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Cropping systems in the Great Plains are changing because of no-till. Rotations now include a diversity of crops in contrast with rotations in tilled systems that grow only one or two crops. This diversity of crops often leads to yield increases because of the rotation effect. We have observed that crop response to the rotation effect can be categorized as either improving resource-use efficiency or increasing plant size and yield capacity. An example of the efficiency response, which we term synergism, is that some crops improve water-use efficiency (WUE) of following crops. Crops produce more grain with the same water use in synergistic sequences; in contrast, crops that increase yield capacity consume more water to produce more grain. Further, synergism among crops increases tolerance of weed interference because of improved resource-use efficiency. Synergism is more prominent in low-yielding environments due to stresses such as drought, and appears to be rare among crop sequences. The cause of synergism among crops is likely related to a multitude of interacting factors such as microbial changes, growth-promoting substances, and altered nutrient cycling. Quantifying WUE as affected by preceding crop may provide a method to identify synergism among crops. No-till rotations that include synergistic crop sequences are improving land productivity, farm economics, soil health, and resource-use efficiency in the semiarid Great Plains.
