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

a reverse-phase column. Ether-linked PCA and FA were calculated as the difference
between total and ester-linked phenolic acids. Phenolic acids and respective amounts of
their different linkages and lignin types were used in a multiple regression to select
those factors explaining most of the variation in kd, 24hr and 96hr NDFD.
Since most esterified PCA on lignin are not covalently attached to other cell wall
polymers, they should not directly influence cell wall degradability. However some cell
wall models show how they can interfere with ferulate-lignin cross linking and in some
cases reduce the proportion of lignin bound to cell wall. Figure 2 and 3 show the
relationship between 24hr NDFD and PCA content in NDF and ADF residues
respectively. Forage groups demonstrated different relationships for digestibility from
positive to negative in NDF residues. ADF residues were instead characterized by a
consistent negative relationship among all forage groups and similar results were
obtained for 96 hr NDFD.
Relationship between 24hr and 96hr NDFD with esterified FA content in NDF
residues are shown in Figures 4 and 5, respectively. Ester-linked FA had generally a
negative relationship except in bmr corn for 24hr and positive for 96hr NDFD. Ferulates
primarily form as esters of arabinoxylans and later they cross-link through ether linkages
with lignin. So esters of FA should not necessarily limit NDF degradation. This has
probably more to do with the degree of arabinoxylans substitution.
Ether linkage between FA and lignin has been used a measure of cross-linking
between lignin and arabinoxylans and defined as the most important factor limiting
energy utilization (Casler, 2001). However in our case we were not able to obtain
consistent negative relationship with NDFD. Figure 6 and 7 show the relationship for 24
and 96 hours respectively with divergent relationship for 24 hr and consistent positive
ones for 96 hr NDFD. Negative effect of etherified FA on NDFD has been found in
elongating internodes in maize but not in internodes that had stopped the elongation
process and confirms the hypothesis that secondary cell wall development may mask
the negative impact of etherified FA on NDFD. Interestingly also, bmr shows higher
content of etherified FA compared to conventional corn in NDF residues, demonstrating
that etherified FA is not always a good indicator of cross-linking between lignin and
arabinoxylans and this was also shown recently by Marita et al. (2003). However this
relationship changes when ADF residues were analyzed for ether linked FA, showing
how the solubulization or branching of the lignin structure has in this case more
importance than linkages. Acid detergent solution in this case might dissolve those FAs
that only etherified (instead of having and ester-ether linkage).
A multiple regression with stepwise selection was run within each forage group using
the independent variables lignin type (ADL or Klason lignin) and their difference on an
NDF basis and phenolic acids with their specific linkages also on an NDF basis. We
wanted to determine how much variation in 24hr and 96hr NDFD and kd was explained
by the above mentioned variables. The procedure was not able to find any significant
variable for the alfalfas (P < 0.10), showing how NDF digestibility cannot be easily
explained in legumes by many of these factors. All the other forage groups reached