Multipurpose Brassica cover crops for sustaining Northeast farmers

Milestone 1: About 450 farmers will hear about potential benefits of Brassica cover crop systems and farmer research through the initial project flier, extension, Future Harvest newsletters, farmer conference presentations, extension visits and direct phone calls.
Milestone 2: One hundred of those farmers request more information on Brassica cover crops and on-farm research through the flier response form, emails, phone or web site responses.
Milestone 3: Forty farmers who requested more information or otherwise heard about the project decide to try some Brassica cover crops on their farm.
Milestone 4: Fifteen of those requesting more information decide to implement on-farm trials and evaluate cover crops as solutions to their specific farming system problems. These farmers work closely with project personnel and are well documented in the database.
Milestone 5: Ten of the 15 farmers conducting trials document (measure and report) significant economic or environmental benefits from using Brassica cover crops.
Milestone 6: Six of the farmers conducting trials on their farms work on educational efforts to spread their results about Brassica cover crops and develop guidelines for farmer cover crop research. This group is in close contact with the project leaders.

1. 1. Of the 450 horticultural and grain crop farmers reached by this SARE project, 40 will transition to the use of cover crops for two or more of the benefits demonstrated by the project research and 15 farmers will 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 required that information on the Brassica cover crops be gathered and/or new information be generated by field research before we could provide practical information and recommendations to farmers. However, while we were gathering and generating new information, we were 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 also used 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 conducted 12 additional field experiments on these stations and have worked to collaborate with 15 commercial farmers in conducting 13 replicated on-farm experiments, some running for multiple years.
Each on-station experiment was designed to investigate one or more 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 were aimed at evaluating one specific cover crop function that addresses a problem identified by the farmer, or testing the adaptability and productivity of one or more of the brassica cover crops to the farmer’s situation. While some of the on-farm experiments were more successful than others, all yielded important lesson and experiences, both for the project researchers and for the farmers.

Specific accomplishments with respect to the project milestone are discussed in the "Impacts" section , below.

1. N UPTAKE BY COVER CROPS

Forage radish, oilseed radish, and rape were compared to rye, a popular cover crop in the region, with regard to nitrogen uptake N at three sites CMREC (Hayden), WREC and LESREC.

In late April 2004 at LESREC, the Rape (both Essex and Humus cultivars) shoot dry matter had about the same tissue C/N as did rye (23 to 29), but the C/N ratio in the rape roots was about 45, high 4. enough to cause N immobilization. The clover shoot dry matter, as expected, had a sigificantly lower C/N ratio (about 15) that would cause immediate N mineralization to occur. Perhaps because of the high root C/N ratios present, only the plots that had clover cover crops showed moderately high levels mineral nitrogen (15 to 20 mg/kg) in the upper 30 cm of soil collected in mid June. For cover crops sown in August 2004, the rapid growth of the Brassicas, especially the radishes, resulted in almost 2 fold higher production than rye, if only shoot dry matter is considered, and 3 fold higher if the fleshy tap root is included. The Brassicas also produced about 5 fold higher dry matter than weeds.

Our data 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 to more than 7,000 kg/ha by late fall, their potential for N uptake appears to be even larger than that of rye, which is the standard N capture cover crop in the Mid-Atlantic region. At CMREC (Evesboro loamy sand), plant N uptake by late fall 2004 ranged from 151-214 kg N/ha in the Brassicas (shoot + fleshy root). On a finer textured soil at WREC (Matapeake silt loam), the average shoot N uptake from November 2003 and January 2005 (deep sampling targeted for December 2004 was delayed by frozen soils until early January) ranged from 121-160 kg N/ha for the Brassicas and rye.

To summarized our N uptake results, we analsyzed the data on the shoot biomass and nitrogen content (kg ha-1) for rye, rape and forage radish for the four site-years where all three species were grown under the same conditions and N uptake was determined. By late November, the mean uptake of N in the foliage (not including above ground fleshy roots) was 119 kg ha-1 for forage radish and 111 kg ha-1 for rape, both values significantly higher than the mean of 79 kg ha-1 for rye planted on the same dates. Rye roots, which are fibrous in nature and completely underground, were not measured in these experiments. However, the fleshy root of forage radish occurs partly aboveground, and is easily collected. It could be considered partly above ground tissue. The easily harvested fleshy forage radish roots contained a mean of 60 kg ha-1 N in 2003 at four field sites (BARC, WREC, LESREC, CMREC) and an mean of 84 kg ha-1N in 2004 at three field sites (WREC, LESREC, and CMREC). Rape and rye both usually had less total N in their shoot dry matter by cover crop termination time in spring than they had the previous November, despite a large amount of biomass increase during the spring. The mechanisms of N loss during the winter for these non-winter killed cover crops is not clear, but could include leaf senescence and drop as well as leaching.

Our work suggests that if planted by mid September, rape, forage radish and oilseed radish are at least as effective as cereal rye in capturing residual soluble nitrogen in the soil profile. In agreement with European studies published after the initiation of our project (Kristensen and Thorup-Kristensen, 2004; Thorup-Kristensen et al., 2003), the Brassicas appear to exhibit a very rapid and deep rooting habit that allows them to uptake large quantities of N from deep in the soil profile early in fall before the N can leach beyond their reach. For example, data from a highly permeable sandy soil near Beltsville, MD, suggest that within 6-7 weeks of planting, the Brassica cover crops can capture exceptionally large amounts of nitrogen that might otherwise leach to groundwater over the winter. When planted by September 1, good stands of forage radish or rapeseed cover crops produced up to 6,000 kg/ha dry matter containing over 140 kg/ha of nitrogen by mid-November.

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 very sandy, unmanured soils in a strictly grain rotation. We have not conducted nitrogen rate fertilizer trials on the Brassica cover crops, however, our experience with virtual cover crop failures from N deficiency on some very low N sandy soils leads us to believe that where needed to ensure vigorous cover growth, a small nitrogen application 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 often fertilized the radish with N both to assure vigorous growth and enable the N uptake potential to be expressed. We believe that application of 15 to 20 kg N/ha as fertilizer in fall would be justified, if it is necessary in order to get the cover crops off to a rapid, deep rooting start that allows them to recover 100 to 150 kg N/ha from deep in the profile before it can leach completely below the potential root zone.

Productivity and N uptake for forage radish planted in early Fall by commercial farmers averaged 3023 kg ha-1 dry matter (7 site years of replicated on-farm studies) and 84 kg ha-1 N (mean of 5 site years of replicated on-farm studies). These on-farm forage radish cover crops were always planted without fertilization, but in most cases on fields with a history of manure or compost application. The data show that the on-farm N uptake was very substantial - in the same ballpark as that for our experiment station plots that did receive N fertilizer, attesting to both the impressive N uptake potential of the forage radish and to the large amount of N that can become available during the fall in many farm soils.

Our project also conduscted trials in which cover crops were planted in early spring instead of fall. These studies were carried out because we received requests from grain farmers for information on spring planting of the Brassicas as a way to get around the difficulties of making the early fall planting date needed for best winter hardiness and performance of the Brassica cover crops. They wanted to know if it would make sense for them to plant the cover crops in late March/early April and kill them about two months later in late May before planting soybeans. The results were rather unimpressive. The means of three site years of replicated trials were 1381, 506 and 985 kg ha-1 of shoot dry matter for forage radish, rape and rye. The aggressive growth seen for fall planted covers, especially for the radish, was not seen with spring planting. Both rape and radish flowered early on and produced relatively little foliage and even less root dry matter, even when fertilized. The N taken up in 2006 (37 and 23 kg ha-1 N uptake fro radish and rye) barely exceeded the 20 kg N ha-1 applied as fertilizer to the spring planted covers on the sandy soils at CMREC. Part of the spring growth limitation on these sandy soisl may have been that the available N had leached during the winter to below the reach of the young cover crop roots in early spring. The CMREC 2006 study also gave a good idea of root to shoot ratios for rye, rape and radish under these conditions. For the Brassicas, the shoot to root ratios were near 6:1, indicating far less root allocation than for the same species planted in fall (for which the shoot to root ratios varied from near 1:1 to about 3:1).
We therefore conclude that early spring planting, as envisioned by some farmers, does not seem to be a viable option. Because of the relatively slow growth and especially small proportion of root growth, there appears to be little chance that any compaction alleviation or weed suppression benefits could be realized from spring planted Brassicas, and the N uptake benefits were very modest and undoubtedly occurred too late to prevent most N leaching.

2. RESIDUE DECOMPOSITION IN LAB STUDY

In the laboratory incubation study comparing the mineralization of residues of rye, rape and forage radish in two soils, evolution of C was rapid in both soils within 24 h following incorporation of plant materials into soil but declined thereafter, and the C evolution rate was low from day 16 to the end of the study. Average cumulative C mineralization from all materials was approximately 425 to 650 g kg-1 added residue C over the 48-day period. Rye roots (sampled from spring –planted rye) decayed the most slowly and radish roots and shoots (from fall planted radish) decayed most rapidly. The rates of CO2 evolution from rape shoots and roots and rye shoots were in the middle.

After 48 days incubation in both soils, only the rape and radish shoot tissue (C/N ratios of 16 and 12) produced significantly more mineralized N than the unamended control soil. Between 10 and 37% of N added to soil with the forage radish and rape shoot residues was measured as soil mineral N on the last day of the incubation study. In contrast, the cumulative N mineralized from rye shoots and Brassica roots (C/N ratios > 21) was not significantly greater than from the unamended control soils. These data are in agreement with our observation in the field of greater N availability in spring following radish than following rye (see below).

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. 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.

3. NITROGEN RELEASE IN THE FIELD

In addition to reducing leaching losses (discussed below), cover crops that capture residual soil N in fall can also reduce fertilizer needs if N release by mineralization from cover crop residues is synchronous with N use by subsequent crops. The potential of forage radish (Raphanus sativus L.) and rape (Brassica napus L.), to act as N catch crops as well as to provide subsequent summer crops with inorganic N mineralized from their residues was compared to that of rye. Cover crop treatments (rye, forage radish, rape, forage radish + rye mixture and an unweeded no cover control) were compared in replicated experiments at three locations from 2003 to 2005. Each experiment used a rotation consisting of winter cover crop/soybean (Glycine Max L.)/winter cover crop/corn (Zea mays L.) beginning in the fall of 2003. Nitrogen uptake and dry matter production by cover crops and main crop seedlings (prior to any N side-dressing) were measured along with mineral N (NO3-N and NH4-N) and total soluble organic N (TSON) content of the soil at 0-15 and 15-30 cm depths.

Fall and spring N uptake by Brassica cover crops ranged from 100 to 250 kg N ha-1 (roots and shoots combined, see above discussion) and equaled or exceeded that of rye at all sites in both study years. We observed no effects of cover crops on soluble organic nitrogen in the surface soil in either field season (data not shown).

Except for a single date at one site, we did not observe any cover crop treatment effects on inorganic N levels in the upper 30 cm soil in spring 2004. The relatively low mineral N levels (generally less than 20 kg N/ha in the upper 30 cm) were probably due to microbial immobilization stimulated by the high C/N ratio in the corn residue present in spring 2004. By contrast, the soybean residue present in spring 2005 had a much lower C/N ratio and as a result higher mineral N levels were observed in the upper 30 cm of soil.

More importantly, in this lower carbon environment, the cover crop residues did have some marked effects on soil mineral N. On most sample dates from February to April 2005, soil NO3-N concentrations in forage radish plots were higher than those in rape, rye and no cover plots. The higher mineral N levels in the radish plots probably reflect the fact that the rye and rape were still alive, while rapidly decaying radish residues (but no living plants) were present in the radish treatment plots. In May and June 2005, following the termination in April of the rye and rape cover crops, soil NO3-N concentration following rape generally exceeded that following rye. Compared to rye or no cover, Brassica residue decay increased early June dry matter and N content of V6 stage corn or seedling soybean plants at most sites.

Modest amounts of N from decomposing forage radish tissues were observed in surface soils as early as February or March when this cover crop was frost killed in December. Rates of N mineralization accelerated considerably with warming temperatures in the period from March-June. Significantly more N as nitrate from the forage radish cover crop was available for uptake by following main crops in early May and mid-June than from rye (killed in April at early-boot stage). Heavy rain preceded soil sampling in early April 2005 (~day 100). The dip in surface soil nitrate levels at CMREC in April 2005 corresponded to a sharp rise in leaching water nitrate levels in the subsoil.

These data illustrate the greater risk of nitrate leaching from early mineralized N in a very sandy soil compared to a finer textured soil. The low soil nitrate levels in spring 2004 suggest that much of the N taken up by the Brassicas (and rye) in fall was probably immobilized as microbes decayed the high C/N ratio residues from the previous corn or wheat crops along with the lower C/N residue of the cover crops. Only in the plots with spring-killed crimson clover (at LESREC Exp. 1) was there a significant increase in soil mineral N in the upper 30 cm by mid June compared to the no-cover crop controls. In the 2004-2005 cover crop seasons, the soil system was less carbon dominated because of the use of a soybean green manure before planting the cover crop the previous fall. This relatively high N, low C soil environment allowed net N mineralization from the cover crop residues. The Brassicas decomposed and released N more rapidly than the rye, according to both field and lab incubation measurements.

4. NITRATE-N IN LEACHING WATER FROM SOIL PORES

Porous ceramic cup tension lysimeters were also installed at depths of 75-120 cm to monitor NO3-N in soil porewater at CMREC and WREC. In spring 2004, NO3-N concentrations in subsoil porewater samples were very low (< 2 mg L-1) and were unaffected by cover crop treatments at either site, except that on the earliest sampling date in 2004 at CMREC (February 23), the no-cover control plots had slightly, but significantly higher nitrate levels in the porewater. It is possible that early leaching took place and that cover crop may have had effects prior to this first sampling date. Based on a simple model that used the average monthly concentrations and assumed that no surface runoff took place, evaporation and precipitation data suggest that approximate leaching losses for March 2004 (2.22 kg NO3-N ha-1 mo-1) were less than those for April 2004 (5.01 kg NO3-N ha-1 mo-1) at CMREC1. WREC1 had similar leaching losses in April and March (3.01 kg ha-1 mo-1).

In spring 2005 subsoil porewater NO3-N concentrations were markedly affected by the cover crop treatments. At CMREC2, when porewater sampling began on 5 February 2005, porewater NO3-N in all the cover crop plots was significantly lower than in the control plots. Thereafter, porewater NO3-N in the control plots following a declining trend while that in the forage radish plots increased during March and April. In the rape and rye plots, porewater NO3-N was low (0.06-0.25 mg L-1) for all sample dates. During the spring on coarse textured soil, porewater NO3-N concentrations in plots with freeze-killed cover crop (forage radish) were greater than in control plots or plots with overwintering cover crops (rape and rye). On fine textured soil, all cover crops provided a similar decrease in porewater NO3-N concentration compared to control.

At CMREC, 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. The spring 2005 porewater NO3-N levels at WREC2 were similar to those at CMREC2, but except that the levels in the forage radish plots were not increasing and averaged 0.602 mg L-1 throughout the sampling period. Rape and rye plots averaged 0.052 mg L-1 of porewater NO3-N while that in the control plots was much higher, averaging 3.94 mg L-1. Therefore, we recommend that on coarse textured soils, freeze-killed Brassica cover crops should be followed by an early-planted spring main crop.

5. NITRATE-N REMAINING IN THE SOIL PROFILE

Soil samples from the soil surface to depths of 105 to 180 cm depth were obtained in fall (2003-2004) and spring (2004-2005) for NH4-N and NO3-N analyses. The depth of soil core sampling for a given site varied according to soil conditions at that site. Brassica and rye, compared to a weedy fallow control, caused similar decreases in soil profile NO3-N in fall and spring throughout the sampled profile. Cover crops had no effect on extractable NH4-N in the soil profile during the course of the experiment (data not shown), with the exception of fall at WREC2 when rye plots had 77.4 kg ha-1 NH4-N compared to the Brassica and control plots (86.4-94.8 kg ha-1).
Ammonium-N in the soil profile during the first year of the study decreased from fall to spring 2004, but increased from fall 2004 to spring 2005.

Cover crop treatments significantly reduced soil profile NO3-N in fall compared to control plots, with significant effects in several depth increments to 1.0 m depth. By spring, soil NO3-N in the upper profile (0-60 cm) had increased in forage and oilseed radish plots (especially in 2005) while plant uptake by rape and rye continued to maintain low soil profile NO3-N. When conditions allowed us to sample as deep as 180 cm (CMREC, October 2003), we found a large and significant reduction in nitrate-N from the 150-165 cm and 165-180 cm depth increments under both oilseed radish and forage radish as compared to the control plots. These data show that the radish roots apparently reached deeper than 180 cm by November. This observation is in agreement with the recent literature from northern Europe (Kristensen and Thorup-Kristensen, 2004) that suggests that forage radish is more effective than rye at capturing nitrogen from deep in the profile in fall. It should be kept in mind that the deeper the N is located in the soil, the more likely it is to leach to groundwater before spring crops can take it up. Therefore, N captured in fall from deep in the profile probably is of greater environmental value than the same quantity captured from more shallow soil layers (Thorup-Kristensen et al., 2003).

Although in fall, all cover crop plots had significantly less soil NO3-N than control plots due to N uptake by the growing plants, during the spring, control and radish plots had greater soil NO3-N concentrations than rape and rye, as the later cover crops continued to grow and capture nutrients. In the spring samples for three of the four site-years, NO3-N concentrations of in the upper 60 cm of the soil profile were greater in forage radish plots than in the rape or rye plots, and in one site year, greater than in the control plots. We ascribe these higher NO3-N concentrations to N released from radish tissue by mineralization between the time of freeze-kill in late December and the time of soil sampling in April-May. These data are in agreement with most of the data we obtained on mineral N in the surface soil layer.

The increased spring soil NO3-N concentrations in radish plots was most pronounced and deeper in the profile in the second year than in the first, possibly because January 2005 was considerably warmer and wetter than January 2004. The rapid appearance of NO3-N in the surface soil of radish plots could be an advantage from the viewpoint of providing early N fertility for spring-planted main crops. Overwintering non-legume cover crops, such as rye, can reduce N availability to the following crop if termination occurs when their C/N ratio has increased above 25/1. However, the data from spring 2005 shows that on a very sandy soil (CMREC), NO3-N concentrations in 120 cm deep porewater increased dramatically by early April, so some of the N released would probably have been lost to leaching before the roots of even a very early-planted main crop like corn or potato could capture it. Weinert et al. (2002) noted a similar early N release from a freeze-killed mustard cover crop on irrigated sandy soils in Washington State.

6. OVERALL NITROGEN CYCLING CONCLUSIONS

This research demonstrated that forage radish and rape performed as well or better than rye as N cycling cover crops. Our data suggest that N in freeze-killed, decomposing forage radish residues may be at some risk for leaching in excessively drained very sandy soils during late winter and early spring. The Brassica cover crops cleaned up the nitrate in the soil profile in fall as well as rye. In fall 2003 at CMREC we were able to obtain soil cores to 180 cm and document that the two radish cultivars caused significant reductions in nitrate levels as deep as 150 to 180 cm (rye plots were not sampled in that site-year).

An important objective of our project was to evaluate the N leaching risk presented by winter-killed, decomposing forage radish in late winter and early spring months. The temporal patterns of N release for control and forage radish treatments during spring 2005 were nearly opposite from each other. 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 plots. 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 cover crop is followed by an early-planted crop such as corn or early vegetables. If planting is delayed until May, on very sandy soils (loamy sands), significant N may be lost by leaching before the spring crop can capture it.

To avoid N leaching in spring, we would suggest two methods for retaining and/or reusing the N conserved by the forage radish cover crop: (1) following the forage radish cover crop with the planting of a nitrogen-demanding cash crop in April, or (2) fall-planting of forage radish as a mixture with an over-wintering cereal cover crop (rye or oats). Since low levels of nitrate in leaching water in all treatments in Exp. 1 (spring 2004) lead to inconclusive results on using the forage radish and rye mixture as a way to conserve N mineralized from forage radish residue in late winter/early spring, additional investigation into this mixture is warranted.

Our farmer collaborators have successfully grown combinations of forage radish with rye or oats by drilling the two species in alternate rows. However, we do not have deep profile N data for those on-farm plots.

Even with the potential risk for inorganic N leaching following the winter-kill of forage radish, in Exp. 2, the forage radish cover crop consistently provided the greatest N availability to main crops in spring, indicating that a substantial portion of N from forage radish residues can be retained in surface soils until it can be used by subsequent main crops.

In summary, our finding that greatest N release in spring comes from forage radish, followed by rape and then rye, is consistent with research conducted in northern Europe (Thorup-Kristensen et al., 2003). Peak nitrogen release from forage radish residues in May could coincide with the acceleration of corn N uptake beginning about 1 month after planting in early April. Other questions not addressed by this research include the possible inhibition of nitrification by glucosinolate degradation products from decomposing rape and forage radish residues. Also not studied were possible effects on nitrification and denitrification by the high amount of calcium recycled to the surface soil environment through the high-Ca content of Brassica cover crop residues.

1. SUPPRESSION OF PLANT PARASITIC NEMATODES

Soil nematodes in grain crop agroecosystems were studied in three experiments, LESREC Exps.1 and 2 and CMREC Exp. 1. The hypothesis tested was that glucosinolate-containing Brassicaceous cover crops would suppress plant-parasitic nematodes. Cover crops tested included mustard blend (Brassica juncea and Sinapis alba) ‘Caliente’, rapeseed (B. napus) ‘Essex’, rapeseed (B. napus) ‘Humus’, oilseed radish (Raphanus sativus) ‘Adagio’/’Colonel’, and forage radish (R. sativus) ‘Daikon’. These were combined with rye (Secale cereale) ‘Wheeler’ and crimson clover (Trifolium incarnatum) in LESREC Exp. 1. Soybean cyst nematode (Heterodera glycines) was studied by monitoring the active second juvenile stage individuals (J2). A survey of J2 H. glycines and soil properties showed an uneven distribution of the nematode and soil water content in the experimental field prior to establishing the experiments. This information was used to design an efficient blocking arrangement in the field, and as covariate data for later analysis.

The abundance of Heterodera glycines increased more than ten-fold from less than 100,000 to more than 1,400,000 nematodes/ m2 during the two years in which susceptible soybeans were grown; it was not suppressed by Brassicaceous cover crops. Dolichodoridae (known as awl nematodes) declined over the two years in all treatments of the same experiment. Rye had opposite effects on Dolichodoridae in two experiments.

Nematodes in the family Trichodoridae are commonly called "stubby-root" nematodes, because feeding by these nematodes can cause a stunted or "stubby" appearing root system (Crow, 2004). Trichodoridae nematodes were 2-4 times higher in mustard plots than in other Brassicaceous treatments during cover crop growth and 1.8 times higher than in oilseed radish plots during the entire two years, in Exp. 1. In two of the three experiments, rye favored high abundances of Trichodoridae in June. The combination of Brassicaceous cover crops with rye and clover decreased H. glycines J2 abundances, and/or increased non-parasitic nematode abundances on one or more sample dates.

In laboratory bioassays, all cover crop tissues reduced survival of Meloidogyne incognita (root knot nematodes) or H. glycines J2 compared to unamended controls. Rapeseed biomass production (mg/kg soil) in 2005 in field Exp. 1 was lower than the minimum amount needed for suppression in the bioassay. Radish biomass production in 2005 was high enough to create residue to soil ratios as high as those that suppressed H. glycines J2 in the bioassay. The failure to observe suppression in the field by radish cover crops may have been related to the freeze-termination of this cover crop at a time when cold soil temperatures depress nematode activity. Future Brassicaceous biofumigation studies in Maryland should target high value horticultural production systems that allow for more intensive and flexible management of cover crops.

3. EFFECTS ON HETERODERA GLYCINES AND SOYBEAN YIELDS

Heterodera glycines juveniles (J2) increased in abundance dramatically over the two years of susceptible soybean, whether or not Brassica winter cover crops were grown. Among the five Brassicaceous cover crop alone treatments and the control, oilseed radish (P < 0.03) and forage radish (P < 0.09) had higher abundances of H. glycines J2 compared to the control across dates in the second experiment year, though significant differences were only detected on the November sample date. Main effect means of H. glycines J2 abundances were significantly higher in forage and oilseed radish compared to treatments without Brassica cover crops on only one sample date -in June 2005. Egg densities were not measured in this study. Higher H. glycines J2 populations in radishes on one date may not be more problematic than populations in other treatments, if egg production was equivalent or less.

Cysts were counted on several dates. There were no treatment effects on cyst abundances in 2004 (mean=139 ± 13 103/m2) or in 2005 (mean=78 ± 8 103/m2). A lower abundance of H. glycines J2 across treatments in August, is probably a temporal effect on J2 activity in soil rather than a decrease in reproductive potential, because soils in August were warm (mean measured temperature =25 ˚C) and dry (8.4 g water/g dry soil) and there was no rain or irrigation for six days prior to sampling. Considering the more than ten-fold increase in H. glycines J2 over the two years, this study agrees with other recent studies that suggest Brassica cover crops do not decrease H. glycines reproductive potential either during cover crop growth or after green manure incorporation (Miller et al., 2006;Warnke et al., 2006).

Combination of Brassica cover crops with rye and crimson clover, however, resulted in lower H. glycines J2 abundances in June samples in both years. Rye suppressed H. glycines J2 abundance compared to Brassica cover crops (main effect means) by 38% (P < 0.09) in June 2004 and 57% (P < 0.0001) in June 2005. Clover main effect means for H. glycines J2 populations were 43% lower compared to Brassica main effect means in June 2005. However, neither rye nor clover alone was different from the weedy control plot alone in either year (with mean abundances ranging from 72,000 to 117,000 m2).

There were no treatment effects on soybean yield in 2004, however main effects means of soybean yield were 59% and 25% higher in rye (1851 kg/ha) than in the control (1166 kg/ha, P < 0.001; Brassicaceous cover crops alone + weedy control) or crimson clover (1480 kg/ha, P < 0.10), respectively in 2005. Low yields across treatments in 2005 (1503 kg/ha), compared to 2004 (3579 kg/ha), can be explained by only 10 cm total precipitation, including irrigation, during pod-fill in August and September, followed by high rainfall in October (20 cm), which delayed harvest and caused bean rot. Some yield loss, however, may be attributable to damage from H. glycines. Nonlinear regression of soybean yield and H. glycines J2 suggests that yield decreases exponentially with increasing abundance of H. glycines (Y = 3174 –e-9E-07x, R2=0.54**). This supports current management recommendations in Maryland to take preventative measures if one cyst is found per 250 cm3 of soil (Sardanelli et al., 1983. While the trend appears to be primarily a difference in years, reduced yields in the same plots in block 1 where H. glycines J2 populations were high in both years, suggests that the nematodes contributed to significant yield reductions in 2005. In both years, H. glycines J2 abundance in June was negatively correlated with yield (2004 r=-0.451, P < 0.0001; 2005 r=-0.436, P < 0.0001). Overall, more significant effects on H. glycines J2 and yield were detected in 2005 than in 2004, possibly as a result of weather, accumulated cover crop effects on soil properties, increased root density in continuous soybean, or as a function of higher nematode densities, possibly already at equilibrium (Chen et al., 2001).

Bioassay 1
Bioassay results suggest that a rate of 50 g kg-1 plant material is an effective rate for nematode control. Assuming incorporation to a 15 cm depth and 90 g water g-1 dry matter and 85 g water g-1 dry matter for radishes and rapeseed/rye moisture contents, the equivalent biomass production needed for nematode suppression in the field was 5,000 and 7,500 kg dry matter ha-1 for radishes and rapeseed/rye respectively. Oilseed radish leaf tissue, however, suppressed Meloidogyne sp. by 57% compared to the unamended control at a rate of only 10 g kg-1 or 1,000 kg dry matter ha-1 (P < 0.0036). These rates were achieved in the field studies. Interaction with soil properties, incomplete hydrolysis of glucosinolates, differing sensitivities of nematodes observed in this study, or insufficient penetration of the soil during winter may explain the general lack of effects on plant parasitic nematodes by radishes.

Root tissue was generally more suppressive than shoot tissue, within an amendment application rate. Rapeseed root suppressed Meloidogyne sp. at the lower rate (1500 kg dry matter ha-1) by 86% compared to the unamended control (P < 0.0007). This supports studies showing higher glucosinolate concentrations in rapeseed roots than shoots (Eberlein et al., 1998; Gardiner et al., 1999). Few studies have reported oilseed or forage radish root and shoot glucosinolates contents. Suppression of Meloidogyne sp. with rye shoot at the 5% rate may be a result of hydroxamic acids (McBride et al., 2000; Zasada et al., 2005) or may indicate that oxygen was depleted during decomposition of the plant material. Meloidogyne was not present in our field experiments and may be more sensitive to Brassicaceous decomposition products than the genera observed in this study.

Bioassay 2
Rapeseed cultivars ‘Essex’ and ‘Humus’ were similar in their suppressive effect on H. glycines in bioassay 2. Nematodes used in this experiment were extracted from LESREC Exp. 1 field plot soil in April 2005, from the rhizosphere of the rapeseed cultivar corresponding to that of the tissue amendment. Lower abundances of H. glycines in the rapeseed ‘Humus’ control (unamended) may indicate some degree of suppression by ‘Humus’ during growth of that cover crop. Equal suppression of H. glycines by all amendments may indicate a general plant tissue effect, such as oxygen depletion, rather than specific chemical fumigation. Use of plant tissue known to be non-allelopathic, instead of rye, as the non-Brassica control might have made a better comparison. Rapeseed roots suppressed non-parasitic nematodes as much as H. glycines, but shoots did not. Bioassay results show that even in ideal conditions, Brassicaceous cover crop tissue has potential for only partial suppression of H. glycines. Non-plant parasitic nematodes were most sensitive to rapeseed root amendment.

2. EFFECTS ON SOIL NEMATODE ECOLOGY

In addition to the work on plant parasitic nematodes, analysis of nematodes grouped by function and indices of nematode community structure were used to study the effects of several of the cover crops on the total soil nematode community in three field experiments (LESREC Exps. 1 and 2 and CMREC Exp. 1) with loamy sand surface soils. Soil properties were also studied in relation to the observed effects. Nematodes provide a relatively simple way to assess the soil biological condition because they occupy every level of the food web and are easy to extract from the soil. Since nematode community dynamics reflect combined soil chemical, physical, and biological properties over time, nematode community analysis offers insight into how particular agricultural tools, such as cover crops, may be managed to optimize their impact.

The objective of this study was to determine the effects of Brassica and rye cover crops on the nematode community, via analysis of nematode genera, trophic group and community indices. A secondary objective was to evaluate the influences of the timing of cover crop termination versus the cover crop type on nematode response parameters. As these research objectives went beyond those of the funded project, the results will be only briefly summarized in this report.

The enrichment index (EI), channel index (CI), structure index (SI), bacterivore and fungivore maturity indices (BaMI, FuMI), and total community maturity index 2-5 (∑MI25, MI25) were calculated (as described in Gruver, 2007 and Ferris et al., 2001) to quantify the nematode community responses to cover crops. In summer 2004 at CMREC (Hayden farm), the oilseed radish had a large positive effect on beneficial (non-parasite) soil nematodes. Plant parasitic nematode populations were small , and unaffected by the cover crops. Forage radish plots were not analyzed for nematodes in that site-year.

Nematode genera identified were similar among the three experiments. Total nematode abundance ranged between 1.9 and 2.7 billion/m2 in Exp. 1; 1.5 and 3.2 billion/m2 in Exp. 2, and 1.3 and 1.8 billion/m2 in Exp. 3 (data not shown). Large populations of dormant (dauer) bacterivore Rhabditidae nematodes were found in radish cover crop plots four to nine months after radish winter freeze-kill, and EI values were higher in radish plots than in control plots in 2005 experiments, six months after radish was killed by freezing. Spring-terminated cover crops favored fungivore decomposition channels, evidenced by high CI values. Large abundances of the plant associate (facultative hyphal feeder), Coslenchus, in rapeseed and rye plots contributed to this effect.

Despite repeated agronomic disturbances such as tillage, N applications, and herbicide treatments, SI, BaMI, FuMI, and ∑MI25 values were frequently higher in winter-terminated cover crop plots compared with spring-terminated cover crop plots. Dauer larvae and EI effects appeared to be associated with cover crop type, while changes in the fungivore activity and maturity of the community appeared to be associated with the timing of cover crop termination.

On a sandy soil under no-till management near Beltsville, Maryland, frost killed radish cover crops (‘Daikon’ and ‘Colonel’) increased bacteria-eating nematodes while rye (‘Wheeler’) and rapeseed (‘Essex’) cover crops killed by herbicide in spring increased the proportion of fungal feeding nematodes. The unweeded control plots without cover crops tended to have nematode communities with characteristics in between those of the two cover crops effects. The Enrichment Index, which indicates a greater abundance of opportunistic bacteria – eating nematodes, was 23% higher in soils that had Brassica cover crops (Radish ‘Daikon’ and ‘Colonel’ and rapeseed ‘Essex’) than the unweeded control plots.

These results, averaged over samples taken in November when the covers were growing, in June about a month after spring cover crop kill, and in August while corn was growing, suggest that the cover crops, living or dead, increased bacterial activity and possibly enhanced nitrogen cycling through the food web.

Our weed suppression research focused mainly on the forage radish because of the rather remarkably weed-free conditions observed following this cover crop. Although forage radish winter-kills and leaves little residue, we have observed it to consistently leave the ground virtually weed-free through the winter and early spring. In response to this observation, several dairy farmers in Maryland and Pennsylvania have already successfully no-till planted directly onto the virtually weed-free and almost completely decomposed forage radish residue without any burn down herbicide. Anecdotally, they report that stands were actually better and early growth more vigorous for corn planted into radish without burn-down herbicide than that planted into herbicide-killed small grain cover crop (or killed weeds with no cover crop).

Because the Beltsville site was the wettest of the four sites, the soybeans showed rank growth and heavy lodging, and matured the later than expected. These factors made it very difficult for interseeded cover crops to establish. On 06 November 2004 vegetative cover was scored in the interseeded plots. Relatively little cover (5 to 35%) was provided by the Mustard, Rape and interseeded Rye. However the Forage Radish, whether sown alone or with Rye, provided a significantly greater 55 to 65% ground cover.

In early November 2004 ground cover in Beltsville Exp. 2 with August drilled cover crops was very high. The results showed that the forage radish and rape plots had more than 90% cover while the rye plots had only 70% cover and the unplanted no-cover control plots averaged about 10% ground cover by weeds (glyphosate was applied just prior to drilling the cover crop seed in August).

Weed suppression research for the period of January 2005 to August 2005 attempted to quantify the observed effects of Brassica cover crops, in particular forage radish. This was primarily done using ground cover ratings and plant counts in plots of cover crops planted in fall 2004. In order to test the hypothesis that weed suppression is due to allelochemical release from Brassica cover crops (especially forage radish), a greenhouse study was designed to determine if the cover crop treatments influenced the weed seed bank in the soil. A growth chamber experiment was also set up to determine if lettuce and weed seed germination would be inhibited by the presence of chemicals in forage radish residue or a combination of soil and residue.

Effect of cover crop treatment on percent soil cover and weed density
Winter cover crop plots were rated for percent ground cover in March and May of 2005 and for weed density in May of 2005 to determine the degree of weed suppression following different cover crop treatments. Weed cover was monitored in four replicated experiments (described in the material and methods section) that included forage radish, rapeseed, rye, and no cover treatments (USDA Beltsville Exp. 2, CMREC (Hayden) Exp. 1, WREC Exp.2, and LESREC Exp. 2). Data were normalized with a square root transformation for analysis. Analysis was performed using the mixed procedure of SAS

In both March and May, percent weed cover was highest in the no cover treatment but there was no difference between the three cover crop treatments (α = 0.05). Despite the fact that forage radish was dead and mostly decayed, it was able to achieve weed suppression in March comparable to that of the living rye and rapeseed. However, percent exposed soil was highest in the forage radish plots in both March and May (α = 0.05). No difference in weed densities was found in May of 2005. At this time the corn crop was in the two leaf stage and it appears that there was no longer any difference in weed suppression among cover crop treatments. Weed control (glyphosate) was applied only after these data were collected.

Effect of forage radish residue on seed germination
The purpose of this experiment was to quantify the effect of forage radish residue on seed germination. The materials collected for the experiment included forage radish residue, intact soil from beneath forage radish residue, and both soil and forage radish residue. These materials were collected in March of 2005 from four replications of winter cover crop experiments at four locations (Beltsville, CMREC (Hayden), WREC, and LESREC). A thin layer of these materials was placed over moist filter paper in Petri dishes, each with 50 seeds of a particular species. The dishes were then incubated in a growth chamber at 25oC for 7 days. Seeds of lettuce, lambs quarters, and redroot pigweed were used in the incubation. Percent germination of seeds in the experiment was corrected for the germination percentages of the seed lots of lettuce, lambs quarters, and redroot pigweed used which were 83, 93, and 99 %, respectively. The statistical model accounted for variation between sampling locations and the blocking of the experiments from which the samples were collected. The data were normalized using a square root transformation prior to analysis.

Percent germination of all seed species was at least 20% lower than their respective germination tests for all material types. Significant differences in percent germination among treatments and seed species were found but there was no significant interaction. Unexpectedly, soil + residue from the forage radish plots had the highest percent germination and soil from no cover control crop plots with no forage radish residue had the lowest percent germination. We speculate that the decayed forage radish may have released nitrates, a compound long known (Steinbauer and Grisby, 1957) to signal germination for some weed seeds.

Effect of cover crops on weed seed germination
The objective of this experiment was to quantify the effect of cover crops on the weed seed bank in the soil. It was hypothesized that allelochemicals released by cover crops would kill weed seeds and reduce weed seed germination. Soil samples were collected in March of 2005 from four blocks of Belts Exp. 2 and LESREC Exp. 2. The Beltsville site was no-till while the LESREC site was tilled conventionally. Cover crop treatments included forage radish, rapeseed, rye, and no cover. Soil samples were spread out over trays of a fine sand material and watered to keep the soil moist. These trays were laid out in a completely randomized design on benches in a greenhouse. Germinating weed seedlings were counted once a week for a period of four weeks.

Emergence data from each location were analyzed separately because of differences in tillage between the two sites from which the soil had been sampled. The statistical model accounted for variation among blocks in the field experiments from which the samples were collected. The data were square root transformed prior to analysis. Average germination counts were expressed on a per cm3 of soil basis.

Average germination counts were less than 1 plant per cm3 for all cover crop treatments at both locations. It was expected that weed seedling emergence would be lowest following forage radish. This trend was observed in the samples from LESREC, where weed seedling emergence was highest in the no cover treatments. However, this trend was not seen in the samples from Beltsville. At this location, weed seedling emergence was highest following forage radish. As nitrates have been long known to stimulate the germination of many seeds (Hendricks and Taylorson, 1974; Steinbauer and Grisby, 1957), we speculate that the apparent stimulation in germination may have been due to higher levels of nitrate from the release of N by the decaying forage radish residues (see above). Although once these effects were observed in these studies, it was too late to obtain a meaningful measure of the nitrate content of the soil. We are currently conducting studies aimed at monitoring the nitrate fluctuations and their impact on weed seed germination following forage radish cover crops.

In all our experiments, we have observed that radish winter cover crops suppressed weeds from the time they were planted in late August until some time in April. In our 2005-2006 weed suppression studies, during April, henbit and chickweed emerged in the forage radish plots, while horseweed emerged in no cover plots. 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. Our 2006-2007 experiments (which carry on beyond the project grant period reported on here) will also include test crops (corn and potato) that are planted in early April to determine if suppression of horseweed following forage radish cover crops will remain effective after the soil is disturbed during seeding.

Since after every good stand of radish we had observed the soil to be weed free at planting time in early spring, it was hypothesized that forage radish might provide weed control in two ways. 1) it might suppress weeds by severe competition during the rapid growth of the cover crop in the fall. 2) during the decomposition of forage radish residue over the winter months, allelochemicals might be released that prevent weed seeds from germinating.

As just mentioned, in the 2005-2006 and 2006-2007 seasons, forage radish cover crops suppressed weeds from the time they were planted in late August until mid- to late-April in two experiments in Beltsville, MD. Horseweed (Conyza canadensis) resistant to glyphosate is the most recently reported new type of resistant weed in Maryland first reported in 2002 (Ritter, 2005). In these experiments, forage radish successfully suppressed horseweed (Conyza canadensis) suggesting that it may be a new integrated management tool for herbicide resistant horseweed in the Mid-Atlantic region. 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.

Although spring germination of horseweed was suppressed by forage radish cover crops, no such suppression of lambsquarters, pigweed, or green foxtail was observed at the Beltsville site in 2006. Post-project experiments are being conducted in 2007-2008 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. Continuing research spun off from this grant is on sites selected to have a known history of weeds that are of interest, rather than depending on introducing weed seed.

Experiments designed to understand the mechanism of forage radish weed suppression were conducted and were described above as Beltsville Experiments 7 and 8. Although these first year studies were not considered conclusive because of the small plot size, our results showed that adding or removing forage radish residues had no effect on weed suppression. In spring 2007 (after this grant period had ended) data from larger plot experiments showed the same thing. In growth chamber studies, 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. Therefore, we conclude that our data strongly suggest that radish suppresses spring weeds by smothering in fall, rather than by allelopathy during residue decomposition. However, the mechanism for horseweed suppression in spring is still unclear, hence our continuing research into that aspect.

The duration and extent of forage radish weed control

Forage radish suppressed weed growth in the fall when the cover crop was growing and in the spring as the residues decomposed when compared to no cover treatments. In April, henbit and chickweed were the first weeds to emerge in forage radish treatments. These two winter annual weeds had previously dominated the no cover treatments in January and March, suggesting that their emergence had been delayed by the forage radish cover crop. While henbit was the dominant weed in the forage radish plots, horseweed began to emerge in the no cover plots in April. Percent ground cover ratings for weeds in the no cover plots decreased relative to the forage radish plots later in May and June as weeds (especially horse weed) grew tall and shaded soil surface. Henbit began to die out in the forage radish plots by mid-June to early July, however no horseweed germinated in the forage radish plots. It is not known if forage radish had an allelopathic effect on the horseweed that prevented it from germinating or if the canopy of henbit created an unfavorable environment for horseweed germination.

One hypothesis was that forage radish would suppress winter annual weeds better than it would suppress summer annual weeds. Forage radish was observed to delay the emergence of winter annual weeds relative to no cover and oat treatments rather than inhibit them from germinating. By delaying the emergence of winter annuals, summer annual weeds were also delayed from emerging. If the emergence of these summer annual weeds (which are harder to control) could be pushed past a critical weed free period for a summer crop, this might give the summer crop a competitive edge.

This experiment was more controlled than Beltsville Experiment 5 because it dealt with weed seeds that had been planted under forage radish residue. Other weeds from the natural seed bank were frequently removed to decrease their interference with the introduced weeds. Weeds began to emerge in these plots in April. Lambsquarters and pigweed emerged in larger numbers than horseweed and green foxtail in both the no cover and forage radish treatments. As green foxtail and horseweed emergence was poor in both forage radish and no cover plots, the results may reflect poor seed rather than any effect of forage radish. In contrast to experiment 5, forage radish in experiment 6 had a stimulatory effect on pigweed and lambsquarters emergence compared to no cover treatments.

In Beltsville Experiment 6, lambsquarters and pigweed began emerging well before these weeds were germinating from the natural weed seed bank in Experiment 5. Lambsquarters was never observed to germinate in forage radish plots of Experiment 1 and pigweed began only in July to germinate in both forage radish and no cover treatments. Some of the differences observed between Beltsville Experiments 5 and 6 may have been due to mechanical weeding that was done in these plots to facilitate weed emergence counts. Differences may also have resulted from the placement of weed seeds on the soil surface in close contact with the forage radish residues that could alter the temperature and/or nutrient environment these seeds experience relative to Experiment 1.

An occasional horseweed seed (2 per m of row) did emerge in the forage radish treatment from Beltsville Experiment 6, while no horseweed seeds from the natural weed seed bank emerged in Beltsville Experiment 5. While the number of seeds emerging was so low as to be inconclusive, it is possible that the canopy of henbit present in Experiment 5 played a role in preventing the emergence of horseweed, rather than an allelopathic suppression of horseweed forage radish.

Beltsville Experiment 7: Effect of forage radish residues on weed suppression
Increasing cover crop residues did not increase weed suppression in forage radish treatments. Neither removing forage radish shoots, doubling forage radish residues, nor adding forage radish roots had any significant effect on weed suppression. Thus the results of this experiment fall short of explaining the relative contributions of forage radish roots and shoots to weed suppression, as well as the relative effect of fall competition and the release of allelochemicals from decomposing residues. These results may be due to the large edge effect in the small plots that were used as well as the orientation of the residues that were added to the plots. It may be necessary to use larger plots to detect significant differences in weed emergence due to the patchy distribution of weeds.

Beltsville Experiment 8: Effect of forage radish extracts on lettuce seed germination and growth

Forage radish plant tissue, residue, and soil extractions influenced lettuce root and shoot lengths rather than lettuce seed germination. Extracts of forage radish root and shoot tissues harvested in November inhibited lettuce root and shoot growth relative to the distilled water control. This inhibition did not occur when using extracts of forage radish residues collected in March. Extracts of soil collected below forage radish residues in March increased lettuce root and shoot length while extracts from soil samples collected below forage radish residues in May had no effect on shoot or root length.

Conclusions regarding 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 the Beltsville 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 the Mid-Atlantic region 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 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 the Mid-Atlantic.

Soil compaction reduces productivity and profitability to some degree on nearly every farm in the mid-Atlantic region. It does so mainly by limiting root penetration and elongation, thus reducing plant access to water stored in the subsoil. Subsoil compaction also promotes surface runoff and ponding after heavy rain, increasing the potential for crop damage and loss of nutrients to streams. Tap rooted Brassica cover crops may have the potential to help ameliorate soil compaction problems, especially in no-till farming, by creating root channels and altering soil structure by the addition of organic matter and protection of the soil surface. We hypothesized that roots of cash crops following a radish cover crop will more rapidly and frequently penetrate compacted soil than where a rye cover crop or no cover crop was grown. To test the above hypotheses, two types of field studies were conducted, the first imposed cover crop treatments on existing soil compaction conditions (CMREC Exp. 2 and Beltsville Exp. 3) while the second type of experiment imposed compaction treatments in a factorial combination with cover crop treatments (Beltsville Exp. 4).

Alleviation of subsoil compaction was one of the first research goals when our group began studying the Brassica cover crops. We hypothesized 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.” Documentation of possible ameliorative effects of cover crops on soil compaction presents a research challenge. The normal approach to evaluating the effect of tillage or traffic on soil compaction does not apply because the traditionally used measures of soil compaction, bulk density and penetration resistance are not expected to be influenced by the root bio-drilling mechanism hypothesized. The root channels formed by biodrilling are likely to be small enough (~ 1mm) that their presence would not be detectable by 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 on compaction alleviation in terms of water use by crops and crop rooting patterns. This makes sense because the main reason that soil compaction is a problem is that is restricts crop root growth, especially downward into subsoil layers where water and nutrients can be accessed during times of drought stress. The main advantage of improved deep rooting, then, is that it should give crops more access to subsoil moisture. Plants especially need access to subsoil water during dry, hot periods during the summer that create high transpiration demand.

Because direct root measurements in compacted soils is extremely difficult and labor intensive, our first approach to measuring the effect of biodrilling was to monitor soil water above and below the compacted layer. We hypothesized that biodrilling by cover crops would allow more roots to access the subsoil and therefore the crop would use subsoil water more rapidly than where there was no biodrilling. The situation is complicated by the likelihood that the root channels can also serve as pathways for rapid preferential flow of percolating rainwater causing the subsoil to recharge more rapidly after a rainfall event. Therefore, effective biodrilling might result in greater or smaller subsoil water content at any given time, making soil water data interpretations somewhat uncertain.

We monitored soil water content above (A horizon) and below (B Horizon) the compacted plow pan as a means of studying the “bio-drilling” effects of fall cover crops. We expected to see the A horizon water content respond rapidly to rainfall events and drying periods, but we also expect to the presence of plant residue surface mulch to moderate these fluctuations. We expected to see subsoil water depleted only during longer dry periods during which A horizon water would be mostly used up. We expected that increased crop rooting in the subsoil would result in greater use of subsoil water during such periods. We also expected that channels from biodrilling might allow more rapid recharge of subsoil water after rain events.

To monitor how cover crop influenced the ability of summer crops to obtain subsoil moisture, during 2004-2005 we installed almost 200 soil moisture sensors consisting of concentric electrodes embedded in gypsum blocks (electrical resistance blocks). We installed the soil water sensors at 15 cm (above the 25 to 40 cm deep compacted layer) and 50 cm (below the 25 to 40 cm deep compacted layer). The blocks respond to the soil water potential (or tension) by soaking up water from the soil as the soil becomes moister. As the soil dries, it draws water out from the blocks. The water in the blocks, which is effectively a saturated CaSO4 solution due to the gypsum matrix, controls the electrical conductivity or the resistance to the passage of a know current between two concentric metal mesh cylindrical electrodes embedded in the gypsum.

Wires from these electrodes which were positioned above the soil surface so they could be attached periodically to a meter to obtain soil readings indicative of the soil moisture at the two depths. We used a Delmhorst meter (KS-D1 Digitak Soil Moisture Tester) made specifically for the model GB-1 Gypsum Soil Blocks. This meter is factory calibrated to read near 95 to 98 when the soil is at field capacity (about 10 kPa tension) and near zero when the soil water tension is near the wilting coefficient (Delmhorst Instrument, 2003). When the soil was very dry, readings were as low as -30. We checked each individual block for a consistent reading under saturation. We also calibrated blocks in duplicate pots of soil material obtained from each field site in which gypsum block were used in the field experiments. The soil represented approximately the two soil layers that were monitored (15-20 cm in the A horizon and 50-55 cm in the B Horizon). A typical pair of calibration curves for the soil in Beltsville Exp. 1 relating gypsum block meter readings to gravimetrically measured soil water content were shown in the Methods section. We fit the data to polynomial functions with R2 > 0.98. However, since relative soil water is the variable of interest, we analyzed and herein present the gypsum block data as raw meter readings to avoid adding additional error and to maximize sensitivity.

The data suggest that the sensor array installed was capable of closely reflecting soil water use by the summer crops and replenishment by rain. Although conditions caused relatively little water stress during summer 2004, the gypsum resistance blocks distinguished differences in mean summer subsoil water potential among cover crop treatments at two of the three sites monitored. The gypsum block data clearly shows that, compared to the subsoil, the surface soil dried more rapidly during dry periods and wetter much more rapidly after rain events (, however cover crop effects were not easily detected, partly because of the lack of severe moisture stress and partly because of the limited number of readings that could be taken with the hand held meter.

Some difference in subsoil and surface soil water content during late summer was observed following radish compared to the no-cover crop plots, but the exceptionally ample and well distributed rainfall during most of summer 2004 did not allow for extensive depletion of subsoil moisture that might have maximized these effects. Two drought stress periods did occur, first in early July and then from late August and until substantial rain events between September 8 and 12. During these periods water was lost more rapidly from the surface soil in the radish treatment plots than in the no cover or in the rye treatment plots, probably because the radish plots had almost no water conserving mulch from either weed or cover crop residues. No significant effects were detected in the rate of subsoil water depletion.

In 2005 and 2006, we replaced most of the gypsum block sensors with Watermark granular matrix resistance sensors. Not only were the Watermark sensors more sensitive to changes in soil moisture, they could also be wired to multi-channel, data loggers that recorded hourly temperature-corrected readings directly as kPa soil water tension. This capability, combined with some extended dry periods, allowed us to obtain data that more clearly showed how summer crop soil water use was influenced by cover crop treatments.

Interesting soil ater data was obtained from the 50 cm depth of the sandy soil at CMREC Exp. 2 during the period 01 June to 16 August 2005 during which corn was growing on the plots. In early June the subsoil for all treatments was in the same range of low subsoil moisture tensions (a few erratic peaks are from one sensor which had a loose connection). Subsequently, when the weather became hot and evapotranspiration demand increased, the subsoil moisture tension increased (that is, the soil dried out) more rapidly in the forage radish treatment plots than in the rye or no-cover plots. The subsoil moisture also rebounded more dramatically in the forage radish plots after each of the four rain events, including one event ( 7/27/05) that caused no reduction of moisture tension in the rye and no-cover treatment plots. These patterns of water depletion tend to support our hypothesis of greater subsoil rooting by corn following the forage radish cover crop. In addition, it appeared that the cover crop biodrilling also opened channels by which rain water could more rapidly recharge the subsoil. The differences in soil water content could not have been due to water use by the cover crops themselves because all plots were uniformly completely recharged with water at both depths at several times during the soybean growing season.

Due to the rapid decomposition of the winter-killed residues, the surface of forage radish plots was almost devoid of residue or weed cover by late March, while rye cover crop plots were densely covered with residues all spring and summer because the rye was not killed until April and the rye residues decomposed much more slowly than did radish residues. The difference in surface residue cover is likely to have influenced the water content of the surface soil layer (0-15 cm) by decreasing evaporative losses compared to those occuring following the radish cover crop. This residue cover effect probably explains why the soil water content at 15 cm depth during the rainless, drought stressed period of July 20 to 26, 2006 differed among the cover crop treatments in the order of rye-treated plots > FR-treated plots > NC-treated plots.

During the same July 20-26 period, soil at 50 cm depth dried faster in the forage radish plots than in the no-cover or rye plots, but surface soil evaporation could not be a direct cause. Rather, we interpret the greater subsoil water depletion to indicate that during a critical time for the soybean growth, plants in forage radish-treated plots used more subsoil water than those in rye- or no cover treatment plots. During subsequent dry periods, the soil at 50 cm was drier than the measurement limit of the sensors (data not shown), but the soybeans in forage radish treatment plots showed fewer foliar water stress symptoms than those in rye or no cover treatment plots. This observation suggests that the soybean roots in the forage radish plots had access to more subsoil water deeper than 50 cm. Our preliminary conclusion is that cover cropping with forage radish has the potential to help ameliorate the effects of soil compaction on agricultural land and give crop root better access to subsoil moisture. These results also suggest that a mixed cover crop that combined both rye and forage radish might combine the benefits of both to result in the highest crop yields and least soil erosion.

Although the soil water depletion data were apparently sensitive to cover crop effects, the complex patterns of water depletion and recharge in two soil layers made it difficult to interpret the implications regarding the effects of cover crops on summer crop rooting depths. Direct measurement of crop roots would be a desirable complimentary data set. However, the measurement of plant roots in the field is notoriously difficult and intrusive. During our preliminary research prior to obtaining the project grant, we arranged to use a state of the art minirhizotron fiber-optic camera is capable of imaging roots in situ repeatedly over time. We were successful in obtaining images of soybean roots following the channels made by Brassica cover crops and these data were published in 2004. Later in 2004, we had the opportunity to collaborate with several colleagues in the purchase (using non-project funds) of a state-of-the art minirhizotron camera apparatus.

Installing the minirhizotron observation tubes in compacted soils proved very difficult, so we were limited to one or two tubes per plot in selected plots, and in most cases the depth was limited to 75 cm. While we captured thousands of root images, obtaining quantitative root data with the minirhizotron camera has proved to be quite challenging and difficult to interpret. To obtain more unequivocal data, we eventually resorted to measuring root growth directly using deep soil cores.

Therefore, we also approached the problem by using large (~9 cm diameter) soil cores and the core-break method (Bohm, 1979) to measure plant root distribution. Root counting by the core break technique gave results that were less time consuming, less variable and more repeatable than root length measurements based on images of roots taken with a minirhizotron camera or on roots washed from soil cores. With the core break root counting 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 as compared to the no cover plots. The cover crop treatments were repeated in the fall of 2005 and, during summer 2006, soybeans were grown on the plots. The soybean roots, like the corn root in the previous year, were much more numerous where the crop had been preceded by the forage radish cover crop than where either a rye cover crop or no cover crop had been used. The second greatest number of deep roots occurred in the rye cover treatments. The fewest deep roots occurred in the no-cover crop control plots. In contrast to the deep roots, shallow roots (above the plow pan) of both corn and soybean were most abundant in the rye cover crop treatment.

In 2006 we also initiated a compaction experiment in which we applied compaction treatments by driving a heavy front end loader over a loam soil near field capacity. The loader made either no pass (uncompacted), a single pass (light compaction, Wt: 1.19*104 kg, force: 7.44*104 N) or two passes, (heavy compaction, the second pass had 926 kg gravel in the loader, force: 8.02*104 N). The compaction treatments resulted in severe surface soil compaction down to about 35 cm depth and simulated the kind of compaction that harvest and manure spreading machinery often cause on commercial farms during wet weather. 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 weeds in the control plots) responded dramatically to the severe compaction levels even though the upper 8 cm had been loosened. Root and shoot biomass were both dramatically influenced. Large soil cores were taken to 60 cm depth using hydraulic equipment. These cores were utilized for core-break root counts and root washing. The compaction treatment caused all three cover crops to increase their rooting in the surface soil layer (0-8 cm) which was loosened by tillage after the compaction treatment. Unlike rye and rapeseed, forage radish roots in subsoil (30 -65 cm) were not significantly reduced by compaction. As a result, in the 30 to 55 cm layer of the compacted soil, the radish produced 2 or 3 times as many roots as did rapeseed and rye, respectively.

Because they are non-hosts for mycorrhizae, Brassicaceous plants are known to have evolved other mechanism for accessing soil phosphorus. For example, in a recent study by Zhang et al. (1997), root exudates were collected from radish and rapeseed in P-sufficient and P-deficient nutrient solutions. In radish, the dominant acids were tartaric, malic and succinic acids; the production of these increased between 15 to 60 times under P deficient conditions. In sand culture with either Ca(PO4)2 or AlPO4 radish used P from AlPO4 much better than from Ca(PO4)2, but the opposite was true for rapeseed. These results help to explain why radish is preferentially used in China on acid soils and rapeseed on calcareous soils.

In our preliminary investigations, we found that after three fall seasons with cover crops a corn-soybean rotation with Brassica species, soil test P by Mehlich 3 extract increased between 40% and 80% compared to no cover cropping. The forage radish plots (Mehlich3 =85) were significantly higher in P in the A horizon than were either the rye plots (Mehlich3 =60) or the no cover plots (Mehlich3 =48). . Below 45 cm depth all plots had the same Mehlich3 levels (5-20). The fact that the significantly increased P occurred as deep as 45 cm suggests that the increase was not due simply to vegetative pumping that left high P residue on the soil surface. We hypothesize that unique radish root exudates may have mobilized P in the upper soil layers. The increased soil test P is also due in part to the fact that the radish cover crops can accumulate between 30 and 40 kg/ha P in their biomass before freezing in winter.

We also observed that forage radish tissue concentration (median = 5.5 with values as high as 0.9) and total uptake of P greatly exceeded that of other agricultural plants, including other Brassica cover crops. These data suggest the need for further study on the potential of the forage radish to improve low phosphorus soils by making P more available. Just as important for Maryland, research should evaluate the potential of forage radish cover crops to accelerate the remediation of soils excessively high in phosphorus. In uncompacted plots in the compaction experiment, radish shots took up 36 kg P ha-1 during September and October, while rape took up 17 kg P ha-1 and rye took up 13 kg P ha-1. Therefore a forage radish cover crop may present the opportunity to harvest nearly 40 kg P/ha each year in addition to the 20 to 30 kg P removed by a crop of corn or soybeans.