Nutrient Removal and Allocation in Napiergrass Treated With Swine Lagoon Effluent.

Animal waste in Mississippi is typically disposed of by application on subtropical forage grasses, but yield data using this management on napiergrass (Pennisetum purpureum Schum.) is lacking. Napiergrass is a dual-purpose, perennial forage and bioenergy crop candidate for the lower southeastern USA. Field studies were conducted in 2011 and 2012 to examine yields and nutrient removal by ‘Merkeron’ napiergrass grown in a swine-effluent spray field located in north-central Mississippi. Plants were transplanted in mid-April 2011 at a spacing of 1.5 by 2.0 m using nine plants in each of two blocks. The plot area received regular applications of swine effluent from April to October that provided approximately 318 kg N, 47 kg P, and 470 kg K per ha. Final biomass was determined in November and partitioning of nutrients into leaf and stem fractions was determined for tillers (n=3) harvested biweekly beginning in June 2011 and in May 2012. A 17% increase in final biomass from approximately 35 Mg ha^-1 in 2011 to 41 Mg ha^-1 in 2012 was associated with a 69% increase in tiller number per plant (range = -10 to 130%). Estimates for N, P, and K removal in 2011 were approximately 380, 68, 862 kg ha^-1, respectively, and increased considerably in 2012 to approximately 458, 82, and 978 kg ha^-1, respectively. Leaf and stem contents of N and K (g tiller^-1) reached their maximum levels in late-September to mid-October, when approximately 0.8-1.0 g more N and 3.0-3.8 g more K was allocated to the stem than the leaf fraction. These results are consistent with reports of high K uptake (luxury consumption) in napiergrass, and the need for regular applications of N, P, and K to enable rapid growth and high biomass yields. Observations will continue in 2013, as napiergrass yields typically reach their peak in the third year following establishment.