Extending the Fall Harvest Window of Switchgrass on the Basis of Phosphorus and Potassium Tissue Concentrations

It is hypothesized that switchgrass (Panicum virgatum L.) remobilizes phosphorus (P) and potassium (K) to belowground plant organs during maturation and senescence. Consequently, recommended biomass harvests occur after the first killing frost or early November, although field curing conditions at that time may be undesirable. Therefore, the objectives of this study were to determine i) if harvests can occur earlier based on removal of P and K (in leaves and stems) for two standard ('Alamo' and 'Kanlow') and eight new upland and lowland cultivars during the fall (i.e., mid-September, mid-October, late October, and early November) in Exp. 1, and ii) changes in P and K concentration in above-ground versus belowground biomass (roots, crowns, and shoots) in standard cultivars during mid-September, late October, and mid-November in Exp. 2. Both experiments were performed in a randomized complete design with three blocks at Knoxville and Springfield, TN, USA, in 2009 and 2010. Shoot P and K concentrations in cultivars did not decline from mid-September to late October, nor did P and K in crowns and roots increase from mid-September to mid-November (P > 0.05). However, leaf concentrations of P (across locations and years) were greatest for OK NSL-2001-1 (lowland cultivar), which did not differ from Blackwell (upland cultivar). Cultivar Kanlow had the lowest P concentrations across all plant parts and cultivars. Similarly, stem P was greatest for the cultivar Blackwell. Harvest timing of upland and lowland switchgrass cultivars may therefore be extended to earlier in the fall (mid-September) based on lack of attenuating declining trends of P and K in shoots of switchgrass cultivars in the Southeast.