Harvest Frequency Alters Forage and Feedstock Growth and Quality.

Switchgrass (Panicum virgatum L.) is a native warm season species with biofuel feedstock potential.  Dual-purpose use as forage and feedstock will depend on both yield and nutritive value.  This study conducted during 2008 to 2011 at Raymond, MS on a Loring silt loam soil quantified yield and chemical composition of ‘Alamo’ switchgrass at two N rates (80 and 160 kg N ha^-1) and four harvest frequencies (HF; 1, 2, 3, or 6 harvests annually).  Treatments were assigned in a 4 × 2 factorial arrangement of a randomized complete block design experiment with three replications.  There was a HF effect (P < 0.0001) and an N × year interaction (P < 0.0001) on crop growth rate (CGR).  A single harvest (119.1 kg ha^-1 d^-1) or two harvests annually (91.7 kg ha^-1 d^-1) had the most rapid CGR.  As HF increased to three (14.5 kg ha^-1 d^-1) and six (10.3 kg ha^-1 d^-1) harvests per year, CGR decreased.  In 2008 and 2009, CGR was 65.8 (high N rate) vs. 55.0 kg ha^-1 d^-1 (low N rate) but in 2010 and 2011 there was no effect of N (70.9 kg ha^-1 d^-1).  Acid detergent fiber (ADF) and neutral detergent fiber (NDF) decreased with HF while crude protein, tissue P, K, Ca, and Mg concentration, and total digestible nutrients all increased with HF over all years.  Harvest frequency and year effects were observed for Mg, fructan, sugars and water-soluble carbohydrates.  Total ethanol production (TEP) differed with HF.  Similar to CGR, N effects on TEP occurred only in 2008 and 2009.  A single harvest (2206.8 l ha^-1) TEP was similar to two harvests annually (2304.2 l ha^-1) with TEP decreasing from three harvests (1701.5 l ha^-1) to six harvests (1286.8 l ha^-1) annually.  These results indicate that a two-harvest system (early season and end-of-season) has potential for both forage and feedstock use.