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Milchproduktion und Rindermast
Factors affecting microbial protein synthesis in the rumen
with emphasis on diets containing forages
J. VERBIC
Introduction ME, digestible carbohydrates (DCHO) amounts of concentrates. It has been
Microbial protein contributes about two- or fermentable organic matter (FOM) shown that in diets containing high le-
thirds of the amino acids absorbed by (INRA 1988, AFRC 1992, DACCORD vels of concentrates the efficiency of
ruminants. Although it is characterised 1994, TAMMINGA et al. 1994, MAD- microbial protein synthesis in the rumen
by a relatively high proportion of non- SEN et al. 1995, VERBIC and BABNIK is lower than in well-balanced forage-
protein nitrogen (25%, AFRC 1992) it 1997, GfE 2001). In all these systems a based diets (ARC 1984).
has an invaluable role in the nutrition of constant microbial yield per unit of ME, The preservation of forages as silages
ruminant animals. The amino acid com- NEL, fermentable ME, DCHO or FOM can induce a reduction of microbial pro-
position of microbial true protein is si- is proposed. Some of the above-mentio- tein synthesis in the rumen. A certain
milar to that of protein in the main ani- ned systems already take into account number of the fermentation end products
mal products, i.e. milk, lamb and beef the fact that not all the nutrients availa- to be found in silages do not contribute
(ØRSKOV 1992). Compared with oil ble for ruminant animals can be utilised any energy for microbial growth while
seed meals and legume grains (DLG by the rumen microbes. In other systems the utilization of others is limited. The
1976) microbial protein contains a hig- this fact is neglected. In experiments the theoretical assumption performed by
her proportion of methionine and lysine efficiency of microbial protein synthe- CHAMBERLAIN (1987) suggested that,
(STORM and ØRSKOV 1983). In fact, sis often deviates markedly from values in the case of the most favourable fer-
after the ban of feedstuffs of animal ori- proposed in protein systems. This con- mentation pathway in the silo, i.e. ho-
gin in ruminant diets, there are no protein firms the possibility that several further molactic fermentation, rumen microbes
sources which would meet animal requi- factors affect microbial growth in the can reach only one-quarter of the ener-
rements better than microbial protein. rumen, apart from those which are no- gy that would be available in the case of
Although extensive research work has wadays taken into account. We need to direct fermentation of hexose in the ru-
been carried out during the last few de- be aware that minor changes already men. The problem of low-energy supply
cades with the aim of improving the pre- made to the diet, feeding regime or en- to rumen microbes can be expected first
diction of microbial protein synthesis in vironment (ambient temperature) can of all in extensive fermented direct cut
the rumen, the level of reliability of cur- alter microbial flora and fauna. As a con- silages. In a direct comparison of sila-
rently used models is still low. The aim sequence the microbial protein yield in ges and hay from the same parental grass,
of this paper is to review the limited work the rumen can be changed as well. it was established that microbial protein
on this field in Slovenia and to discuss The maximum potential of rumen micro- yield in highly wilted silage and hay was
some factors which could affect micro- bes to produce microbial protein can be about 15% higher than in direct cut and
bial protein synthesis in diets containing explored only by the provision of high- moderately wilted silage (Table 1). It has
forages. quality forage. The importance of fora- been shown that, even in the case of ensi-
ge quality was clearly pointed out by an ling highly wilted Italian ryegrass, micro-
Factors affecting microbial experiment in which forage digestibility bial protein yield has been reduced by
protein synthesis in the and microbial protein synthesis was about 10% due to ensiling (Table 1).
rumen monitored during the ageing of grass As mentioned above, some protein sys-
clover sward. From the last week of April tems try to overcome discrepancies bet-
The supply of fermentable until the first week of June organic mat- ween energy utilisation in microbes and
energy ter digestibility decreased from 82% to animals by taking into account the fact
60%. During the same period the micro- that only fermentable organic matter can
Energy supply is usually the first limi- bial protein yield decreased from appro- be utilised by rumen microbes. The term
ting factor for microbial growth in the ximately 130 to 90 g per kg of dry mat- efficiency of microbial protein synthe-
rumen. To estimate the microbial prote- ter intake (Graph 1). It can be estimated sis is used when microbial protein syn-
in yield, modern European protein sy- roughly that, along with a decrease in the thesis is expressed per unit of fermenta-
stems use information which is directly NEL concentration in forage for 1 MJ, ble organic matter. Regarding the accep-
or indirectly used in estimating the en- the microbial protein yield decreased by ted concept, constant efficiency in micro-
ergy supply to the animal. The microbi- about 16 g. The problem of low micro- bial protein synthesis might have been
al protein yield can be estimated on the bial protein yield in diets containing low expected. However, the efficiency of
basis of metabolizable energy (ME), net quality forages can not simply be solved microbial protein synthesis varied widely
energy for lactation (NEL), fermentable by supplementing diets with high between forages (Table 1, Graph 1). In
Autor: Dr. Joze VERBIC, Agricultural Institute of Slovenia, SL-1001 LJUBLJANA; email: joze.verbic@kis-h2.si
29. Viehwirtschaftliche Fachtagung, 24. - 25. April 2002
Bundesanstalt für alpenländische Landwirtschaft Gumpenstein, 8952 Irdning 1
J. VERBIC
Table 1: Efficiency of microbial protein synthesis in various forages (VERBIC tein synthesis was measured, it has been
and BABNIK 1998, VERBIC et al. 1999a, VERBIC and BABNIK, unpublished shown that a decline in efficiency of
results) microbial protein synthesis during the
Dry matter (DM) Microbial protein yield Efficiency early maturity stages coincided with a
of microbial protein decline in the concentration of sugars
synthesis (Graph 1). An increase in microbial pro-
g/kg g MP/kg DM g MP/kg FOM tein synthesis in mature grass (Graph 1)
Forage from permanent grasslands: first cut still remains to be explained.
Direct cut silage 213 69,3 139 Considerable progress in the selection of
Formic acid treated silage 236 70,8 132 grass for high sugar concentration has
Wilted silage 432 68,1 115 recently been achieved. It has been
Highly wilted silage 521 78,9 133
Hay 893 79,8 126 shown that a variety of such grass can at
Italian ryegrass: third cut certain times of the year contain up to
Green forage (frozen) 218 82,8 162 350 g of water soluble sugars per kg of
Silage 477 73,6 158 dry matter. When this variety was fed to
Maize silage dairy cows the milk production and ef-
Flint type hybrid 369 113,8 217 ficiency in the use of grass protein
Dent type hybrid 374 99,4 165 increased (MILLER et al. 2001).
200 200 Rumen environment
s 175 An important factor which may alter the
r
160 a
n g )
i u er microbial protein yield in the rumen is
e s t150
t t
o 120 of a
pr n m pH value. Low pH value can be delete-
l o
a i y125
bi at
o r rious to rumen microbes, and especially
80 t g dr
cr k
en /100
Mi c g
n ( sensitive are protozoa. A low pH value
o
40 Yield (g/kg dry matter intake) C 75 is also expected to reduce the digestibi-
Efficiency (g/kg fermentable organic matter intake) Concentration of sugars in herbage lity of fibrous plant tissues. Due to low
0 50
15. Apr. 25. Apr. 5. May 15. May 25. May 4. Jun. 14. Jun. 15. Apr. 25. Apr. 5. May 15. May 25. May 4. Jun. 14. Jun. pH value, energy within the rumen is
Date Date diverted to non-growth functions, i.e.
7,0 n 60 maintaining neutral pH in bacterial cells
o
i
t (STROBEL and RUSSEL 1986). Al-
6,5 a 55
id b
u
u c though forage-based diets are generally
fl
in
n 6,0 h n 50
me 4 e not considered to promote rumen acidi-
2 m
ru u
f 5,5 ) 45 ty, the rumen pH value in diets contai-
o % he r
ue ( t
l y n ning solely immature grass may be well
va 5,0 iliti 40
H b
p a below the optimum. In a fresh forage ex-
d
4,5 a 35 Degradability of standard hay sample as affected by
r
pHof rumen fluid 4 h after the morning feeding g cellulolytic activity of rumen fluid during the ageing of periment (Graph 1) we can speculate that
De grass-clover mixture
4,0 30 the increased efficiency of microbial pro-
15. Apr. 25. Apr. 5. May 15. May 25. May 4. Jun. 14. Jun. 15. Apr. 25. Apr. 5. May 15. May 25. May 4. Jun. 14. Jun. tein synthesis in mature grass could be
Date Date due to the improved rumen environment.
Graph 1: Microbial protein synthesis, concentration of sugars, rumen pH value With advancing maturity, the pH value
and cellulolytic activity of the rumen fluid as expressed by dry matter degrada- of rumen fluid increased. An increase in
bility of a standard hay sample in the rumen during the ageing of grass-clover the rumen pH value was accompanied
herbage (VERBIC et al. 1999b, VERBIC et al. 2002a). by a pronounced rise in the cellulolytic
grass silages it varied from 115 to 158, role of specific compounds which may activity of the rumen fluid (Graph 1).
in hay it was 126, in maize silages it va- stimulate microbial growth. CHAM- The degradability of a standard hay sam-
ried from 165 to 217 and in green forage BERLAIN et al. (1993) reported higher ple, which was incubated in the rumen
from 145 to 199 g of microbial protein microbial protein yield when grass sila- every six days of experiment (24 h incu-
per kg of fermentable organic matter. It ge based diets were supplemented by bation), was relatively constant (45%)
is evident that microbial protein synthe- sucrose, fructose, lactose or xylose in until the middle of May and gradually
sis in the rumen also depends on other comparison to the addition of starch. If increased to 52% thereafter. Another
factors which can not simply be explai- the assumption of beneficial effect of example in which rumen environment
ned by the concentration of fermentable sugars is true, it can at least partially played an important role in the determi-
organic matter. explain the variability in efficiency of nation of microbial protein yield can be
One of the possible explanations for the microbial protein synthesis between found in maize silage diets. Silage made
wide variability in the efficiency of forages. In experiments in which the ef- from the flint type hybrid supported a
microbial protein synthesis in forage- fect of the ageing of a grass-clover mix- higher microbial protein production than
based diets can be associated with the ture on the efficiency of microbial pro- that from the dent type hybrid (Table 1).
29. Viehwirtschaftliche Fachtagung, BAL Gumpenstein 2002
2
Factors affecting microbial protein synthesis in the rumen with emphasis on diets containing forages
This occurred despite similar concentra- 77
100
tions of rumen degradable starch and ) Fresh forage ) Silage
despite the fact that the degradability of % 73 % 90
( (
y y
non-grain fraction per se was higher in ilit ilit
b 69 b
a a 80
d d
dent type hybrid than in flint. Again, it a a
r r
g g
was clearly shown that a higher efficien- e 65 e 70
d d
n n
i i
e e
cy of microbial protein synthesis in sila- t t
o o
r 2 r
ge made from flint type hybrid was rela- P 61 y = -0.002x + 0.61x + 25.97 P 60 y = -0.031x + 86.26
2
R = 0.987 2
R = 0.540
ted to a higher rumen pH value (6.33 vs. 57 50
6.21) and better conditions for cellulo- 70 90 110 130 150 170 190 210 90 290 490 690 890
lysis which were expressed through the Crude protein concentration (g/kg DM) Dry matter concentration (g/kg)
higher effective degradability of insolu- 90
ble non-starch carbohydrate fraction of ) Hay
%
maize silage (35.5 vs. 31.7 %, VERBIC ( 80
y
ilit
and BABNIK, unpublished results). b
a
d
a
r 70
g
In conclusion, it should be pointed out e
d
n
i
that a low rumen pH value may inhibit e
t
o 60
r
microbial protein yield by inhibiting the P y = 0.170x + 50.78
2
degradation of fibrous material as well R = 0.827
as by diverting the available energy to 50
50 90 130 170 210
non-growth functions. Crude protein concentration (g/kg DM)
The supply of nitrogen Graph 2: Most pronounced relationship between forage composition and pro-
compounds tein degradability in green forage, silage and hay (BABNIK and VERBIC 1996,
Protein degradation in the rumen is one VERBIC et al. 1999a, VERBIC et al. 2002a, VERBIC et al. 2002b)
of the main reasons for the inefficient
utilisation of protein in ruminants. On 180
the other hand, nitrogen compounds y = 0,7692x - 5,578
which are released during the protein ) R2 = 0,900
degradation are crucial for microbial M 140
growth in the rumen. In modern protein g D
(g/k
systems it is required that the needs of
n
i
e
rumen microbes for nitrogen compounds t
are fully covered either by degradable pro100
e
dietary protein or by metabolic nitrogen, l
which arise from the oxidation of amino egradab
acids in animal tissues and which can be d 60
n
e Silage
recycled into the rumen. In some systems m
u Fresh forage
it is proposed that the capture of rumen R
degradable protein is not complete Hay
20
(INRA 1988, AFRC 1992) and therefo- 50 100 150 200 250
re a surplus of rumen degradable prote- Crude protein concentration (g/kg DM)
in is required.
As already discussed, the efficiency of Graph 3: Relationship between crude protein concentration and the supply of
microbial protein synthesis is highly va- rumen microbes by rumen degradable protein in green forage, grass silage
riable. Therefore, in practice, the mini- and hay (BABNIK and VERBIC 1996, VERBIC et al. 2002a, VERBIC et al. 2002b)
mal requirements for rumen degradable
protein can be under- or overestimated. ble nitrogen increased to a greater ex- be claimed that forages of similar prote-
On the other hand, a wide variation was tent than can be expected from protein in concentration provide similar amounts
also observed in the protein degradabi- concentration per se. It was established of rumen degradable protein (Graph 3).
lity of forages. It was found that in fresh that in grass silages protein degradabili- There are a number of reports of
forage and hay, protein degradability was ty mainly depends on dry matter concen- increased microbial protein yields in re-
closely related to protein concentration. tration. For each g of increased DM con- sponse to the addition of N in the form
Protein degradability with increasing centration protein degradability de- of amino acids or peptides. Increased
protein concentration in forage increased creased by about 0.031 %. Despite the microbial growth can be expected first
(Graph 2). This means that with increa- pronounced effect of forage preservati- of all in diets containing a high level of
sing protein concentration in fresh fora- on methods on protein degradability starch (RUSSEL et al. 1992). However,
ges or hay the supply of rumen degrada- (VERBIC et al. 1999a) it can generally recently OH et al. (1999) reported that
29. Viehwirtschaftliche Fachtagung, BAL Gumpenstein 2002
3
J. VERBIC
amino acids and peptides are quickly
40 degraded in the rumen and it is questio-
) DEFAUNATED SHEEP REFAUNATED SHEEP nable whether they can really support
n
e microbial growth. A considerably better
rum source of free amino acids and peptides
s e 30
i h
s t may be found in slowly degradable pro-
e
h in
t tein fraction which comprise in green
n ed
y t
s s
forage 74% (5284%), in hay 46% (27
n ge
eii 20 55%) and in silage 35% (1656%) of
ot d
r r
e
pt
t total protein (VERBIC and BABNIK,
ala
obi m unpublished results). Due to the speci-
r
c
i nic fic needs of rumen microbes for amino
M ga 10
r
o
acids and peptides, it can be expected
g
k
/ that by synchronising the availability of
(g fermentable energy and degradable pro-
0 tein in the rumen, the efficiency of micro-
WATER UREA CASEIN bial protein production can be increased.
Typical synchrony indexes, which des-
Graph 4: Effect of intraruminal infusion of ammonia or casein on the efficiency cribed synchrony of crude protein and
of microbial synthesis in the rumen of defaunated sheep which were given dry matter degradation in the rumen, are
ammonial treated straw (VERBIC and CHEN 1989) presented in Graph 5. It is evident that
the response of microbial protein syn- available peptides and free amino acids with advancing maturity the synchroni-
thesis to the inclusion of casein as oppo- can be expected in forages. Plant prote- city of forages decreased. Relative to
sed to urea was even greater at low level in is broken down into peptides and free silages (mean IS=0.69), synchrony inde-
of starch. VERBIC and CHEN (1990) amino acids by the action of plant pro- xes (IS) were generally higher in fresh
for instance infused urea or casein into teases (KEMBLE 1956) while the bre- forages (mean IS=0.84) and hay (mean
the rumen of sheep which were given akdown of amino acids to ammonia and IS=0.80).
ammonia-treated straw (Graph 4). In other forms of non-protein compounds Rumen outflow rate
comparison with urea, casein improved is mainly caused by the action of clostri- One of the factors which affect the ef-
the efficiency of microbial protein syn- dia in the silo (OHSHIMA and McDO- ficiency of microbial protein synthesis
thesis in the rumen. That happened in NALD 1978). Therefore, concentrations is rumen outflow rate. Faster outflow
sheep with a normal rumen population of peptides and amino acids are expec- rate is expected to reduce the mainte-
as well as in defaunated sheep. Consi- ted to be higher in fresh forage and hay nance costs of microbes because they
derable variation in the concentration of than in silage. It is considered that free spend less time within the rumen. From
a theoretical point of view, it would be
1,0 expected that the maximum microbial
yield would occur when the dilution rate
was equal to the multiplication rate of
0,8 bacteria (ØRSKOV 1992). The theory
x has been confirmed experimentally
e (HARRISON et al. 1975, KENNEDY
d 0,6 and MILLIGAN 1978, DEWHURST
in
y
n and WEBSTER 1992, MURPHY et al.
o
r
h 1994) and there are some protein systems
c 0,4
n which already takes it into account. In
Sy AFRC (1992) for instance, it is suppo-
0,2 sed that the efficiency of microbial pro-
Silage tein synthesis can be increased by about
Fresh forage 20% if rumen outflow rate is increased
Hay -1
0,0 from 0.02 to 0.08 h . Rumen outflow rate
150 200 250 300 350 400 450 is a function of dry matter intake and
Crude fibre concentration (g/kg DM) therefore it can be assumed that the ef-
ficiency of microbial protein synthesis
Graph 5: Relationship between crude fibre concentration and synchrony index in the rumen can be increased by an
which describes the synchrony of crude protein and dry matter degradation in increase in dry matter intake. One of the
the rumen for green forage, grass silage and hay (VERBIC and BABNIK, unpu- most important factors which limits in-
blished results). take of low quality roughages is their
29. Viehwirtschaftliche Fachtagung, BAL Gumpenstein 2002
4
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