Nitrogen Fertilization and Burning Effects on Rumen Protein Degradation and Nutritive Value of Native Grasses

Abstract

Abstract Nitrogen fertilization and burning can affect crude protein (CP) concentration of warm‐season grasses. While total CP concentration in these forages may limit animal performance, the amount of protein degraded in the rumen may be equally important. A 2‐yr study was undertaken to determine the effect of N fertilization and burning on protein degradation and quality components of three warm‐season grasses. Monocultures of switchgrass ( Panicum virgatum L.), big bluestem ( Andropogon gerardii Vitman), and indiangrass [ Sorghastrum nutans (L.) Nash] were either burned in mid‐April or left unburned. One half of each burn treatment was either fertilized with 66 kg ha −1 of N or not fertilized. Plots were harvested in mid‐June and again in mid‐July of 1990 and 1991. Crude protein, rumen degradable protein (RDP), in situ organic matter disappearance, neutral‐detergent fiber (NDF), acid‐detergent fiber (ADF), and acid‐detergent lignin (ADL) were determined. Nitrogen fertilization increased (P < 0.01) CP concentration 18%. Sixty‐five percent of the additional CP associated with N fertilization was RDP. The percentage of CP that was RDP was greater in switchgrass and indiangrass than big bluestem in both years. Even though burning increased early‐season CP concentration 7%, it did not affect the RDP percentage. Increased CP levels, particularly RDP, indicate that rumen undegradable protein (RUP) would be a more appropriate supplement with N fertilized switchgrass and indiangrass, while supplementation with RDP would be more appropriate with big bluestem and forages from burned sites.