Unreliable measures of undigested neutral detergent fiber (uNDF) can influence animal nutrition and performance when balancing diets based on flawed forage quality estimates. Two common techniques for in vitro long digestions are the conventional flask method and the ANKOM filtration bag procedure. An ANKOM filter bag has been developed (F58) with an 8 to 10-mu m pore size, decreasing the chance of losing particles during neutral detergent fiber (NDF) procedure, but it has not been evaluated for use with in vitro digestions. Our objective was to compare ANKOM F58 bags with F57 bags and the conventional flask method for in vitro long digestions. Analyses incorporated 24 forage samples representing a broad range of temperate and tropical grasses and legumes. A commercial laboratory analyzed the same samples using the conventional flask procedure. Separate analyses evaluated the effect of Na2SO3 for ANKOM F57 and F58 methods and the effect of ruminal fluid plus buffer refreshing at 2-d intervals with ANKOM F57. Undigested NDF at 240 h on an organic matter basis (uNDF240om) values between methods were different from one another, but rate calculations derived from uNDF240om values were not different. Method pore size was highly correlated (r = -0.993) with uNDF240om values. Results showed that refreshing ruminal inoculum plus buffer at 2-d intervals for ANKOM F57 and the addition of Na2SO3 during the analysis of ash-free NDF on an organic matter basis (aNDFom) after ANKOM F57 and F58 in vitro digestions both had significant effects on lowering uNDF240om values.