Establishing a Fall Harvest Window for Switchgrass Based On Optimum Decline In Phosphorus and Potassium Levels In Shoots.

It is hypothesized that switchgrass (Panicum virgatum L.) remobilizes nutrients such as phosphorus (P) and potassium (K), during maturation and senescence. Currently, the recommendation in TN is to harvest switchgrass biomass after the first killing frost or early November, whichever comes first.  This is partially based on the premise that nutrients can be conserved within the plant if harvest is delayed until senescence. However, curing conditions are problematic during that time and harvesting in September or early October is highly desirable.  The objectives of this research were to (1) determine if an earlier fall harvest window can be established based on optimum changes of P and K in the shoots and (2) compare fall P and K changes in the standard varieties (Alamo and Kanlow) versus new varieties.  In 2007, twelve varieties, comprised of upland and lowland ecotypes, were planted at the East Tennessee Research and Education Center (ETREC), Knoxville. Eight of the twelve varieties were planted at the Highland Rim Research and Education Center (HRREC) in Springfield. Ten plant tillers were collected from each plot at each location in mid-September, mid-October, late October, and early November in 2009 (ETREC) and 2010 (ETREC and HRREC).  Each sample was separated into leaf, stem and panicle subsamples, then ground and analyzed for P and K using ICP.  Although P and K levels varied somewhat throughout the fall, the changes in stems or leaves were not significant (P≤ 0.05) across harvest dates. Phosphorus was typically highest in panicles (range, 1.0-2.0 kg t^-1), while leaves and stems had similar concentrations (0.6-0.9 kg t^-1).  Potassium was 3X-6X greater than P in different tissues and tended to be highest in stems (range, 3.5-5.8 kg t^-1), followed by panicles then leaves. The P in Alamo was similar at the four different harvest dates (range, 0.5-1.8 kg t^-1 ETREC; 0.6-1.0 kg t^-1 HRREC).   In general, P and K levels in Kanlow and the new varieties were similar to those observed in Alamo. Based on changes in P and K levels in switchgrass shoots observed in this study, a fall harvest window starting as early as mid-September could be implemented without removing significantly higher amounts of P and K in harvested biomass compared to November.