Farm Animal Metabolism and Nutrition

from the whole-forage profile may or may
not correspond to a given chemical fraction.

Gas Yield and Microbial Yield

Feeds in the rumen are converted to short-
chain fatty acids (SCFAs), to CO 2 , to CH 4
and to microbial mass plus water. In our
earlier discussion of gas yields, we chose to
ignore the microbial mass. We now return
to consider two different quantitative
measures in forage digestion. The gas yield
is the volume of gas produced by digestion
of 1 g of substrate. The microbial yield is
the microbial mass produced from this
same digestion.
The gas system measures waste
products (mostly CO 2 and CH 4 ). We make
no distinction here between CO 2 and CH 4
since the latter arises directly from the
former. Carbon dioxide comes from two
sources, direct and indirect. The direct
source is the fermentation of glucose by
various pathways yielding VFAs, ATP and
CO 2. The indirect source is the reaction of
VFAs with bicarbonate. Both sources
provide a benefit to the animal. The ATP is
used for microbial growth in the rumen,
the VFA as a substrate for metabolism in
other organs. The gas measured in an in
vitro experiment is thus an indirect
measure of nutritionally important events.
The scheme in Fig. 10.9 shows the
relationship between carbohydrate digested

(CHOdig) and its products. We now con-

sider separately the production of gas and

of microbial mass.

Gas yield

We have already noted that the yield of gas

per g of substrate digested is generally less

than that expected for complete conversion

to direct and indirect gas. A wide range of

yields may be encountered in practice,

ranging from 200 to 480 ml g
^1 (Table 10.1).

Part of the reason for this variation is

that some of the substrate is used to

produce microbial mass. Another reason is

that the actual yield of gas per g of sub-

strate digested may vary with the chemical

nature of the substrate and with the micro-

bial population. Different populations of

rumen microorganisms may use several

different metabolic pathways for digestion

(Van Soest, 1994). Variations in these path-

ways will result in variations in the com-

position of the VFA mixture produced and

will cause a corresponding variation in gas

from both direct and indirect sources

(Beuvink and Spoelstra, 1992). Readily

digested soluble carbohydrates will tend to

produce more propionate and less direct

gas (Sarwar et al., 1992).

A third reason why indirect gas yields

are less than unity is because the in vitro

buffer contains phosphate as well as

bicarbonate (Beuvink and Spoelstra, 1992).

Gas Production Methods 225

CHOdig

microbial

metabolism

VFA + gas

MICROBIAL YIELD GAS YIELD

microbial mass

microbial

YATP growth

ATP

Fig. 10.9.Relationship between carbohydrate digested (CHOdig) and its products.