A Virginia study evaluating weed suppression of a biculture cover cropping system compared to a monoculture system determined that there may not be an advantage to the biculture system. Data from this study suggests that a cover cropping system that utilizes oilseed radish significantly decreases weed means. However, as evidenced with this study, the oilseed radish may pose a weed problem later in the growing season because of its inability to sufficiently winterkill in Virginia’s mild winters.
Weed management in vegetable production systems can be difficult because of the limited number of herbicides available and the degree of control necessary to maintain adequate yields (Creamer & Bennett, 1997). According to a 1986 study by Pimental & Levitan, farmers spend more than $3.6 billion annually on herbicides; however weeds still reduce yields by about 10%. An alternative option to herbicide application is the use of cover crops. Farmers incorporate cover crops into their production practices for several reasons, which include nutrient acquisition, erosion control, and weed suppression (Hill et al, 2006). Cover crops can be used in weed suppression by leaving a layer of residue on the soil that serves as organic mulch (Teasdale, 1993). When these residues remain on the soil surface, they can modify seed germination by limiting light availability, decreasing soil temperatures, and altering moisture content (Creamer et al 1996). Other types of interference can occur, which includes allelopathy. Rice (1974) defined allelopathy as harmful effects on one plant species through the release of toxic chemicals into the environment by another.
Allelopathic weed suppression by several plant families has been well documented. Creamer et al (1996) determined that rye, crimson clover (Trifolium incarnatum L.), hairy vetch (Vicia villosa Roth.), and barley (Hordeum vulgare L.) all suppressed the emergence of eastern black nightshade in a field study. They also found similar results when a mixture of these cover crops was used.
Even though cover crops offer a wide range of benefits, including weed suppression, it is important to note that negative effects from alleopathic cover crops has occurred. Putnam (1986) and Teasdale (1996) both reported results where cover crop residue has reduced establishment, growth, and yield of cash crops. In a 2005 study conducted by Ngouajio and Mennan, marketable yield of cucumber was significantly lower when planted behind hairy vetch compared to other cover crops.
While cover crops are an important and necessary component of sustainable agriculture, it is important to understand how these cover crops not only affect weed species, but cash crop species, as well. Cover crop bicultures, which consist of two cover crops planted together with the hopes of complimenting each other’s characteristics, is yet another aspect of sustainable production systems. It is necessary to understand how one cover crop will affect the other when planted in a biculture. If allelopathic effects occur on the cash crop, then there is potential for negative effects on other cover crops.
Fabacaceae (legume) and Brassicaceae are two families of cover crops that are often recommended in a sustainable production system because of their unique properties and benefits on the soil. The use of allelopathic legume cover crops is of great interest because of their ability to fix nitrogen (Hill et al 2006). Several studies have shown the allelopathic potential of legume cover crops (Njoujio & Mennan 2005; Teasdale 1996; Hutchinson & McGiffen 2000). One such group of legume cover crops that have shown strong allelopathic capabilities is the vetches (Njouajio & Mennan 2005, White et al 1989; Hill et al 2006). Vetches, which include hairy vetch, purple vetch, and lana vetch, perform well over a wide range of soils, can fix over 100 pounds of nitrogen per acre and release about half of it to the following cash crop (Schonbeck & Morse 2006). They also make soil phosphorus more available and provide habitats for beneficial insects.
Cover crops in the brassica family, which includes daikon, oilseed, and fodder radishes, are often chosen as cover crops because they are deep rooted crops that can help open subsoil hardpan (Schonbeck & Morse 2006). This characteristic is especially important in areas where traditional tillage has left a layer of hard soil just under the disturbed soil area. Other advantages include conservation of soluble nitrogen and rapid canopy closure to help prevent weed seed germination (Schonbeck & Morse 2006). These cover crops are also known to have strongly allelopathic root exudates, which can leave behind a weed-free seedbed after winterkill.
Planting a brassica and legume cover crop as a biculture could be very beneficial. In areas where traditional agricultural practices, such as mold-board plowing, have left a hardpan under the soil, the brassica cover crop could help break-up this layer. Incorporating a legume cover crop, that will help fix nitrogen, could prove to be very beneficial, especially in areas where the soil contains very low organic matter. Weed suppression could also be increased by incorporating the two families, instead of planting a monoculture cover crop system. However, it is important to understand how these crops will not only affect one another, but the cash crops that would follow behind this system. If increased weed suppression occurred from the biculture system, it stands to reason that cash crops could be negatively affected, as well. It is necessary to conduct field studies examining this system before it is recommended to growers.
The objectives of this project were to determine: 1. compatibility and weed suppression of a brassica and legume cover crop in a biculture production system and 2. effects on a cash crop when brassica and legume cover crops are planted as a mono- and biculture.
Trials were conducted at the Hare Road Research Farm, which is part of the Tidewater Agricultural Research and Extension Center (Suffolk, VA) and in Dinwiddie County, VA on the farm of Todd Adams. All research plots were identical in terms of materials and methods. Soil tests were taken at the beginning and end of each growing season for both objectives. An individual soil test was taken in each plot.
1. Compatibility and weed suppression of brassica and legume cover crop in a biculture production system
Oilseed radish (Raphanus sativus) and purple vetch (Vicia atropurpurea) were chosen for this study because of similar planting dates and winterkill temperatures. Field design was a randomized complete block design with four replications. Individual plots measured 6.0 feet by 15.0 feet. A 5.0 ft border completely surrounded each treatment plot to avoid interference from other treatments. The treatments were as follows: 1. Bareground control, 2. Purple vetch monoculture, 3. Oilseed radish monoculture, and 4. Biculture (50-50 mix of both cover crops). Cover crop seed were sown at the highest recommended rate with an Earthway® “EV-N-SPRED” broadcast seed spreader. The rate for the purple vetch monoculture was 80 lbs/A and the rate for the oilseed radish monoculture was 20 lbs/A. Cover crop planting occurred on 13-Aug-07 and 15-Oct-08 in Dinwiddie and on 11-Sept-07 and 16-Oct-08 in Suffolk. Once the seed was broadcast over the plot, it was incorporated into the top five centimeters of soil. Approximately three weeks after planting, ground coverage percentages of purple vetch, oilseed radish, and total weeds were taken from each plot at each location. These data were subjected to ANOVA and means separated using Fisher’s Protected LSD test.
In year 1, broadleaf and grass weeds were sampled on a weekly basis at each location. A 20- by 20- inch quadrat was placed randomly within each treatment plot and the weeds inside the quadrat were counted. Two samples were taken from each treatment plot. The weed data were placed into one of the following categories: 1. <2 inches, 2. 2-4 inches, 3. > 4 inches. In year 2, weed data were collected in the same manner, however, sampling occurred once every two weeks. These data were subjected to ANOVA and means were separated using Fisher’s Protected LSD test. A t-test was also used to determine any significant differences in weed means over time.
2. Determine effects on fresh-market tomato yields when transplanted behind brassica and legume cover crops.
In the spring following cover crop planting, fresh-markets tomatoes were transplanted into the same plots as mentioned in objective 1. Seeds for the transplants were grown in the greenhouse located at the Tidewater AREC. Two rows of tomato were transplanted into each plot. Spacing followed the Commercial Vegetable Production Guide for Virginia (VCE pub. 456-420), as did all aspects of production for that particular crop, with the exception of herbicides. No herbicides were applied to the research plots. Vine-ripened tomatoes were harvested from each location in year 1. Tomatoes were transplanted in year 2; however, because of weather conditions, diseases were difficult to control in both locations and tomato data was not collected. Harvest data (total yield, marketable yield, average fruit size) were subjected to ANOVA and means separated using Fisher’s Protected LSD test.
Ground Coverage Percentages
In Dinwiddie, there were significant differences noted in purple vetch percentages between the monoculture(54.38) and the biculture(28.13) (Table 1). Likewise, the same results occurred in Suffolk (Table 2). Percent ground coverage was significantly higher within the purple vetch monoculture (62.38) when compared to the biculture (35.63). There were also significant differences noted at the Dinwiddie location for ground coverage for the oilseed radish when percentages were compared among treatments (Table 1). The oilseed radish monoculture contained a significantly higher percentage when compared to the biculture, 74.38 and 60.23 respectively. However, there were no significant differences noted among the oilseed radish at the Suffolk location.
When weed percentages were compared among the four treatments, significant differences were noted at both locations (Table 1 & 2). Weed percentages were significantly higher in the bareground control when compared to the oilseed radish monoculture and the biculture. At the Suffolk location, weed percentages within the vetch monoculture (9.00) were also significantly different from those that occurred in the bareground control (37.13)
While there does appear to be some type of interaction occurring in the treatments that contain the oilseed radish, it does not necessarily mean negative effects are occurring on the purple vetch as a direct result from a biculture planting with oilseed radish. Both the oilseed radish and the purple vetch exhibited lower ground coverage percentages in the biculture. Percentage of cover crops in the biculture may be lower because the total amount of seed for each crop was lower than what was broadcast in the monoculture plots. Likewise, the lower weed percentages in the plots that contain oilseed radish may be due to allelopathy, however it could also be attributed to an inability to outcompete with the oilseed radish. The oilseed radish grows very rapidly and can quickly take over an area, thus not allowing the weeds to become established.
When total broadleaf means were compared from both years at the Dinwiddie location, there were no significant differences. However, there were significant differences noted at the Suffolk location. The radish monoculture (177.63) and the biculture treatments (147.00) exhibited significantly higher weed means than the bareground control and the vetch monoculture (Figure 1). This is exactly the opposite of what was expected to occur. However, it is important to note that within the oilseed radish monoculture and biculture 76.21% and 69.64%, respectively, of the broadleaf weeds were volunteer cover crops. Most parts of Southeast Virginia experienced an extremely mild winter during 2008. The winter temperatures in Suffolk were not cold enough to sufficiently winterkill the cover crops. The surviving cover crops flowered out in the spring before they could be cut and tilled, thus leading to a weed problem.
When grass means were compared, there were no significant differences noted at the Suffolk location, however, Dinwiddie’s grass means were significantly lower in the oilseed radish monoculture (38.00) and the biculture (42.88) when compared to the other treatments (Figure 2).
When broadleaf and grass means were compared over time, there were no significant differences noted at either location. However, carpetweed occurred very heavily in Suffolk in 2008. Significant differences were noted in carpetweed means when compared over time in Suffolk (Figure 3). Carpetweed means 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 have been a result of shading from “volunteer” oilseed radish.
Effects on fresh-market yield
Yield data for fresh-market tomatoes was only collected in 2008 due to heavy tomato disease pressure that occurred in 2009. There were no significant differences noted in either location for total yield, marketable yield, or average fruit size. Several studies have reported negative impacts on cash crops from allelopathic cover crops; however most of these studies were performed on direct seeded vegetables (Putnam 1986; Teasdale 1996; Ngouajio & Mennan 2005). Tomatoes may not be susceptible to allelopathic chemicals within the soil because they are transplanted and not direct seeded.
Educational & Outreach Activities
A cover crop demonstration was held at the Suffolk location in 2007. Participants were introduced to several different cover crops, as well as the ones used in this study. Hands-on demonstrations were provided for using a direct-seeder or a broadcast seeder to plant cover crops in small areas. Plots were established approximately two months prior to the demonstration with the cover crops used in this study to allow participants first- hand experience with these particular crops. One highlight of the demonstration was the oil-seed radish, which produced many radishes that were over 1.0 ft in length. This furthered the discussion on how certain cover crops can be used to break-up hardpans in the soil.
In May of 2008, a “Sustainable Vegetable Production Short Course” was held. The course met once a week for three weeks and highlighted such areas of sustainability as building organic matter, composting, cover crops, organic insect & disease pest management, and organic certification. Preliminary findings from this study were presented to the group, who responded with much interest. One grower in particular plans to look at cover crops within the mustard family as a way to suppress weeds.
Two field days also occurred in 2008, one in Suffolk and one in Dinwiddie. These field days occurred in August upon completion of the first year of work within this study. Information presented included background of the project, weed suppression data, and any affects seen on tomato yield.
In July 2009, a paper was prepared from the data collected within the scope of this study and presented at the 31st Southern Conservation Agricultural Systems Conference held in Melfa, VA. A poster of the data was also prepared and presented at the National Association of County Agricultural Agents in Portland, Oregon in September.
Although the two cover crops used in this study may not be suitable for use in SE Virginia because of their inability to sufficiently winterkill in our climate, the work accomplished has been used to encourage many positive conversations with local vegetable growers about sustainable vegetable production. During a 2009 SE Virginia vegetable advisory panel meeting, increasing farm input prices and weed suppression were identified as two major issues facing vegetable production in our area. When these panel members were asked if they would consider using cover crops as a means to address these issues, they were genuinely interested. Future on-farm work with local vegetable growers will occur to educate local growers about sustainable practices, as well as using cover crops to suppress weeds and fix nitrogen.
As mentioned before, the two cover crops used in this study did not sufficiently winterkill, thus leading to a weed problem within the treatment plots. Therefore, before these two cover crops could be recommended to growers in the Southeast, more research would need to be done to determine how to prevent cover crop weed issues. Through the numerous outreach events held as a direct result of this study, growers have gained an interest in the use of cover crops to suppress weeds. Many growers were unaware that allelopathic properties existed within plants and that these characteristics can be used to our advantage. Several have expressed an interest in on-farm work and the use of cover crops on their farms in 2010. To date, over 100 growers were reached as a result of this project. In addition, over 1000 Extension professionals across the country were reached.
Areas needing additional study
Additional work in the Southeast needs to be done regarding allelopathic cover crops. Even though the oilseed radish did not sufficiently winterkill, perhaps other management techniques could be researched to develop a means to prevent weed issues with the cover crops. Other biculture cover cropping systems also need to be researched.