MSU Extension

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

The development of stacked trait technology and the increased interest in herbicide tolerant corn has led to an increase in postemergence herbicide applications for weed control. Postemergence programs without a residual herbicide applied at planting have their place, and can provide excellent season long weed control. However, important considerations that should be made include timing, herbicide resistance, and management strategies.

First, let’s quickly discuss herbicide resistance and the role postemergence programs can play in delaying or speeding up the expression of resistance in a field. It is commonly believed that there are resistant genes present in most weeds, only at a very low frequency that may never be noticed. The reliance of a single mode of action will increase the selection pressure for those plants that are resistant, increasing the frequency of expression, resulting in a field population resistant to the herbicide being used. Utilizing multiple modes of action for each weed will reduce the chance of developing resistance. Therefore, it is important to rotate your herbicide modes of action as you rotate crops, or apply multiple modes of action in a season through preemergence and postemergence herbicides.

Postemergence only programs have become increasingly common. A study was begun at Michigan State in 2004 to investigate the effects of postemergence timing on weed control and corn yield. The study included treatments of glyphosate or glufosinate applied postemergence to weeds at 3, 6, or 9 inches, either alone or with an acetamide plus atrazine premix herbicide such as Harness Xtra or Bicep II Magnum. The study also included a 2-pass system also using the acetamide plus atrazine herbicide preemergence followed by a postemergence application of either glyphosate or glufosinate on 3, 6, or 9-inch weeds. The study has been repeated for five seasons, with a wide range of weather conditions, and the results are quite clear on the impact application timing can have on weed control and corn yield.

As is expected, the highest yields and greatest weed control were observed where a residual herbicide was applied preemergence and followed by either glyphosate or glufosinate postemergence. Excellent weed control was also observed when a residual herbicide was applied with the glyphosate or glufosinate postemergence when weeds were 3 to 4 inches in height. The residual herbicides extended the window of control and resulted in overall greater yields. Ironically, some of the greatest weed control was observed when postemergence applications were made when weeds were 9 inches, however significant yield reductions were observed.

Corn yields decreased as the postemergence timing was delayed, whether or not a residual herbicide was included, which shows the competitiveness of early season weeds. Corn yields were lower for all total postemergence treatments (with or without a residual herbicide tank-mix partner) compared to those that had a residual herbicide applied at planting. A 19 to 24 bushel per acre yield reduction was observed when postemergence herbicides were applied to 9-inch weeds compared to a 2-pass program. When comparing total postemergence timings, yields were reduced 13 bushels per acre when the application was delayed from 3-inch weeds to 9-inch weeds when a residual herbicide was included. If no residual herbicide was used, there was a five bushel per acre yield reduction as the application timing was delayed from 3 to 6-inch weeds, and also from 6 to 9-inch weeds, resulting in a 10 bushel yield reduction when the postemergence application was made on 9-inch weeds.

During this time of year, it is easy to see how weeds can get away from you. It only takes a few days for weeds to germinate and grow an inch, and in good growing conditions weeds can grow several inches in a couple of days. Now throw in some timely rains and your weeds jump to 7-8 inches before you can cover all your fields. The most effective weed management program will include residual herbicides either preemergence or postemergence, with postemergence applications being made to 3-inch weeds or smaller to avoid a yield reduction. Postemergence only programs with no residual herbicide will need multiple applications to ensure maximum weed control and yields. Since it is too late to apply a preemergence herbicide in corn, be sure to make your postemergence applications in a timely manner, and include an additional mode of action when possible.