By Dwayne Beck
One of the less well-known crop rotations is a stacked rotation. This describes a rotation where crops, or crops within the same crop type, are grown in succession (normally twice) followed by a long break.
Example: Wheat-Wheat-Corn-Corn-Soybeans-Soybeans or Barley-Wheat-Pea-Canola
Stacked rotation concepts should not be an unfamiliar they are the way that plants sequence in nature. A species predominates a space for a period of time and is succeeded by another species. Eventually (after many such successions) the original species will again occupy the space. The time frame for these "rotations" is much longer than the one usually considered in annual crop production, but the principles are the same.
Humans tend to operate in a different time frame than other species. Days, hours and years have a totally different meaning to a bacteria or fungi than they do to a tree. Some species have very fast growth curves, once they are given the opportunity, while others take a long time to build population. Each species has a "survival strategy" designed to increase the chances that it will continue to exist. Humans learned to build shelters, grow food, etc. because we were not the best adapted species at enduring the elements and hunting or gathering. Many annual weeds produce huge numbers of seeds, increasing the probability that at least one will survive. Other weeds have seeds that contain a range in dormancy allowing them to fit into environments where all years are not good years. Many disease organisms produce resting bodies that require favorable conditions to exist before they attempt to grow.
The universal survival strategy for all species is genetic diversity. This allows some of them to survive in conditions that eliminate the rest of the population. Some of the offspring of these survivors have this same survival advantage. Consequently individuals with this trait will increase as long as the conditions that favor them continue. They may not have an advantage if conditions change. The main reason agriculture faces issues with resistant weed and insect biotypes is that cropping programs create conditions that favored specific individuals amongst the population and keep these conditions in place long enough, frequently enough, and/or predictably enough to allow that biotype to become the predominate population.
The concept behind stacked rotations (as with some of the other types of rotations as well) is to keep both crop sequence and crop interval diverse. Part of the strategy recognizes the fact that rotations containing only one crop sequence or one interval will eventually select for a species (or a biotype within a species) that suits the particular conditions. In the case of a species biotype, the population will continue to grow and purify as long as the specific conditions remain the same.
In the Corn Belt and in irrigated areas on the Plains, it is common for many growers to produce corn on the same land every year. When this is done, the corn rootworm beetle (there are different species with similar habits) feeds on the corn silks and lay eggs at the base of the corn plant. Most of these eggs hatch the next spring. If corn or other suitable hosts are present, the larvae feed on the corn roots and cause significant losses. This required use of insecticides on land devoted to continuous corn production.
When corn was seeded following soybeans this insect was initially not a problem. Interestingly enough, following a long history of corn-soybean rotation in parts of the Corn Belt corn rootworm beetles have devised two known survival strategies. In western areas an extended diapause biotype has become common and in cases predominate. The majority of the eggs laid by this biotype do not hatch the next spring (when soybeans are seeded) waiting instead for corn to predictably return the second year. In reality, eggs laid by some individuals always had a higher proportion with this tendency. They now predominate the population because the persistent and widespread use of the corn-soybean was consistent in the interval between successive corn crops. This gave this biotype competitive advantage.
The second example comes from more eastern areas. This adaptation involves the gravid females migrating to soybean fields to lay their eggs. When these hatch the next spring, corn will most likely be there. In this case the biotype was given an advantage because the corn soybean rotation is consistent in sequence. A similar adaptation would probably occur if all corn in an area was seeded following wheat.
In the stacked Wheat-Wheat-Corn-Corn-Soybean-Soybean example, the sequence for corn and the interval between corn crops is unpredictable in the time frame of an insect. (It looks very predictable to humans). Just as importantly, some of the population with normal habits (feeding on corn, laying eggs in corn, eggs hatching the next spring) has been kept alive due to the corn-corn stack. This will dilute the population of those with aberrant behavior.
Examples can just as easily be found using weeds or diseases. The important point to remember is that these shifts in characteristics do not always occur quickly. Species with only one generation per year, may take a decade or two for a biotype with suitable survival strategy to develop into predominance. During this period the producer becomes convinced that he has developed the ultimate crop rotation, found the perfect chemical, etc. for his operation (it has worked for seven years in a row). Then almost without warning the system fails. Everyone with resistant weed biotypes has witnessed this phenomenon.
Beck is manager of the Dakota Lakes Research Farm, Pierre, S.D.
Next: Part 5 - Stacked Rotations Break Pest Cycles