Identification of superior crop rotations

Short canola rotations in the southeast Peace region bring up concerns of yield losses, long-term economics and sustainability. However, despite the advantages of crop diversity found in other areas (including reducing the risk from crop failure or price fluctuations, minimizing pest issue, optimizing yield, and spreading the workload), there was a lack of local information on this topic.

This seven-year study conducted by Dr. Kabal S. Gill at SARDA compared canola and wheat monocultures to 10 different crop rotations including pea (P), barley (B), flax (F), canola (C) and wheat (W) crops, i.e. WC, PWW, CWW, CCW, PCW, CPW, WBC, BWC, FWC, and FCW. One crop from each of the 12 treatments was grown from 2009 to 2015. Crops were fertilized to 45, 60, 60, 35 and 90 bu/ac of canola, wheat, peas, flax and barley, respectively. Recommended agronomic practices were used for each crop.

There were a few weather challenges throughout the study. Four of the years were drier than average (2009, 2010, 2014 and 2015) and the other three years (2011-2013) were normal or wetter than average (including flooding in June of 2013 which resulted in a few plots being discontinued). The variations in precipitation levels and the distribution of rain throughout the growing season were linked to the huge range in canola yields (especially in the crop following a dry year), regardless of the rotation they were a part of. Frost damage in 2010 may have also affected the canola yields.

Results

Crop rotations were generally superior to continuous canola and continuous wheat systems. Interestingly, this is the result of improvements in several areas, including soil moisture uptake, fertilizer use, root health and yield, which together impacted economic returns.

Yield. Although the yield benefits of canola grown in a crop rotation varied from year to year, the average yield benefit was 11.6 bu./ac. (19.9 per cent) over continuous canola. (See the table.) While canola generally yielded lower on canola stubble than other crop stubble, the difference between a variety of crop stubbles was not consistent. Continuous wheat system also tended to have lower yields than wheat in a rotation.

Fertility. Compared to the continuous canola system, which received the maximum amount of N, P, K and S fertilizers, all other rotations required lower total inputs and therefore had lower total fertility costs.

Soil Moisture. Soil water data in 2013, 2014 and 2015 indicated that cereals, especially barley, used more water and peas and flax used the least. Therefore a rotation which included peas or flax would reduce total water depletion compared to one without, while a rotation with only cereal crops would increase water depletion.

Roots. Canola root length, surface area, projected area, volume and number of tips were higher when grown on wheat stubble compared to canola grown on canola stubble. Flax root growth in 2015 was better on wheat stubble than canola stubble but wheat root growth wasn’t significantly different when grown on the canola or wheat stubble.

Economics

The canola-pea-wheat rotation provided the highest gross returns and contribution margins, followed by the wheat-barley-canola rotation and then continuous canola. The continuous wheat rotation came in with the lowest contribution margin out of all 12 treatments.

However, while these values include crop expenses, they don’t account for the increased risk of pest pressure or impact on soil quality that come with long-term continuous rotations. Similarly, revenues do not account for the potential increase in stability and robustness that may come from improved soil quality and reduced soil moisture depletion.