Multipurpose Brassica cover crops for sustaining Northeast farmers

2006 Annual Report for LNE03-192

Project Type: Research and Education
Funds awarded in 2003: $158,570.00
Projected End Date: 12/31/2007
Matching Federal Funds: $113,673.00
Region: Northeast
State: Maryland
Project Leader:
Dr. Ray Weil
University of Maryland

Multipurpose Brassica cover crops for sustaining Northeast farmers


This project is designed to conduct on-station and on-farm research investigating multiple benefits from cover crops, with a special focus on Brassica species, in order to accomplish two related goals. First, the research has the potential to demonstrate how multiple benefits can make these cover crops profitable enough to encourage wider adoption of this sustainable farming practice by farmers in the Northeast. Second, the process used to conduct this research will empower and encourage farmers to conduct their own research on their own farms, empowering them to generate objective answers to their own questions about cover crops and other farming practices.

The project aims to achieve these goals through four on-station field experiments that investigate a range of cover crop species, management practices and possible mechanisms behind some of the beneficial effects, combined with on-farm research conducted by about 15 farmers. The farmer-research will address simple, focused questions about profitably fitting cover crops into specific farming situations. Results from at least 10 on-farm and 4 on-station experiments will contribute to a regional database on Brassica cover crops. This database will evaluate the practical effectiveness of various Brassica cover crops, grown alone or in mixtures, in capturing residual nitrogen before it can leach away, in providing lower cost and more sustainable alternatives to replace deep tillage for compaction alleviation, to replace fumigation for nematode and fungal disease suppression, and to replace some herbicides and tillage for weed control. Extension educators, farmers and project personnel should be able to use these results to promote appropriate and profitable cover crop practices. Project personnel will work closely with farmers and county extension educators to develop farmer research interests and skills. Farmer participants will share their results and methodologies with other farmers, researchers and extension personnel via newsletters, conferences, presentations, field days, on-farm twilight tours, and discussion groups sponsored by Cooperative Extension and Future Harvest- Chesapeake Association for Sustainable Agriculture (FH-CASA). The project will work with farmers to use the experience and lessons learned to produce a user-friendly, visually-oriented guide to conducting on-farm research this guide will build upon several farmer research publications already in existence. The project aims to develop skills, experience and interests that lead to continuation of farmer-initiated research activity and research support groups even after the project-funding period.

Objectives/Performance Targets

1. The project aims to reach some 400 horticultural and grain crop farmers, interest 40 of these farmers in transitioning to the use of cover crops for two or more of the benefits demonstrated by the project research, and empower 10 to 15 of the farmers to conduct Brassica cover crop research trials on their farms.

2. Six of the farmers who participate in the design and implementation of trials to evaluate multiple benefits of Brassica cover crops on their farms will collaborate in the development of two farmer research guide booklets and participate in continuing farmer-to-farmer support of on-farm research.


This project requires that information on the Brassica cover crops be gathered and/or new information be generated by field research before we can provide practical information and recommendations to farmers. However, while we are gathering and generating new information, we are also reaching out to farmers with our preliminary results to generate interest in possible uses for the Brassica cover crops that are new to our region. We are also using the new (Brassica) cover crops as a vehicle to interest and empower farmers in to conduct their own research that provides reliable answers to their own questions about farming practices. In addition to the originally planned 4 experiments on four different experiment stations, we have conducted or have begun 7 additional field experiments on these stations and have worked to collaborate with 10 commercial farmers in conducting 12 replicated on-farm experiments, some running for multiple years.

Each on-station experiment is designed to investigate several of the proposed Brassica cover crop benefits, as well as to develop or validate information on the practical management of these cover crops (such as planting dates and seeding rates). Most of the on-farm experiments are aimed at evaluating one specific cover crop function that addresses a problem identified by the farmer, or to test the adaptability and productivity of one or more of the brassica cover crops to the farmer’s situation. The main functions under investigation are:

 alleviation of subsoil compaction
 capture excess N in fall to prevent nutrient pollution
 enhanced nitrogen cycling to crops
 suppression of plant parasitic nematodes
 reduction of weed pressure
 improvement of soil organic matter and structure

Because each experiment is focused on a different set of cover crop functions, not all of the Brassica cover crops we are working with are represented in each study. However, in August 2004, we established at each on station site a second experiment that does allow us to compare forage radish, rapeseed, rye and no cover (weeds only) at all four sites. These basic experiments were repeated in fall 2005 at two of the sites (one with loamy sand, one silt loam soils). Three new experiments were established in fall 2005 at Beltsville, MD, two focusing on weed suppression and one on compaction alleviation. In fall 2006, the weed suppression experiments were modified and additional weed suppression experiments initiated. In addition, we initiated a new compaction-alleviation experiment in which treatments of 3 known compactive force levels were imposed and planted to 4 cover crop treatments. Finally, in fall 2006, we also began investigating the effects of the Brassica cover crops on the potential for mycorrhizal symbiosis.
We have now collected four years (effectively, up to 22 site-years) of data on the fall biomass production of five different Brassica cover crops (some more site years than others), plus several combinations of Brassicas with non-Brassica cover crops. For the cover crops with fleshy taproots, we measured the biomass of both the shoot and the fleshy taproot (which accounts for most of the root system dry matter, although very little of the root length or surface area). Data collection is complicated by the fact that some of the studied cover crops are killed by freezing in winter (oilseed radish, forage radish, mustard, oats), while others are not killed (mainly rapeseed and rye), and therefore continue to grow in spring before summer cash crops are planted. We have analyzed the shoot and root biomass and N uptake data for fall 2004 and spring 2005, as well as the soil N forms down to 150 cm deep in fall 2003 and 2004 and spring 2004 and 2005. We have completed the majority of the planned assessments of nematode suppression at two sites and analyzed that data. Data on the degree, timing and mechanisms of weed suppression by forage radish as compared to rye was collected at three sites in 2006. In 2006, we intensively monitored surface and subsurface soil water content changes during the summer using moisture sensors and data-loggers. We collected root images to 60 cm deep at two sites and made some preliminary analyses of these images to asses rooting depth and crop root ability to penetrate compacted zone after various cover crop treatments.

The data show some dramatic and highly significant effect by Brassica cover crops on nematode populations, though little of the hypothesized parasitic nematode suppression. In addition to enumerating the “bad” nematodes, we also characterized the community of “good” nematodes. The brassica cover crops, especially the radishes had much more dramatic effects on the free living nematode community than on the plant parasites. The free living communities, especially the bacteriovorus nematodes, were markedly increased at the Hayden farm (CMREC) site through September 2005 in response to a good radish cover crop stand that winter killed in December 2003–almost two years.

Although we had some excellent stands of the cover crops and some bin-busting yields of soybeans following them, the excellent rainfall distribution in the 2004 growing season meant that the influence of the cover crops on subsoil water in summer and soybean yield was statistically significant at only the sites with sandy soils. At the no-till Hayden Farm site, there was a 7 bushel/acre yield advantage (about a 10% increase) for soybeans growing after oilseed radish or forage radish as compared to soybeans following mustard, rape or no cover. At the conventionally tilled LESREC site, soybeans growing after any of the cover crops tried yielded significantly more than those after no-cover crop. At all sites, forage radish (‘Daikon’) grew the fastest in fall and was very competitive against weeds. Even after the forage radish stand was winter killed, strong suppression of spring weeds was event.

Steve Groff, one of our farmer collaborators, has conducted trials on his farm since we began working with the brassicas and liked the forage radish so much that in 2004 he decided to plant 10 acres of forage radish for seed production in spring 2005. His seed production experiment was a great success, producing about 12,000 pounds of seed, every bit of which was soon sold out at approximately $2-$3/pound. In 2005 Steve sold his seed to as many as 40 farmers (some seed was purchased by extension agents and soil conservationists who then passed it on to their farmers who requested it). In 2006 Steve grew 30 acres of Forage radish for seed and had another successful year, selling about 20,000 pounds of seed while holding about 10,000 for pre-seed harvest sales in summer 2007. His web page about forage radish is at

In January 2006, we participated in the Farming for Profit and Stewardship Conference in Hagerstown Maryland by conducting a pre-conference workshop on farmer research. Extension educators Ronald J Hoover (of Pennsylvania State University), Caragh Fitzgerald, and David L. Almquist helped run the workshop and graduate students Amy Kremen, Lisa Stocking and Yvonne Lawley served as additional resource people. Three farmers (Dick Bitner, Steve Groff and Drew Norman) with experience doing research on their own farms discussed their perspectives and lead three breakout groups focusing on 1) vegetable production, 2) agronomic crops, and 3) animal production. The breakout groups served essentially as three concurrent farmer research circles (addressing performance target 2).

Impacts and Contributions/Outcomes

Outreach and Farmer Interest.

Our project has already stimulated interest in Brassica cover crops and farmer research among a wide diversity of farmers and extensionists. In March 2004, the largest farm newspaper in our area, The Delmarva Farmer, ran a front page story devoted mainly to our investigation the potential benefits of Brassica cover crops. The on-line sustainable farming magazine, New Farm, ran two features in 2005 as a result of our Brassica project collaboration with Pennsylvania farmer Steve Groff. In the first two and a half years of the project, we have reached about 675 people, including 475 farmers, in face to face events. We probably reached twice that many farmers through print and internet media, such as stories in the Lancaster Farmer, New Farms, Delmarva Farmer, NRCS Soil Quality Team Newsletter, etc. Our list of farmer contacts has grown to over 140. Of these, some 83 of whom have received varying amounts of brassica cover crop seed from us (in addition to the 100+ who bought seed from Steve Groff), and 74 contacted us with requests for more information on the cover crops or on research methods.

Approximately 35 farmers that we know of in Maryland, Pennsylvania, Delaware, Virginia and West Virginia have begun to use brassica cover crops, in addition to those who purchased seed from Groff. Mr. Groff informed us that he already has orders for 3,000 pounds of seed and interest is so high he plans to double his seed production to 20 acres in 2006.

Through our collaboration with Groff and local extension and conservation officials, our cover crop message reached dozens of farmers in SE Pennsylvania. Many of these are dairy farmers who are experimenting with planting forage radish after early corn silage harvest to control nitrogen leaching, protect the soil, provide fall forage and improve next years corn yields.

In 2006, we worked with a grain farmer and an extension educator in the Eastern shore to help clarify the regulations in MACS –the Maryland cover crop support program aimed at reducing nitrogen loading to the Chesapeake Bay. As a result, the State of Maryland has broaded the Brassicas category in its MACS so that forage radish, which has become the greatest focus in our project, now qualifies for payments under the program. The September 15th planting cut off for Brassica in that program is in partly based on our experience in field experiments. In late 2006, conservation officials from The State of Virginia contacted us for input and data to help them decide about inclusion of Brassica cover crops in the incentive program they are planning in that state.

The input provided to the research process varies among the farmers who are collaborating with our project, but in most cases the trial aims to address a question posed by the farmer (e.g. can the Brassicas help me with my soil compaction problem? nematode infestation? weed pressure? etc). While it is too early in the project to report on the farmer-adoption and research outcomes, we can report that in year 1, five farmers tried Brassica cover crops in collaboration with our project, and three of these suggested the basic objectives of the experiments on their farms. In year 2, we worked closely with seven farmers, all of whom originated the basic objectives of their experiments. At least 13 (but probably twice that number) additional farmers are experimenting on their own with Brassica as a result of seeds we provided. Most of the later are not conducting replicated trials. In the fourth fall of the project (fall 2006), we again worked with several farmers (some new, some from the previous group) on cover crop trials. However, the January 2006 farmer research workshop did not by itself initiate self sustaining farmer research support groups as initially anticipated. The main reason is not lack of interest, but lack of time on the part of both farmers and project personnel. Setbacks in this regard included the departure for another job of extension educator Caragh Fitzgerald, who was playing a key role in developing the farmers’ research groups.

Nitrogen capture and release.

In 2005 we completed the main soil field work of the project in the area of nitrogen capture and release by Brassicas. However, we continue to expand our data base on plant N uptake by these covers. In 2006 we completed analysis of the data from the deep soil cores and N mineralization samples. Although our research on this topic is continuing, our results indicate that the Brassicas are capable of rapidly taking up large amounts residual soil N in the fall if planted earlier than mid September. With tissue N concentrations ranging from 2.0 to 3.5% and dry matter ranging up to 6,000 kg/ha by late fall, the fall N uptake potential is even larger than rye, which is the standard N capture cover crop in our region. Using soil cores taken to 150 or 180 cm deep, we have seen that the Brassicas rapidly depleted the soil profile of soluble N. An important objective of our project is to evaluate the N leaching risk presented by winter-killed, decomposing forage radish in late winter and early spring months.

The potential of Brassica species as alternatives to winter rye cover crops for reducing N leaching from cropland in the Mid-Atlantic Region can only be assessed with appropriate N leaching studies. In our project the N capture potential was studied, including plant biomass N uptake, soil profile (upper 105 to 150 cm) mineral N (NH4+NO3), and soil porewater NO3-N and total soluble nitrogen (TN) (at 90 or 120 cm) of forage radish, oilseed radish rapeseed, rye and winter weeds (control) at two Maryland Atlantic coastal plain locations, UM CMREC and UM WREC, from August 2003-May 2005. Analysis of the samples was nearly complete by December 2005, so we can report the main results here. At UM CMREC (Galestown-Evesboro loamy sand), plant N uptake by late fall 2004 ranged from 151-214 kg N/ha in the Brassicas (root and shoot) and 65.6 kg N/ha for rye shoots. Total NO3-N in the upper 150 cm of soil (kg N /ha) averaged 492 under control and 138-248 under the Brassicas and rye with significant mineral N reductions occurring as deep as 105 cm. The soil porewater N (sampled with suction lysimeters at 120 cm) under rape and rye averaged over 10 sampling dates in February-April 2005 was 0.28 and 0.13 mg NO3-N/L whereas the control and forage radish had significantly greater concentrations at 4.4 and 6.8 respectively.

However the temporal pattern for these two treatments was nearly opposite. Nitrate in control plot porewater was highest on the first sample date in February and declined thereafter, indicating that our samples caught the tail-end of N leaching from the control. In the radish plots, the porewater was as low as under rye or rape for the first several sample dates, but began increasing in late March and reached high levels only in mid April, suggesting that N was conserved during the winter months, but release in early spring. This pattern was corroborated by the N mineralization study in the upper 15 cm of soil, and suggested that radish would make N available early in spring. This can be viewed as an advantage over rye (which is known for immobilizing N in spring) if the cove crop is followed by an early-planted crop such as corn or early vegetables. If planting is delayed until May on loam sand soils, significant N may be lost by leaching before the spring crop can capture it. On a finer textured soil at UM WREC (Matapeake silt loam), the average shoot N uptake from November 2003 and January 2005 ranged from 121-160 kg N/ha for the Brassicas and rye. Nitrate-N (0-105 cm) was 260 kg N/ha under control and 76-96 under forage radish, rape, and rye–a significant decrease. In March-April 2005, much more NO3-N was measured in the porewater collected under the control plots than under all the brassica and rye cover crops. Porewater NO3-N under control averaged 4.3 mg/L over the sampling period while the cover crops averaged 0.2-0.7 mg/L.

If established by early September, brassicas were at least as effective as rye in capturing excess soil N. No differences in NO3-N due to treatment were observed at any site in spring 2004. Ammonium-N was also unaffected by treatment at both sites regardless of season. Between mid-March and mid-April 2004, soil porewater NO3-N was lower under the cover crops than the control. The data showed that NO3-N and TN in porewater varied with time as a function of decomposition and precipitation.

Summary of Nitrogen Capture Potential of Forage Radish

Our experience and data suggest that the Brassicas offer new tools to capture N in certain situations. We are not suggesting that Brassicas like forage radish can replace rye as the principle cover crop in grain rotations. Forage radish needs to be planted earlier than rye to be effective. Aerial seeding into maturing corn soybeans in early September can be fairly reliable, but not as effective as drilling into an open field. The most practical situations for establishing radish are in vegetable rotations and after corn silage harvest. Even where they can be conveniently established, we would not suggest using the brassicas continuously year after year…rather we suggest rotating cover crops.
Like small grains, radish will not grow vigorously if surface soil nitrogen is very low. In low N soil, deep rooting suffers more than shoot growth. On sandy soils there is often a lot of N left deep in the profile while most has washed out of the surface horizon by fall. We have not applied any N to our fall radish plots on commercial farms and have had excellent biomass and N uptake on all but the sandiest soils. However, a small nitrogen application where needed to ensure vigorous cover growth is very likely to result in much improved nitrogen scavenging from the profile and therefore less N loss by leaching. In our experiment station research plots with low residual N we have fertilized the radish to assure good growth, usually putting down 20 to 30 kg N/ha as fertilizer or as legume residues and getting back 150 kg in the brassica cover crop shoot growth – not a bad trade.

Because forage radish usually winter-kills in zone 7, modest amounts of N from its decomposing tissues can be observed in surface soils as early as February or March when this cover crop frost killed in December. Rates of N mineralization speed up considerably in the period from March-June. Significantly more N as nitrate from the forage radish cover crop is available for uptake by following main crops from early May to mid-June than from rye (killed in April at early-boot stage). Heavy rain preceded soil sampling in early April (~day 100). The data illustrate the greater risk of nitrate leaching from early mineralized N in very sandy soils compared to finer textured soils.

The brassica cover crops cleaned up the nitrate in the soil profiles in fall as well or better than rye, but the winter killed radishes began to release mineral N from root and shoot decomposition in early spring. This early N release is of agronomic advantage compared to the N immobilization often caused by rye in spring. However, to avoid N leaching in spring, we would recommend two methods for retaining and/or reusing the N conserved by the forage radish cover crop: (1) fall-planting of forage radish as a mixture with an over-wintering cereal cover crop (rye or oats), or (2) following the forage radish cover crop with the planting of a nitrogen-demanding cash crop in April.

There may be environmental value in using radish or rapeseed to clean up the nitrate deep down in the soil profile in fall. Rye takes up very little N in the fall when planted at typical early October planting dates. Even when planted in late August/early September as in our studies, rye does not take up N as fast or from as deep in the profile as radish. In terms of environmental protection, a kg of N taken up from 2 m deep is worth much more than a kg taken up from 0.6 m deep because the deep N had much less chance of being used next season and much greater risk of leaching.

The available data certainly appears to warrant including brassicas – both radish and rapeseed –in cover crop programs for capturing nitrogen so long as the brassicas are managed properly and established early (by mid-September in Maryland).

Soil Compaction Alleviation

Alleviation of subsoil compaction was one of the first research goals when our group began studying the brassica cover crops. It is also one of the most difficult effects to measure because the normal approach to evaluate the effect of tillage or traffic on soil compaction does not apply when the method of alleviating the compaction is the rooting action of cover crops. We hypothesize that the cover crop roots would penetrate compacted subsoils in fall and winter when the soil was wet and relatively soft, and that these cover crop roots would leave channels that the summer crop roots could follow to traverse the compacted zone when it was dry and very hard. This process has been termed “bio-drilling.” However, the root channels are likely to be small enough (~ 1mm) that there presence would not effect penetrometer measurements. Nor would traditional bulk density measurements be affected since the opening of large pore spaces (channels) by root action would of necessity further compact the soil adjacent to the channels. Therefore we decided to measure the effects in terms of crop rooting patterns. However, the measurement of plant roots in the field is notoriously difficult and intrusive. Therefore we arranged to use (and later purchased on non-project funds) a state of the art minirhizotron fiber-optic camera which is capable of imaging roots in situ repeatedly over time. We were successful in obtaining images of soybean root following the channels made by brassica cove crops and these were published in 2004.
Obtaining quantitative root data with the minirhizotron camera has proved to be quite challenging. Therefore, we also approached the problem by using Bohm core-break method to measure cash crop root distribution. With this method we were able to show that corn in mid summer 2005 grew about twice as many roots below the compacted plowpan of a Galestown loamy sand where a rye cover crop was used as compared to where no winter cover had been grown. Moreover, nearly 10 times as many summer crop roots were able to penetrate the plow pan where the forage radish had been grown fall 2004 and 2005, as compared to the no cover plots. Improved deep rooting will give crops more access to subsoil moisture which the plant will need to use during dry, high transpiration periods during the summer. Therefore we also studied the “bio-drilling” of fall cover crops rooting by monitoring soil water above and below the compacted plow pan.

In 2006, we installed several multichannel data loggers and were able to obtain hourly readings. This capability, combined with some extended dry periods, allowed us to obtain data that clearly showed that the subsoil moisture was used much more rapidly in by corn following forage radish than by corn followng no-cover or rye. The rye cover, however, provided much more surface mulch than the radish and so conserved more water in the surface soil above the plow pan. These results suggest that a mixed cover crop that combined both rye and forage radish might confirm the benefits of both to result in the highest crop yields and least soil erosion.

In 2006 we also initiated a compaction experiment in which we applied compaction treatments by driving a heavy solid tired loader over a loam soil in a wet condition, making either no passes (uncompacted), a single pass (compaction level 1) or two passes (compaction level 2). This resulted in severe surface soil compaction down to about 20 cm depth and simulated the kind of compaction that is often caused by harvest and manure spreading machinery. After the compaction treatments were applied, the whole field was disked to about 8 cm and rye, rapeseed and forage radish cover crops were drilled. All three cover crops (and the weed in the control plots) responded dramatically to the severe compaction levels. Root and shoot biomass data were obtained and are being analyzed. Large 10 cm cores were taken to 60 cm depth using hydraulic equipment. These cores were utilized for core-break root counts. Preliminary data suggests that while the roots of rye and rapeseed were severely restricted by the compaction, forage radish had as many roots penetrate to 50 cm in the compacted as in the uncompacted soils. Corn will be planted on these plots in early spring 2007.

Nematode Suppression

Brassica cover crops contain compounds reputed to suppress plant-parasitic nematodes when incorporated into soil. One of our original hypotheses was that the brassica cover crops would provide useful suppression of plant parasitic nematodes, as has been reported in other types of environnment and agro-ecosystems elsewhere. We were especially interested in soybean cyst nematode (SCN) suppression as that is a common nematode pest on Maryland’s Eastern Shore. We conducted experiments at the Lower EASTERN Shore Education and Research Center (LESREC) for three years, using susceptible soybeans on a SCN infested soil to compare the effects of rye, two cultivars of rape, a mustard mix and two types of radish, grown either alone or in combination with rye or clover. In those studies, the cover crops were mowed to macerate the tissue as suggested in the literature and then disked in prior to spring planting. However, we did not find any significant supression by the brassica cover crops. We did find that mustard seemed to actually host certain parasitic nematodes; for example mustard increased population of stubby root nematode. There were no effects of specific Brassica cove crops on soybean yields, but the no-cover system always yielded less than the systems with some cover crop over the winter, the effect appearing to be related to factors other than nematode damage. Several farmers tried brassicas for nematode control on strawberries, on cucumbers or soybeans, but the brassica covers did not produce noticeable effects. In some case the lack of effects may have been to poor stands and growth of the cover crops.

Little is known about brassica cover crop impacts on nematodes in no-till systems or on free-living nematodes. On of our original hypotheses was that the Brassica would suppress the parasites, but without inhibiting the beneficial nematodes. Since free-living nematodes are mainly beneficial to nutrient cycling and plant growth in the soil system, we wanted to also study effects on these non-plant parasitic groups of nematodes. Effects of winter-killed oilseed radish versus an unweeded, no cover control an Experiment at eh CMREC/Beltsville from 2003-2005. Summer crops, soybean and corn were planted no-till or after tillage. As in the LESREC experiment, no differences among treatments were found for plant-parasitic nematodes (mean of 1,106 kg-1 dry soil). However, bacterial-feeding nematodes populations – especially of Rhabditidae dauer larvae– were much higher in oilseed radish ‘Adagio’ (1,760 nematodes kg-1 dry soil) compared to the weedy control (256 nematodes kg-1 dry soil) in September 2004, 10 months after the fall 2003 cover crop had winterkilled.

In another experiment established in fall 2004 at the same site, only no-till management with cover crop treatments of rapeseed ‘Dwarf Essex’, forage radish ‘Daikon’, rye ‘Wheeler’, oilseed radish ‘Adagio’ and an unweeded, no cover control. Economically important plant-parasitic nematode populations did not differ among treatments (mean of 1910 kg-1 dry soil); however, averaged across dates, bacterial-feeding nematodes, especially Rhabditidae dauer larvae, again increased dramatically (from 1,901 kg-1 dry soil in the control to 5,265 and 4,720 kg-1 dry soil in forage and oilseed radish, respectively). Although no effects of Brassica cover crops on economic plant-parasitic nematodes were observed, results demonstrate that cover crops impact nematode communities throughout the year. The radishes appeared to favor a bacterial decomposition pathway, while rapeseed and rye appeared to favor a fungal decomposition pathway, as indicated by the Enrichment and Channel indices.

The implications of these effects are not yet understood. One possibility os that the bacteriovorus nematodes could enhance N cycling. We are therefore attempting to integrate these nematode effects with our data on nitrogen mineralization on the same plots. Use of cover crops, such as the radishes, to increase dormant populations of bacterial feeding nematodes may have practical benefits for farmers, but more knowledge of their ecology is needed.

Nematode Community Response to Winter Cover Crops

Cover crops grown in fall or winter, during normally fallow periods, have potential to create beneficial effects on nematode communities which endure into the cash crop season. We evaluated the effects of cover crops (forage radish (Raphanus sativus) ‘Daikon’, oilseed radish (Raphanus sativus) ‘Adagio/Colonel’, rapeseed (Brassica napus) Essex’, mustard blend (Sinapus alba and B. juncea) ‘Caliente’, rye (Secale cereale) ‘Wheeler’ on the total nematode community at two sites in Maryland (LESREC, CMREC). Samples were taken two or three times per year and nematodes were identified to genera or family. The enrichment index (EI), channel index (CI), structure index (SI) and bacterivore and fungivore maturity indices (BaMI, FuMI) were calculated to identify nematode response to nutrient enrichment, stress, or stability.

The EI is calculated by weighting the abundances of bacterivores and fungivores that are low (1-2) on the colonizer-persister scale. Bacterivores classified as cp-1 respond rapidly to nutrient enrichment related to reproductive strategies and fast metabolic rates. Bacterivores and fungivores classified as cp-2 comprise the basal component of the food web. They also respond to food resources, but are more adaptable under nutrient depleted conditions and therefore more persistent and ubiquitous. The EI weights cp-1 bacterivores highest and evaluates their abundance with cp-2 fungivores (numerator) in proportion to weighted cp-2 bacterivores and fungivores (denominator).

The Structure Index (SI) is a proportion of all free-living cp 3-5 (weighted 1.8, 3.2, and 5.0 respectively) nematodes to the sum of those nematodes with weighted cp-2 fungivores and bacterivores. High SI values, similar to the MI, suggest community complexity (more food chain linkages) and stability (based on attributes associated with high cp-ranked nematodes).
Maturity indices weight the proportion of cp-2, cp-3, and cp-4 bacterivores or fungivores (out of total bacterivore or fungivores community) by their respective cp value. High MI values indicate either relatively higher ranked cp nematodes or relatively fewer low ranked cp nematodes. Both circumstances suggest a more successional mature nematode community.
At LESREC the experimental treatments included three cover crops that winter killed: the mustard blend, and the two radishes. We compared the nematode communities in plots with winter killed cover crops to those in plots with spring-kill cover crop (rye and the two rapeseed cultivars). On all spring and summer dates on which a significant difference was observed, the values for the bacteriovore and fungivore maturity indices, as well as the structure index were higher for the plots with winter killed cover crops. Higher BaMI, FuMI, and SI means for winter-killed cover crops than spring-killed cover crops, suggests earlier timing of cover crop additions stimulates greater community succession during the cash crop season. In agricultural ecosystems, lower maturity indices suggest greater fertility, since abundant lower ranked cp-2 nematodes signify decomposition activity. High SI values may be favorable in agriculture however, since it is known that higher trophic groups regulate lower groups through predation, stimulating more rapid organic matter turnover.
Evidence of nutrient enrichment was primarily observed via large abundances of the dormant (dauer) bacterivore Rhabditidae nematodes after cover crop termination. Dauer larvae are known to form when food resources are depleted and bacterivores overpopulated, often an effect of rapid nutrient enrichment. Residues from N rich winter-killed radishes resulted in high dauer larvae populations in all three experiments, with these effects diminishing by late summer. The EI (excluding dauer larvae) was highest in radishes and rye at LESREC in June, and 23% higher in cover crops (radishes and rapeseed ‘Essex’) than the weedy control at CMREC across time (Nov/Jun/Aug). High CI values in rapeseed ‘Essex’ and rye in all three experiments suggest greater fungal food web activity. High populations of facultative root-hair/fungal feeding Coslenchus sp. in rapeseed and rye may also suggest more fungal activity as a result of these treatments; however, further research on Coslenchus feeding habits is needed.

Future studies may improve on the ecological signification of these indices in agricultural systems by approximating nematode biomass rather than abundance. There were no differences found in total predator or omnivore abundance. Future studies may also investigate the potential benefit of combining cover crops which are winter susceptible and winter hardy, thus priming the decomposition food web in early spring and late spring. This study showed that the timing and resource quality of cover crop biomass has a significant impact on the nematode community, which may last from 4-8 months after cover crop termination. To better understand the ecological and agronomic implication of these results the life characteristics and ecology of Coslenchus and dauer larvae should be studied further.

Weed suppression.

As alternative weed management strategies become needed to reduce the environmental impacts and expense of herbicides, weed suppressive cover crops may play an important role. Forage radish (Raphanus sativus) is a member of the Brassica family and is being researched as a new winter cover crop in Maryland. When planted in late August, forage radish rapidly produces biomass in the form of leaves and a swollen tap root, a portion of which can grow above ground. In the mid-Atlantic region, forage radish cover crops are usually killed during the first extended period of temperatures below – 3oC, usually in late December. The residues then rapidly decompose during the winter months. At planting time the following spring, a thin film of leaf residue coats the soil and the tap roots have shriveled up with large holes left behind.
Glucosinolates are secondary metabolites characteristic of plants in the Brassica family. Breakdown products of glucosinolates formed during decomposition of Brassica cover crop residues are believed to exhibit bio toxic effects. Greater understanding of the weed suppressive mechanisms of Brassica cover crops and interactions with the soil environmental are needed to develop reliable weed management practices for Maryland cropping systems.
Our preliminary observations indicate that forage radish provides weed control in two ways. 1) It rapidly produces a dense canopy in the fall which suppresses weeds during the growth of the cover crop. 2) During the decomposition of forage radish residue over the winter months, very few weeds are able to germinate when compared to no cover control plots. In response to these observations, and to comments by farmers who also observed the nearly weed-free condition following the radish cover crop, we have initiated research that will focus on quantifying the weed suppression of forage radish cover crops and determining the mechanism of its suppression. Two independent hypotheses, resource competition and allelopathy, must be isolated and considered when testing mechanistic hypothesis for weed suppression of forage radish. In 2006 we initiated several lab and field experiments designed to:

1. Quantify the duration of weed suppression following forage radish cover crops into the next cropping season.

2. Evaluate the ability of forage radish cover crops to suppress horseweed, lambsquarters, redroot pigweed, and green foxtail both during and after the cover crop growing season.

3. Quantify the effect of increasing forage radish cover crop residues on weed suppression.

4. Determine the allelopathic effects of forage radish cover crop tissue, residue, and amended soil on seed germination and growth.

Summary of Findings on Weed Suppression by Forage Radish.

Forage radish winter cover crops suppressed weeds from the time they were planted in late August until April. In April, henbit and chickweed emerge in the forage radish treatments, while horseweed emerged in no cover treatments. Horseweed was suppressed following forage radish cover crops for the duration of the experiment but the mechanism to explain why this suppression occurred is still unknown. Future experiments should also include test crops that are planted in April to determine if suppression of horseweed following forage radish cover crops remains effective after the soil is disturbed during seeding.

Although horseweed was suppressed by forage radish cover crops, no suppression of lambsquarters, pigweed, or green foxtail was observed at this site in 2006. Experiments should be conducted at other sites to confirm the results of this experiment, especially in fields where horseweed is known to be a problem.
Experiments that tested the effect of forage radish cover crops on the natural weed seed bank in the field were more successful than those using planted weed seeds or lab experiments. Future research sites should be selected that are known to have a history of weeds that are of interest rather than introducing them.

Experiments designed to understand the mechanism of forage radish weed suppression were not conclusive. Adding or removing forage radish residues had no effect on weed suppression in the small plot experiment. An experiment that uses larger plots should be used to confirm the findings of this small plot experiment. Forage radish tissue, residue, and soil extracts had no effect on lettuce seed germination and had little effect on lettuce seed root and shoot growth. Germinating seeds in soil collected from forage radish plots may be a more biologically meaningful way to conduct bioassays to test for effects on seed germination and growth. Understanding the mechanism of forage radish weed suppression will be critical to predicting how effective forage radish will be in suppressing horseweed in other locations and under different environmental and management conditions.

Farmers in Maryland wanting to take advantage of forage radish cover crops to suppress weeds should plan their crop rotations so that early seeded crops and crop varieties that can be planted in April following forage radish cover crops. This study may be the first to indicate that forage radish could be used to manage horseweed without herbicides. This is encouraging given the difficulty of killing horseweed using herbicides and the increasing occurrence of herbicide resistant horseweed in Maryland.

Cover Crop Planting Methods

The optimum planting method for our region appears to be drilling seed in mid to late August. This practice is most practical for diversified vegetable growers and for dairy farmers with early harvested corn silage. Two dairy farmers that we are working with (and many that we are not) have tried forage radish planted immediately after corn silage harvest. These farmers needed to apply manure to the corn field right after silage harvest and wanted a cover crop that would: 1) capture the N released in fall, 2) provide potential late fall forage, 3) improve soil quality (specifically with regard to compaction) for the following years corn crop. One farmer I northern Maryland, Dan Magness, planted a 2.5 acre field to side by side strips (32 corn rows wide) of forage radish, barley and no-cover crop. He told us that the radishes grew so fast and produced so much biomass in 2005 that he was really wondering if he would have problems planting into them the next spring… his concerns increased when they survived the first couple frosts. However, his concerns transformed into satisfaction in spring of 2006, when the radish strip had essentially no winter annual weeds and was very easy to no-till plant…without using any burn down herbicide application. He said the corn planted into radish residue established more rapidly and then tasseled ~ 1 week earlier and more uniformly than the corn planted following no cover and burned down barley. With the help of a dealer with a weigh wagon, he measured corn grain yields of 180, 193 and 221 bu/acre in the no cover crop barley, and radish strips, respectively. He was so impressed that in fall 2006 he planted 30 acres of forage radish after corn silage.
Most grain farmers in the mid-Atlantic region do not plant cover crops until several weeks later (mid-October following corn harvest, and mid-November following soybean harvest).

Grain farmers, who typically account for the largest acreages, have expressed interest in finding alternative approaches to planting the brassica cover crops. They have suggested either flying the seed on into standing corn or soybean crops before mid September, or planting the covers early in spring, instead of in fall.

In fall 2004, two large-scale farmers collaborated with and hired an airplane to fly on replicated strips of forage radish seed into standing corn crops in mid August. Both of these farmers were interested in the soil compaction effects of the forage radish and both used the same airplane applicator flight to seed three widely spaced strips 300 feet long in their respective fields of corn. In fall 2004 and fall 2005, we also implemented “aerial seeding” in several of our on-station experiments by spinning the seed into soybeans at leaf yellowing or corn at beginning of dry down. In some cases, the aerial seeding (real or simulated) resulted in very good stands that eventually produced 50 to 80% as much dry matter as the August drilled covers. In other cases, especially when seeding was done too far ahead of crop senescence, the Brassica understory seedlings suffered from lack of light, resulting in thin stands, poor initial growth and a low root to shoot ratio that might compromise the cover crops’ ability to improve soil quality.

In spring 2005, we worked with two farmers (one in Maryland and one in Pennsylvania ) who wanted to try planting one or more of the Brassicas at spring “green up” time (late March) and kill them just before planting soybeans around June 1. Also, because of expressed interest in the feasibility of spring planting the brassica, we conducted a replicated experiment at the Central Maryland Research and Education Center, Beltsville in spring 2005 and 2006. These experiments showed us that the brassicas go to flower early and do not produce large roots when planted in early spring. We concluded that planting a brassica cover in late March to be followed by a late May planted soybean crop is not an effective option.

In summary, although there is need for much more research on the influences of the Brassica cover crops on the soil ecosystem and on their practical management, we believe we are stimulating significant interest in the Brassicas among regional farmers, are beginning to make some progress in encouraging farmers to do research on their farms, but have not made much head way in empowering or convincing farmers to conduct their own research at an appropriate level of complexity and replication.


Weil, R.R., G. Chen, J. Dean, A. Kremen, L. Stocking, Y. Lawley, B. Momen, S. Sardanelli, I. Zasada, J. Teasdale, and S. Williams. 2006. Integrating multiple soil quality impacts from Brassica cover crops. International Union of Soil Science, World Congress of Soil Science, Philadelphia, PA, July 09-14, 2006

Kremen, A., and R.R. Weil. 2006. Monitoring nitrogen uptake and mineralization by Brassica cover crops in Maryland. International Union of Soil Science, Philadelphia, PA

Dean, J.E. 2006. Brassica cover crops for nitrogen retention in the Maryland coastal plain. M.S., University of Maryland, College Park, Md.


Steve Groff
Cedar Meadow Farm
679 Hilldale Road
Holtwood, PA 17532
Office Phone: 7172845152
John Teasdale
Weed Scientist and Research Leader
USDA/ARS, Sustainable Agric. Systems Lab
Wallace Agric. Research Center
Bldg. 001 Room 245
Beltsville, MD 20705
Office Phone: 3015045504
Sandra Saardanelli
IPM Coordinator and Nematologist
University of Maryland
H. J. Patterson Hall, Rm. 2105
University of Maryland
College Park, MD 20742
Office Phone: 3014057877
David Almquist
County Extension Director & Extension Educator, Ag
Harford County Extension Cooperative Extension
Caragh Fitzgerald
Extension Educator, Agriculture & Natural Resource
Howard County Cooperative Extension
Office Phone: 4103132710