Allelopathic potential of a biculture cover cropping system utilizing Fabaceae and Brassicaceae cover crops

2008 Annual Report for OS07-037

Project Type: On-Farm Research
Funds awarded in 2007: $12,840.00
Projected End Date: 12/31/2009
Region: Southern
State: Virginia
Principal Investigator:
Janet Spencer
Virginia Cooperative Extension

Allelopathic potential of a biculture cover cropping system utilizing Fabaceae and Brassicaceae cover crops


This research investigated the effects on individual cover crops and weed suppression in both monoculture and biculture cover cropping systems, as well as possible effects on transplanted tomatoes. The four systems were as follows: 1. Bare-ground control, 2. Monoculture of purple vetch, 3. Monoculture of oilseed radish, and 4. Biculture of 50% purple vetch and 50% oilseed radish.

In 2008, there were significant differences in total weed counts when individual species of weeds were compared at the Tidewater AREC location. However, there were no significant differences in total weed counts for individual species of weeds at the Dinwiddie location. At the Tidewater AREC, there were three predominant species of weeds present: carpetweed (Mollugo verticillata), yellow nutsedge (Cyperus esculentus), and grasses. Carpetweed counts between the untreated control (41.00) and the vetch monoculture (32.75) were not significantly different from one another. Likewise, weed counts in the oilseed radish monoculture (0.75) and the biculture (2.75) were not significantly different. However, when weed counts for the untreated control and the purple vetch monoculture were compared to weed totals in the oilseed radish monoculture and the biculture, the treatments that contained oilseed radish had significantly lower carpetweed counts. Significant differences in total weed counts did not occur with any of the other predominant weed species at the Tidewater AREC.

Weed data were also analyzed to determine if weed suppression properties of the cover crops diminished over time. At the Tidewater AREC, carpetweed counts were significantly higher approximately one month after the initial counts in both the untreated control and the vetch monoculture. There were no significant differences in weed numbers at different days in the radish monoculture and the biculture, indicating weed suppression up to eight weeks after incorporation of the cover crop. There were no other significant differences in weed counts over time at the Tidewater location. While it is possible that carpetweed totals were significantly lower in treatments containing oilseed radish because of allelopathy, it may actually be a result of shading. The Tidewater location experienced very warm temperatures in early spring. This caused the oilseed radish to bolt and ultimately seed out before the cover crop was incorporated into the soil. High numbers of “volunteer” oilseed radish germinated in all treatment plots that contained this cover crop. Early in its development, carpetweed is a very low growing weed and may have been shaded by the oilseed radish plants and were therefore unable to grow as well as in the treatment plots that did not have an oilseed radish weed problem.

As with total weed counts, there were no significant differences in weed species as a function of time at the Dinwiddie location.

Yield data was also collected in 2008 from tomatoes that were transplanted into the cover crop residues. Data were analyzed for total number of plants that survived to harvest, total fruit weight, and marketable fruit weight. There were no significant differences with any of these data sets at either location.

Objectives/Performance Targets

  • Determine the compatibility and weed suppression of oilseed radish and purple vetch cover crops when planted in both a mono- and bi-culture system.

    Determine the effects on tomato yields when transplanted into residues of oilseed radish and purple vetch in both a mono- and bi-culture cover cropping system.


The cover crop plots that were used for data collection in 2008 were planted in the fall of 2007. The cover crops were allowed to grow throughout the winter and early spring. Approximately two weeks before transplanting tomatoes, the cover crops were incorporated into the soil with a garden tiller. Any excess plant material not fully incorporated was allowed to remain on the soil surface. Prior to tillage, weed data were taken from each plot. Data was collected utilizing a 20- inch by 20- inch cardboard quadrat that was randomly placed within the plot. Weeds were identified, counted, and ranked according to size (< 2 inches, 2-4 inches, and > 4 inches). Two quadrat counts were taken from each treatment plot. ‘Crista’ tomatoes were transplanted at the Dinwiddie location on May 30, 2008 and at the Tidewater AREC on June 12, 2008. Just prior to transplanting, soil samples were taken from each plot to obtain soil nutrient levels, pH, and percent organic matter. Weed data were collected weekly at the Dinwiddie location for a total of eight weeks. Weed pressure at the Tidewater AREC was lower than the Dinwiddie location, thus only four sets of weed data were taken. Data were collected in the manner described above.

In October 2008, cover crop plots were again planted at the Dinwiddie and Tidewater AREC locations. Different locations on the individual farms were chosen to avoid soil borne diseases and pests, as well as high seed banks within the soil that may have occurred as a result of the previous year. Prior to planting, soil samples were taken from individual treatment plots at each location and evaluated for soil nutrient levels, pH, and percent organic matter. These cover crops will be incorporated into the soil in May 2009 and tomatoes will once again be transplanted into the cover crop residues. Data collection will follow the same protocol as described for 2008.

Impacts and Contributions/Outcomes

In May 2008, a Sustainable Vegetable Production Short Course was held at the Tidewater AREC. The course met once a week for three weeks and introduced area vegetable growers to more sustainable practices for their farming operations. Topics included soil management, cover crops, insect and disease management, and organic certification. During the cover crop portion of the course, information regarding this study was presented to the group, as well as basic information regarding cover crops and their benefits on soil life and soil physical properties.

In July 2008, two field days were held at each location of this study. The purpose of the field days was to discuss various cover cropping practices and to present information collected from the research plots throughout the summer. Demonstrations were also conducted during the field day on different planting techniques for cover crops. As a result of the short course and two field days, 15% of the participants adopted at least one sustainable production practice for their farming operation. In the summer of 2009, two additional field days will be conducted that will include information obtained through this work.

Upon completion of this project in late summer 2009, Extension publications will be generated from the collected data. The publications will be posted on the Virginia Cooperative Extension website. These Extension publications will relay the benefits of cover crops and possible effects (positive and negative) of biculture cover cropping systems.

In September 2009, a poster will be presented at the National Association of County Agricultural Agents annual meeting in Portland, Oregon. The poster will describe the work accomplished with this study.


Mike Parrish

[email protected]
Virginia Cooperative Extension
P.O. Box 399
Dinwiddie, VA 23841-0399
Office Phone: 8044694514