Planting corn after a crop of canola is generally not recommended, and the reason why has to do with beneficial fungi living in the soil. However, some studies and anecdotal experience in Ontario have shown that this rotation is not always problematic.
Arbuscular mycorrhizal fungi (AMF)
Many crops we grow, including corn, have a symbiotic relationship with AMF in the soil. AMF colonize plant roots and form an extensive network of hyphae (thread-like filaments) that transports nutrients to the plant. The hyphae explore much more soil volume than plant root systems can, and mycorrhizal plants can absorb more phosphorous at lower concentrations in soil solution than nonmycorrhizal plants. In exchange for this vast nutrient highway, AMF depend on plants for their carbon nutrition. AMF are abundant in most soils, although cropping practices such as tillage and rotation can impact their associations with crops.
Corn seedlings have small root systems and utilize AMF to explore more of the soil to obtain adequate phosphorous to satisfy early demand. Phosphorous (P) is not very mobile in soils and in the absence of AMF, corn seedlings may exhibit P deficiency; seedlings may be short with weak stalks and may be a darker shade of green or purple in colour. Typically, the main issue is delayed corn growth (Figure 1). Flax is another crop highly dependant on AMF.
Canola, like all Brassica plants (e.g. cabbage, broccoli, mustard), is nonmycorrhizal and does not utilize or host AMF. After a crop of canola, AMF networks in the soil will be reduced and the fungi have to re-colonize from spores (reproductive cells), which can take about 50 days in the presence of a host. This is the key reason why corn after canola is not recommended. Other nonmycorrhizal plants include sugarbeet (Beta vulgaris) and common lambsquarters from the Amaranthaceae/Chenopodiaceae family.
Figure 1. A corn field in Manitoba (2021) exhibiting two different stages of maturity, where the only difference in management is the previous crop. Corn following peas in rotation (left) is at the tasseling stage, while the corn following spring canola (right) has not yet tasseled because of delayed growth related to lack of AMF in the soil. Photo credit: Anastasia Kubinec.
Examples of Success and Failure
Deb Campbell, CCA with Agronomy Advantage Inc, has described seeing severely stunted corn following spring canola on 4 or 5 occasions. In these cases, the producers did not consider potential P deficiency or apply fertilizer to compensate for low AMF function. The worst case was corn following cabbage, where the field had high soil P levels but starter P was not applied with the corn.
Ryan Benjamins, CCA with Benjamins Agronomy Services, works with sugarbeet growers who are well aware of the risks of this rotation but are making it work. Most have access to manure and have high soil test P levels. Others ensure they broadcast a strong rate of P ahead of the corn and include about 13 lbs of P2O5/ac in furrow. They have sometimes noted delayed growth and purple colouring of seedlings which suggests P deficiency, but there is typically no yield loss associated with the rotation in Ryan’s experience. He did note that there may be differences in corn hybrid response.
With winter canola, there is ample opportunity to grow another crop after canola harvest. Oats are highly mycorrhizal and are the top recommended cover crop choice, but many crops and weeds support AMF. Soybeans are mycorrhizal but not as highly reliant on AMF, so this rotational crop works well for those interested in double cropping. One winter canola grower has had their highest yielding corn in rotation after canola, but most years there were double-cropped soybeans in between. These fields have sandy loam soils, high soil test P levels, and starter P fertilizer in furrow at corn planting. The grower also offered the reminder you will always have volunteer canola after harvest, and it may need to be controlled to establish a cover crop.
What is unclear with winter canola rotations, is if you can successfully grow corn in the spring if canola is winterkilled. Generally speaking, John Heard, soil specialist with Manitoba Ag, feels that high soil P levels and starter P fertilizer have reduced our reliance on AMF in corn. Studies in Manitoba conducted by Dr. Don Flaten and Magda Rogalsky have shown that starter P narrows the yield and maturity penalty of corn grown after canola. Starter P treatments increased corn maturity in 2 of 3 years and increased corn biomass, with the greatest response in corn after canola. In these trials there was a 10% increase in corn grain yield where MAP (monoammonium phosphate) was banded at a rate of 53 lbs P2O5/ac, regardless of the crop rotation.
There are few examples in the literature of corn planted directly after terminating canola. In a weed control study in Indiana, published in 2020, winter canola seeded in September and terminated 2 weeks prior to planting corn did not decrease corn grain yield compared to fallow (fallow from September to spring) but there was no information on soil P levels or starter fertilizer. In a corn silage study conducted in the late 1990s in British Columbia, where corn followed corn, canola or summer fallow, the effect of previous crop on silage yield was greater for unfertilized corn compared to corn with P side banded at 60 lbs of P2O5/ac. Silage yields were similar following summer fallow and canola (note that fallow fields may also have low AMF), and in one year yields were lower after canola than they were in the corn-on-corn rotation.
Conclusion
The safest recommendation is to not plant corn after a Brassica crop because of the risk of P deficiency and delayed corn growth. If possible, grow something else between canola and corn to help re-establish AMF. It is difficult to know what conditions and management practices will completely mitigate this rotational issue, but high soil test P levels and starter P in furrow or banded beside the corn seed will likely support early corn growth.
AMF Fun Facts:
hyphae can extend more than 10 cm from root surfaces, which is 100x further than most root hairs
hyphae can access smaller soil pores than roots + root hairs because they have a smaller radius (0.005 mm vs 0.15 mm)
the volume of soil explored by roots with mycorrhizal associations can be up to 100x greater than nonmycorrhizal roots
mycorrhizae can acidify the rhizosphere or excrete chelating agents to mobilize P and increase availability to plants
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