Farm Animal Metabolism and Nutrition

of analysis determines the amount of
marked material administered in order to
obtain acceptable results, without exceeding
the marker-binding capacity of the feed
particles. Minor changes in marker concen-
tration may have marked effects on kinetic
estimates obtained from complex mathe-
matical models.
No existing marker totally satisfies all
requirements. Some particulate markers
tend to migrate to particles not initially
labelled. In this respect, liquid markers are
generally more satisfactory than particulate
markers. Marker migration is of concern
because the label being followed would be
unrelated to the component originally
marked. Unless the integrity of the marked
feed particle has been established, pool
sizes and rate of passage should be dis-
cussed in terms of the marker employed,
and not the fraction initially marked. Some
markers such as lignin may be partly
metabolizable and may therefore not be suit-
able for estimating residence time, since dis-
appearance is not proportional to residence
time. Some markers are slightly absorbable
in the gastrointestinal tract, and are there-
fore not totally recoverable in the faeces.
Finely milled particles used for labelling
may pass from the rumen more quickly than
feed particles as they are not delayed for
reduction in size. Mordanted particles may
also pass very rapidly from the rumen, due
to their high density. Because binding
affinities vary among feed components,
feeds may be labelled unevenly. The
importance of marker criteria such as
indigestibility, however, depends entirely
on the use to which the marker is put.
Markers should therefore be selected with
discretion in order to avoid serious errors in
interpreting results. Markers are classified as
internal or external markers.

Internal Markers

An internal marker is a chosen substance
which forms an integral part of the forage or
feed consumed by the animal. Internal
markers have the advantage of being cheap
and convenient, and may be particularly

useful in experimentation with wild or free-

ranging animals, which are difficult to dose

with external markers. Although many

internal markers appear promising, few

have gained acceptance, mainly because the

marker in the diet may be different from the

marker analysed in the faeces due to

changes occurring during digestion. Further-

more, erratic results frequently have been

reported in evaluation studies of internal

markers by different laboratories, or when a

marker is applied across a wide range of for-

ages. Of the internal markers, the alkanes

have the most widespread application for

estimating organic matter digestibility and

forage species selection under grazing con-

ditions, while purines appear to be the most

promising for predicting microbial protein

synthesis in the rumen.

Indigestible acid detergent fibre (IADF)

The usefulness of indigestible fibre as an

internal marker has been limited by a lack

of standardized analytical procedures. The

treatment of plant material with neutral and

acid detergents produces a fraction known

as lignocellulose or acid detergent fibre

(ADF) (Van Soest, 1963). IADF is deter-

mined gravimetrically after incubating ADF

with cellulases for 10–14 days (Penning and

Johnson, 1983), after rumen fluid fermenta-

tion in vitrofor 4 days or after incubation in

nylon bags in the rumen for 6–8 days. These

fractions should, theoretically, be totally

indigestible, rendering them suitable as

markers, but in practice variable results

are often obtained. Furthermore, faecal

recoveries of in vitroIADF of 0.46 and 0.99,

and recoveries of cellulase IADF of 0.47 and

0.97, were obtained with immature tall

fescue and lucerne cubes, respectively,

indicating that IADF fractions should be

used with caution as markers in immature

forage (Cochran et al., 1986).

Indigestible lignin

Because mammals do not appear to contain

enzyme systems capable of degrading

256 J.P. Marais