Harvest Management Effects On Yield, Composition, and Nitrogen Dynamics of Perennial Bioenergy Grasses.

Harvest frequency and timing affect yield and composition of perennial bioenergy grasses. In addition, harvest timing may impact crop N dynamics, particularly when harvest is delayed until after physiological maturity. Objectives were to determine the effects of i) harvest frequency and timing on biomass yield and composition of three perennial bioenergy grasses, and ii) the effect of late-season harvest timing on the distribution and recycling of N in the plant-soil continuum. Field experiments were conducted in North Central Florida in 2010 and 2011. The harvest management treatments were: i) two harvests yr^-1 (2X; summer and fall), ii) one harvest ^-1 in fall (FALL; after first flower), and iii) one harvest ^-1 in winter (WINTER; within 1 wk after first freeze). Two elephantgrass (Pennisetum purpureum Schum.) entries (‘Merkeron’ and the breeding line UF-1) and one energycane (Saccharum spp.; ‘L79-1002’) were compared. Labeled N (15N) was used to quantify the effect of fall harvest timing on N recovery and N recycling. Averaged over 2 yr, total annual biomass yield was not affected by harvest frequency or timing, but UF-1 (30.0 Mg ha^-1) outyielded both Merkeron and L79-1002 (25.0 and 24.5 Mg ha^-1). Only UF-1 had increased biomass in the second year (27.5 and 32.4 Mg ha^-1 in 2010 and 2011, respectively). During the first year, harvest management did not affect stem total structural carbohydrate (TSC) concentration, but differences occurred in leaves (572, 650, and 639 mg g^-1 for 2X, FALL, and WINTER, respectively). Stem TSC was affected by entry (542, 592, 597 mg g^-1 for L79-1002, Merkeron, and UF-1, respectively), but there was no entry effect on leaf TSC. First year isotopic N tracer analysis showed no difference between fall and winter harvest treatments in fertilizer recovery, but L79-1002 above-ground biomass contained 46% of applied N compared to 36% for Merkeron.