By Dwayne Beck
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 establish.
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 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 stated that 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.
Still another concept in stacked rotations involves allowing the use of more diverse herbicide programs, specifically those utilizing long-residual compounds. Relatively high rates of atrazine can be used in the first year corn (or sorghum or millet) of a stack since another tolerant crop will follow. This provides the time necessary for the herbicide to degrade before sensitive crops are grown.
Similarly, products like Command or Scepter can be used in first year soybeans in areas where these products could not be used in other rotations.
The following is a typical herbicide program at Dakota Lakes for a Spring Wheat-Winter Wheat (double crop forage sorghum-Corn-Corn-Soybean-Soybean rotation starting following the second crop soybean harvest):
Year one -- Spring Wheat, no burndown followed by Bronate (Buctril M). Year two -- Winter wheat would have a burndown between spring wheat harvest and winter wheat seeding. No herbicide is normally required in the winter wheat. Two pounds of atrazine would be applied either to the double crop forage sorghum or after it is harvested in the fall. This is dependent on the weeds present. The first year corn usually does not need a burndown, but normally receives an early post-emergence application of bromoxynil and perhaps a grass herbicide. Second year corn receives a traditional program. A GMO, such as Liberty-Link or Roundup Ready, could be used. We do not always use Roundup-Ready in this slot at Dakota Lakes.
First year soybeans receive a long residual program like Scepter plus Command. Second year soybeans are Roundup Ready. With this program, we have used ALS chemistry once in 6 years, triazines once in six years, Roundup Ready once or twice in six years (and perhaps a burndown between wheat crops also but this could be paraquat). The Roundup is applied at different times with different companion herbicides.
It is obvious that weeds (viewed from their perspective of time) will find it difficult to develop resistance or tolerance to any of the modes of action employed.
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 minimizes the chance of tolerance, resistance, and biotype changes.
Disadvantages of stacked rotation: Not well tested. Some crop sequences may not be ideal. Fewer crops mean less workload spreading.
Beck is manager of the Dakota Lakes Research Farm, Pierre, S.D.
Next: Part 6 - Hybrid stacked rotations