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EAP Publication - 72
Mechanical weeding versus herbicide
Stale Seedbed (or False seeding technique)
Weeds and soil fertility
Sources of equipement
Mechanical weed control allows farmers to reduce or even eliminate herbicide use, and contribute to a better environment. In cereal crops, it costs the same or less than chemicals while still providing a satisfactory weed control.
Mechanical weeding is not widely used for cereal crops because many farmers believe it can reduce yields. In fact, mechanical weeding may do the opposite as it aerates the soil, therefore stimulating crop growth.
During mechanical weeding, weeds are mostly destroyed when buried by moving soil. Tools like the rotary hoe or the finger weeder, used at pre-emergence and/or post-emergence of the crop do this well. The number of passes required varies with the level of infestation. One to two passes are generally needed. Very weedy fields may require three passes while others may require none.
Mechanical weeding can be used for all spring and winter cereals in any type of soil except organic (black) soil. It can also be used for mixed grains even with peas in the mixture. However, it is not ideal for underseeding unless done very early before hayseed germination. Other options for underseeding are to mount a grass seed box on the mechanical weeding device to seed while weeding or to broadcast the seeds immediately prior to the weeding operation.
Mechanical weeding is effective against most annual weeds like wild mustard, lambsquarter, annual grasses, etc. It will also be effective against young biennial seedlings like dandelion. It is however not effective against perennials like quackgrass or deep-rooted annuals like wild oats. Wild oats can be controlled mechanically but this requires a lot of experience (see Hanley, 1984). It is therefore important to get rid of these weeds in a field before adopting mechanical weeding.
Get experience with mechanical weeding on small acreages first and on lightly infested fields because it takes a few years to get used to it. Use mechanical weeding as part of a general weed control strategy that includes good crop rotation, competitive varieties and cover crops.
You should increase seeding rate by 10% to compensate for losses during weeding. Moreover, a higher rate of seeding provides better competition against weeds. Although not necessary, some farmers prefer to plant deeper (3-4 cm) so that they can go in the field later for pre-emergence mechanical weed control.
MECHANICAL WEEDING VERSUS HERBICIDES
Advantages Disadvantages Mechanical weeding
* Maintains yields
* Maintains or reduces cost of weed control
* Aerates soil, stimulates crop growth
* Reduces pollution
* Breaks soil crust
* Leaves on average 20% more weeds in the field than herbicides
* Timing is critical
* Stony surfaces reduce degree of coverage
* Needs drier soil conditions to operate
* Give nearly complete weed control
* Cover large areas in less time
* Result in pollution
* Are subject to price fluctuation
* Create weed resistance problems which reduce herbicide effectiveness
* Involve health risks for the applicator
CHOOSING THE EQUIPEMENT
The most common tools for mechanical weed control in cereals are finger weeders and rotary hoes. Chain harrows can also be used on loam to sandy soils. In dry regions, the rodweeder can be used for pre-emergence weed control.
Width of the equipment is another factor to consider. Width depends on acreage to be covered (present and future) and tractors available.
Finger weeders versus rotary hoes
Advantages Disadvantages Finger Weeders
* Have adjustable tine tension
* Compared to rotary hoes can destroy weeds at later stage than cotyledon because of stronger action
* Are not good in high residue situations
* Should not to be used before two-leaf stage
* Require more time than rotary hoes
* Are more expensive to use than rotary hoes
* Can be used as early as two-leaf stage of cereals
* Cover larger area in the same time than finger weeder
* Require twice as much HP as finger weeders for same working
A rotary hoe, with its dozens of closely adjacent wheels, works by throwing the soil as it rolls shallowly over the field. Some models are made specifically for high residue conditions such as those found under minimum tillage systems. The most common brands are John Deere, M & W, Case International and Yetter. The prices vary from $ 4 000 to $ 12 500 CDN depending on the width and whether the model can work in high residues. Widths vary from 4.6 to 9.1 meters, the wider model requiring up to 100 HP.
The rotary hoe has a mild impact on the soil surface. It is therefore very important to use it when the weeds are just coming through the soil (white stage) or starting to emerge (cotyledon stage). This last weed stage lasts 4 to 8 days depending on the climate. The stage of the crop is not as critical as for the finger weeder because the rotary hoe has a more gentle action. The best results are obtained in slightly dry and crusted soils. Results will be better in a sandy soil if it is still a little moist. Two to three passages are usually necessary.
Weeding strategy for the rotary hoe
Crop Stage Weeding Pre-emergence Almost obligatory, 3 to 7 days after sowing at a speed of 12-20 km/h. 1 leaf 6-8 km/h. Be very cautious. Only for high weed populations 2 to 5 leaves 12-30 days after sowing at a speed of 10-14 km/h. Crop will look terrible but yields will not be reduced. May be done, even if weed pressure is not high, to stimulate crop growth and possibly increase yield.
Adjusting the rotary hoe
Drive about a hundred meters noting the speed you are going at. Check the crop making sure that roughly less than 10% of the plants are affected (e.g.: exposed seeds or broken seedlings). Partly buried plants are not considered damaged. Verify that 80-90% of the weeds at cotyledon stage are buried and that the whole surface is disturbed by comparing it with an adjacent non-weeded surface.
Then, decide to adjust either:
1. Working depth (2-5 cm): There is no need to go deep with the hoe. You are looking for the minimum depth that will bury weeds. Generally, the later the weed stage, the deeper the hoe should go. In light soils, the wheels of the rotary hoe could be adjusted deeper for better breaking up of clods, making sure that it is not damaging the crop.
2. Speed: Go as fast as you can to save time and get better weed control but do not exceed 20 km/h.
3. Weights: Weights should be added to the hoe only if the soil is compacted on the surface.
4. Number of passes: If you decide to go over the field again after one pass because the weed pressure is high, it is preferable to wait a few days rather than doing it right away.
5. Driving direction: weed control is often more effective if a second pass is made in the opposite direction of travel to the first pass.
Finger weeders have long tines that vibrate in the soil. They are made by European companies like Lely, RabeWerk and Hatzenbichler. They range in price from $ 4 000 to $ 12 500 CDN depending on the width (4,5 to 9 meters). The wider model requires up to 70 HP. The best models of finger weeders are made in several sections, allowing the implement to follow the surface of the field. They also have a lever which permits adjustment of all tines in a section at once. Depth wheels permit better precision when adjusting.
Crop stage is more critical for the finger weeder than for the rotary hoe because of its stronger action. Ideally, you should aim at doing only one pass, preferably at the 3-5 leaf-stage. In a Swedish experiment, similar results were obtained regarding final grain yield and weed population whether harrowing at 3-4 leaves was done along the rows or across them. Avoid if possible earlier stages. The finger weeder is not appropriate if a lot of residue is present on the soil surface. It will drag along the residue and damage the crop. Finger weeder effectiveness is reduced after first leaf-stage of weeds.
Crop Stage Weeding Pre-emergence 10-15 km/h. 3 to 7 days after sowing but only if white "threads" are present. 2 to 3 leaves 5-6 km/h. Only if weed pressure is high and most plants have two leaves. Set finger pressure at minimum. 3 to 5 leaves 6-9 km/h. 15-30 days after sowing. Crop will look terrible but yields will not be reduced. Do not go over twice. May be done, even if weed pressure is not high, to stimulate crop growth and possibly increase yield.
Adjusting the finger weeder
If you have a multi-section finger weeder, adjust the tines of each section at different pressure. Adjust the depth wheels so that all tines are slightly into the soil and the supporting chains well extended. Adjust the three point hitch so that the sections are horizontal. Drive for a small distance, noting speed. Stop and check that less than 10% of the crop seedlings are damaged. Check also that 80-90% of the weeds are buried and that the entire surface is disturbed.
Then adjust either:
1. Tine tension: Choose the right tension according to the results observed under each section. There is no need to go deep with the tines as they only wear off faster.
2. Speed: High speed will destroy more weeds but will also affect the crop more. If you want to go fast with a finger weeder, it is better to decrease tine tension or adjust depth wheels. In crusted clay soils, more tension should be put on the tines and the tractor should move faster. Avoid double passes.
Don't look back
The advice of experienced farmers to newcomers with mechanical weed control is "don't look back". You should not worry if the crop looks a little "rough" and buried after weeding. However, it does not mean you should not adjust the tools. You have to try them out on short distances, evaluate if they are doing a good job and adjust. Take the 5 to 15 minutes that this requires as it will pay off.
If you want to buy second hand, compare with a new one and check these points:
- Spoons on each wheel are thumb-like rather than slim and are well-aligned. Note that spoons or the entire wheel can be replaced if worn out;
- Springs are still functioning;
- Wheel bearings are in good condition;
- Tines are long and not too distorted.
STALE SEEDBED (or FALSE SEEDING) TECHNIQUE
Tillage operations, like preparing the seedbed, favor germination of weed seeds. Therefore, if you sow right after preparing the seedbed, weeds will exert strong pressure on the crop. The principle of stale seedbed is to prepare the soil as usual, then wait 5 to 10 days (the weeds should just be coming out) and rework the soil very lightly (to not expose more weed seeds) with a rotary hoe, a finger weeder or a chain harrow, and then sow. This first generation of weeds will be controlled and the crop will be able to compete better.
Preparing your seedbed at night will inhibit annual weed seed germination. Research done in Europe has demonstrated that this technique can reduce by 50 to 60% weed seeds germination. When working the soil in the dark, only seeds left on the surface will get the light necessary to initiate germination. This technique works well against lambsquarter, ragweed, redroot pigweed, black nightshade, mustard and field bindweed. It does not work against barnyard grass and other annual grasses like foxtail or against weeds that do not need light to germinate like velvet leaf.
Start working the soil one hour after dusk, ideally on a cloudy day with no moon (that's dark!). Do only the last tillage-harrowing at night. You want to encourage weed seeds germination in the preceding tillage operations to exhaust the seed bank. Keep on the front light of the tractor only. To get the same effect, an alternative is to install an opaque tarp on top of the harrows for working during the day. However, weed control will not be as good with a tarp as with night tillage.
Weeds in large-seeded oilseed crops like sunflower can be controlled the same way as in cereals with pre-emergence and 4-6 leaf stage harrowing. Cross-harrowing is preferable to harrowing parallel to the rows in the case of sunflowers and has been a standard practice in Manitoba for many years. If the field is very weedy, earlier post-emergence operations can be done without problems as sunflowers have very strong seedlings. You can even do several post-emergence weedings without problems. If grown in large spacings, row cultivation can also be done.
For small-seeded oilseed crops like canola and flax, mechanical weed control is best done with false seeding. Pre-emergence and post-emergence weeding is risky and seldom needed due to the good competitive ability of these crops when the seeding rate is high enough. Underseeding with clover is also a good strategy for flax.
WEEDS AND SOIL FERTILITY
Overfertilization (that is fertilization above plant needs), whether with mineral or organic fertilizers, favors weed growth. Weed density is often directly related to soil nitrogen levels. High populations of weeds, like chickweed, lambsquarter, mustard family weeds, often indicate an excess of nitrogen fertilization. Certain weeds like wild oats have a better response than cereal crops to soluble fertilizers. A well-planned rotation and improving soil fertility by means other than fertilizers (e.g.: organic amendments, drainage, liming) will allow the crop to get a head start on weeds.
Douville, Y. and P. Jobin. 1995. Le sarclage des céréales. Centre de développement d'agrobiologie, Ste-Élizabeth-de-Warwick, Québec. 4 pages.
Douville, Y. and P. Jobin. 1996. Peignes et houes rotatives. Centre de développement d'agrobiologie, Ste-Élizabeth-de-Warwick, Québec. 8 pages.
Hanley, P. 1984. Harrowing for cultural weed control. Watershed, no.5:58-61.
Rydberg, T. 1994. Weed harrowing - the influence of driving speed and driving direction on degree of soil covering and the growth of weed and crop plants. Biological Agriculture and Horticulture, 10:197-205.
SOURCES OF EQUIPMENT
Case International and John Deere rotary hoes are easy to get from your local dealers. However, other equipment may be harder to find. Here is a list of suppliers.
Acres Tillage and Planting Systems
12054 Main St. West, Winchester, Ontario K0C 2K0
Yetter rotary hoes
Agri-Distribution J.M. inc.
23, de la Station, Ste-Martine, Québec, J0S 1V0
Hatzenbichler finger weeder
HWE Agricultural Technology Ltd.
P.O.Box 1515, Embrun, Ontario K0A 1W0
Different brands of finger weeders (Einböck, Schonberger)
5353 North Service Road, Burlington, Ontario, L7L 5H7
Lely finger weeder
Machinerie agricole St-Césaire inc.
650, route 112, C.P. 399, St-Césaire, Québec, J0L 1T0
Rabewerk finger weeder
Manufarm Specialties Ltd.
RR1, Wheatley, Ontario, N0P 2P0
Ho-Bits, replacement spoons for rotary hoe
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