Farm Animal Metabolism and Nutrition

2.The insoluble but potentially degradable
fraction is considered to be degraded by
microorganisms according to first-order
kinetics, implying that substrate digested at
any time is proportional to the amount of
potentially digestible matter remaining at
that time. An important assumption in
using first-order kinetics is that this pool of
slowly degradable material is homogeneous.
3.The rapidly degradable fraction corre-
sponds to the fraction which disappears
prior to the earliest removal of bags from
the rumen. This fraction includes not only
soluble material but also undegraded
small particles that are washed out of the
bags.

The degradation kinetics of DM or any
component of the incubated feedstuff may
be described by curvilinear regression of
the amount which has disappeared from
the bag with time:

D(t) = a+ b(1 
exp(
ct)) (11.1)

where D(t) is the amount of feedstuff which
has disappeared from bag at incubation
time t, ais the rapidly degradable fraction,
bis the slowly degradable fraction and cis
the fractional digestion rate constant (h
^1 )
of the fraction b.
When substrate is incubated in the
rumen, degradation is usually not con-

sidered to begin instantaneously. The

period during which either no digestion

occurs or digestion occurs at a greatly

reduced rate is generally referred to as the

lag phase. A modification of the model of

Ørskov and McDonald (1979), incorporating

a lag phase time, has been given by Dhanoa

(1988):

D(t) = afor t≤ (11.2)

D(t) = a+ b(1 
exp(
c(t
)))

for t>  (11.3)

where D(t), a, b, cand tare as defined in

Equation 11.1, and is the discrete lag time

(h). This model is used mainly to describe

the kinetics of forage DM and cell wall

degradation.

Subsequently, these simple models

were improved and completed in order to

take better account of the dynamic

processes of digestion. A possible method

for correcting degradation data for the

small particles lost at time zero is dividing

the a-term into two subfractions: (i) the

water-soluble fraction; and (ii) the particles

sufficiently small to pass through the bag

pores, this subfraction being considered as

degraded at the same rate as the particles

within the bag (Huntington and Givens,

1995). The amount of residue in the bag at

each incubation time is thus corrected by

In Sacco Methods 241

Fig. 11.3.In saccodegradation of forage DM: adjustment to first-order kinetics.