Abstract
An experiment to evaluate the effects of planting season and harvest dates on biomass production and nitrogen accumulation by sunn hemp was conducted at the Agricultural Research Station, Isabela, Puerto Rico. Sunn hemp 'Tropic Sun' was established in an Oxisol soil (Cotito series) at a seeding density of 10 kg/ha. Dry matter yield (DMY) was estimated at 71,92 and 121 days after planting (DAP) in June and November. Effects of both major variables were significant (P > 0.05) for DMY, the June planting far surpassing that of November (15,831 vs. 5,665 kg/ha); DMY increased progressively for the three harvest dates (7,704; 11,141; and 13,398 kg/ha). Maximun biomass production (20,734 kg/ha) was obtained at 121 DAP in June, thus exceeding by threefold that of November at 121 DAP (6,062 kg/ha). Nitrogen accumulation followed a similar pattern, with that of June surpassing that of November (352 vs. 147 kg/ha) and values ascending for the successive harvest dates (193, 254, and 303 kg/ha). Differences (P < 0.05) in N concentration were found by comparing the upper plant canopy with the lower. Expressed as crude protein (CP) the maximum value corresponded to that of the upper canopy at 92 DAP in November (19.0%), whereas the corresponding lower canopy value was only 4.72%. Based on its high biomass production and N accumulation, this legume could provide multiple beneficial effects when included in short crop rotations of sustainable production systems, taking advantage of the most favorable season and harvest stage. The present results suggest that 'Tropic Sun' planted in summer has good potential as a cover crop and green manure.