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 iignin (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.

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