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Performance of livestock grazing warm-season, perennial grasses is generally greater than would be expected given their relatively low protein concentrations. Two experiments were conducted to assess ruminal escape protein using an in situ rumen technique for switchgrass (Panicum virgatum L.) and smooth bromegrass (Bromus inennis Leyss.). Whole-plant, leaf, and stem samples were harvested at specific stages of maturity in 1987. Duplicate samples of each grass were incubated for 12 h in situ. Escape protein values were expressed as concentration [grams escape protein per kilogram dry matter (DM) adjusted for acid detergent insoluble nitrogen (ADIN)] and as a percentage of total plant protein concentration (grams protein per kilogram DM adjusted for ADIN) to characterize protein composition. Whole-plant escape protein concentration was greater (P < 0.05) in switchgrass (31.8 g kg-1 DM) than smooth bromegrass (22.3 g kg-1 DM), averaged across all growth stages, and generally declined with maturity in both species. Escape protein concentration was consistently greater (P < 0.05) in leaves (50.8 g kg-1 DM) than in stems (193 g kg-1DM) averaged over species and growth stages. As the proportion of stem to leaf increased during maturation, whole-plant escape protein concentration decreased. Escape protein percentage was similar for smooth bromegrass leaves and stems while switchgrass stems were generally higher than leaves. Escape protein percentage of whole-plant switchgrass (50.9%) was greater (P = 0.08) than smooth bromegrass (20.5%) over all harvest dates. Anatomical differences between switchgrass (C4) and smooth bromegrass (C3) may partially explain differences in ruminal protein degradation and subsequent animal performance.