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

The variation associated with using a single time point was primarily related to the
estimation of the iNDF.
ADL AND iNDF
According to the Lucas principle, the iNDF is an ideal fraction since by definition it is
digested at a predictable rate of zero. The estimation of the indigestible fraction is not a
mathematical or modeling contrivance, but is a critical biological principle upon which
the concept of digestion kinetics and rates are based (Mertens, 1994). Often digestion
rates are calculated without subtracting the indigestible residue or by subtracting one
that is determined at too short a fermentation time. The purpose of long digestion times
in kinetic analysis is to obtain a value that is used to estimate an indispensable kinetic
parameter. According to Ellis et al. (1999) determination of iNDF should be included in
all basic feedstuff analysis because it has a predictable digestibility; it can be used for
the estimation of the potentially digestible NDF (pdNDF) as NDF-iNDF and it has an
important role in contributing to rumen digesta load. Furthermore, a close empirical
relationship between silage iNDF and OM digestibility (Nousiainen et al., 2003)
indicates that iNDF is a useful entity for the prediction of the nutritive value of forages.
The importance of the iNDF estimation on OM and NDF digestibility, rumen NDF pool
and microbial N flow is also demonstrated by the Nordic model of dairy cow metabolism
―Karoline‖ (Danfær et al., 2005). Simulation results clearly demonstrate profound effects
of these parameters on OM digestibility and consequently on the supply of energy and
microbial protein.
Mertens has shown (1973, 1977) the effect of the fermentation time chosen to
represent the iNDF on digestion rate, determined using ln-linear. Subtraction of large
iNDF (early time points) results in prediction of greater than true digestion rates.
Conversely, subtraction of small or no iNDF results in less than true rates of digestion,
because high value of residues at later fermentation times cause a counter-clockwise
rotation of the semi-logarithmic regression line. Therefore any error in estimating
indigestibility can bias the estimates of fractional rate and lag time as they are
sequentially estimated using ln-linear regression (Mertens and Loften, 1980; Moore and
Cherney, 1986). Observations of long in-vitro fermentations (up to 216 hours) have
shown how digestion was not completed at 96 hr (Van Soest et al., 2005). However,
Huhtanen et al. (2006) have suggested that the ultimate extent of NDF digestion may
not be reached with in-vitro batch systems and the in-situ approach might be biased due
to crucial drawbacks of the traditional nylon bag procedure as discussed by Nousiainen
et al. (2004). The close relationship between in-vivo digestibility and the potential extent
of digestion (Nousianen et al., 2003) suggests that using prolonged incubations and
bags with a small pore size may allow the extent of NDF digestion (and iNDF) to be
accurately measured. Nousiainen et al. (2004) determined iNDF by 12 days in situ
incubations using nylon bags of small pore size.