Timing applications of preemergence herbicides

A recent trend in weed management has been earlier applications of preemergence herbicides, with products being applied four to six weeks ahead of planting. Although there can be benefits to this strategy, putting the herbicide on too early may result in decreased performance. Growers need to carefully evaluate the risks and benefits of different application timings to develop the most efficient weed management system.

Benefits of early application

All preemergence herbicides require rainfall to move them into the soil profile and make them available to emerging weeds. Applying the herbicide ahead of planting can reduce the risk of poor performance due to lack of rain after application because the herbicide is applied prior to major weed emergence events.

In no-till systems, early preplant applications can eliminate the need for burndown herbicides such as Roundup, Touchdown, or Gramoxone Extra. The herbicides are applied prior to emergence of most species, and in many cases, the preemergence products also have some foliar activity on emerged weeds and will kill small seedlings.

Convenience is also a major driving force in the movement towards earlier applications. Completing herbicide application in early April frees growers from this task when conditions become favorable for planting.

Risks of early application

Applying the herbicide ahead of planting reduces the length of weed control provided after the crop is planted. If the herbicide breaks down too quickly, the grower will be faced with making supplementary control strategies or accepting poor weed control. The length of control provided by the herbicide will be influenced by the herbicide's persistence, environmental conditions, and target weed species. Early applications pose a greater risk in fields with difficult-to-control species (woolly cupgrass, waterhemp, etc.).

Balancing risks and benefits

The optimum application time for preemergence herbicides is dependent upon both the grower's management skills and environmental conditions. In systems where tillage is used to prepare a seedbed and the grower is willing to use a rotary hoe when needed, applications made shortly before or after planting will provide the most consistent control. The tasks of final seedbed preparation, planting and herbicide application must be completed in a relatively narrow time frame (2-3 days) for optimum performance.

In no-till systems, or systems in which growers are unable or unwilling to supplement preemergence herbicides with rotary hoeing, more consistent control can be achieved by applying the herbicide one to three weeks ahead of planting. If tillage is used following application for seedbed preparation, operate the tool as shallow as possible while maintaining uniform mixing of the soil.

The benefits of early applications generally occur in the two to three weeks prior to planting. Applications made earlier than this should be viewed primarily for convenience rather than performance. The level of risk associated with early applications is dependent largely upon weather conditions. If planting or crop development is delayed due to weather, fields where herbicides were applied in early April will be more prone to late-season weed escapes than fields where herbicides were applied closer to planting.

This article originally appeared on pages 25-26 of the IC-478 (3) -- April 7, 1997 issue.

Updated 04/06/1997 - 1:00pm