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

2004 Annual 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

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


We have completed our third year of this program and finished three complete cropping cycles of sweet corn at the Citra field site in north central Florida. At the Tifton (South Georgia) and Boynton Beach (South Florida) field sites, we are in the middle of our third annual crop rotation cycle. Therefore, we have requested and received a no-cost extension to allow us to complete the analysis and compilation of all research results. It is concluded that use of summer cover crops in S-Florida seems to provide the greatest short-term soil fertility benefits since summer cover crops are directly followed by fall vegetable crops. Use of summer cover crops for this system reduced N requirements of peppers by 50-75%. In on-farm trials, use of sunn hemp as a cover crop preceding tomato also increased yield potential of tomato by 27 to 44% for total yield and larger-sized fruits, respectively. In this case nitrogen benefits were as high as 130 lbs of N/acre. At northern locations we opted to follow summer cover crops with leguminous winter cover crops but in most cases N losses from both summer and winter crops was substantial (upto 80-90% within two weeks after crop senescence). Most leguminous winter cover crops are poorly adapted to Florida coarse sandy soils, resulting in poor stands and erratic growth during the first few years of their cultivation. Overall net N benefits from cover crop based systems for these systems were on the order of 30-60 kg N. We have observed that even for sandy soils growth and performance of leguminous cover crops improved over time, which may be related to changes in soil ecology. On more fine-textured soils leguminous-based winter cropping systems appeared to perform better and more consistently. We have also observed that the use of cover crop mixtures may provide a more robust winter cover crop systems that would be better adapted to a wider range of soil en climatic conditions and also providing more favorable carbon to nitrogen ratio’s and higher nitrogen accumulation compared to the use of either vetch or winter rye by itself. During a second phase of this project we aim to modify cropping system design to increase nutrient retention of summer cover crops. This could be accomplished by using either a mixture of fast-growing non-leguminous crops with leguminous winter cover crops or by planting a commercial fall crop such as broccoli directly in the cover crop residue. Although there are some significant benefits to the use of cover crops, cost savings due to reduced inorganic fertilizer use as such does not appear to warrant their use by most conventional growers. It appears that use of cover crops may only be economically viable for conventional farmers if other functions such as the reduction of soil erosion and/or leaching of excessive residual soil nutrients during fallow periods are included in a cost benefit analysis. During the final part of the first program phase we will quantify these benefits for a commercial operation using economic analysis and crop modeling approaches. Making cover crops integral part of conservation plans, NMP’s, BMP’s and providing growers with financial incentives (such as cost sharing) also 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 be very cost-effective. 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. We are currently designing and developing a web-based expert system that will provide growers with a better understanding what cover crops may work best for their specific production environments.

Objectives/Performance Targets

A1) Determine which (combination of) green manure 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 green
manure based production systems and compare their yields with conventional systems;
A3) Determine nitrogen uptake efficiencies and N leaching for green manure-based cropping
systems in comparison with conventional vegetable cropping systems; and
A4) Develop a regional research and outreach program for improved integration of green
manure crops in commercial vegetable systems in close collaboration with local growers.
B1) Evaluate the long-term effects of included treatment combinations on soil quality and the
abundance and diversity of weed, arthropod and nematode populations;
B2) Improve the exchange and integration of information on the use of green manure 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 green manure 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.


During the first three years our main focus was to determine the benefits of cover crops with respect to soil quality and crop nutrient requirements. We therefore aimed to keep the crop ping cycle constant (with the exception of winter cover crops being used) by using sweet corn and sunn hemp during 3 consecutive years in the same plots. The advantage of this approach was that we could assess the more long-term benefits in terms of reduced crop nutrient requirements and that we could also monitor the effects of cover crops on soil organic matter accumulation.

At northern locations we opted to follow summer cover crops with leguminous winter cover crops. On the more fine-textured soils (Southern Georgia), leguminous-based winter cropping systems seem to perform satisfactory (Kahaba white vetch and Crimson Clover yielded 4.5 and 5.1 MT in 2002/03 and 2003/04, respectively). However, most leguminous winter cover crops are poorly adapted to Florida sandy soils and the initial performance of these systems was rather poor on these soils. During 2002/03 we used Kahaba white vetch in Citra (N-Florida) which yielded 1-3 Mt/ha but stand uniformity was rather poor and overall N accumulation was only 20-30 kg N. During 2003/04 we used a 70-30% hairy vetch/winter rye mix resulting in much more uniform growth and higher overall biomass production (2.0 + 3.5 = 5.5 Mt/ha total biomass) and N accumulation. During 2004/05 we used a 30-70% hairy vetch/winter rye mix which resulted in 8 + 3.0 = 11.5 Mt/ha total biomass. It is proposed that the use of cover crops on these sandy soils may gradually improve soil quality/ecology over time and that this in combination with using a rye/vetch mix greatly enhanced the overall performance of this winter cover crop production system. Although after two years of cover crops total soil organic matter was not significantly increased, there was an increase in newly formed organic matter (POM fraction) and the soil organic matter also had a lower C:N ratio, and there was also reduced incidence of soil nematodes. Use of cover crops also resulted in a reduction of weed growth by up to 90%.

Sunn hemp performed extremely well in all locations and all soil types. Parallel studies have shown that this is one of the best summer cover crops for the Southeastern region. However, repeated cultivation of sunn hemp appears to result in a build up of verticillium wilt on some soils and resulted in a gradual decline in biomass production (yields were 8, 12 and 7 MT/ha in Citra during 2001, 2002 and 2003, respectively). In Tifton the incidence of this disease was not observed and yields were 8, 7 and 6 Mt/ha during 2002, 2003, and 2004, respectively). Lower yields at this location may be related to a later planting date. Planting sunn hemp past the 1st of August will shorten its growth cycle (which is typical dictated by the first frost) and thereby will greatly reduce total biomass production. Although Sunn Hemp may accumulate up to 130-200 lbs of N, overall 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%) for both locations. This is related to appreciable N losses (upto 80%) within 2 weeks after crop senescence.

During 2004, disease incidence and water stress impacted growth of sweet corn growth in Tifton severely, and yield analysis was inconclusive. In Citra, use of cover crops with 133 kg N resulted in sweet corn yields comparable to conventional yields with 200 lbs N/acre. However, it was observed that cover crop based system should be managed slightly different to get maximum benefits and that benefits may be greater at low plant density compared to very high plant density of sweet corn. For experimental purposes, we maintained N distribution between different N applications constant for all treatments. Growth analysis showed that cover crop-based treatments showed superior growth at 67% N application rate compared to much higher N application for conventional treatments. However, cover crop-based treatments ran out of N towards the end of the growing season which may have diminished yield benefits. So based on this, it is proposed that cover crop-based systems would greatly benefit if early N applications were reduced and late season applications were increased. It may also be desirable to delay mowing and/or killing of cover residue as much as possible for leguminous cover crops. Currently, we are only moving strips of the hairy vetch/rye mix where we are planting water melons. Leaving the remainder of the cover crop senescing naturally will allow us to better match nutrient release patterns with crop nutrient uptake patterns of the commercial crop. It was also observed that crop residue greatly enhanced root growth of sweet corn. Since we used a zero-tillage system most of the residue accumulated in the upper soil layers thereby also favoring root proliferation near the surface. Based on this, it is also concluded that cover crop based systems may thus require different irrigation practices (more frequent irrigation with smaller volumes of water). On the other hand, based on field observations the presence of a mulch layer may greatly reduce soil water loss and both water and wind erosion.

Following sunn hemp directly with either tomato or pepper crop in S-Florida resulted in a reduction of crop N-requirements of peppers by 50-75%. Results for the 2004 on-farm trials showed that with use of sunn hemp as a cover crop preceding tomato also increased overall yields and larger-sized fruits by 27 to 44% respectively compared to conventional production treatments. In this case nitrogen benefits were as high as 130 lbs of N/acre. 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. In N-Florida and S-Georgia, most commercial vegetable crops are grown during the spring season, so substantial amount of nitrogen released from summer cover crops may be lost during the 4-5 months prior to planting. Based on sampling of sunn hemp 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.

Based on the information from the first 3 years we decided to further refine the cover crop systems in Citra to enhance the ecological and economic viability of these systems during the 4th cropping cycle. We replaced sunn hemp with either, Sesbania, Cowpea or Pearl millet, resulting in respective biomass production of 6, 4 and 1 MT/ha. Although Sesbania may do well under certain conditions (it was suggested to us by a local organic grower) it did rather poor under our experimental conditions. Since it is sensitive to nematodes and may become a weed problem, we opted to discontinue its use and may use velvet bean instead. The use of cowpea provided additional benefits (seed yield was 1 MT/ha). However, using Pearl millet resulted in highest broccoli yields and a reduction in crop N requirements by 33%. Planting broccoli directly into the cover crop residue was also very effective in repressing weeds and reduced herbicide application costs compared to conventionally-managed crops.

During the spring of 2005 we will plant water melon into a winter rye/vetch and assess if this system will provide us with additional benefits (such as weed control, mulching and wind protection) compared to conventionally managed systems.

Laura Avilla, the second student working on the grant has been collecting and compiling critical economic parameters for the on-farm studies in South Florida and will conduct cost benefit analysis and to assess the fossil fuel input requirements of different production systems.

Corey Cherr will be using results from growth analysis to calibrate crop growth models which then can be used to simulate long-term yield and environmental benefits of the use of cover crops.

Corey Cherr, the graduate student participating in SARE program, has written 3 refereed publication (including a comprehensive literature review on improved use of cover crops) up to date and he is currently working on two additional publications. He was awarded best thesis award of the University of Florida, Agronomy Department on march of 2005. He has also presented our results at both professional and growers meetings. 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. Johan Scholberg presented research findings at an invited guest seminar at the School of Natural Resources and the Environment in Gainesville Florida. On March 10th 2005 we conducted a field day for improved use of cover crops for organic growers and had a group of growers tour the field sites. Participating growers were extremely interested in the critical role of cover crops for weed management and there is strong increase in interest in our program especially from organic growers. On April 3rd
2005, Johan Scholberg gave a field tour of the field sites for undergraduate students interested in sustainable agriculture. At this point we are in the process to integrate all this information into a web-based expert system for improved use of cover crops in collaboration with Howard Beck (Information Technology specialist at the Agric. & Biol. Eng. Dept, Gainesville). This expert system will be available via the world-wide web and will provide users with access to a comprehensive and interactive data base outlining performance of a range of cover crops for different production environments and management practices. The goal of this software application is to provide users with interactive information what cover crop is expected to work best for specific production environments. We were invited by Louise Buck and Laurie Drinkwater to contribute to the SSARE 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. Based on our initial result 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. In conclusion, we like to state that participation and feedback from growers and grower’s organization will remain essential for us to fine-tune our research program. In this manner we anticipate to further increase the program impact and general awareness of growers, students and scientists of our cover crop program.


Robert McSorley

[email protected]
Professor Nematology
Entomology and Nematology Dept., Univ. of Florida
Natural Area Drive PO Box 110620
Gainesville, FL 32611-0620
Office Phone: 3523921901
Sharad Phatak

[email protected]
Professor of Horticulture
Department of Horticulture, University of Georgia
P.O. Box 748
Tifton, GA 31793-0748
Office Phone: 2293863355
Marty Mesh

[email protected]
Executive Director FOG
Florida Certified Organic Growers & Consumer Inc.
PO Box 12311
Gainesville, FL 32604
Office Phone: 3523776345
Website: http://www.foginfo.org/
Nancy Roe

[email protected]
Director of Farming Systems Research Inc.
Farming Systems Research Inc.
5609 Lakeview Mews Dr.
Boynton Beach, FL 33437
Office Phone: 5616382755
Website: www.vegies4u.com
Odemari Mbuya

[email protected]
Assistant professor
Florida Agricultural and Mechanical University
College of Engineering Sciences Technology and
Agriculture Center for Water Quality
Tallahassee, FL 32307-4100
Office Phone: 8505993383