Forage Quality and Digestion Kinetics of Switchgrass Herbage and Morphological Components

Abstract

As switchgrass (Panicum virgatum L.) matures, whole plant nutritive value declines. In field studies, ‘Cave‐In‐Rock’, ‘Pathfinder’, and ‘Trailblazer’ switchgrass were seeded into a Blount silt loam soil (fine, illitic, mesic Aeric Ochraqualf) to determine the interaction between morphological components and maturation and its impact on forage quality. Herbage was initially harvested when the flag leaf was just visible with subsequent harvests occurring 14 and 28 d later. Cave‐In‐Rock had higher (25 g kg−1) in vitro dry matter disappearance (IVDMD) and lower (35 g kg−1) neutral detergent fiber (NDF) concentrations than the other cultivars (P < 0.05). Leaf blades, which comprised 47% of total forage at the first harvest and 26% at the last harvest, were highest in N concentration, IVDMD, and extent of NDF digestion for all harvest dates. Leaf sheaths and stems had similar N concentrations, but for IVDMD, NDF, lignin, and extent of cell wall digestion, leaf sheath values were intermediate between blades and stems. Lignin concentration of stems between the first and last harvest increased 38.8 g kg−1, while stem IVDMD and extent of stem NDF digestion declined 210 g kg−1 and 228 g kg−1 NDF, respectively. No significant whole plant cultivar differences were found for either rate or 96‐h extent of NDF digestion (P > 0.05). The in vitro rate constant for stem NDF declined 0.024 h−1 with maturation (P < 0.05), while leaf blade and sheath rate constants did not change. These results indicate that changes in the proportion and composition of the stem fraction and composition of the sheath fraction are the major factors causing a decline in switchgrass herbage quality and rate of NDF digestion with maturation.