This technique can also be used to
estimate the apparent digestibility of
nutrient components within the forage,
using Equation 12.6.
Digestibility of nutrient component
(g kg^1 ) = 1000 1000(Mh/Mf)(Nf/Nh)
(12.6)where Mh= the marker concentration in
the herbage, Mf= the marker concentration
in the faeces, Nh= the concentration of the
nutrient component in the herbage and
Nf= the concentration of the nutrient com-
ponent in the faeces.
These equations are based on the
assumption that the marker is completely
inert and not absorbed, and that marker
concentration reached a steady-state in the
digestive tract. Furthermore, if an external
marker is used, feed intake should be
known in order to calculate feed marker
concentration.
Markers with incomplete faecal
recoveries, such as the even-chain alkanes,
can be used if the equation contains a
correction factor to compensate for incom-
plete recovery. Since the loss of alkane
through absorption from the small intestine
tends to decrease with an increasing
molecular weight of the alkane, the con-
centration of the natural alkane C 35 , which
has a constant recovery of 95% (Mayes et
al., 1986), is used to estimate digestibility,
provided it is present in the forage in
sufficient quantities (Equation 12.7).
Dry matter digestibility (g kg^1 )
= 1000(1 (Mh/Mf) 0.95) (12.7)where Mf= the concentrations of C 35 in the
faeces and Mh= the concentration of C 35 in
the herbage.
The accuracy of the estimates depends
on the degree of variation in recovery.
Errors in digestibility estimates may be
larger in cattle than in sheep due to an
apparent lower and more erratic recovery
of alkanes (Dove and Mayes, 1991). A
disadvantage of C 35 as marker is the rela-
tively low C 35 content of many plants and
forages (Dove and Mayes, 1996). However,
any of the natural alkanes present in
sufficient concentrations may be used as a
marker provided its recovery is known.
Comparing alkane C 31 and chromium oxide
as markers gave digestibility values of 605
and 633g kg^1 , respectively (Ohajuruka et
al., 1991).
The recovery of a marker can be
estimated by dosing grazing animals with
known mixtures of even-chain alkanes and
either comparing relative concentrations of
alkanes in a faecal sample with the
amounts dosed or by measuring even-chain
alkane output by total faecal collection.
Because of the low concentration of even-
chain alkanes in forage, the recoveries of
the dietary odd-chain alkanes can be
calculated by interpolation. Recovery can
also be estimated from a separate group of
housed animals with known intakes and
on which total faecal collections have been
made (Mayes et al., 1994; Salt et al., 1994).
Penning and Johnson (1983) obtained
variable results using acid-insoluble ash as
marker for digestibility estimates. Accurate
estimates of the digestibility of grasses
have been obtained using acid-insoluble
ash as marker (Van Keulen and Young,
1977; Sunvold and Cochran, 1991), but
highly variable results have been obtained
on hays. Hays were also ranked in a
different order than that obtained with in
vitromethods or in vivoprocedures with
beef cattle. The observed variation could
have been partly due to contamination of
samples with sand.
Acid detergent lignin and chromium
oxide added to the feed as external
digestibility marker in cows gave digesti-
bility values 11.0 and 12.4% lower, respec-
tively, than that calculated from total faecal
collection (Morse et al., 1992). A com-
parison of the suitability of the internal
markers, rumen-indigestible NDF, cellulase-
indigestible ADF, Klasons lignin, potassium
permanganate-soluble and insoluble Klasons
lignin, and permanganate-insoluble ash, and
the external markers, chromium-mordanted
NDR and Co-EDTA, for estimating the
digestibility of feed components showed
that only chromium-mordanted NDR, Co-
EDTA and cellulase-indigestible ADF can
be regarded as suitable (Tamminga et al.,
1989).268 J.P. Marais