Utilizing Decline of P and K Levels In Shoots to Establish An Earlier Harvest Window for Switchgrass Biomass.

Switchgrass (Panicum virgatum) remobilizes nutrients, especially phosphorus and potassium, during senescence. Identifying the appropriate harvest window in a one-cut biomass system based on the remobilization of these nutrients can be economically beneficial for producers. The objective of this research was to determine if a one-cut harvest can be executed earlier in the fall based on the optimum timeframe of translocation of phosphorus and potassium from stems and leaves to the crown and roots.  The current recommendation is to harvest after the first killing frost or early November, whichever comes first.  In 2007, ‘Alamo’ and ‘Kanlow’ cultivars were planted at the East Tennessee Research and Education Center in Knoxville. Ten plant tillers, clipped at 3-5 cm above ground level, were collected from each plot (50 total for each sample date) throughout the fall (July through November, 2008; September through November, 2009).  Each 10 tiller sample was separated into panicles, leaves, and stems then ground and analyzed for P and K. Although P and K levels declined numerically throughout the fall, they were not significantly lower in stems or leaves from September through November for both varieties. The P and K levels were: panicle > leaves > stems. Potassium ranged from 3X-9X higher than P in different tissues. Data from crowns, roots and above ground biomass (2009) followed the same trends observed in the tillers (2008 and 2009).  Based on these results, harvest can be executed earlier in the fall (e.g., late September) without removing significantly more P and K.