6
Volatile Fatty Acid Production
J. France
1
and J. Dijkstra
2
1
Centre for Nutrition Modelling, Department of Animal & Poultry Science,
University of Guelph, Guelph, Ontario N1G 2W1, Canada;
2
Animal
Nutrition Group, Wageningen Institute of Animal Sciences, Wageningen
University, PO Box 338, 6700 AH Wageningen, The Netherlands
Introduction
Volatile fatty acids (VFAs), principally acetate, propionate and butyrate but also
lesser amounts of valerate, caproate, isobutyrate, isovalerate, 2-methylbutyrate
and traces of various higher acids, are produced in the rumen as end-products
of microbial fermentation. During the fermentation process energy is con-
served in the form of adenosine triphosphate and subsequently utilized for the
maintenance and growth of the microbial population. As far as the microbes
are concerned the VFAs are waste products but to the host animal they
represent the major source of absorbed energy and with most diets account
for approximately 80% of the energy disappearing in the rumen (the remainder
being lost as heat and methane) and for 50–70% of the digestible energy intake
in sheep and cows at approximately maintenance, the range being 40–65% in
lactating cows (Sutton, 1972, 1979, 1985; Thomas and Clapperton, 1972).
Dietary carbohydrates, i.e. cellulose, hemicellulose, pectin, starch and
soluble sugars, are the main fermentation substrates. They are degraded to
their constituent hexoses and pentoses before being fermented to VFA via
pyruvate (Fig. 6.1). Pentoses are converted to hexose and triose phosphate
by the transketolase and transaldolase reactions of the pentose cycle so that the

to VFAs in the rumen. The contribution from lipids is very small as lipids
normally represent a small proportion of the diet and only the carbohydrate
moiety, i.e. glycerol and galactose arising from lipid hydrolysis, and not the long-
chain fatty acids, are fermented. Dietary proteins on the other hand may be a
significant source of VFA when diets having a high rumen-degradable-protein
content are fed. The proteins are hydrolysed to amino acids, which are deami-
nated before conversion to VFA. Of particular importance in this respect is the
formation of isobutyric, isovaleric and 2-methylbutyric acids from valine, leucine
and isoleucine, respectively, as these branched-chain VFAs are essential growth
factors for certain of the rumen bacterial species (Cotta and Hespell, 1986).
The majority of the VFAs produced in the rumen are lost by absorption
across the rumen wall, although a proportion (10–20% in sheep and up to 35%
in dairy cattle) pass to the omasum and abomasum and are absorbed from these
organs (Weston and Hogan, 1968; Dijkstra et al., 1993). Absorption across
the rumen wall is by simple diffusion of the undissociated acids (Stevens, 1970;
Dijkstra et al., 1993). It is a concentration-dependent process and therefore
Pyruvate
Acetyl CoA
Cellulose
Starch
Soluble sugars
Pectin
Pentoses
Hemicellulose
Hexoses
Pentose
cycle
Embden−Meyerhoff
pathway
Formate

2000).
The concentration of VFA in the rumen at any given time reflects the
balance between the rate of production and rate of loss. Immediately after
feeding, production exceeds loss and the concentration increases, but subse-
quently the situation is reversed and the concentration falls. The total VFA
concentration may fall as low as 30 mM or be in excess of 200 mM but is
normally between 70 and 130 mM. The relative concentrations of the individ-
ual acids, commonly referred to as the fermentation pattern, is a reliable index
of the relative production rates of the acids when forage diets are given but
would appear less reliable with concentrate diets (Leng and Brett, 1966; Esdale
et al., 1968; Sharp et al., 1982; Sutton, 1985). The fermentation pattern is
determined by the composition of the microbial population, which in turn is
largely determined by the basal diet, particularly the type of dietary carbohy-
drate, and by the rate of depolymerization of available substrate (review by
Dijkstra, 1994). High-fibre forage diets encourage the growth of acetate-
producing bacterial species and the acetate:propionate:butyrate molar propor-
tions would typically be in the region 70:20:10, whereas starch-rich concen-
trate diets favour the development of propionate-producing bacterial species
and are associated with an increase in the proportion of propionate at the
expense of acetate, although acetate is almost always the most abundant of the
acids. Under certain conditions, concentrate diets may encourage the develop-
ment of a large protozoal population and this is accompanied by an increase in
butyrate rather than propionate (Williams and Coleman, 1997). If levels of
substrate available for fermentation are high, either from increased intake or
increased rates of depolymerization, a shift in fermentation pattern from acetic
acid to propionic acid occurs to dispose of excess reducing power (Dijkstra,
1994). In addition to the type of dietary carbohydrate, other factors such as the
physical form of the diet, level of intake, frequency of feeding and the use of
chemical additives may also affect the fermentation pattern (Ørskov, 1981;
Thomas and Rook, 1981; Nagaraja et al., 1997). Some examples of the

amounts lost by absorption and passage) are less than the total production rates
(Bergman et al., 1965). In this and subsequent sections of the chapter, the
term production is synonymous with net production unless total production is
specified.
Zero-time in vitro method
A sample of rumen contents is taken and subsamples incubated in vitro under
anaerobic conditions. The rate of production of individual or total VFAs is
calculated from the increments in acid concentration obtained by incubating
the subsamples for different periods and extrapolating back to zero time to give
the rate of VFA production per unit volume at the time the sample was
removed. Equations for performing the calculation are given by Whitelaw
et al. (1970). If the rumen volume is known, total ruminal production can
160 J. France and J. Dijkstra
Table 6.1. VFA concentration, molar proportions and production rates in the rumen of sheep, steers and cows given various diets.
Animal
species Diet
Intake
(kg/day)
Total VFA
concentration
(mmol/l)
Acetate
(molar %)
Propionate
(molar %)
Butyrate
(molar %)
VFA
production
(mol/day) Reference

a
103 73 18 9 50.1 Siciliano-Jones and
Murphy (1989)
Lucerne hay:lucerne
pellets:concentrate (1:3:1)
8.29
a
100 72 18 10 42.4 Siciliano-Jones and
Murphy (1989)
Concentrate:lucerne hay
(4:1)
8.56
a
108 67 22 12 54.1 Siciliano-Jones and
Murphy (1989)
Concentrate:lucerne
hay:lucerne pellets (16:1:3)
8.94
a
118 63 26 12 42.3 Siciliano-Jones and
Murphy (1989)
Maize silage:concentrate
(1:1)
5.19
a
123 55 34 11 14.3 Rogers and Davis
(1982a)
Concentrate:maize
silage (3:1)
7.7

145 49 34 17 51.4 Sharp et al. (1982)
Ground maize:other (5.25:1) 6.22
a
141 41 49 10 42.0 Sharp et al. (1982)
Dairy cows Lucerne hay:grain (1:1.3) 19.1
c
109 67 21 12 37.52 Davis (1967)
Lucerne hay:grain (1:6.6) 17.27
c
121 49 40 11 44.58 Davis (1967)
Maize silage 3.5
a
83 64 19 17 30.9 Esdale et al. (1968)
Lucerne hay 3.9
a
77 73 17 10 26.7 Esdale et al. (1968)
Ryegrass
hay:concentrate (6:4)
12.9
a
85 68 19 13 79.8 Sutton et al. (2003)
Ryegrass
hay:concentrate (1:9)
12.7
a
89 52 38 9 90.0 Sutton et al. (2003)
a
Dry matter.
b
Organic matter.

V
0
(6:2)
where U
0
, C
0
and V
0
denote acid utilization, acid concentration and ruminal
volume, respectively, in the new steady state. Subtraction of Eq. (6.1) from
Eq. (6.2) yields an expression for the constant of proportionality:
k ¼ I=(C
0
V
0
À CV)(6:3)
Substituting for k in Eq. (6.1) gives the rate of production:
P ¼ I=[C
0
V
0
=(CV) À 1] (6:4)
The steady-state volumes V and V
0
can be determined using one of the methods,
based on digesta markers and intraruminal sampling, described in France et al.
(1991a). This approach of raising the steady-state level was used by Bath et al.
(1962) though they assumed a constant ruminal volume and expressed the acid
concentration relative to that of the other acids. Martin et al. (2001) adopted the

temporarily isolated rumen of sheep. To explain the differences, Kristensen
et al. (2000) suggested substantial microbial utilization of VFA. Also, measure-
ments of blood flow show considerable variability (Dobson, 1984).
Methane production
Methane production is an index of rumen fermentation, which has been used to
obtain indirect estimates of VFA production. Total methane production can be
measured in intact, non-fistulated animals using indirect calorimetry (McLean
and Tobin, 1987) or the polytunnel method (Lockyer and Jarvis, 1995).
Calorimetry and the polytunnel, however, overestimate the ruminal contribu-
tion; Murray et al. (1976), for example, showed that the production of
methane in the rumen of sheep fed lucerne chaff accounted for 87% of the
total production. Alternatively, ruminal methane production can be measured
with fistulated animals using isotope dilution techniques (Murray et al., 1976,
1978; France et al., 1993). Also, non-isotopic tracer techniques have been
developed to measure ruminal methane production in free-moving, intact
animals, such as the sulphur hexafluoride (SF
6
) method (Johnson et al.,
1994). The value obtained for methane production is then multiplied by the
164 J. France and J. Dijkstra
ratio of individual or total VFA produced to methane produced. This ratio may
either be determined in vitro using rumen samples, or calculated stoichiome-
trically (Murray et al., 1978), provided the VFA proportions are known. The
method relies on a close relationship between VFA and methane produced,
based on the need to maintain redox balance in the rumen. However, a number
of other factors, including the uptake of hydrogen for biohydrogenation of
unsaturated long-chain fatty acids and the uptake or release of hydrogen for
microbial protein synthesis, may impair this relationship (Mills et al., 2001).
Tracer Methods of VFA Production Measurement
The tracer methods developed in this section are described for radioactive iso-

À F
ov
(6:5)
dq
dt
¼ I À sF
ov
(6:6)
VFA, Q
F
vo
F
ov
(a)
q
I
sF
ov
(b)
Fig. 6.2. Single-compartment model for estimating
VFA production: (a) tracee and (b) tracer. The scheme
assumes no re-entry of label into the rumen. Q, total
VFA; q, quantity of tracer; F
vo
, rate of de novo VFA
production; F
ov
, rate of VFA removal; s, plateau
specific activity of total VFA; and I, infusion rate.
Volatile Fatty Acid Production 165

calculated from Eq. (6.7) using either the mean specific activity or the specific
activity of a pooled sample or, alternatively, by multiplying the infusion rate by
the mean reciprocal specific activity. Although with steady-state conditions all
three procedures should give the same result, Morant et al. (1978) found in
simulation studies with non-steady-state conditions that estimates obtained
using the latter procedure were closer to the true production rates and recom-
mended its use in preference to the other two. (Note: Eq. (4) in Morant et al.
(1978) should read M
R
¼ (I
R
=n)
P
n
i¼1
M
i
=I
i
:)
Weller’s method can be adapted for single-dose injection of tracer, rather
than continuous infusion. Equation (6.6) reduces to:
dq
dt
¼ÀsF
ov
(6:9)
where s is now the instantaneous specific activity. Integration of Eq. (6.9) with
respect to time between time zero and infinity gives:
ÀD ¼ÀAF

Equation (6.12) is derived using the rate:state equations for Weller’s method
in non-steady-state (i.e. from Eqs. (6.5) and (6.6) not equated to zero) and
eliminating the flow F
ov
. It applies from the instant of commencement of
infusion.
The instantaneous production rate may be determined by varying the
rate of isotope infusion in synchrony with the rate of VFA production so that
the specific activity remains constant, and therefore, the differential term in
Eq. (6.12) is equal to zero. Gray et al. (1966) used this method to measure
VFA production in sheep fed twice daily but, since it is dependent on prior
knowledge of the rate of VFA production, it is unlikely to be of general
applicability.
An alternative approach, proposed by Morant et al. (1978), is to infuse the
isotope at a constant rate, and monitor the variable liquid volume of the rumen
and its isotope and total VFA concentrations (thus permitting determinations
of total VFA pool size Q and its specific activity s at time t). Variable volume can
be determined using one of the methods described in France et al. (1991a).
The differential term in Eq. (6.12) is given by the slope of the curve of
inverse specific activity against time. A way of determining this slope is to fit
a polynomial of the form:
f(t) ¼
X
n
i¼0
a
i
t
i
(6:13)

Such differences in rumen volume resulted in only minor differences in esti-
mates of net production rates of VFA obtained by continuous infusion of
acetate, propionate and butyrate in a three-pool scheme (next section, this
page). This suggests that, in practice, attempts to make accurate measurements
of diurnal changes in rumen volume may not be necessary.
Three-pool scheme
Weller’s method has the advantages that only one infusion (or single injection)
experiment needs to be undertaken and the specific activities of the individual
VFAs do not have to be determined. However, it is dependent on the produc-
tion rate of the acids being proportionally the same as their concentration in
rumen liquid and this may not always be so (Sutton, 1985).
An alternative method for estimating VFA production rates in steady state,
which is not dependent on the proportionality between VFA production and
concentration and also provides a more detailed description of VFA metabol-
ism in the rumen (thus permitting total rather than just net production to be
estimated), is to use interchanging compartmental models to interpret isotopic
tracer data. The models may be complete – i.e. exchange between all pools
(plus the external environment) included – or incomplete (i.e. exchange be-
tween some pools excluded). Tracer is administered into each pool in turn and
on each occasion the specific activity of all pools is determined. A unique
solution to the model is obtained by deriving a series of n simultaneous equa-
tions (where n is the number of flows included in the model) to describe the
movement of tracer and tracee between pools.
Consider the fully interchanging three-pool model for acetate, propionate
and butyrate (Fig. 6.3). This scheme was proposed by Bergman et al. (1965)
using sheep but with no interconversion between propionate and butyrate
(i.e. F
bp
¼ F
pb

Following the infusion of labelled acetate, I
a
(mCi=h), the movement of label
through the acetate pool, q
a
(mCi), is described by:
dq
a
dt
¼ I
a
þ s
p
F
ap
þ s
b
F
ab
À s
a
(F
oa
þ F
pa
þ F
ba
) ¼ 0, (6:16)
through the propionate pool, q
p

F
ba
þ s
p
F
bp
À s
b
(F
ob
þ F
ab
þ F
pb
) ¼ 0(6:18)
Similar equations may be derived to describe the movement of tracee propi-
onate and butyrate and the movement of label when labelled propionate and
butyrate are infused into the rumen. The resulting 12 simultaneous linear
equations may be solved using a simple computational procedure (France
et al., 1987).
The method can also be adapted for single-dose injection of tracer. The
system is now in non-isotopic steady state so the rate:state equations for
labelled material are non-zero. In the three-pool scheme, movement of label
through the acetate pool following injection at time zero of a single dose of
labelled acetate, D
a
(mCi), is given by:
dq
a
dt

p
(F
op
þ F
ap
þ F
bp
)(6:20)
and through the butyrate pool by:
Butyrate
Butyrate
Propionate
Propionate
Acetate
Acetate
F
ao
F
ba
F
ap
F
bo
F
pb
F
bp
F
op
F

bp
À s
b
(F
ob
þ F
ab
þ F
pb
)(6:21)
The s terms now refer to instantaneous specific activities. Integrating these
three equations with respect to time between the limits zero and infinity yields:
ÀD
a
¼ A
p
F
op
þ A
b
F
ab
À A
a
(F
oa
þ F
pa
þ F
ba

þ F
ab
þ F
pb
)(6:24)
where A
a
, A
p
and A
b
are the areas under the acetate, propionate and butyrate
specific activity–time curves, respectively (i.e. A
a
¼
Ð
1
0
s
a
dt, etc.). Eqs (6.22)–
(6.24) can be derived for the movement of label when labelled propionate and
butyrate are injected into the rumen. The system of equations for single dose is
therefore the same as for constant infusion, but with dose and area replacing
infusion rate and plateau specific activity, respectively.
The method can also be extended to the non-steady-state. Under
non-steady-state conditions and constant infusion, movements of tracee and
label in the three-pool model are described by the same set of 12 equations as
represented in Eqs (6.15)–(6.18), but with the derivatives not now equated to
zero. Instantaneous values of the derivatives may be determined in a similar way

Annison et al. (1974) and Lebzien et al. (1981) obtained results for only two
labelled VFAs in dairy cattle. Other authors have used variations of the three-
pool scheme (Esdale et al., 1968; Armentano and Young, 1983) or a four-pool
170 J. France and J. Dijkstra
model (Wiltrout and Satter, 1972; Sharp et al., 1982) with cattle, but in all cases
some interconversions were omitted. Generally, a large amount of C exchange
between acetate and butyrate is reported. However, whilst several authors
observed very little exchange between propionate and butyrate (Bergman
et al., 1965; Annison et al., 1974; Sharp et al., 1982), Sutton et al. (2003)
reported 10–13% of propionate C to be derived from butyrate, whereas 2–4%
of butyrate C was derived from propionate. This argues against omitting the
propionate:butyrate C exchange from three-pool schemes.
The tracer methods described in this chapter employ radioactive isotopes
such as 1-
14
C acetate. Stable isotopes such as 1-
13
C acetate could be used
equally well, though they have to be administered in larger amounts in order to
bring ruminal enrichments up to detectable levels, and hence their use is more
costly. The models presented, together with the associated mathematical for-
mulae (Eqs (6.5)–(6.24)), remain the same for stable isotopes, though minor re-
definition of the entities used in the models is needed. These are presented in
Table 6.2.
Conclusions
The fermentation pattern and total supply of VFA are major determinants of
feed utilization by the ruminant. Many attempts have therefore been made to
estimate the rates of individual and total VFA production in and removal from
the rumen. Originally, non-tracer methods such as the zero-time in vitro and
the perturbation of steady-state methods were employed. These have now

i
(mmol) Quantity of labelled VFA i in rumen liquid
s
i
Enrichment of pool i (¼q
i
=Q
i
): mmol labelled VFA i /(mmol total VFA i )
Volatile Fatty Acid Production 171
frequently fed animals. The methods, however, can be adapted for non-steady-
state conditions and for single injection of label, and extended to any number of
pools.
References
Annison, E.F., Bickerstaffe, R. and Linzell, J.L. (1974) Glucose and fatty acid metabol-
ism in cows producing milk of low fat content. Journal of Agricultural Science,
Cambridge 82, 87–95.
Armentano, L.E. and Young, J.W. (1983) Production and metabolism of volatile fatty
acids, glucose and CO
2
in steers and the effects of monensin on volatile fatty acid
kinetics. Journal of Nutrition 113, 1265–1277.
Barcroft, J., McAnally, R.A. and Phillipson, A.T. (1944) Absorption of volatile acids
from the alimentary tract of sheep and other animals. Journal of Experimental
Biology 20, 120–129.
Bath, I.H., Balch, C.C. and Rook, A.J.F. (1962) A technique for the estimation of the
ruminal production of volatile fatty acids in the cow. Proceedings of the Nutrition
Society 21, ix–x.
Bergman, E.N. (1975) Production and utilisation of metabolites by the alimentary tract
as measured in portal and hepatic blood. In: McDonald, I.W. and Warner, A.C.I.

reference to VFA absorption from the rumen. Journal of Theoretical Biology
125, 193–211.
France, J., Siddons, R.C., Dhanoa, M.S. and Thornley, J.H.M. (1991a) A unifying
mathematical analysis of methods to estimate rumen volume using digesta markers
and intraruminal sampling. Journal of Theoretical Biology 150, 145–155.
France, J., Siddons, R.C. and Dhanoa, M.S. (1991b) Adaptation of compartmental
schemes for interpreting isotope dilution data on volatile fatty acid metabolism in
the rumen to the non-steady state and for single-dose injection. Journal of Theor-
etical Biology 153, 247–254.
France, J., Beever, D.E. and Siddons, R.C. (1993) Compartmental schemes for esti-
mating methanogenesis in ruminants from isotope dilution data. Journal of The-
oretical Biology 164, 207–218.
Gray, F.V., Weller, R.A., Pilgrim, A.F. and Jones, G.B. (1966) The rate of production
of volatile fatty acids in the rumen. III. Measurement of production in vivo by
two isotope dilution procedures. Australian Journal of Agricultural Research
17, 69–80.
Hungate, R.E. (1966) The Rumen and its Microbes. Academic Press, New York.
Hungate, R.E., Phillips, G.D., Hungate, D.P. and MacGregor, A. (1960) A comparison
of the rumen fermentation in European and Zebu cattle. Journal of Agricultural
Science, Cambridge 54, 196–201.
Johnson, K.A., Huyler, M., Westberg, H., Lamb, B. and Zimmerman, P. (1994)
Measurement of methane emissions from ruminant livestock using a SF
6
tracer
technique. Environmental Science and Technology 28, 359–362.
Kristensen, N.B., Ga
¨
bel, G., Pierzynowski, S.G. and Danfaer, A. (2000) Portal recovery
of short-chain fatty acids infused into the temporarily-isolated and washed reticulo-
rumen of sheep. British Journal of Nutrition 84, 477–482.

of sheep fed on two levels of intake. British Journal of Nutrition 86, 331–340.
McLean, J.A. and Tobin, G. (1987) Animal and Human Calorimetry. Cambridge
University Press, Cambridge.
Volatile Fatty Acid Production 173
Mills, J.A.N., Dijkstra, J., Bannink, A., Cammell, S.B., Kebreab, E. and France,
J. (2001) A mechanistic model of whole-tract digestion and methanogenesis in
the lactating dairy cow: model development, evaluation and application. Journal of
Animal Science 79, 1584–1597.
Morant, S.V., Ridley, J.L. and Sutton, J.D. (1978) A model for the estimation of volatile
fatty acid production in the rumen in non-steady-state conditions. British Journal
of Nutrition 39, 451–462.
Murray, R.M., Bryant, A.M. and Leng, R.A. (1976) Rates of production of methane in
the rumen and large intestine of sheep. British Journal of Nutrition 36, 1–14.
Murray, R.M., Bryant, A.M. and Leng, R.A. (1978) Methane production in the
rumen and lower gut of sheep given lucerne chaff. British Journal of Nutrition
39, 337–345.
Nagaraja, T.G., Newbold, C.J., Van Nevel, C.J. and Demeyer, D.I. (1997) Manipulation
of rumen fermentation. In: Hobson, P.N. and Stewart, C.S. (eds) The Rumen
Microbial Ecosystem, 2nd edn. Blackie Academic and Professional, London,
pp. 387–443.
Ørskov, E.R. (1975) Manipulation of rumen fermentation for maximum food utilisation.
World Review of Nutrition and Dietetics 22, 152–182.
Ørskov, E.R. (1981) Recent advances in the understanding of cereal processing for
ruminants. In: Haresign, W. and Cole, D.J.A. (eds) Recent Developments in
Ruminant Nutrition. Butterworths, London, pp. 258–267.
Re
´
mond, D., Ortigues, I. and Jouany, J.P. (1995) Energy substrates for the rumen
epithelium. Proceedings of the Nutrition Society 54, 95–105.
Rogers, J.A. and Davis, C.J. (1982a) Rumen volatile fatty acid production and nutrient

Thomas, P.C. and Clapperton, J.L. (1972) Significance to the host of changes in
fermentation activity. Proceedings of the Nutrition Society 31, 165–177.
Thomas, P.C. and Rook, A.J.F. (1981) Manipulation of rumen fermentation. In: Hare-
sign, W. and Cole, D.J.A. (eds) Recent Developments in Ruminant Nutrition.
Butterworths, London, pp.157–183.
Warner, A.C.I. (1964) Production of volatile fatty acids in the rumen: methods of
measurement. Nutrition Abstracts and Reviews 34, 339–352.
Weekes, T.E.C. and Webster, A.J.F. (1975) Metabolism of propionate in the tissues of
the sheep gut. British Journal of Nutrition 33, 425–438.
Weigland, E., Young, J.W. and McGilliard, A.D. (1972) Extent of propionate metabol-
ism during absorption from the bovine ruminoreticulum. Biochemical Journal
126, 201–209.
Weller, R.A., Gray, F.V., Pilgrim, A.F. and Jones, G.B. (1967) The rates of production
of volatile fatty acids in the rumen. IV. Individual and total volatile fatty acids.
Australian Journal of Agricultural Research 18, 107–118.
Weston, R.J. and Hogan, J.P. (1968) The digestion of pasture plants by sheep. I.
Ruminal production of volatile fatty acids by sheep offered diets of ryegrass and
forage oats. Australian Journal of Agricultural Research 19, 419–432.
Weston, R.J. and Hogan, J.P. (1971) The digestion of pasture plants by sheep. V.
Studies with subterranean and berseem clovers. Australian Journal of Agricul-
tural Research 22, 139–157.
Whitelaw, F.G., Hyldgaard-Jensen, J., Reid, R.S. and Kay, M.G. (1970) Volatile fatty
acid production in the rumen of cattle given an all-concentrate diet. British Journal
of Nutrition 24, 179–195.
Williams, A.G. and Coleman, G.S. (1997) The rumen protozoa. In: Hobson, P.N. and
Stewart, C.S. (eds) The Rumen Microbial Ecosystem, 2nd edn. Blackie Academic
and Professional, London, pp. 73–138.
Wiltrout, D.W. and Satter, L.D. (1972) Contribution of propionate to glucose synthesis
in the lactating and non-lactating cow. Journal of Dairy Science 55, 307–317.
Ziemer, C.J., Sharp, R., Stern, M.D., Cotta, M.A., Whitehead, T.R. and Stahl, D.A.