The behaviour of the herbicide atrazine was compared in conventionally tilled corn grown continuously year after year versus corn grown in three different crop rotations. The various levels of tillage and irrigation, including no irrigation were tested.

The amount of irrigation used including a total absence of irrigation had no impact on the rate of degradation of atrazine by soil microbes in the top foot of soil. The factors that made a difference were prior herbicide use and the choice of crop sequences, with prior herbicide use the most important factor by far. The previous applications of atrazine can predispose soil to more quickly degrade later applications of the herbicide. There are two consequences of the more rapid dissipation of atrazine in the plots with a history of use. The first consequence is a loss in weed control. In the plots with the most rapid dissipation, weeds began to re-infest the plots within four weeks after treatment, while the plots with the slowest rate of dissipation remained weed-free throughout the growing season. The second consequence is that atrazine leached more deeply in the soil in the plots where it did not dissipate rapidly, but the herbicide did not move below the top three inches of the soil in the plots where it was degraded rapidly. Thus, crop and herbicide use history are more critical to herbicide efficacy and environmental safety than the timing and amount of irrigation water used.