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Postemergence Options for Waterhemp Management in Soybean

June 28, 2002
The time has arrived for applications of postemergence soybean herbicides. Questions about postemergence management of waterhemp in soybean are still common, so this article will summarize available management options, as well as some new findings related to this problematic weed species.

One of the most important factors for effectively managing waterhemp is to understand its germination and emergence characteristics. The germination and emergence patterns of waterhemp are characteristics that contribute significantly to management problems. While the peak emergence of other, more familiar summer annual weed species generally occurs during the early portion of the growing season, waterhemp emergence easily can occur during the middle to late portions of the growing season. Research at the University of Illinois has indicated that waterhemp can emerge during late June and even into early July. This characteristic generally results in a range of waterhemp plant sizes when postemergence herbicide applications are made. It is not unusual to treat waterhemp plants in a given field, ranging in size from 6 inches to less than 1 inch in height, for example.

Application Considerations

The factors governing the effectiveness of postemergence herbicides are critically important when dealing with waterhemp. Herbicide rate, application timing, and spray additive all influence how well postemergence herbicides perform on waterhemp. Several application considerations deserve special attention, as these can greatly influence the level of waterhemp control achieved.

1. Waterhemp plant size. As previously described, waterhemp size can vary greatly in a given field when post-emergence herbicides are applied. Smaller plants (4 to 5 inches or less) are generally more effectively controlled with a given herbicide than are larger plants. The likelihood of water-hemp "recovering" from a postemer-gence herbicide increases as the plants become larger (greater than approximately 5 to 6 inches) prior to application.

2. Carrier gallons. Generally speaking, the more water the better. Applications of postemergence contact herbicides should be close to 20 gallons per acre. While glyphosate applications are usually made at 12 to 15 gallons per acre, increasing the carrier volume to 20 gallons may help provide better coverage, especially if the waterhemp population consists of a mixture of large (4 inches or larger) and small (1 inch or less) plants.

Often, producers like to wait as long as possible to apply postemergence herbicides, especially those that lack any significant soil residual activity, in order to have as many waterhemp plants emerged as possible. Remember that, because waterhemp can germinate and emerge for an extended time, a wide range of plant sizes typically exists by the time postemergence herbicides are applied. This can present problems with spray interception by smaller plants under the protective canopy of larger plants. Adjustments in spray volume (around 20 gallons per acre) and pressure (40 to 50 pounds per square inch) can help overcome some of the problems with coverage.

Herbicide Options

Only four active ingredients for postemergence waterhemp control in soybean are available, and three of these belong to one chemical family. The diphenyl ether herbicides (PPO inhibitors) acifluorfen (Ultra Blazer), fomesafen (Flexstar), and lactofen (Cobra/Phoenix) are mostly contact in nature, and thus thorough coverage of the target vegetation is essential for good control. Waterhemp plants 4 to 5 inches in height (or less) are usually controlled better than larger plants. Each product label has some flexibility with respect to application rate and spray additives. When conditions have been hot and dry, application rates should be increased, and crop oil concentrate may enhance activity more than nonionic surfactant. Keep in mind that soybean injury may also increase when using crop oil concentrate.

The other postemergence soybean herbicide option is glyphosate. Glyphosate is very effective on waterhemp, but attention to plant size and application rate is still important. If waterhemp has been growing under good conditions (especially adequate soil moisture), control at a given application rate is frequently better than if the plants were growing under dry soil conditions. With no significant soil residual activity from glyphosate, waterhemp plants that emerge after application will not be controlled and may require further management considerations.

Recent Problems

Diphenyl ether herbicides have been used extensively for waterhemp control in soybean, and while these herbicides in general are still effective for waterhemp control, some problems have developed. In particular, we have been investigating a population of waterhemp in Illinois that was not controlled by a postemergence application of lactofen, under greenhouse conditions. We applied Cobra at a rate equivalent to 20 fluid ounces per acre, plus crop oil concentrate, and did not control most of the waterhemp plants from this population. We currently are conducting field experiments at the site from which our greenhouse population was collected, and will keep you informed as additional results become available. If resistance to this chemical family becomes more common, that could potentially reduce the number of viable postemergence soybean herbicides to one--glyphosate.

What else could go wrong, you ask? Over the past two growing seasons, we have received an increasing number of reports of glyphosate failing to provide adequate control of waterhemp. Other states have reported similar observations. While perhaps not always meeting the "criteria" for being designated resistant to glyphosate, lack of control for whatever reason presents a problem.

These examples are given to illustrate an important point. Waterhemp is a very diverse plant species, as is evidenced by the selection of biotypes resistant to ALS inhibitors, triazine herbicides, and PPO inhibitors. Anecdotal observations suggest the effectiveness of glyphosate for waterhemp control is not always as consistent as it once was. In years past, many new herbicide active ingredients were commercialized for the soybean market, but that has changed also. It is unlikely that many (indeed, if any) new active ingredients, with good efficacy on waterhemp, will be introduced into the soybean market during the next few years. If the effectiveness of any of the currently available postemergence soybean herbicides for waterhemp control (only four active ingredients, three of which are in the same chemical family) is reduced, new active ingredients may not be available to fill the need, at least for the foreseeable future.

Where can you go to learn more about waterhemp's adaptability to our current management options? At Agronomy Day 2002 (August 22), Dr. Pat Tranel, from the Department of Crop Sciences, will present contemporary research findings on the herbicide-resistance characteristics of Illinois waterhemp populations.--Aaron Hager and Christy Sprague

Author: Aaron Hager Christy Sprague


The Pest Management and Crop Development Bulletin
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