Early Preplant and Preemergence Applications Not Season Long Control

May 23, 2011
ICM News

By Micheal Owen, Department of Agronomy

A few Iowa growers were able to get herbicides applied prior to planting and emergence of the crop but now some are expressing concerns that the herbicidal activity has diminished. In many instances, the rate of herbicide application was not the full labeled rate. Thus, the fact that weeds are beginning to emerge through the herbicide treatments should be expected. Many weeds emerge opportunistically and as the amount of herbicide in soil diminishes due to degradation, new weeds are likely to emerge despite the earlier herbicide application. Remembering some important weed management expectations and perspectives will help growers get the highest potential crop yield.

First, the goal of early preplant (EPP) and preemergence (PRE) applications is to control early weed growth. Generally, this goal was successful and the crop was provided a (relatively) weed free environment early. However, regardless of what some advertising suggests, no herbicide treatment will provide full season control consistently and more importantly, meet grower expectations. It is unreasonable from an environmental, ecological and economic perspective to expect otherwise. Herbicides begin to degrade the moment they are applied.  An appropriate expectation for a soil-applied herbicide treatment would be six to eight weeks of effective control. If rates of application were reduced, expect less weed control. As indicated, the goal of effective early season weed control was generally met. 

Second, considering that early season weed competition is most costly to crop yield potential, there was significant economic value to the EPP and PRE treatments even though control may be declining.  Now, the next round of weed management treatments must be established. Appropriate mechanical and/or POST herbicides should be timed just as carefully as the EPP/PRE, perhaps even more carefully given the effects of weed size and crop stage of development. Generally, early application timing on small weeds and crops is better than late and large. Scouting is an important weed management strategy.

Third, the mechanisms of herbicide action (MOAs) of the second round of weed control must be reviewed and provide diversity so as not to repeat the same MOAs as the EPP and PRE herbicide treatments. Refer to the herbicide label and the herbicide family number that is listed, or if a particular product label does not have the MOA number listed; refer to 2011 Herbicide Guide for Iowa corn and Soybean Production, WC- 94. It is critically important that MOAs be diversified as much as possible in order to provide stewardship and mitigate the evolution of herbicide-resistant weed populations. Repeated applications of the same MOAs will inevitably result in herbicide-resistant weeds; do as much as possible to diversify weed management tactics as possible. 

Micheal Owen is a professor of agronomy and weed science extension specialist with responsibilities in weed management and herbicide use. Owen can be reached by email at mdowen@iastate.edu or by phone at (515) 294-5936.


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