Promoting Tropical Cover Crop Mulch Systems for Minimum-Till Crop Production in the U.S. Virgin Islands
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/yr gross annual income) with 63% of farms totaling 9 acres or less and 81% of farms totaling 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 6 months. Both surface and subsurface ground water is severely limited and farmers rely strictly upon precipitation for agronomic production and the use of water conservation practices such as micro irrigation from harvested rainwater stored in tanks/ponds or pumped from shallow wells for fruit and vegetable production. Therefore, the supply of sufficient soil-moisture levels for crop production during the dry season is often a limiting factor for crop production in the USVI.
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. Soil fertility and pest management create unique challenges to USVI farmers in that imported fertilizers and pesticides are 2-to 3-times the cost of inputs in the continental USA and they 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 and are often not available at all. Therefore, low-external-input agriculture must utilize alternative production methods which will increase soil fertility and reduce pest incidence by implementing green manure cover crops, sheet composting, crop rotation, and conservation tillage technologies.
Cover crop benefits to agroecological systems 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, especially when conventional soil tillage methods are practiced. This is due to a 12 month hot and humid climate and the lack of seasonal changes that result 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. Minimum- 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. Roller crimper implementation that mechanically kill tropical cover crops by “rolling” and “crimping” plant material into surface residue results in an organic mulch 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 intensive tillage practices that fully incorporate cover crops. This management technique combines sustainable farming techniques of cover crops, green manure, organic mulch, and conservation tillage for sustainable low-external-input agriculture.
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 roller crimper termination and their vegetative regrowth potential has not been documented. This project will provide crucial baseline information on the potential of roller-crimper technology for its applied use in tropical/subtropical agroecosystems to reduce tillage, increase soil quality, and provide a number of different ecosystem services to increase overall farm sustainability.
Key performance targets include:
- To conduct and demonstrate on-farm cover crop research at Sejah Farm, St. Croix, USVI.
- To conduct on-farm cover crop research that tests roller-crimper technology to increase knowledge for the integration of no-till tropical cover crop systems.
- 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.
- To evaluate and compare three 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.
- To measure the selected cover crops ability to suppress weeds prior to termination.
- To determine the effectiveness of using a roller-crimper to terminate the selected crops by measuring cover crop regrowth.
- To evaluate the rolled cover crop residue sheet mulch for weed suppression.
- To evaluate four different mechanical cover crop termination methods on CC regrowth and weed suppression.
- To determine nutrient content in sunn hemp and lab lab cover crop residue as surface mulch (roller crimper) or fully incorporated (disk harrow).
On October 15, 2012, one production field (1 acre) was identified at Sejah Farm to be the location to conduct the 2012/2013 experimental cover crop trials for the second year of the On-Farm project. The fields selected were different from the fields used for the trials held in the previous year during the 2011/2012 season and were part of the farm’s primary production fields. Farm records indicated that the selected field was in need of a cover crop rotation. The field was mowed with a rotary deck mower and then tilled with a disc harrow. Prior to planting, the field was disc harrowed for the second time to prepare the seed bed for cover crop planting. The field was divided into 9 sub-fields and each subfield was randomly assigned to one of three cover crop treatments. As in year one, sorghum-sudan grass (Sorghum bicolor x S. sudanense cv. Mega Green, SS) was selected for the cover crop grass species since it is highly adaptable to tropical climates and has been a common cover crop selection among temperate farmers. The legume species would consist of one that represents a cover crop that has climbing/vining stems with a low lignocellulosic content (pliable stems) for which lab lab (Lablab purpureus cv. Rongai, LL) was selected and the second leguminous cover crop would be characterized as having erect stems with a high lignocellulosic content for which sunn hemp (Crotalaria juncea cv. IAC-1, SH) was selected. This design resulted in three randomly assigned cover crop treatments with three replications.
On November 1, 2012, SS, SH, and LL was planted at Sejah Farm by broadcast seeding. Sorghum sudan was seeded at a rate of 40 lbs./acre, SH at 50 lbs./acre, and LL at 65 lbs./acre. After seeding all plots where rolled with a culti-packer to increase seed germination rate. Sorghum sudan and SH germinated within 7 to 10 days after planting while LL had poor germination, poor establishment, and was negatively affected by heavy army worm herbivory. Sorghum sudan and SH fully established and LL failed to establish in 2013. The LL plots were therefore treated as weedy fallow treatment plots due to high weed establishment and development in the absence of a suppressive cover crop. Sorghum sudan and SH reached vegetative maturity with the plant stand exhibiting greater than 85% flower development at 50 days after planting (DAP). Prior to termination, CC biomass and weed biomass (from the CC understory) samples were collected from each treatment plot. The LL treatments were sampled for weed biomass and then mowed with a rotary deck mower followed by tillage with a disc harrow to simulate conventional tillage. Cover crop plant tissue samples were analyzed for nutrient content to assess cover crop nutrient contribution to the system. After all data was collected SS and SH were terminated with the roller crimper. Sejah Farms then planted vegetable crops (transplants) into the cover crop residue (SS and SH surface sheet mulch) and into the conventionally tilled plots. At 4 and 6 weeks post termination, all treatment plots were assessed for cover crop regrowth and weed development. Data was compiled, analyzed, and interpreted for reporting results and findings.
In a second experiment SH and LL was planted in two, 1 acre fields. Cover crop establishment, biomass yield, weed biomass yield and cover crop plant tissue nutrient analysis was determined prior to termination. Both SH and LL were terminated 120 days after planting and each field was then subdivided into 4 treatment subplots with 3 replications each. Each treatment subplot for SH and LL was then randomly assigned to one of four mechanical termination methods that represented standard cover crop termination practices that included both full tillage and no tillage management methods. Post-termination treatments were randomly assigned to each subplot and consisted of 4 termination methods that included; 1) full incorporation with a disc harrow (3 passes), 2) minimum incorporation with a disc harrow (1 pass), 3) mowing with a rotary brush mower (1 pass), and roll down with a roller crimper (1 pass). Cover crop regrowth and weed biomass was determined at 6, 9, and 12 weeks post-termination. Weed species were separated by weed class and designated either a grass or broadleaf, no sedges were encountered in this trial. Litter bags containing either SH or LL crop residue were placed in the field on day 1 after termination in treatments 1 (buried below the soil at 2 in. in depth) and 4 (left on the soil surface) to measure decomposition rate and nutrient release. Litter bags were collected at 4, 6, and 9 weeks after CC termination and analyzed for plant fry matter disappearance and plant chemical properties.
In a separate on-farm cover crop trial located on SunCroix Farm, SH was planted on farm to serve as a cover crop for soil conservation, soil improvement, and weed control. Sunn hemp was planted in November 2012 with a Great Plains No-Till Drill at a rate of 45 lbs. per acre into lightly disked soil. The SH cover crop fully established and provided the farmer with a cover crop from November, 2012 until July, 2013. Sun hemp biomass and weed biomass was collected in July, 2013 prior to cover crop termination. On July 17, 2013 the SH was terminated with the roller crimper to produce a thick layer of surface sheet mulch which continued to protect the soil for 12 weeks into the rainy season. During this time SH regrowth and weed establishment was measured at 3, 6, 9, and 12 weeks after termination.
Impacts and Contributions/Outcomes
Project Impacts, outcomes, and lessons learned from working with tropical cover crop systems
Year 2 results from the on-farm cover crop trial conducted at Sejah Farm differed in some aspects compared to year 1 results, however, the general trends and major findings were similar in year 1 and 2. Overall CC biomass was significantly lower in year 2 and there was no difference in CC biomass between SH and SS with 3,589 and 4,424 kg/ha, respectively. This is a result of the shortened CC rotation from 110 day in year 1 to 55 days in year 2. Lab lab failed to establish due to environmental and biological conditions (lack of rainfall and intense army worm herbivory). Sunn hemp contributed 79 kg/ha of nitrogen and SS contributed a total of 96 kg/ha of nitrogen which were not significantly different. Weed development during cover crop establishment and growth was similar between SH and SS for both grass and broadleaf weeds. However, due to the fact that LL failed to establish, the LL treatments served as a weedy fallow which demonstrated what the true weed pressure would be in conventional full tillage systems where the soil is left fallow without a cover crop to suppress weed development. The weedy fallow treatments (which were the failed LL subplots) developed extremely high broadleaf weed populations resulting in high broadleaf and total weed biomass during the same period of low weed development in the SH and SS cover crop treatments. The weedy fallow treatments had a mean broadleaf weed biomass of 1,404 kg/ha compared to broadleaf weed biomass levels of 93 and 162 kg/ha for SH and SS treatments, respectively. Total weed biomass (grass and broadleaf weeds combined) reflected the same trend with a total of 1,533, 193, and 184 kg/ha for the weedy fallow, SH, and SS treatments, respectively, This indicates the benefit of CCs to suppress weed development during the cover crop rotation which has numerous benefits to the overall farming system.
Both SH and SS regrowth biomass and their ability to suppress weed development after termination was similar to year 1 results. At 4 weeks after termination SS had the highest level of CC regrowth with 340 kg/ha which was higher than that for the SH and weedy fallow treatments. The SH treatments had the same level of total weeds and CC regrowth as the weedy fallow at 4 weeks after termination which indicates that the SH surface sheet mulch residue was equally effective as suppressing weeds as full soil tillage with multiple passes with a disk harrow. At 6 weeks post termination, year 2 results showed an increase in SH and SS regrowth. However, the dominant weed class in the SS treatments was the SS regrowth and the dominant weed class in the SH was a combination of regrowth and broadleaf weeds. Overall, at 6 weeks post termination, the SS had the highest total CC regrowth and weed biomass (2,467 kg/ha), followed by SH (1,478 kg/ha, and then the weedy fallow (309 kg/ha). This indicates that SS crop residue effectively suppressed weed development, however, SS was not effectively terminated with a roller crimper and future research needs to identify SS cultivars that exhibit determinant characteristics. Overall, SH providedthe greatest level of weed suppression as a cover crop and as surface sheet mulch when terminated with a roller crimper and provided equal weed suppression to fully tilled soil up to 4 weeks after termination.
Differences in results occurred between SH and LL cover crop performance and in their response to the four mechanical termination practices evaluated that included; 1) full incorporation with a disc harrow (3 passes), 2) minimum incorporation with a disc harrow (1 pass), 3) mowing with a rotary brush mower (1 pass), and roll down with a roller-crimper (1 pass). Sunn hemp yielded the highest amount of CC biomass at termination with 6,800 ± 683 kg/ha compared to LL at 3,126 ± 683 (p=0.002). Lab lab had greater plant tissue nitrogen (N) content than SH at 2.3% ± 0.1 compared to 1.7 ± 0.1, respectively. However, due to the greater SH biomass, total estimated N contribution was greater for SH (117 kg/ha) than for LL (70 kg/ha). At 6 weeks after termination, SH had 0 regrowth across all treatments compared to LL which had the greatest measured regrowth from treatment 2 (1,229) and similar regrowth in treatments 2, 3, and 4 (11, 91, and 498, respectively). This trend continues at 9 and 12 weeks after termination, indicating that SH regrowth is not as vigorous compared to LL, thus, SH may be better suited for use as a CC in reduced tillage tropical agroecosystems. Sunn hemp controlled broadleaf and grass weeds across all treatments through week 9 after termination. Treatments 1, 2, and 4 had similar levels of grass weeds at week 9 with 0, 0, and 196 kg/ha, respectively compared to treatment 3 with 573 kg/ha. Broadleaf weeds followed similar trends in weeks 6 and 9 until week 12 when broadleaf and grass weed levels exceeded 1000 kg/ha in all treatments except for treatment 1 which had the lowest level of broadleaf and grass weeds at 631 kg/ha and 44 kg/ha, respectively. Sunn hemp crop residue N content after termination was not influenced by either treatment 1 or 4, but did change over time by increasing by 19 percent in week 4 from 1.7 to 2.1 percent N, and then returning to 1.7 percent N at 6 and 9 weeks after termination. Nitrogen content in LL crop residue was influenced by treatment and time with greater N levels in LL residue from treatment 4 (2.1 to 3.0 percent N) compared to treatment 1 (2.1 to 2.5 percent N).
Impacts and contributions from the SunCroix farm are being analyzed and results interpreted and will be reported in the final report.
Outcomes resulting from this project that apply to tropical/subtropical cover crop management:
Sunn Hemp provided the greatest level of weed suppression as a cover crop of the three cover crops evaluated while lab lab had the poorest
The use of a roller-crimper for cover crop termination was effective for sunn hemp, but was not effective for either cultivars of sorghum sudan or lab lab.
Sunn hemp cover crop residue formed a dead layer of surface sheet mulch that suppressed weeds for 6 weeks after termination in year 1 but not in year 2.
Sunn hemp stage of maturity contributed to the effectiveness of the cover crop to be terminated with a roller crimper to form a weed suppressive residue barrier. Lignification of the sunn hemp stem at the time of flowering increased termination effectiveness at 110 days after planting compared to less lignified stems at 55 days after planting.
Sorghum Sudan cv. Mega Green and lab lab cv. Rongai were not killed by termination with a roller-crimper and continued to produce plant regrowth through the development of new shoots.
Cover crops can be a valuable management tool in the tropics that require few if any external inputs.
Cover crop regrowth may cause problems when using a roller crimper in tropical or extended warm season environments.
For vining/twining and Poaceae cover crops, roller-crimper termination may not be viable without herbicide application.
Lab lab had higher plant tissue nitrogen content than sunn hemp, but due to higher biomass production there was greater nitrogen contributed to the farming system through sunn hemp.
Post termination sunn hemp crop residue showed an increase in nitrogen content compared to levels at termination. Surface mulch crop residue of both sunn hemp and lab lab exhibited a delayed release of nitrate nitrogen compared to fully incorporated cover crop residue.
Surface cover crop residue can provide a delayed release of nitrate nitrogen that is better timed with the following vegetable crop rotation nitrogen nutrient demands. Sunn hemp surface sheet mulch has a more pronounced delayed nitrate nitrogen release than lab lab surface sheet mulch.
American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America National Conference Abstract Submission and Oral Presentation.
Weiss, S.A. and Kenneth P. Beamer. 2013. Reduced Tillage Termination Of Cover Crop Systems In The Tropics. In: International Annual Meetings ASA-CSSA-SSSA; Nov. 3-6, 2013, Tampa, Fl. University of the Virgin Islands, Kingshill, US Virgin Islands.
Stuart A. Weiss and K.P. Beamer, Agriculture Experiment Station – Agronomy Program, University of the Virgin Islands, St. Croix.
Reduced tillage termination of cover crop systems in the tropics
Cover crop (CC) use is increasing around the world and their use is considered a valued component of sustainable agricultural production systems. Cover crops provide a range of agricultural and ecosystem benefits which range from soil protection and improvement to pest reduction. Tropical agroecosystems require cover crop management strategies to be modified to meet environmental and cultural conditions. Farm operators limited to low-external-input agroecosystems often rely exclusively on farm-derived resources for soil fertility management and reduced tillage practices which have been promoted for soil conservation and to reduce on-farm expenses. At the University of the Virgin Islands in St. Croix, sunn hemp [(Crotalaria juncea cv. IAC-1) SH] and lab lab [(Lablab purpureus cv. Rongai) LL] were planted on October 3, 2012, evaluated as CCs, and then terminated 120 days after planting. Post-termination treatments were randomly assigned and consisted of 4 termination methods that included; 1) full incorporation with a disc harrow (3 passes), 2) minimum incorporation with a disc harrow (1 pass), 3) mowing with a rotary brush mower (1 pass), and 4) termination with a roller-crimper (1 pass). Cover crop and weed biomass were determined prior to termination and subsequent CC regrowth and weed biomass was determined at 6, 9, and 12 weeks post-termination. Weed species were separated by weed class and designated either a grass or broadleaf, no sedges were encountered in this trial. Litter bags containing either SH or LL crop residue were placed in treatments 1 and 4 on day 1 after termination and were collected at 4, 6, and 9 weeks and analyzed for plant chemical properties. Sunn hemp yielded the highest amount of CC biomass at termination with 6,800 ± 683 kg/ha compared to LL at 3,126 ± 683 (p=0.002). Lab lab had greater plant tissue nitrogen (N) content than SH at 2.3% ± 0.1 compared to 1.7 ± 0.1, respectively. However, due to the greater SH biomass, total estimated N contribution was greater for SH (117 kg/ha ± 15) than for LL (70 kg/ha ± 15) (p≤0.05). At 6 weeks after termination, SH had 0 regrowth across all treatments compared to LL which had the greatest measured regrowth from treatment 2 (1,229 ± 198) and similar regrowth in treatments 2, 3, and 4 (11 ± 198, 91 ± 198, and 498 ± 198 respectively) (p≤0.05). This trend continues at 9 and 12 weeks after termination, indicating that SH regrowth is not as vigorous compared to LL, thus, SH may be better suited for use as a CC in reduced tillage tropical agroecosystems. Sunn hemp controlled broadleaf and grass weeds across all treatments through week 9 after termination. Treatments 1, 2, and 4 had similar levels of grass weeds at week 9 with 0, 0, and 196 ± 127 kg/ha, respectively compared to treatment 3 with 573 ± 127 kg/ha (p≤0.05). Broadleaf weeds followed similar trends in weeks 6 and 9 until week 12 when broadleaf and grass weed levels exceeded 1000 kg/ha in all treatments except for treatment 1 which had the lowest level of broadleaf and grass weeds at 631 ± 260 kg/ha and 44 ± 260 kg/ha, respectively (p≤0.05). Sunn hemp crop residue N content after termination was not influenced by either treatment 1 or 4, but did change over time by increasing by 19 percent in week 4 from 1.7 ± 0.2 to 2.1 ± 0.2 percent N (p≤0.05), and then returning to 1.7 percent N at 6 and 9 weeks after termination. Nitrogen content in LL crop residue was influenced by treatment and time with greater N levels in LL residue from treatment 4 (2.1 ± 0.2 to 3.0 ± 0.2 percent N) compared to treatment 1 (2.1 ± 0.1 to 2.5 ± 0.1 percent N) (p≤0.05).
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