good marking system for spraying into standing stubble and gives farm experiences in controlling specific weeds not discussed here.

SPRAY WATER QUALITY

Some herbicides are severely inactivated by poor water quality, especially glyphosate.4 Calcium and magnesium, in particular, reduce glyphosate activity. Other ions, like sulphate, chloride, sodium, and bicarbonate can cause problems.

It is important to test your water for hardness to see if there is a problem. The hard water problem can be reduced by using full herbicide rates, mixing ammonium sulphate fertilizer (21-0-0-24) to the spray tank before adding the glyphosate and by keeping water volumes low (Figure 6).5

HERBICIDE GRANULES

Ethylfluralin, trifluralin and triallate applied as granules work well in conventional tillage, especially in canola. Given their spectrum of weed control and the need to mix herbicides groups to avoid herbicide resistance, they would also be useful in zero tillage.

These granular herbicides are not registered for surface application without incorporation. However, they have been effective in zero tilled field trials without incorporation (Table 1).5 In a worst case scenario of; early seeding, cool soil and a non-competitive crop, this technique has given 80% control compared to 90% control with post emergent herbicides.

Why should these granular herbicides work without tillage? Incorporation with tillage puts the herbicides near germinating weeds while reducing atmospheric losses. However, with zero tillage, weeds emerge from near the surface. Therefore, these herbicides may not need to be incorporated to get them close to weed seeds. The greatest success with non-incorporated herbicides occurs when they are applied to a field that has been zero tilled for at least three years. In these cases, the weed seeds are near the soil surface. The granules are applied just before freeze up to reduce herbicide losses.

Non-incorporated surface application works just as well as incorporation with harrows. But for best results, use these herbicides with mid-season sown crops, so that early escapes are controlled by pre-seeding burn-off treatments. In low crop residue situations in Australia, the liquid formulation of these products has been very effective when applied at seeding, ahead of narrow point seed openers.6

CONTROL OF WINTER ANNUALS

Recommendations for the control of common winter annuals like stinkweed and flixweed are available in most weed control guides and spraying can occur up until freeze up.

The control of "new" winter annuals like cleavers, night flowering catchfly, storkÌs bill, common peppergrass, and blueburr is less well understood, with few recommendations available. These weeds are protected by the insulating layer of snow trapped by zero tillage stubble.

In-crop control methods are recommended for these weeds, but they do not work well for plants that have overwintered in zero tillage. Control of these weeds may require early or late post-harvest use of higher rates of phenoxy herbicides than are commonly used to control other winter annuals or fall use of herbicides normally used in-crop or with glyphosate. More research is required here.

SOIL HERBICIDE RESIDUES

Using 2,4-D or dicamba in the fall or spring can damage following crops. However, in western Canada, zero tilled canola, peas and lentils have not been affected by registered rates of 2,4-D that were used in the previous fall.7

However, fall or spring use of dicamba or spring use of 2,4-D has thinned crops and reduced yields (Table 2). In sensitive crops, residues from chlorsulfuron and metasulfuron may last 1-2 years longer in zero tillage.8 Some farmers with high pH soils have seen residual problems lasting 9 years. On the other hand, imazamethabenz carry-over effects on canola and wheat were less in zero tillage than in conventional tillage.9 It is best to be cautious with herbicide residue risks.

For potentially sensitive crops, choose pre-seeding herbicides like glyphosate that have no soil residues.

"Herbicide resistance won't beat us! It can't! We will just have to do creative things and be determined to find new ways around the problem. There are more tools coming available every year. Zero till offers too many benefits for us to just let it go"

Bill Crabtree, West Australia

STUBBLE AND HERBICIDES

Extensive research in standing stubble or with surface residues shows that post-emergence herbicides work just as well with zero, minimum or conventional tillage systems.10 However, to be sure of good herbicide activity use properly calibrated, good spraying equipment.

Interestingly, crop residues can control or suppress weeds. The residues create a poor seedbed for weeds to establish in and they have a negative chemical effect on weeds. The residues also promote increased pathogens and insects that can feed on weed seeds.

PERENNIAL WEED CONTROL

Controlling most common perennial weeds like Canada thistle, quackgrass, and perennial sow-thistle is almost the same in zero as in conventional tillage. These weeds should be suppressed or controlled at every opportunity. Use targeted measures, such as high rates of clopyralid on Canada thistle, when necessary. Newly registered pre-harvest RoundupÆ works well on these weeds and is an effective part of a perennial weed management system.

Dandelion: can be effectively controlled with pre-harvest glyphosate (Figure 7). Fall applied 2,4-D at normal rates for winter annuals improves dandelion control, especially several years after the application of pre-harvest Roundup(R). Pre-seeding and post-harvest glyphosate use are also registered for dandelion control.

Foxtail barley: seedlings can be killed with pre-seeding glyphosate which will also suppress established plants. Using pre or post-harvest glyphosate gives better control than pre-seeding. Other useful tools include using higher seeding rates and banding rather than broadcasting fertilizer.11 A multi-year approach is required to manage this weed in conservation-tillage systems. If densities become too great tillage may be required.

Yellow toadflax: is an increasing weed problem for no-till farmers in parts of Alberta and Saskatchewan. Yellow toadflax can be controlled by the use of pre-harvest RoundupÆ. Increased crop seeding rates also helped suppress toadflax.12

SNOW SEEDED CROPS

Canola sown during fall freeze-up, will germinate in the spring and develop normally.13 Using this technique with glyphosate and glufosinate resistant canolas is a useful new weed management tool. Since the crop emerges early in the spring it helps to keep weeds off balance by effectively competing against them. The canola crop can finish flowering before the summer drought sets in. True winter-type canola does not survive in the northern plains.

HERBICIDE RESISTANCE

Prevention is the best cure. Since zero-tillage farmers have a greater reliance on herbicides, following good IWM practices by rotating herbicide groups and applications among all herbicide windows and avoiding the over-use of any one herbicide is important. Remember, herbicide resistance could occur to any product.

TILLAGE

Some zero tillers have used some tillage to control foxtail barley or dandelion. At times this may be necessary, but watch your weeds the next year. In wet years, some growers have used light tillage to help dry out wet areas. However, they have often found that weeds that had not been major problems since they stopped tilling, such as green foxtail, were back in high numbers. These weeds can be controlled with herbicides but quick management changes may be needed.

YIELD LOSS DUE TO WEEDS

Recent research determining if yield losses from weeds are with equal with all tillage systems.14 Where the weeds were removed and no herbicide used, zero tilled wheat and lentils gave higher yields than conventional tillage in most years.

This could be due to fewer and also later emerging weeds and vigourous crop competition due to more soil moisture in zero tilled field. This work may help explain why yields from zero tilled fields are generally higher than conventional tillage, despite the presence of similar numbers of weeds at harvest.15

FALLOW IN ROTATION

Fallow is not a neutral force on weeds. Wild buckwheat can increase in rotations with tilled fallow, wild tomato can increase in minimum-tillage fallow (tillage and herbicides), and dandelion and foxtail barley may increase in chemical fallow.16 Chemical fallow should be minimized and not relied on to avoid the build up of hard to control weeds, higher herbicide use and herbicide resistant weeds. Chemical fallow is difficult to sustain as weeds that survive may set many seeds.

FERTILIZER PLACEMENT

Banding fertilizer greatly reduces the density of green foxtail, especially in zero tillage.17 With similar wild oat densities in a flax crop, banded nitrogen reduced yield loss.18 High soil disturbance at seeding also increased yield loss.

HERBICIDE RESISTANT CROPS

These crops allow the use of broad spectrum herbicides from different groups and give new weed management options. For example, early planting of crops resistant to these herbicides could mean that pre-seeding burn-off treatments are not needed. Herbicide resistant crops that volunteer the following season may require special attention.

REDUCED HERBICIDE RATES

Why will reduced rates work in some situations and not others? Herbicide rates are registered to work in average environmental conditions and cropping situations. If conditions favour herbicide activity, or if the crop has a competitive advantage over the weeds (IWM) then reduced rates can work. However, there is not a 1:1 relationship between herbicide rate and weed control. In other words, a 20% reduction in rate does not necessarily mean 20% less weed control. It could mean no weed control difference, in good conditions or 50% less weed control. Experience is required to make this approach work consistently. Be aware that reducing rates leaves you without company support.

FUTURE

Cropping systems in the northern plains are moving toward less soil disturbance with efficient use of herbicides and fertilizers.19 Successful weed control in zero tillage will require multi-year IWM strategies. Future good weed control lies not in a new "silver bullet" but in a systems approach.

Spraying Equipment

Pesticides should be applied efficiently, at the right time and with minimal spray drift. The nozzle is probably the most important part on the sprayer. It controls the flow rate, produces the spray pattern and uniformly directs the chemical to the sprayed surface. Nozzles wear so they need to be replaced regularly. For good results operating pressure should be uniform over the boom, nozzle spacing should be uniform and the boom should operate at the proper height above the sprayed surface for the nozzle being used. It should also be operated at a constant speed, unless using a spray controller. Even then, speed changes should not exceed the capabilities of the controller.

On occasions, pesticides will drift and cause damage to susceptible nearby crops. It is not possible to eliminate all drift but it can be reduced. Using new low drift nozzles, lowering the boom, increasing application rates and using spray shields and drift control agents will help. Spray shields are becoming popular and they can reduce drift by 50%. Spray shields will not stop all drift so caution is still sensible when spraying near susceptible crops.

A new type of sprayer is being introduced in the northern plains. It is the air assist sprayer that uses a high velocity air stream to help carry chemical to the sprayed surface. Limited information shows more spray is carried into the plant canopy but there may also be more spray drift than conventional sprayers.

Vern Hofman, Extension Agric. Engineer

Fargo, North Dakota

"Competition, sanitation and rotation are my three cultural practices. Herbicides to me are only something that aids my crops competitiveness - we're not going to eradicate all weeds. What I'm out there to do with a herbicide, when I need it, is to give my crop the competitive advantage"

Dwayne Beck, Dakota Lakes Res. Centre, Pierre, South Dakota

Prepared from information provided by:

Doug A. Derksen, Agriculture and Agri-Food Canada

Box 1000A, Brandon, MB R7A 5Y3

Telephone (204) 726-7650 Fax (204) 728-8358

Email "dderksen@em.agr.ca"

References:

1. Kelner DL et al (1996). MB & SK Ag Dept. ISSN 0840-9447
2. Shirtliffe S and Entz (1996). U of Manitoba research
3. Derksen DA (1995). Proc Agron Wkshp. Red Deer, AB
4. Holm FA et al (1994). Sask Agric and Food. Farm Facts
5. Kirkland KJ (1996). Proc Direct Seed Wkshp Feb 14, SK
6. Curtin S (1995). W Australian no till newsletter, May
7. Holm FA (1994). Proc Crop Rotations Conf Sask. Jan 24
8. Loeppky HA and Derksen (1992). Soil & Crop Wkshp. U of SK
9. Derksen DA (1995). Proc Soils and Crops. U of Sask
10. Derksen DA et al (1995). Weed Res 35:311
11. Blackshaw et al (1997). PARI fact sheet (in press)
12. Harker KN et al (1995). Cons Wkshp Nov 26, SK
13. Kirkland K (1996). Scott Exp Farm, SK
14. Derksen DA and Hume (1997). PARI fact sheet (in press)
15. Derksen et al (1997). PARI fact sheet (in press)
16. Derksen DA et al (1994). Weed Sci. 42:184
17. OÌDonovan JT et al (1995). Cons Wkshp Nov 26, SK
18. Derksen DA and Lafond (1995). Proc D Seed Wkshp, SK

Lafond GP and Derksen (1996). Can J. Pl. Path 18:151