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Is one-pass weed control a realistic goal?
by Bob Hartzler
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November 21, 1996 - In recent years there has been a movement towards one pass weed management systems. This trend towards one-pass weed control can be attributed to two major factors. As farm size increases without additional labor resources, the need for reducing trips over the field is obvious. This has resulted in a significant decline in mechanical weed control strategies that once were primary components of most weed management programs. The second factor is the increased competitiveness of the herbicide market. Manufacturers have created unrealistic expectations for product performance in order to gain market share. While everyone can cite examples where one-pass weed control has provided effective control, most also recognize the inherent variability in these systems.

Risks associated with one-pass weed control

A one-pass weed control program can be built around preemergence or postemergence herbicides. While there are situations where either of these strategies will work, both have inherent weaknesses that result in inconsistent performance. The effectiveness of preemergence programs is strongly influenced by the timing of rainfall events. Herbicides applied at planting generally do not provide effective control if rain does not occur within three to five days of application. The resistance of growers to supplement preemergence programs with rotary hoeing has resulted in increased variability in performance of these programs.

Early preplant applications have increased in the upper Midwest to help manage problems associated with lack of rain following application. However, this limits herbicide options since only those products with relatively long residual activity will provide full-season control when applied ahead of planting.

Most one-pass preemergence programs also have poor activity on many important weed species. Large seeded broadleaf weeds, such as velvetleaf, cocklebur and morningglory, will not be controlled consistently by many preemergence herbicide programs. Perennial weeds, which are increasing due to reduced tillage systems, also are unaffected by most soil-applied herbicides.

The second approach to one-pass weed control is a total postemergence program. Total post programs have become more popular with the introduction of several highly effective herbicides and the increase in reduced tillage. Application timing plays a key role in the effectiveness of postemergence herbicides. Herbicides must be applied before weeds exceed the size where they will be controlled and before early season competition occurs between the crops and weeds. Most fields are infested with several weed species that exhibit different emergence patterns. Thus, an application that is made when giant foxtail is at the appropriate growth stage may be too late for controlling lambsquarter or too early for cocklebur. Single-pass treatments usually result in a compromise for application timing that may reduce overall weed control.

The second problem with one-pass postemergence programs is the potential for herbicide interactions when products are tank-mixed. Reduced grass control is often observed when products such as Poast Plus, Assure II, Select or Fusion are tank-mixed with herbicides for broadleaf weed control. The likelihood of antagonism is dependent upon target weed species, weed size and environmental conditions. Other problems may be encountered due to differences in requirements for spray additives among the products used in tank-mixes. The additive required by one product for effective weed control may significantly reduce crop tolerance to the second product and result in serious crop injury.

Common waterhemp: a case study

One consequence of simplified weed management systems, such as one-pass programs, is the potential for rapid shifts in problem weed species. The recent emergence of common waterhemp as a widespread problem in Iowa and surrounding states is an excellent example of a weed shift brought on by changes in weed management strategies. Common waterhemp is a native species that was consistently controlled by weed management programs used in the 1970's and 1980's. However, current crop production practices have created a niche that has allowed waterhemp to thrive. There are at least three reasons for the increase in waterhemp problems: 1) changes in tillage systems, 2) adoption of total postemergence programs, and 3) selection of herbicide resistant biotypes.

Small seeded weeds such as waterhemp and lambsquarter are better adapted to reduced tillage than large seeded weeds, such as velvetleaf and cocklebur. The expansion of reduced tillage has created millions of acres of habitat favorable for waterhemp. Concurrent with the adoption of reduced tillage has been the growth of total postemergence programs and a decrease in the use of preemergence herbicides and inter-row cultivation. Waterhemp is ideally adapted to one-pass programs since it germinates later in the growing season than many other common weed species. The emergence patterns of four weed species during 1995 in central Iowa is shown in Table 1 and illustrates the problem caused by differential emergence of weed species. For example, if soybeans were planted in mid-May and a total post program was applied during the first week of June, over 85% of the giant foxtail, woolly cupgrass and velvetleaf would be exposed to the herbicide. However, only half of the waterhemp population had emerged at this time. Since most postemergence products have little residual activity, the plants that emerged after the first week of June would have been unaffected by the herbicides used in a total post program.

Table 1. Emergence profiles of four weed species in Iowa during 1995.


Date of first emergence

% Emerged on May 18

% Emerged on May 31

% Emerged on June 8

Giant foxtail

May 15




Woolly cupgrass

May 2





April 28





May 22




The final reason for the rise in waterhemp problems is the selection of waterhemp biotypes that are resistant to ALS-inhibiting herbicides. Repeated use of ALS products has caused a shift in waterhemp populations so that the dominant biotype found in many fields is resistant to most ALS-inhibiting herbicides. The rapid rate that this occurred indicates the resistance biotype was present at a relatively high frequency in native waterhemp populations.

Waterhemp is an example of the species shifts that can occur when changes in crop production practices result in increased reliance on a single type of management strategy. Other examples of weed species that are found on increasing acres include Venice mallow, common ragweed, woolly cupgrass and wirestem muhly. In most instances, these weeds can be brought back into control with minor adjustments to the weed management program.


Today's herbicides are highly effective tools, allowing changes in production systems that have improved profitability and lessened environmental impacts of agriculture. Many growers have designed weed management programs that are almost totally dependent upon herbicides. This type of weed management program is a high risk approach, even though farmers are said to adopt practices that avoid risk. While there is nothing wrong with trying to simplify weed control to as few operations as possible, one must be aware of the limitations of these systems. One-pass weed management programs will always be more inconsistent than programs that include multiple strategies. There will also be many fields that have a mixture of weed species that no single tactic program will control. Developing successful weed management programs requires the ability to recognize the limitations of specific strategies and determining how best to incorporate additional practices that fit the economic and time constraints of individual growers.

Prepared by Bob Hartzler, extension weed management specialist, Department of Agronomy, Iowa State University

For more information contact:
ISU Extension Agronomy
2104 Agronomy Hall
Ames, Iowa 50011-1010
Voice: (515) 294-1923
Fax: (515) 294-9985
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Common chemical and trade names are used in this publication. The use of trade names is for clarity by the reader. Inclusion of a trade name does not imply endorsement of that particular brand of herbicide and exclusion does not imply nonapproval.