University of Illinois

No. 5/April 25, 1997

Management of Waterhemp in Corn and Soybeans

In the last issue of this Bulletin, we provided some information on the biology and growth habits of common and tall waterhemp. For control programs to be effective, this basic information of how waterhemp grows and develops can be helpful. In this issue, we provide some information on waterhemp control programs in corn and soybean production systems.

There are several soil-applied and postemergence herbicide programs that can provide good control of waterhemp, but each type of application timing has some basic considerations that can influence the degree of success achieved.

Considerations with soil-applied programs
There are numerous soil-applied herbicides that possess good activity on waterhemp and other small-seeded species. Time of application can have a significant impact on the success of soil-applied herbicides for waterhemp control. A common practice in no-till systems is to apply the herbicide several weeks before planting to receive sufficient precipitation for incorporating the herbicide. Keep in mind, however, that the earlier a herbicide is applied, the earlier within the growing season that the level of weed control begins to decline. Waterhemp does not appear to have much difficulty emerging well after the time that most other summer annual species have emerged. If the herbicide has been on the ground for several weeks, there may not be enough herbicide remaining to control emerging waterhemp.

What can be done to extend the length of control afforded by soil-applied herbicides? Three possible options:

  • If allowed by label, increase the rate when applications are to be made several weeks ahead of planting.
  • Apply the herbicide in a split application (generally two-thirds early, with the remaining one-third at planting).
  • Apply the herbicide closer to planting time.

In our research, we have had better and more consistent results with soil-applied herbicides that were applied within 1 to 2 weeks of planting or at planting, compared with the same herbicides applied several weeks (up to 5 weeks) before planting. It's not likely that all soil-applied herbicides can be applied immediately before planting due to constraints of time and equipment, but fields with a significant waterhemp problem would be excellent candidates for applications just prior to planting.

Considerations with postemergence herbicides

Similar to soil-applied programs, there are several postemergence herbicides that are very effective on waterhemp. 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.

Often, producers like to wait as long as possible to apply postemergence herbicides, especially those that lack any significant soil-residual activity. 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 can present problems with spray interception by smaller plants under the protective canopy of larger plants. Adjustments in spray volume and pressure can help to overcome some of the problem with coverage. Spray volumes of 20 gallons per acre with application pressures of 40 to 60 pounds per square inch generally provide a very uniform coverage of the target vegetation.

The following outline lists chemical families and representative members that we have evaluated for waterhemp control. Rates should be based upon label recommendations; when attempting to control waterhemp, reducing rates below those on the respective label is generally not advisable.

Whatever program you decide to use, keep in mind that the most consistent program to control waterhemp includes an integrated approach using soil-applied herbicides, postemergence herbicides, and mechanical cultivation.

Control programs for field corn

The most consistent control programs for field corn are those that combine a soil-applied herbicide with either one or more cultivations or the application of a postemergence herbicide.


Triazines: atrazine and metribuzin applied no earlier than 2 weeks before planting.

Chloroacetamides: alachlor, metolachlor, acetochlor, and dimethenamid. Even though these herbicides are primarily for grass control, they afford some control of waterhemp. By themselves, however, they generally do not provide sufficient residual control. Selection of these herbicides likely should be based upon the need for grass control: All have performed similarly with respect to controlling waterhemp.

Growth regulators: Dicamba or dicamba + atrazine can be used as soil-applied treatments, but these products usually perform more consistently on waterhemp when applied postemergence.

ALS-inhibiting herbicides: These herbicides should be used as premixed or tank-mixed treatments because of the presence of waterhemp biotypes resistant to this class of herbicides.


Growth regulators: dicamba, dicamba + atrazine, 2,4-D. These herbicides usually provide the most consistent level of waterhemp control with respect to postemergence corn herbicides. Many ALS-inhibiting herbicide labels recommend tank-mixing reduced rates of dicamba to enhance control of waterhemp. The rate of dicamba used in the tank mix should be high enough to control waterhemp if it were being applied alone.

Triazines: Atrazine and metribuzin are the two available options. Both of these contact herbicides provide better control when applied to waterhemp under 4 inches in height. Atrazine must be applied before corn reaches 12 inches in height, and metribuzin must be applied with a tank-mix partner.

Control Programs for Soybeans
The most consistent control programs for soybeans are those that combine a soil-applied herbicide with either one or more cultivations or the application of a postemergence herbicide.

pendimethalin and trifluralin. Pendimethalin may be surface applied without subsequent incorporation, whereas trifluralin requires mechanical incorporation. If pendimethalin is surface applied (no earlier than 2 weeks before planting in waterhemp fields) and no precipitation is received between application and planting, a shallow incorporation may prove beneficial.

Chloroacetamides: alachlor, metolachlor, dimethenamid. These herbicides can provide some control of waterhemp in soybeans, but use rates are sometimes lower than those used in corn.

Triazines: metribuzin applied within 2 weeks of planting.

Phenylureas: linuron applied preemergence (after planting).

ALS-inhibiting herbicides: These herbicides should be used as premixed or tank-mixed treatments, due to the presence of waterhemp biotypes resistant to this class of herbicides.

Authority Broadleaf and Canopy XL have attracted a great deal of attention as new soil-applied options for water hemp control. Both of these premixes contains Authority and Classic at equivalent ratios. The Authority component will contribute the most toward control of waterhemp, especially in fields containing ALS-resistant biotypes. Based on our limited experience evaluating these premix products in fields with a significant waterhemp population, we would recommend these products be applied no earlier than 2 weeks before planting.

Cover is a product marketed by DuPont for use in STS soybeans. This prepacked product contains Authority alone and Synchrony STS. The Authority component is soil-applied for control of nightshade and waterhemp.

Diphenyl ethers: lactofen, fomesafen, acifluorfen. These herbicides have provided consistent control of water hemp. Best control is achieved when applications are made to waterhemp less than 4 inches in height. With all these herbicides, crop injury should be expected and is generally more severe under conditions of high temperature and relative humidity and when crop oil or 28% UAN solution is included.

Glyphosate: Roundup Ultra only on soybeans designated as Roundup Ready. Rates of Roundup Ultra should begin at 1 quart per acre and be increased when large waterhemp is present. We would caution producers not to wait too long after crop and weed emergence to make postemergence applications of Roundup Ultra.

ALS-inhibiting herbicides: These herbicides should be used as premixed or tank-mixed treatments, due to the presence of waterhemp biotypes resistant to this class of herbicides.

The key points to remember for effectively managing waterhemp are

  • Utilize an integrated control program, which could include soil-applied herbicides, postemergence herbicides, and tillage.
  • When using soil-applied herbicides, do not apply these products several weeks before planting. Reducing the interval between application and planting can often increase the length of control.
  • When using postemergence herbicides, do not wait too long after crop and weed emergence to make the application. The variable growth habit of waterhemp usually ensures that a wide range of plant sizes are present when postemergence herbicides are applied.

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 Vocational Agricultural Services, Information Services, College of Agricultural, Consumer and Environmental Sciences, University of Illinois, 1401 S. Maryland Dr., Urbana, IL 61801; fax, (217)333-0005; phone, (217)333-3871.

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