Nitrogen Allocation In Low-External-Input Tropical Cover Crop Livestock Forage Systems.

Tropical smallholder farmers that operate under low external input (LEI) conditions often have integrated agricultural systems that include different combinations of agronomic, horticultural, and livestock interests.  Where external inputs are not economically feasible and may not be locally available, dual-purpose cover crops (CC) that can supply organic matter, soil nitrogen, and serve as livestock forage in LEI crop production systems are of particular interest.  Field plots were established on St. Croix, U.S. Virgin Islands in August 2009 in a split plot design to evaluate the effect of three cropping systems and two harvest dates on harvested forage and non-harvested root/stubble plant tissue nitrogen (N) and CC biomass. Systems tested were: 1) high crop complexity with 1 legume (HIGH-L) and 1 grass (HIGH-G) CC biculture, 2) intermediate crop complexity with 1 legume (MED-L) or 1 grass (MED-G) CC monoculture, and a control of 3) low crop complexity (LOW) of native grass monoculture.  The legume CC was sunn hemp (Crotalaria juncea L.) [SH], the grass CC was sorghum-sudangrass (Sorghum bicolor x S. sudanense L. cv Mega Green) [SS], and the control plots were volunteer native guinea grass (Panicum maximum Jacq.) [GG] CC.  Harvest 1 was conducted 60 days after planting (DAP) and harvest 2 took place 105 DAP, both harvests represented a CC with forage removal.  Statistical analysis was performed by system complexity and harvest using PROC MIXED (SAS).  The SS (Med-G) monoculture harvested biomass yield was 4,787 and 4,698 kg ha^-1 at harvest 1 and 2, respectively, which was greater than all other CC forage biomass at harvest 1, but was similar to GG forage biomass with 4,565 kg ha^-1 at harvest 2 (P ≤ 0.05).  Root/stubble biomass for SS (Med-G) produced 1,296 kg ha^-1 in harvest 1; however, harvest 2 GG produced the greatest root/stubble biomass at 3,448 kg ha^-1 (P ≤ 0.05).  Sunn hemp (Med-L) had the numerically highest vegetative N level (3.9 % N DM basis), but had the lowest root/stubble N (1 % N DM basis).  Guinea grass had the highest root/stubble N percent at all harvests with a range of 2.1 to 2.8 % N DM basis.  Overall, N concentrations declined for forage biomass and root/stubble from harvest 1 to harvest 2 (P ≤ 0.05).  Neither soil organic matter nor NO₃-N showed any major shifts over the 4-mo crop cycle and averaged 2.5 % and 27 ppm, respectively.  Vegetative biomass removal from traditional CC may provide valuable livestock forage, and simultaneously provide soil cover and contribute to soil fertility through root/stubble N contribution.