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

Final Report for LNC02-220

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
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Project Information


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.


Materials and methods:

An advisory board was formed to facilitate on-farm data collection, to help assess the relative advantages and limitations of different vegetable systems, and to identify continuing and emerging problems in weed management. The board assisted in the development of a questionnaire and in identifying farmers who might participate in this project. A detailed questionnaire regarding weed management practices and farming systems was developed and disseminated in 2003 and 2004 to a group of twenty-five production, fresh market, and organic tomato growers in Indiana. Growers were recruited during extension meetings, during conferences, and based on recommendations from the advisory board. We used on-farm sampling of emergent weeds and of the soil seedbank during 2003 and 2004 to determine the identity and relative abundance of weed species on fields managed by farmers who completed our questionnaire. We used multivariate statistical analyses to classify tomato farms into weed management systems based on the questionnaires and canonical correspondence analyses and species indicator analyses to identify relationships between weed communities and management systems.

Research results and discussion:

Management systems. The results of our multivariate analyses (Ward’s method and non-metric multidimensional scaling) indicated that the tomato fields of the surveyed growers could be grouped into five management systems. Irrigated Processing (IPROC) and Rain-fed Processing (RPROC) groups consisted entirely of fields grown with conventional management practices for the processing tomato market. Growers in these two groups used 84 cm rows, did not use plastic mulch or stake tomatoes, did not use cover crops and averaged less than 2 hrs ha-1 season-1 hand-weeding. Tomatoes in these fields were typically grown in rotation with agronomic crops like field corn, seed corn, and soybeans. IPROC fields differed from RPROC fields primarily in the application of irrigation. The fields that grew fresh market tomatoes were grouped into three management systems: Irrigated Mixed Fresh Market (IMFM), Rain-fed Mixed Fresh Market (RMFM), and Irrigated Organic Fresh Market (IOFM). Four organically managed fields were included in the RMFM group and two organically managed fields were included in the IMFM group. The IOFM group consisted solely of organically managed fields. The IMFM group consisted primarily of irrigated fields and averaged 46 hrs ha-1 season-1 of hand-weeding per season. Cover crops were used on 26% of the fields and plastic mulch was used on 85% of the fields in the IMFM group. Growers in the IMFM and RMFM groups rotated an average of 3.3 to 3.4 crops, primarily sweet corn, cucurbit vegetables, and peppers, over a five year period. The RMFM group differed from the IMFM group in that growers did not irrigate or use plastic mulch for weed control, averaged 75 hrs ha-1 season-1 of hand-weeding, and planted cover crops on 55% of the fields in this group. All of the growers in the IOFM group used irrigation, more than half used plastic mulch, and averaged 280 hrs ha-1 season-1 of hand-weeding per season. Growers in this group used cover crops, staked their tomatoes, and averaged three crops in their rotations over a five year period. IOFM fields had fewer crops in rotation than the other fresh market systems; this reflects the use of perennial legumes, like alfalfa and red clover, by organic growers for soil building and mineralization for future crops. Field size was not considered a management practice and was not included in our statistical analyses; however, field size may be related to hours spent hand-weeding. Fields in the IPROC and RPROC groups averaged 28.0 ha ± 2.6. Fields in the IMFM group averaged 1.8 ha ± 0.3 while fields in the RMFM and IOFM groups averaged 0.4 ha ± 0.1 and 0.6 ha ± 0.1, respectively.

Our study suggests that conventional tomato growers in Indiana who are interested in transitioning to organic production may face substantial hurdles related to weed management. Fields in the organic group (IOFM) required many more hours of hand-weeding to produce tomatoes than fields in the other groups. It seems unlikely that conventional growers, particularly those who grow processing tomatoes (IPROC and RPROC) would be willing or able to adopt the level of hand-weeding apparently needed for fields in IOFM. Thus these growers are unlikely to adopt the IOFM weed management system. Some organic fields had lower hand-weeding requirements and were placed in the IMFM and RMFM groups, suggesting that it may be possible for hand-weeding to be reduced for organically grown fresh market tomatoes. Additional research is needed to identify and test alternatives to hand-weeding for growers interested in organic tomato production.

Weed communities. No single weed species had a density greater than 3 plants m-2 in any system. The average weed densities summed across species ranged from 7.1 plants m-2 ± 2.8 for the RPROC system to 29.6 plants m-2 ± 7.2 for the IMFM system. Weed densities were significantly greater in the IMFM than the RPROC system but no differences were detected between the organic and fresh market systems. The average number of species per field ranged from 5.8 species m-2 ± 0.5 for IPROC to 13.1 species m-2 ± 1.1 for the IOFM system. The number of species was significantly greater in the IMFM, RMFM, and IOFM systems than in the IPROC and RPROC systems. The IOFM system had greater diversity (H’), evenness (E), and species richness (Margalef’s index) than the IPROC systems but did not differ significantly from the other fresh market systems. The processing and mixed fresh market systems did not differ significantly in diversity, evenness or species richness.

Several broadleaved species, including velvetleaf (Abutilon theophrasti Medicus.), common lambsquarters (Chenopodium album L.), and prickly sida (Sida spinosa L.), were found in all systems. Velvetleaf and common lambsquarters were identified as problematic weeds in corn and soybean fields throughout Indiana (Gibson et al. 2005). Prickly sida is common throughout Indiana; it germinates later in the season and may avoid control practices in tomato fields (Stubbendieck et al., 1995; Uva et al., 1997). Grass species densities were generally lower than broadleaf densities but some grasses were associated with specific systems. Giant foxtail (Setaria faberi Herrm.) was present in all management systems; however barnyardgrass (Echinochloa crus-galli (L.) Beauv.) and goosegrass (Eleusine indica (L.) Gaertn.) were only found in the fresh market systems. While all of these grasses germinate into the midsummer period (Stubbendieck et al., 1995; Uva et al., 1997; Myers et al., 2004; Davis et al., 2005), these species differ in their growth habit and responses to resource availability. Barnyardgrass thrives in moist environments (Stubbendieck et al., 1995; Uva et al., 1997) and the presence of irrigation but absence of chemical controls may contribute to the strong association of barnyardgrass with the IOFM system. Giant foxtail is capable of growing above the soybean canopy but goosegrass has a prostrate growth habit. Since soybean is grown in rotation with tomato by the processing growers but not by fresh market growers, goosegrass may be at more of a disadvantage than giant foxtail in the processing systems than in the fresh market systems. Only two perennial weed species [horsenettle (Solanum carolinense L.) and yellow nutsedge (Cyperus esculentus L).] had densities high enough to be included in the analyses. The use of multiple tillage practices in all five systems may limit the regeneration niche of most perennial weeds in tomatoes.

Five species (barnyardgrass, yellow nutsedge, green foxtail [Setaria viridis (L.) Beauv] goosegrass, and redroot pigweed) were significant indicators of the IOFM systems. Since mechanical removal is the primary method of in-season control for the IOFM system, it is likely that the grass and sedge species, with their protected growing points and multiple stems, were better adapted than broadleaf species to survive hand-weeding and cultivation practices used by the IOFM growers. Common purslane was a significant indicator of the RMFM system. Common purslane can re-root at nodes following in-season cultivation (Stubbendieck et al., 1995). This ability and the succulent nature of common purslane may prove advantageous for survival following cultivation in non-irrigated environments like the RMFM systems where other species might struggle to survive.

Eastern black nightshade continues to be one of the more problematic weeds in tomato (McGiffen et al., 1992; Eizenberg et al. 2003; Whale and Masiunas 2003) and its competition and control has been studied extensively (Weaver et al., 1987; McGiffen et al., 1992; Eizenberg et al., 2003). Eastern black nightshade was absent from the IOFM system but among the most abundant species in remaining systems. Researchers have shown that herbicides can influence the selection for particular species (Hume 1987; Barberi et al., 1997; Hilgenfeld et al., 2004; Vitta et al., 2004). Eastern black nightshade and tomato are both members of the Solanaceae family and control of eastern black nightshade by most herbicides used in tomatoes is generally low. It is possible that herbicide use in conventional fields has selected for eastern black nightshade, causing an increase in the eastern black nightshade populations. An alternate explanation is that the IOFM system strongly suppresses eastern black nightshade. Fields in the IOFM system included perennial legumes and cover crops in their crop rotation. Creamer et al (1996) found significant seedbank suppression of eastern black nightshade from the use of rye, crimson clover (Trifolium incarnatum L.), hairy vetch (Vicia villosa Roth.), and barley (Hordeum vulgare L.), and a when used as a fall-seeded cover crop. Further research is needed to identify the mechanisms which control populations of eastern black nightshade in Indiana IOFM fields.

Both the CCA and species indicator analyses suggest that strong associations exist between the farm management systems and specific weed species. These differences along with differences in the species density and diversity among the management systems support the hypothesis that weed communities are strongly affected by management systems, even within a single crop.

Information on how management practices cumulatively affect weed community attributes such as species composition and abundance should aid in the development of integrated weed management systems. Our work raises a number of questions about the relationships among weed species and management systems. First, why are certain weeds associated with particular systems? More specifically, why did the IOFM system have lower populations of eastern black nightshade? Second, can practices which limit problematic species such as eastern black nightshade in one system be adapted for other systems? Finally, do the weed species associated with a given system have similar biology, (i.e. late-emerging weeds as observed in the IMFM) and does this similarity point to a single management solution, (i.e. more late season control options)? Our research also highlights the need in all five systems to focus on reducing escapes and limiting the number of weeds that survive to produce seed.

Literature Cited
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Research conclusions:

This project involved 25 growers and required substantial interaction with the growers to collect information for the questionnaires and to sample their fields for weeds. Thus, in the short-term, we increased interaction between university researchers and growers. We anticipate that the long-term impact of this project will be increased use of on-farm research to study system properties in organic and conventional crops. Our research has also identified areas within tomato management systems where improvements could be made (see below).

Economic Analysis

Not applicable.

Farmer Adoption

Our work identified two areas in which growers might improve their production systems. First, fields in the organic group (IOFM) required many more hours of hand-weeding to produce tomatoes than fields in the other groups. It seems unlikely that conventional growers, particularly those who grow processing tomatoes would be willing or able to adopt the level of hand-weeding apparently needed for fields in IOFM. Fresh market growers might be more likely to transition to organic production but would be more likely to make such a transition if hand-weeding could be reduced. Additional research is needed to identify and test alternatives to hand-weeding for growers interested in organic tomato production. Second, all fields were relatively weedy after control practices were concluded for the season. Weeds that survive or escape control practices can contribute seed to the soil seedbank and necessitate substantial inputs to achieve control in subsequent years. An increased focus on preventing seed production could lead to reductions in the seedbank and might reduce the need for intensive weed management efforts earlier in the season.

Participation Summary

Educational & Outreach Activities

Participation Summary

Education/outreach description:

Hillger, D. E., S. C. Weller, E. Maynard, and K. D. Gibson. 2006. Differences in weed communities among weed management systems in tomato production. Weed Science.

Hillger, D. E., S. C. Weller, E. Maynard, and K. D. Gibson. In press. Weed management systems in Indiana tomato production. Weed Science 54:516-520.

Hillger, D. E., S. C. Weller, E. Maynard, and K. D. Gibson. Submitted. Weed seedbanks differ among Indiana tomato management systems. Weed Research.

Gibson, K. D., D. Hillger and S. C. Weller. 2004. Weed management systems in organic vegetable production. The New Agriculture Network Vol. 1, No. 6 – June 24, 2004.

In addition to publication, our research was presented at the annual meetings of the North Central Weed Science Society and the Weed Science Society of America.

Project Outcomes


Areas needing additional study

Our research, and research conducted on seedbanks in tomato systems, suggests the need to reduce late-season weed survival and seed production. Growers have a number of tools at their disposal ranging from herbicides to cover crops to address this need. However, it seems unlikely that growers would make this additional investment in time and money without quantitative evidence showing the effectiveness of various management practices on reducing both the seedbank and the need for intensive management in subsequent years. Thus research is clearly needed to provide this information to growers. This research might be particularly useful for organic growers interested in reducing hand-weeding.

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.