Waterhemp Management in Corn and Soybeans

April 16, 1999
An article in the previous edition of the Bulletin described the biology and identification of waterhemp and provided some basic management considerations. This article examines in more depth management options that producers may consider when attempting to control this weed.

Several herbicide options are available that can provide good control of waterhemp, but each may require additional management considerations. Although this article does not attempt to discuss every conceivable herbicide option for waterhemp control, it does focus on those products we have evaluated for several years in our waterhemp research trials. It has been our frequent observation that the most consistent waterhemp management programs in corn or soybean production systems involve a sequential management approach.

ALS-inhibiting herbicides have long been used for broad-spectrum weed control in both corn and soybean production systems. Waterhemp response to this family of herbicides can range from acceptable control to complete failure. There are numerous instances of waterhemp biotypes demonstrating resistance to this herbicide family, and we generally do not recommend exclusive reliance on this chemistry to manage waterhemp. While still effective on many other weed species, ALS-inhibiting herbicides should not be relied on exclusively for effective waterhemp control. Including alternative herbicide modes of action, through tank mixes or premixes, with ALS-inhibiting herbicides should result in more consistent waterhemp control.

No-till production fields may have waterhemp emerged prior to planting, even though waterhemp is frequently observed to germinate later in the season. Every effort should be made to control any emerged waterhemp prior to planting. 2,4-D, glyphosate-type products, Gramoxone Extra, or triazines (other than cyanazine) can provide good burndown activity on waterhemp. Any waterhemp that is not controlled prior to planting may present significant management problems later in the season.

Soil-Applied Herbicides for Corn

Triazine herbicides can provide good residual waterhemp control. Atrazine probably has the most activity on waterhemp, whereas cyanazine (Bladex) has virtually no activity on waterhemp. The other triazines used in corn, simazine (Princep) and metribuzin (Sencor), also have activity on waterhemp, but may not possess the degree of efficacy as atrazine. We have, however, identified waterhemp biotypes in Illinois that are resistant to triazine herbicides. In fields with these biotypes, triazine herbicides would obviously not provide an acceptable level of waterhemp control. We do not feel that triazine resistance is extremely widespread in the Illinois waterhemp population at this time, especially in the northern half of the state.

Chloroacetamide herbicides (metolachlor, acetochlor, dimethenamid, alachlor) are used primarily for grass control, but each of these herbicides can afford some level of waterhemp control. Axiom, a premix of a grass herbicide (flufenacet) and a broadleaf herbicide (metribuzin) can also be used to achieve some level of waterhemp control. Our research indicates that when these products are used alone, the level of waterhemp control can be expected to decline by about one month after application (we applied the herbicides immediately after planting). Are there any major advantages to selecting one of these chloro
acetamides over the others for waterhemp control? Our data suggest that all these herbicides perform similarly, and we do not see any distinctly clear advantages of one over another. These herbicides are used primarily for grass control, and selection should likely be based primarily on their respective performance on grass species. Tank-mixing these herbicides with atrazine or using premixes containing atrazine can increase the level of waterhemp control over what can be achieved with these chloroacetamide herbicides alone.

Balance (isoxaflutole) is a new corn herbicide that can be used to manage waterhemp. It has performed reasonably well in our waterhemp trials, both alone and as a tank-mix partner. Use rate will influence the length of residual control. As Balance will likely be tank-mixed with some other herbicide(s) to broaden the weed control spectrum, several potential tank-mix partners could increase the level of waterhemp control.

Dicamba is used primarily as a postemergence herbicide to control emerged broadleaf weed species, but it can also be used as a preemergence treatment. Dicamba has a high water solubility and can easily move through the soil profile with adequate moisture. In certain situations, dicamba may actually move too deeply in the soil profile to provide acceptable control of a shallow-germinating species such as waterhemp. Dicamba, alone or with atrazine, generally provides more consistent waterhemp control when used as a postemergence treatment.

Postemergence Herbicides for Corn

Triazine herbicides such as atrazine and metribuzin may be used for postemergence control of waterhemp. When applied postemergence, these herbicides translocate little within the plant following absorption and thus are more "contact" in nature. Best control can be achieved when these herbicides are applied to waterhemp
less than 4 inches in height. Atrazine should be applied with a crop oil concentrate, whereas metribuzin is usually tank-mixed with another herbicide.

Growth regulator herbicides may afford the most consistent postemergence control of waterhemp. In particular, dicamba has generally provided the best waterhemp control in our field research. 2,4-D can also be used to control waterhemp, but many producers are reluctant to use 2,4-D because of crop response concerns. Dicamba is often the tank-mix partner of choice for other postemergence herbicides that may be somewhat weak on waterhemp. Reduced rates of dicamba may be applied by directed application later in the growing season, which may help suppress waterhemp growth and seed production. Distinct (dicamba + diflufenzopyr) can also provide postemergence waterhemp control.

Glyphosate-resistant corn hybrids allow for the postemergence use of glyphosate-type products such as Roundup Ultra. Glyphosate-type products can be effective on waterhemp, and rates can easily be adjusted to control larger plants.

Glufosinate (Liberty) used in conjunction with glufosinate-resistant corn hybrids is another effective postemergence herbicide option for waterhemp control. Liberty is primarily contact in activity, but limited translocation within the plant following absorption can occur. Postemergence applications of Liberty should be made before waterhemp exceeds 4 to 5 inches in height. Similar to glyphosate, Liberty has little soil residual activity, so only waterhemp that was emerged at the time of application will be controlled. Additional waterhemp emergence after application may require subsequent management considerations. Liberty ATZ is a premix of Liberty and atrazine that can provide some limited soil residual activity following application.

Carfentrazone (Aim) is a new postemergence corn herbicide that can provide some limited control of waterhemp when applied alone. Enhanced control may be achieved by tank-mixing Aim with another postemergence herbicide that also has activity on waterhemp.

Soil-Applied Herbicides for Soybeans

Dinitroaniline herbicides such as Prowl or Treflan may be used for waterhemp control in soybean. Treflan requires mechanical incorporation, whereas Prowl may be surface applied without incorporation (south of Interstate 80). With application followed by no mechanical incorporation, precipitation is required to move the herbicide into the soil solution, where it becomes available for plant uptake. Surface applications of Prowl with no accompanying plan to mechanically incorporate should probably be made within 2 weeks of planting to increase the likelihood of receiving the required precipitation. If sufficient precipitation is not received between application and planting, mechanical incorporation can still be performed. Prowl applications of 2.5 to 3 pints can provide several weeks of waterhemp control, but a postemergence herbicide application is often needed to provide an acceptable level of waterhemp control.

Sulfentrazone-containing herbicides (Canopy XL and Authority) have performed well in our research trials, and have generally afforded the longest residual waterhemp control of any soil-applied soybean herbicide we have evaluated. Data in Table 2 and Table 3 are from a 2-year study examining the influences of herbicide rate and application timing on waterhemp control. The data suggest that good waterhemp control may be achieved for several weeks after application. Keep in mind that these research plots were established on a soil with less than 3 percent organic matter, and results may vary when these same treatments are applied to soil with a higher organic matter content. Soybean response to sulfentrazone has been observed in some of our research, and evidence is accumulating which suggests that differential varietal tolerance may exist. Adverse environmental conditions may also influence this response.

The chloroacetamide herbicides Frontier, Dual II Magnum, and Lasso or Micro-Tech can also afford some waterhemp control in soybeans. Axiom may also be used in soybean for waterhemp control. Similar to their activity in corn, the level of waterhemp control may begin to decline about 1 month following application of these herbicides, and a postemergence herbicide application is often needed to provide an acceptable level of waterhemp control.

Sencor/Lexone (metribuzin) and Lorox (linuron) are other herbicides that can provide some waterhemp control in soybean. Lorox is better suited to the lower organic matter soils of southern Illinois.

Postemergence Herbicides for Soybeans

Postemergence herbicides for waterhemp control in soybean are essentially limited to two herbicide families, the diphenyl ethers and glyphosate-type products.

The diphenyl ether herbicides include Blazer/Status, Flexstar/Reflex, and Cobra. Several management considerations need to be addressed with
these herbicides. First, waterhemp size has a significant influence on the level of control that can be achieved with these herbicides. Applications made to plants in excess of 6 inches in height will generally provide only marginal results. About a week after application to large plants (larger than 6 inches), the initial results often appear good. However, our observations have been that, by about 2 to 3 weeks after application, many of these larger plants have recovered from the herbicide application and have resumed active growth. This observation is evident by the data in Table 4. By 3 weeks after application, the data indicate that waterhemp control has declined. In both years, this decline in waterhemp control was attributed to regrowth of larger plants, not to additional waterhemp emergence after application. Second, environmental conditions can influence the level of waterhemp control. Overall, in our studies waterhemp control was better 1 week after application in 1998 than in 1997. Finally, herbicide application rate can influence the level of waterhemp control achieved. When these herbicides were applied at near full labeled rate, waterhemp control was generally enhanced compared to when rates were reduced.

Glyphosate-type products are other postemergence options for waterhemp control when used in conjunction with glyphosate-resistant soybean varieties. Application rates can easily be adjusted to match waterhemp size. With no significant soil residual activity, waterhemp plants that emerge after application will not be controlled and may require further management considerations. Table 5 has data from a 2-year study examining the influence of Roundup Ultra application rate and timing on waterhemp control. As with other postemergence herbicides, control can vary across years, as is evidenced by the overall better control with Roundup Ultra in 1997 as compared to 1998. Additionally, each year when treatments were applied to waterhemp 2 inches in height, additional waterhemp emergence occurred after this early season application, and a sequential application was needed to achieve acceptable control.

In summary, several herbicide options are available that can provide good waterhemp control. However, consistency of control can vary across years and depends on factors such as application timing, rate, and environmental conditions. Whereas waterhemp may, in some instances, be adequately controlled by a single soil-applied or postemergence herbicide, this is generally not considered the most consistent method to manage this weed. Because of the biology of waterhemp and environmental variability across years, we feel that the most consistent waterhemp-management programs in either corn or soybean production systems consist of a sequential management approach. By sequential, we are referring to utilization of multiple control options, including tillage, cultivation, soil-applied herbicides, and postemergence herbicides.--Aaron Hager, Loyd Wax, Marshal McGlamery

Author: Aaron Hager Marshall McGlamery Loyd Wax