Lessons from Long-Term Irrigated Crop Rotation Research

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 Introduction | Experimental treatments | Results | Soil microbial properties | Other measurements | Summary | Acknowledgements

Irrigated cropping offers a dual challenge of producing high value crops while maintaining soil quality. Common irrigated crops (e.g. potatoes, beans, sugar beets) produce little crop residue for return to the soil and tight rotations may have long-term detrimental effects on our soil resource in terms of diminished soil quality and increased erosion risk.

An irrigated rotation study was initiated in 2000 to examine the impact of conventional and sustainable rotations for potatoes, sugar beets, beans, soft wheat and timothy. The merits of each of six rotations are judged using data on crop yield and quality, weed, insect, and disease pressures and soil quality.

Our objectives are to devise crop sequences and tillage management systems for irrigated land that: (1) optimize crop response; (2) reduce soil erosion, enhance soil quality and promote long-term sustainability; and (3) minimize weed, insect and disease pressures.

Experimental Treatments

The following crop rotations were established in spring 2000 at Vauxhall, AB. The 2005 growing season represented the 6th crop year of this study.

Rotation Management
1 Yr W Cont. wheat (baseline)
3 Yr (P-B-W)cConventional
3 Yr (P-B-W)sSustainable
4 Yr (W-SB-B-P)c Conventional
4 Yr (W-SB-B-P)s Sustainable
5 Yr P-W-SB-W-BSustainable (cereal break)
6 Yr O(t)-T-T-SB-B-P Sustainable (forage-based)
W = wheat; P = potatoes; B = beans; SB = sugar beet; O(t) = oats harvested as green feed in July, timothy seeded in late August, T = timothy.

Each phase of each rotation was represented resulting in 26 treatments. These were replicated four times to give 104 plots. The plot dimensions were 10 x 18.3 m with a 2.1 m interplot area between each plot.

Agriculture and Agri-Food Canada, Lethbridge, AB, Canada
The sustainable rotations are built around four specific management practices:
  1. direct seeding or reduced tillage where possible
  2. fall-seeded cover crops where possible
  3. composted cattle manure as a substitute for inorganic fertilizer
  4. straight cutting of solid seeded rather than undercutting of wide-row seeded beans

Table 1 shows that in 2005, the 3 yr conventional rotation with wide row beans (1319 kg ha-1) produced lower yields than the narrow row beans in the 3 yr sustainable rotation (2941 kg ha-1). Also in the 4 yr rotation the conventional treatment (1726 kg ha-1) was significantly lower-yielding than the sustainable rotation (2890 kg ha-1). Averaging all 6 years, the longest 6-yr rotation (narrow row) produced the highest yields (2306 kg ha-1), while the 5 yr rotation (narrow row beans direct seeded into shredded wheat stubble, 1888 kg ha-1) and the conventional 3-yr rotation (wide row, 1906 kg ha-1) yielded lowest.

Table 1.  Rotation effect on bean yields (kg ha-1) at Vauxhall, 2000-05.
6-yr avg 190620612138223018882306
†Within rows, means followed by the same letter are not significantly different from each other according to LSD = 0.05.

For potatoes (Table 2), there was no significant effect of rotation on yield in the first five years of the study. Therefore on the compost-amended plots, fertilizer inputs of P could be cut back to zero and N by one third and yields were not significantly different that the rotations receiving all their nutrients from fertilizer. However in the 6th year (2005), the 4-yr sustainable rotation (43.9 Mg ha-1) yielded significantly higher than both the 3-yr conventional (28.6 Mg ha-1) and sustainable (32.7 Mg ha-1) rotations. Averaging all six years, the overall lowest rotation was the 3-yr conventional one which was about 10% lower yielding (37.7 Mg ha-1) than the average of the other five rotations (41.9 Mg ha-1).

Wheat yields (Table 3) were highest overall on the 3-yr sustainable rotation for 2001, 2002 and 2003. The continuous wheat (Cont. W) was the significantly lower in all years except the initial one (2000). In 2005, the wheat following sugar beet in the 5-yr rotation was significantly higher yielding (5.63 Mg ha-1) than all other treatments except the 3-yr conventional treatment (4.53 Mg ha-1). Over the 6-year period, continuous wheat yielded about 70% of the average yield of the other rotation treatments.

Table 2.  Rotation effect on potato yields (Mg ha-1) at Vauxhall, 2000-05.
6-yr avg.37.742.541.94141.842.5
†Within rows, means followed by the same letter are not significantly different from each other according to LSD = 0.05.

Table 3.  Rotation effect on wheat yields (Mg ha-1) at Vauxhall, 2000-05.
6-yr avg.3.815.665.555.445.225.135.58
†Within rows, means followed by the same letter are not significantly different from each other according to LSD = 0.05.

Rotation treatment had no significant effect (P = 0.05) on extractable sugar yield in the first 5 years of the study (Table 4). However, in 2004, there was a trend (P = 0.13) of higher extractable sugar yield in the 5-yr sustainable rotation (12,637 kg ha-1) than in the 4-yr conventional rotation (11,181 kg ha-1). The 2005 beet yield data analysis has not been completed at the time of writing.

Table 4.  Rotation effect on extractable sugar yields (kg ha-1) at Vauxhall, 2000-04.
2005Not available at time of writing
5-yr avg9075951896249384
†Within rows, means followed by the same letter are not significantly different from each other according to LSD = 0.05.

Soil Microbial Properties

Each July, beginning in 2002, soil samples were taken from the wheat phase of each rotation for measurement of soil microbial biomass carbon and soil bacterial diversity. Wheat plants were excavated from six random 0.5-m lengths of row from each plot. Loose soil was shaken off the roots, and the soil that adhered strongly to the roots was carefully brushed and kept as ‘rhizosphere’ soil. Non-rhizosphere (‘bulk soil’ (0-7.5 cm depth) was sampled from the middle of two adjacent wheat rows near each of the six locations per plot.

Table 5.  Rotation effect on soil microbial biomass C (mg kg-1 soil), 2002-05. 
Cont. W.
Bulk soil
†Within rows, means followed by the same letter are not significantly different from each other according to LSD = 0.05.

Table 6.  Rotation effect on soil bacterial diversity (Shannon index, H’), 2002-05.
Cont. W.
Bulk soil
†Within rows, means followed by the same letter are not significantly different from each other according to LSD = 0.05.

Microbial biomass carbon (C) is an estimate of both the size of the total microbial community and the mass of potential plant nutrients contained within the cells of the microorganisms. In the bulk soil, the 3-yr and 4-yr sustainable rotations had significantly more microbial biomass C than their conventional counterparts (Table 5). There was also evidence that the longest 6-yr rotation had the highest level of microbial biomass C. The high level of microbial C in continuous wheat indicates that total organic inputs into the soil are also an important factor. For rhizosphere soil, the rotational effect on microbial biomass C was non-significant (Table 5).

The metabolic diversity of soil bacteria, which was analysed by assessing carbon substrate utilization patterns (community-level physiological profiles), is characterized by the Shannon index (Table 6). A higher Shannon index denotes a more diverse or complex soil ecosystem in terms of microbial functions, which makes it generally more stable and more resistant to stress. For bulk soil, even though effects were non-significant for individual years, the cumulative effect, as evidenced by the overall average value, showed that the 3-yr sustainable rotation had significantly higher diversity than its conventional counterpart (Table 6). The 5-yr sustainable rotation had a significantly higher Shannon index (2.50) than the 4-yr conventional rotation (2.22). For rhizosphere soil, as with microbial biomass C, the rotational effect on microbial diversity was non-significant (Table 6).

Other Measurements

Other parameters measured include weed density and diversity, disease pressure (e.g. Sclerotinia on beans); beneficial vs. non-beneficial insects, soil fertility and quality properties.


A trend of better performance in the sustainable vs. conventional rotational practices is starting to emerge in crop yield patterns and soil microbial properties as the study progresses. We need long-term rotation studies to fully understand the interactions that occur when crop choice and sequence are varied. Since the longest rotation is 6 years, the study needs to run for 12 years in order to complete two full cycles and gather meaningful results. It is hoped to continue this experiment up to and including the 2011 field season.


We thank the Alberta Agricultural Research Institute, Alberta Pulse Growers, Potato Growers of Alberta, Rogers Sugar Ltd. and Agriculture and Agri-Food Canada’s Matching Investment Initiative for funding contributions. Andrew Olson, Paul DeMaere, Mandy Collins, Andrea Eastman and Jim Sukeroff provided technical and field help.

Prepared by:
F.J. Larney
Agriculture and Agri-Food Canada, Lethbridge, AB

N.Z. Lupwayi
Agriculture and Agri-Food Canada, Beaverlodge, AB

R.E. Blackshaw
Agriculture and Agri-Food Canada, Lethbridge, AB

J.J. Nitschelm
Rogers Sugar Ltd., Taber, AB

H.A. Carcamo
Agriculture and Agri-Food Canada, Lethbridge, AB

H.H. Mündel
Agriculture and Agri-Food Canada, Lethbridge, AB

H.C. Huang
Agriculture and Agri-Food Canada, Lethbridge, AB

D.C. Pearson
Agriculture and Agri-Food Canada, Lethbridge, AB

D.D. Reynolds
Agriculture and Agri-Food Canada, Lethbridge, AB

G.H. Dill
Alberta Agriculture and Food, Lethbridge, AB

This document is an excerpt from the 2006 Irrigated Crop Production Update Conference Proceedings.

Other Documents in the Series

  2006 Irrigated Crop Production Update Conference Proceedings
Lessons from Long-Term Irrigated Crop Rotation Research - Current Document
Seeding Practices for Irrigated Cereals and Canola
Soil Fertility and Crop Nutrition -- A Balanced Approach
Nitrogen Fertilizer, Forms and Methods of Application
Micronutrients for Irrigated Production
Understanding How to Use Manure or Compost to Optimize Irrigated Crop Production
Re-Cropping Practices after Residual Herbicide Use
Field Crop Disease Review for 2005 and Forecast for 2006
Irrigation and Plant Disease Management
The R.A.T. of Irrigation Management: The Future for Improving Irrigation Efficiencies
Irrigating to Enhance Quality and Yield
New Irrigated Crops of the Future
For more information about the content of this document, contact Ross McKenzie.
This document is maintained by Judy Lee.
This information published to the web on May 24, 2006.
Last Reviewed/Revised on July 7, 2008.