Prevention
The focus of this element of weed management is to prevent the introduction of new weed species and prevent the establishment of new weeds in the seed bank, should a new weed be accidentally introduced. Many weeds have seeds or other reproductive parts that may remain viable in the soil for several years, or even decades. Because organic producers cannot take for granted that a new weed will be easily manageable once established, preventing the introduction of new weeds should be a cornerstone of organic weed management.
Prevention, also known as exclusion, begins with securing clean seed and feed sources. Every opportunity should be taken to prevent new weeds from being introduced to the farm. Care should be taken to select seed and feed sources that are free of weed species that are not currently present on the farm, particularly invasive or noxious weeds. Be aware of the sources of these inputs, and if necessary, visit the source and conduct testing to confirm weed content.
The contamination of seed is the single most important source of introduction of new species. Some seed cleaning facilities may not be equipped to do a good job of seed cleaning, and can be a source of new weed species if proper care is not taken in cleaning the equipment between batches. Elevators do not provide adequate cleaning for seed and, because of cross contamination from the seed of other producers, may result in a greater risk of introducing a new weed to your farm. Care should always be taken not to import new weed species when purchasing seed from outside the immediate farming area.
Prevention includes monitoring for new weed problems which, in addition to the field itself, may develop along fence lines, shelterbelts, road allowances or in other non-crop areas adjacent to cultivated fields. Pastures, silage towers or bunks, feed troughs and manure piles should also be monitored for the introduction of new weeds.
Proactively monitoring for the occurrence of new weed species allows producers an early opportunity to eradicate the problem before it becomes established. Eradication of an early weed infestation may be accomplished by pulling (for simple tap rooted plants), or digging up perennial plant colonies by the roots. Other techniques for small areas include solarization or mulching. Solarization involves laying clear plastic over an infested area to raise soil temperatures and kill all roots or seeds in the upper layer of soil. Mulching involves laying down an impermeable barrier to new plant growth in order to starve new shoots. Once infestations become larger, these methods are less practical. Spread of the weed can be minimized using intensive mowing and multi-species grazing.
Municipal weed inspectors, who are responsible for the enforcement of The Noxious Weeds Act, should be alerted to the occurrence of any new weeds to the area so that they may also monitor for other occurrences to prevent spread.
Sanitation
Sanitation, from the perspective of weed management, refers to the practice of minimizing the movement of weeds that are found on the farm to other parts of the farm. The primary activities of sanitation involve keeping equipment free of seed or other reproductive parts of weeds, and ensuring that field boundaries and other non-crop areas are kept free of weeds capable of reproducing. There is significant overlap between the activities carried out in both prevention and sanitation.
Attention should be directed to tillage and harvesting equipment. Equipment used for custom work should be closely scrutinized, as it has the highest likelihood of introducing new weed species. Equipment cleaning before leaving a field is important to prevent the deposit of weeds to the next field, as well as along the transport route. The spread or re-introduction of perennial weeds such as quack grass (Elymus repens) or Canada thistle (Cirsium arvense), as a result of rhizomes or roots on tillage equipment, is an example of how equipment cleaning could prevent future weed problems. Harvesting machinery, if not properly cleaned before leaving the field, can easily introduce new weed seeds to a field, as well as along the route between fields. Depending on the design of the machine, the header, rocktrap, and grain tank are all major reservoirs of material from previous fields, as well as any catch points on the outside of the combine.
Manure is an important source of nutrients, particularly phosphorus, in organic cropping systems. Composting manure prior to field application is important in limiting the movement of weeds in manure. A large percentage of the weed seeds fed to animals in infested hay or straw will pass through the digestive tract unharmed. Manure from animals fed weedy forage should be composted for several weeks. During composting, the heating process that occurs in well managed compost will destroy many weed seeds. Proper composting involves frequent turning to aerate the pile and maintain high pile temperatures. See the Saskatchewan Agriculture and Food factsheet Composting Solid Manure available online, or call the Agriculture Knowledge Centre at 1-866-457-2377 for details on composting manure.
Cultural Control
Crop Rotations
An effective crop rotation is absolutely critical for weed, disease, insect and fertility management. Rotations will vary depending on the individual producer, crop/livestock mix, soil characteristics, as well as regional constraints such as moisture and fertility. Research has indicated that crop rotation can have a greater effect on weed species and densities than tillage practices. In a survey, 84 per cent of the farmers surveyed indicated that a rotation, including forage crops, significantly reduced weed problems. Fifty per cent noted a reduction in annual grasses, while 35 per cent reported a reduction in broadleaf weeds. A later study assessed weed populations in fields that had been taken from alfalfa production the previous year, and compared those populations to fields under continuous cereal production. The fields that had been removed from alfalfa production had moderately lower levels of wild mustard, wild buckwheat, green foxtail, annual smartweed, and greatly lower populations of cleavers, wild oat, perennial sow-thistle, Persian darnel, and Canada thistle. At the same time, other weeds were found at higher levels after alfalfa, such as volunteer alfalfa, dandelion, and winter annual weeds such as stinkweed. Weeds with high seed dormancy levels, such as Lamb's quarters, did not differ between fields previously in alfalfa or cereals.
In designing a crop rotation for weed control, the overall key to success is diversity. The following principles or practices are commonly incorporated into successful rotations.
- Alternate competitive crops, such as alfalfa or barley, with less competitive crops, such as flax or lentil, in the rotation.
- Alternate between broadleaf and grass crops in the rotation. Weeds are more competitive with crops to which they are most closely related.
- Use perennial phases in crop rotations combined with mowing (cutting for hay) or intensive grazing to control perennials and to interrupt cycles of annual weeds in cultivated crops. Consider the growth stage of the weed, as well as the forage, when timing the cut.
- Vary seeding dates to prevent weeds from adapting to static management practices. Alternating winter and summer crops within the rotation is the ultimate in seeding date diversity.
- Where possible, use weed suppressing cover crops such as fall rye or sweetclover.
Crop Competition
Within a selected cropping rotation, many additional agronomic or management decisions can be used to increase the competitive advantage of a crop. In many cases, the effectiveness of these strategies depends on regular field monitoring and adjusting management practices on an ongoing basis depending on crop, climatic and weed situations.
The time of weed emergence relative to the crop is critical. For every day that wild oat (Avena fatua) emerges before a spring wheat or barley crop, yield losses increase by three per cent. By the same logic, crop yield improves by three per cent for each day wild oat emerges after the crop, until eventually the effect of weed competition on yield is insignificant when compared to plots kept free of weeds. Research indicates that there is a critical period following crop emergence when yields are reduced the greatest by weed growth. This varies between year, crop and weed species. In the case of wheat, this period is between two and four weeks after emergence. Attention to the following management factors can assure this critical period is as weed free as possible.
A) Crop Selection - Small grains, broadleaf and legume plants - Each of these plant groups have specific rooting habits, nutrient and moisture requirements. Within a rotation, the competitive ability of crops varies greatly. The most competitive crop is fall rye, followed closely by winter wheat, barley, oat, spring wheat, mustard, canola, peas, flax and, finally, low lying crops such as lentil, chickpea and dry beans. Most pulse crops, like lentil, are poor competitors against weeds. Canola offers poor competition to weeds in the seedling stage, but can be effective once it becomes established.
Within any specific crop, there may be varieties that differ in competitive ability. The use of varieties that achieve canopy closure early, or that have a more competitive architecture (taller or have more tillers) should be considered. Semi-dwarf varieties are significantly less competitive than taller lines. Research studies involving semi-dwarf winter wheat varieties resulted in a 14-30 per cent greater yield reduction from downy brome (Bromus tectorum), than did taller cultivars. When selecting a variety that is competitive against weeds, we should look for the following traits: rapid early growth, many tillers, leafy and tall stature.
The competitive ability of a nurse crop when used in forage crop establishment should be considered. Some of the above factors that make a crop competitive against weeds may not be desirable when used as a nurse crop.
The weed cycle may be further disrupted by introducing a green manure crop. The most common option is a pulse or legume forage crop. The forage crop will usually offer very good competition against weeds and tillage opportunities while the crop is incorporated into the soil. Removing the green manure crop early also allows a partial fallow to occur. A rotation of this nature would aid in controlling perennial weeds such as quackgrass or Canada thistle (Cirsium arvense). There is also the option of cutting and spreading the green manure without actually incorporating it, thereby reducing soil degradation concerns.
The frequency that a crop is grown within a rotation can also affect the population of certain weed species. Weeds tend to be more successful when they are growing among closely related crops. A research study involving a two-year canola-fallow rotation had a stinkweed infestation of 190 plants/m2. Extending this rotation to a canola-barley-hay-fallow system reduced the density to 23 plants/m2.
B) Seeding Date - The time at which a crop is planted is probably the main factor determining the composition of weed flora. In the case of cereals, early seeding has consistently produced the most competitive crop with the highest yields. As a general rule, crops such as flax, pulses, and canola should not be seeded into cool soil. Under such conditions, the seedlings of these crops will be slow to develop and prone to disease. Since these crops are not good competitors, it is essential that they develop a strong uniform stand. Crops such as tame buckwheat and certain spice crops should be seeded considerably later than any of the above crops, to avoid the risk of late spring frosts.
Delayed seeding can be an effective weed control practice, offering tillage options either before seeding or after harvest. Tillage should begin early, in an attempt to warm the soil and stimulate weed growth. The number of tillage operations that can be performed will depend on the soil type, moisture conditions, erosion concerns and specific crop. In spite of reduced yields usually associated with moisture loss, wild oat control can be better than 80 per cent. Early germinating broadleaf weeds such as wild buckwheat (Polygonum convolvulus), and certain mustard species, may also be controlled using this approach. In practicing delayed seeding, it is important that crop maturation, quality, and optimum yield are not unduly sacrificed, or put at risk.
Varying seeding dates will limit the ability of any one particular weed species to adapt and become problematic. Early seeded crops should be well established before species like green foxtail (Setaria viridis) emerge. Delayed seeding will allow the largest flush of wild oat (Avena fatua) to occur, providing an opportunity for tillage control before seeding. For optimum germination, green foxtail prefers higher soil temperatures than most cereals, and wild oat germinates best in cool soils.
Planting winter crops such as fall rye or winter wheat will offer competition to spring annuals since they generally out-compete these weed species. But these crops will become vulnerable to winter annuals like stinkweed (Thlaspi arvense), shepherd's-purse, or flixweed (Descurainia sophia). The fact that fall seeded crops are harvested early provides an opportunity to conduct a tillage program which will also help manage perennial weeds.
C) Seeding Rate - Increasing seeding rates 20-50 per cent above normal can increase the competitive ability of some crops. If moisture conditions are adequate, higher seeding rates will also result in earlier maturity (two to three days), shorter plant height, reduced tillering, and possibly higher yields. Green feed and silage crops should be seeded at higher rates to increase crop competition and feed quality. Increased seeding rates should also be used where either post-seeding or post-emergence tillage is planned. This will help compensate for any damage caused by the in-crop tillage.
Under certain environmental conditions, higher seeding rates may increase the disease incidence and result in higher lodging losses.
D) Row Spacing - In general, narrow row spacing in cereal, pulse and most oilseed crops offers the best competitive advantage against weeds. From a crop-weed perspective, any system that places seeds equal distances apart in all directions is the ideal arrangement. This allows plants an equal opportunity to compete for water and nutrients. In addition, there is less open space for weeds to grow. Less competitive crops, such as lentil, will tend to benefit most from narrow rows.
Some crops, such as dry beans, corn, potatoes and vegetable crops, have been traditionally grown in wide rows in order to facilitate weed control through tillage after the crop has emerged. This is generally referred to as row-cropping. Row spacing can range from 40 to 90 cm (15 to 36 in.) and can be done by blocking off runs in a typical seed drill to achieve the desired spacing, or with the use of specialized precision row planters. See In-Crop Tillage below for information on row-crop tillage.
E) Seeding Depth - Optimum seeding depth will vary between crops. This will depend on seed size, soil type and moisture conditions. Seeding depth should not be deeper than required to achieve a quick and even emergence, ensuring optimum competition. Pre-seeding tillage should be as shallow as possible, since deep tillage will result in excessive soil moisture loss, and make seeding at a uniform depth difficult. If the soil is loose, consideration may be given to packing prior to seeding. This will firm the soil and help bring moisture closer to the soil surface.
F) Plant Nutrition - The level of soil fertility changes the relative competitiveness between the crop and weed. It is important that an imbalance of any nutrient does not occur. A balance of nutrients assures optimum crop emergence and healthy plants. At higher levels, or an imbalance favouring higher levels of nitrogen, weeds like wild oat are generally more competitive than a crop. This has been confirmed for wheat, barley and flax. A similar situation can be stated for numerous broadleaf weed species.
Soil available nitrogen has been shown to stimulate the germination of wild oat, green foxtail and possibly other weed species. Conversely, nitrogen may discourage individual species such as horsetail (Equisetum arvense) which is only prevalent on low fertility areas. Barnyard grass (Echinochola crus-galli) may be stimulated to grow where high levels of phosphate occur.
An imbalance of nutrients can readily occur where large applications of manure have been applied. Soil testing should be conducted on a regular basis to monitor nutrient levels, which will help in making appropriate management decisions.
Mechanical Control by Tillage
Soil cultivation has exerted a considerable influence on the evolution of weed flora. This has resulted in an increase in both perennial weeds, which rely on fragmentation and dispersal for successful regeneration, and for annual species that can produce dormant seeds of a potentially long life span. Survival of these annual species depends on incorporation into a buried seed bank. Subsequent germination of this seed is dependent on cultivations that return this seed to, or near, the soil surface.
The type and frequency of cultivation will influence the composition and density of weed species. For certain weeds, such as wild oat, timing of tillage is more important than the type of cultivation. In other situations, like Canada thistle, the type of implement, timing and frequency are all important.
In general, reduced cultivation will discourage the growth of broadleaf plants and encourage grass species such as wild oat and green foxtail.
Out-of-Crop Tillage
A) Pre-Seeding Tillage - The objectives of this practice are to prepare a seed bed so planting can occur at a uniform depth and to control weeds that germinate prior to seeding. Although generally discouraged because of moisture and erosion concerns, if a number of tillage operations are planned prior to seeding, the first operation should be the deepest, with each successive one shallower. The first operation should be to aerate and warm the soil, or in many cases, to expose small seeds to light in order to stimulate weed growth. The following operations should destroy weed growth while conserving as much soil moisture as possible. The loss of soil moisture due to tillage can be a critical problem, hindering successful crop establishment. Delayed seeding can also cause difficulties in areas where short growing seasons exist. It is important to balance these negative aspects against the benefit in weed control.
B) Post-Harvest Tillage - Tillage following harvest is effective in controlling winter annual and biennial species. Tillage should be shallow (less than 10 cm) to avoid burying weed seeds in the soil.
A certain amount of control of perennial weeds, such as quackgrass and Canada thistle, can be achieved, but this is weather dependent. Successful control depends largely on having freezing temperatures shortly after the tillage operation.
C) Summerfallow Tillage - Tillage operations should be as shallow as possible to avoid bringing new weed seeds to the soil surface, in addition to preventing moisture loss. The initial operation should always be the deepest, with subsequent ones progressively shallower. Tillage is most effective when the soil surface is dry and air temperature is high. Tilling small seedlings when the soil surface is moist will usually produce poor results, as many of the seedlings are transplanted, rather than being killed.
Summerfallow tillage is particularly effective for the control of perennials such as Canada thistle, quackgrass and sow thistle (Sonchus arvensis). If these weeds occur in patches, they should be worked separately to avoid spreading the weed over the entire field, as well as to avoid excessive and unnecessary tillage. In addition, weeds like Canada thistle are best left undisturbed until they reach the green bud or early flower stage. The plant is weakest at this stage of growth, and thus most vulnerable to tillage. Once tillage begins, it should continue each time Canada thistle reaches the bud stage until August 1, then afterward each time the plant reaches a height of about 7.5 cm, until freeze up. This approach will starve the root system and prevent it from forming any food reserves. It will enter winter in a very weakened state, and many will not survive.
For field bindweed, experiments conducted in Kansas by Phillips and Timmons between 1936 and 1943 found that repeated tillage 12 days after the emergence of new shoots resulted in control of established plants within two fallow seasons. Furthermore, their experiments also showed that this strategy required the fewest number of tillage operations over that two year period to completely eliminate the perennial root.
Quackgrass problems should be handled in a somewhat different manner. Tillage to control this weed depends on physically damaging the root system. In dry years, a cultivator with narrow spikes will be effective, as this will drag roots and rhizomes to the surface where they will dry out and die. In wet years or areas, the first tillage operation should be with a disc implement that cuts the rhizomes into small pieces. Each of these smaller sections of rhizome will try to establish a new plant, which in turn has to be destroyed by subsequent tillage. New plant growth should not be allowed to get taller than 7.5 cm before being tilled.
In either of the above cases, tillage should not be deeper than required to do an effective job. Shallow tillage will concentrate the rhizomes on or near the soil surface, resulting in a more uniform emergence and better control from future tillage operations.
Extensive tillage should always be approached with caution. It can create serious soil erosion problems, particularly if it is done on a large area. Where soil erosion is a problem, tillage should be integrated with other methods to ensure the soil is protected.
In-Crop Tillage
Weeds which emerge ahead of the crop can reduce yields significantly more than those that emerge following the crop. Depth of seeding should be at the deeper end of the recommended range to ensure crop plants are well anchored at a uniform depth.
A) Post-Seeding Tillage - Post-seeding tillage may be done by harrowing or using a rod-weeder. Best results will be obtained on a warm, sunny day when the soil surface is dry. It is important to ensure that the crop has not sprouted into the tillage zone. Experiments conducted at the Agriculture and Agri-Food Canada Research Farm at Scott suggest that a rod-weeder works very well when used about five to seven days after seeding peas in mid-May. An early spring tillage operation should take place about 10 to 12 days prior to seeding.
B) Post-Emergence Tillage - Weeds which emerge shortly after the crop can be controlled by a harrowing operation. Small seeded species such as green foxtail, Lamb's-Quarters (Chenopodium album) and redroot pigweed (Amaranthus retroflexus), which usually emerge from shallow depths, can be well controlled. The effectiveness of this strategy is dependent on many factors: competitive ability of the crop, the seeding depth, crop stage, weather and soil conditions, implement and tractor speed. A certain amount of crop damage is likely to occur; therefore, seeding rates should be increased to compensate for the damage. Wild oat can germinate from substantially deeper in the soil. Under these circumstances, control may not be as satisfactory. A field inspection of weeds and the depth from which they are emerging should be done prior to any post-emergent tillage.
This is also dependent on soil surface conditions being dry, and the weather warm and windy. It is generally recommended that the harrowing operation should be in the direction of the rows. However, recent research has indicated that the direction is not as important as once thought.
This technique should only be considered for use during the following stages of crop development:
- Wheat - From the two to four leaf stage.
- Barley - From the two to four leaf stage (before tillering). Barley is more sensitive than wheat.
- Oat - Oat is more susceptible than wheat or barley to damage. Post-emergent harrowing is not recommended.
- Sunflower - Safe up to the six leaf stage. Harrow across the rows. Adjust the seeding rate to make up for the approximately 5,000 plants/acre that will be lost each time the crop is harrowed.
- Fababean - Harrow across the rows when the crop is between five to 15 cm tall and the plants are dry.
- Lentil, Field Pea - Harrow in the seedling stage (no more than 10 cm tall). The crop must be dry to minimize the spread of disease. Use tine harrows only. Lentil is more sensitive to damage than pea.
