aNDF, NDFd, iNDF, ADL and kd: What have we learned?

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Table 1: Comparison between the non-linear reference method and the 24 hours single point approach with both fixed and variable lag, and using alternative iNDF estimations. The values highlighted are the ones falling inside the 95% confidence interval. Nonlin 95% c.i. 24 hr with fixed lag 6-24 hr with estimated lag time Forage kd lower upper kd2.4 kd216 kdD kd2.4 kd216 kdD FULLTIME 7.64 6.81 8.47 6.60 6.66 6.75 7.01 7.08 7.69 MYCOGEN 6.81 5.59 8.04 5.23 5.36 6.17 5.39 5.54 6.43 HARDIES BMR 8.18 7.38 8.98 6.59 7.43 7.68 6.85 7.79 8.06 HARDIES NORMAL 6.15 5.20 7.09 4.82 5.53 5.76 4.87 5.64 5.89 CORNELL T&R 6.14 5.16 7.11 5.27 6.09 6.21 5.47 6.36 6.49 CPM 238 5.05 4.20 5.90 4.57 5.07 5.35 4.49 5.01 5.31 CPM 249 5.95 4.55 7.35 4.78 5.49 6.68 4.84 5.61 6.91 CPM 250 5.54 4.65 6.42 4.58 4.88 5.22 4.68 5.00 5.37 CPM 251 6.79 6.19 7.39 6.16 6.43 6.83 6.45 6.76 7.20 CPM 252 5.75 4.82 6.68 4.56 4.86 6.00 4.57 4.88 6.11 CPM 259 5.29 4.10 6.47 4.71 5.41 5.66 4.69 5.43 5.70 CPM 260 4.89 4.14 5.63 4.42 5.32 5.43 4.45 5.40 5.75 CPM 261 5.45 4.17 6.73 4.36 5.46 5.89 4.51 5.70 6.18 ALFALFA 83 6.35 5.18 7.52 5.43 7.22 6.67 5.45 7.39 6.79 ALFALFA 93 8.18 6.48 9.87 7.69 8.72 9.46 7.18 8.28 9.08 TIMOTHY 68 3.89 3.00 4.79 2.96 3.55 3.87 2.81 3.41 3.74 TIMOTHY 93 7.01 5.69 8.32 6.27 6.73 7.12 6.22 6.73 7.15 COCKSFOOT 76 4.40 3.73 5.08 3.20 4.09 4.37 3.28 4.24 4.54 WHEAT STRAW 68 2.39 1.88 2.91 1.87 2.20 2.42 1.72 2.04 2.25 WHEAT STRAW 92 3.41 3.01 3.81 2.61 3.04 3.09 2.72 3.18 3.23 slope 1.02 0.92 0.90 1.01 0.91 0.89 Correlation 0.97 0.94 0.95 0.98 0.95 0.97 mean bias 0.86 0.08 0.00 0.77 0.04 0.05 regression bias 0.00 0.01 0.02 0.00 0.02 0.03 unexplained variation 0.12 0.28 0.23 0.09 0.23 0.14 MSPE (%/hr)squared 0.98 0.38 0.26 0.87 0.29 0.22 RMSE 4.15 1.27 0.31 3.93 0.84 1.04
Lignin is generally accepted as the primary entity responsible for limiting the digestion of forages (Besle et al., 1994; Van Soest, 1994). Assuming that any estimation based on long fermentations and made at any time other than infinity is an overestimate of the true asymptotic indigestible residue, several attempts to predict iNDF from lignin concentration have been made (Mertens, 1973; Chandler, 1980; Conrad et al., 1984; Weiss et al., 1992; Traxler et al., 1998). As previously mentioned Chandler et al. (1980) estimated the indigestible fraction as lignin times 2.4, after fermentation between 90 and 120 days in methane digesters. The Cornell Net Carbohydrate and Protein System uses the 2.4 value as ratio between ADL and NDF to estimate iNDF in forages. Despite the small size and origin of the database, the Chandler equation performed well in prediction of iNDF in our observations (Van Soest et al., 2005), resulting in satisfactory regression between predicted and observed (R 2 = 0.94). Data from Huhtanen et al. (2006) shows a general relationship between permanganate lignin and iNDF measured by 12 day in-situ fermentation, with an overall slope of 2.4, but that relationship does not hold among all the values from Huhtanen et al. (2006). This was also observed by Nousianen et al. (2004) who could not develop an acceptable prediction equation (R 2 < 0.40) for iNDF based on lignin content on different grass silage types. The previous findings from Nousianen et al. (2004) and Huhtanen et al. (2006) refer to cold climate grasses that might result in different relationship between lignin and iNDF due to environmental and agronomic interactions. Our next objective was therefore to evaluate the consistency of the 2.4 ratio among various forage families. Previous tests for nutritional uniformity indicated an average recovery of 86% for ADL and sintered glass filters with a 40 µm aperture might not achieve complete recovery of fine particles (Robertson, unpublished results; Udén, 2006). Thus, we evaluated ADL and iNDF recovery to assess the ratio between ADL and NDF after improved recovery in order to estimate iNDF. Thirty forage samples of various species and ADL content were analyzed for ADL content in Gooch crucibles of porosity of 40 µm, with or without glass microfiber filters (1.5 µm; Whatman, 934-AH). The same samples were also fermented in situ for 16 d using bags of PPT monofilament fabric with porosity of 15 µm and an open area of 8.5% (Ankom Technology, Fairport, NY). The bags were also inserted in the rumen of two fistulated non-lactating cows. The same samples were also fermented for 16 d in the same bags in a Daisy Ankom System where the medium and rumen fluid were renewed every 4 d. All bags were analyzed for NDF after 16 d. Ratios between ADL and NDF, for estimation of iNDF, were back calculated with the iNDF obtained after the fermentations. Recovery of ADL varied among samples, but was generally higher using the filter paper (Table 2). Improved recoveries for iNDF followed the ADL recoveries, with higher recoveries for lower iNDF forages, when using the filter papers from the invitros rather than the bags from the in-situ or from the Daisy. Long fermentations were consistent within specie for both in-situ and in-vitro procedures. The combined improved recoveries of ADL and iNDF resulted in the back-calculated ratio not being constant and different than the 2.4 used so far. The observed ratios were in general always larger than 2.4. Forages lower in ADL/NDF had larger ratios and vice-versa for forages with larger ADL/NDF values. The data suggest the lignin procedure needs to be revised to
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aNDF, NDFd, iNDF, ADL and kd: What have we learned?

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