Native warm-season grass response to nitrogen fertilization

Authors: BISANGWA, ERIC - University Of Tennessee; KEYSER, PATRICK - University Of Tennessee; WALKER, FORBES - University Of Tennessee; RICHWIINE, JOHNNY - University Of Tennessee; Ashworth, Amanda

Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/1/2024
Publication Date: 1/14/2024
Citation: Bisangwa, E., Keyser, P., Walker, F., Richwiine, J., Ashworth, A.J. 2024. Native warm-season grass response to nitrogen fertilization. Agronomy Journal. 14(1). Article 180. https://doi.org/10.3390/agronomy14010180.
DOI: https://doi.org/10.3390/agronomy14010180

Interpretive Summary: Native warm-season grasses including switchgrass, big bluestem, and eastern gamagrass have gained significant attention due to their potential contribution to forage and bioenergy bioeconomies. These species have long been known for various desirable attributes such as their broad adaptation, drought tolerance, low nutrient requirements for productivity and conservation potential. In addition, native grasses offset against the reduction in quantity and quality of available cool-season forage during the summer. However, little is known about optimum nitrogen fertilization rates in dual use forage-bioenergy production systems in the Mid-South. This study documented optimal N fertilizer levels for three native forage species (switchgrass, big bluestem, and eastern gamagrass) based on yield. Results showed no single fertilizer rate performed the same for all species across sites, but all native grasses responded to fertilization and an optimal N rate for each species was identified. A consistent improvement in forage quality occurred with increased fertilization for all species. Overall, this study indicated that the efficiency of nitrogen use varies by site, species, and weather conditions. Results can be used to inform soil fertility recommendations for native grass forage-bioenergy production.

Technical Abstract: Identification of optimum nitrogen fertilization rates for native warm-season grasses (NWSG) is needed to develop sustainable dual-harvest forage systems in the Mid-South. Experiments were conducted in Knoxville and Springfield, TN, from 2015-2019 to evaluate NWSG dry matter (DM) yield, forage nutritive value (FNV), and partial factor productivity (PFP), as well as evaluate weather pattern responses to varying nitrogen (N) rates. Three NWSG species (big bluestem [BB; Andropogon gerardii Vitman], switchgrass [SG; Panicum virgatum L.], and eastern gamagrass [EG; Tripsacum dactyloides L.]) were grown at each location in marginal soils. Plots were harvested twice annually – June and August, or June and post-dormancy in some years. Annual N-rates (0, 34, 67, 135, 202, and 269 kg N ha yr-1 for BB and SG and 0, 67, 135, 202, 269, and 404 kg N ha-1 for EG) were applied in split applications in late April and again following the first harvest (i.e., mid-June). At both sites, N increased yield for all species (P < .001), except BB at Springfield. In addition, N interacted with year for EG at Knoxville. Nitrogen rate and harvest time affected FNV (P < .001), crude protein, and harvest time for SG (P = .031). Based on regression models, all species at Knoxville responded positively to precipitation (P < .001) and temperature (P < .001), while at Springfield, all species responded positively to temperature (P < .001). Use of N amendments can enhance productivity of NWSGs, but site conditions and weather interact to affect primary productivity and forage quality.