Crop Rotations Should be Planned

(continued from last issue)

By Ken Sapsford, P.Ag.

SSCA Soil Conservationist

In the last issue of the Prairie Steward I started to look at crop rotations. In there I covered a number of weed control options that can be tied into a rotation. I will continue to look at the disease implication of this rotation and a few additional weed control options.

I was looking at a 4-year crop rotation with Cereal - Pulse - Cereal - Oilseed. This rotation can be used in all soil zones, as the specific crops that are plugged into the rotation will vary according to the region. This rotation has some advantages:

  1. Volunteer weed control is made simple, as it is easy to remove a broad leaf out of a cereal crop and a grassy weed out of a broadleaf crop.
  2. Crop diseases are kept in check as all crops have a 4-year span before they are planted back on the same piece of land, provided there are two different cereal crops used. Sclerotinia is the one disease that can carry across between pulses and oilseed crops so it has to be watched in wet years.
  3. Crop residues are easy to manage, as there is always a low residue crop, pulse or oilseed following a high residue crop, cereal. This helps avoid large residue build up that may cause plugging problems with seeding equipment. It also protects the soil with enough residues to avoid erosion.
  4. Soil fertility is enhanced with the inclusion of a pulse in the rotation. This can either reduce some of the required fertilizer input or increase yield and/or protein in the following cereal crop.

It was once believed that disease would be worse in a direct seeded crop than in conventional till and that the disease would stay around on the field longer because the straw is decomposing at a slower rate. At Indian Head research station Dr. Karen Bailey showed that disease was definitely worse on a direct seeded field with no crop rotation. This was wheat on wheat and the tanspot and septoria were worse on the direct seeded field compared to the conventional till wheat on wheat stubble. When she looked at wheat on oilseed or pulse stubble, the degree of disease infestation was the same whether the field was direct seeded or in conventional tillage.

Dr. Bailey rated a number of factors as to how they affect disease:

  1. Environment - If the year is wet and humid and ideal for disease growth we will see disease in our crops.
  2. Rotation - A poor crop rotation will have higher incidence of disease than a good rotation
  3. Location in field - The field borders are more likely to be infected with a disease than the centre of a field. This is due to the neighbouring crop. Even if you have a good crop rotation there are 8 fields adjacent to it (1 on each side and 1 on each corner) and some of these fields could have had a similar host crop last year.
  4. Tillage system - This had the lowest rating of any of the factors she looked at in disease rating.

Of all the factors, the one with the biggest impact on disease and the one we have control over is rotation.

Some of the diseases that were a problem in a given year may not be controlled by rotations but rotations may have an impact on their severity.

Ergot

Ergot is caused by a fungus that can infect all cereals and grasses. It is usually more severe in rye and it is seldom found in oats. The ergot fungus survives from year to year as sclerotia that have fallen to the ground or as sclerotia sown with seed. The incidence of ergot varies from year to year and it depends upon how closely the spore release coincides with the flowering period of susceptible plants.

If you had ergot in a cereal field last year you know that you will have sclerotia in the soil because they are lighter than the grain and many of them blew out of the combine with the chaff. We can't avoid ergot with rotation because the spores will blow in with the wind from the field borders but we may be able to reduce its severity by not planting a cereal crop on that field next year. Ergot sclerotia can only survive one year in the soil so the oilseed - cereal - pulse - cereal rotation will help reduce infection.

Sclerotinia

Sclerotinia has a similar life cycle as ergot but the sclerotia stay viable in the soil for a longer period of time. The crop rotation of oilseed - cereal - pulse - cereal will not reduce the incidence of Sclerotinia significantly as it is recommended that we have a 4 year break in having a host crop in rotation. All pulse and oilseed crops can be a host. Sclerotinia does usually not affect flax but it can be a host that will allow for the disease to remain on that field. We don't have enough crop selection to choose from to avoid Sclerotinia. We can improve our rotation with the inclusion of a winter cereal that will extend the rotation to 5-years from our 4-year rotation. Our rotation would now be oilseed - cereal - winter cereal - pulse - cereal. This rotation would give us two years of no host for Sclerotinia. This will not eliminate the disease from the rotation but it will help.

If we continue to look at this 5-year rotation as to its weed control options, we find it has some benefits there as well.

This chart is similar to the one I had in the last Prairie Steward but this time I have included some specific crops and added Winter Wheat as my 5th crop. Many farmers have been seeding winter wheat on canola stubble but some also have been seeding it on barley stubble. The advantage of barley stubble is the increased straw cover helps insulate the soil so we are not completely dependent on snow cover for the survival of the winter wheat. Many times the only weed control that is required for winter wheat is a fall application of 2,4-D to control the winter annuals. The winter cereals get off to a very early start in the spring so the need for in crop weed control may not be needed. Don't assume that you don't need in crop weed control, field scouting is required and you may need to spot spray for weeds like wild oats.

If you like to get into longer-term rotations to assist in weed control and disease management some farmers are looking at including forages in rotation. One example may be:

Year 1 - Smart canola under seeded with alfalfa (use Pursuit to control weeds in canola and give some residual weed control in alfalfa stand)

Year 2 to 5 - Alfalfa - This will help break disease and weed cycles

Year 5 - Remove alfalfa with glyphosate sprayed prior to second cut while there is good regrowth

Year 6 - Seed competitive cereal crop that can be sprayed with a broad leaf herbicide to control alfalfa (oats may be good as there should be very few wild oats)

Continue with annual crop rotations for a few years. You may want to include a Roundup Ready canola in the rotation if dandelion or other perennials have developed while the land was in alfalfa.

There is no perfect rotation. However taking the time to plan your rotation may reap you some benefits. Any increase to production that is achieved from additional management is all profit. There is no input cost to management besides the time you put into it. Like everything else, the more you put into it, the more benefit you receive from it.