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

improve recovery of lignin particles and the development of forage family and speciespecific
equations for the estimation of the iNDF.
The original AOAC lignin procedure for crucibles relied on the use of asbestos as a
filtering agent, but the asbestos was rendered a health hazard and removed and
another filtering agent was never instituted, thus the variation in the lignin assay is
partially a function of the filtering step. Data in Figure 1 shows examples of the
lignin/NDF relationships for conventional and bmr corn, grasses and alfalfas. The two
types of corn silages and the grasses present differing ratios, while the alfalfas are
characterized by a less variable ratio (avg=2.8), with later (4 th and 5 th ) cuts having a
different behavior and larger values. More data are needed to allow specific equations
to be used for groups of forages. The improved recoveries alter the previous lignin to
NDF relationships primarily because of the increased recoveries of both lignin and
fermented NDF and this does not invalidate the previous ratio of 2.4 but enhances our
understanding to be more forage specific. More accurate and precise iNDF values will
allow for better estimations of rates of NDF digestibility to be used in CNCPS.
Table 2. Average increases in recoveries of ADL for different classes of forages using
small pore size (1.5 µ) filter papers in Gooch crucibles.
Class of forage % Increase in recovery
Conv. corn silage 25
Bmr corn silage 36
Alfalfa 3
Grasses 9
Early cut grasses 28
NDFd, kd and Chemical Structure of Lignin Bonds
Finally, when comparing different plants, the negative correlations observed between
the extent of digestion and lignin content are relatively good for the same variety among
stages of maturity; however, they are weaker within a family and poor when data from
plant families are pooled (ie grasses and legumes). The action of lignins seems to
depend not only on their amount but also on other factors like cross linking and because
of the chemical nature of this heterogeneous compound, it is nearly impossible to
extract lignin in any pure form – especially once it polymerizes into ADL.
Lignin action on digestibility depends therefore not only on its content but its
qualitative structural aspects. In general lignin is associated with carbohydrate (in
particular with hemicellulose) through covalent bonds, hydroxycinnamic acids are
attached to lignin and hemicellulose through ester and ether bonds as bridges between
them forming lignin/phenolic-carbohydrate complexes. It is generally accepted that the
association of phenolic components with carbohydrates presents the greatest barrier to
their utilization. It is well established that hydroxycinnamic acids attach to arabinoxylans
through ester bonds. A simple dimerization of these esters, even if there is no further
attachment to lignin, may be enough to substantially reduce the biodegradability of
digestible cell walls. Any further association (ether bonds) between this