A recent Agriculture and Agri-Food Canada study found that growing peas and, to a lesser degree, lentils the year prior to canola can enhance canola yield. Study results showed a significant and consistent yield advantage when canola was grown on pea compared to wheat stubble. The 2-year study also found that growing canola on canola resulted in statistically significant reductions in canola yield at Beaverlodge and Lethbridge. Results from other locations also showed trends towards reduced canola yields but differences were not statistically significant.

Crop insurance data from Manitoba and Alberta for 2000-10 provides more evidence that canola on canola will reduce yields. In a presentation from 2011, Murray Hartman, Alberta Agriculture canola specialist, and Anastasia Kubinec, Manitoba Agriculture oilseed specialist, used crop insurance data to show that canola on canola stubble yielded 15% to 20% lower, on average, than canola on wheat and barley stubble. Not only that, but the crop insurance data showed that yields for canola on canola are actually declining.

A quick explanation of why rotation matters for pest management: Lack of crop diversity can increase the density of pests in a field. The higher the population of a pest in a field, the higher the risk of developing resistance to the pesticides (herbicide, fungicides or insecticides) used for controlling the pest in those fields. Why? The genetic coding for resistance to pesticides is found in very low numbers within a pest population, but the higher the density of the pest, the more likely a resistant individual will be present. The goal of resistance management, regardless of the class of pest, is to maintain that frequency of resistant individuals at a very low level within the total population. Once the frequency of that resistant type makes up the majority of the total population the chemistry that it is resistant to will no longer provide effective control. By the time resistance in the pest population becomes obvious, it may be too late to manage effectively.

Historically, disease management — blackleg specifically — was the key driving factor behind the recommended one year in 4 canola rotation. Blackleg lives on stubble for 18-24 months. Since the disease is initiated from infected canola residue, a break of 3 years between canola crops was needed for that residue to decompose to significantly reduce the blackleg threat the next time growers seeded canola on that field. Then came blackleg resistance in canola varieties in the mid-‘90s, which reduced the need for rotation to manage blackleg. Growers took advantage and went to tighter rotations. But blackleg resistance is starting to show cracks in some areas where tight canola rotations have become normal practice.

Canola growers need to asses the risk that blackleg poses to their production. Scouting is essential to identify the disease and to determine its severity and the risk it poses to production. If the disease is identified in a canola crop, and it is severe, then this is a high risk field and additional measures are needed. (See blackleg photos below.) Growing the same variety will put the grower at even greater risk. Using a fungicide can bring the risk down. Extending the rotation between canola crops will bring it down further. Rotating to a new variety with a different source of resistance even further.

Clubroot. A similar rotation plan will help preserve clubroot resistant canola varieties, which have become the key clubroot management tool for growers will heavy clubroot infestation.

Clubroot-resistant varieties are effective, but to keep that single-gene resistance viable for the long term, growers may need to use longer canola rotations and rotate between varieties with different sources of resistance. Clubroot resistance is not expected to be durable and can be put at risk within two cycles of canola crops on a field. Extending the rotation length between canola crops and rotating between the 2 major clubroot resistance suppliers will reduce this risk. For growers who have discovered clubroot recently, taking measures to isolate infested areas within a field can have a long term financial benefit. Effective measures include reduced tillage on those fields, seeding those fields last to limit spread of soil to other fields, and cleaning equipment before leaving those fields.

Rotating fungicides. Pathogens can become resistant to certain active ingredients, and fungicide rotation is often recommended — especially when applying fungicide multiple times per year. This may be more important for potato and chickpea growers, but in the case of sclerotinia — a pathogen that infects a variety of crops — growers may want to consider rotating between fungicides if they grow a number of sclerotinia-susceptible crops in their rotation. If you have limited fungicide group options for sclerotinia control in other crops, canola may be your best opportunity to rotate fungicide groups. Registered canola products include fungicide groups 2, 3, 7, 9, 11, 12 and biofungicide.

Blackleg spores on stubble. Most spores will break down in 18-24 months.

Blackleg damage on canola leaves, Manitoba July 2011. Source: Anastasia Kubinec, Manitoba Agriculture, Food and Rural Initiatives.

For many canola growers, their most challenging weed has become volunteer canola, especially with tighter canola rotations. Growers may need to consider a rotation of herbicide-tolerance systems as a method to help manage the volunteer canola seed bank. And volunteer canola management becomes a goal in non-canola years.

Crop planning and canola rotations also require growers to think about the herbicides used in the 12 months before seeding canola. This may be especially important for growers who tried different products or rates this fall to clean up fields that didn’t get a timely spray in the spring/summer. Many herbicides can cause carryover damage in canola when used in the fall ahead of canola seeding. Click here to see an article that lists products that can be used in the fall ahead of canola.

Benefits of canola for weed control: For growers in less traditional canola growing areas, an extra benefit of canola in the rotation is the opportunity to hit weeds with a different system. Canola can clean up fields for other crops in the rotation that may not have weed control options that are as broad spectrum or cost effective as canola products. With a rotation of wheat and canola and pulses, growers can hit weeds with a wide variety of herbicide groups from year to year. This is good, as long as these products do not become overused as a result.

Risks with tight rotations: The success of herbicide tolerant canola technology to control weeds is one reason why canola rotations have tightened. Economical weed control has increased canola profits. But the reduction in crop diversity can allow some weeds (and insects and diseases) adapted to growing with canola to increase in density and intensity. This places increased pressure on the chemical tools used to manage these pests, thereby increasing the risk of resistance to those products. Some examples of resistance have have already been documented in North America, such as glyphosate resistant kochia in central North Dakota. A predictive model by Hugh Beckie, research scientist with AAFC in Saskatoon, ranks kochia first among weeds at risk of developing glyphosate resistance on the Prairies. Read the article on page 11 of the March 2011 Canola Digest.

When using glyphosate as a pre-seed treatment ahead of non-canola crops, tank mixing with 2,4-D or Heat will help reduce the risk of glyphosate tolerant weeds developing. Do not use these tank mixes ahead of canola. CleanStart with Aim chemistry is a pre-seed option ahead of canola.

In the end, growers have to consider profitability when making rotation decisions. Taking a risk on a tight canola rotation may be the better economic decision for their farm that year. The key is to consider the overall approach to insect, disease and weed management and, ultimately, how all rotation decisions will influence the long-term profit potential for the farm. This is also where risk management related to differences among crops in nutrient use, moisture use and stress tolerance come into play. Relative performance of crops will vary year to year depending on which challenges are most limiting to yields in that year, so some having some diversity can help manage economic risk in the long term.

As stated in the intro, recognize the value of diversity. The more growers do to vary their practices — including rotation of crops, pest control products, and varieties — the harder it will be for disease, insects and weeds to adapt and increase.

Crop Production Week, Saskatoon, January 9-12. SaskCanola’s meeting is Thursday, January 12. The Canola Council of Canada will join SaskCanola at its booth at the Crop Production Show all week.

AgDays, Brandon, January 17-19. Manitoba Canola Growers Association hosts Canola Day on Tuesday, January 17.

Farm Tech, Edmonton, January 24-26. Alberta Canola Producers Commission holds its AGM Tuesday, January 24, but there are canola presentations each day. Register by January 6 for early bird savings.

Excess moisture agronomy meeting, February 1 in Moosomin, Saskatchewan and February 2 in Elgin, Manitoba. Growers and agronomists will learn how to deal with fertility, weeds, disease and insects following a year with excess moisture. Contact CCC agronomy specialists Kristen Phillips (phillipsk@canolacouncil.org) for more on the Manitoba meeting and Shawn Senko (senkos@canolacouncil.org) for more on the Saskatchewan meeting.