Stewardship, Yields, Money and Resistance

April 15, 2011
ICM News

By Mike Owen, Department of Agronomy and Tamsyn Jones, Corn and Soybean Initiative

A consistent theme the last few years has been the need to provide stewardship for glyphosate- and glufosinate-resistant crops in order to preserve the value these traits bring to agriculture. Consider that there are a number of ways to provide stewardship, and one way that does not. Unfortunately, the one way that does not provide stewardship — recurrent use of the herbicide to which the crop is resistant — continues to prevail. I anticipate that we will have a breakout year for glyphosate-resistant weeds, particularly waterhemp, in 2011. Thus, the many ways to provide stewardship must be considered — or crops and growers alike will suffer the consequences.

Soil-applied residuals for weed stewardship

One option is the use of a soil-applied residual herbicide(s) that is/are cleverly selected to control the most problematic weeds (i.e. waterhemp). However, appropriate application timing is an important part of effective stewardship.

The worst way to use a residual herbicide is to apply it post-emergence to the crop and weeds alone or in combination with a post product such as glyphosate. While this type of application is convenient and simple, it results in loss of most of the stewardship benefits (e.g. yield protection and better time management) accrued by residual herbicides. Applying soil-applied residual herbicides post-emergence allows weeds to compete with crop yield potential and does not provide any time management benefits relative to the other suggested herbicide application timings.  

The best and least risky application timing is early pre-plant (EPP) — which means now is the time to make applications. An EPP application results in the herbicide being in place to control weeds as they begin to germinate and precludes the loss of yield potential attributable to early-season weed interference. Importantly, the EPP timing does not interfere with planting, thus providing the greatest time management benefit.

The next best timing is pre-emergence (PRE) application timing. The PRE application timing also potentially provides similar weed control benefits as the EPP — but there is greater risk of insufficient rainfall to provide the appropriate environment for effective weed control. Also lost is the time management benefit provided by the EPP application timing.

Considerable data generated from a five-year field-scale on-farm project (Benchmark Study) conducted in Iowa clearly and consistently demonstrates the benefits of soil-applied residual herbicides when the application timing is correct. The benefits include:

  • greater yields compared to post treatments, regardless of whether the latter included a residual herbicide
  • more profitability; and
  • other stewardship benefits resulting from this tactic, such as mitigation of herbicide-resistant weed populations.

There is one other bit of troubling news to consider: Waterhemp populations have evolved resistance to HPPD herbicides (e.g. Callisto, Impact, and Laudis).  While not yet widely distributed, unless appropriate stewardship of this class of chemistry is provided, it is inevitable that HPPD resistance will become prevalent across Iowa.  Act now to steward HPPD herbicides.


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 or by phone at (515) 294-5936. Tamsyn Jones is a communications specialist with the Corn and Soybean Initiative. Jones can be reached at (515)294-7192 or email at



Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Integrated Crop Management News, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on April 15, 2011. The information contained within may not be the most current and accurate depending on when it is accessed.