AMMONIA-LABILE BONDS IN HIGH-DIGESTIBILITY AND LOW-DIGESTIBILITY STRAINS OF SWITCHGRASS

Improvement in the forage quality of switchgrass (Panicum virgatum L.) through phenotypic selection for increased in vitro dry matter digestibility (IVDMD) has been demonstrated. This study tested the hypothesis that genetic improvement of fiber digestibility in switchgrass has been achieved by selection for a strain with a decreased quantity of ammonia-labile bonds. Tissue samples of a high-digestibility (high-IVDMD) and a low-digestibility strain (low-IVDMD) of switchgrass were ammoniated at rates of 0, 10, 20, and 40 g kg-1 dry matter. Fiber composition and in vitro rate and extent of neutral-detergent fiber (NDF) digestion were determined on control and ammoniated samples. The high-IVDMD strain had lower (P < 0.05) concentrations of NDF and acid-detergent lignin (ADL) than the low-IVDMD strain. Lignin concentrations averaged 53 and 71 g kg-1 for the high- and low-IVDMD strains, respectively. The high-IVDMD strain had a greater (P < 0.05) extent of NDF digestion when compared with the low strain; however, the rate of NDF digestion did not differ (P > 0.05) between strains. Increased digestibility of the high-IVDMD strain was primarily attributed to increased cell-wall (NDF) digestibility. Ammoniation at 20 and 40-g kg-1 resulted in small decreases (P < 0.05) in NDF concentrations when compared with the control; however, ammoniation had no effect on hemicellulose, cellulose, or ADL concentrations. Ammoniation increased (P < 0.05) both the rate and extent of NDF digestion. Extent of NDF digestion averaged 0.395 for the control and 0.465, 0.498, and 0.493 for the 10, 20, and 40-g kg-1 treatments, respectively. Strain X ammoniation rate interaction was not significant for rate and extent of digestion, suggesting that genetic improvement in digestibility of switchgrass was not related to the number of ammonia-labile bonds.