Should you plant corn after canola?

Planting corn after canola can result in corn being impacted by phosphorus deficiency, or what is commonly called “corn after canola syndrome”.  Phosphorus enters the corn plant through root hairs, root tips, and the outermost layers of root cells.  Beneficial fungi, called mycorrhizae, enhance P uptake in corn early in the season because the mycorrhizae strands increase the effective rooting volume of plants.  This is extremely important for uptake of immobile nutrients, such as P.

In corn, up to 80% of early season P uptake is by mycorrhizae since the strand network may extend 8-12 inches from root.  Crucifer plants, such as canola, sugarbeets, and mustard, are not hosts for mycorrhizae so the mycorrhizae must regrow from spores.  This is why early in the growing season you might see corn suffering from P deficiency.

Although the mycorrhizae eventually regrow and colonize the roots, the damage done by lack of P early in the growing season may have already occurred.  Yield may be impacted by the early season P deficiency, maturity of the crop may be delayed and grain moisture content at harvest may be higher (leading to higher drying costs).

How Common is Grain Corn Planted After Canola in Manitoba?

In Yield Manitoba 2014, Anastasia Kubinec (Provincial Oilseed Crops Specialist) updated her crop rotation tables based on information MASC contract holders provide to MASC.  These tables provide trends that can be used to help with crop rotation choices.  If we dig into the data from the 2008 to 2012 time period, 22% of the grain corn acres were planted into canola stubble.  In fact, it was the most popular choice for producers, followed by planting corn into soybeans (16%) and into spring wheat (10%).  So the data illustrates there are other factors producers look at when planning their grain corn crop rotations, and not necessarily the influence of beneficial fungi.

What is the potential impact to yield?

The same data source provides details on the yield response of those rotations (see Table 1 below).  Grain corn  yields are lower following canola than soybeans or spring wheat.

Table 1: Relative Yield Response (per cent of 2008-2012 average) of Manitoba crops sown on previous crops (stubble >120 acres)

Previous   Crop  Crop   Planted – Grain Corn 
Hard Red Spring Wheat 100
Winter Wheat 87
Barley 99
Oat 93
Canola 95
Soybean 103
Sunflower 99
Grain Corn 87
Yield (bu/ac)  95

The initial phosphorus uptake can be an issue but is this practice manageable?

To avoid ‘corn after canola syndrome’, producers should grow a crop less dependent on mycorrhizae for P uptake after canola (corn & flax are two of the most dependent crops).

If rotation requires corn after canola, a “Plan B” is to supply high starter P levels to try and overcome any P deficiency problems that might occur.

Variable results are seen with in-crop treatments to remedy phosphorous deficiency.   If products are tried, it is recommended to leave a check strip to determine effectiveness on crop recovery and yield.

For more information, please see the complete article by Anastasia in Yield Manitoba 2014 at:

Submitted by:  Pam de Rocquigny, Provincial Cereal Crops Specialist

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