| | Continuous Cropping | Fall-seeded Crops | Perennial Forages | Permanent Cover | Green Manuring
Although growing a more varied crop rotation requires increased management skills, it can provide many rewards. For example, it helps to reduce diseases, insect pests and weeds in the rotation, in comparison to a cropping system that relies on one or two crops. As well, varied rotations can help to diversify the operation and widen the windows for seeding and harvesting, lowering the production risk.
Rotations can be selected to achieve a variety of objectives, such as to: help manage crop residues, make the most of available moisture, build organic matter, reduce nitrogen fertilizer inputs, vary herbicide types (to avoid creating herbicide-resistant weeds), and lower excessive levels of soil nutrients.
Table 3.2 provides information on the typical amounts of crop residues produced per bushel of grain when planning a rotation to manage residue amounts. For example, after growing a crop that produces very little straw, you may wish to grow a crop that produces more straw.
Cropping rotations can help to diversify the operation and widen the windows for seeding and harvesting.
Courtesy of ARD
Table 3.2. Typical amounts of straw and chaff per bushel of grain
| Crop | Soil zone | Pounds of straw per
bushel of grain * | Pounds of chaff per
bushel of grain ** |
| HRS wheat | Brown | 50 | 20 - 25 |
| Dark Brown | 65 |
| Black, Gray | 80 |
| CPS Wheat | Brown | 40 | 20 - 25 |
| Dark Brown | 50 |
| Black, Gray | 60 |
| Barley | Brown | 30 | 5-10 |
| Dark Brown | 35 |
| Black, Gray | 45 |
| Oats | Brown | 30 | 5-10 |
| Dark Brown | 35 |
| Black, Gray | 45 |
| Canola | Brown | 40 | 15 - 20 |
| Dark Brown | 50 |
| Black, Gray | 60 |
| Peas | Brown | 40 | 20 - 25 |
| Dark Brown | 50 |
| Black, Gray | 60 |
* Amount of harvestable straw, assuming about 80% recovery in cereals, and 50% in peas and canola, with 2- to 4-inch stubble left.
** Amount of harvestable chaff, assuming little or no weed chaff.
Adapted from: Hartman, M. 1999. Estimating the Value of Crop Residues. Alberta Agriculture and Rural Development, Agdex 519-25.
Crop rotations in drier areas can be designed to improve moisture use efficiency. A logical water-based rotation alternates shallow-rooted crops with deep-rooted crops and combines good agronomic and economic performance while making the most efficient use of water over a wide range of moisture conditions. Table 3.3 provides the average moisture use efficiency for four crops.
Table 3.3. Average moisture use efficiency (bu/ac/in) for four common crops
| Soil zone | Wheat | Barley | Canola | Oats |
| Brown | 3.75 | 5.70 | 2.60 | 7.10 |
| Dark Brown | 4.00 | 6.20 | 2.80 | 7.75 |
| Black | 4.25 | 6.40 | 3.20 | 8.20 |
| Gray | 4.75 | 7.20 | 3.60 | 9.10 |
Adapted from: EnviroTest Laboratories, Soil Climate Zones of the Canadian Prairies.
Continuous Cropping
In continuous cropping, crops are grown every year with no fallow years in between. As noted in Tillage and Seeding Practices, summerfallow decreases soil organic matter content and soil quality in most situations. Where summerfallowing is used to conserve soil moisture, direct seeding and reduced tillage systems may allow summerfallow to be used less often because these systems conserve soil moisture by reducing evaporation and increasing infiltration.
Rotations benefit all crops in the rotation. For example, wheat or barley grown after peas or canola usually performs better by about 10 to 20% than a cereal grown after a similar cereal crop.
In annual crop rotations, it is best to avoid planting a field with the same crop two or more years in a row. Alternating long-season crops with short-season crops can improve weed control because pre-seeding and in-crop herbicides can be applied earlier or later in the spring. Also, a short-season crop may be harvested early enough to allow the seeding of winter wheat or fall rye, which can take advantage of early spring moisture (see Fall-seeded Crops).
Rotating cereals with broad-leaved crops, such as oilseeds or pulses, allows weeds to be controlled with herbicides from different herbicide groups, reducing the risk of developing herbicide resistance. This type of rotation can also break the cycles of most diseases, except for those diseases that remain dormant in the soil or persist on crop residues for long periods.
Fall-seeded Crops
Like any crop, fall-seeded crops have their challenges, but they can be a valuable part of a crop rotation, providing a range of agronomic and environmental benefits. Fall-seeded crops grown in Alberta include winter wheat, winter triticale and fall rye.
Fall-seeded crops provide erosion protection during fall and winter, especially if the crop is seeded into stubble. The stubble also traps snow that insulates seedlings against cold temperatures and increases spring soil moisture. As well, a fall-seeded crop seeded into stubble can provide spring nesting cover for waterfowl and other ground nesting bird species.
Fall-seeded crops also help in reducing pest problems. For example, seeding a fall cereal into stubble from broad-leaved crops, such as canola, mustard and peas, reduces the risk of insect, disease and weed problems developing in the rotation. Fall-seeded crops are also able to outcompete some weeds that emerge in spring, reducing the need for herbicides.
Although these crops can suffer winterkill and be damaged by spring frosts, they are better able to take advantage of early spring moisture than spring-seeded crops. And when spring weather conditions delay seeding, the fall-seeded crop is already in place. Since these crops mature earlier than spring-seeded crops, they may avoid damage from late summer droughts or frosts. In addition, the earlier seeding and harvesting dates for these crops help to spread out the farm workload and to increase marketing options.
Perennial Forages
Perennial forages in long-term rotations help to build up soil organic matter, prevent wind and water erosion, and to reduce pest problems in subsequent crops. Several years of forages break some disease and insect cycles, and help to control perennial and annual weeds as long as the forage stand is healthy and vigorous. Weed control is improved if the forage crop is harvested as hay or silage since the weed seeds are often not yet viable at the time of cutting.
Perennial forages protect and improve the soil.
Courtesy of ARD
Hay, pasture, and grass seed crops can be grown in rotations for varying lengths of time. The longer the forage cycle, the lower the cost of rotating into and out of forages in a rotation with annual crops. However, when the forages include legumes, particularly alfalfa, the marginal benefit to subsequent crops decreases after the stand is three years old.
Forages can be successfully established by direct seeding. The higher soil moisture content in direct seeded fields allows these small-seeded crops to be seeded near the soil surface for better germination and emergence. A cover crop can be used to protect an establishing forage crop. To improve forage establishment and to decrease competition for nutrients, light or water, it is best to remove the cover crops as greenfeed or silage.
Methods to remove forages and the choice of crop to be seeded after forage cycle depend on the type of forage stand. Terminating forage stands using herbicides and then direct seeding an annual crop is a recommended practice.
Permanent Cover
Permanent cover refers to forage, grass or tree cover. Areas at high risk for such problems as erosion or soil salinity are usually better suited to permanent cover than annual crops because permanent cover protects the soil all year long, year after year. In addition, permanent cover enhances soil organic matter, carbon storage and wildlife habitat.
Lands benefiting most from permanent cover include the following:
- Areas with steep slopes or very erodible soils can be used for forage production.
- Wooded areas with poor soils and steep slopes can be managed as woodlots.
- Non-irrigated corners of pivot-irrigation fields tend to be focal points for wind erosion. Growing forages or grass on these corners reduces erosion.
- In areas prone to soil salinity, both the recharge areas and the discharge areas benefit from permanent cover.
Green Manuring
Green manuring is the practice of growing a short-term crop to improve soil tilth, add organic matter and nutrients (especially nitrogen) to the soil, and reduce erosion by providing soil cover. After eight to 10 weeks of growth, the green manure crop is worked into the soil, desiccated with herbicides, or hayed. If a green manure is allowed to grow too long, it will deplete the soil moisture reserve for the next year.
While almost any crop may be used as a green manure crop, annual legumes, such as peas and lentils, or biennial legumes, such as sweet, red or alsike clover, are preferred because these crops can fix nitrogen; that is, with the help of Rhizobium bacteria, taking nitrogen from the air and convert it into a form that plants can use.
Tilling down the crop returns most of the fixed nitrogen to the soil. However, it is important to leave some of the crop residues on the soil surface to reduce the risk of soil erosion. Desiccating or haying the crop returns about 60% of the plant material and nitrogen to the field, so these options provide almost as much nitrogen benefit as tilling down the crop. Desiccating the crop maintains a crop residue cover to reduce erosion and enhance snow trapping over the winter. Haying the crop also leaves stubble to prevent erosion and trap snow, and it offers the added advantage of economic returns from the hay.
Green manures add organic matter and nutrients to the soil.
Courtesy of ARD
A well managed green manure crop boosts the yield of a subsequent cereal crop. Over the long term, the practice of green manuring will improve soil organic matter content and productivity. In addition, a well established cover crop can provide good weed control. Sweet clover and its residues maintain excellent weed control without cultivation or herbicide applications into the year following the green manure crop.
Back to Chapter 3 - Cropping Practices |
|