Season and Date of Harvest Effects On Biomass Production of Sunn Hemp (Crotalaria juncea L.) in Puerto Rico.

Sunn hemp (Crotalaria juncea L.) could be an excellent cover crop for degraded soils in Puerto Rico because of its fast growth and potential N₂ fixation capacity. However, there is little information on season effects and harvest date effects on biomass and N contribution that would enhance its use as a cover crop in Puerto Rico.  This study assessed season (summer and fall) and date of harvest effects on biomass and  N accumulation of sunn hemp cv. Tropic sun  at 12 and 17 weeks after planting (WAP) on a vertisol and 6 and 12-WAP on an oxisol.  Tropic sun was seeded (10 kg ha^-1) on a fine, smectic, isohyperthermic Typic Haplusterts (Fraternidad series) and a cotito series (oxisol) on mid-May and mid-October 2008 and 2009, respectively. Biomass was estimated in marked 2 m² by clipping at 15-cm height, and subsamples (500g) were taken and dried in an open aired oven at 60° for 48 h.  Ground samples were ground to pass 1 mm sieve and N was determined. On the vertisols, maximum biomass (P<0.05) was produced at the May planting (7.2 Mg ha^-1) and was 18% higher than for the October planting (5.9 Mg ha^-1). On the oxisols, there was also a time of planting effects (P<0.05) with biomass was produced at the May planting and 12-wk harvest (17 Mg ha^-1) and this was three times higher than for the October planting (5.9 Mg ha^-1).  There was no season by harvest date interactions (P<0.05) for the vertisosl on biomass with mean yield of 5.3 and 7.9 Mg/ha for the 12 and 17 WAP, respectively. Season and harvest date did not affect (P>0.05) N concentration on vertisols. The mean values were 1.83 and 1.78% for summer and fall, respectively, and 2.02 and 1.55% for 12 and 17 WAP, respectively. N contribution was 127 and 106 kg ha^-1 for the May and October panting, respectively. On the oxisols, when comparing date of harvest, biomass at the 6-wk in May exceeded the 6-wk October harvest by 6 Mg ha^-1, suggesting that Tropic sunn is highly photoperiodic. Because of the higher yield for the May plantings, N contribution differed (P<0.05), surpassing 400 kg  ha^-1 for the 12-wk harvest. N concentration differed when the upper and lower canopy was compared. Mean values were 2.2 and 0.88% for upper and lower canopy, respectively.  Sunn hemp is a productive legume that can be used for weed suppresion and also incorporated in rotational cereal grain planting in Puerto Rico.  However, its potential for N mineralization on both alkaline and acid soils needs to be assessed.