Lead Scientist: Joanne Chee-Sanford
Martin M. Williams II

Introduction

Management of weeds is essential for successful crop production. In the U.S. Corn Belt, the primary method for control of weeds involves the use of herbicides. Over 95% of the acreage is treated each year with herbicides. A substantial portion of the acreage is treated with combinations of herbicides, applied either together or sequentially. Repeat applications of herbicides are common. The acreage of no-till production is increasing and the use of postemergence cultivation is decreasing, resulting in further reliance on herbicides for control. Under these conditions, herbicides may sometimes be used when not needed economically and the herbicides in use may contribute on some occasions to off-target drift and to environmental contamination. The database regarding weed biology (germination, emergence, competition, seed production and longevity, etc.) is not adequate and more work is needed. The weed spectrum in many fields is shifting from species sensitive to commonly used herbicides to ones that are somewhat tolerant of these herbicides and practices being used. Weed biotypes that are resistant to some classes of herbicides are evolving and are becoming predominant in some areas.

We are conducting research on weed biology to enhance the database, with emphasis on the most troublesome weeds now and anticipated in the future. We believe these data will help to plan improved and more efficient weed management systems. Other work is aimed at optimizing the application and use of new and standard herbicides and combinations in a systems approach. Our investigations include research on the distribution, mechanism of resistance, and development of strategies for managing herbicide resistant weed biotypes and other hard-to-control species, with emphasis on conservation tillage systems.

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Highlights of Recent Accomplishments

Postemergence applied herbicides often vary in effectiveness with weather and growth stage of crop and weeds, but can be made more consistent with the use of adjuvants. Our fundamental and applied research with adjuvants helped to explain the role of adjuvants in improving herbicide efficacy, and showed ways for improving application techniques to improve weed management with less herbicides inputs.

In regional cooperative work, we researched hemp dogbane, an important, and hard-to-control perennial weed in the Corn Belt, and described the biology of biotypes of this species from several areas. Other studies investigated the efficacy and fate of several herbicides in this species and found improved means of management

Soil-applied herbicides vary in effectiveness depending on soils and weather, among other things. We conducted cooperative research in normal rainfall situations, in rainout shelters, and in the greenhouse to assess the optimum amounts and timing of rainfall needs after application for improved performance with a variety of herbicides. These results allowed for improved guidelines for more effective use of these herbicides.

Cooperative work on the chloroacetamide herbicides helped to define their relative persistence and need for rainfall, and provided guidelines for time of application for optimizing their effectiveness and reducing potential for environmental contamination.

Regional cooperative research resulted in our weed emergence data being incorporated into an area-wide database for generating a mechanistic model of weed seedling emergence timing in the Corn Belt. These results are helpful in planning integrated weed management systems.

A series of studies described the comparative uptake and translocation of various postemergence applied herbicides and related the results to effectiveness of control with these herbicides.

Field experiments over several years determined emergence patterns, seed longevity and seed production for several of the more troublesome weeds infesting corn and soybean crops.

Several experiments were conducted to determine and fine-tune the role of herbicide resistant cropping systems in weed management and provided guidelines as to the most appropriate use of these systems, especially with regard to tillage, row spacing, timing of applications as related to effectiveness and competition, and combinations with other herbicides.

Cooperative studies with University of Illinois scientists investigated the crop injury and weed control of new herbicides, combinations and adjuvants, over several years and locations. The combined database allows for the improved selection of herbicides that are more environmentally benign, have shorter residual, are effective on selected weeds not controlled by standard herbicides, and often allow for more effective mixtures that are more consistent and often allow for reduced herbicide inputs.

In a series of cooperative studies over several years, we conducted considerable field and greenhouse research on the Amaranthus complex of weeds, resulting in a number of reports on identification of the various species, identification and confirmation of herbicide resistant biotypes, determination of the mechanism of action, and development of strategies for improved management. An in vivo method was developed for assaying plants for the ALS enzyme activity, thus facilitating determination of herbicide resistance in weed to ALS-inhibiting herbicides. In other studies, we found that various biotypes of Amaranthus species showed resistance and cross resistance to selected herbicides. Improved management strategies were developed for resistant biotypes, with special emphasis on common waterhemp, using combinations of chemical and non-chemical control. We cooperatively located , identified, determined mechanism of resistance of a common waterhemp biotype that has evolved multiple resistance to two different classes of herbicides. Another cooperative study also found biotypes of the weed kochia that had evolved multiple resistance.

In a 3-year field study, we investigated the efficacy and economic viability of using ultra low rates (ULR) of herbicides for weed suppression in corn and soybeans. The ULR system suppressed weeds enough to allow economical soybean production all three years of the study but this system proved viable only during the wettest year for corn.

The herbicide sulfentrazone is relatively new and can provide excellent control of some of the waterhemp biotypes resistant to other types of herbicides. On some occasions, however, it can produce undesirable injury to soybeans. In a series of greenhouse and field studies, we worked with this herbicide and a newer, related herbicide, flumioxazin, to determine if variety differences could be causing some of the injury noted. Our findings show considerable variety difference in response to both of these herbicides, but also show in field trials that at normal use rates, soybean yields will not likely be reduced. Sulfentrazone was shown to cause more injury at normal use rates than did flumioxazin.

We conducted cooperative research to be able to predict absorption, translocation and activity of foliar applied pesticides in order to enhance pesticide efficacy and to aid in the development of new products that have desired biological properties that will allow expression of maximum activity without adverse environmental consequences. A computer simulation model was developed and validated that combines and describes all of the processes of pesticide absorption into the leaf, transport within the plant and allocation throughout the plant and predicts how the pesticide will be distributed in the plant following foliar application. This model should be useful to researchers in the public and private sectors that develop and design pesticides for maximum biological activity and want to understand all the ramifications for weed management.

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Proposed Research

Under our current project plan, our research is aimed toward providing improved knowledge regarding weeds and their interaction with crop plants. This information should help in our attempt to develop improved and more environmentally friendly and cost effective weed management systems, based on decision aids and improved integrated use of non-chemical means of management in combination with judicious use of new and standard herbicides and combinations as appropriate. Other research is directed toward improving our knowledge and database regarding the fundamental biology of weeds so that possible weaknesses in their life cycle can be exploited in developing more environmentally friendly and sustainable weed management systems.

In the meantime, we are focusing on continuing some current new research and beginning new preliminary research in several areas, including the following, most of which are cooperative with University of Illinois and with other ARS scientists. Overall goals of these projects include: 1) collecting fundamental data regarding the biology and ecology of current and potentially troublesome weeds, including work on seed banks, emergence patterns, biotype variation and competitive relationships, 2) researching new herbicides and combinations with the aim of finding improved weed management with reduced herbicide inputs and more effective and efficient results, 3) establishing cooperative research on biocontrol of selected troublesome weeds in crop and non-crop situations, including microbial effects on invasive and troublesome weeds, 4) investigating current and new biotypes of weeds that evolve resistance to one or more classes of herbicides, 5) investigating the potential for the use of remote sensing to map weed populations and herbicide injury, and 6) researching systems for weed management that include chemical and non-chemical means of control, herbicide by disease and insect interactions, and optimization of herbicide resistant cropping systems, with emphasis on conservation tillage, that aim toward minimizing undesirable effects of herbicides, reduction of herbicide use, and economical weed management.

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