| | Seeding rates | Seeding depth | Timing of seeding | Seedbed preparation | Fertility | Crop rotations
Strong, healthy crops have a competitive advantage over weeds, but crop species vary in their ability to compete. Perennial crops, after the year of establishment, provide competition for weeds, particularly annual weeds. Fall rye and winter wheat compete well because they resume growth in early spring before weeds have emerged. Cereals are generally the best annual crop competitors. Barley is more competitive than spring wheat. Canola is the most competitive oilseed with flax being the poorest. Crops such as lentils, peas and beans are generally poor competitors, owing to low plant populations and slow initial growth. Varieties also differ in competitiveness. For example, the semi-leafless pea variety Tipu is less competitive than Century in areas infested with wild mustard.
Crops have some defense mechanisms to deter pests; many can withstand a certain amount of pest damage when they are well nourished and healthy. Crops are susceptible to disease when they are stressed or poorly nourished. A well-prepared seedbed in good fertile soil will enable crops to resist many pests. Place fertilizer where it benefits the crop more than the weeds. Shallow seeding and packing will establish a vigorous stand. Avoid seeding too early; cold soils will retard seedling growth and make the crop susceptible to disease. Heavy seeding rates to establish early competition for weeds will also compensate for some plants being lost to pests. A vigorous crop can carry more insect damage without yield reduction, so fertility is doubly important if pest problems are anticipated.
The seedling is the most vulnerable stage of growth, and because many insects emerge from hibernation just when seedlings emerge, a variety of special seeding practices have been developed.
Seeding Rates
Seeding at rates 25 per cent greater than normal will help crops compete with weeds by providing a lush crop canopy. Heavier seeding rates of cereals allow a margin for crop damage from post-seeding tillage. However, under extremely dry conditions, heavier seeding rates cause the crop to compete with itself for moisture. For this reason, heavy seeding should not be practised in very dry areas.
Heavy seeding rates can compensate for a certain degree of seedling mortality by insects, especially in wheat when wireworm damage is possible. If the infestation is patchy, doubling the seeding rate in susceptible areas is a possible control option. A thorough field scouting program is required.
Seeding Depth
Planting shallow into moisture will encourage rapid crop emergence. This will prevent weeds from emerging before the crop. Weeds that emerge before the crop cause significantly greater yield losses than those that emerge after.
Some recommended seeding depths:
Crop | cm |
Cereal, lentils | 4 to 5 |
Canola, flax | 1.3 to 4 |
Forage grasses, legumes | 1.3 to 2 |
Peas, beans (large seed) | 6 to 10 |
Timing of Seeding
Crops seeded shallow into warm (5 degrees Celsius or warmer), moist soils germinate and emerge quickly and evenly. Tillage at or before seeding will destroy weed seedlings and give the crop a head start.
By seeding when the soil is both warm and moist, the crop gets a head start and establishes itself before the pest does. Moisture is especially important. For instance, wireworms can destroy up to 95 per cent of a crop sown in dry soil compared with up to 10 per cent in moist soil.
Early seeding is sometimes used so that crops are advanced when grasshoppers, cutworms and wireworms hatch. Another advantage of early seeding of a fast growing variety is that the crop matures before the development of late season pests and weeds such as aphids and green foxtail.
Use delayed seeding to control weeds such as wild oats, which germinate under cool soil conditions. Early spring tillage encourages weed germination. Till and plant after weed seedlings emerge. Late-planted crops usually yield less and are vulnerable to frost in the fall. Choose an early maturing crop or variety to minimize yield losses caused by frost. Do not use delayed seeding when moisture is extremely limited.
Delayed seeding may also be used to time seedling emergence to occur after a destructive insect stage has passed. For instance, if a heavy cutworm infestation is expected, seeding can be delayed until the cutworm larvae have stopped feeding. It is also possible to reduce sawfly damage by delaying seeding, because the stems of young plants are not developed enough to be attractive to egg-laying female sawflies.
Certain risks are associated with late seeding. If spring weather is cold and wet, pests will be delayed right along with the crop. Late seeding results in lower yields, and early maturing varieties usually yield less. This loss of yield must be balanced with the expected cost of control and the reliability of predicting insect damage.
Disease development is affected by the date of planting, soil temperature at planting and depth of planting. If Tobin canola is planted too early or too deep, the incidence and severity of seedling blight increases markedly. Seed treatments will be of little help in reducing seedling disease if the soil is below 5 degrees Celsius and dry.
Seedbed Preparation
A firm, well-packed seedbed maximizes soil moisture contact with the seed. This promotes uniform seeding and seedling emergence. Before seeding, the seedbed should be firm enough so that heels sink no deeper than the thickness of the sole on a normal work boot. This will also provide better depth control with the seed drill.
Fertility
Nitrogen (N), phosphorus (P) and potassium (K) are the major macro-nutrients essential to crop production. N and P limit crop yields in most regions and a few areas are low in K. Soil tests identify the levels of all soil nutrients.
Sulphur (S) is an essential element. Where sulphur levels are deficient, yield losses will result in crops such as canola without the addition of sulphur. Sulphur deficiency occurs in patches, most often on Gray Wooded soils. Canola on sulphur-deficient soil is stunted, with yellowish cupped leaves, red discolorations and poorly filled or empty seed pods. Soil tests identify sulphur levels and deficiencies can be corrected by applying fertilizers that contain sulphur.
Copper (Cu) deficiency has been identified on sandy soils and soils with a high organic matter content. Copper deficiency contributes to a condition known as melanosis in wheat, where the grains fail to form. Wheat varieties such as Park, Biggar and Roblin appear to be very susceptible to this condition. On high organic soils in west-central Alberta, Cu deficiency is associated with low yields of barley. Research is underway to find economical ways of adding available Cu to soils to correct these conditions.
Other micro-nutrients that have occasionally been identified as deficient are manganese (gray speck disease of oats), boron (in canola and alfalfa), zinc (in beans), molybdenum (in alfalfa) and iron (on alkali soils). If any of these nutrients are limiting, they become the weak link in the production chain and limit yield even though other nutrients may be present in ample quantities.
Vigor of crop plants increases with the use of fertilizers. Phosphorus promotes root development and allows crops to compete more effectively with most weeds. Weeds such as green foxtail respond to nitrogen fertilizer applications thus reducing the crop's competitive advantage. Generally, under wet and cool conditions, the use of fertilizer is an effective tool in promoting crop growth. Broadcasting nitrogen stimulates weed growth because the fertilizer is readily available to the weeds, as well as the crop. Banding fertilizer is more advantageous to the crop.
Improper fertilization practices may cause crop problems. Placing too much phosphorus or nitrogen fertilizer with the seed will damage tender seedlings by a simple salt (burning) effect. This reduces the plant stand and yield. Crops under stress from too little or too much fertilizer become more susceptible to disease. Good plant nutrition is one of the best ways to avoid disease problems.
Consider the choice of fertilizer. For example, the number of sugar beet plants affected by seedling blight (Rhizoctonia solani) can double when ammonium-type nitrogen is used as opposed to nitrate-type nitrogen.
Soil fertility affects the vigor of both crops and weeds. Many weeds use fertilizers as effectively as or better than crop plants. If most of the weeds are suppressed or killed by tillage or herbicides, the extra vigor given to the crop by fertilizers will make the crop plants better able to resist disease.
High nutrient levels in the soil can lead to crop lodging because most crop varieties were developed to cope with lower levels of fertilizer inputs. New on the scene are growth regulators that can dwarf naturally tall-growing crops, reducing their tendency to lodge.
Using Crop Rotations to Reduce Pest Problems
Spring-seeded annual crops
The first flush of weeds can be destroyed by tillage before spring seeding. Weeds that germinate later in the season such as green foxtail and shepherd's-purse, may increase if a field is repeatedly seeded in the spring. Spring-seeded annual forages are an alternative to perennial forages, especially on weedy land. Annual forages should be cut before weeds flower, to prevent formation of viable seeds.
Legumes
Leguminous cover crops prevent erosion, fix nitrogen in the soil, reduce the need for nitrogen fertilizer, loosen the subsoil, improve soil structure and organic matter, smother weeds, and starve insects and diseases of non-legume crops.
Underseeding or companion seeding
The principle of underseeding or companion seeding is to crowd weeds out. For example, planting clover beneath cereals provides strong competition for weeds. After cereal harvest, the clover can be used as forage or green manure. Companion seeding means growing several varieties at the same time, such as a grass and a legume, mixed grains, or a silage mix of peas and oats. These plantings ensure a variety of habitat for beneficial insects and use light and soil nutrients to the detriment of weeds.
Row crops
Summer annuals, like wild oats, are usually a problem when spring grains are grown continuously or in rotation with summer fallow. Problems with summer annuals can be reduced by using row crops such as sugar beets, potatoes, field corn, peas and beans. When planning a rotation with grain crops, select a row crop compatible with the climatic conditions in your area. As a good rotation for weed control, include both summer row crops and winter or early spring grain crops such as fall rye, winter wheat, or early barley. Row crops that are unrelated to common oilseeds and grains are helpful in preventing the buildup of many common insects and diseases.
Perennial forage crops
Perennial forage crops or pastures seeded with introduced grasses effectively reduce populations of annual weeds and some insects. Annual weeds must be controlled during the seedling stage of the crop. Once perennial forages are established, mowing and competition will suppress weeds. Eventually, vigorous competition will eliminate annual weeds almost completely.
The forage stand must be left for a number of years to allow buried weed seeds to rot. Some weed seeds will stay dormant and germinate when the forage stand is broken. The first year after breaking a forage stand is critical, because the field will be re-infested if prompt control measures are not taken. With careful management, none of the weeds that grow in the year after breaking should ever produce seed. This land can remain clean for many years. Leaving parts of a field in forage and unharvested provides a lasting habitat for beneficial insects and birds. Some perennial grasses, especially crested wheatgrass, compete better than perennial legumes.
Fall-seeded crops
Fall rye and winter wheat are effective in weed control programs where it is practical to grow them. Fall rye will grow much further north than winter wheat, but even rye can be killed by snow mold fungi in the Peace region when snow cover remains late into April. These crops give early season competition to weed seedlings and their early harvest date enables a partial summer fallow for the balance of the year. This two-pronged approach provides for control of both annual and perennial weeds.
Winter annual weeds are a problem in fall-seeded crops. These weeds emerge with the crop and form overwinter rosettes. In the spring, the winter annual weeds flower and produce seed long before the crop can be harvested. Some winter annuals can be controlled in the crop by using 2,4-D. Others must be controlled by rotations.
In the year of establishment of a perennial crop, weed control can be as simple as mowing or cutting before weeds go to seed. Weeds contribute to the feed value of hay; after they are cut the crop will gain a competitive advantage.
Perennial crops should be left down for three to five years. This allows time for the crop to compete against weeds and deplete the number of buried weed seeds. Weed control must immediately follow breaking of sod because the dormancy of many seeds will be broken and they will germinate as soon as the soil is plowed.
Greenfeed
Greenfeed crops can be used when weed infestations are very heavy and profitable cereal production is questionable. Oats are generally used because they make better hay or silage than other cereals. The greenfeed crop should be cut or thoroughly grazed before the weeds bloom. By cutting at this time, the greenfeed will be free of weed seeds and a population of weeds will have been harmlessly grown out. Weeds must be watched carefully if the weather is good for regrowth. A second cutting or tillage may be necessary to prevent weeds from producing seed because seed production can take place without much additional top growth. Alternatively, the forage can be turned under for both green manure and weed control.
In the wetter parts of Alberta, a combined greenfeed and summer fallow operation may be effective. Seed greenfeed early in the spring. Then till after the infested crop has been used for pasture, hay or silage. The strategy is to begin to summer fallow before the seed-set of annuals and when perennials have depleted their root stores of nutrients.
Variety selection
Enhance the benefits of crop rotation by selecting varieties that will discourage local pests. Pick a variety suited to the growing conditions, and thus vigorous enough to withstand some pest damage, or select a variety that matures earlier (or later, in some cases) than the pest does, and so avoids damage.
Some varieties have been bred to resist insect attack. For instance, solid-stemmed wheat varieties are resistant to wheat stem sawfly and most of the alfalfa grown in the western United States is resistant to spotted alfalfa aphid. |
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