In Vitro And In Vivo Analyses Of Hays Of Switchgrass Strains Selected For High And Low In Vitro Dry Matter Digestibility

In a previous report the progress made in altering the in vitro dry matter digestibility (lVDMD) of switchgrass (Panicum oir­ gatum L.) in one cycle of divergent selection was described. In this study, the detergent system of analyses was used in an at­ tempt to determine what plant constituents had been altered by the selection for IVDMD. Hays from five switchgrass strains differing in IVDMD harvested over a 3-year period were ana­ lyzed for protein content, neutral detergent fiber (NDF), acid detergent fiber (ADF), ash, and lignin. Although there were dif­ ferences (p :s 0.05) among strains for IVDMD as determined by two different procedures (with and without additional N), there were no differences among strains for ADF, ash, lignin, and protein. The strains ranked the same with the two IVDMD procedures but the IVDMD·N (with additional N) test averaged 6.7 percentage points higher in digestibility. There were small differences (p -s 0.1) among strains for NDF in 2 of the 3 years. The strain with the highest IVDMD had the lowest NDF, sug­ gesting that, in part, the improved IVDMD was due to a decrease in the cell wall component of the harvested forage. Hays ofthree of these strains, high-IVDMD PC, low-IVDMD PC, and 'Path· finder' from the 1980 harvest were ground and fed to sheep to compare in vivo and in vitro digestibilities. After grinding, the hayshad IVDMD-N values of49.9,45.5, and 46.7%, respectively. There were no differences among strains in the sheep feeding trial for in vivo digestibility, dry matter intake, and fiber di­ gestibility. The in vivo digestibilities for these strains were: high­ IVDMDPC 50.4%, low-IVDMD PC 49.0%, and Pathfinder 50.8%. In contrast to a previous report in the literature, the IVDMD­ N values were generally similar to in vivo digestibilities for switchgrass hays. Parameters of the conventional detergent sys­ tem of analyses, while useful in identifying factors responsible for changes in digestibility of switchgrass hays, would probably be of little value as selection criteria in a breeding program. Because of the large number of animals required to obtain sta­ tistical significance in in vivo digestibility studies, the IVDMD procedure would be preferable in studies in which breeding ma­ terial is being evaluated and small differences in digestibility are expected.