Agronomic Information - Crop Rotation

Chapter I - Keys to Success

When developing a crop rotation, several factors must be considered:

  1. How much residue is produced by each crop being considered? How will that residue be managed?
  2. How will the weeds be controlled in each crop? Is there any risk of herbicide carry-over that will affect the next year's crop?
  3. How can volunteer crops be controlled? Is the herbicide you plan to use on volunteers safe for use on this year's crop?
  4. Will two similar crops, such as oilseeds, be grown back-to-back? What's the risk of increased disease and insect infestations?
  5. Is there a market for the crop?

The key to a successful rotation is one that alternates between a cereal and a broadleaf. An oilseed-cereal-pulse-cereal rotation allows for a diversity of crops and prevents similar crops from being grown back-to-back. There are many advantages to this basic rotation.

The first is that the crop residue is more manageable. Cereals produce large volumes of residue. Alternating cereals with broadleaf crops that don't have a lot of residue ensures that no mat of chaff and straw is built up over the years. Another advantage to having crops with low residues in the rotation is that the fields with little residue cover on the surface will warm up and dry sooner than the fields with high amounts of residue. The low residue fields can be seeded first in the spring.

The second advantage is that volunteer crops can be controlled easily. A grassy herbicide may be applied to broadleaf crops while a broadleaf herbicide can be applied to a cereal.

Controlling weeds is a challenge for all farmers. In the early days of direct seeding or zero tilling, it was a commonly held opinion that weed management and control were more difficult than under an intensive tillage system. After extensive research, Dr. Doug Derksen (AAFC Brandon) concluded "Crop rotation has a larger impact on weed communities than tillage systems".

In a diverse crop rotation where a variety of herbicides are being used, record keeping becomes even more important. Many of today's herbicides have more than season-long residual. Keeping a detailed notebook of the herbicides applied and at what rates will help to stave off any establishment problems the following spring.

Extending the rotation by adding different crop types further diversifies the crop base and enhances weed control. For example, introducing a winter cereal into the rotation is advantageous in two ways. The first is that an establishing winter cereal crop often requires only one 2,4-D application late in the fall to control winter annual weeds. No other herbicide may be required the following spring as the winter cereal, if well established, is likely to be more competitive than the spring weeds. The second advantage to a winter cereal seeded after a spring cereal, such as barley, is that it increases the interval between two broadleaf crops.

A forage crop, such as alfalfa, is another crop type that has advantages in a rotation. It's easy to establish alfalfa by underseeding it to an imazethapyr-tolerant canola variety. Many weed species actually decrease in incidence under a healthy alfalfa stand. The stand can then be removed after 3 to 5 years with an application of glyphosate applied just prior to a cut. In many instances, the herbicide has eliminated the need for tillage to remove the alfalfa. The following year, a competitive crop, such as oats, could be seeded so any alfalfa regrowth can be controlled. Including Roundup Ready canola in the rotation will help to control any dandelions remaining from the alfalfa stand.

A third advantage to the broadleaf-cereal-broadleaf-cereal rotation is that it lowers the risk for soil-borne and residue-borne diseases. Alternating a broadleaf with a cereal serves to "confuse the pests". The pests can also be confused if the cereals are different each year such as wheat in Year 2 and oats in Year 4.

At one time, it was believed that direct seeding increased disease risk because of the extra crop residue. Dr. Karen Bailey (AAFC Saskatoon), initiated a study at Indian Head that looked at rotation x tillage and how these 2 factors affected disease pressure. After several cycles of the different rotations, she concluded that a monoculture (ie. wheat on wheat) in a direct seeding system did indeed increase the presence of disease. Where there was a rotation, however, such as wheat on pulse or oilseed, the degree of infestation was the same on both conventionally seeded and direct seeded fields.

Dr. Bailey has since rated 4 factors that affect disease. The most important factor is Environment. If the growing season is wet and humid, conditions are ideal for disease growth. The second most important factor is Rotation. A diverse rotation is less affected by disease than a monoculture. Location in the field is ranked third. Typically, one field has eight fields adjacent to it (one on each side and one on each corner). Some of the fields could have had a similar host crop last year so no matter how good your rotation, some diseases can move in from neighbouring fields. The lowest ranking went to the tillage system. Conventional tillage or low disturbance seeding had the least bearing on whether a field would be affected by disease.

Finally, when planning a rotation, the marketability of each crop must be considered. There's no point in growing the crop if there's no market for it. A well planned rotation has to have enough flexibility in it that some substitutions can be made when the market for a crop appears to be peaking or bottoming out.

The farm's crop rotation is just one component of a direct seeding system. Getting the rotation right will enhance the other components, creating an efficient, effective seeding system.

Chapter II - Specific Crop Types

In the Introduction to Rotations, reasons for rotating crops in a direct seeding system were discussed and the basic crop rotation oilseed-cereal-pulse-cereal was introduced. In this chapter, more specific information on different crop types and their place in the rotation will be reviewed. "Confusing the Pest" by alternating dates of seeding and timing of herbicide application will also be discussed further.

Stew Brandt, researcher with Agriculture and Agri-Food Canada (AAFC), once stated at an SSCA Annual Conference (February, 2000), that rotation planning begins with deciding on the proportions of the various crops to be grown in the rotation over the long term and then developing a rotation strategy for the entire farm. Ken and Don Sapsford, long time direct seeders from near Perdue Sk, believe the key to their rotation is planning. Before including a crop in their rotation, they considered the impact each crop would have on the farm's bottom line, crop residue management, weed control and fertility requirements.

Clint Steinley, another experienced direct seeder from near Empress, Ab said that when he developed his rotation, he did so with the idea that the crop this year had to in some way benefit the crop next year. For instance, Clint grows winter wheat because he believes it benefits the succeeding pea crop in 2 ways. The first is that the winter wheat stubble tends to be clean (free of weeds). The second benefit comes from the straw and chaff which Clint believes suppresses early spring weed growth in the peas.

Each crop in the classic oilseed-cereal-pulse-cereal rotation benefits the succeeding crop in that it "confuses the pests". The diseases common to broadleaf crops are not common to cereals. Likewise, cereal diseases are not found in broadleaf crops. Alternating between a cereal and a broadleaf helps to reduce the incidence of disease in the succeeding crop.

Volunteer weed management is also made easier by alternating crop types. With herbicides, it's easy to remove a volunteer broadleaf from a cereal and a volunteer cereal from a broadleaf.

The following chart illustrates how rotating crops, seeding dates, herbicide groups and timing of herbicide application throws the weeds "off-guard".

sapsford1

If we take the Wheat as Year 1, Roundup is applied in late April, early May just prior to seeding the wheat. In June, the wheat receives an in-crop herbicide application. The wheat is treated with Roundup pre-harvest for perennial weed control.

Year 2 is the canola year. If the variety is Roundup Ready it can be seeded in late October, just prior to freeze-up, or seeded in late April. Roundup may be applied twice in May. The canola will likely be harvested fairly early in the fall. If the field doesn't have a lot of perennial weed pressure, then it might receive only an application of 2,4-D about the 15th of October. If the field does have a lot of perennial weeds, then a post-harvest application of Roundup, soon after the field has been harvested may be warranted. The application of 2,4-D on the 15th of October will help to control winter annuals and any late germinating dandelions.

If the canola in Year 2 is a different variety, Roundup should be applied prior to seeding and the crop may very well require an in-crop herbicide application. Again, 2,4-D applied in mid October is a good idea for the succeeding crop.

Barley is the crop for Year 3. Seeded in early May, it's likely to require an in-crop herbicide. If the field has been direct seeded for more than a couple of years, granules such as ethalfluralin, may be spread on the surface without any incorporation (contact your local granular rep for instructions).

The peas in Year 4 receive season-long protection from the granules against some broadleaf and grassy weeds. Seeding the peas early and applying Roundup post-seeding, pre-emergence will ensure good weed control early in the growing season. An in-crop herbicide may be required in June. Again, an application of 2,4-D in late October will help to control the winter annuals prior to seeding a cereal the next spring.

Following this 4 year cycle, the wheat and barley were seeded about the same time, and later than the canola and the peas. Roundup was applied pre-seed each year but at slightly different times. In-crop herbicides were used in the wheat, barley and peas and possibly in the canola if it was not a Roundup Ready variety. Roundup was applied post-harvest following the wheat crop and possibly following the fall seeded or early seeded canola. 2,4-D was applied twice, prior to cereals the following spring and granules were applied once, prior to the peas. In summary, this rotation shows a variety of seeding dates, a variety of herbicides and a variety of times when the herbicides were applied.

To introduce winter wheat into the rotation is relatively simple, if you plan ahead. In Saskatchewan, the ideal window for seeding winter wheat is between the 20th of August and the 10th of September. That's usually in the middle of harvest. To avoid conflicts between seeding and swathing/combining, the seeding implement needs to be ready for use on those mornings when it's too damp to harvest. As well, the crop has to be harvested early enough that seeding of the winter wheat can occur in the appropriate time frame. Once those two conditions have been met, winter wheat can be added to the rotation.

In the classic rotation, there are two options for inserting the winter wheat. The first is following the fall seeded or very early seeded canola. An application of 2,4-D in early spring may be the only weed control needed in the winter wheat as the winter wheat, if well established in the fall, is highly competitive against wild oats the following spring.

Another option for introducing winter wheat into the rotation is to seed the winter wheat after the barley crop as barley is usually off fairly early in the fall. Again an application of 2,4-D early in the spring to the winter wheat is recommended.

A real benefit to including a winter cereal in a rotation is that it adds an extra year between broadleaf crops which will help to reduce the incidence of sclerotinia in susceptible crops.

Introducing forages into the rotation is another way to confuse the pests and reduce the use of herbicide. At an SSCA Annual conference (February, 2000), Dr. Martin Entz of the University of Manitoba said that a stand of alfalfa has a strong impact on the weed species and composition. In one Manitoba study, fields under annual crop production were compared to fields with alfalfa in the rotation for the presence of and numbers of a variety of weeds. The results showed that the number of wild oat, wild mustard, Canada thistle, sow thistle and cleavers tended to be much lower in the succeeding crop in the fields with alfalfa than those fields under annual crop production. Wild buckwheat, red root pig weed and lamb's quarters numbers tended to be the same under both systems while dandelion and stinkweed populations tended to be higher after an alfalfa crop.

Dr. Rene Van Acker of the University of Manitoba related the results of one study conducted near Glenlea, MB at another SSCA Conference (February 2001). This study has three basic rotations ranging from simple annual to two to four years in alfalfa. The rotations also received the following treatments: both pesticides and fertilizer; pesticides only; fertilizer only; and no pesticides or fertilizer. In the 8th year of the rotation, all plots were seeded to flax and weed infestations were assessed. Weed densities were significantly lower in rotations containing legume forages (sweet clover plough-down or 2 years of alfalfa). Flax yields in the alfalfa rotation without either pesticide or fertilizer applied, were the same as flax yields in the annual rotation when both fertilizer and pesticides were applied.

Including alfalfa in the rotation is easy now that Pursuit (imazethapyr) is registered for use on alfalfa seedlings. Underseeding the alfalfa to a Clearfield canola variety allows for a cash crop to be harvested in the year the alfalfa is establishing. An application of Pursuit controls a variety of weeds, both broadleaf and grassy, reducing competition for the alfalfa. After 3 - 5 years, the alfalfa can be removed without tillage by applying Roundup prior to a cut when there's lots of vegetation. (Check the Crop Protection Guide or call your local Monsanto Rep for application rates). The following spring, a competitive crop such as oats or Roundup Ready canola should be seeded so that alfalfa regrowth may be controlled.

In terms of disease control, crop rotations are most effective at helping to reduce infestations of diseases that originate from the soil or on crop residue. Diseases that are wind blown or borne on the seed will not be affected by crop rotation, therefore, every field on the farm is at some risk for a disease infestation due to the crops that were on or are on the neighbouring fields.

While a crop rotation must have some flexibility, designing a rotation based on the oilseed-cereal-pulse-cereal model will ensure a good mix of high and low crop residues and a better defense against weeds and diseases.

Farming without a crop rotation plan is like driving a car in a blizzard without a winter survival kit. It doesn't cost anything to throw some blankets, matches and candles into the car and these may save your life. It doesn't cost anything to plan your rotation and it may save your farm.

Thanks Thom Weir, Extension Agrologist, SAFRR and Ken Sapsford, Research Assistant, University of Saskatchewan for their assistance with this section.