A System Approach for Improved Integration of Green Manure in Commercial Vegetable Production Systems

Final Report for LS02-140

Project Type: Research and Education
Funds awarded in 2002: $171,800.00
Projected End Date: 12/31/2005
Region: Southern
State: Florida
Principal Investigator:
Johannes Scholberg
Agronomy Department, University of Florida
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Project Information


We determined the benefits of green manures to increase yields and/or to reduce N-fertilizer rates at Tifton (S-GA), Citra (N-FLA), and Boynton Beach (S-FLA). Sunn hemp performed well at all three locations and it accumulated 120-160 kg N/ha. However, most of this N was lost within 2 wks so it should be followed by a commercial crop or by crimson clover (Tifton) or a rye/vetch mix (Citra). Use of green manures reduced crop N requirements by 25-50%, increased potential yields and cost-benefit ratio of a CSA operation. They also increased soil quality while suppressing weeds, lesion and ring nematodes.

Project Objectives:

Vegetable cropping systems have experienced some of the highest levels of diversity loss. We designed innovative cover crop-based vegetable production systems specific to the southeastern US by combining a selection of summer and winter legumes with minimum tillage practices. Our main objective was to complement existing information on improved use of cover crops as a nitrogen source for commercial vegetable production systems and to integrate this information in web-based nutrient management and information tools for organic amendments. In order to achieve this objective we aimed to strengthen regional collaborative efforts between scientists and participants from the Univ. of Florida, Florida A&M Univ., the Univ. of Georgia, farm-based research organizations, and regional growers groups. Our overall goal was to develop a regional group of scientists and innovative farmers who are committed to develop sustainable production practices and guidelines for both conventional and organic farmers.

Short-term objectives:

A1) Determine which (combination of) cover crop(s) when used with minimum tillage will result in optimal nitrogen supply to subsequent vegetable crops;

A2) Assess the amount of supplemental N fertilizer required for optimal yields for cover crop based production systems and compare their yields with conventional systems;

A3) Determine nitrogen uptake efficiencies and N leaching for cover crop-based cropping systems in comparison with conventional vegetable cropping systems; and

A4) Develop a regional research and outreach program for improved integration of cover crops in commercial vegetable systems in close collaboration with local growers.

Long-term objective:

B1) Evaluate the long-term effects of included treatment combinations on soil quality, crop disease levels and abundance and diversity of weed, arthropod and nematode populations;

B2) Improve the exchange and integration of information on the use of cover crops in
commercial production systems in the SE region;

B3) Development of web-based nutrient management systems that will allow producers to make more efficient use of organic nutrient sources such as cover crops; and

B4) Use of the Decision Support System for Agrotechnology Transfer (DSSAT) model for risk assessment (environmental and economical) of above cropping systems for a number of farming systems and locations throughout the southeastern US.


The last century of American agriculture has been characterized by a shift from highly diversified low-input systems to highly specialized cropping systems that greatly depend on external non-renewable resources. Dramatic increases in farm size, along with an erosion of farm and crop diversity, has resulted in degraded agroecosystems that are extremely vulnerable to perturbances associated with urbanization, climate change, and volatile global markets. Designing agricultural systems that mimic or encompass natural ecosystems will improve their resilience, inherent diversity, and sustainability in rapidly changing production environments.

With a production area of 420,000 acres and a crop value of 2.4 billion dollars, vegetable production is one of the most important agricultural activities in both Georgia and Florida. Many soils in the southeastern region are low in organic matter and have poor water and nutrient holding capacities. Conventional vegetable cropping systems therefore require continuous application of large amounts of external nutrients. Cost-effective and environmentally sound use of crop nutrients constitutes a major concern for both producers and consumers. Due to high transportation cost of organic amendments, producers mainly depend on inorganic fertilizers to maintain soil fertility and crop productivity. Excessive use of agrochemicals and/or non-renewable resources such as inorganic fertilizers and their negative impact on biodiversity, environmental quality, and food safety has resulted in increased interest in sustainable and/or organic production systems.

Legumes fix their own nitrogen and can correct phosphorus imbalances typically associated with excess applications of animal waste products. Nutrient release from crop residues depends on a large number of interactive factors, such as soil temperature, residue composition and soil water content. Green manure crops may not always supply adequate N to meet actual crop nutrient requirements of subsequent vegetable crops, and additional inorganic fertilizer may be required to prevent yield reductions associated with N immobilization. Current guidelines provide only very crude estimates as to what fraction of nutrients from organic materials will be available for uptake by subsequent crops. However, improved integration of green manure crops in vegetable cropping systems will require more precise and detailed information on their nutrient release over time for specific conditions.

During the past four years we have, in close collaboration with extension specialists and growers’ groups, designed and evaluated green manure and minimum tillage-based vegetable cropping systems that can provide economically viable alternatives for existing conventional systems. Studies were implemented at locations throughout Florida and S-Georgia for different soil types, climatic conditions, and vegetable production systems. We have evaluated the interactive effects of different combinations of green manure crops, inorganic fertilizer, and minimum tillage on N availability, potential N leaching, soil quality, and subsequent yields and returns of vegetable crops differing in N uptake dynamics.


Click linked name(s) to expand
  • Odemari Mbuya
  • Robert McSorley
  • Marty Mesh
  • Sharad Phatak
  • Nancy Roe


Materials and methods:

Research was conducted at field sites in Tifton (Location 1, South Georgia), Citra (Location 2, North Florida), and on a Community Supported Agriculture (CSA) operation in Boynton Beach (Location 3, south Florida). These sites represent a range of differential soil and climatic conditions. We evaluated the benefits of summer cover crops (CC) with the main emphasis being on sunn hemp (SH which was tested at locations 1-3). Although sunn hemp may accumulate up to 280 kg N ha-1 in 12-16 weeks, at more northern locations sunn hemp will senesce 3-4 months prior to the planting of a subsequent spring vegetable crop and a substantial N losses may occur. Therefore, a subsequent cold-hardy CC, such as winter rye, lupine, vetch, or clover, was planted into standing sunn hemp residues using no-till to minimize such N losses and/or to fix additional N during the early spring.

At the two more northern locations, CC were planted during the fall/winter season, where as in S-Florida, we only included sunn hemp as summer CC crop. Winter CC included crimson clover (location 1), cahaba white vetch (location 1+2), lupine (location 2), and rye + vetch (location 2). At Tifton and Citra, we contrasted summer cover crops (S) vs summer fallows (F) followed by either a winter cover crop (W) and/or winter fallow (F) followed by spring-grown sweet corn (C) resulting in four main crop rotation systems: 1) S-W-C; 2) S-F-C; 3) F-W-C; and 4) F-F-C (conventional) control. In order to better evaluate N availability we compared different combinations of cover crops (CC), fallow, and inorganic N-sources. At the two more northern locations, CC were planted during the fall/winter season. Via on-farm studies in South Florida we also investigated how pepper and tomato differed in terms of N uptake patterns and requirements. By including double cropping systems (S-Florida) and by continuing experiments for a minimum of three years we also attempted to evaluate the long-term nutrient dynamics and residual effects of CC on crop N requirements and soil quality.

The crop rotation at northern locations started with sunn hemp planted in mid August. In November, when the sunn hemp stopped growing, half of the sunn hemp plots were planted with a winter cover crop using minimum tillage in order to retain nutrients and/or to provide additional nutrients during the winter season. Sweet corn was amended with 0, 67, and 133 kg N/ha for CC-based systems vs 0, 67, 133, 200, and 267 kg N/ha for conventional treatments, which allowed us to develop a general fertilizer curve for sweet corn for each cropping season. In South Florida, sunn hemp was planted in June/July and mowed within 7-8 weeks to prevent it from developing overly thick stems which can interfere with bed formation and may also damage the plastic mulch. The land was then disked to incorporate the sunn hemp residue, and rototilled since this is required for this specific production system. Winter cover crops were replaced by either tomato or pepper grown on raised mulched beds with drip irrigation. All fertilizer was banded and applied preplant.

During the first year pepper and tomato were amended with 0, 75, vs 150 kg N/ha of the recommended N rate and yield was compared to conventional plots (no sunn hemp and 225 kg N/ha). These crops were harvested from the end of December to the end of January and sweet corn was planted in the existing beds during the beginning /middle of February which was amended with either 67 vs 100% of the recommended N rate. During the next two years we maintained the overall crop rotation but N rates for tomato and pepper were 0, 112 and 224 kg N/ha for both conventional and sunn hemp-based systems while all corn treatments received 112 kg N/ha to determine if N benefits from sunnhemp extended over time. All treatments were replicated four times within a complete randomized block design that fitted within a production block. Field plots consisted of 5 beds of 15 m and there were a total of 48 plots.

Field plots for northern locations were 9m x 7.5m and the trial included a total of 60 plots separated by 7.5-15 ft wide alleys. All treatments were replicated four times within a complete randomized block design and the field study occupied 2-3 acres. During the spring all plots were planted with sweet corn using zero tillage. To avoid “edge” effects, crop yields were measured in the 3m x 6 m central plot section. Since continuous cultivation of Lupine at Citra was expected to promote diseases, we alternated it with cahaba white vetch, and hairy vetch/rye mix during subsequent years. Detailed growth and soil measurements were recorded for at least three complete rotations. After the spring of 2004, the cropping system of Citra was modified since continuous cultivation of sunnhemp resulted in a build up of soil born diseases. Therefore, sunn hemp was replaced by cowpea, pearl millet, sesbania, and sorghum sudan grass during subsequent years. We also included a fall vegetable crop (broccoli) while sweet corn was replaced with water melon. At Tifton, there was no significant build up of soil born diseases in sunn hemp plots but during 2004, sweet corn yields were greatly impacted by pest and diseases. nd we completed three cropping cycles, with including sunn hemp planted at the end of July or beginning of August of 2002,2003, and 2004 followed by either a fallow or Cahaba white vetch (planted on 12-12-2002) and Crimson clover (planted on 12-1-2003 and 11-19-2004).

Sunn hemp was planted in late July or early August and killed by herbicide towards the beginning/ middle of November. Crops were irrigated with overhead irrigation. Three equal split applications of ammonium nitrate and potassium chloride were applied approximately 1, 3 and 5 weeks after planting. Fields were mowed only just prior to corn planting and after final corn harvest. Sweet corn was planted during the middle of April using a rip-strip planter with 0.76 m row spacing and a 0.15-0.18 m plant spacing. Weeds were controlled with synthetic herbicides using standard recommendations.

Crop yield was determined for an inner plot area, which was 4.6-6.0 m long. Harvested produce was graded using USDA standards. At northern locations, crop biomass was also sampled at 2-3 weeks intervals throughout the growth season outside the inner plot area but away from plot edges using a 0.9 m long representative row section. In south Florida, dry matter and N content of above-ground biomass was determined at the end of each cropping season for a representative sampling area. Biomass samples were used to assess how treatments affected plant growth and crop N accumulation rates over time. All tissues were bagged, dried for 72 h at 65 C, and then weighed. Afterwards, tissues were ground in a Wiley mill to pass through a 2-mm screen, and a thoroughly mixed 5-g portion of each grinding was subsequently stored. Grindings were digested and analyzed for total Kjeldahl N. Total plant dry weight, N concentration, and N content were calculated by summing individual tissue types.

Soil samples were collected prior to each fertilizer application using 0.3 m depth increments to a soil depth of 1.2 m with a 5 cm diameter soil auger. A 10 g subsample was extracted with 50 mL of 2 M KCl and filtered. Samples were analyzed using an air-segmented automated spectrophotometer (Flow Solution IV, OI Analytical, College Station, TX) coupled with a Cd reduction approach (modified US EPA Method 353.2). After sweet corn harvesting, additional soils samples were collected from the upper 15 cm of the soil profile. Samples were homogenized and subsamples of 30 to 40 g were weighed, dried at 100 C for 24 hours, and reweighed to determine gravimetric water content. Particulate organic matter was separated using a procedure adapted from Dr. Alan Franzluebbers (USDA Agricultural Research Service, Watkinsville, GA). Microbial biomass was determined during the first year using the chloroform fumigation method. Microbial biomass was calculated as the difference in C and N concentrations between fumigated and non-fumigated samples.

Weeds were sampled from all treatments at the end of every GM crop except blue lupine. For sweet corn grown in Citra during 2003 and 2004 we also determined the effect of cover crops on root distribution during 2003 and 2004. Soil cores were extracted at 3, 5 and 8 weeks after emergence (WAE) with a 5 cm wide soil auger (Forestry Suppliers, Inc; Jackson, MS). In each plot, soil was extracted in the row and between rows from three different depths: 0-15 cm; 15-30 cm; and 30-60 cm. Soil cores were gently washed above a grain sieve with 2 mm in diameter to remove soil. Roots were suspended in water and placed on a Plexiglas plate, scanned and subsequent image were analyzed with the Winrhizo software (Regent Instruments, Quebec City, Canada). At Citra, weed samples were also taken from representative areas of each plot using a 1 m2 PVC frame, with shoots and roots of all weeds rooted in the frame taken. Weeds were dried at 65 C for 72 hours, before recording dry weights. Soil samples for nematode analysis were collected on six occasions during the two-year study. Each sample consisted of six soil cores (2.5 cm diameter x 20 cm deep) from a plot. After thorough mixing of the aggregate sample, a 100 cm3 subsample was removed for nematode extraction using a sieving and centrifugation procedure. All data were analyzed using SAS software (Statistical Analysis Systems; Cary, NC).

Economic analysis was based on the “Green Cay Farm” a CSA operation located in Boynton Beach, in south Florida. Aside from yield measurements, analysis included labor and cost of production data for the different years and systems. This data allowed for an economic budget analysis (IFAS, 2005) and energetic analysis. Replacement scenarios for sunn hemp use were created by substituting the amount of N mineralized, which amounted to 65-75 kg N/ha, with either broiler litter (BL) or compost (C). Literature values were used to complement measured data. Given the particular nature of the CSA studied (equipment used in the study was also used for non-experimental operations) and because its special market dynamics (captive markets, buffered prices and produce baskets packaging system), fixed and post-harvest costs were not evaluated. Analysis was based on operating cost which considered inputs, labor and machinery rental. The scenarios did not consider the option of replacing the applied fertilizer, but analyzed what was the cost of replacing cover crop mineralized N. The cost per kg of chicken broiler litter and compost were calculated as $0.06/kg and $0.07/kg delivered on farm. Gross returns and returns before fixed costs were calculated based on current average prices for tomato, pepper and sweet corn ($0.80, $0.90 and $0.64/kg, respectively) from the Food and Resource Economics Vegetable Budgets from the University of Florida.

Research results and discussion:

North Florida (Citra)

In North Florida and South Georgia, most vegetable crops are grown during the spring season, so substantial amount of nitrogen released from a summer CC, such as sunn hemp, may be lost during the 4-5 months prior to the planting of a spring vegetable crop. Based on sampling of over-wintering sunn hemp residue it was determined that 4-6 weeks after the senescence less then 60% of the weight and less then 20% of the N were retained in the residue itself. This underlines that following a summer crop with a relatively rapid growing winter CC should increase its benefits via improved retention of previously fixed N and other nutrients. During the first year (2001) of our cropping cycle we examined the use of Lupine as a winter cover crop to minimize N-losses and/or to fix additional nitrogen. Lupine produced 3-4 Mg biomass per ha (1 Mg/ha ~ 900 lbs/ac) and contained to 40-60 kg N/ha. Initial growth of lupine appeared to benefit from the presence of sunn hemp crop residue. Based on Dr. Phatak’s positive experience with Cahaba white vetch, we included this winter cover crop during 2002 in N-Florida. However, this crop was poorly adapted to Florida sandy soils and its production ranged from 1-3 Mg/ha in 20 weeks and it accumulated only 20-40 kg N/ha. Because of the inconsistent performance of Cahaba white vetch we substituted this crop with radish aiming to more efficiently utilize residual soil N from sunn hemp residue. Considerations for its use includes its rapid initial growth and its very deep root system. It was anticipated that this should enhance the recycling of nutrients from greater soil depths, and could also enhance soil structure at greater soil depths for zero tillage systems. However, germination of radish was erratic due to the presence crop residue on the soil surface, so we replanted with a mixture of hairy vetch and winter rye instead.

During 2003/04 we used a 70-30% winter rye/ vetch CC mix which resulting in more uniform growth and higher overall biomass production (2.4 + 5.4 = 7.9 Mg/ha) and appreciable N accumulation 130-140 kg N/ha compared to mono-cropped systems. Biomass and N accumulation of winter rye were 6.4 Mg/ha and 61 kg N/ha in sunn-hemp based systems compared to 3.5 Mg/ha and 50 kg N/ha if rye followed a summer fallow while the growth of vetch was similar in both systems. During 2004/05 we used a 30-70% rye/vetch mix which resulted in 3.0 + 9.6 = 12.6 Mg/ha total biomass which contained up to 265 kg N/ha. The luxuriant growth during 2005 was related to unseasonably cool and wet spring which greatly enhanced continuous growth of vetch. Overall N concentrations were 0.7-1.3% and 2.6-3.6% for vetch and the overall C:N ratio for the crop mix ranged from 18 to 22, which seems to be very favorable. Instead of applying herbicides, plots were mowed (life mulch), which enhanced N retention and weed suppression. However, continuous and exuberant growth of vetch after mowing, hampered initial growth of watermelon and delayed fruit harvest by up to 2 weeks but it also greatly increased N content of water melon plants.

During 2005/06 we planted 0-100, 30-70, 70-30 and 100-0% rye/vetch mixes. Corresponding biomass accumulation was 4.7 + 0 = 4.7 Mg/ha; 3.4 + 2.4= 5.8 Mg/ha; 2.9 +3.1 = 6.0 Mg/ha; and 4.4 + 0 Mg/ha, respectively. These results, demonstrates the synergistic benefits of mixed CC systems. Relatively low biomass production during 2006 was related to extensive droughts during the early spring which shortened the expansive growth stage of hairy vetch and herbicides were very effective in killing the residue. Based on our results we conclude that continuous cultivation of winter CC on sandy soils gradually improved soil quality/ecology and that this in combination with using a rye/vetch mix greatly enhanced the overall performance of winter CC-based production system.

Sunn hemp produced 8.5, 12.2, and 7.2 Mg/ha during 2001, 2002, 2003, respectively. Crop residue contained 1.4-1.6% N and had a C:N ratio of 24. Sunn hemp accumulated between 111 and 172 kg N/ha in 12-14 weeks. Parallel studies have shown that sunn hemp is one of the best summer leguminous CC for the Southeastern region. Although sunn hemp accumulated large amounts of N, due to appreciable N losses (upto 80% within 2 weeks after crop senescence), only 35-50 kg N were left in crop residues by the time of corn planting. Repeated cultivation of sunn hemp resulted in a gradual build up of Verticillium wilt, a soil-born disease which resulted in steep decline in biomass production during the third year. At the end of the growing season of the third year sunn hemp was planted it affected 50-81% of the plants compared to 1-10% during the second year. Use of sunn-hemp, on the other hand, reduced nematode populations compared to the conventional treatment. Although sunn hemp clearly outperformed any other CC by far, alternating it with another less productive summer CC crop may be required to prevent disease build up. As a result, starting the summer of 2004 we used a more dynamics crop rotation including cowpea, pearl millet, sesbania, sorgum sudangrass, and velvet bean. Biomass accumulation of cowpea, pearl millet and sorghum Sudan grass were 3.0, 9.4, and 10.9 Mg/ha, respectively and these crops accumulated 50-75 kg N/ha. The performance of sesbania and velvet bean was poor due (< 0.7 Mg/ha) due to poor stand establishment and susceptibility of Sesbania to nematodes. Overall N accumulation (in kg N/ha) by CC residues 1-2 wks prior to planting of sweet corn during 2002 and 2003, was 120-125 (S-W-C), 30- 60 (S-F-C), 44-115 (F-W-C) for systems 1 through 3, respectively. Prior to the first N application, inorganic soil N in conventional system for 0-30 cm, 30-60, 60-90, and 90-120 cm soil depth layer was 27, 16, 15, and 15 kg N/ha, respectively. Corresponding increases for CC-based systems were 14, 6, 2, and 0.4 kg N/ha and these systems only accumulated 14-20 kg N/ha more in the effective root zone at the beginning of the growing season. Nitrogen release from winter CC residues appeared to be greatest during the first 4-6 wk after corn emerged. Soil solution nitrate values for the 133 kg N/ha treatment in the upper 30 cm of the root zone after the first N application, were 49 ppm and 27 ppm for CC-based vs conventional systems, respectively. Growth benefits for sweet corn in CC-based systems were typically greatest during initial growth. However, CC-based systems amended with 67-133 kg N/ha often produced similar corn plant growth compared with use of 200 to 267 kg inorganic N ha-1 only. In terms of total plant N concentration and N content, CC-based systems tended to fall behind conventional systems receiving 200-267 kg N/ha by the time of maximum crop growth (6-8 wk). On poor sandy soils corn yields for CC-based systems in the absence of fertilizer were invariably very low (< 1-2 Mg/ha). At final harvest, ear yields for conventional systems receiving high N rates were always numerically, if not statistically, greater than from any GM treatment. Although sunn hemp accumulate between 115-200 kg of N, overall net N benefits from cover crop based systems for sweet corn were only on the order of 11-60 kg N (a reduction in overall N requirements by 6-30%). This was related to appreciable N losses (upto 80%) within 2 weeks after crop senescence and supplementary N-rates of 130-160 kg N may be required to attain yields comparable to those for the 266 kg N/ha non-CC treatment. Averaged sweet corn yields (2002-2004) for the 0, 67 and 133 kg N/ha rates for cropping systems 1 through 4 were 1.0, 8.6, 14.5 Mg/ha (S-W-C); 0.5, 7.4, 13.6 Mg/ha (S-F-C); 1.3, 7.3, 12.9 Mg/ha (F-W-C); and 0.1, 4.9, 12.3 Mg/ha for the convention system (F-F-C), respectively. At lower plant populations (2002) yield for the conventional 200 and 267 kg N rates were similar (13.9 vs 14.4 Mg/ha) while at higher plant densities corresponding values were 17.3 vs 19.4 Mg/ha (2003) and 16.9 and 20.1 Mg/ha (2004). Discrepancies between CC-based systems fertilized at 133 kg N/ha and conventional systems receiving 267 kg N/ha were most pronounced at high plant densities (2003 and 2004). Towards the end of the growing season nitrate concentrations in the soil solution below the effective rootzone were < 1ppm and 36 ppm for the 133 and 267 kg N/ha N rate while values for the 133 kg N/ha treatments were similar for both CC-based systems and conventional systems. It thus may be concluded that although yield responses may occur up to 200-267 kg N/ha, excessively high N rates may also impact N loading rates of groundwater in an adverse manner. Based on N release patterns of winter CC it is also concluded that partitioning a greater fraction of supplemental N towards the end of the growing season may benefit CC-based systems. Root length densities (RLD), which is critical for nutrient interception and efficient N utilization, was greatest near the plant row, and it increased over time but decreased with soil depth. For the conventional system receiving 133 kg N/ha (C133) RLD for sweet corn at week 4 in the upper 0-15 cm was 1.9 cm/cm3 in the row vs 0.7 cm/cm3 between the row. Corresponding values for SH-based system (SH-133) receiving equal N rates were 3.1 and 2.3 cm/cm3, respectively. Root densities showed an exponential decay pattern and RLD for the C 133 treatment for the 0-15, 15-30 and 30-60 cm soil depth were 1.3.,0.3, 0.1 cm/cm3 (wk 4), 1.7, 0.9, 0.4 cm/cm3 (wk 6), and 2.4, 0.8 and 0.4 cm/cm3 (wk 9). Corresponding values for the SH133 system were 2.7, 0.9, 0.2 cm/cm3 (wk 4); 2.9, 1.2, 0.2 cm/cm3 (wk 6); and 2.5, 1.4 and 0.4 cm/cm3 (wk 9). For conventional treatments, root density increased with N-rate and in-the-row root densities in the upper 15 cm of the soil at 4, 6 and 9 wks were 1.9, 3.1, 3.3 vs 2.0, 3.9, and 3.4 cm/cm3 for the 133 and 266 kg N rates, respectively. Covercrop residue greatly promoted root growth in the upper soil layer and these effects were most pronounced during the first 6 weeks. At week 6, the effective root depths (ERD), the depth at which 90% of the total RLD occurs, was 27 cm for SH-based systems vs 32 cm for conventional systems. Broadcasting fertilizer increased between-row root proliferation in the upper soil layer by 37-97%. Starting 2004, we changed the crop rotation and for 2-3 systems we planted Broccoli following a summer CC crop (pearl millet (PM) vs cowpea (CP) in 2004 and Sorghum Sudan grass (SS) vs Cowpea (CP) in 2005) while the other cropping systems were planted with a rye/vetch mix during the fall followed by water melon (2005) or sweet corn (2006). Broccoli yields in CP-based systems amended with 200 kg N/ha were equal or better that those for conventional plot receiving 300 kg N/ha. At equal N rates CP-based systems had higher yields (25-34% yield benefits) while PM or SS-based systems reduced crop yields (20-31% yield reduction). Estimated N benefits from cowpea residue ranged from 30 to 100 kg N/ha while potential yields appeared to be also higher for CP-based systems. The negative effect of PM and SS residue on Broccoli yield were related to their high C:N ratio (58 vs 12 for Cowpea) which would result in reduced short-term N availability due to N-immobilization by the PM or SS residue. However, CC residues did appear to reduce weed incidence in Broccoli. For water melon, N response trends were reversed. For conventional systems water melon yield increased linear with N rates and yields were 0.0, 13.2, 18.6, 23.5, and 29.4 Mg/ha for the 0, 84, 126, 168 and 210 kg N/ha N-rates, respectively. At equal N rates, PM-based systems had 2.0 and 6.6 Mg/ha higher yield at the 84 and 168 kg N/ha rate. Yields for PM-based systems receiving 168 kg N were comparable or higher to those for conventional systems receiving 210 kg N and overall N benefits were on the order of 17-52 kg N/ha for PM-based systems. For CP-based systems yields were similar or lower compared to conventional systems. Although, water melon following hairy vetch had much greater leaf N, the continuous growth of vetch even after repeated mowing delayed yield by 1-2 weeks. Since all plots were harvested at the same time, vetch-based systems did not attain their maximum yield yet. Although management of vetch as a “life-mulch” appeared to greatly reduce premature N losses, use of strip application of herbicides may be required to reduce competition with a crop like water melon that is very sensitive to (weed) competition. Alternatively, use of taller and/or more competitive crop such as tomato or watermelon may be better suited for vetch-based live mulched systems. Overall crop carbon addition rates were 8.5-12.6, 7.2-10.7, 5.9-9.4, and 4.3 to 7.1 Mg C per year for systems 1 through 4, respectively. However, due to the fact that our crop rotation followed a bahia pasture overall soil organic matter (SOM) values across time decreased from 1.44% (2002) to 1.36% (2003) and 1.20% (2004) before stabilizing at 1.22% in 2005. Overall SOM values were 10% greater in System 1 (double cover crop) compared to conventional systems while systems that only included a winter CC showed a 5% increase. Total TKN N concentrations were 0.065, 0.065, 0.62 and 0.56 for Systems 1 through 4, respectively. The soil C:N ratio was 12.3 for CC-based systems compared to 13.0 for conventional systems. Soil particulate organic carbon (POC) content, which is indicative of recently added soil organic matter, increased from 0.27% in 2002 to 0.30% in 2003. Overall POC values were 0.3, 0.3, 0.28 and 0.26 for systems 1 through 4, respectively and SH-based systems showed a 15% increase, which is relatively low considering that these systems had almost twofold greater crop addition rates. There was no significant difference between systems in soil microbial biomass and overall values were 105 mg C/kg soil. Due to shading and allopathic suppression sunn hemp and rye/vetch mixes reduced weed growth by 24-80% and 34-100%, respectively. However, presence of crop residue typically enhanced subsequent growth of either crops and/or weeds. Overall weed biomass was greatest during the summer and lowest during the winter. Weed suppression typically was inversely related to overall cover crop growth and performance. Thus, more vigorous growing cover crops that maintained dense canopies for a prolonged period of time and/or had more recalcitrant crop residues, were more effective in suppressing weeds. Overall weed biomass for Systems 1 through 4 were: 0.9, 2.0, 0.4, and 1.5 Mg/ha (1 Mg ~900 lbs/acre) in March vs 1.8, 1.1, 4.7, and 4.4 Mg/ha in October, respectively. At the beginning of spring, weed growth was always greatest in sunn hemp-based systems that did not include a winter cover crop, which was related to weeds benefiting from N release from sunn hemp residue. At the beginning of fall, sunn hemp-based system had 36 to 90% less weed growth compared to summer fallow treatments. Continuous cultivation of sunn hemp resulted in a gradual build up of soil borne disease over time and crop performance and weed suppression was relatively poor during the third year. While winter-cover crop based systems had 34 to 100% less weed growth compared to fallow systems. Soil populations of root-knot, lesion, stubby-root, and ring nematode increased in plots planted with sweet corn. Soil populations of root-knot nematode (Meloidogyne spp.) and lesion nematode increased after the first sweet corn crop, while soil populations of stubby-root nematode (Paratrichodorus spp.) and ring nematodes did not significantly increase until the second or third year of study. During the first two years, nematode numbers at the end of the corn growing season typically were not affected by CC treatments. The only exception occurred during the first year of study, when use of any CC combination reduced lesion nematode soil population compared to conventional plots. During the third year, the double CC system (S-W-C) had lower numbers of lesion and ring nematodes compared to conventional treatment where as single cover crop systems (S-F-C and F-W-C) did not reduce soil nematode populations.
South Georgia (Tifton)

In Tifton, Verticillium wilt occurred in sunn hemp but it did not impact growth significantly. Biomass accumulation by sunn hemp during a 10 wk growth period was 8, 7 and 6 Mg/ha during 2002, 2003, and 2004, respectively. Lower yields at this location may be related to a later planting date and a shorter growing season. Planting sunn hemp earlier will extend its vegetative growth cycle resulting in taller plants with thicker stems. Planting it past the beginning of August will shorten its growth cycle (which is typical dictated by the first frost) and thereby will reduce total biomass production. Sunnhemp contained 2% N and thus accumulated 130-140 kg N/ha. Cahaba white vetch generated 4.5 Mg/ha in 20 weeks, but since the N concentration was 3.1% the overall N accumulation was still 140 kg N/ha. During the first year of planting crimson clover yielded 5.1 Mg/ha and accumulated 110 kg N/ha. During 2004/05 Crimson clover was fertilized with 450 kg/ha of a 10-10-10 fertilizer to enhance its growth, and biomass production was increased to 8.5 Mg/ha. With a N concentration of 2.0-2.2%, overall N accumulation was 175-185 kg N/ha and net N benefits (accounting for fertilizer N added) were up to 130-140 kg N. Overall soil organic matter content at Tifton was 2.3% and use of cover crops resulted in relatively small (<5%) annual increase in organic matter. Overall crop carbon sequestration rates were 8.1-11.4, 5.8-8.5, 4.8-7.9, and 2.5 to 5.0 Mg C/year. Soil microbial biomass and residual soil nitrate values were 47% and 181% greater in CC-based systems compared to controls. During 2003, double CC amended systems increased marketable corn yields of 8.1 Mg/ha compared to 1.5 Mg/ha for control treatments and N benefits from cover crops were on the order of 50-70 kg N. Sweet corn yields (2003) for the 0, 67 and 133 kg N/ha rates for cropping systems 1 through 4 were 8.1, 9.0, 19.6 Mg/ha (S-W-C); 2.8, 8.2, 13.1 Mg/ha (S-F-C); 6.4, 6.2, 10.2 Mg/ha (F-W-C); and 1.6, 10.7, 12.3 Mg/ha for the convention system (F-F-C), respectively. Yield for the conventional 200 and 267 kg N rates were similar (13.0 vs 13.9 Mg/ha). Soils at Tifton were more fine-textured, higher in organic matter (2.3% vs 1.2% at Citra) and thus have a higher inherent soil fertility. This was reflected in much higher (8.1 Mg/ha) sweet corn yields for the S-W-C systems that did not received supplemental inorganic fertilizer compared to those in Citra which yielded only 1.0 Mg/ha. At higher N-rates CC-based systems performed similar in both Tifton and Citra. During 2004, disease incidence and water stress impacted growth of sweet corn growth in Tifton severely, and yield analysis was inconclusive since most plots yielded less than 6 Mg/ha. Amending crimson clover with 450 kg/ha of a 10-10-10 fertilizer during the 2004/2005 growing season, greatly enhanced the growth and N content of the Crimson clover. Sweet corn yields (2005) for the 0, 67 and 133 kg N/ha rates for cropping systems 1 through 4 were, 22.3.6 Mg/ha (S-W-C); 3.0, 7.8, 7.8 Mg/ha (S-F-C); 4.2, 11.1, 20.9 Mg/ha (F-W-C); and 0.3, 2.7, 7.5 Mg/ha for the convention system (F-F-C), respectively. Yield for the conventional 200 and 267 kg N rates were similar (9.8 vs 10.4 Mg/ha). The relatively low yields of plots at intermediate and higher N-rate for systems that did not include crimson clover (yields were 30-50% lower compared to 2003) were related to N-fertilizer toxicity due to fertilizer being applied too close to the plants. Due to the buffering action of the crimson clover residue, these plots were not adversely affected by N toxicity. It is concluded that the modification of crop management practices was appropriate since it also resulted in a tremendous increase of sweet corn potential yields which greatly off-set the additional cost associated with application of moderate amounts of fertilizer to the crimson clover. Observations for a parallel study at Citra have shown that, especially on sandy soils with low inherent soil fertility, Crimson clover will require adequate levels of residual soil K in order to attain maximum production. In this case, in the absence of K, biomass production was only 1 Mg/ha due to stunted plant growth compared to 1.9-5.0 Mg/ha with the application of 50 kg K2O/ha.
South Florida (Boyton Beach)

On-farm work in S-Florida showed that sunn hemp produced 4.7, 3.3 and 3.9 Mg/ha in two months during 2002, 2003 and 2004, respectively. Detailed growth analysis at Citra during previous years had shown that the majority (60-75%) of the N accumulation of sunn hemp occurs during the first 8-10 wks. After this time, there is a shift towards the formation of more recalcitrant plant material with low C:N ratio’s such as stems. The participating grower had indicated that it was undesirable for plants to become excessively large since plants would get too “woody”. She stated that this could potentially interfere with bed preparation of mulched vegetable cropping system. Keeping the cover crop small by mowing, resulted in smaller plants with less stems, yet proportionally more leaves and overall N accumulation was on the order of 75-105 kg N/ha.

During the first year we contrasted sunn hemp-based tomato and pepper production systems amended with 0, 75 and 150 kg N/ha (SH0, SH75 and SH150) with a conventional system without sunn hemp that received 225 kg N/ha (C224). During this first season, sunn hemp supplemented with 75-150 kg N/ha had similar or higher tomato yields compared to the conventional treatment and respective fruit yields for the 4 treatments were 49.7, 55.0, 61.2, and 58.5 Mg/ha. For Pepper, there was no significant response to N rate for sunn hemp-based systems and all treatments had similar yields ranging from 22.9 to 26.8 Mg/ha. A subsequent sweet corn crop amended with 224 kg N/ha planted in the existing beds, showed an overall yield benefit of 1.5 Mg (12%) in SH-based plots while overall yields ranged from 11 to 15 Mg/ha.

Since the intrinsic variability of on-farm studies tends to be greater, we modified the design to a complete balanced factorial design to gain better insight in treatment effects. During the next two years tomato and pepper received 0, 112 or 224 kg N/ha for both conventional and sunn hemp-based systems. We also reduced the N rates of corn systems to 112 kg N/ha in order to determine if N benefits from sunn hemp extended over time since such difference are most obvious at intermediate N-rates.

For tomato, yields during 2004 for the C0, C112, and C224 treatments were 29.5, 42.1 and 64.0 Mg/ha. Corresponding values for the SH0, SH112, and SH224 systems were 48.2 (+63%), 57.0 (+35%), and 67.1 Mg/ha (+5%). During 2005, overall production was lower due to hurricanes and crops had to be replanted and corresponding values for the C0, C112, and C224 treatments were 21.2, 29.9 and 22.2 Mg/ha compared to 24.8 (+17%), 21.4 (-30%), and 27.0 Mg/ha (+9%) for the SH0, SH112, and SH224 systems, respectively. Pepper yields during 2004 were 23.7, 37.2 and 40.8 Mg/ha. for the C0, C112, and C224 treatments, respectively. Corresponding values for the SH0, SH112, and SH224 systems were 28.8 (+22%), 42.4 (+14%), and 44.5 Mg/ha (+9%). During 2005, similar to tomato overall pepper yields were was lower due to hurricanes and corresponding values for the C0, C112, and C224 treatments were 8.9, 12.1 and 22.7 Mg/ha compared to 17.9 (+101%), 20.5 (+69%), and 18.9 Mg/ha (-20%) for the SH0, SH112, and SH224 systems, respectively. Growing sunn hemp prior to tomato or pepper, reduced inorganic nitrogen fertilizer requirements of these vegetable crops by up to 50%, and plots amended with 112 Kg N/ha had similar or only slightly lower yields compared to the conventional treatment with 225 kg N/ha.

The greater efficiency of the Sunn hemp residue in replacing inorganic N-fertilizer in S-Florida compared to N-Florida was related to tomato and peppers being grown directly after Sunn hemp. Where as in N-Florida and S-Georgia, vegetable crops are typically grown during the spring season, so substantial amount of N is lost prior to the planting of a subsequent crop.

Participation Summary

Educational & Outreach Activities

Participation Summary

Education/outreach description:

This program provided partial support for two graduate students being Corey Cherr (2001-2004) and Laura Avila (2004-2006). Monica Cooper (University of Florida, Doctor of Plant Medicine Program) and Amy Van Scoick (University of Florida, BSc program Agronomy Department) completed internships working on special studies that were part of this program during the spring and summer of 2002, respectively. Kari Reno, an undergraduate student at the School of Natural Resource and Environment at the Univ. of Florida completed an internship with our group during the summer of 2004 and participated in environmental quality assessment studies.

Up to date, these students participated in development of 5 refereed publications and 13 scientific/grower presentations. The on-line version of the Agronomy Journal publication generated from this program received the most hits of newly released papers during March 2006 and within the first week of it release we have received over a dozen international requests for reprints. During the next year program results will generate another 3-5 refereed papers. We were invited by Louise Buck and Laurie Drinkwater to contribute to the SARE systems research handbook and our field studies and research approach are being included in a book chapter on systems approaches to help illustrate important characteristics and issues in systems research.

This program also allowed us to make a direct contribution to the adaptation of the decision support system for agrotechnological transfer (DSSAT) model to sweet corn via collaborations with Ken Boote and Jim Jones. During the next two years this model will be used to validate the effectiveness of Best Management Practices, including the use of cover crops, to meet environmental quality standards.

On January 24, 2002 a group of thirty farmers and extension agents from Columbus and Robeson counties (South Carolina) visited the project site in N-Florida during a field tour. The main focus of this group was to explore alternatives for conventional agriculture and improved use of green manures in agriculture. Some of the visiting farmers were interested in sunn hemp. We provided Milton Parker, the county extension agent, with 10 lbs of seed to evaluate this crop under local growing conditions. On February 21-22, Johan Scholberg was invited to participate in a roundtable conference sponsored by SARE in Statesboro, GA to assist organic farmers and researchers with the development of research strategies. During this workshop concepts develop within the context of this SARE grant were shared with growers and scientist from several states throughout the southeastern US. Corey Cherr, the graduate student participating in SARE program, presented our results at the Florida Agricultural Conference and Trade Show (FACTS) in Lakeland on April 30, 2003 and at two scientific presentations during the national annual meetings of the American Society of Agronomy/Crop Science Society of America/Soil Science Society of America in Denver on November 4, 2003. On January 23, 2004 we hosted a field tour showing to 40 growers participating in the Southern Sustainable Agricultural Working group and developed a brochure that outlines our initial results. On March 15, 2004 Johan Scholberg outlined potential benefits of cover crop based systems to a vegetable group meeting of 40 growers in Immokalee, S. Florida. On March 10, 2005 we conducted a field day for improved use of cover crops by organic growers and toured the field site. Participating growers were extremely interested in the capacity of cover crops for weed management and there is strong increase in interest in our program especially from organic growers.

On February 4, 2002, project results were presented to the Junior Science, Engineering and Humanities Symposium (JSEHS) at the University of Florida to participating high school students to increase their awareness of the importance of sustainable agriculture via improved use of cover crops in conventional production systems. Johan Scholberg presented our initial research findings for an invited guest seminar lecture for Doctor of Plant Medicine Program at the University of Florida in November of 2003. The DMP program is a unique program that trains students to become crop consultants and work directly with farmers. On February 2 of 2004, we presented our program again at the Junior Science, Engineering and Humanities Symposium (JSEHS) at the University of Florida to participating high school students. During the second week of April in 2003 research trials was shown to 25 students participating in the southern regional student activities subdivision of the ASA/CSSA/SSSA. Field tours for undergraduate students were also conducted on April 3 2005 and March 12, 2006 for students interested in sustainable agriculture and use of cover crops.

Refereed Publications: (*=graduate student)

Cherr, C.M.*, J.M.S. Scholberg, and R.M. McSorley. 2005. Green manure growth and decomposition in a warm-temperate environment. Agron. J. (accepted).

Cherr*, C.M, J.M.S. Scholberg, R.M. McSorley, and O.S. Mbuya. 2006. Growth and yield of sweet corn following green manure in a warm temperate environment on sandy soil. J. Agron. Crop Sci. (accepted).

Cherr, C. M.*, L. Avila*, J.M.S. Scholberg, and R.M. McSorley. 2006. Effects of green manure use on sweet corn root length density under reduced tillage conditions. Renewable Agriculture and Food Systems (in press).

C.M. Cherr*, J.M.S. Scholberg, and R.M. McSorley. 2006. Green manure approaches to crop production: A synthesis. Agron. J. 98:302-319.

Student Thesis (2):

Cherr, C.M. 2004. Improved use of green manure as a nitrogen source for sweet corn. M.S. thesis. Univ. of Florida, Gainesville. Available at: http://etd.fcla.edu/UF/UFE0006501/cherr_c.pdf

Avila, L. 2006. Potential benefits of cover crop based systems for sustainable production of vegetable crops in Florida. M.S. thesis. Univ. of Florida, Gainesville (available July 2006).

Scientific Presentations and Abstracts (10):

Avila, L.*, J.M.S. Scholberg, L. Zotarelli, and R.M. McSorley. 2006. Can cover crop based systems reduce vegetable crop fertilizer requirements in the southeastern U.S.A. Amer. Soc. Hort. Sci. (in press).

Avila, L.*, J.M.S. Scholberg, N. Roe, R.M. McSorley, and C.M. Cherr*. 2005. Use of cover crops for enhancing soil quality, productivity and sustainability of vegetable production systems in the Southeastern U.S. Agronomy Abstracts. 1 pp.

Cherr, C.M.,J.M.S. Scholberg, K.J. Boote, O.S. Mbuya, and M.D. Dukes. 2005.Application of DSSAT for simulating nitrogen response of sweet corn. Soil and Crop Science Society of Florida Abstracts. 1 pp.

Avila, L.*, J.M.S. Scholberg, C.F. Kiker, and N. Roe. 2005. Economic, energetic, and environmental (Emergy) analysis of cover crop based systems in Florida. Agronomy Abstracts. 1 pp.

Cherr*, C.M., L. Avila*, J. Linares*, J.M.S. Scholberg, S. Phatak, N. Roe, O.S. Mbuya, and H.W. Beck. 2005. Green manure and cover crop research: Interpretation of current findings as a basis for future program perspectives in the southeastern region. Agronomy Abstracts. 1 pp.

Cherr*, C.M., J.M.S. Scholberg, and H.W. Beck. 2005. Greencover: An information access tool to support the use and research of cover crops and green manure-based systems. Agronomy Abstracts. 1 pp.

Cherr, C.M*, J.M.S. Scholberg, N. Roe, S. Phatak, R.M. McSorley, and N.B. Comerford. 2003. Improved integration of cover crops in vegetable production systems in Florida and Georgia: Considerations and limitations. Agronomy Abstracts. 1 pp.

Cherr, C.M.*, J.M.S. Scholberg, K. Woodard, R.M. McSorley, and N.B. Comerford. 2003. Do cover crops affect nitrogen availability, root growth, and N uptake efficiency of sweet corn? Agronomy Abstracts. 1 pp.

Cherr, C.*, J.M.S. Scholberg, R. McSorley, K.L. Buhr, N.B. Comerford, and S. Phatak. 2002. Can a sequence of green manures replace chemical N as a fertilizer source for sweet corn? Agronomy Abstracts. 1 pp.

Cooper, M.L.*, C. Cherr**, and J.M.S. Scholberg. 2002. Effects of Sunn hemp residue on N-fertilizer requirements of a subsequent sweet-corn crop. Agronomy Abstracts. 1 pp.

Professional Presentations (3)

Scholberg, J.M.S. 2004. Improved integration of cover crops in vegetable cropping systems, Vegetable Growers Meeting, SW Florida REC, Immokalee, FL, March 2004.

Scholberg, J.M.S. 2004. Use of cover crops in citrus and vegetable production, Plant Medicine Program Seminar Series, Gainesville, January 21, 2004.

Cherr, C.M.* J.M.S. Scholberg, R.M. McSorley, S. Phatak, N. Roe, O.S. Mbuya, and M. Mesh, 2003. A system approach for improved integration of green manures in vegetable production systems in the Southeastern US. Organic Farming Sessions, Florida Agricultural Conference and Trade Show, Lakeland, FL, April 2003.

Project Outcomes

Project outcomes:

The key to successful implementation of green manure based systems lies in appropriate synchronization between N release from cover crop residues and subsequent crop N demand. Reduced tillage, green manure based sweet corn production systems at S-Georgia appear to benefit greatly from Crimson clover fertilized with 400 lbs/ac of 10-10-10 fertilizer followed by sweet corn fertilized with 100-125 lbs N/acre. Based on results with Cahaba white vetch it appears that use of this crop may hamper/delay initial corn growth and use of Crimson clover based systems may thus be preferable. This crop is also more suited for a life mulch strip-till corn systems. Allowing the Crimson clover to produce seed heads and reseed can further reduce the cost associated with cover crop establishment. Double cropping with sunn hemp may provide some extra yield benefits, which are most articulated at lower N-rates to a subsequent corn crop.

Based on results from this and parallel study we argue that sunn hemp may be one of the best leguminous summer cover crop for the southeastern U.S.A.. In order to prevent build up of soil born diseases on sandy soils, this crop may be alternated with other crops such as “Iron Clay” cowpea, Sorghum Sudan grass, and Pearl Millet. However, the residues of the last two crops have a high C:N ratio and may adversely affect soil N availability of subsequent planted crops. Following sunn hemp directly with a commercial transplanted commercial fall/winter vegetable crop, such as broccoli may afford growers to make better use of the N accumulated by this crop. Alternatively, if followed by a spring vegetable crop such as sweet corn, double cropping with winter cover crop may be required to enhance N retention. However, even in this case N benefits may be low and between 120 to 180 lbs N/ac and/or addition of manure may be required for maximum sweet corn yields.

Use of a rye/vetch winter CC systems appear to provide superior results in North Florida. Winter rye itself has higher initial growth. It is more efficient in utilizing residual soil nutrients and it also more robust and will grow on relatively poor/sandy soils. However, it has a very high C:N ratio and it may induce N deficiency via N-immobilization when it is followed directly with a crop that has high initial N requirements. Initial growth of vetch is very slow and it benefits greatly if it can utilize the rye to expand its canopy. Using a 30:70% vetch/rye mix planted at 50-100 lbs/ac appears to provide multiple benefits and this system provides a nice mulch with a favorable C:N ratio while system components complement each other very well resulting in more efficient resource utilization. Combined with zero tillage, it may interfere with planting operation so either strip tillage or combining it with a transplanted crop may be preferable. Use of a vetch-based live mulch can be tricky since cool weather may prolonged the growth of vetch and it may end up hampering initial growth of slow growing crops. Spray killing 1-1.5 ft wide strips and/or use strip tillage may be beneficial to overcome this problem.

Crimson clover has a more compact growth habit and may be less “viny” and aggressive and may be easier to manage for live mulch systems. Although the performance of Hairy Vetch and Clovers may be disappointing during the first year they are planted on sandy soils, over time as the soil ecology builds, their performance will greatly improve. Supplementing clover mixes with 50-100 lbs/ac of potassium sulfate on soils low in potassium may greatly enhance their performance. Planting crop mixtures including crimson clover, vetch, winter rye, radish, and/or black oats, may greatly increase their performance and these mixes tend to perform better since at least 1-3 crops in the mix will take optimal advantage of site- and season specific conditions.

The use of sunn hemp as a summer cover crop in south Florida in south Florida will generate 3-5 Mt/ha. Broadcasting the seeds or planting in narrow rows (12-15 inch apart) combined with timely mowing (6-8 wks) will prevent plants from developing overly thick stems which may interfere with subsequent cultural practices, including bed formation and mulching. Based on our findings and other studies, it is suggested that inorganic fertilizer N requirements of well-managed sunn hemp based systems may be reduced by 40-70% and that these systems may provide additional yield benefits to subsequent crops.

Although cover crops can enhance nutrient retention and reduce inorganic fertilizer requirements, and environmental impacts, and our dependence on foreign oil imports, their use is an art which takes time to master while benefits may not always be apparent right away. However, integration of well-designed/ managed cover crop systems in existing conventional production systems can provide a more sustainable basis for agriculture since these systems may provide multiple services including preventing soil erosion, enhancing soil fertility, and they may also provide (partial) control of both weeds and nematodes. It appears that use of cover crops may only be economically viable for conventional farmers if they provide multiple services and these other functions are also included in a cost benefit analysis. A parallel research program showed that appropriate use of cover crops provided the most cost-effective mechanism for weed management in organic production systems. Providing farmers with incentives to integrate cover crops into existing production practices as part of Best Management Practices, may facilitate a much broader adaptation of cover crops/green manures in conventional production systems.

Economic Analysis

Tomato production had the highest operating cost for the CSA operation (ranging from $9,543 at 0 N-rate conventional to $10,037 when growing after sunn hemp at 224 N-rate). The cost related to seed and establishment of a summer cover crop ($410/ha for sunn hemp), was comparable to the combination of the cost derived from the ammonium nitrate fertilizer (synthetic form of N) plus the summer weed control ($210 for 112 N-rate, $421 for 224 N-rate, and $198/ha for summer weed control). Yield for all crops increased and reached their maximum with the highest N-rate, even with the additional N from sunn hemp. Yield benefits from mineralizing sunn hemp in un-fertilized treatments reached 44% for tomatoes and 43% for peppers. While for tomato yield benefits at both 100 and 200 N-rates were on the order of 9%. Pepper however, when amended with 100 kg N/ha only presented 27% higher yields, while there was not benefit at 200 kg N/ha. Sweet corn had between 12 to 14% yield benefits from the summer cover crop. Operating cost for Florida tomato, pepper and sweet corn cost are $12,235/ha, $11,282/ha, and $4,025/ha, respectively (Food and Resource Economics, 2005). Production costs for this CSA, were consistently lower than those reported in extensive commercial operations. This might be due to the double cropping system in place, where the structures from one crop, such as beds, plastic much and irrigation lines are used for the following crop.

Returns before fixed costs were high for tomato ($34,480 ha for conventional and $37,680 for S based-system, at 224 N-rate). However fixed, harvest and marketing related costs for tomato, pepper and sweet corn were not pertinent to this analysis, reported values for South Florida are $21,069, $18,894 and $5,590 accordingly (Food and Resource Economics, 2005). Using these values, scenarios for tomato and pepper production systems showed positive net return under conventional 224 N-rate system and for the alternative systems (sunn hemp, broiler litter, and compost) at 112 and 224 N-rates.

The benefit to cost ratio (B:C ratio) before fixed costs was positive for all crops in all cropping scenarios. Maximum ratios to N-fertilizer input were obtained by cover crop system in tomato at all N fertilizer rates with 3.0, 3.2 and 3.8 fold return per dollar invested compared to a 2.1, 3.0 and 3.5 return for conventional systems. Corresponding values for pepper were 2.0, 3.0, and 3.7 vs 2.9, 3.8 and 3.7. Marginal returns ($/ kg N) were the lowest for CC based systems and replacement scenarios, demonstrating that excessive use of inorganic N is such system is less profitable to farmers. When comparing production cost of sunn hemp against the replacement scenarios with either broiler lither or compost, these replacement options were 1.5-3% higher. Inputs that require large amounts of fossil fuel for their production, such as fuel, herbicides, fertilizer, and plastic mulch dominated the overall energy budgets. In terms of energetic analysis, if crop residues from cover crops based systems can substitute the function of synthetic mulches, as has been demonstrated with hairy vetch/rye mixtures, will render CC-based systems much more energy efficient and cost-effective.

Farmer Adoption

We like to state that participation and feedback from growers and grower’s organization has been essential for us to fine-tune our program and it also allowed other research groups at U.F., including researchers interested in organic production systems, to benefit from this information.
In this manner we were able to further increase the program impact and general awareness of growers, students and scientists of our cover crop program. We are currently implementing a web-based expert system that will provide growers throughout the U.S.A with a better understanding what cover crops may work best for their specific production environments.

A total of 110 farmers attended field days and growers meetings. Parallel studies with the use of cover crops in organic citrus have shown that cover crops provided the most effective and least expensive control of most obnoxious weeds and several local organic farmers have adapted the use of sunn hemp into their production systems.

Based on our program finding we were also able to secure local funding via the Florida Department of Agricultural and Consumer Services (FDACS) to study environmental benefits of cover crops for conventional systems. Making cover crops integral part of conservation plans, NMP’s, BMP’s and providing growers with financial incentives (such as cost sharing) may afford conventional growers to adapt cover crops more readily into their farm management practices. Alternatively, if cover crops are used as a cornerstone for alternative production systems to provide a complete package of services such as mulching, protection of young transplants from wind damage, enhance soil fertility, and weed and nematodes control, their use will be more cost-effective which will be critical for their adaptation by farmers.


Areas needing additional study

The current program greatly enhanced our understanding of how cover crops affected some of the processes that control growth and yield of vegetable crops (e.g. “what works where and why it may not work somewhere else”). The next step will be to translate that information to a set of optimal production practices and to assess potential benefits using a farming systems approach. In this case using a “Ceteris Paribus” approach, where by a large number of small plots allow assessment of the effects of changing 1-2 parameters while al other production practices are being kept constant for each system, may no longer be adequate or appropriate. Instead information generated by previous research should be used to integrate a suite of best management practices into a limited number (4-6) of production systems that are replicated across the landscape and/or farm operations using large and representative production units.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.