What is herbicide stewardship?

A consistent theme these days is the need to provide stewardship for weed control when planting corn and soybean varieties that are resistant to glyphosate and glufosinate herbicides. The goal of promoting stewardship is to preserve the value these traits and herbicides bring to agriculture.

Importantly, the stewardship theme should apply for all crop management decisions, but weed scientists warn that the inevitability of weeds evolving and developing resistance to all herbicides makes stewardship that much more important for all weed management decisions.

Key Points

Consider the ways to provide stewardship when using herbicides to control weeds.

Continuous use of certain herbicides results in weeds developing resistance.

Using soil-applied residual herbicides is one of the stewardship options.

The continuous, widespread use of certain herbicides has led to the evolution of more herbicide-resistant weed populations. “Consider there are a number of ways to provide stewardship, and one way that does not,” said Mike Owen, an Iowa State University Extension weed scientist. “Unfortunately, the one way that does not provide stewardship — recurrent use of a specific herbicide to which the crop is resistant — continues to prevail.”

He added, “I anticipate we will have a breakout year for glyphosate-resistant weeds, particularly waterhemp, in 2011. The many ways to provide stewardship must be considered, or crops and growers alike will suffer the consequences.”

Owen was one of the discussion leaders at a recent meeting in Davenport focusing on weed resistance to herbicides. The meeting, sponsored by weed scientists from ISU and University of Illinois and sponsored by Syngenta, Bayer CropScience, BASF and AMVAC, was attended by about 75 people — crop consultants, seed company agronomists and herbicide industry representatives.

Use residuals right way

Owen pointed out that stewardship is a complex issue involving decisions that impact crop yields, money and weed resistance. In an era when postemergence herbicide applications are extremely popular, he is strongly recommending a return to the use of soil-applied residual herbicides as a key part of every farmer’s weed management strategy.

“One option is to use soil-applied residual herbicides that are cleverly selected to control the most problematic weeds in your fields, such as waterhemp,” he said. “However, appropriate application timing is also an important part of effective stewardship.”

The worst way to use a residual herbicide, said Owen, is to apply it postemergence 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 (such as yield protection and better time management) accrued by residual herbicides.

Applying residual herbicides postemergence allows weeds to compete with crop yield potential and does not provide any time management benefits relative to the other suggested herbicide application timings.

Use least-risky timing

Owen said the best and least-risky application timing is early preplant, or EPP. An EPP application made in late March or early April results in the herbicide being in place to control weeds as they begin to germinate and it avoids the loss of yield potential attributable to early-season weed interference. Importantly, the EPP timing doesn’t interfere with planting, thus provides the greatest time management benefit.

The next best timing is preemergence application timing. PRE application timing also potentially provides similar weed control benefits as the EPP, but there is greater risk of insufficient rainfall to provide the right 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 research project (Benchmark Study) conducted in Iowa clearly and consistently demonstrates the benefits of soil-applied residual herbicides when the application timing is correct.

Owen cited these benefits:

greater yields compared to post treatments, regardless of whether the latter included a residual herbicide

more profitability

other stewardship benefits resulting from this tactic, such as mitigation of herbicide-resistant weed populations

There is one other bit of troubling news: Waterhemp populations have evolved resistance to HPPD herbicides (such as Callisto, Impact and Laudis). “Although 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,” warned Owen. “Farmers, dealers and everyone involved in weed management needs to act now to practice good stewardship of HPPD herbicides.”

This article published in the May, 2011 edition of WALLACES FARMER.

All rights reserved. Copyright Farm Progress Cos. 2011.