AbstractSunn hemp (Crotalaria juncea L.) is a fast-growing, multipurpose legume of high N2 fixation ability, and an option for use in improving the fertility of degraded tropical soils. The objective of this study was to compare the effects of two planting seasons and three harvest dates on biomass yield (DMY) and N accumulation in the vegetative material of sunn hemp Tropic Sun'. Sunn hemp was seeded at the Lajas Agricultural Experimental Substation of the University of Puerto Rico in a Mollisol soil (San Antón Series) at a density of 7 kg/ha in May and October, and harvested at 84, 121, and 177 days after planting (DAP). The 177 DAP date proved to be not feasible because of previous senescence of the plants from the October planting; therefore, it was excluded from the analysis. Planting in May rather than October and harvesting at 121 DAP rather than 84 DAP resulted in higher DMY (kg/ha) (7,248 vs. 5,975 and 7,946 vs. 5,277, respectively). However, only the difference between planting seasons was significant (P < 0.05). The same tendencies were observed in N accumulation (kg/ha), means of 126.55 vs. 105.09 and 120.65 vs. 110.99, respectively, but without differences at P = 0.05. No interactions (P > 0.05) were detected between seasons of planting and date of harvest in these two dependent variables. Differences (P < 0.05) were found in N content of plant tissue, the upper canopy mean surpassing that of the lower canopy (2.12 vs. 1.00%). The highest crude protein content of 16.05% was found in the upper canopy for the May planting. These results suggest that sunn hemp has good potential as a cover crop and green manure in the Lajas region, and that summer planting is preferable.
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