This series of experiments answers a few important questions regarding velvetbean’s effectiveness as a suppressor of some weeds and possibly fungi, as well as the potential of ‘GA Bush’ velvetbean and sunn hemp for improving the organic matter content and fertility of soils in two important agricultural areas of Georgia. The data collected and presented also provides a reference for farmers in the Southeast region regarding planting dates and specific effects that can be anticipated when using velvetbean as a cover crop and as part of a comprehensive crop rotation plan. Significant biomass and nutrient accumulation was seen when velvetbean and sunn hemp were grown for 60- 120 days in the two locations, with more variability as days after planting increases. In Watkinsville, it may be best to plant ‘GA Bush’ velvetbean after April, unless a grower has the ability to grow the crop for 120 days. The data began to show a trend of increasing biomass with planting after April, however, this is through extrapolation of existing data due to damage to the crop from pests that terminated the project before all data collection was complete. This shows a need for some type of protection of ‘GA Bush’ velvetbean from deer and other pests when grown in the Watkinsville area.
Sunn hemp also accumulated substantial biomass and nutrients after 60 DAP, however, biomass accumulation began to level off for the later plantings (June and July) harvested in the late Fall. Allowing 90 days for growing sunn hemp was optimal for both locations.
Velvetbean proved to be effective in suppressing the growth and germination of the common horticultural weeds of the Southeast that were tested in this series of experiments. Though, the stirred extract of velvetbean seed produced better weed suppression results, a technique that sterilizes the extract without destroying the allelopathic effects needs to be explored due to the considerable fungi that contaminated the petri dishes in this portion of the study. The biomass residue collected from the April planting in Watkinsville also exhibited weed suppression, although better inhibition was seen for the biomass harvested 60 DAP than for that harvested 90 DAP for redroot pigweed and for sicklepod. For Florida beggarweed and crabgrass significant inhibition of weed growth was seen for the residue harvested 90 DAP. The best suppression was seen using the stirred velvetbean seed extract.
Velvetbean may have some suppressive effect on P. capsicii, however insignificant to no inhibition was seen for R. solani. This series of experiments begins to explore ‘GA Bush’ velvetbean as a biological control of weeds and fungi and shows that ‘GA Bush’ velvetbean may be a good alternative for farmers interested in growing a summer cover crop that needs only a short time to accumulate significant biomass and nutrients and at the same time will help inhibit weed growth through allelopathy and competition.
For centuries, people have been growing cover crops as part of an agricultural system to improve the fertility and structural composition of their soil. Today, cover crops are still grown as part of a total agricultural system that promotes sustainability. Some of the long-term benefits obtained from the use of cover crops include weed suppression through competition or allelopathy, shorter fallow periods, possible insect control through rotation, and less monetary input through the decreased use of herbicides, pesticides, and water. Velvetbean (Mucuna spp.) and sunn hemp (Crotalaria juncea) are two tropical legumes in the Fabaceae family that have been used for many years in agriculture and may fit well in a sustainable vegetable production system in the Southeastern Region of the U.S. Traditionally used in agricultural systems in places such as Hawaii, the Philipines, and Meso-America, velvetbean was also once used in the early 1800s in the Southeastern United States. Here it was used as a green manure in orange orchards and in rotation with cotton and corn, because it helped lower external inputs and created a more sustainable system. Sunn hemp (Crotalaria spp.), also in the Fabaceae family, is one of the earliest, most distinctly named fibers of India; and one of the most widely grown green manure crops throughout the tropics (Cook et al., 1996). Grown usually in rotation with several different crop species, sunn hemp shows promise as a green manure/cover crop for the Southeast region due to its high N content, fast-growing habit and ability to prevent soil erosion. In recent years little research has been done on sunn hemp and velvetbean as cover crops in the United States. This results in a lack of information regarding when to grow and when to harvest sunn hemp and velvetbean for the most nutrients and biomass production as part of a sustainable vegetable production system in the United States. Though some research exists on velvetbean as a weed suppressor, this study examined the allelopathy in velvetbean seed and above-ground biomass. This study also sought to examine velvetbean’s effect or non-effect on common horticultural fungi, since very little research is published on the topic.
One objective of this series of experiments was to analyze the biomass and nutrient accumulation of ‘GA Bush’ velvetbean and sunn hemp (Crotalaria juncea) through field experiments. This part of the study included growing Sunn hemp (Crotalaria juncea) and ‘GA Bush’ Velvetbean (Mucuna pruriens) to collect the previously mentioned data on these somewhat common cover crops from the Fabacea family. This experiment was performed Spring/Summer/Fall of 2002. Lab experiments utilizing the residue from the field experiments will also help to determine velvetbean’s allelopathic affects toward four common southern horticultural weeds (sicklepod, redroot pigweed, crabgrass, and beggarweed). This study also examined velvetbean seed as a suppressor as weeds. These experiments began in May of 2002 and were completed May 2003. Another objective of this series of experiments was to determine any suppression velvetbean may have toward Phytophthora capsici and Rhizoctonia solanii, common southern horticultural fungi. These experiments began in summer 2002 and were completed May 2003.
Biomass Accumulation and Nutrient Cycling
A biomass accumulation and nutrient content study of Velvetbean (Mucuna sp.) and Sunn hemp (Crotoleria juncea) took place during the growing season in Watkinsville and in Tifton, GA. In Watkinsville, one plot measuring 10’ x 40’ of each cover crop was planted in a block design in April, May, June and July of 2002. The layout was essentially the same in Tifton, except the dimensions of the plots were slightly different due to the planting equipment available. Velvetbean was planted at a rate of 2.5 seed/foot at a depth of 1.5 inches and Sunn hemp was planted at a rate of 9-12 seed/foot (12 lbs./acre) and at a depth of .75 inch using a standard planter set for 30” row widths in Watkinsville and 36” row widths in Tifton. Each plot contained four treatments of different harvest dates (30, 60, 90, and 120 D.A.P.). Fifteen soil core samples were collected per plot at a 4in. depth in-row on each harvest date. The core samples are to be analyzed in the near future, but are not included in this report. Four above ground biomass samples per treatment were taken randomly from three linear feet per two rows for a total of 6 linear feet per replicate sampled within each plot. Fresh/green weight was recorded, as well, as oven-dried weight after 72 hours in a 150 degree F oven. The previous study was conducted at the USDA experiment station on the Piedmont in Watkinsville, GA as well as at the Coastal Plain USDA experiment station in Tifton, GA.
For the tissue nutrient analysis portion of this study, 1-2 plants were harvested as above at each sampling, these plants were dried and ground and analyzed for nutrient content at the University of Georgia Dept. of Horticulture Soil Testing Laboratory, Tifton campus.
Weed Suppression Experiments
The following series of experiments began in May 2002 and were concluded May 2003. Lab experiments were conducted using the Richards test standards and other experimental methods based on novel indexing of Richards’ function (Lehle and Putnam 1982; Richards 1959). The materials and methods used by Lehle and Putnam in their study of the quantification of the allelopathic potential of sorghum residues were also included in this portion of the study (Lehle and Putnam 1982). The materials and methods used in this section also include those used by Fujii in his experiments to assess the allelopathic activities of Mucuna pruriens (1999). The weeds tested in our study include redroot pigweed (Amaranthus retroflexus), Florida beggarweed (Desmodium tortuosum), sicklepod (Senna obtusifolia), and crabgrass (Digitaria sanguinalis). An aqueous extract was prepared with two separate parts of the velvetbean plant (the bean and the whole above-ground biomass). This will help in the identification of which part of the plant contains the highest concentration of allelopathic compounds particularly effective in the suppression of the Southern weeds tested.
For this portion of the study, treatments included whole velvetbean seed autoclaved for 45 minutes at 121 degrees C with a ratio of 10 g of velvetbean seed (including speckled, white, and ‘GA Bush’) to 100 ml distilled H2O. For the other extraction method, velvetbean seed were ground in a Wiley mill to pass through a 40-mesh screen. Five grams of each velvetbean seed type were stirred with 50 ml of water overnight at 4 degrees Celsius. Simple aqueous extracts of the whole above-ground dried plant residue from the Watkinsville plots (including the April planting harvested 60 and 90 DAP) were made using 100 ml distilled water and 1 gram of plant residue according to Fujii (1999). This mixture was then placed in a mechanical shaker for 2 minutes to complete the transfer of chemicals from the residue to the water.
The resulting extracts were each filtered through Whatman No. 4 filter paper. Two ml of each resulting solution were placed individually on Whatman No. 1 filter paper set in 9 cm petri dishes. Ten weed seeds (of the same species) were placed in the petri dishes with the saturated filter paper having 4 replications per treatment including distilled water as the control. After 96 hours of incubation in the dark at 25 degrees Celsius the dishes were removed and the number of seeds germinated counted, the hypocotyls or radical growth (whichever was greater) were measured (Fujii 1999; Lehle and Putnam 1982).
Pathogen Experiment I.
For this experiment, ‘GA Bush,’ speckled, and white velvetbean seed were surface disinfected and cut vertically. The seed were then placed cut side down and deeply into potato dextrose agar containing an anti-bacterial. Seventy two hours later the petri dishes containing the seed and agar were inoculated with 3 mm2 cuts of active Rhizoctonia solani and Phytopthora capsicii. A week later the dishes were visually inspected for signs inhibition of growth, particularly near the cut seed. The seed coats were also examined using this method.
Pathogen Experiment II.
This experiment was done in the lab with the following methods. Ground velvetbean seed were added to potato-dextrose broth in a dilution sequence, with 3 grams of bacto agar before auto-claving. Three dilutions were used; 100,000 ppm, 10,000 ppm, and 1,000 ppm of each type of velvetbean (‘GA Bush,’ speckled, and white) extract to water. The same dilution series was used with pharmaceutical L-Dopa, with the control being no velvetbean or L-Dopa. Five replications of each dilution were used. The Petri dishes containing the agar with velvetbean extracts were then innoculated with a 3 mm2 cut of active Rhizoctonia solani and Phytopthora capsicii. The growth of each fungus was then measured to evaluate the rate and extent of its growth.
Pathogen Experiment III.
The methods followed for the greenhouse pathogen experiment come from Williams-Woodward et al. (1997). Methods included growing a strain of P. capsici on a 70:30 cornmeal/sand mixture that had been autoclaved at 121degrees Celsius for 30 minutes. R. solani was grown on barley oats that had been soaked for 1 hour and then autoclaved at 121 degrees Celsius for 30 minutes two times. Premier Pro-Mix™ containing peat moss, perlite, vermiculite, dolomitic and calcitic limestone, and a wetting agent, was used to grow velvetbean in plastic containers (6 x 25 cm; Deepotst™, Steuwe and Sons, Corvallis, OR). Five replications of four treatments were used: velvetbean inoculated with R. solani, velvetbean inoculated with P. capsici, velvetbean without fungi (control I), and no velvetbean inoculated with each fungi (control II). Roughly 6 W.A.P. the velvetbean above- and below-ground biomass were evaluated. The velvetbean was then chopped with scissors and mixed into the soil; which was placed back into its original pot. A dixie hybrid of squash was sown in the soil and residue mixture inoculated individually with P. capsici and R. solani. At 6 W.A.P. each squash plant was weighed per above ground, root, and fruit biomass. This experiment was duplicated simultaneously on opposite ends of one greenhouse in Athens, GA.
Biomass Accumulation and Nutrient Cycling
For in depth results and discussion on the biomass study please refer to ‘GA Bush’ Velvetbean (Mucuna pruriens) and Sunn Hemp (Crotalaria juncea): Biomass Accumulation and Nutrient Content (Martini, 2004). Although soil samples were taken, these need to be analyzed for nutrients. Essentially, in Watksinville, GA velvetbean may perform best when planted after April and allowed to grow 60 – 120 days, with dry weight biomass accumulation ranging from 3.0 t acre-1 – 6.6 t acre-1.(maximum dry weight biomass was accumulated in the April planting harvested 120 DAP). In Watkinsivlle, the velvetbean suffered damage from deer browsing in September and either the fall army worm or the velvetbean caterpillar in October. The July planting appeared to suffer severely stunted growth due to what is suspected to be a virus. For Tifton, GA, maximum dry weight of velvetbean was accumulated in the April planting harvested 120 DAP (12.0 t acre-1), although this was similar to the dry matter accumulated in the May and July plantings. The June planting did not perform as well, most likely due to a suspected virus which stunted its growth.
Weed Suppression Experiments
The growth of all of the weeds tested was significantly less with autoclaved velvetbean extract than with water as a control (Figures 1- 3). The germination of the weed seed treated with velvetbean extract that had been stirred overnight was also lower than for the control treatment. The only treatment where the water treatment produced more growth was in crabgrass seed treated with white and bush velvetbeans. This might be an effect of considerable fungi contamination in the petri dishes. More fungi might have been a result of the stirring action that brings out the starches in the velvetbean seed, as well as a lack of autoclaving or sterilizing effectively since the seed were ground (Figures 4 – 6).
For the extract prepared from above ground field residue of velvetbean grown in Tifton, Significant suppression of weed seed germination was seen for the April planting residue harvested 60 DAP. The results for the residue planted in April and harvested 90 DAP, was not significantly different for the redroot pigweed or the sicklepod versus water as the control. However, for the florida beggarweed and the crabgrass, significant weed seed germination suppression was seen when the extract was tested against water (Figures 7 & 8).
Pathogen Suppression Experiments
Pathogen Experiment I.
The plates were visually analyzed. Complete colonization of the seed and plate was the result. The velvetbean seed had no effect on the fungi tested, including lack of any sort of ring of inhibition around the seed or the seed coat. This data suggests untreated velvetbean seed does not exude a substance inhibiting growth of P. capsicii and R. solani.
Pathogen Experiment II.
For this section of the study, plates were measured using two measurements across the area covered with fungi and an average of the two measurements was taken. While for the plates infected with Rhizoctonia solani, no inhibition was seen, all of the plates inoculated with P. capsicii showed some inhibition at 10,000 ppm and 100,000 ppm velvetbean seed extract to agar. This data suggests velvetbean seed that has been autoclaved may provide some inhibition of the growth of P. capsicii, but not R. solani.
Pathogen Experiment III.
In this section of the study none of the velvetbean seed planted in pots inoculated with R. solani germinated. When velvetbean plants were weighed before planting squash, the velvetbean seed sowed in the pots inoculated with P. capsicii produced significantly less velvetbean biomass than the control pots. However, also in both trials, the control plants performed significantly better than the plants grown in pots inoculated with fungi. In trial one, more squash biomass was produced (including fruit, above ground biomass, and roots) in the R. solani infected pots than in the P. capsicii infected pots. This data suggests that some inhibition of R. solani may be seen in pots planted with velvetbean at the time of inoculation, however, further related studies need to be done to answer this question.
Educational & Outreach Activities
‘GA Bush’ Velvetbean (Mucuna pruriens) and Sunn Hemp (Crotalaria juncea): Biomass Accumulation and Nutrient Content (Martini, 2004)
A thesis submitted and accepted for partial fulfillment of requirements for the degree M.S. in the Department of Horticulture, School of Agricultural and Environmental Sciences, University of Georiga, Athens, GA
Biomass Accumulation of ‘GA Bush’ Velvetbean on the Piedmont and the Coastal Plain of Georgia (Martini et al., 2004)
A paper submitted to the Southern Conservation Tillage Conference in Raleigh, N.C. June 2004
These studies will provide an idea of the effectiveness of Velvetbean as a suppressor of common southeast horticultural weeds, as well as biomass accumulation and nutrient cycling data on Velvetbean and Sunn hemp as grown in the southeast. Roughly, the most effective planting dates and harvest dates will be determined (through analysis of collected data) for maximum organic matter input, as well as maximum nitrogen input from each cover crop. Weed control was determined visually and noted at each planting date and harvest date. The analyzed data will help determine how each cover crop can most effectively fit into a farming rotation for the southeast. Velvetbean (Mucuna pruriens) shows great promise as a source of L-Dopa (L-3,4-dihidroxyphenylalanine), a precursor of the neurotransmitter dopamine. It proves to be effective as a treatment of Parkinson’s disease and continues to be the subject of medical research today. Mucuna pruriens is also found in health food stores as a natural remedy for depression. Hence, it appears velvetbean may be an economically viable crop in many ways when grown in rotation with a vegetable crop, by providing an economically viable cover crop that can be used in a short fallow rotation with vegetable and other cover crops. Sunn hemp also provides a good summer alternative as a short fallow cover crop that accumulates significant biomass and nutrients to improve soil structure and fertility.
Areas needing additional study
In this study, velvetbean appeared to be effected by a virus when planted in June in Tifton, GA and in July in Watkinsville, GA. Additional studies need to be done to determine if there are particular months for planting during the growing season that should be avoided in order to deter viral infection.