Developing Sustainable Tropical Leguminous Cover Crop and Green Manure Mulch Systems for Low-External-Input crop Production in the U.S. Virgin Islands, Puerto Rico, and Florida

Objective 1; U.S. Virgin Islands: Experiments were conducted at the University of the Virgin Islands Agricultural Experiment Station (UVI AES), Kingshill, U.S. Virgin Islands (lat. 17^◦43'08'' N, long. 64^◦47'46'' W, 100 ft. elevation). UVI AES is located on the island of St. Croix, which is the largest of three primary U.S. Virgin Islands. The territory presents an equatorial climate with dry summers. St. Croix is characterized as having a tropical/subtropical environment with annual rainfall averages of 1041 mm and a mean monthly temperature range of 72 to 91^◦F. Rainfall patterns exhibit inter annual variability with an initial early rainy season beginning in May that can extend into June, then a dry period in July and August followed by the primary rainy season from August to November (concurrent with the Atlantic hurricane season); and finally an extended dry season that runs from December/January. Highest temperatures occur in August–October, and the lowest in January–February. The annual mean temperature on St. Croix is 83^◦F. The soil on the experimental site is a Sion clay (SiB) according to the USDA soil survey

The experimental design was a complete randomized block with three replications. Experiments were conducted during the primary rainy season when cover crops are needed most for soil conservation, when pests are at their greatest, and when they can be established and grown without irrigation. Experiments took place from August, 2012 through February, 2013, repeated during the same time period in 2013/2014, and again in 2014/2015. The experimental design was a complete randomized block with three replications. Planting dates varied due to fluctuating environmental conditions. Cover crops evaluated were: velvet bean (Mucuna pruriens L. cv. Vine 90; VB), sunn hemp (Crotalaria juncea L. cv. IAC-1; SH), pigeon pea (Cajanus cajan L. cv. BRS Mandarim; PP), sesame (Sesamum indica L. cv. ns; SE), and sunflower (Helianthus annus L. cv. Black Oil; SF).

Cover crops were planted by broadcast sowing prior to the start of the heavy rainy season and terminated when 90% flowering was observed for SH, SE, SF, and VB via roller crimping 70 days after planting (DAP). PP took longer to mature and was terminated at 120 DAP. CCs were evaluated pre-termination on biomass, understory weed development and PNC nitrogen, phosphorus, and potassium estimated from CC biomass yield and CC tissue nutrient content and expressed as kg ha^-1. Post termination CC plots were evaluated for CC regrowth and weed development 3, 6, 9 and 12 weeks after termination (WAT). Weed categories were designated as broadleaf weeds (BW) or grass weeds (GW) and were separated from any CC regrowth present. All plant samples were collected by sampling a 0.25-m² area clipped at ground level with three samples collected per plot. Dimensions of treatment plots measured 5 x 10 m. PP results are reported separately from the other CC results and were not a part of the statistical analysis for SH, SF, SE, or VB data.

Data Analysis. Data were analysed using a general linear model and least square means separate with SAS (Cary, NC Ver. 9.4) with significance of alpha = 0.05.

Objective 1; Puerto Rico: Experiments were conducted for three planting dates; April and July 2013, and February 2014. Three studies were conducted to compare the effects of two crimped legumes (CL), dwarf velvet bean (mucuna pruriens), and sunn hemp (Crotalaria juncea cv. Tropic Sun) vs. conventional planting (CT; plow and disk passes) on ear yield and size, insect damage and seed yield of sweet corn (Zea mays L. cv. Suresweet 2011).

Objective 2; U.S. Virgin Islands: Experiments were conducted at the University of the Virgin Islands Agricultural Experiment Station (UVI AES), Kingshill, U.S. Virgin Islands (lat. 17^◦43'08'' N, long. 64^◦47'46'' W, 100 ft. elevation). UVI AES is located on the island of St. Croix, which is the largest of three primary U.S. Virgin Islands. The territory presents an equatorial climate with dry summers. St. Croix is characterized as having a tropical/subtropical environment with annual rainfall averages of 1041 mm and a mean monthly temperature range of 72 to 91^◦F. Rainfall patterns exhibit inter annual variability with an initial early rainy season beginning in May that can extend into June, then a dry period in July and August followed by the primary rainy season from August to November (concurrent with the Atlantic hurricane season); and finally an extended dry season that runs from December/January. Highest temperatures occur in August–October, and the lowest in January–February. The annual mean temperature on St. Croix is 83^◦F. The soil on the experimental site is a Sion clay (SiB) according to the USDA soil survey

The experimental design was a complete randomized block with three replications. Experiments were conducted during the primary rainy season when cover crops are needed most for soil conservation, when pests are at their greatest, and when they can be established and grown without irrigation. Experiments took place from September, 2012 through May, 2013, repeated during the same time period in 2013/2014, and again in 2014/2015. The experimental design was a complete randomized block with three replications. Cover crop planting dates varied due to fluctuating environmental conditions. Cover crops evaluated were: sunn hemp (Crotalaria juncea L. cv. IAC-1; SH), pigeon pea (Cajanus cajan L. cv. BRS Mandarim; PP), and sunflower (Helianthus annus L. cv. Black Oil; SF).

Cover crops were planted by broadcast sowing prior to the start of the heavy rainy season and terminated at flowering via roller-crimping between 110 to 129 days after planting (DAP) in mid-January. No external inputs were applied to the cover crops (no irrigation, fertilizer, or pesticides) Biomass sampling of cover crops and weeds took place prior to termination. Cover crops were evaluated pre-termination on biomass, understory weed development (separated by weed class). Weed categories were designated as broadleaf weeds (BW) or grass weeds (GW) and were separated from CC biomass. Potential nutrient content for nitrogen, phosphorus, and potassium was estimated from CC biomass yield and CC tissue nutrient content and expressed as kg ha^-1. All plant samples were collected by sampling a 0.25-m² area clipped at ground level with three samples collected per plot. Dimensions of treatment plots measured 5 x 10 m.  

Jalapeno peppers (Invicto-F1) were seeded into seedling trays and placed in a greenhouse. Following cover crop termination, Jalapeno peppers were then no-till hand transplanted directly into the SH, SF, and PP residue in situ mulch between 3 and 7 days after cover crop termination. Pepper seedlings were between 45 and 60 days old at time of transplant. A weedy fallow control plot was included during the cover crop period that was then prepared for pepper transplanting using full tillage methods and represented a conventional vegetable production system. Therefore, there were a total of four vegetable cropping systems evaluated:

• Sunn hemp (Crotalaria juncea IAC-1)
• Pigeon pea (Cajanus cajan BRS Mandarim)
• Sun flower (Helianthus annuus Black Oil)
• Weedy Fallow Control – Conventional full-till seed bed preparation

Six weeks after pepper transplant, all plots were split in half to compare pepper yields from weekly weeding vs. reduced-weeding at three week intervals. Post-termination CC plots were evaluated for CC regrowth and weed development 3, 6, 9 and 12 weeks after termination (WAT). Weed categories were designated as broadleaf weeds (BW) or grass weeds (GW) and were separated from any CC regrowth present (no sedges were present). All plant samples were collected by sampling a 0.25-m² area clipped at ground level with three samples collected per plot. Peppers where harvested from data rows, graded (marketable or unmarketable), and weighed.

Data Analysis. Data were analysed using a general linear model and least square means separate with SAS (Cary, NC Ver. 9.4) with significance of alpha = 0.05.

Objective 2; Puerto Rico: In a laboratory experiment, the allelopathic effects of aqueous extracts 3.3% w/v of crotalaria (Crotalaria juncea L. cv. Tropic Sun), jack bean (Canavalia ensiformis L.) and pigeon pea (Cajanus cajan L. cv. Lazaro) were determined for their effects on seven agronomic crops: corn (Zea mays cv. Mayorbela 2011), sorghum (Sorghum bicolor), bean (Phaseolus vulgaris cv. Verano), soybean (Glycine max), cowpea (Vigna unguiculata L.), lettuce (Lactuca sativa L. cv. Nevada) and pigeon pea (Cajanus cajan cv. Lazaro).

In a greenhouse study, sunn hemp (Crotalaria juncea cv. Tropic Sun), jack bean (Canavalia ensiformis L.) and pigeon pea (Cajanus cajan cv. Lazaro] were evaluated on their phytotoxic effects on germination percentage at three dates (4, 7 and 14-d) and the dry weight (14-d) of agronomic crop seedlings. Agronomic crops evaluated included in the study were corn (Zea mays cv. Mayorbela 2011), sorghum (Sorghum bicolor L.), bean (Phaseolus vulgaris L. cv. Verano), soybean (Glycine max L.), cowpea (Vigna unguiculata L.), lettuce (Lactuca sativa L. cv. Nevada) and pigeon pea (Cajanus cajan L. cv. Lazaro).

Objective 3a; Florida and the U.S. Virgin Islands: In both Florida and the US Virgin Islands, the Experimental design was a randomized split plot design; treatments were replicated four times. The experiment was conducted two years in Florida and one year in the U.S. Virgin Islands. Based on our experience, additional adjustments were made to the system, and the experiment was repeated for an additional two years in the U.S. Virgin Islands (see Objective 3b below). While there were specific adaptations to each location, treatments in Objective 3a were similar as follows:

Vegetable Crop Mulch Treatments:

1. ROLLER-CRIMPER: Sunn hemp terminated by crimper, residue remains on soil surface
2. NO MULCH: Sunn hemp mowed and soil incorporated
3. PLASTIC: Sunn hemp mowed and soil incorporated, Plastic mulch applied.
4. CUT-N-CARRY: Sunn hemp residue mowed and soil incorporated, straw applied to surface.

Two Weed Removal Frequencies:

Each plot was divided in half perpendicular to tractor direction, and weeding treatments were randomly assigned to each plot six weeks after planting (WAP).

1. LOW INTENSITY weeding (every 3^rd week)
2. HIGH INTENSITY weeding (every week)

Florida

Experimental location and design. The experiments were located on an Alpin-Blanton-Foxworth fine sandy soil at the UF-IFAS Suwannee Valley Research and Demonstration Center near Live Oak, FL. The unit (2.5 acres) was certified organic by Quality Certification Services in 2013, so experiments in 2012 was on transitional land, while 2013 and 2014 were on certified land. The experiment was situated on different locations within the unit each year. Treatments were as follows: 1) surface sheet mulch of sunn hemp terminated by a roller crimper; 2) sunn hemp mowed and incorporated followed by a surface application of locally purchased Bahia hay in 2012 and winter annual rye straw grown in situ in 2013 and 2014; 3) sunn hemp mowed and incorporated followed by an application of white on black plastic mulch; and 4) mowed and incorporated sunn hemp with no mulch application. Each plot was split and randomly assigned one of two weed removal frequencies: low intensity (weeding every three weeks) or high intensity (weeding every week). Treatments were arranged in a randomized complete block design and replicated four times. Each plot consisted of three rows, five feet (1.5) on center for a total width of 15 ft (4.5 m) by 25 ft (7.6 m) in length. Alley ways between replicates were 18 ft (5.5 m). Total experimental area including alleys was 0.35 acre (0.14 ha). In Florida in 2012, the cultivar of pepper selected did not perform well and multiple challenges following transplanting resulted in termination of the experiment to conserve grant funding. The experiment was executed without any significant challenges in 2013 and 2014. The following methods, results and impacts are presented for 2013 and 2014 only. Additional information on the 2012 season is located in the 2013 annual report. 

Cover crop management. Prior to sunn hemp seeding (12-14 days), 2 tons/acre of poultry litter was applied and incorporated providing a seasonal potentially available NO3-N level of 72 kg ha-1. Sunn hemp (Crotalaria juncea L., unnamed cultivar) was inoculated with N-Dure garden blend inoculant (Intex Microbials) prior to planting. Sunn hemp was seeded on May 19, 2013 and May 20, 2014 with a Great Plains no-till drill at 7/8 56 kg ha-1 using drive type 1 box set 60. Sunn hemp was terminated either by flail mowing or roller-crimping according to treatment. Termination occurred on July 26, 2013 and August 19, 2014.

Transplant management. Untreated jalapeno pepper ‘Tormenta’ (US Agriseeds) were seeded in a 73-unit cell tray with compliant seed starting mix (Organic Mechanics). Transplants were fertilized weekly with Drammatic One liquid fertilizer (4-1-1) (Dram Corporation) when root systems were established and two to three pairs of true leaves emerged. Pyganic was sprayed twice as needed for whiteflies. Seeds were planted on May 7, 2013 and May 29, 2014.

Treatment establishment. Except the plastic mulch treatment, all other treatments were planted on the flat. Beds were pulled in plastic mulch plots with a tractor mounted bed shaper. Based on pre-plant soil analysis, our experimental site rated medium response for additional potassium and medium high response for additional phosphorus each year. Dry granular fertilizer (Ag-Life 6-2-6, Rhizogen, Woodlands, Tx) was applied by hand at 75% of the recommended UF/IFAS rate for specialty peppers a rate of 125 lb a^-1 (63.5 kg ha^-1) nitrogen (7.25 lb per bed) in two bands, and lightly incorporated with a roto tiller in all plots except those with roller-crimped sunn hemp mulch. In roller crimp plots, fertilizer was applied with an applicator fitted with an eight-inch knife. Remaining nitrogen was applied through the drip using the same liquid fertilizer (4-1-1) following transplant establishment. Weekly fertigation supplied 2 lb a-1 N (2.24 kg ha-1). Drip tape was installed in all plots (RoDrip 12 in (30 cm) emitter spacing, John Deer, Moline IL). Cut and carry mulch was provided by bailed rye straw grown on location the previous winter in 2013 and 2014. Straw was applied by hand at a rate equal to sunn hemp dry weight above ground biomass each year. White on black plastic mulch (1.5ml) was installed by hand on raised beds in appropriate plots, and bare fallow plots were lightly evened prior to installing drip tape (John Deere 12 in emitter spacing).

Pepper Production. Peppers were transplanted by hand 80 and 84 days after seeding in the greenhouse on July 27, 2013 and August 19, 2014, respectively, in two offset rows per bed, with 24 in (61 cm) in-row spacing and 6 in (15 cm) from each side of the drip tape for a total of 25 plants per split plot. In 2013, pepper plant population was 21,529 plants per acre (53,176 plants ha-1). The area was experiencing record high temperatures for several weeks prior to planting in both years, and air temperatures were around 98 ⁰F with a heat index of 105 ⁰F. To compensate for high rates of evapotranspiration, additional overhead irrigation at planting was provided by a water wagon at a rate of 500 g to the experimental area immediately following planting.

Data Collection. Weed density was measured with a half meter frame placed randomly in each treatment prior to weed removal. Weeds were separated into broadleaf, grasses, and sedges, counted, dried in a 60 C forced air oven until constant weight, and weights recorded. Sunnhemp post-termination regrowth was documented by counting occurrences, removing actively growing stems and recording dry weight. Man-hours required for weed removal for each treatment were estimated twice each year in all treatments to estimate the labor demands of hoeing and removal of weeds by hand. Peppers were harvested weekly for 11 weeds in 2013, and eight weeks in 2014. Each year, harvest was interrupted by frost. Fruit was removed from the plant if mature or damaged. Damaged fruit were recorded separately as culls. All mature fruit was considered marketable. Fruit were counted and weighted from each plot. At the final harvest, fruit were categorized into number ones and small sizes.

Data Analysis. Data were analysed using a general linear model and least square means separate with SAS (Cary, NC Ver. 9.4) with significance of alpha = 0.05. Interactions of year occurred throughout data sets, therefore results are presented by year.

The U.S. Virgin Islands: Objective three of this study was carried out in Florida in 2013 and 2014 to provide background operational and technical information prior to the start of objective 3 studies performed in the U.S. Virgin Islands. The purpose of this was to generate experiential support that would benefit and improve the U.S. Virgin Islands project operations. Objective 3 was conducted in cooperation with local farmer Grantley Samuel and carried out at his farm on St. Croix. The study followed similar methods to the studies conducted at the UF-IFAS Suwannee Valley Research and Demonstration Center near Live Oak, FL. In the USVI studies, sunn hemp was planted on December 15, 2014 and terminated at full bloom with a roller-crimper on February 26, 2015. Prior to termination, CC and weed biomass was determined. Cubanelle peppers (Capsicum annuum cv. Naples) were germinated in a greenhouse and transplanted (46 days after planting) on Jan 30, 2015. Peppers were planted via no-till hand planting into sunn hemp residue in situ mulch and into conventional full till plots for all other treatments. Below is a summary of vegetable crop system treatments.

Vegetable Crop Mulch Treatments:

1. ROLLER-CRIMPER: Sunn hemp terminated by crimper, residue remains on soil surface
2. NO MULCH: Sunn hemp mowed and soil incorporated
3. PLASTIC: Sunn hemp mowed and soil incorporated, Plastic mulch applied.
4. CUT-N-CARRY: Sunn hemp residue mowed and soil incorporated, straw applied to surface.

Two Weed Removal Frequencies:

Each plot was divided in half perpendicular to tractor direction, and weeding treatments were randomly assigned to each plot six weeks after planting (WAP).

1. LOW INTENSITY weeding (every 3^rd week)
2. HIGH INTENSITY weeding (every week)

Six weeks after pepper transplant, all plots were split in half to compare pepper yields from high intensity weeding at weekly intervals vs. low intensity weeding at three week intervals. Post-termination CC plots were evaluated for CC regrowth and weed development 3, 6, 9 and 12 weeks after termination (WAT). Weed categories were designated as broadleaf weeds (BW) or grass weeds (GW) and were separated from any CC regrowth present (no sedges were present). All plant samples were collected by sampling a 0.25-m² area clipped at ground level with three samples collected per plot. Peppers where harvested from data rows, graded (marketable or unmarketable), and weighed.

Objective 3a; Puerto Rico: Two experiments were conducted to determine the effect of three leguminous cover crops (LCC) jack bean (Canavalia ensiformis L. DC.), sunn hemp (Crotalaria juncea L. cv. Tropic Sun) and pigeon pea (Cajanus cajan L. cv. Lazaro) and conventional planting (CP) on weed control, N content, vegetative growth, mineral composition (N, P, K, Ca, Mg, Fe, Mn, B and Zn), production parameters, and weevil incidence in the false-horn type Maricongo plantain. In June 2013, corms of Maricongo were established at the Isabela Agricultural Experiment Stations at Isabela, PR (very fine, kaolinitic, isohyperthermic and Typic Eutrustox) and Corozal, PR (mixed semiactive, isohyperthermic and Aquic Haplohumults). Four rows of leguminous cover crops were intercropped between plantain rows 60-cm spacing. A randomized complete block (RCB) design with three and four replications in Isabela and Corozal, respectively, was used. Ninety days after planting, weeds and LCC samples were cut at ground level, fresh weight taken and subsamples taken (500 g) to determine DM yield (kg ha^-1). The remainder of the LCC vegetative residue was left on the soil surface for two weeks and upon decomposition the LCC were replanted. During the experimental time frame, three LCC plantings occurred. During the growth cycle of the Maricongo clone at (3, 5, and 7 months post emergence, and at flowering) plant height (m) was determined (3, 5, and 7 months post emergence, and at flowering). Additional measurement collected include pseudostem diameter (cm). Leaf tissue samples were collected for leaf tissue nutrient analysis. At fruit maturity, plantains were harvested to measure production yield parameters. Banana weevil borer (Cosmopolites sordidus) were counted (larvae and adults) in the plantain corms to determine pest population density.

Data Analysis. Data were analysed using a general linear model and least square means separate with SAS (Cary, NC Ver. 9.4) with significance of alpha = 0.05.

Objective 3b; U.S. Virgin Islands: In 2015/2016 objective 3 was modified and repeated in the U.S. Virgin Islands at two separate field locations at the University of the Virgin Islands Agricultural Experiment Station on St. Croix, USVI. Similar methodology was used in this experiment as was used in previous objective 3 studies conducted in Florida and the U.S.V.I. The primary change in this study was that all mulch treatments utilized sunn hemp cover crop residue as in situ surface mulch and in each of the three mulch treatments pepper transplants were transplanted via no-till. The control plot still represented a full till conventional vegetable crop system where the sunn hemp residue was mowed and soil incorporated prior to pepper transplant. Two additional changes included the use of landscape fabric mulch instead of plastic film mulch and the use of both jalapeno and serrano peppers as the vegetable test crop.

Sunn hemp was planted as a cover crop on September 21, 2015 and allowed to reach full bloom prior to termination with a roller-crimper on January 4, 2016. Following termination, experimental treatment plots measuring 5x10 m were prepared. Four treatments were arranged in a RCBD and split to two levels of weeding intensity (high and low) subplots after vegetable transplanting to evaluate the effectiveness of weed management among treatments. Jalapeno and serrano peppers were then transplanted into treatment plots on January 7, 2016. Peppers were transplanted 60 days after planting. Below is a summary of vegetable crop system treatments.    

Vegetable Crop Mulch Treatments:

1. Sunn Hemp Mulch: Sunn hemp terminated by crimper, residue remains on soil surface (SHM)
2. Sunn Hemp + Hay: Sunn hemp terminated by crimper, residue remains on soil surface; hay mulch applied (SH+H)
3. Sunn Hemp + Fabric: Sunn hemp terminated by crimper, residue remains on soil surface; landscape fabric mulch applied (SH+F)
4. NO MULCH: Sunn hemp mowed and soil incorporated (SH+None)

Two Weed Removal Frequencies: Each plot was divided in half perpendicular to tractor direction, and weeding treatments were randomly assigned to each plot after pepper transplanting.

1. LOW INTENSITY weeding (every 3^rd week)                       B.    HIGH INTENSITY weeding (every week)

Post-termination CC plots were evaluated for CC regrowth and weed development 3, 6, 9 and 12 weeks after termination (WAT). Weed categories were designated as broadleaf weeds (BW) or grass weeds (GW) and were separated from any CC regrowth present (no sedges were present). All plant samples were collected by sampling a 0.25-m² area clipped at ground level with three samples collected per plot. Peppers where harvested from data rows, graded (marketable or unmarketable), and weighed.

Data Analysis. Data were analysed using a general linear model and least square means separate with SAS (Cary, NC Ver. 9.4) with significance of alpha = 0.05.

Objective 4; All Locations: Discover the Cover Workshop series: Multiple workshops followed by practical field demonstrations were conducted in the U.S. Virgin Islands, Puerto Rico, and Florida. Results will be disseminated by online webinars, professional conferences, and additional outreach activities.