Comparisons of in vitro fermentation and high moisture forage processing methods for determination of neutral detergent fiber digestibility

Neutral detergent fiber digestibility (NDFD) determined in vitro with rumen inoculum is widely used to assess digestibility and potential energy contributions of feedstuffs. An in vitro fermentation system (IVFS) with potential to improve sample throughput and ease of handling for NDFD determination was investigated. Additionally, methods for preparing high moisture forages and their effect on in vitro NDFD were evaluated. In the IVFS study, a commonly used method that uses Erlenmeyer flasks under continuous CO2 pressure in water baths (GV) was compared to a system that uses sealed glass tubes in a shaking incubator (TU). Fibrous feeds (alfalfa hay, maize silage, soyhulls, and ryegrass hay) were incubated in duplicate for 24, 30, and 48 h in three fermentation runs (run) in both IVFS. Overall, NDFD was greater for TU than GV at 24 h, and greater for GV than TU at 48 h. Maize silage had lower values with TU than GV, with the difference increasing with fermentation time, possibly due to low pH related to amount of fermentable substrate used. Within-run variability at 48 h was less with GV than TU. Variability of NDFD across runs was or tended to be less for TU at 24 and 30 h, and tended to be less for GV at 48 h. In the forage processing study, silages (alfalfa, maize) and pasture grasses (meadow fescue, orchardgrass, reed canarygrass, ryegrass) were ground with dry ice through a meat grinder to pass 4.5 mm openings. Subsamples were dried in a 55 °C forced-air oven (OD), freeze dried (FD), or retained as undried frozen (UF) material. Samples were fermented in duplicate in TU for 24, 30, and 48 h in two runs. NDFD response to processing varied by forage with FD for orchardgrass and UF for maize silage lower than other treatments for those forages. Overall, NDFD did not differ among processing methods at 24 h, was greatest for OD and UF at 30 h, and for OD at 48 h. Based on NDFD and analytical variability results, TU may be recommended at 24 and 30 h and GV at 48 h, however, substrate amount may need to be restricted in TU to avoid depressing NDFD. Methods of processing high moisture forage samples for NDFD analysis showed no clear advantage for using freeze dried or undried forage over oven dried materials.