Diversity - Intensity of Cover Crop Systems: Managing Weed Seed Bank - Soil Health

Final Report for LNE01-141

Project Type: Research and Education
Funds awarded in 2001: $155,937.00
Projected End Date: 12/31/2005
Matching Non-Federal Funds: $73,290.00
Region: Northeast
State: Maine
Project Leader:
Dr. Eric Gallandt
University of Maine
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Project Information


This project addressed two related issues on low-input and organic small-scale diversified vegetable farms: recurrent and ubiquitous weed problems, and the need for long term soil improvement. Cover crop species and the frequency, timing, and depth of soil disturbance can affect soil quality and contribute to weed management by imposing stresses at multiple points in weed life cycles.

In a cropping systems study, we compared no cover cropping in two year rotation of broccoli and winter squash, to fall cover cropping, a two-year red clover cover crop followed by the cash crops, and short duration cover cropping with periods of summer fallowing in alternate years with cover crops. Soil quality improved by cover cropping, as evidenced by decreases in bulk density and increases in water holding capacity, water stable aggregates, and particulate organic matter in select cover-cropping systems. Similar benefits of a preceding red clover cover crop were noted when the above ground biomass was removed prior to incorporation, indicating important contributions from the below ground biomass and opening up many questions regarding the role of roots in sustainable cropping systems.

Despite an overall decline in the total weed seed bank in three of the four systems over the four years, common lambsquarters, a particularly pernicious species in most northeast cropping systems, increased in each of the systems. However, compared to a system without cover crops, common lambsquarters increased only slightly in the disturbance-intensive, alternate year cover crop system. The perennial cover crop system, despite an abundance of invertebrate seed predators, experienced no net reduction in the germinable weed seedbank over the four-year period. Abundant weed seed rain in the winter squash, as well as within select cover crops, prevented greater progress in depleting the weed seedbank in these cover cropping systems, and suggests that fall weed seed rain management should be carefully considered as part of a comprehensive weed seedbank management plan.


Strategies for weed management and soil improvement can become antagonistic on farms attempting to reduce or eliminate farm chemical use, as herbicides are replaced with mechanical control, which may involve repeated and intensive soil cultivation. Such soil disturbance regimes can badly damage soil structure, decrease soil organic matter levels, and decimate soil biological activity, thus pitting short-term non-chemical weed control against long-term soil health. Furthermore, because each cultivation event controls a constant proportion of weed seedlings present, to maintain a constant, acceptable yield loss the intensity of cultivation must increase with increases in weed pressure (Figure 1). Cover crops can play a key role in an ecologically-based weed management strategy. They may substantially offset or reverse losses of soil quality by reducing the need for recurrent intensive cultivation, by adding crop residues to rebuild soil structure and organic matter, as well as by maintaining soil cover through a greater part of the year, thus reducing erosion losses. Cover crops can have additional benefits in the overall farming system of recycling nutrients, adding biologically-fixed N, acting as a mulch to reduce evaporative losses and attracting beneficial organisms to the field. Weed control from cover crops is derived from their competitive ability during growth, their function as a physical barrier or mulch when killed and left on the surface, as well as allelopathic properties of certain incorporated and surface residues. Perhaps even more important from a weed management perspective are changes in the frequency, timing, and depth of soil disturbances throughout the year, and the period during which the cover crop is actively growing, occupying key niches for certain weed species. This project aimed to better understand how to best exploit these numerous weed-suppressive and soil-improving properties of cover crops in the implementation of cover cropping systems that limit weed biomass and seed production in diverse vegetable rotations (Figure 2).

Performance Target:

Performance Target—Experiment Station and on-farm research will yield a decision-aid matrix including weed species or functional groups, timing and intensity of disturbances, and diversity of cover crops deployed; management strategies will be identified based on their ability to reduce the germinable portion of the weed seed bank while maintaining or improving soil health.

Working towards this Performance Target, two meetings were held in the early spring of 2001. Project participants designed an on-station systems trial that provides replicated comparisons of regionally relevant cover cropping strategies (Figure 3). The team chose first to look in detail at two systems modeled after well-known Northeast farms: New Leaf farm of Dave and Christine Colson in Durham, ME (USDA Hardiness Zone 5b), and the farm of Ann and Eric Nordell in Trout Run, PA (Zone 5a), both of which utilize intensive cover cropping systems on farms that have sufficient land to dedicate to fallow periods. The Colson’s use a two-year block of red clover (Trifolium pratense), followed by two years of vegetables to build the soil, suppress weeds and supply fixed-N to their crops. The Nordell’s alternate years of annual cover crops (including vetch, rye, field peas and oats) and cash crops, and include several weeks of summer fallow during the cover crop year, during which the weed seed bank is depleted through repeated cultivation, creating fields that are virtually weed-free fields. The third system chosen was a more typical New England small farm, which is relatively land-poor and more likely to utilize a winter fallow period for cover cropping.

On-farm sampling was conducted in 2001, with the intention that the cover cropping practices used by the participating growers would be monitored in parallel with the cover cropping system trial established at the Rogers Farm. Ultimately, however, the workload of maintaining and sampling the large systems trial was greater than originally anticipated, precluding our ability to establish, maintain and sample several related on-farm trials.

In the context of regionally relevant organic vegetable cropping systems, we have successfully identified salient features of fundamentally different cover cropping strategies employed by both land-limited growers and growers with sufficient land to implement periods of soil-improving and weed-controlling fallow. While we did not produce a decision matrix as originally envisioned, e.g., as may result from a more reductionist study of weed species by individual cover crop treatments, the larger systems context in which we decided to work has the benefit of a data set that includes the cropping system complexity operating at the farm level. Within this systems context we have identified key management principles related to weed seed rain, maintenance of weed seedbanks in perennial cover crops, the potential benefits of cover crop vegetation as habitat for weed seed predators, and the use of short duration cover crops with summer fallowing to encourage seedbank depletion.

Performance Target—Fifteen farmers will implement intensified/diversified cover cropping strategies on particular fields to reduce the seed bank of problematic weed species.

Lacking a dedicated survey, it is difficult to determine how close our project came to reaching this Performance Target was reached. Through our extensive outreach efforts, many farmers in the region were made aware of key issues related to cover cropping strategies for weed management and soil improvement. As the comprehensive analysis of this systems experiment is completed and distributed throughout the region the change resulting from this project will be evident. Our knowledge of impact on growers to date, however, is evidenced by invitations to present project results at meetings and requests for additional information on the project.

For example, results of this study were used to validate a organic matter budget, which was presented to a group of 20 agriculture educators and extension personal in September, 2005, as part of a SARE grant “Advanced Training in Organic Crop Production.” Since that time, 10 of those who participated in the training have requested further help in using the organic matter budget when working with organic farmers on their own rotations.


Click linked name(s) to expand/collapse or show everyone's info
  • David Colson
  • Mark Guzzi
  • Mark Hutchinson
  • Rick Kersbergen
  • Tom Molloy
  • Suzanne Morse
  • Marianne Sarrantonio
  • Eric Sideman
  • Lucian Smith


Materials and methods:
Cover Cropping Systems Trial

This experiment was designed by Project Participants, a group made up of farmers, extension personnel and University of Maine faculty and was established in 2001 to compare the following four vegetable/cover cropping systems (Table 1): (1) a “control” system (Control), a conventionally-managed 2-year rotation of broccoli and winter squash; (2) Fall cover crop system (Fall CC), an organic, “land limited” system, also a 2-year rotation of broccoli and winter squash, but with winter cover crops (e.g., rye/hairy vetch and winter rye alone) planted following harvest of the cash crops; (3) Perennial cover crop system (2-Yr. CC), an organic, 4-year rotation of broccoli, winter squash, oats/red clover, red clover sod; and (4) Alternate year cover crop system (Alt-yr CC), an organic, 4-year rotation of broccoli/winter rye, winter rye/summer fallow/oat-hairy vetch.

We expected soil quality parameters to improve with increasing frequency of cover crops in the rotations. We expected weed seed inputs to be least, and seed bank depletion will be greatest in the 2-Yr. CC and Alt.-Yr. CC Systems. Specifically, we expected this common endpoint to result from different stresses affecting weed dynamics—mowing and natural mortality within the red clover phase of the 2-Yr. CC reducing weed seed rain and increasing seed mortality; shallow tillage and “stale seedbed” driving weed dynamics in the Alt.-Yr. CC system. In the Control and Fall CC systems, weed seed inputs will be greater resulting in maintenance of the weed seed bank.

Site. This trial was located on a 1.2 ha field at the University of Maine Rogers Farm in Stillwater Maine. The area was cropped with red clover in 1996-1998; dry bean and a cover crop of spring wheat were grown in 1999 and 2000, respectively. Soil at this site is a Lamoine silt loam, pH 6.1, and 3.9% organic matter.

Experimental design. The experiment was established in a randomized complete block design in which all of the rotation entry points were included for a total of 48 plots. Individual plots were 7.3 by 21.2 m; plots were separated by a 2-m of mowed ryegrass (Lolium perenne L.), and headlands were maintained free of vegetation by periodic fallowing. Prior to individual treatment establishment all plots received a one-time application of soft colloidal rock phosphate (16% total P2O5, 2% P2O5 available) at 1840 lbs. per acre.

Broccoli management. Broccoli (Brassica oleracea (italica group) ‘Packman’ 50 day F1 hybrid) transplants were started in 72 cell flats using Pro-Mix BX soil-less growing medium in mid-April each year. Plants were fertilized using an organic fish/seaweed emulsion (3-2-2). Broccoli plots were chisel or moldboard plowed, disked twice, and, depending on the cover cropping system, fertilizers were broadcast applied to a target of 100 kg N ha-1, 67 kg P2O5 ha-1, and 157 K2O ha-1. In the Control treatments nutrients were supplied with ammonium nitrate, triple superphosphate and muriate of potash; blood and bone meals and sul-po-mag were used in the remaining systems. Seedlings were transplanted in late May, with eight rows per plot. Rows were spaced 80 cm apart; seedlings were spaced 30 cm apart. In the Control weeds were managed with napropamide and trifluralin (2.24 kg and 1.12 kg ai ha-1, respectively) incorporated by disking prior to transplanting. Fall CC, 2-Yr. CC, and Alt.-Yr. CC treatments were cultivated twice with sweeps. Weed-free subplots four rows wide (3.5 m) and 3.3 m long were established within each plot. Broccoli harvest began in mid-July each year and lasted about two weeks.

Winter squash management. Squash plots were chisel or moldboard plowed, depending on residue conditions, and nitrogen (84 kg N ha-1) was applied in a band centered over each bed; remaining fertilizers were broadcast, by system, to a target of 157 kg P2O5 ha-1, and 224 kg K2O ha-1. Nutrient sources used in each system were as described above. After disking to incorporate the fertilizers, two black plastic (1 mil) covered beds were formed in each plot. Beds were centered at 2.4 m and buttercup squash (Cucurbita maxima L. ‘Burgess’ 95 day squash seeds were planted with a jab-planter at 46-cm in-row spacing in early June. Squash plants were thinned to one plant per location. In each system the inter-bed area was cultivated twice to control weeds both between the beds and along the edges of the plastic. The final cultivation was performed as close as possible prior to vine elongation. Fruits were harvested in the third week of September each year.

Cover crop management. The Fall CC, 2-Yr. CC, and Alt.-Yr. CC Systems each have cover crop components as part of their rotations (Table 1). Tillage operations used to prepare the soil for cover crop planting varied somewhat depending on residues and soil conditions, but generally relied on disking for preparation of a seedbed. Additional tillage occurred in the Alt.-Yr. CC plots to initiate and maintain the summer fallow period. This tillage consisted of a deep moldboard plowing (approximately 17 cm) and two disking operations in June. Two subsequent disking operations in early and late July were used to maintain the plots weed free. All cover crops were planted using an 8-ft. Massy-Harris grain drill with the seedbed culti-packed after planting.
Monitoring Cover Crop Performance, Soil Quality and Weed Dynamics
Cover crops
Cover crop sampling occurred at various times in the different systems. For example, in the Perennial CC system, above-ground biomass was sampled in mid-July prior to mowing of the oats. All of the plant material in three 0.25 m2 quadrats was clipped at ground level and separated by plant species. Weeds were counted and had any maturing reproductive material removed. The non-reproductive weed biomass was dried for several days in a drying room and weighed. The reproductive biomass was allowed to air dry for several weeks and then weighed. This reproductive biomass (primarily common lambsquarters) was then sub-sampled to determine the number and biomass of mature seeds.

All plots were sampled in mid- to late-October for cover crop biomass and weed density and biomass. Three 0.25 m2 areas were sampled out of each plot. All plant material was clipped and removed from each area and separated by species. Weeds under 2-cm tall were counted by species but not included in the estimate of above-ground biomass. All weeds that were removed were counted and those weeds without reproductive material were dried at 60 C for several days and weighed. Any weeds with maturing reproductive material were air dried for several weeks, weighed, and sub-sampled as described previously to determine mature seed numbers and biomass.
Soil quality
Soil quality determinations were performed each year in all broccoli and squash plots in the spring just prior to tillage. In alternate years, soil quality parameters were measured again in mid-season. Assessments were performed by inserting a 15 cm aluminum ring into the soil to a depth of 7.5 cm. This allowed the measurement of saturated water infiltration rates (time for 440 ml - 2.5 cm - of water to infiltrate the area of the ring), bulk density (dry weight of soil removed from ring/volume) and field water holding capacity (weight of water in the ring volume after 24 hr/dry weight of soil). The 2005 sampling season allowed an assessment of cumulative changes in the four cropping systems over the previous four years of rotation in terms of soil quality and N cycling. All previous soil quality sampling was repeated in 2005 on broccoli and squash plots, as in previous years.

Soils were sampled bi-monthly during the growing season each year of the trial and analyzed colorimetrically for ammonium and nitrate in all broccoli and squash plots. Continuing the experiment into a fifth year also allowed a second year of sampling cover crop roots and assessing the effect of roots on N dynamics and crop yields, by removing aboveground biomass from subplots within the main plots. Cover crop roots (cereal rye and red clover) were sampled in 2004 and 2005 at the following depths: 0 to 5 cm, 5 to 20 cm and 20 to 50 cm. Soil cores were excavated and roots separated in small aliquots from soil particles from soil material with a root elutriator. Root length density was then determined using a WinRhizo® scanner and software that calculated length and diameter of roots per cubic cm.

Three new soil quality assessments were made in 2005: percent water stable aggregates (WSA), particulate organic matter (POM), and overall percent organic matter (OM) in all plots. These parameters change more slowly over time than the other soil quality parameters measured, so it was appropriate to take a final assessment at the end of the rotations. Assessment of WSA was performed using a wet sieving method, which separated soil aggregates after gentle agitation in water into the following sizes 0.005 to 0.05 mm, 0.05 to 2.00 mm, and 2 to 4 mm. Particulate organic matter is a measurement of organic material in the 0.05 to 2.00 mm size fraction and is well correlated with the active fraction of soil OM. POM is determined by dispersion of soil aggregates and sieving to capture the fraction of soil minerals and organic matter of the correct size. The organic matter is then burned off at 375 C and the difference in weight before and after ignition is the POM fraction.

Because the decision was made prior to the beginning of the field season that 2005 would be the final year of the trial, it allowed us to split the plots in half, applying a nitrogen source to one half (ammonium nitrate or blood meal) and no N to the other half. This afforded a rare opportunity to see the cumulative effect of the previous four years of rotation on the N availability to broccoli and squash.
Weed dynamics
Weed seedbank sampling. To estimate the germinable portion of the weed seedbank, soil samples were collected from each plot in the spring following primary and secondary tillage operations. Ten soil cores (8-cm dia. by 10 cm deep) were taken from each plot. Samples were spread out on fine vermiculite in flats in a greenhouse and watered daily (Figure 4). Emerged weeds were identified, counted and removed and the flats were allowed to dry out for several days. When dry, the soil was lifted, crumbled and redistributed back in the flats and watered again. This cycle of watering and drying encourages additional flushes of the germinal weeds and was repeated five times.

Weed density and biomass. Weed density and biomass were sampled in the spring, prior to unique disturbance events in individual systems, and in the fall of each year. At least three 0.25 m2 quadrates were taken from each plot. Weeds were identified, counted, and dried at 60 C for several days before being weighed.

Research results and discussion:

The 2001 through 2005 growing seasons ranged from record drought conditions in 2001 and 2002 to more than double the long term average precipitation in May and June of 2005 (Table 2). The extreme conditions prevented broccoli transplanting in certain years, and prevented sowing of certain cover crop treatments in other years, but nevertheless represents realistic growing conditions for these systems in Maine.

Over the period of 2001 through 2004 the cover crop systems received a similar amount of total cover crop biomass, but due to unpredictable performance of the winter legume (hairy vetch), considerably different proportions of legume and non-legume biomass (Table 3).
Soil quality
Decreases in soil bulk density and increases in soil water holding capacity became more detectable over time and were related to the total biomass of cover crop incorporated in each system, with the Control (no cover crop) showing the least improvement, and the 2-Yr. CC system showing the most (Figure 5). Surprisingly, the Alt.-Yr. CC system, which had a low bulk density and high WHC in 2004, actually showed a reverse trend in 2005. This characteristic may have been due to the extra tillage performed on this rotation and could have led to some of the other negative consequences in terms of crop growth seen in this treatment.

Water stable aggregation was significantly greater following two years of red clover than in any of the other treatments within the 2005 broccoli plots (Figure 6 A). Treatment effects on aggregation was not as evident in the second crop following red clover (data not shown): WSA determinations in the squash plots showed a small increase in aggregation in the 2-Yr. CC system, especially in the larger size aggregates, but the effect was not significant.

Compared to the other systems, POM was greater in the 2-Yr. CC system across all 4 years of the rotation (Figure 6 B). POM was highest during the years that clover was in the field, as expected, but showed only slight reduction during the broccoli and squash years. The Control treatment was lower across both years of the rotation than the other treatments.

Nitrogen Dynamics. Nitrogen cycling was somewhat confounded over the five years of the trial by the fact that three of the four systems had external N inputs for broccoli each year, whereas the 2-Yr. CC treatment, in which broccoli followed two years of clover, relied entirely on indigenous N. In 2005, soil cores for inorganic N were taken from both +N and –N subplots and allowed an examination of how well each system had stored N over the previous four years of rotation. As expected, red clover plots accumulate more nitrate N during the season than winter rye plots (Figure 7). Both systems had very low ammonium levels throughout the season (data not shown). All of the cover crop treatments showed a disturbing trend of accumulating nitrate in the soil after broccoli uptake had ceased.

After 4 years of rotation and coming out of two years of red clover, soil N was 2 to 4 times higher in red clover plots than in cereal rye plots (Figure 7). Of greater interest was the fact that both red clover and rye treatments displayed strikingly similar patterns of nitrate accumulation in the roots only (RO), vs. Total biomass (TB) plots.

Effects of cover crop roots. There was little difference between the fine root length density of red clover and cereal rye either year, except at the 20-50 cm depth, which had more rye roots (Figure 8). Also, the fine root length density was greatest in the top 5 cm of the soil for both species, indicating that the very upper layers of the soil stratum may be even more important than previously thought in organic systems.

There was no significant broccoli yield differences in 2005 between subplots with roots- only incorporated and those with the total biomass incorporated (Table 4), indicating that, in a year such as 2005 with flooded soils early in the season that delayed planting, either long-term soil quality changes were of greater importance than short-term N availability, or that organic N had accumulated sufficiently over previous years to meet the needs of the current crop. The fact that the rye crop also showed no difference between TB and RO broccoli yields, and that neither cover crop system showed any broccoli yield differences between N and no N plots, further suggests that N was not the limiting factor in 2005 broccoli yield. These results are counter to the current recommendations that cover crops should be allowed to grow to maximum biomass before incorporation and opens up an array of new research questions about the role of roots in sustainable cropping systems.
Broccoli and winter squash yield
As a consequence of poor growth and over winter survival of hairy vetch, in two of three years, broccoli yield was considerably lower in the Alt.-Yr. CC compared to the 2-Yr. CC system in which broccoli followed two years of red clover (Table 5). In the final year of the study broccoli yield was equal in the 2-Yr. and Alt.-Yr. CC systems, and both were greater than the Fall CC and Control treatments. Weeds reduced broccoli yield an average of 12%, but yield loss was similar across the four cover cropping systems (data not shown).

Winter squash yield was greatest in the Control system in 2001, equal in all systems in 2002 through 2004, and greatest in the Alt.-Yr. CC system in 2005 (Figure 9). In first year, the yield advantage with synthetic fertilizer is not surprising given that squash plots were not preceded with cover crops. Although the Alt.-Yr. CC squash yield was top-ranked in two of five year, the lack of consistent ranking of squash yields in the cover crop systems suggests that factors other than our treatments have a greater affect on squash yield.
The weed seedbank
Over the course of the experiment there was a decline in the relative abundance of marsh yellow cress (Rorippa islandica), the dominant weed found in 2001, and a corresponding increase in common lambsquarters (Chenopodium album). Interestingly, this shift in the weed community was evident the Fall CC and 2-Yr. CC systems, but not in the Control and Alt.-Yr. CC systems (data not shown).

Despite an overall decline in the total weed seed bank in three of the four systems over the four years, common lambsquarters, a particularly pernicious species in most northeast cropping systems, increased in each of the systems (Figure 10). Compared to the Control, common lambsquarters increased only slightly in the disturbance-intensive Alt.-Yr CC system (an increase of 1,900 vs. 150 germinable seed m-2, respectively). Interestingly, and in contrast to our original hypothesis, the red clover-based 2-Yr. CC system apparently maintained weed seeds (Figure 10). Despite an abundance of invertebrate seed predators (see below), the fall tillage following winter squash likely incorporates a heavy weed seed rain. Two years of red clover, with no soil disturbance, maintains these weed seeds in “cold storage.”

Seed predators. The predominant invertebrate seed predator at this field site was a carabid beetle, Harpalus rufipes. Pitfall trap counts revealed greater density-activity of H. rufipes in vegetated plots, particularly red clover, compared to those recently tilled and planted to a fall cover crop of oat. To determine the rate of seed predation caused by resident invertebrates, we conducted a typical “feeding” trial in which 25 seeds of each of six weed species were placed in the field. Treatments excluded either no predators or used wire screen (1.2 by 1.2 cm openings) to exclude vertebrate predators. Invertebrates were responsible for 71% and 69% of the measured predation in 2002 and 2003, respectively, and while there was evidence of preference for yellow foxtail seeds in 2003, a high level of predation was observed for all six weed species (Figure 11). Based on these feeding assays, predation rates averaged 56% seed removal over 11 days in 2002 and 58% seed removal over 4 days in 2003.

Seed rain. Although the Alt.-Yr. CC system effectively maintained the seedbank over time, consistent weed seed rain in the late-harvested winter squash crop prevented what would have likely been a considerable decline in the seedbank. Common lambsquarters seed rain in the Alt.-Yr. CC system was consistently ranked the lowest of the four systems, but nevertheless reached an average of 30,000 to 40,000 seeds m-2 in two of the four years (Figure 13). Thus, a comprehensive strategy to manage the weed seedbank with cover crops and related disturbance must consider the likelihood of seed rain within the rotation crops. Because late-season vine crops like winter squash cannot be cultivated or hand weeded, the likelihood of weed seed rain is high. A rotation strategy should therefore take this into account, grouping such crops into a rotation block in which the long-term goal is not related to a declining seedbank and ultimately more efficient and cost effective weed control.

Fall cover crops also contributed to weed seed rain, particularly in the 2-Yr. CC system (Table 6). Although the red clover was routinely mowed, with the intention that this would prevent reproduction of annual weeds, in warm fall conditions certain species (e.g., common lambsquarters) will produce flowers and seed from lateral branches that are below mowing height. Compared to the perennial cover cropping strategy of the 2-Yr. CC system, the Alt.-Yr. CC cover crop consistently had the least weed biomass in the fall (Table 6).

A further problem with the perennial cover cropping system (2-Yr. CC) that has, in recent years, prompted a modification of this strategy at the New Leaf Farm, is the increase in perennial weeds in the clover years. Observation suggested that quackgrass was increasing in the 2-Yr. CC system. In the spring of 2005, soil samples were collected from each entry point of the 2-Yr. CC and Alt.-Yr. CC plots. The samples, totaling 500-cm-2, were sieved to recover quackgrass (Elytrigia repens) rhizomes. The 2-Yr. CC systems had an average of 1.6 g dry wt. of rhizomes 500 cm-2, compared to 0.01 g 500 cm-2 in the Alt.-Yr. CC system (P = 0.04).

Benefits of seedbank management to weed control in cash crops. Ultimately, a comprehensive strategy to manage the weed seedbank will have to contribute to improved weed control in cash crops. The theoretical basis for this project is that a reduction in the weed seedbank will result in lower pre-cultivation weed densities, thus a lower density of weeds surviving cultivation (e.g., Figure 1), less weed biomass in the cash crop, greater crop yield, less weed seed rain, and overall an improving weed problem.

Common lambsquarters was the predominant weed in both broccoli and winter squash crops. Prior to cultivation we measured common lambsquarters density in both broccoli (Figure 13) and winter squash (Figure 14). Although pre-cultivation densities were affected by cover cropping systems in four of five years in broccoli, but only in one of five years in squash, the density was not consistently least in a particular treatment. Noteworthy, however, is the apparent pre-cultivation threshold of about 100 seedlings m-2 in broccoli. With the exception of the Fall CC treatment in 2002 (Figure 13 B), each remaining instance in which the pre-cultivation density is below 100 seedlings m-2 results in a correspondingly low density of lambsquarters at harvest, and less than 10 g dry weed biomass m-2, which would not likely affect broccoli yield. In squash, even a low density of weeds may produce considerable biomass because of the long growing season, demonstrating the need for an exceptionally low starting seedbank to avoid seed rain (e.g., Figure 14 D, Alt.-Yr. CC treatment).

Participation Summary


Educational approach:

There has been considerable grower interest in the results of this unique cover cropping systems comparison in Maine and elsewhere as evidenced by invitations to speak on this and related experiments at regional grower meetings. We have made a considerable effort to make growers and educators aware of the results of the systems trial through volunteered presentations featuring or including results of the field experiment.

We estimate that key features of this project were presented to a combined audience of nearly 1000 individuals at the many presentations featuring or discussing this project since it was initiated in 2001. Conservatively, this included 250 growers who would be the primary beneficiaries of the project. Furthermore, the project has already gained some degree of national/international exposure through the peer-review journal publications. We consider this extensive outreach effort to be highly successful in raising awareness of cover cropping strategies for weed management and soil quality improvement.

Although the results and methodologies of the trial have been reported numerous times over the 5 years from 2001 to 2005, the outcomes of the trial became more valuable and easier to link to other activities in 2005, after 4 years of rotation were ended. The soil quality results of the trial were presented in September 2005 to a group of 20 agricultural educators, extension staff and NRCS staff who were undergoing intensive training in organic agriculture through the SARE Professional Development grant “Advance Training in Organic Crop Production.” Some of the results were used to develop an organic matter calculator which was presented to the group as a way to estimate the effects of various crop rotations on accumulation or loss of soil organic matter. There was a great deal of interest in the prototype calculator, and plans have been made to seek funding to expand and validate the calculator in the next few years. Our vegetable rotation trial was of itself of great interest to the group and has led to various communications about our results and methodologies from the training group since the training ended.
Presentations to Growers, Agricultural Educators, and Agricultural Scientists
Invited Presentations Featuring Project No. LNE01-141
1) Gallandt, E., M. Sarrantonio, T. Molloy, E. Sideman, and D. Colson. Two and four-year cover cropping strategies to manage the weed seed bank and soil quality. Maine Organic Farmers and Gardeners Association, Spring Conference. Unity, ME (February 25, 2006). This grower meeting, to be held in February 2006, will feature Eric and Anne Nordell speaking on their use of cover crops for weed control and maintaining soil quality. Results of our cover cropping systems trial will be presented in one of the three hours of this workshop.

2) Gallandt, E., E. Sideman, D. Colson and T. Molloy. Diversity and intensity of cover crop systems to manage the weed seed bank. Maine Organic Farmers and Gardeners Association, and the University of Maine Cooperative Extension annual Farmer-to-Farmer Conference. Bar Harbor, ME (November 6, 2004). This three-hour session featured a presentation of data from the cropping systems trial, discussion of the participating growers experiences with the cover cropping practices, and an hour of audience/participant discussion (45 attending).
Invited Presentations Including Project No. LNE01-141
1) Gallandt, E.R. Managing the weed seedbank. Northeast Organic Farming Association. Invited presentation. August 13, 2005 (40 attending).

2) Gallandt, E.R. Soil-improving practices for ecological weed management. Biology Department, Colby College, Waterville, ME. Invited presentation. November 19, 2004 (55 attending).

3) Gallandt, E.R. How can we target the weed seedbank? North Central Weed Science Society Annual Meeting. Invited presentation. December 15, 2004 (45 attending).

4) Gallandt, E.R. Integrated weed and soil management. United States Department of the Interior Fish and Wildlife Service National Integrated Pest Management Coordinators annual meeting. Ellsworth, ME (August 29, 2002; 12 in attendance).

5) Gallandt, E.R. Controlling Problem Weeds With Cover Crops. New England Vegetable and Berry Conference, Sturbridge MA. December 11, 2001 (ca. 300 growers in audience).
Volunteered Presentations Featuring Project No. LNE01-141
1) Gallandt, E. and T. Molloy (2004) Diversity and Intensity of Cover Cropping Systems: Effects on Weed Seedbank Dynamics. Northeast Weed Sci. Soc. Am. Abstr. 58:158.

2) Lynch, R. and E. Gallandt (2004) Effects of Cover Cropping Systems on Resident Weed Seed Predators. Northeast Weed Sci. Soc. Am. Abstr. 58: 155.

3) Sarrantonio, M. Soil Quality in a Vegetable Rotation with Varying Levels of Cover Crops. Poster Presentation. Annual meeting of the American Society of Agronomy, Seattle Washington. November 2004.

4) Diversity and Intensity of Cover Crop Systems to Manage the Weed Seed Bank. E. Gallandt and M. Sarrantonio. Northeast SARE Conference. Burlington, VT (October 19-21, 2004).

5) Cover Cropping Systems Effects on the Soil Quality. M. Sarrantonio. Rogers Farm Field Day. Stillwater, ME (July 7, 2004; 45 in attendance).

6) Cover Cropping Systems Effects on the Weed Seedbank. E. Gallandt. Rogers Farm Field Day. Stillwater, ME (July 7, 2004; 45 in attendance).

7) Cover Crops for Weed Management and Soil Quality Improvement in Vegetable Systems, M. Sarrantonio, E. Gallandt, and Tom Molloy. Rogers Farm Field Day. Stillwater, ME (July 10, 2002; 100 in attendance).

8) Diversity and Intensity of Cover Crop Systems: Managing the Weed Seed Bank and Soil Health. Maine Organic Farmers and Gardeners and University of Maine’s Farmer to Farmer Conference. Poster presentation. Bar Harbor, ME (November 2-4, 2001)

9) Cover Cropping Strategies to Manage Weeds and Improve Soil Quality, E. Gallandt, M. Sarrantonio, and T. Molloy. Rogers Farm Field Day presentation. Stillwater, ME (July 10, 2001).
Volunteered Presentations Including Project No. LNE01-141
1) Cover Crops, Eric Gallandt and Tom Molloy. Maine Organic Farmers and Gardeners and the University of Maine Cooperative Extension Small Farm Field Day. Unity, ME (August 11, 2002; 12 in attendance).

2) Effects of Cover Cropping Systems on Resident Weed Seed Predators, Eric Gallandt and Frank Drummond. Rogers Farm Field Day. Stillwater, ME (July 10, 2002; 100 in attendance).
Peer-Reviewed Publications
1) Gallandt, E.R. (in review). How can we target the weed seedbank? Weed Science. Submitted: May 31, 2005.

2) Gallandt, E.R., T. Molloy, R.P. Lynch, and F.A. Drummond. (2005). Effect of cover cropping systems on invertebrate seed predation. Weed Science 53:59-76.

3) Gallandt, E.R. (2004). Soil-improving practices for ecological weed management. in Weed Biology and Management, Inderjit, ed. Kluwer Academic Publishers, The Netherlands. pp. 267-284.

4) Sarrantonio, M. and E. Gallandt. (2003). The role of cover crops in North American cropping systems. Journal of Crop Production 8:53-73.

Three publications are in the preparation stage for publication from the soil quality portion of the field trial. The first will detail soil physical and chemical changes in the rotations over time and correlate changes to the timing, quantity and quality of cover crop residues incorporated. The second will specifically follow the N dynamics of the 4 rotations over 5 years, and the third will present cover crop root length density, root composition and the soil N and broccoli yield data presented in this report. It is anticipated that these three publications will be submitted to well-known scientific journals during the summer of 2006.

A manuscript documenting the weed dynamics over the five years of the project, including community dynamics not discussed in this Final Report, is in preparation and will be submitted for publication in Weed Research, an international journal of the European Weed Research Society.
Non-Peer-Reviewed Publications
We intend to draft the key elements of this final report into a photo- and graphic-rich bulletin for growers that that will be distributed as a .pdf file through the PI’s website and by MOFGA.
Field Demonstrations
Cover crop demonstration plots were established at MOFGA’s Common Ground Fair site in 2002 and 2003. Two cycles of cover crops were planted demonstrating early and late season cover crops. This cover cropping demonstration trial was used by project personnel for several talks given over the course of the weekend of the Common Ground Fair.

No milestones

Additional Project Outcomes

Project outcomes:

Impacts of Results/Outcomes

Cover cropping and crop rotation are widely listed by organic growers as, along with cultivation, their foremost weed control tools. This cropping systems study has demonstrated how cover cropping practices may contribute to improving soil quality, and benefit weed management efforts, but without accompanying strategies to avoid or manage weed seed rain, little progress will be made over time. Thus, a “cleaning strategy” of short-duration cover cropping will encourage depletion of the weed seedbank, but a long-season, weedy crop like winter squash may negate much of this progress in a subsequent year. Furthermore, while perennial cover crops are clearly a benefit to soil quality, weed seeds may persist, or despite mowing, may be replenished by seed rain. Perennial cover crops and potentially weedy, long-season crops should be considered within a rotation after the seedbank has been reduced. Likewise, fall cover cropping practices, while clearly a benefit for capturing unused nutrients and protection of the soil from erosion, are incongruous with fall weed seedbank management goals. The high level of potential weed seed predation demonstrated in this project supports this contention and the conclusion that fall weed management, i.e., “weed seed rain management,” should be a priority for future research.

The results of this 5-year study will go along way toward answering some of the most common questions regarding cover crops, such as “How much is enough?” We will be able to break down the reply into categories that address weed control, soil quality and N dynamics separately, giving the grower some options about changing rotations to fit their most pressing needs. The data from the root study will be of particular interest to growers who would like to harvest some aboveground biomass from cover crops and still reap benefits belowground.

The high level of invertebrate predation, and apparent persistence of the weed seedbank in perennial cover crops, indicates that fall weed management should have been given a greater priority in the design of these treatments. Mowing and no-till sowing of any late-season cover crops would maintain seed rain on the soil surface while offering a desirable habitat for seed predators.

Assessment of Project Approach and Areas of Further Study:

Areas needing additional study

The results of this study cover a five-year period, which is the time period often cited as “transition phase” for those wishing to convert to organic crop production. Ideally, some of these treatments should be followed on organic farms to see at what point improvements in soil quality and N dynamics begins to level off. It is possible that, once that period is reached, intensity of cover cropping can be lessened and still maintain high quality soil. It is also possible that the reason no differences were seen between cover crop roots-only and total biomass plots was that this point had already been reached in those system. Cover crop roots warrant a great deal more study, as indicated by the surprising results that we saw.

It became apparent that results that we saw were not entirely similar to those on the farms that we modeled, particularly in soil quality changes in the alternate years cover system. It would be of great interest to determine the role of soil texture and climate on these outcomes. Hairy vetch cover crops overwintered poorly in the Maine climate during this study. In response to this, a concerted screening/breeding program should be launched to find more cold tolerant varieties or species of winter annuals for New England.

Late-season weed seed rain and fall tillage—which incorporates weed seed into the soil thereby preventing predation, reducing natural mortality, and enforcing dormancy—ensure that an equal or greater intensity of weed control will be required the following year. In contrast, management practices that reduce weed seed rain or increase seed predation offer better weed control in a subsequent crop, and continually improve weed control. Thus, to facilitate “progress” regarding weed control on organic and low-input farms, priorities for future research should focus on fall weed management strategies that maximize predation, natural mortality, and maintenance of the seedbank in upper soil strata to facilitate germination losses.

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.