Promoting Tropical Cover Crop Mulch Systems for Minimum-Till Crop Production in the U.S. Virgin Islands

View the project final report

• 2012 annual report
• 2013 annual report

Commodities

• Fruits: melons
• Vegetables: beets, cabbages, cucurbits, eggplant, peppers, tomatoes
• Additional Plants: herbs

Practices

• Crop Production: conservation tillage
• Education and Training: demonstration, extension, on-farm/ranch research, participatory research
• Farm Business Management: whole farm planning, cooperatives
• Natural Resources/Environment: carbon sequestration, biodiversity, soil stabilization
• Pest Management: cultural control, mulches - killed, mulches - living, mulching - vegetative
• Production Systems: agroecosystems, holistic management, organic agriculture
• Soil Management: green manures, organic matter, soil analysis, nutrient mineralization, soil quality/health
• Sustainable Communities: ethnic differences/cultural and demographic change, local and regional food systems, sustainability measures

Tropical cover crops representing three morphological plant types were tested followed by the effect of no-till roller-crimper termination (without herbicide) to provide residual surface sheet mulch for extended weed suppression. Cover crops with rapid soil cover and high levels of shoot biomass suppressed weeds prior to termination and included the legumes sunn hemp, lablab, and cowpea; and the grasses sorghum sudangrass and pearl millet. Of the cover crop cultivars tested, only sunn hemp was successfully terminated using a roller-crimper resulting in minimal cover crop regrowth with a sufficient quantity of vegetative crop surface residue that effectively suppressed weeds after termination.    

Introduction

Virgin Island’s farmers face unique challenges to sustainable agriculture as a result of geographic, environmental, and socio-economic conditions that exist in the USVI. Low-external-input farming is a reality for farmers in the USVI because conventional agricultural inputs are costly or difficult to acquire.  Farm size is classified as small (<$250,000/year gross annual income) with 63% of farms totalling 9 acres or less and 81% of farms totalling 19 acres or less (USDA, 2007). 

Tropical and subtropical environments pose specific challenges for the management of soil quality, agricultural pests, and water resources. Virgin Island’s farmers have limited options to address these management concerns. Rainfall in the USVI is bimodal with an extended dry season which can last up to six months. Both surface and subsurface ground water is severely limited and farmers rely predominantly upon precipitation for agronomic production. Water conservation is practiced by all farmers and micro-irrigation is relied upon with water sourced from harvested rainwater that is stored in tanks/ponds or pumped from shallow wells. In the USVI, the supply of sufficient soil moisture levels for crop production during the dry season often limits crop production to micro-irrigated fields and requires the growing of agronomic crops, such as cover crops that have a high water demand, to coincide with the rainy season. The rainy season typically lasts from August through November and is associated with the height of the Atlantic hurricane season.

In areas of the world that do not have access to reliable, readily available, or economically feasible external sources of synthetic or organic agricultural inputs, as is the case in the USVI, alternative agricultural production systems based on local resources must be utilized. Imported synthetic fertilizers and pesticides are generally 2-to 3-times more expensive than that of prices in the continental USA. Conventional fertilizers and pesticides require costly specialized application equipment to which USVI’s farmers have little or no access. Synthetic fertilizers/pesticides and bulk organic soil amendments are not economically feasible for smallholder farmers in the USVI and are often not available at all. Therefore, smallholder farmers must utilize alternative production practices to maintain soil fertility and reduce pest incidence. Alternative low-external-input management practices readily available to smallholder farmers include cover crops, sheet composting, crop rotation, and conservation tillage technologies. Cover crop and green manure systems are one of the most promising technologies which farmers could adopt to support sustainable soil fertility levels, and agricultural improvement (Snapp et al. 1998). In order for this to be put into practice, protocols for cover crop management along with the resulting crop residue as surface mulch for use in conservation tillage, low-external-input cropping systems is needed.  

 Crop rotation, cover crops, nitrogen fixation by legumes, conservation tillage, and organic mulch from plant residues are all considered sustainable agriculture management techniques, and when employed in tandem, result in a production system with increased resilience and overall sustainability. Sustainable systems promote multiple crop rotations per year which increase biodiversity.

The use of leguminous cover crops can promote nitrogen fixation via a symbiotic relationship with mycorrhizum bacteria that can convert atmospheric N² into plant available NO^-₃.  Cover crops have the ability to increase nutrient recycling and their residues result in nutrient mineralization into labile forms that would otherwise not be available to crop roots alone (Marchner 1998). In the tropics, biologically fixed N from legumes can supply greater than 110 kg N ha^-1 and have been shown to achieve crop yields equivalent to yields in conventional synthetic fertilizer-driven systems (Tonitto et al. 2006). Smithson and Giller (2002) found that the utilization of legume cover crops can reach levels up to 450 kg of N per ha^-1 crop^-1. At this level of N production, legume based cover crop systems in the tropics can meet crop N requirements if N is conserved and environmental losses are minimized. 

Decomposition rates occur much faster in tropical climates due to elevated air and soil temperatures, as the annual mean temperature on St. Croix is 28.3°C with a mean high temperature of 32.8°C (Godfrey and Hansen 1996). Nitrogen mineralization and volatilization increases due to higher ambient temperatures which results in greater N loss for warm climates, compared to cooler climates. Soil temperature plays an important role in soil microbial populations that aid in organic matter decomposition and nutrient cycling, as biodiverse active soil can increase organic matter decomposition and speed organic N mineralization rates (Smithson and Giller 2002). High rainfall can further exacerbate N losses through increased N leaching. Conventional tillage practices greatly increase the amount of soil particle surface area in contact with environmental elements, resulting in increased N loss. Therefore, conservation-till systems that include cover crop residues may greatly reduce N and other nutrient loss through decreased soil temperatures, decreased atmospheric soil particle exposure, and by slowing the rate of water infiltration into the soil (Akanvou et al. 2001; Marchner 1998). 

In repeatedly tilled soils beneficial soil organism populations are decreased due to organic matter exhaustion, and in these cases it is more likely that harmful species of bacteria, fungi and nematodes will develop in the root zone (Sullivan 2003); thereby, increasing the reliance on fungicides and pesticides. Organic mulch from cover crop residues left on the soil surface creates "sheet composting" that more efficiently converts the carbon in the crop residue into soil organic matter (Hoorman et al. 2009; Sullivan 2003). In comparison, mowing or chopping the residue increases decomposition and percent transformation of organic molecules into carbon dioxide. Mowing or chopping the cover crop residue may result in the production of less soil organic matter than what would be produced from the same quantity of cover crop biomass if the cover crop would have been killed with a roller-crimper and left as a surface mulch (Curran et al. 2010; NRCS 2002). Therefore, overall sustainability of the agriculture production system can be increased by the use of improved tropical cover crop rotations where the resulting biomass residue is utilized as an organic surface mulch in conservation-till cropping systems.  

Cover crop benefits to agroecosystems are well documented, but the adoption of cover crops become even more important in tropical and subtropical climates where soil degradation occurs at an accelerated rate compared to temperate climates. This is attributed to 12 months of continuous hot and humid conditions and a lack of seasonal changes resulting in elevated air and soil temperatures, increased solar radiation, high evapotranspiration and soil moisture loss, increased nutrient volatization, rapid organic matter decomposition, and increased soil macro and microorganism activity. Conservation-till, mechanical-kill systems for cover crops that utilize roller-crimper technologies have proven to be cost efficient, all while increasing soil organic matter, improving overall soil quality, and providing weed suppression (Curran et al. 2010) . Roller-crimper implementation that mechanically kill tropical cover crops by "rolling" and "crimping" plant material into surface residue results in an organic surface mulch layer that creates an organic soil cover to reduce soil erosion, lower soil temperature, reduce soil nutrient loss, reduce weed germination and development, and increase soil moisture compared to conventional tillage practices that fully incorporate cover crops that require mechanized soil disturbance.

Roller-crimper technology was developed in temperate climates where either a winter or summer kill assists in the termination of the cover crop. This is not an option in tropical/subtropical climates and many cover crops have not been evaluated under tropical conditions for termination with a roller-crimper without the use of herbicides. Additionally, the resulting vegetative regrowth potential of cover crops has not been documented and needs to be quantified to determine which cover crops are adaptable for use with a roller-crimper. This project provides crucial information on tropical cover crops and their ability to be roller-crimper terminated in tropical/subtropical cropping systems.

Literature Cited

Akanvou, R., L. Bastiaans, M. J. Kropff, J. Goudriaan and M. Becker. 2001. Characterization of growth, nitrogen accumulation and competitive ability of six tropical legumes for potential use in intercropping systems. J. Agronomy & Crop Science. 187: 111-120.

Curran W., M. Ryan, and S. Mirsky. 2010. Cover crop rollers for Northeastern grain production. Penn State University USDA?ARS. Retrieved from: http://extension.psu.edu/pests/weeds/cover-crop-rollers-for-northeastern-grain-production

Godfrey, R. W. and Hansen, P. J. 1996 Reproduction and milk yield of Holstein cows in the US Virgin Islands as influenced by time of year and coat color. Arch. Latinoam. Prod. Anim. 4:31-44

Hoorman James J.  Alan Sundermeier,  Rafiq Islam, Randall Reeder. 2009. Using cover crops to convert to no-till. Ohio State University: Agriculture and Natural Resources, Fact Sheet. Available at: http://ohioline.osu.edu/sag-fact/pdf/0011.pdf

Marchner, H. 1998. Soil-root interface: biological and biochemical processes. Soil Chemistry& Ecosystem Health, Soil Science Society of America.

Smithson, P.C., and K.E. Giller. 2002. Appropriate farm management practices for            alleviating N and P deficiencies in low-nutrient soils of the tropics. Plant and Soil. 245.1: 169-180.

Snapp, S.S., P.L. Mafongoya, and S. Waddington. 1998. Organic matter technologies for integrated nutrient management in smallholder cropping systems of Southern Africa. Agric. Ecosyst. Environ. 71: 185-200.

Sullivan, P. 2003. Overview of Cover Crops and Green Manures. NCAT/ATTRA Publication No. IP 024. Retrieved from: https://attra.ncat.org/attra-pub/summaries/summary.php?pub=288

Tonitto C., M.B. David, and L.E. Drinkwater. 2006. Replacing bare fallows with cover crops in fertilizer-intensive cropping systems: A meta-analysis of crop yield and N dynamics. Agriculture, Ecosystems and Environment. 112: 58-72. 

U.S. Department of Agriculture. 2007 Census of Agriculture.  National Agriculture Statistical Service. Virgin Islands of the United States. Vol. 1. Geographic area series. Part 54. U.S. Summary and State data.  USDA, Washington, DC. AC07-A-54

USDA-NRCS. 2002. U.S. Department of Agriculture, Natural Resource Conservation Service. Soil Quality Institute. Technical Note No. 13. The Knife Roller (Crimper): An Alternative Kill Method for Cover Crops. Retrieved from: http://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs142p2_053284.pdf

This Southern SARE On-Farm grant provided the financial resources necessary to evaluate promising tropical cover crops, evaluate and demonstrate a custom built roller-crimper for advances in alternative tropical cover crop management practices, to quantify cover crop response to termination with a roller-crimper, and to measure the effectiveness of the roller-crimper to suppress cover crop regrowth and weed development following cover crop termination. Specific project objectives are as follows: 

Key performance targets include:

1. To conduct and demonstrate on-farm cover crop research at Sejah Farm, St. Croix, USVI.
2. To conduct on-farm cover crop research that tests roller-crimper technology to increase knowledge for the integration of conservation-till tropical cover crop systems.
3. To develop recommendations for farmers for the use of tropical cover crop rotations that integrate roller crimper termination to reduce tillage and suppress weeds in vegetable production systems.

Project Objectives:

1. To construct a roller-crimper from locally available recycled materials for use in on-farm research.
2. To evaluate and compare five tropical crop species as cover crops by measuring and determining key cover crop performance indicators prior to termination (establishment rate, vegetative biomass, plant vigor, and plant tissue quality.
3. To measure the selected cover crops ability to suppress weeds prior to termination.
4. To determine the effectiveness of using a roller-crimper to terminate the selected crops by measuring cover crop regrowth.
5. To evaluate the rolled cover crop residue sheet mulch for weed suppression.
6. To evaluate four different mechanical cover crop termination methods on cover crop regrowth and weed suppression.