Weed control in crops should rely first on tillage and later on the crop as competition. But a "window of opportunity" still exists during which weeds are difficult to control without the use of herbicides. No dependable, long-term, sustainable weed control systems currently exist. The consequent widespread use of the herbicides atrazine and alachlor for corn production in Iowa poses a serious environmental concern because of their potential to pollute drinking water. Because of this possible hazard, an alternative weed control system is clearly desirable. However, any alternative practice must be developed in accordance with when yield losses due to weeds begin and when that weed interference ends