Managed Rotations

Stewart Brandt

AAFC Scott Research Farm, Scott, Sask.

Crop rotations are simply a planned sequence of crops, repeated over time on the same land base. Historically, summerfallow and wheat were the cornerstones of rotations on the Canadian prairies. Typical rotations in the dry prairie were fallow-wheat and fallow-wheat-wheat. In the parkland, rotations of fallow-canola followed by one to three crops of wheat and/or barley were more typical.

With the introduction of other crops, reduced summerfallow and improved seeding and pest control practices, rotations have become much more diverse. Deciding on the most appropriate rotation to use is often confusing due to the large number of options. Further complicating matters are the rapidly changing technologies used with various crops. Rather than selecting the perfect rotation, growers need to focus on developing a rotation plan that is appropriate to their farming operations. To do so requires a good understanding of what is to be accomplished by rotation, and knowledge of the principles underlying crop rotation.

There are numerous benefits to rotation of crops. Some that are currently important are; improved soil and crop productivity; reduced disease and weed losses and control costs, and better marketability and economic returns.

Research on crop rotations began in the early 1900's, and continues to the present. Most of the recent work has been conducted in Saskatchewan. These studies have generated a wealth of information about the principles of crop rotation. As with all scientific principles, there are exceptions. However these principles do apply on most situations where best management practices are used. Some of the more important principles are as follows:

Agronomic principles;

• 1. Crop rotation influences yield.

The practice of summerfallow was developed as a means of increasing and stabilising yield. This practice increased stored soil water and available soil nitrogen while reducing pest populations in some cases. The result was generally increased yield. The advantage of fallow tends to be greater in the dry prairie than in the parkland, because moisture is a more limiting on yield. More recently other technologies have been developed that provide alternatives to fallow, and the benefit of this practice is less evident, particularly in the parkland.

As summerfallow is eliminated, the importance of one crop on those that follow is increased. Several recent studies have evaluated the impact of various crops on the yield of those that follow. In general, crops of one kind (cereal, oilseed, pulse) yield more on stubble of another crop kind (Table 1). These responses can often be attributed to fewer pest problems, soil fertility, or soil moisture effects of the previous crop. In other cases, the cause is less evident. Occasionally small yield decreases (<10%) are noted (particularly in dry regions), but increases are frequently greater than 50% (usually in the parkland). While information for all crop sequences across soil climatic zones is limited, data for pulse crops serves to illustrate this trend, where wheat yield responses increase with increasing moisture for the Brown soil zone (dryer) through to the Black (wetter) (Table 2).

Table 1. Preceding crop stubble effect on yield of various crops at Melfort and Aylsham (Black soil zone) during 1990-92.

Source: Adapted from Townley-Smith, report to Sask Pulse Crop Development Board, 1994.

Table 2. Wheat yield increase on pulse crop stubble across soil zones.

Adapted from various crop rotation and crop sequence reports for the prairie region

2. Some diseases can be managed by growing crops on stubble of other crop types.

(Intervals between similar crops vary depending on disease, crop, and climate).

Crop rotation has long been recognised as a means of reducing incidence of soil and residue borne diseases. Where the same crop or same crop kind is grown in the same field for several years in succession, these diseases usually increase in severity. Rotating to other crop kinds prevents the build up of disease, and allows time for any disease inoculum present to decline. Early results from a canola and pea rotation study under favourable moisture in 1999 illustrate this effect (Table 3). Where pea was grown on pea stubble, mycosphaerella disease infection was higher, and yield was reduced compared with pea grown on wheat stubble. Use of fungicides recovered some but not all pea yield loss on pea stubble. Pea yield on wheat stubble also tended to be higher with fungicide treatment. However, the difference was much smaller than on pea stubble. These observations agree very closely with what is known about rotation effects on disease; rotation will not prevent disease but can substantially reduce yield loss when a disease outbreak occurs.

A similar response was noted for canola, where Blackleg played a major role in reducing yield on canola stubble (Table 4). The highly susceptible variety Westar suffered a yield loss greater than 50%, when grown on canola stubble without fungicide. Fungicide treatment resulted in some yield recovery, but yield was still much lower than for Westar on wheat stubble. Even the resistant variety Invigor 2473 suffered a yield loss when grown on canola stubble. This loss was not fully recovered with fungicide application. This is a typical response for a disease resistant variety, where yield loss is very low under low disease pressure, but does occur where disease pressure is sufficiently high. Even with resistant varieties and fungicides, rotation plays an important role in minimising losses from diseases like Blackleg.

Table 3. Influence of previous crop and fungicide treatment on disease severity and yield of pea at Scott in 1999.

Yield or disease rating values followed by the same letter do not differ significantly at P=0.05

Table 4. Influence of previous crop and fungicide treatment on disease severity and yield of two canola varieties at Scott in 1999.

Yield or disease rating values followed by the same letter do not differ significantly at P=0.05

Similar observations have been made for leaf diseases of wheat and barley grown on the same as opposed to stubble of a different crop type, although responses are not always this consistent, and yield losses are not usually as high. A general rule of thumb is to allow 3-4 years before growing the same crop on the same field. In continuously cropped rotations, this may not be feasible due to a lack of suitable different crop types. Where this is the case, growing two successive cereal crops is usually preferable to growing two broadleaved crops in succession, because disease losses would normally be lower.

3. Weed control can be enhanced.

Taking full advantage of weed control benefits of rotations requires careful record keeping, and more intensive management. Accurate records are required to know what weeds are present, to be alerted when new ones invade, and to avoid damage from soil residual herbicides. Species that are difficult or costly to control in each crop of the rotation can be identified and targeted in crops where control is less costly and/or control is more consistent, thereby reducing populations so that losses or costs are reduced in the problem weed-crop combinations. In general, broadleaf weed control is less costly and more consistent in cereals, and grassy weed control is better in broadleaf crops. In addition, broadleaf treatments are more limited for some broadleaf crops. Thus, the weed control benefits of rotation are more likely to be evident in these crops. Two examples where weeds can be influenced by rotation are green foxtail in wheat and stinkweed in canola (Table 5).

Table 5. Populations of stinkweed in canola and green foxtail in wheat from rotations at Scott.

Other benefits such as improving crop residue management or making more efficient use of moisture are not as well understood, but can be equally important.

Soil Quality Principles:

1. Soil quality is improved by

• reducing summerfallow
• reducing/eliminating tillage
• perennial forages

At the time of first cultivation, the organic matter of our soils provided a vast storehouse of nutrients for crop growth. Over time this resource has been greatly depleted, and nutrient deficiencies are a normal occurrence. Some of these nutrients were exported off the land with the crop, but the major losses occurred as a result of decomposition, erosion, and leaching. Frequent summerfallow and intensive tillage were major contributors to these losses. Long-term yield trends in wheat-fallow and wheat-wheat fallow rotations provide some insight into how soil productivity has been affected. Since the 1940's wheat yield on summerfallow in a wheat-wheat-fallow rotation continued to increase over time for both the phosphate fertilised and unfertilised crops (Figure 1 &2). This trend reflected improved management and varieties, and the ability of the land to mineralize sufficient nitrogen during the summerfallow period. Yield on stubble stopped increasing in the mid-1970's, and now appears to be declining in the absence of fertiliser. This is most probably a reflection of declining capacity of the soil to supply nitrogen on stubble. Application of N and P fertilisers in recent years has largely restored yield, but at ever increasing rates and cost.

From the 1940's till the mid-1980's, yield of unfertilised wheat on fallow was similar for a wheat-fallow and a wheat-wheat-fallow rotation (Figure 3). Since then yields for the wheat-fallow rotation have lagged behind, probably reflecting greater losses in N supplying capacity with more frequent fallow. Applying fertiliser P (Figure 4) increased yield for this rotation, but did not restore it to the levels for P fertilised wheat in the wheat-wheat-fallow rotation.

Other, more direct evidence of the impact of fallow and tillage on soil organic matter is provided in Table 6. After 15 years, organic matter was lowest for a conventional tillage oilseed-wheat-fallow rotation, and increased where either or both fallow or tillage were eliminated. Cropping contributes fresh crop residues to the pool of soil organic matter, while little fresh material is generated on fallow. Tillage, particularly on fallow, speeds the breakdown of soil organic matter, and may contribute to soil erosion.

Table 6. Soil organic matter content (%) after 15 years (1979-94) of cropping to two tillage and two rotation systems at Scott, Sask.

2. Improved soil quality increases yield and reduces fertiliser N requirements.

In the Dark Brown soil zone at Scott where wheat was grown continuously, soil quality was increased compared with wheat-fallow. Where wheat-fallow plots were converted to continuous wheat in 1987, wheat yield was almost 4 bu/ac lower and required 15 lb/ac more fertilizer N than where wheat was grown continuously since 1963 (Table 7). When the capacity of the soil to supply N is increased, more N is released late in the growing season where it can contribute to increased grain protein. Grain protein was higher for the longer term continuous wheat.

In the same rotation study, wheat-fallow plots were converted to a canola-wheat-barley-pea-wheat-wheat rotation in 1992. Wheat yield following canola or pea was 15-19% higher than the long-term continuous wheat and only 11-14% lower than after fallow (Table 8). The second crop of wheat after pea showed little yield benefit over continuous wheat. However, both wheat crops after pea had protein contents similar to long term continuous wheat, and higher than for wheat on fallow. Responses from this rotation suggest that much of the benefit of long term continuous wheat can be achieved and even enhanced in a much shorter time with diverse rotations of cereals, oilseeds and pulses.

3. Pulses improve N supplying power of the soil.

Including nitrogen-fixing pulses in the rotation can also increase the nitrogen supplying power of the soil. In a comparison of a wheat-lentil rotation with a continuous wheat rotation at Swift Current in the Brown soil zone, soil tests revealed that less fertilizer N was required for wheat after lentil (Table 9). The difference tended to increase with more years of lentil in the rotation. Unlike wetter areas of the prairies, lentil did not provide a consistent yield increase in the wheat crop compared to continuous wheat. However, wheat protein was increased after lentil, and the difference tended to get larger over time.

Table 7. Influence of long term vs shorter term continuous cropping on grain yield, protein content and fertilizer N requirements at Scott, Sask.

• *From 1963-86, this treatment was in a wheat-fallow rotation.

  Yield or protein values followed by the same letter are not significantly different at P= 0.05.

Table 8. Yield of wheat from wheat only and diversified rotations at Scott during 1993-99.

Yield [bu/ac] for canola=24.9; barley=62.9; and pea=44.3

• This rotation was in a wheat-fallow rotation from 1963-91, and values are for the phase noted in bold.
• Yield or protein values followed by the same letter are not significantly different at P= 0.05.

Table 9. Influence of lentil in rotation with wheat vs. continuous wheat on fertilizer N requirements and wheat protein at Swift Current, Sask .

Source: C.A. Campbell and R.Zentner, Swift Current, Sask.

Economic Principles:

• 1. Fallow reduces net income variability (risk).

  2. Economics of continuous wheat can be risky.

  3. Continuous cropping increases input costs.

  4. Economics of continuous rotations of cereals, oilseeds and pulses are;

  • more variable than fallow-wheat.
  • typically higher than fallow-wheat. (Oilseeds and pulses must be well adapted).

Economics of crop rotation are discussed in depth in other part of the proceedings, and readers are urged to read these papers for a more complete understanding of the subject. However it is difficult to separate other rotation principles from economics, because in the final analysis, rotation practices must make economic sense. For this reason, a brief discussion of economics is provided here.

Numerous Economic comparisons between rotations with various combinations of fallow and wheat have been conducted. The general trend is for net income to be more stable over years for rotations with a high frequency of fallow (Table 10). This reflects more uniform yield and reduced input costs (Table 11) for wheat grown on fallow than on stubble. With continuous wheat, income can be highly variable, again reflecting high input costs and variable yield.

One strategy to overcome some of the risk of continuous cropping is to grow a diversity of adapted crops. At Scott, overall yield in a zero tillage canola-wheat-barley-pea-wheat-wheat rotation was more uniform over years than a conventional tillage continuous wheat rotation, but still much more variable than for wheat-fallow (Table 8). However when economics were applied to the same rotations, the canola-wheat-barley-pea-wheat-wheat rotation had the greatest year to year variability (Table 11). Offsetting this increase in risk, was a substantially higher average net return than for wheat-fallow or continuous wheat.

Table 10. Net income ($/ac) for three rotations of wheat and fallow at Melfort and Indian Head, Sask.

Source: adapted from Zentner et al 1988, and Zentner et al 1990.

Machine and machine operation costs were high for the conventional tillage continuous wheat, but reduced for the zero till canola-wheat-barley-pea-wheat-wheat rotation. Offsetting reduced machine costs for C-W-B-P-W-W were increased costs for herbicides for pre-seed burnoff and desiccation. Herbicide costs for in-crop weed control were lower for the C-W-B-P-W-W rotation compared to continuous wheat. Including an N fixing pulse crop in the rotation resulted in a small reduction in fertilizer N costs compared with continuous wheat. The overall impact was a slight reduction in total costs for C-W-B-P-W-W compared to continuous wheat. The big impact of the C-W-B-P-W-W was on gross returns which were much higher than for the other rotations, reflecting the higher value of canola and pea as opposed to wheat, plus increased yield of wheat.

Table 11. Six year average (1993-99) economic performance for three rotations at Scott, Sk.

Rotation planning

Rotation planning begins with deciding on the proportions of various crops to be grown in the rotation over the long term. Once this is done, these principles can be applied to begin to develop a rotation strategy for the whole farm. Some considerations that need to be added are:

• how to deal with residual herbicides to avoid crop damage;
• how to deal with control of volunteer crop;
• whether all blocks of land are suited to the same rotation;
• how to incorporate some degree of flexibility into the rotation to deal with new problems that arise, make minor changes to the crop mix and incorporate new technologies.

Residual herbicides present a special challenge, since they may require drastic changes to the rotation. Well-planned and executed rotations should manage weeds at low levels and provide a variety of options that reduce the need for residual herbicides.

Generally, volunteer crop control is readily achieved where cereals are rotated with broadleaved crops, and vice versa. Where cereal-cereal or broadleaf-broadleaf sequences are planned, it is usually better to use the same crop, but disease losses become a major concern. Rotation can be used to facilitate changing from one crop to another without major volunteer crop problems, or greatly increased costs.

Soil characteristics and pest populations associated with individual fields can influence rotations. Lighter textured soils are better suited to continuous cropping, possibly including perennial forages. Highly productive fields are well suited to crops where high yields are needed to offset high input costs.

Changing the crop mix can be facilitated either by lengthening or shortening the rotation, or by substituting crops without changing rotation length. Examples are substituting wheat, barley, and oat in cereal years of the rotation, canola, mustard, and flax in oilseed years or pea, lentil or chickpea in pulse years.

Summary

Successful crop rotations are based on applying the principles of crop rotation to maintain a high level of soil productivity, to keep pest problems at manageable levels, and to allow for production of crops that provide adequate returns. Above all, crop rotations of the future must be based on a sound strategy that allows for enough flexibility to deal with problems as they arise, and allows growers to take advantage of market opportunities.