LIGHT-SOURCE AND NUTRIENT REGIME EFFECTS ON FIBER COMPOSITION AND DIGESTIBILITY OF FORAGES

The negative correlation between lignification of forage plant cell walls and digestibility has been confounded with growth stage, plant age, or comparison across species. Growth environment was manipulated to alter cell-wall development in forages to produce experimental materials of the same age for the study of cell-wall composition effects on digestibility of fiber. Birdsfoot trefoil (Lotus corniculatus L.) and orchardgrass (Dactylis glomerata L.) were grown in sand culture under fluorescent-incandescent or low pressure sodium (LPS) lamps. Nutrient solution treatments (total, high NH4+:NO3-, high NO3-:NH4+, and low S) were imposed on each forage in each light regime. Four-week regrowth of leaf and stem plant parts was harvested, lyophilized, and analyzed for detergent fiber components, cell-wall neutral sugars, uronic acids, esterified and etherified p-coumaric and ferulic acids, Klason lignin, and lignin nitrobenzene oxidation products. In vitro degradation by a rumen fluid inoculum of detergent fiber components after 48 h of fermentation was determined. Exposure to LPS lamps caused small changes in neutral sugar proportions of the forage cell walls, and in birdsfoot trefoil, plant parts composition of core and noncore lignin shifted. The LPS lamp treatment resulted in higher in vitro degradability of detergent fiber components in birdsfoot trefoil, but depressed degradability of orchardgrass fiber. Nutrient regime caused changes in cell-wall composition, but these were inconsistent and resulted in minor effects in fiber degradability. Detergent fiber analysis was less sensitive to treatment differences than individual component analysis, and gave different concentration estimates of the same cell-wall fractions. Explanation of the changes in cell-wall degradability was not possible based on the compositional data, but rather may require cell-wall structural analysis.