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

indicating where changes will most likely occur in the near future when estimating rates
of digestion. This involves refinements in methodology and a more complete
understanding of the extent of digestion.
Sample Analyses
Conventional and bmr corn silages, perennial ryegrasses, wheat straw, and alfalfa
hays were analyzed for fiber fractions using the procedure of Van Soest et al. (1991).
Sulfuric lignin was also used as the standard lignin procedure (Van Soest and
Robertson, 1980). A small preliminary study indicated that fermentations in-situ for 16
days may not be enough to reach the maximum extent of digestion and this is still being
investigated. Extended fermentations were also carried using the ANKOM in-vitro
apparatus (DAISY III; ANKOM Technology Corp., Fairport, NY), because of the ease of
re-inoculating the samples. This latter system allowed a better estimation of the intrinsic
iNDF of the forage after 16 days. Fermentations were then carried for 16 days using the
DAISY system and bags of polyester polyethylene terephthalate with 15 μm porosity
and 8.5% open area (ANKOM Technology Corp., Fairport, NY). Jars were re-inoculated
with medium every 4 days. Fermentations in-vitro were carried out for 6, 12, 24, 30, 36,
48, 72, 96, 120, 144, 168, 192 and 216 hours, in 125 ml Erlenmeyer flasks in a 39 °C
water bath under constant CO2 in Goering and Van Soest buffer (1970), and with
renewed medium after 96 hours. In-vitro flasks were inoculated with rumen fluid from
the same cows used for the DAISY fermentations, fed hay and grain. Blank samples
were run for both fermentations and used to correct for any contamination at each
fermentation time. All samples at the end of the fermentations were analyzed for NDF.
Further Evaluations of Rate Estimations for NDF Digestion – Comparison to a Non-
Linear Model for a First Order Approach
The earliest attempts to describe the kinetics of digestion have been reviewed by
Mertens (1993a, 1993b). The term ―rate of digestion‖ appeared in the 1950s, but the
assessments were mainly based on the visual interpretation of digestion curves.
Waldo (1970) suggested that if the indegistible residue was subtracted, digestion of
potentially digestible cell walls might follow first order kinetics. Two primary methods are
used for fitting data to the first order kinetic models: linear regression on logarithmic
transformations of undigested residues (ln-linear) and nonlinear estimation of
parameters using multiple time points. The regression between the natural logarithm of
the potentially digestible NDF (pdNDF) against time results in supporting the hypothesis
that pdNDF follows the first order digestion kinetics. In this case, it is critical the
estimation or measurement of the indigestible fraction (iNDF) is accurate in order to
accurately describe the digestion kinetics.
A further problem in describing NDF digestion and kinetics is that bias can also be
associated with early measurement of digestion. Usually, digestion of the pdNDF
fraction does not appear to start instantaneously at time zero. Instead there is a lag
period during which digestion occurs slowly or not at all (Mertens, 1993a, 1993b) while
the fermentation stabilizes and bacterial attachment and enzyme synthesis has