University of Illinois

No. 12/June 12, 1998

Postemergence Herbicides
for Waterhemp Control
in Corn and Soybeans

Waterhemp will continue to be a significant weed problem in many corn and soybean fields across central and southern Illinois during the 1998 growing season. The late planting in this area has helped to eliminate some of the earliest waterhemp flushes, as tillage operations and burndown herbicide applications were also delayed long enough to control the earliest emerging waterhemp. When warm air temperatures return, coupled with adequate soil moisture, waterhemp can make an incredible amount of growth in a very short period of time.

Several postemergence herbicides are effective against waterhemp. The factors that govern the effectiveness of postemergence herbicides are critically important when dealing with waterhemp. Herbicide rate, application timing, and spray pressure all influence how well these herbicides perform on waterhemp.

Often, producers like to wait as long as possible to apply postemergence herbicides, especially those that lack any significant soil activity, so that as many weeds as possible are emerged. Because waterhemp can germinate and emerge for an extended period of time, there typically exists a wide range of plant sizes by the time postemergence herbicides are applied. This situation can present problems with spray interception, due to smaller plants growing under the protective canopy of larger plants. Adjustments in spray volume and pressure can help overcome some problems related to coverage. Spray volumes of 20 gallons per acre, with application pressures of 40 to 60 pounds per square inch, generally provide uniform coverage of the target vegetation.

What about using ALS-inhibiting herbicides for waterhemp control? Many of these herbicides (either used in corn or soybeans) can provide some control of waterhemp. However, the instances of waterhemp biotypes resistant to certain ALS-inhibiting herbicides are widespread enough in Illinois that this herbicide mode of action should not be considered as a primary option to manage waterhemp. Although these herbicides are still effective against many other weed species, most require either a tank-mix partner or to be part of a sequential program where another herbicide is relied upon to control waterhemp.

Postemergence Corn Herbicides
Growth regulators--dicamba (Banvel, Clarity), dicamba + atrazine (Marksman), and 2,4-D. These herbicides have provided the most consistent waterhemp control over the past several seasons, with dicamba generally providing slightly better control than 2,4-D. Other postemergence corn herbicides with different modes of action, in particular ALS-inhibiting herbicides, often recommend adding dicamba (usually at 2 to 8 fluid ounces) to improve waterhemp control. With the widespread prevalence of waterhemp biotypes resistant to many ALS-inhibiting herbicides, the rate of added dicamba in these tank mixes to control waterhemp should be as high as if it were applied alone.

Triazines--atrazine and metribuzin (Sencor). Both are contact herbicides when applied postemergence and provide better waterhemp control when applications are made to waterhemp under 4 inches in height. Atrazine + crop-oil concentrate must be applied before corn reaches 12 inches in height, and Sencor must be applied with a tank-mix partner and usually without any additional spray additives.

What about controlling large waterhemp in corn? Dicamba can provide control or suppression of larger waterhemp. When used alone, broadcast applications can be made until the 5-leaf stage (8 inches). After this, drop nozzles are needed to keep the herbicide out of the corn whorl. According to the Banvel label, a late postemergence application of up to 1/2 pint can be made to corn between 8 and 36 inches tall or 15 days prior to tassel emergence, whichever comes first. Be sure to follow other label precautions with respect to applicationsmade near soybean fields. Some formulations of 2,4-D allow application with drop nozzles to larger corn and can provide some control or suppression of waterhemp. Rates vary somewhat by formulation, and drop nozzles should be used when corn is larger than 8 inches.

Postemergence Soybean Herbicides
Diphenyl ethers--acifluorfen (Blazer, Status), lactofen (Cobra), and fomesafen (Reflex, Flexstar). These herbicides have been extensively used for postemergence waterhemp control in soybeans with generally good results. Each is a "contact" herbicide, so thorough coverage of the target vegetation is essential for good control. The greatest level of waterhemp control with diphenyl ether herbicides is achieved when applications are made to plants 4 inches or less in height. When applied to larger waterhemp, the plants may appear to be adequately controlled within 7 days after application but can sometimes initiate new leaf development within 2 weeks of the application. With all these herbicides, some degree of crop injury should be expected and is generally more severe under conditions of high temperature and relative humidity, and when crop-oil concentrate or 28 percent UAN solution is included.

Glyphosate--Roundup Ultra only on soybeans designated as Roundup Ready. Roundup Ultra has provided good waterhemp control in field research trials. As with the diphenyl ether herbicides, Roundup Ultra does not have significant soil-residual activity, so only waterhemp that has emerged at the time of application will be controlled. University research has indicated good waterhemp control can be achieved with 1 pint per acre of Roundup Ultra, butthese applications were very timely and made to waterhemp no more than 2 inches in height. For consistent control, Roundup Ultra rates should be at least 24 fluid ounces, and increased to 32 fluid ounces when larger waterhemp is encountered or dry growing conditions have persisted for several days prior to application.

What about controlling larger waterhemp in soybeans? Few good options exist to control large (12 inches or greater) waterhemp in soybeans. Roundup Ultra at 1 to 2 quarts can provide control of larger plants, but adequate spray coverage of the target vegetation can become more of a problem with late-season applications. Diphenyl ether herbicides can burn off waterhemp leaves but the plants are generally able to recover. Additionally, if the soybean leaves are burned off, this may allow enough light to reach the soil surface and, with adequate soil moisture, allow another flush of waterhemp to emerge.

Cultivation, where possible, is another effective strategy for controlling waterhemp. Cultivation should be delayed 5 to 7 days after a postemergence herbicide application, or a postemergence herbicide application delayed 5 to 7 days after cultivation. With the current growing conditions, don't expect many miracles from postemergence herbicides for waterhemp control. In many instances, a cultivation would be the preferred alternative to a second postemergence herbicide application.

For those interested in obtaining further information on waterhemp, a color brochure has been developed through a collaborative effort between the University of Illinois and the USDA/ARS. Waterhemp Management in Agronomic Crops (publication number X855) is available for purchase ($2 per copy) from University of Illinois, 1401 S. Maryland Dr., Urbana, IL 61801, FAX (217)333-0005, phone (217)333-3871 or (800)345-6087.

Aaron Hager (, Loyd Wax (, and Marshal McGlamery (, Crop Sciences, (217)333-4424