Weed Community Shifts and Management Options in the Conversion to Organic Production Systems

Project Overview

Project Type: Research and Education
Funds awarded in 2002: $93,375.00
Projected End Date: 12/31/2006
Region: North Central
State: Indiana
Project Coordinator:
Kevin Gibson
Purdue University

Annual Reports


  • Agronomic: soybeans
  • Vegetables: peppers, sweet corn, tomatoes


  • Crop Production: cover crops, crop rotation
  • Education and Training: extension, farmer to farmer, on-farm/ranch research
  • Pest Management: integrated pest management, mulching - plastic, prevention, weed ecology
  • Production Systems: transitioning to organic
  • Soil Management: nutrient mineralization


    Five weed management systems were identified in Indiana tomatoes. More time was spent hand-weeding in the organic system than in the other systems but weed densities in the organic system were not greater than in the other systems. Species composition for both the emergent and soil seedbank communities differed among the management systems. However, large weed populations were present in all systems after control measures were completed for the season suggesting that new approaches are needed in tomatoes to reduce within-season weed survival and seed production in order to lessen the need for intensive weed management efforts in subsequent years.


    Weed populations persist or increase when a set of species-specific environmental conditions are met. Grubb (1977) referred to this set of conditions as a “regeneration niche.” The goal of integrated weed management (IWM) is to assemble a set of practices that maintain crop yields while reducing grower reliance on herbicides by: 1) limiting or preventing the growth of weed populations, 2) minimizing the effect of weeds on the crop, and 3) controlling within-season weed outbreaks (Mortensen et al., 2000). When complementary practices are assembled into a system, IWM can limit regeneration niches for weeds. Researchers have examined the relationships between weeds and management practices in vegetable crops (Bonanno 1996; Smeda and Weller 1996; Sankula et al., 1999; Baumann et al., 2000; Melander and Rasmussen 2000; Bond and Grundy 2001; Haar et al., 2002; Ngouajio et al., 2003; Rasmussen 2003; Brainard and Bellinder 2004; Madden et al., 2004). However, researchers have rarely examined the cumulative effect of management systems, as opposed to individual practices, on weed species and weed communities.

    On-farm studies can allow researchers to examine management systems and weed communities in a manner that is not possible with the small-plot experiments typically used by weed scientists (Shennan et al., 1991). Leeson et al. (1999) used minimum variance classification and non-metric dimensional scaling (NMDS) to group 28 Canadian farms into seven farm management systems based on the responses of the farmers to detailed questionnaires regarding cropping history, pesticide, tillage and fertilizer use. The authors conducted on-farm sampling of the weed communities and used canonical correspondence analysis (CCA) to determine if associations existed between farm management systems and weed communities. There were significant correlations between management systems and weed communities suggesting that weed species were being selected for by management practices. The correlation of weed species with management practices is of concern because these species have the potential to increase in abundance and threaten the sustainability of the crop production system (Leeson et al., 2000).

    Researchers have also examined the effect of management practices on weed community structure (Magurran 1988; Derksen et al., 1995) and have speculated that management practices that promote more diverse weed communities and weed seed banks (greater species richness, more even distribution of species relative abundance) might be more sustainable than practices that promoted weed communities with a few dominant species (Clements et al., 1994). However, Clements et al. (1994) noted that relatively little is known about the relationship between weed community characteristics and the sustainability of weed management practices. The authors called for more detailed studies of weed communities and management systems to provide information necessary to improve our ability to understand and predict the effect of IWM on weed communities.

    Bell et al. (2000) noted the need for increased research on chemical and non-chemical weed management systems in vegetable crops. However, we are aware of no published studies which specifically address the relationship between farm management systems and weed communities in vegetable crops. Our primary objective was to characterize the relationship between weed communities and management systems in tomatoes. We were particularly interested in comparing conventional and organic systems and weed communities.

    Project objectives:

    Objective 1. Increase interaction among farmers, extension personnel and researchers by organizing an advisory board committed to facilitating research on organic vegetable crop production and by conducting on-farm research.

    Objective 2. Quantify the effect of conventional, transitional and organic farm management systems on weed species composition and abundance.

    Objective 3. Provide information on the potential advantages and limitations of weed management in organic systems to farmers and extension personnel.

    Objective 4. Stimulate on-farm research on weed management systems in organic crop systems.

    Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.