The phosphate anion reacts with some of
the protons that otherwise would generate
CO 2. Feeds with a high protein content
may also produce some ammonia on
fermentation that reacts with VFAs and
reduces the indirect gas yield. If samples
with widely different protein content are to
be compared, it may be necessary to
measure and correct for ammonia produc-
tion (Cone, 1998) and for direct gas from
protein.
Microbial yieldThe pathways used by the microorganisms
for CHO degradation will determine the
amount of ATP available for microbial cell
production. The microbial yield, YAT P, is
the mass of microorganisms produced from
1 mol of ATP, and can vary from about 10
to 20 g under rumen conditions. This yield
varies with the microbial species and tends
to be higher when bacteria are growing
quickly, slower when growing slowly
(Hespell and Bryant, 1979).
It is clear that the relationship between
microbial mass and gas volume is complex.
It may vary with the type of substrate, with
the nature of the inoculum, with the
growth conditions and with the time of
observation (Blümmel et al., 1997a). This
relationship was first explored in the gas
system by Krishnamoorthy et al. (1991).
Using a syringe technique and defined sub-
strates (starch, cellulose and a glucose–
starch–cellulose mixture), these authors
recorded gas volumes before and 2 h after
pulse-labelling the culture with^32 P. The
total microbial protein synthesis during
this 2 h period was calculated from the
specific activity of the extracellular phos-
phate pool and from the N:P ratio in the
bacterial pellet. They found a curvilinear
relationship between the rate of microbial
protein synthesis and the rate of gas
production. High rates of gas production
corresponded to high rates of protein
synthesis, but the curvature of this rela-
tionship varied with the type of substrate;
cellulose produced a steep upward curve,
starch a less steep curve. With the mixed
carbohydrate substrate, they also found a
linear relationship between microbial pro-
tein synthesis and cumulative gas produc-
tion over an 8 h time span. The important
conclusion from this work was that one
should not rely solely on cumulative gas
production as an index of the microbial
growth potential of feeds.
Blümmel et al. (1997a) used different
methods to measure microbial yield in the
digestion of roughage. Incubations were
carried out in syringes for 24 h. The
insoluble residue was then collected by
centrifugation, washed, lyophilized and
weighed. A known mass of this material
was then refluxed with neutral detergent
solution and the residue again washed and
weighed. The microbial mass was calcu-
lated as the loss of mass caused by the
detergent treatment (Blümmel et al.,
1997a). These data were then used to relate
microbial mass to gas volumes and gave226 P. Schofield
Table 10.1.Gas yield from different carbohydrates.
Substrate Gas yield (ml g^1 ) Reference
Non-starch polysaccharide 200 Longland et al.(1995)
Various mixed 215–365, mean = 288 Blümmel et al.(1997)
Aqueous soluble fraction from grass, legume hay 240–280 Stefanon et al.(1996)
Cellulose, processed 320 Schofield et al.(1994)
Glucose 369 Cone et al.(1997)
NDF 360 Stefanon et al.(1996)
NDF 390 Pell and Schofield (1993)
Cellulose 384 Cone et al.(1997)
Pectin 437 Cone et al.(1997)
Cellulose, bacterial 440 Schofield et al.(1994)
Cellulose + R. albus 480 Taya et al.(1980)