Comparing three filter-bag typesfor accuracy and precision of <i>i</i><i>n sacco</i> undigestible neutral detergent fibre determination of various dicotyledon forages

<b>Context</b>: Internationally, undigestible neutral detergent fibre (uNDF) has become a standard feed analysis to quantify the potential digestible NDF fraction and to be an internal marker to estimate diet total-tract digestibility in animal studies. However, this analysis is labour-intensive/expensive and not commercially available in New Zealand and many other countries.<b>Aim</b>: To compare the uNDF after rumen incubation determined for various forages by using filter bags that can be used sequentially for NDF and uNDF analysis with the standard method using Saatifil bags.<b>Methods</b>: Freeze-dried material of 15 forage samples (mainly dicotyledons) was weighed into three types of bags (Saatifil, 12 μm; F57, 25 μm; F58, 6–9 μm) in sextuplicate and then incubated across two runs for 12 days in the rumen of a fistulated non-lactating pasture-fed dairy cow. After incubation, the NDF of the residue was determined.<b>Key results</b>: Overall, the average (±within forage sample CV%) uNDF was 5.4% (9.8), 6.7% (15.2) and 6.3% (16.0) of DM for Saatifil, F57 and F58, respectively, and the mean bias (±95% confidence interval, CI) was 1.3 (0.3–2.3) and 1.2 (−0.1–2.5) for F57 and F58 versus Saatifil respectively. The 95% CI indicated that the intercept and slope for the orthogonal regression of F57 and F58 versus Saatifil were not different from zero and one, respectively, and the correlation for F57 with Saatifil was stronger than for F58 (<i>r</i> = 0.65 and 0.40 respectively).<b>Conclusions</b>: Both F57 and F58 bags resulted in similar uNDF values as with the Saatifil bag; however, numerically uNDF values were greater and with a large within-sample CV.<b>Implications</b>: The results of the current study suggest that the F57 bag is suitable for uNDF determination, but that some further modifications to the protocol need to be made to improve the accuracy and precision of the uNDF determination.