Yield and Nitrogen Removal By Bioenergy Crops on Swine Effluent Sprayfields in Coastal Plain Region North Carolina.

The swine industry in North Carolina produces large quantities of waste effluent, which is commonly applied to fields (“sprayfields”) of coastal bermudagrass (Cynodon dactylon L.) for hay production. Dedicated bioenergy crops, such as switchgrass (Panicum virgatum) and giant miscanthus (Miscanthus x giganteus L.), are promising alternatives to coastal bermudagrass if these grasses can maintain high yields and adequate nitrogen (N) removal. Unlike more common bioenergy cropping systems, where minimum nutrient input and maximum yield are preferred, maximum N removal is an advantage on sprayfields due to nutrient management guidelines. The primary objective of this experiment to determine the biomass yield and nutrient uptake/removal of dedicated biomass crops, including switchgrass, giant miscanthus, sweet sorghum, and high-biomass sorghum (Sorghum spp.) grown on swine effluent sprayfields in the North Carolina Coastal Plain region. Research plots were established in 2011 and 2012 at 3 on-farm locations (Goldsboro, Cottle, Prestage) in eastern NC. Treatments were arranged in a split-plot randomized complete block design with main plots of five crop cultivars: coastal bermudagrass (CB), giant miscanthus (GM), switchgrass (SG, cv. ‘Colony’), high-biomass sorghum (FS, cv. Blade ‘ES5200’) and sweet sorghum (SS, cv. MSU ‘M81-E’). For perennial grasses, subplots were six different harvest times (May & Oct, June & Oct, July & Oct, Aug & Oct, Oct, and Dec). Each of the treatments was replicated three times. Sorghum plots were harvested in September each year. Swine effluent application was managed by the farm cooperators. Plant tissues were collected from subplots at each harvest to determine N uptake and changes in soil nutrient status. An additional site was established at the Williamsdale Biofuels Field Lab in 2011, where five annual N rates (0, 67, 134, 202, and 269 kg N ha^-1) were included as an additional subplot (split-split plot design) to further investigate the N use efficiency of these bioenergy crops. Dry biomass yields of both switchgrass and giant miscanthus were significantly greater than traditionally-planted coastal bermudagrass. Data thus far have indicated that harvest timing is critical for determining N removal and total biomass production. In 2013, across all three farm sites, the June & Oct cutting treatment for both miscanthus and switchgrass removed the most N. Switchgrass achieved highest yield with the Aug & Oct harvest in two of the three farm sites, with maximum yields around of 19.6 Mg ha^-1 across all sites. Giant miscanthus yield was highest with the December-only harvest, with averaged yield around 23 Mg ha^-1 across all sites. However, the Dec only harvest removed the least amount of N compared to other harvest treatments because of the nutrient remobilization after senescence. Sorghum yields for both cultivar varied among years and sites. The highest yield of sorghums was 25.8 Mg ha^-1 with N removal of 302 kg ha^-1 by forage sorghum at Goldsboro farm site in 2012.