Comparison of in Vitro Long Digestion Methods and Digestion Rates for Diverse Forages

Long-term in vitro digestions of forages provide digestion rate estimates of neutral detergent fiber (NDF) and undigested NDF (uNDF) at some time point short of infinity. The indigestible portion is required for an accurate estimate of the feed’s potentially digestible fraction. Depending on the filtration system, potentially digestible particles could remain trapped, or indigestible particles could mistakenly dislodge. Unreliable measures of uNDF can impact animal nutrition and performance when balancing diets based on flawed forage quality estimates. Two common techniques for in vitro digestion are the conventional flask method and the ANKOM filtration bag procedure. Small indigestible particles might escape the ANKOM F57 filter bag in long-term digestions because of its 25 µm pore size. An ANKOM filter bag has been developed (F58) with an 8-10 µm pore size, decreasing the chance of losing indigestible particles during NDF procedure, but it has not been evaluated for use with in vitro digestions. Our objective was to compare ANKOM F58 bags to F57 bags and the conventional flask method for in vitro long digestions. Analyses incorporated twenty-four forage samples representing a broad range of temperate and tropical grasses and legumes. Duplicate samples for both F57 and F58 filter bags digested for 30, 120 and 240 h. Cumberland Valley Analytical Services (CVAS) analyzed the same samples using the conventional flask procedure three times on separate days. Correlation between uNDF of F57 and F58 bags was 0.997, between F57 and CVAS was 0.964, and between F58 and CVAS was 0.965. Correlation between digestion rate of F57 and F58 bags was 0.956, between F57 and CVAS was 0.616, and between F58 and CVAS was 0.546. All methods were relatively similar in terms of uNDF. F57 and F58 were more related to one another than CVAS results for rates of digestion. Rates of digestion varied considerably between forages; however, they were consistent over procedures.