Due to the wet weather this spring, 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," says Mike Owen, Iowa State University Extension weed scientist.
"Thus, the fact that weeds are beginning to emerge through the herbicide treatments should be expected," he notes. "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 considerations will help farmers get the highest potential crop yield." Owen provides the following weed control information and observations.
Preplant and preemergence herbicides don't give season-long control
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 weed 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, you should expect less weed control. As indicated, the goal of effective early season weed control was generally met this spring in Iowa.
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 treatments, and 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 your fields is an important weed management strategy.
Third, review the mode of action of herbicides before you apply them. The mechanisms of herbicide action (MOAs) of the second round of weed control must be reviewed and chosen with careful consideration. These herbicide treatments must 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 there, 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 in your corn and soybean fields.
Click on this link and listen to ISU's Mike Owen for more information
The mp3 audio file is available at http://www.agron.iastate.edu/cropminute/.
This May 23, 2011 "Crop Minute" interview features ISU Extension weed specialist Mike Owen talking about the expectations corn and soybean growers should have for their preplant and preemergence herbicide applications.