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Reduced tillage termination of cover crop systems in the tropics Stuart A. Weiss and K.P. Beamer University of the Virgin Islands, Agriculture Experiment Station, St. Croix, US Virgin Islands Introduction: 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. Low-external-input farmers rely heavily on farm-derived resources such as cover crops for soil and pest management. Tropical agroecosystems require cover crop management strategies to be modified to meet environmental and cultural conditions and the use of reduced tillage practices have been promoted to increase soil conservation and reduce on-farm expenses. Conventional cover crop management strategies were developed for temperate climates where plant senescence is timed with seasonal transition for effective CC termination. Mechanical cutting followed by full incorporation of CCs in the tropics has been the accepted practice for CC termination. While an effective termination tool, this method relies on conventional soil tillage that can result in decreased soil conservation. The alternative method of rolling/crimping CCs to produce surface sheet mulch has gained attention as a progressive practice that reduces tillage and provides additional agroecosystem benefits. However, tropical environments have a 365 day warm growing cycle which promotes re- growth capabilities of many indeterminate CCs through potential crown and bud meristematic activity. Assessment of different mechanical CC termination methods is needed to avoid having CCs become weed pests. A CC termination study was conducted on St. Croix in the U.S. Virgin Islands to test 4 mechanical termination methods and their effects on CC regrowth, as well as broadleaf and grass weed suppression. Objectives: To evaluate sunn hemp [(Crotalaria juncea cv. IAC-1) SH] and lablab [(Lalab purpureus cv. Rongai) LL] as CCs and their ability to suppress weeds. To evaluate 4 different types of mechanical CC termination and their effect on CC regrowth and weed development. To monitor the physical and chemical decomposition of SH and LL residue. Materials and Methods: At the University of the Virgin Islands in St. Croix, sunn hemp and lablab were planted on October 3, 2012, evaluated as CCs, and then terminated 120 days after planting. No additional external inputs were applied to the fields. Termination treatments tested consisted of: • 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), • 4) Roll down 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 28, 42, and 63 days after termination and analyzed for plant chemical properties. Results and Discussion: 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). Lablab 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 1, 3, and 4 (11 ± 198, 91 ± 198, and 498 ± 198 respectively) (p≤0.05). At 9 and 12 weeks after termination, SH regrowth was effectively controlled in all termination treatments with the only measurable regrowth occurring in plots terminated with the roller-crimper (Table 2). In contrast, LL had higher levels of regrowth across all treatments for all three post-termination harvests and termination treatments 1, 3, and 4 resulted in similar LL regrowth for each respective post-termination harvest date. Results indicate that SH has a favorable response to all reduced tillage termination methods tested compared to LL, thus, SH may be better suited for use as a CC in reduced tillage tropical agroecosystems. Sunn hemp controlled grass weeds in treatments 1, 2, and 4 through week 9 which had similar biomass accumulation of grass weeds at week 9 with 0, 0, and 196 ± 127 kg/ha. At 12 weeks after SH termination, broadleaf and grass weed levels exceeded 1000 kg/ha in all treatments except for treatment 1 which had the lowest levels at 631 ± 260 kg/ha and 44 ± 260 kg/ha, respectively (p≤0.05). Therefore, full incorporation with 3 passes with the disc harrow resulted in the most effective termination and weed suppression method for SH. Sunn hemp crop residue N content after termination was not influenced by either treatment 1 or 4, but did change over time by increasing from day 1 to day 28 by 19 percent from 1.7 ± 0.2 to 2.1 ± 0.2 percent N (p≤0.05), and then returning to 1.7 percent N at 42 and 63 days after termination. Total N 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 ± 0.2 percent N) compared to treatment 1 (2.1 to 2.5 ± 0.1 percent N) (p≤0.05). Nitrate-N content in SH surface residue resulting from termination with the roller-crimper increased over time to a high of 348 ppm at 63 days after termination. In comparison, nitrate-N content of SH residue fully incorporated peaked at 180 ppm at 42 days after termination and then decreased to 158 ppm at 63 days after termination. Lablab surface residue and fully incorporated residue were highest at 42 days after termination at 228 and 240 ppm nitrate-N, respectively. From 42 to 63 days after termination both surface and fully incorporated LL residue decreased, however, nitrate-N content in surface residue only dropped to 189 ppm while fully incorporated LL residue dropped to 133 ppm. Nitrate-N content of SH surface residue (termination with a roller- crimper) increases over time and provides a slower, delayed conversion of nitrate-N compared to SH residue that is fully incorporated. Lablab responds in a similar way at 63 days after termination where nitrate-N content of surface residue is 30% greater than that of fully incorporated LL residue. These findings will allow farmers to more accurately align cover crop residue nitrate-N availability with peak crop demand. Results of this study provide farmers information to make improved cover and cash crop management decisions to improve production efficiency. CC BL GW Total Weeds Sunn Hemp 6,800 ± 684a 196 ± 130a 413 ± 619a 609 ± 614a Lablab 3,127 ± 684b 238 ± 130a 1,480 ± 619a 1,718 ± 614b Values within the same column group followed by different letters differ (p<0.05) according to a least significant range seperation. Cover crop (CC), broad leaf (BL) weed, and poacea (GW) weed biomass (kg/ha-1) of Sunn Hemp and Lablab at termination 0 50 100 150 200 250 Surface Residue Incorporated Residue 103 125 228 240 189 133 NO3-N Days After Termination Nitrate-nitrogen (NO3-N) content (ppm) of Lablab residue as surface mulch after roller-crimping or full incorporation at 28, 42, and 63 days after termination 28 42 63 0 50 100 150 200 250 300 350 Surface Residue Incorporated Residue 103 163 128 180 348 158 NO3-N Days After Termination Nitrate-nitrogen (NO3-N) content (ppm) of Sunn Hemp residue as surface mulch after roller-crimping or full incorporation at 28, 42, and 63 days after termination 28 42 63 Treatments (TM) TM TM Sunn Hemp CCRG CCVol BL GW CCRG CCVol BL GW CCRG CCVol BL GW 1) Full Disc (FD; 3 passes) 0 ± 47a 264 ± 47a 11 ± 47b 0 ± 47a FD 0 ± 127b 1,111 ± 127a 9 ± 127a 0 ± 127b FD 0 ± 260b 2,613 ± 260a 631 ± 260a 44 ± 260a 2) Disc (D;1 pass) 0 ± 47a 138 ± 47b 87 ± 47ab 29 ± 47a D 0 ± 127b 740 ± 127b 482 ± 127ab 0 ± 127b D 0 ± 260b 2,418 ± 260a 1,084 ± 260ab 1,389 ± 260b 3) Mow (M; 1 pass) 0 ± 47a 102 ± 47b 151 ± 47ab 142 ± 47a M 84 ± 127ab 362 ± 127c 411 ± 127ab 537 ± 127a M 0 ± 260b 478 ± 260b 1,613 ± 260b 2,231 ± 260b 4) Roller-Crimp (RC; 1 pass) 0 ± 47a 58 ± 47b 198 ± 47a 38 ± 47a RC 211 ± 127a 0 ± 127b 696 ± 127b 196 ± 127b RC 367 ± 260a 67 ± 260b 1,202 ± 260ab 1,967 ± 260b Lablab CCRG CCVol BL GW CCRG CCVol BL GW CCRG CCVol BL GW 1) Full Disc (FD; 3 passes) 11 ± 198b 0 33 ± 198a 40 ± 198a FD 264 ± 233b 0 322 ± 233a 7 ± 233b FD 1,109 ± 288b 0 1,147 ± 288b 878 ± 288a 2) Disc (D;1 pass) 1,229 ± 198a 0 229 ± 198a 118 ± 198a D 1,756 ± 233a 0 429 ± 233a 604 ± 233ab D 2,178 ± 288a 0 36 ± 288a 867 ± 288a 3) Mow (M; 1 pass) 91 ± 198b 0 267 ± 198a 302 ± 198a M 484 ± 233b 0 702 ± 233a 1,113 ± 233a M 736 ± 288b 0 611 ± 288ab 1,384 ± 288a 4) Roller-Crimp (RC; 1 pass) 498 ± 198b 0 149 ± 198a 869 ± 198a RC 924 ± 233ab 0 687 ± 233a 411 ± 233ab RC 1,098 ± 288b 0 431 ± 288b 1,869 ± 288a Cover Crop Regrowth = CCRG Volunteer Cover Crop = CCVol Broad Leaf Weeds = BL Grass Weeds + GW Values within the same column group followed by different letters differ (p<0.05) according to a least significant range seperation. 6 Week Harvest 9 Week Harvest Cover crop regrowth and weed biomass (kg/ha-1) at 6, 9, and 12 weeks after termination 12 Week Harvest 6 Week Harvest 9 Week Harvest 12 Week Harvest Days After Termination Rolled Full Till 1 2.1 ± 0.2ac 2.1 ± 0.2ac 28 2.5 ± 0.2abc 1.9 ± 0.2ad 42 2.8 ± 0.2bc 2.1 ± 0.2ad 63 3.0 ± 0.2bc 2.5 ± 0.2bc a, b values within the same column group differ and c, d values within the same row group differ (p<0.05) according to a least significant range seperation. Lablab Nitrogen (percent) content of Lablab vegetative residue left on the surface or soil incorporated N% P% K% N% P% K% 1 1.7 ± 0.2b 0.09 ± 0.03a 1.3 ± 0.05a 0.08 ± 0.01a 2.2 ± 0.1a 38 2.1 ± 0.2a 0.21 ± 0.03b 0.7 ± 0.05b See Table 4 0.15 ± 0.01ab 1.3 ± 0.1b 42 1.7 ± 0.2b 0.19 ± 0.03b 0.5 ± 0.05c 0.14 ± 0.01b 0.8 ± 0.1c 63 1.7 ± 0.2b 0.2 ± 0.03b 0.6 ± 0.05c 0.18 ± 0.01c 0.8 ± 0.1c Differences in values observed by week, not by treatment. Values within the same column group followed by different letters differ (p<0.05) according to a least significant range seperation. Sunn Hemp Nutrient content of Sunn Hemp and Lablab vegetative residue at 1, 28, 42, and 63 days after termination Dayss After Termination Lablab N Contribution Plant P % P Contribution Plant K % K Contribution Sunn Hemp 6,800 ± 684a 1.7 ± 0.1a 117 ± 15a 0.09 ± 0.006a 6 ± 0.5a 1.3 ± 0.07a 85 ± 15a Lablab 3,127 ± 684b 2.3 ± 0.1b 70 ± 15b 0.08 ± 0.006a 2.3 ± 0.5b 2.2 ± 0.07b 71 ± 15a Plant N % Cover crop plant tissue nutrient content (percent) and estimated nutrient contribution (kg/ha-1) for nitrogen (N), phosphorus (P), and potassium (K) based upon total vegetative biomass (kg/ha-1) Vegetative Biomass Values within the same column group followed by different letters differ (p<0.05) according to a least significant range seperation.