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United States Department of Agriculture

Agricultural Research Service


Location: Global Change and Photosynthesis Research Unit

2007 Annual Report

1a.Objectives (from AD-416)
1) Gain knowledge of the ecology of important weeds in vegetable and agronomic crops to understand fundamental principles affecting weed emergence, growth, interference, seed production, and crop yield and quality;.
2)Investigate biotic and abiotic factors linked to seed predation and microbiological activities regulating weed seed and seedling survival in soil ecosystems; and.
3)Identify effective combinations of weed management components through application of new and existing knowledge and technology that exploit useful plants, natural enemies, and environmental interactions. The objectives will address the need to develop new strategies that include more efficient use of herbicides combined with increased use of alternative, biologically based weed management for agroecosystems.

1b.Approach (from AD-416)
Processes that regulate weed population density will be investigated, with particular focus on spatiotemporal variation in demographic parameters and population growth rates at multiple levels of scale. As a means of unifying observations, whole life cycles of weeds will be the unit of study whenever possible. Important microbial seed bank relationships will be identified along with the underlying mechanisms of interactions between microbes and weed seeds. Factors that control these interactions and natural plant defenses will be examined. Long-term rates of weed seed loss due to predation will be measured and used to validate mathematical models that predict long-term predation rates from point measurements of seed predation. Attention will be paid to cropping system effects on seed predation rates, with the goal of understanding how crop habitat affects the seed-feeding activity of different seed predator taxa. Mechanisms underlying synergistic effects of combinations of weed management tactics on weeds will be examined within the context of both vegetable and agronomic production systems. Options for managing weeds in vegetables are limited, thus focused efforts using sweet corn as a model, along with application of the larger knowledge base gained from field corn and soybean studies, will be beneficial for minor crop systems. A long-term goal of this work is to develop practical guidelines, supported by ecological understanding, for creating multi-tactic weed management systems that are effective, perform consistently, and use herbicides to tune, rather than drive the system. Initially, the weed species of focus are giant ragweed, velvetleaf, giant foxtail, and wild proso millet. This research will improve our understanding of fundamental factors associated with multi-trophic processes and interactions regulating weeds throughout their life cycles. These data will contribute to systematic prediction of the impact of weeds on agronomic and vegetable crops, leading to more effective synergistic combinations of weed management tactics.

3.Progress Report
A one-year parallel study of a natural- and artificial giant ragweed seed bank was concluded and a second seed burial study was initiated in Fall 2006. Results from year one focuses efforts on correlating seed density and soil disturbance effects on seed fate. Physical characterization of the seed bank, seeds, and microbial analysis are currently underway. The results will be used to identify key environmental variables for controlled chamber studies to be conducted in Fall 2007.

Isolation and purification of antimicrobial molecules in kochia and jimsonweed seed exudates began in Summer 2007 and will continue through Spring 2008. Results from these experiments will be used to formulate and test hypotheses regarding the potential utility of these compounds to target specific plant pathogens.

Weed seed destruction by seed predators may play an important role in keeping weed populations manageable. Long-term seed predation studies were initiated in Fall 2004 to determine the impact of crop habitat on seed predation rates of giant ragweed, velvetleaf and giant foxtail by small vertebrates and insects. Point estimates and quarterly measurements of seed predation rates began in Fall 2005 and will continue through Fall 2008. Results will help guide the design of cropping systems with increased rates of weed seed loss due to predation.

No-till soybean production requires intensive herbicide use to replace the weed suppressive benefits of tillage and field cultivation. A new tool for managing cover crop residues in no-till soybean systems, a cover crop roller-crimper, was compared in 2005 and 2006 to the traditional burndown herbicide treatment for cover crop residue management. Weed growth, soil properties and crop yield loss were measured to understand how the different systems function. Measurements will be repeated in 2007. These results will benefit all field crop producers, but will be of special benefit to organic producers, for whom no-till production is currently unfeasible.

1. Demographic models inform selection of biocontrol agents for garlic mustard. Garlic mustard is one of the most widespread, problematic invaders of temperate forest ecosystems in North America. Measurements of garlic mustard population dynamics in managed forests across the North Central region indicated that biological control agents which limit the survival of first year rosettes to reproductive maturity would have the greatest impact on reducing garlic mustard populations in this area. Computer simulations of feeding impacts of several candidate biocontrol agents currently being held in quarantine indicated that the weevil Ceutorhynchus scrobicollis shows the greatest promise for achieving reductions in garlic mustard populations over the range of demographic variability observed in these studies. The results of this work helped to prioritize C. scrobicollis for release in the USDA APHIS Technical Advisory Committee's petition for a new biological control agent for garlic mustard, and provides testable hypotheses about the links between demographic variability and biocontrol agent success. This accomplishment addresses National Program 304, Crop Protection & Quarantine, Component VII - Weed Biology and Ecology, Problem Statement VIIf - Population Dynamics.

2. Sweet corn competitive ability as a tool in weed management. Weed management systems in sweet corn are characterized by extensive use of soil-applied herbicides, many facing increased regulatory scrutiny, and recent surveys indicate that over one-half of commercial sweet corn fields suffered yield losses due to weed interference. Although sweet corn hybrids vary extensively in plant architecture, the implications of sweet corn canopy variation on crop/weed interactions were until recently unstudied. As a result of research from ARS locations in Illinois and Washington, commercially-available sweet corn hybrids were found to vary considerably in competitive ability with weeds, and several crop traits responsible for this variation were identified. Sweet corn traits conferring competitive canopy architecture make consistent contributions to weed management. The potential impact from this research is that growing commercially-available hybrids with canopies that are competitive with weeds, or breeding for hybrids with more competitive canopy architecture, offer tangible improvements to weed management systems. This accomplishment addresses National Program 304, Crop Protection & Quarantine, Component X – Weed Management Systems, Problem Statement Xb - Integrated Weed Management in Cropland.

3. Seed-borne antimicrobial compounds from invasive weed species. One notable characteristic of invasive weed species is their ability to form soil seed banks that are seemingly more resistant to infection than their cultivated counterparts. This trait may partially account for the persistence of weed species in agricultural plots. To determine the mechanism by which weed seeds fend off infection, water-soluble exudates from 10 plant species were tested for antagonistic activity against 15 fungi and 7 bacteria. Of the plants tested, exudates from two weed species demonstrated antimicrobial activity against Clavibacter michiganense ssp. nebraskense, an important bacterial phytopathogen that infects tomato plants or inhibited the fungi Colletotrichum graminicola and Taphrina deformans (the causative agents of maize anthracnose and fruit tree leaf curl, respectively). It has long been assumed that phenolic acids, which disrupt microbial cell membranes, constitute the seed’s primary defense against infection. However, even seeds that lacked demonstrable antimicrobial activity had significant phenol concentrations, and there was a clear lack of correlation between phenol content and antimicrobial activity. Further physical and chemical characterization of seed exudates revealed that the antimicrobial compound(s) are likely proteinacious in nature. This work suggests that weed seeds utilize a specific suite of antimicrobial proteins to fend off microbial infection and degradation, and this may play an important role in weed persistence. The results have led to investigation of possible interest in the private sector and future cooperative research relationships. This accomplishment addresses National Program 304, Crop Protection & Quarantine, Component VII - Weed Biology and Ecology, Problem Statement VIId - Reproductive Biology and Seed Bank Dynamics.

5.Significant Activities that Support Special Target Populations

6.Technology Transfer

Number of web sites managed1
Number of non-peer reviewed presentations and proceedings10
Number of newspaper articles and other presentations for non-science audiences7

Review Publications
Davis, A.S., Landis, D.A., Nuzzo, V., Blossey, B., Gerber, E., Hinz, H. 2006. Demographic models inform selection of biocontrol agents for garlic mustard. Ecological Applications. 16:2399-2410.

Pataky, Jerald K., Norby, John N., Williams, M., Riechers, D.E. 2006. Inheritance of cross-sensitivity in sweet corn to herbicides applied postemergence. Journal of the American Society for Horticultural Science. 131:744-751.

Williams, M. 2006. Planting date influences critical period of weed control in sweet corn. Weed Science. 54:928-933.

Lundgren, J.G., Lehman, R.M., Chee Sanford, J. 2007. Bacterial communities within the digestive tracts of ground beetles (Coleoptera: Carabidae). Annals of the Entomological Society of America. 100:275-282.

Williams, M., Boydston, R.A., Davis, A.S. 2007. Wild proso millet (Panicum miliaceum) suppressive ability among three sweet corn hybrids. Weed Science. 55:245-251.

Williams, M., Boydston, R.A., Davis, A.S. 2006. Canopy variation among three sweet corn hybrids and implications for light competition. HortScience. 41:1449-1454.

Koike, S., Krapac, I.G., Oliver, H.D., Yannarell, A.C., Chee Sanford, J.C., Aminov, R.I., Mackie, R.I. 2007. Monitoring and source tracking of tetracycline resistance genes in lagoons and groundwater adjacent to swine production facilities over a three-year period. Applied and Environmental Microbiology. 73(15):4813-4823.

Raghu, S., Anderson, R.C., Daehler, C., Davis, A.S., Wiedenmann, R.N., Simberloff, D., Mack, R.N. 2006. Adding biofuels to the invasive species fire? Science. 313:1742.

Williams, M., Masiunas, J.B. 2006. Functional relationships between giant ragweed (Ambrosia trifida L.) interference and sweet corn yield and ear traits. Weed Science. 54:948-953.

Ortiz Ribbing, L.M., Williams, M. 2006. Conidial germination and germ tube elongation of Phomopsis amaranthicola and Microsphaeropsis amaranthi on leaf surfaces of seven Amaranthus species. Biological Control. 38(3):356-362.

Last Modified: 4/18/2014
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