CAUSAL EFFECTS AMONG FORAGE YIELD AND QUALITY MEASURES OI SMOOTH BROMEGRASS

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Future improvement of forage quality in smooth bromegrass (Bromus inermis Leyss.) and other species will require a knowledge of the chemical and structural factors most limiting to digestibility and intake of dry matter by ruminants. The objective of this study was to estimate direct and indirect effects from path analysis of several forage constituents on in vitro dry matter digestibility (IVDMD) and forage yield of smooth bromegrass. Six hundred and fifteen spaced plants of B8HD smooth bromegrass were harvested at Arlington, Wis. in June 1982 to assess dry matter yield and forage constituents. Near-infrared reflectance (NIR) spectroscopy was used to estimate IVDMD, crude protein, neutral detergent fiber, acid detergent fiber, cellulose, acid detergent hemicellulose, acid detergent lignin, and ash based on a random calibration subset of 75 plants. Five independent variables comprised the path analysis model: crude protein, cellulose, hemicellulose, lignin, and ash. None of the variables directly or indirectly affected forage yield. Lignin and cellulose concentrations had the overwhelmingly largest direct effects on IVDMD, while hemicellulose was important only when expressed as a percentage of the cell wall. Crude protein was important, but only to a small extent, while ash had no effect on IVDMD. The largest indirect effects on IVDMD were generally indirect effects of each character through correlated changes in lignin and indirect effects of lignin through correlated changes in other characters. The large effects of lignin support previous work establishing lignin as the primary factor limiting IVDMD. The importance of cellulose compared to hemicellulose, a result contrary to many current theories on the role of cell walls in rumen digestion, emphasizes at least three factors contributing to differences between this study and previous studies: (1) species differences, (2) genetic vs. species and maturity effects, and (3) analytical procedures. Current evidence suggests a cell wall ideotype should have reduced cellulose and lignin with either reduced or unchanged hemicellulose concentration.Key words: Bromus inermis Leyss., cellulose, hemicellulose, in vitro dry matter digestibility, lignin, forage yield