In Vitro Digestibility of Dry Matter and Cell Wall Constituents of Smooth Bromegrass Forage1

The availability of forage cell walls to ruminants is dependent on the concentration and digestibility of cell wall constituents. The objectives of this study were to quantify genotypic variation for in vitro digestibility of individual cell wall constituents and relate this variation to variation in concentration of cell wall constituents of smooth bromegrass ( Bromus inermis Leyss.). Eighteen clones were grown at Arlington, WI, and sampled in 1982 and 1984. Forage samples were harvested to simulate a normal first cutting. Samples were analyzed for cell wall constituents of dried forage and forage that had been previously digested by rumen microorganisms. In vitro digestibility (IVD) of cell wall constituents was computed from differences obtained between undigested and digested forage samples. Genotypic variation was significant for IVD of neutral detergent and acid detergent fiber (NDF and ADF, respectively), cellulose, and hemicellulose, but not for acid detergent lignin (ADL). In vitro digestibilities of dry matter, NDF, ADF, cellulose, and hemicellulose were mutually correlated, with r ≥ 0.87. Concentration and IVD of cell wall constituents generally were not correlated, with ‒0.52 ≤ r ≤ 0.22. Selection for divergent IVD of dry matter (IVDMD) resulted in similar changes to IVD of NDF, ADF, cellulose, and hemicellulose; differences between high and low IVDMD clone groups ranged from 37 to 42 g kg −1 for IVD of cell wall constituents. The additional information provided by determining IVD of cell wall constituents makes these selection criteria potentially valuable. Because of their close interrelationships and similar correlated selection responses, IVD of NDF, which is the easiest and least expensive to determine, would satisfactorily represent the other IVD variables in smooth bromegrass.