There are wide variations in fertilizer recommendations for switchgrass (Panicum virgatum L.) as biofuel feedstock or forage. Inconsistent yield responses to fertilization are common. Nutrient translocation, from aboveground leaves and stems to belowground roots, is a contributor to variable, and sometimes the absence of, fertilizer response. A field study evaluated how major nutrients are cycled within switchgrass during the growing season. Aboveground (AG) and belowground (BG) biomasses were harvested and analyzed separately for yield (AG) and nutrient concentrations (AG and BG). Maximum yields were 26.3 (2008), 17.5 (2009) and 29.3 (2010) Mg ha−1 until senescence. In all years, the N concentration of the AG biomass decreased as the season progressed (p < 0.0001). The belowground biomass N concentration increased over time in 2008 and 2010 (p < 0.05). Phosphorus (P) and potassium (K) displayed similar trends to nitrogen concentration. Temporal changes of other nutrients in the AG and BG biomass concentrations were inconsistent and varied compared with N. The dynamics of the macronutrients suggested translocation from AG to BG as the switchgrass matured. Nutrients stored in roots can be beneficial for regrowth in the following growing season, which may reduce response to fertilizers. A greater understanding of nutrient cycling and harvest timing is needed to better manage different switchgrass production systems.

Early season switchgrass (Panicum virgatum L.) can be used as animal feed and mature late-season biomass as a biofuel feedstock. However, nitrogen (N) application and harvest timing effects on the quality of both end-use need further evaluation. This study evaluated the changes in nutritive quality for animal feed and biofuel feedstock, under different N application rates (0 to 235 kg N ha−1 yr−1) and different harvest times at a fixed N rate. Plant N removal increased with increasing N application rate (P < 0.05). The largest single difference (27%) was found between 0 and 33.6 kg N ha−1 application rates. Nitrogen removal decreased during subsequent harvests at a fixed N rate (P ≤ 0.0001). Forage quality was affected by N rates, although it was especially impacted by harvesting time. Fibers and most minerals in the biomass increased as accumulated growing degree days (AGDD) increased (P ≤ 0.0001), but N and total digestible nutrients (TDN) decreased as AGDD increased (P ≤ 0.0001). High crude protein and minerals with low fiber are desired forage qualities and the opposite is true for biofuel feedstock. Earlier harvests are beneficial for hay production or livestock grazing, and late-season harvests are better for biofuel production.

Best management practices for switchgrass (Panicum virgatum L.) fertilization in the production of biofuel feedstock and forage hay are variable depending on climate, harvest timing, and variety. A study of N fertilization was conducted to evaluate the yield response, nutrient removal, and nitrogen use efficiency (NUE) in 12 yr old switchgrass stands in Stillwater, OK. Seven rates of N fertilizer were applied ranging from 0 to 235.2 kg N ha−1 for 3 consecutive years (2008, 2009, and 2010). Positive linear trends in yield with increasing rates of N fertilizer (P < .05) were observed in two out of three years. Differences in N removal due to N fertilization were found in all studied years although responses were inconsistent. Nitrogen concentration in harvested biomass increased as N rates and biomass yield increased. Nitrogen use efficiency increased with N rate (P < .05) although many NUE values were negative up to application rate of 100.8 kg N ha−1. Better understanding of nutrient uptake and use efficiency, removal with harvests, and cycling within the plant could provide more refined fertilization practices in switchgrass production.