Switchgrass forage yield and compositional response to phosphorus and potassium

Authors: Ashworth, Amanda; Moore, Philip; KING, RANDY - Natural Resources Conservation Service (NRCS, USDA); DOUGLAS, JOEL - Natural Resources Conservation Service (NRCS, USDA); Pote, Daniel; JACOBS, ALAYNA - Natural Resources Conservation Service (NRCS, USDA); PRATT, EDDIE - Natural Resources Conservation Service (NRCS, USDA)

Submitted to: Agrosystems, Geosciences & Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/3/2019
Publication Date: 5/23/2019
Citation: Ashworth, A.J., Moore Jr, P.A., King, R., Douglas, J.L., Pote, D.H., Jacobs, A., Pratt, E. 2019. Switchgrass forage yield and compositional response to phosphorus and potassium. Agrosystems, Geosciences & Environment. 2:190010. https://doi.org/10.2134/age2019.02.0010.
DOI: https://doi.org/10.2134/age2019.02.0010

Interpretive Summary: Switchgrass, a native grass that grows well under low nutrients and drought conditions, may be used in cattle operations for supplying potentially low-cost hay. Although, little information exists on appropriate various fertilizer rates for this forage crop. The purpose of this work was to gain greater insight into switchgrass yield response to nutrients (phosphorus and potassium). Researchers determined what hay quality to expect per fertilizer rate. This study found that low amounts of phosphorus and potassium fertilizers are required for a yield response, as there were very few detectable yield increases beyond the lowest tested rate. From this research, phosphorus and potassium recommendations were developed for switchgrass hay production in the Southeast.

Technical Abstract: Finite nutrients, including phosphorus (P) and potassium (K) are required in large quantities for many warm-season forages. However, native perennial grasses such as switchgrass (Panicum virgatum L.) have less of an affinity for fertilizers, although recommended P and K rates are largely unknown. Therefore, objectives of this study were to test switchgrass forage yield response to P and K rates representative of warm-season forages and identify forage quality impacts in order to develop fertilizer recommendations for switchgrass as a forage crop. Switchgrass cv. ‘Alamo’ was subjected to two forage harvests per year after a single applications of 0, 28, 56, 112, and 224 kg P ha-1; and split applications of 0, 134, 269, 404, and 538 kg K ha-1 yr-1, from 2014 to 2016. Two sets of controls were utilized in this study, one being represented by a 0 P and K rate and a nutrient control, which received no supplemental (nitrogen, P, K, and S) fertilizer. Harvest timing (first or second cuts) did not influence (P=0.05) P or K removal or forage yields. Greatest P removal tended to occur under higher K rates (> 134 kg ha-1) and the 112 and 224 kg P ha-1 applications, with K removal also peaking under P applications. For all study years, the control (no supplemental fertilizer) resulted in substantially lower (P=0.05) yields than the 0 P and K kg ha-1 application, meaning that yield did benefit (44 and 49% increases) from the supplemental nutrient applications. Therefore, minimal P and K inputs are required for a yield response, as there were very few detectable yield increases beyond the lowest tested application rates (134 K and 28 P kg ha-1). These results indicate that P and K are required in low amounts and that nitrogen is far more a limiting nutrient in switchgrass forage systems on marginal soils of the southeast.