Waterhemp Management Programs for Soybean

by Aaron G. Hager and Loyd M. Wax
This research was funded by the Illinois Soybean Program Operating Board, Project No. 97-23-181-2

• Research Objectives
• Findings
• Impact of Research
• Table 1
• Table 2
• Table 3
• Table 4
• Table 5

Research Objectives

Waterhemp (common and tall) continue to pose significant challenges to Illinois soybean producers. The biology and growth characteristics of waterhemp are such that more traditional control strategies, based primarily on herbicides, are not as effective as for other summer annual species. In particular, waterhemp germination and emergence tend to persist longer into the growing season than for other, more familiar annuals. Resistance to triazine and ALS-inhibiting herbicides is another characteristic of waterhemp that contributes to management difficulties. Developing effective waterhemp management options in soybean production systems requires research involving several possible management scenarios. Thus, we proposed the following objectives for our research:

1. Determine the optimal application timing and rate of selected soil-applied herbicides for satisfactory waterhemp control in soybean

2. Determine optimal application timing and rate of selected postemergence herbicides for satisfactory waterhemp control in soybean

3. Investigate sequential herbicide applications as a reliable approach to waterhemp management in soybean

Findings:

Objective 1: Determine the optimal application timing and rate of selected soil-applied herbicides for satisfactory waterhemp control in soybean

We investigated six soil-applied herbicides growing seasons with respect to efficacy and length of residual control. Each of these herbicides was applied at two rates and two application timings. Rates selected corresponded to label recommendations based on soil texture and organic matter content: one rate was considered a full rate while the other was reduced to approximately two-thirds of the full rate. Application timings were five weeks prior to planting and preemergence.

In each year, only sulfentrazone (Authority) provided an acceptable level of waterhemp control regardless of rate or application timing (Tables 1 and 2). All other herbicides provided good initial waterhemp control, but control several weeks into the growing season was generally not acceptable. Herbicides (except sulfentrazone) applied five weeks prior to planting provided good waterhemp control for approximately two weeks after soybean planting. Preemergence applications provided approximately one to two weeks of additional control, depending upon the herbicide.

The rates of sulfentrazone used in this research were higher than currently labeled rates. Additionally, evidence of adverse soybean response to sulfentrazone is growing. Based on our research as well as other data, we recommend sulfentrazone as an effective herbicide for waterhemp management, but also caution growers about the potential for soybean response. Application of sulfentrazone two weeks prior to planting and avoidance of sensitive varieties are two strategies which may reduce the potential for adverse soybean response. Other herbicides evaluated in this research were also effective in controlling waterhemp. However, based on our results we recommend applications of most soil-applied herbicides be made as close to planting time as possible. Additionally, our results suggest that in many instances some other management option, such as a postemergence herbicide or cultivation, will be needed in addition to a soil-applied herbicide.

Objective 2: Determine optimal application timing and rate of selected postemergence herbicides for satisfactory waterhemp control in soybean

Postemergence options for waterhemp control in soybean are limited. Diphenyl ether herbicides (Blazer, Cobra, Reflex/Flexstar) and Roundup constitute the available options with the highest level of efficacy. Often, these herbicides are applied at less than labeled rates, either alone or as a tankmix component.

A study was initiated in 1996 and repeated in 1997 to compare the efficacy of three diphenyl ether herbicides as influenced by application rate and waterhemp size. Rates selected corresponded to label recommendations for the full rate, one-half the labeled rate, and one-quarter the labeled rate. Application timings were based on waterhemp size, with the initial application made when waterhemp averaged two-to-four inches (EPOST) in height while the late application was made when waterhemp averaged four-to-six inches (POST). Spray additives with each treatment were included according to label recommendations.

Waterhemp control was generally better when herbicides were applied at the EPOST timing (Table 3). Cobra tended to provide the greatest level of control at the EPOST timing regardless of application rate. For all three herbicides, waterhemp control declined during the three weeks after application when ratings were taken. A large portion of this decrease could be attributed to additional emergence of waterhemp that occurred following application. When applications were delayed until the POST timing, waterhemp control was generally reduced for all herbicides. Similar to the EPOST timing, waterhemp control declined over time, however, much of the decline was attributed to lack of control as few additional plants emerged following application.

An additional study evaluating Roundup as a postemergence option for waterhemp management was initiated in 1997 and repeated in 1998. Roundup was applied at 4 separate rates when waterhemp was two, four, eight, or 14 inches in height. Results from each year were generally good (Table 4). Roundup provided good to excellent control regardless of rate in 1997 at all but the 14 inch application timing. Results in 1998 were again good, but control overall was less than in 1997. The two inch application timing was made early enough during the season each year that additional waterhemp flushes occurred after application. A sequential Roundup application was needed to provide an acceptable level of control.

Based on our data, we recommend that diphenyl ether herbicides be applied at near full labeled rates when waterhemp are four to five inches in height or less. Applications made to waterhemp larger than this range are often ineffective and can result in significant soybean injury.

Objective 3: Investigate sequential herbicide applications as a reliable approach to waterhemp management in soybean

We frequently recommend a sequential herbicide approach to waterhemp management in soybeans. Many soybean producers have utilized a "rescue" postemergence herbicide application to attempt to control waterhemp that survived an initial postemergence application, often with less than satisfactory results. A sequential approach utilizing a soil-applied herbicide followed by a postemergence herbicide will, in many instances, provide a more consistent level of control. A study was established in 1997 and 1998 to evaluate the efficacy and consistency of several sequential herbicide programs. Each of the ten treatments utilized a soil-applied herbicide (applied at planting) followed with a postemergence herbicide approximately four weeks after planting.

Averaged across years, seven of ten treatments provided 85 percent or greater control of waterhemp by four weeks after the postemergence application (Table 5). While this level of control may appear less than desirable, none of these treatments had a reinfestation of waterhemp as severe as in our soil-applied or postemergence studies. The results of this study would suggest that a sequential herbicide program can afford a high level of waterhemp control that, in many instances, may be more consistent than a soil-applied or postemergence only approach. Further, the results indicate that there are several viable sequential programs that soybean producers may select.

Impact of Research

Waterhemp continues to present significant management problems for Illinois soybean producers. This research has contributed to refined herbicide recommendations for managing this problem.

There are numerous soil-applied herbicide options that can effectively control waterhemp in soybean. Our work has indicated that herbicide selection, application rate, and application timing all influence the length of residual waterhemp control. Specifically, we inform soybean producers that several soil-applied herbicides can provide control of waterhemp, but that few if any provide sufficient residual control that no further management need be considered. Sulfentrazone provided the highest level of residual control, however the current labeled rates of Authority are much lower than those utilized in our research. This is due to the fact that our research was underway prior to the commercialization of this herbicide and the research was done at rates suggested by the manufacturer as those which would be commercialized. It was during this research that we first noted soybean injury from preemergence sulfentrazone applications.

Postemergence options for waterhemp management include three diphenyl ether herbicides (Blazer, Cobra, Reflex/Flexstar). When applied at full or near full labeled rates, our work suggests that all can be effective, but reduced control can be expected if applications are delayed until waterhemp exceeds approximately five inches in height. We currently recommend against making "rescue" applications of these herbicides for waterhemp control, as our data suggest control will be only marginal while soybean response may be severe.

Roundup can be another effective postemergence option for waterhemp management when used in conjunction with glyphosate resistant soybean varieties. Our data suggest good initial control can be achieved with rates as low as one pint per acre when applications are made to small waterhemp (2 inches). However, when these early postemergence applications are made a second postemergence application may be needed if additional waterhemp emergence occurs. Contrary to observations in the diphenyl ether herbicide study, regrowth of waterhemp generally did not occur following Roundup applications and effective control of larger plants was achieved more consistently.

Sequential herbicide programs may be the most consistent management option for waterhemp control in soybean. Our data indicate several soil-applied herbicide options can provide effective waterhemp control, but may not provide sufficient residual control. Postemergence herbicides can provide good waterhemp control, but generally lack soil residual activity which may allow additional waterhemp emergence. Combining a soil-applied herbicide with a postemergence herbicide provides for waterhemp control early in the growing season as well as additional control immediately prior to closure of the soybean canopy. Utilizing a soil-applied herbicide as a component of a sequential herbicide program may also result in a more uniform size range of waterhemp when the postemergence herbicide is applied, possibly resulting in more consistent control.

Table 1. Waterhemp control with sulfentrazone products, St. Elmo 1997

Table 2. Waterhemp control with sulfentrazone products, St. Elmo 1998

Table 3. Waterhemp control with diphenyl ether herbicides, St. Elmo 1997 and 1998.

*NIS = nonionic surfactant; COC = crop oil concentrate.

Table 4. Waterhemp control with glyphosate, St. Elmo 1997 and 1998.

Table 5. Soybean Sequential Herbicide Programs for Waterhemp Control, St. Elmo 1997-1998