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 NE. Second, the process used to conduct this research will empower and will 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 aimed 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. As the project has progressed, the number and complexity of the on-station experiments has expanded to address new questions about the effects and managment of the Brassica cover crops raised by researchers and farmers. The farmer-research attempts to address simple, focused questions about profitably fitting cover crops into specific farming situations. Results from at least on-farm and 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 suppression, to enhance beneficial nematode communities, 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.
- 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.
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 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 the following:
-alleviation of subsoil compaction
-capture of 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.
We have now collected three years (effectively, up to 16 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 for N uptake and have taken soil cores to 150 cm deep to study the degree to which the cover crops have taken up residual nitrate from soil the profile 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 have collected preliminary weed suppression data at three sites. Data on cover crop effects on subsoil water supply has been collected at sites for 2 years (WREC and CMREC) and 1 site for 1 year (USDA, Beltsville). In 2005 the soil water use monitoring was more intensive (more sensors and data-loggers) and the results were much more conclusive at the most intensively monitored sites.
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 community, 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 earlier.
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 use 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 or no cover. At the conventionally tilled LESREC site, soybeans growing after any of the cover crops yielded significantly more than those after no-cover crop. At all sites, the radishes grew the fastest in fall and were very competitive against weeds. Even after the radish stand was winter killed, strong suppression of spring weeds was evident.
We continued outreach efforts by working to update a fact sheet first produced during year 1 of the project to include newly generated knowledge and to reflect feedback from farmers and extensionists. We also ran a series of stories in the Future Harvest newsletter which reaches some 300 farmers. On May 23, 2005 we conducted an evening field day (twilight tour) with 39 attendees, about half being farmers and half extension and research or industry personnel. To date we have received over 60 responses requesting more information by phone and email. Some 43 farmers have expressed interest in participating in on-farm research, mainly with the Brassica cover crops. Thus, we have actually reached Milestone #2. On his reasons for wanting to do farmer research, Christopher Waesche of Jubilee Organic Farm in Martinsburg, WV wrote: “I don’t necessarily want to be a scientist but I do want to be a farmer and provide an example for my kids so that they can still stay on the land.”
We held two sessions in the annual Farming for Profit and Stewardship conference put on by Future harvest and Maryland Cooperative Extension in January 2005: one on the Brassica cover crops themselves and the other on the potential for farmer research. The response at both was excellent, with 65 farmers requesting further information or asking to collaborate on research on their farms. Among the participants at the farmer research session, 52 farmers submitted answers to a questionnaire about doing research on their farms, of which 43 (87%) said they currently use cover crops to some degree and 48 (93%) answered “yes” to the question “Would you be interested in learning how to conduct your own experiments on your farm?” We worked with 7 of the responding farmers to help them make detailed plans for Brassica cover crop research on their farms, addressing one or more of the aforementioned problem areas that Brassicas may have the potential to ameliorate. To 60 farmers with whom we could not work directly, we sent a package of seed (usually enough forage radish or rape to seed about 1/3 acre) and a request that they fill out a brief form to report what they did with the seeds and their observations on their performance. By December 1, we had received 13 of these completed forms.
Steve Groff, one of our farmer collaborators, has conducted trials on his farm since we began working with the Brassicas. He liked the forage radish performance 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. He soon sold out at approximately $2-$3/pound. Steve sold his seed to as many as 40 farmers (some seed was purchased by extension agents who then passed it on to several dozen farmers who requested it). In November, we presented some of our research results on cover crop impacts on nitrogen leaching and soil nematodes in two poster presentation at the annual meetings of the Soil Science Society of America in Salt Lake City , Utah.
We are scheduled to participate at the Farming for Profit and Stewardship Conference in Hagerstown Maryland in January 2006 where we will conduct a pre-conference workshop on farmer research, most of which will essentially be four concurrent farmer research circles to encourage discussion by farmers with a farmer resource person who has been doing research on his/her farm. (this will address the performance target).
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 Farm, Delmarva Farmer, NRCS Soil Quality Team Newsletter, etc. Our list of farmer contacts has grown to over 140. Of these 140, 83 have received varying amounts of brassica cover crop seed from us (in addition to the 30+ who bought see from Steve Groff), and 74 contacted us with requests for more information on the cover crops or on research methods. Approximately 22 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 Mr. 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. Distribution of seed, including small free packets, has proved to be a very effective tactic in spreading the word about cover crops.
The State of Maryland has now included Brassicas (rape) in its MACS cover crop support program aimed at reducing nitrogen loading to the Chesapeake Bay. The September 15th planting cut off for rape in that program is in part based on our experience in field experiments. 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 Brassicas as a result of seeds we provided. Most of the latter are not conducting replicated trials. In the first half of year 3 of the project (fall 2005), we are again working with 7 farmers (some new, some from the previous group) on cover crop trials. The January 2006 farmer research workshop should be instrumental in the initiation of one or more farmer research support groups and the development of appropriate, new farmer research guide materials.
Nitrogen capture and release.
Although our research on this topic is continuing, our results indicate that the Brassicas are capable of rapidly capturing large amounts of 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 mineral 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 role of Brassica species as attractive alternatives to rye winter 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 released 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 cover 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 compared to 76-96 under forage radish, rape, and rye. In March-April 2005 at this site, 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. The data showed that NO3-N and TN in porewater varied with time as a function of decomposition and precipitation. We conclude that, if established by early September, brassicas were at least as effective as rye in capturing excess soil N.
Cover crops that capture residual nitrogen from the soil profile in fall may reduce fertilizer need and minimize nitrate leaching if N mineralization from the cover crop residues in spring is synchronous with main crop demand. Our research focused on forage radish and rape to study the N the synchronicity between N release from the cover crops and N need by summer crops. Frost kills forage radish in December/January under Maryland conditions and we killed rape with glyphosate or tillage in April before summer crop planting. The ability of these species to capture N and then release plant-available N into soils was compared to that for rye, currently the most commonly planted cover crop in Maryland. Four cover crop treatments (rye, forage radish, rape and no cover) were applied in randomized complete block field experiments at four research stations in Maryland using a cover/soybean/cover/corn rotation from 2003-05. Two experiments used no-till, conventional tillage was used in one experiment, and no-till was compared to spring disking in one experiment. Surface soil (0-15 and 15-30 cm) mineral N (NO3–N and NH4+-N), and soluble organic N (SON) were monitored using monthly composite samples. Soil and air temperatures were monitored throughout the study and soil water content, bulk density, and texture were determined at each sampling date. Also, in late November 2005, we began a lab incubation to evaluate N mineralization rates in contrasting soils (Evesboro loamy sand and Mattapex silt loam) amended with forage radish, rape, or rye root or shoot residues. Amended soil was incubated at 25.0 ± 0.2 °C for 1, 2, 4, 8, 16, 32, and 48 days before determining the CO2 evolved and the NH4+ and NO3- produced.
For the field work, we hypothesized that the rapid decay of forage radish residues in winter would result in a large flush of mineral N in early spring and that leaching would move this flush from the 0-15 cm layer to the 15 to 30 cm layer soon thereafter. However, mineral N levels were higher at 0-15 cm than at 15-30 on all sampling dates. At all research sites and most sampling dates, cover crop had no effect on surface soil NH4+ or SON. There was, however, a significant cover crop treatment effect on surface soil NO3- on most sample dates. Forage radish plots were significantly higher in NO3- than the rape, rye and no cover plots as early as February (~6 weeks after forage radish frost-killed), but concentrations did not exceed 8 mg kg-1. In May, NO3- in rape plots exceeded that of rye at most sites, following the kill of both in late April. Soil in Brassica plots generally exceeded that in rye plots in June, though NO3- levels overall were moderate to low (<20 mg kg-1). Fall N uptake by roots and shoots of forage radish (190 kg N ha-1) significantly exceeded N uptake by rape (135 kg N ha-1). N uptake by rape shoots did not change from fall to spring, and significantly exceeded N uptake by rye shoots during both periods. In mid June 2005, young corn plants (5-leaf stage) following decomposition of forage radish and rape had higher dry matter N content compared to those following rye at most sites. Brassica root and shoot material decomposed more quickly than rye root material, based on CO2 evolution over the initial 8 days of incubation. Additional research should investigate whether a forage radish and rye interplanted combination might be most ideal for successful N budgeting and management.
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.
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. However, obtaining quantitative root data with the minirhizotron camera 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 roots were able to penetrate the plow pan where the forage radish had been grown fall 2004, 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 2004 the data were inconclusive because we used only two sensor per plot and because the rainfall was mostly adequate to prevent any serious drought stress that would force crops to rely on subsoil moisture. However, in 2005, we purchased several multi channel data loggers and were able to obtain hurly 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. We would like to do future research to address such mixed radish/cereal cover crops. To some degree we have already begun to look at these as most of the trial plots Steve Groff has used on his farm feature a combination of cover crops.
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 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 two years, using susceptible soybeans on a SCN infested soil to compare the effects of 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 either mowed and disked, or just disked in prior to spring planting, to macerate the tissue as suggested in the literature. 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 cover 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, before orchard planting, on cucumbers or soybeans, but the brassica covers did not establish well due to extremely wet weather, so most of the experiments were abandoned. Those that did survive the weather did not produce noticeable effects. In some case the lack of effects may have been due to greater effects of crop management .
Little is known about brassica cover crop impacts on nematodes in no-till systems or on free-living nematodes. One of our original hypotheses was that the Brassicas would suppress the plant-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 were studied in an Experiment at 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 nematode 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, under only no-till management, we tested cover crop treatments of rapeseed ‘Dwarf Essex’, forage radish ‘Dikon’, rye ‘Wheeler’, oilseed radish ‘Adagio’ and an weedy, 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 (June and August 2005), 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 have unique impacts on nematode communities throughout the year. Dauer nematodes are bacterial feeding nematodes in a state of temporary dormancy, likely because of dense nematode populations, low bacterial food supplies, high temperatures, or a combination of those factors. We are not sure what the implications on nitrogen cycling are, though potentially they conserve N in the system by immobilizing it in their bodies. Bacterial feeding nematodes as a whole, active and dormant, were greater in the radish plots, suggesting that these nematodes could also stimulate decomposition by stimulating bacterial growth through intensive grazing. The radishes favored a bacterial decomposition pathway, while rapeseed and rye appeared to favor a fungal decomposition pathway, as indicated by the Enrichment and Channel indices of Ferris et al.. We will be integrating nematode and nitrogen data where samples were taken on the same plots. Use of cover crops to manipulate the nematode community may have practical benefits for farmers, but more knowledge of their ecology is needed.
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 Maryland, 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. We have established a series of replicated experiments at Beltsville, MD, on coarse and fine textured soils. We are also collecting weed scoring data in several on-farm experiments. The observed radish weed suppression, combined with lack of significant residue in spring, opens the exciting possibility of developing a system of organic no-till planting for early scoring crops, an idea that several farmers have expressed an interest in trying. Andy Andrew of Colchester Farm (a CSA) in Kent County, MD, set up replicated strips of radish and no-cover in fall 2005 to test the organic no-till planting concept in spring 2006.
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. 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 much larger acreage than the vegetable growers, 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. One of the farmers wanted to avoid having to deal with killing a cover crop in the spring, and was therefore attracted by the winter kill characteristic of the forage radish. In fall 2005 we worked with two other large scale farmers who had airplanes spread rape or radish seed in replicated test strips on their farms. 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 new replicated experiment at the Central Maryland Research and Education Center, Beltsville. This was one of the experiments featured in a late May field day attended by 35. A small study was also conducted on the soil temperature requirements for seed germination of the varius cover crops, as farmers asked this question and no information appears to be available in the literature for some f the brassica cove crops we used. The results from this lab study showed that the ability to germinate quickly in cold soils was in the order rye> forage radish > rape, and these results helped explain what we and the three farmers experienced with regard to early spring establishment.
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 and are beginning to make some progress in empowering farmers to do their own research at an appropriate level of complexity.
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
IPM Coordinator and Nematologist
University of Maryland
H. J. Patterson Hall, Rm. 2105
University of Maryland
College Park, MD 20742
Office Phone: 3014057877
County Extension Director & Extension Educator, Ag
Harford County Extension Cooperative Extension
Extension Educator, Agriculture & Natural Resource
Howard County Cooperative Extension
Office Phone: 4103132710