Stacked rotations break pest cycles
Editor’s note: This is the fifth article in a series about crop rotations written by Dwayne Beck, manager of the Dakota Lakes Research Farm, Pierre, S.D.
The second part of the stacked rotation concept is to have a long break in the rotation. To provide maximum protection against pests with short cycles, one of the intervals must be sufficiently long to allow populations of certain diseases or weeds to drop to low levels.
Careful study of growth and decay curves demonstrates that “first-year” crops on a given piece of land experience few crop-specific pest problems.
If the crop is planted a second time in succession on this “virgin” site, it often does nearly as well — or maybe occasionally even better. It is only during the third year (or more) that problems begin to appear. These problems often grow very quickly once they begin.
The reason this happens is that growth and decay curves for biological systems follow geometric or logarithmic patterns. (Examples: 2, 4, 8, 16, 32, 64 or 1, 10, 100, 1000). Since decay works the same as growth in reverse, a short break is not sufficient to decrease some problems to economic thresholds. This is especially true if they have survival mechanisms like seed dormancy.
The power behind a perennial sequence is the long break. The theory behind stacked rotations is to provide a long break somewhere in the system.
In the “old days,” it was common to have a perennial sequence followed by several years of the same crop. That is why homesteaders were initially so successful when they began farming (and the fact that they had a huge no-till history preceding them).
In Argentina, it was common until recently to rotate seven years of pasture with seven years of cropping. On rented land, this may be seven years (or less if disease strikes) of continuous soybeans.
Plants develop associated positive biology just as they develop associated negative biology. These associated species can sometimes benefit crops when they are planted in the same field in subsequent years. The most commonly cited example includes VAM, the vesicular arbuscular mycorrhyzal fungi that help crops like corn and sunflowers obtain moisture and nutrients from the soil. It is thought that these organisms might be the reason for corn-on-corn and sunflower-on-corn sequences performing better than expected.
Another example is the N-fixing rhizobia bacteria associated with legume crops. Soybeans grown following soybeans are capable of fixing more nitrogen because higher rhizobia populations exist in the soil. The soil is also lower in mineral N sources, since the previous year’s legume crop scavenged these prior to beginning the fixation process. Part of the theory of stacked rotations involves taking advantage of these positive associations before negative associations can build to harmful levels.
There probably are positive associations involving predatory insects as well, but this has not been thoroughly studied.
Recently, I saw an agronomist give what he thought was a negative example of a producer’s rotational planning.
He said the gentleman would seed a particular field to wheat every year, until jointed goatgrass pressure became sufficient to preclude wheat. He would then seed it continuously to sorghum until shattercane overwhelmed him. At that point he would seed sunflowers in successive years, until white mold became a major problem. At that point he began again with the wheat program. My response was that the producer was at least responding to the natural cycles in his field. It might be better if he anticipated these occurring, so that the switch could be made in advance. However, he probably was doing a better job than someone who blindly planted a corn-soybean, wheat-canola-wheat-pea, or wheat-corn-soybean rotation and was surprised when he had to keep changing technology to deal with “new” problems.
Summary of stacked rotations
Advantages: Stacked rotations attempt to keep pest populations diverse (confused) through diversity in the sequences and intervals used. Diversity is gained while keeping the number of crops smaller. They allow a mix of long and short residual herbicide programs. This approach can reduce costs and lesssens the chance of tolerance, resistance and biotype changes.
Disadvantages: Not well-tested. Some crop sequences may not be ideal. Fewer crops mean less workload spreading.
Next: Hybrid stacked rotations