Farm Animal Metabolism and Nutrition

expected to change the digestion profile
and the feed value of silage compared with
forage. Doane et al.(1997a) used a curve
subtraction approach to compare the gas
yield from the NDS (neutral detergent-
soluble) fraction of freeze-dried, oven-dried
and ensiled forages and reported decreases
in gas yield of 7–36% in this fraction on
ensiling.

Correlation with in saccomethod

In saccomethods are discussed in Chapter

11. Degradation rates of a grass (Lolium
    perenne), measured using nylon bags in
    situand also measured using an open gas
    system, were reasonably well correlated
    (r^2 = 0.74) (Cone et al., 1998). For this com-
    parison, the gas curve corresponding to the
    fibre fraction (extracted using a three-pool
    Groot equation (Cone et al., 1997)) was
    fitted to an exponential model so that these
    data could be compared with the in sacco
    results. Some of the lack of perfect agree-
    ment may lie in the assumption that the
    digestion curve extracted from the three-
    pool equation is equivalent to that
    measured directly in sacco.

Intake predictions

Dry matter intake depends upon so many
different factors (see Part III of this volume,
Intake and Utilization), many of them
strongly dependent on the individual ani-
mal, that it is difficult to correlate in-
take and any single measured feed pro-
perty (Van Soest, 1994). Blümmel and
Bullerdieck (1997) have attempted to make
this correlation using intake data from
legume and grass hays (all high NDF) and
in vitrogas production data on the same
forages. They found no significant correla-
tion between intake and the gas parameters
a, band c from the exponential equation
y(gas) = a + b  (1
e
ct) (compare
Equation 10.4). However, if a partitioning
factor (the reciprocal of the gas yield
discussed above, measured at 24 h) was
included, the r^2 increased to 0.74. The

inclusion of low NDF feeds would provide

a more critical test of this correlation.

Summary

The gas system is an alternative analytical

technique to study feed digestion in vitro.

Instead of measuring the dissolution of

insoluble plant components, as in the

Tilley–Terry and in sacco methods, we

measure the appearance of gaseous

products. This change of focus yields two

principal advantages. The first is that, with

electronic pressure sensors, measurements

can be recorded by computer. The second

advantage is that digestion of soluble feed

components can be studied in a way that

permits a direct comparison with insoluble

components.

There is general agreement that the

limited data from in saccomethods are best

interpreted using a simple exponential equa-

tion. No such agreement presently exists for

the richly detailed data from the gas system.

Questions have been raised about the

relevance of gas data to microbial cell

production. The answer seems to be that

gas data need to be supplemented with

measurements of substrate disappearance,

VFA profiles and microbial yield before

they can supply the maximum nutritional

information.

In this brief review, we have discussed

only a few of the possible applications of gas

data. These have included the study of the

feed soluble components, the measurement

of digestion rates for model testing and

the investigation of ‘anti-nutritional’ plant

secondary compounds such as tannins. One

important nutritional topic that has received

little attention by investigators using the gas

technique is that of ‘associative effects’, the

interaction between feed fractions during

digestion. The gas technique is well suited

to this type of investigation.

Acknowledgements

The author wishes to thank Dr Michael

Blümmel, University of Hohenheim,

Stuttgart, Germany, Dr John Cone, ID-DLO,

228 P. Schofield