Mother Nature deplores a vacuum

Crop rotations help prevent the buildup of weeds adapted to a particular cropping system. Certain weeds are more common in some crops than others. Pigweed, lamb’ squarter, common ragweed, velvetleaf, cocklebur, foxtail species, and crabgrass are found in summer-cultivated crops such ascorn. Mustards, wild oat, wild garlic, chickweed, and henbit are associated with fall-sown small grains. Pastures often contain perennial weeds such as ironweed and thistles. Changing crops changes the cultural conditions (planting date, crop competition, fertility, etc.) that a weed must tolerate. Rotating crops also often means that a different set of management tools (especially herbicides) will be used. The overall success of crop rotation in managing weeds depends on the ability to control the weeds in each crop grown in the rotation.

Rotation will prevent a weed species from becoming dominant in a field but will also maintain a diversity of weed species in the same area.  Crop rotation historically was very important for managing weed problems. Today, rotation is used more for managing diseases and insects than weeds. Rotation requires the farmer to have additional knowledge and to use additional equipment to manage the various rotational crops. Even with an abundant supply of fertilizers and diverse herbicides that make it possible to minimize the need of crop rotation for weed control, there are still sound reasons to rotate crops for environmental and pest management reasons. For example, corn rotated with soybeans consistently yields more than corn grown continuously in the same field. Rotation of vegetable crops is important to avoid buildup of soil diseases that reduce crop yields. However, rotation is not an option with long-term perennials such as orchards, forest trees, nurseries, and perennial forages. Some of the benefits of rotation can be retained in monoculture cropping systems by the selection of a variety of herbicides, especially those differing in mode of action, and the use of various cultural practices, especially cultivation.

Herbicide diversity and cultivation help prevent the development of resistant weed populations that are adapted to an unchanging herbicide program and crop.  Problems tend to arise when farmers do not rotate their crops and pest management strategies in an integrated manner. For example, in the past the corn–soybean rotation avoided the buildup of corn rootworm in the corn cycle, as rotation for 1 year to soybean broke the insect life cycle. However, the insect has adapted to these cropping strategies to be able to survive on soybean and has once again become a major corn problem. Similar examples are available in weed control. With the availability of a variety of glyphosate-resistant crops, there will be a tendency to continually use glyphosate for weed control even as we rotate crops. This is poor management, and it will become necessary to rotate herbicide-resistant crops with nonresistant crops to avoid a buildup of weeds not well controlled by glyphosate. The same holds true for herbicides that inhibit branch chain amino acids and can be used in many of our major acreage crops. There is a law of nature that holds true for agriculture that one should always remember: “Mother Nature deplores a vacuum.” Repeated use of any successful pest management practice without appropriate integration with a variety of other tactics and rotation over time will result in that tactic’s selecting for its own extinction. There are many good examples of this phenomenon in weed science, and they are called herbicide-resistant weeds