Harvest Frequency and Timing Affect Perennial Bioenergy Grass Yield and Composition.

Frequency and timing of harvest are important management practices affecting yield, yield distribution, and chemical composition of bioenergy grasses. The objective of this research was to compare harvest management practices for tall grasses in the USA Gulf Coast Region. The experiment was conducted in North Central Florida in 2010. The harvest management treatments were i) two harvests per year (2X; summer and fall), ii) one harvest per year in fall (FALL; before freezing temperatures), and iii) one harvest per year in winter (WINTER; within 1 wk after the first freeze). Two elephantgrass (Pennisetum purpureum Schum.) entries (‘Merkeron’ and a breeder’s line referred to as Schank) and one energycane (Saccharum spp.; ‘L79-1002’) were compared. In the first year of harvesting, total annual biomass yield was not affected by harvest management and averaged 25.7 Mg ha^-1. Schank (27.6 Mg ha^-1) outyielded both Merkeron and L79-1002 (24.8 and 24.8 Mg ha^-1, respectively). Neutral detergent fiber (NDF) concentration of harvested biomass was not different among grass entries at either harvest for 2X (~ 680 g kg^-1),  but L79-1002 NDF was lower in both FALL and WINTER harvests (667 and 637 g kg^-1, respectively) than for Merkeron (703 and 708 g kg^-1, respectively) and Schank (701 and 673 g kg^-1, respectively). Acid detergent fiber (ADF) concentration response to treatment was similar to NDF. Acid detergent lignin concentration (ADL) in the dry matter was lower for energycane (59 g kg^-1) than for Schank or Merkeron (66 and 69 g kg^-1, respectively) at the first harvest of the 2X treatment but did not differ among grasses at the second harvest (55 g kg^-1). FALL and WINTER treatment ADL did not differ among elephantgrass entries and averaged 80 g kg^-1, but energycane ADL was least for both of these treatments (avg. of 65 g kg^-1). These data show that yield of these grasses in the first year of harvesting was not affected by harvest frequency (one vs. two harvests yr^-1) or timing of harvest relative to first freeze. Fiber composition of the grasses varied minimally across defoliation treatments. Species differences occurred as energycane had lower concentrations of NDF, ADF, and ADL than elephantgrass for FALL and WINTER. Additional years of measurements will provide much-needed information on long-term elephantgrass and energycane yield and stand persistence responses to defoliation treatments.