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 [(Lalab 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 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 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).