Farm Animal Metabolism and Nutrition

Intrinsically^14 C- and^3 H-labelled

plant material

Plant cell walls can be labelled with^14 C by
multiple-pulse [^14 C]carbon dioxide dosing
of plants grown in a plastic tent (Smith,
1989), or by immersing excised leaves in a
10 μMsolution of^14 C-labelled acetate and
exposing the leaves to a high-intensity
tungsten light source for several 30 min
periods alternated with 30 min dark periods
(Kolattukudy, 1965). Tritium-labelled grass
can be obtained by spraying young grass at
regular intervals with tritiated water, as
described by Van den Hoek et al. (1985).
As the label is incorporated into poten-
tially digestible and indigestible com-
ponents, tritium and^14 C-labelled plant
material do not meet the criterion of
indigestibility, as required for markers used
for rumen kinetic studies. Digestible com-
ponents should be removed before employ-
ing relatively indigestible fractions such as
NDF (after chromium or cobalt mordan-
ting) and alkanes as isotopically labelled
markers.

Tritiated gypsum

Gypsum labelled in its water of crystalliza-
tion is prepared by mixing gypsum powder
with an appropriate amount of water, the
tritium content of which is adjusted to suit
the level of supplementation required.
After mixing to a smooth paste, the gypsum
is spread out and allowed to dry. It is then
milled to a powder, freeze-dried and added
to the feed ingredients (Dove, 1984).
Activity is measured by means of liquid
scintillation counting.

Estimations Using Markers

Since marker techniques tend to give
variable results depending on experimental
conditions, the marker employed, the
material analysed and the model used for
processing the data should be verified, if
possible, using alternative markers or
procedures.

Digesta flow

Digesta consist of particles of various sizes

suspended in a fluid, forming a number of

interacting pools. Labelling and tracing each

pool through the digestive tract is complex

and a marker suitable for labelling one pool

may not be suitable for another. By prepar-

ing animals with cannulas, digesta flow can

be estimated at the points of cannulation.

Since the estimation of flow requires

measurement of the amount of marker con-

sumed by the animal, both analytical and

dosing errors will influence results. Further-

more, estimations depend on regular faecal

sampling, but will only give accurate results

if the flow of the marker is constant.

However, marker concentration often shows

considerable fluctuation depending on the

feeding pattern of the animal.

To estimate digesta flow, a limited

amount of marker such as chromium-

mordanted or rare earth-labelled fibre is

administered, usually as a single pulse

dose followed by time-sequence sampling

of digesta. However, the continuous dosing

of marker as a component of the normal

diet is preferred to pulse dosing because of

poor mixing and differential flow of pulse-

dosed marker from the rumen. After with-

drawing the continuously injected marker,

the declining marker concentration cur-

ves are fitted to appropriate models.

Comprehensive comparisons of mathe-

matical models used for estimating passage

kinetics have been made (Lallés et al.,

1991; Ellis et al., 1994; Huhtanen and

Kukkonen, 1995). The most frequently

used models are two-compartmental with

two rates of decay, the slowest associated

with rumen passage and the fastest with

post-rumen flow (Grovum and Williams,

1973). The Grovum and Williams model

assumes passage to have an exponential

lifetime distribution and implies that there

is an equal probability of particles leaving

the rumen, regardless of their size or age. It

is recommended for the estimation of the

passage of liquids (Quiroz et al., 1988). The

gamma age-dependent models of Ellis et al.

(1994) assume a gamma lifetime distribution

and imply that the probability of a particle

264 J.P. Marais