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Interciencia
ISSN: 0378-1844
interciencia@ivic.ve
Asociación Interciencia
Venezuela
Ortega Cerrilla, M. Esther; Mendoza Martínez, Germán
Starch digestion and glucosemetabolism in the ruminant: a review
Interciencia, vol. 28, núm. 7, julio, 2003, pp. 380-386
Asociación Interciencia
Caracas, Venezuela
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STARCH DIGESTION AND GLUCOSE
METABOLISM IN THE RUMINANT: A REVIEW
M. ESTHER ORTEGA CERRILLA
and GERMÁN MENDOZA MARTÍNEZ
arbohydrates are as im- Starch Digestion in the Ruminant Ruminococcus bromii, Succinimonas amy-
portant to the ruminant lolytica and Lactobacillus sp. (Clarke and
animal as they are to In the non-ruminant, Bauchop, 1977; Church, 1979; Kotarski
non-ruminants, since they provide the starch digestion occurs mainly in the et al., 1992).
glucose necessary for the adequate func- small intestine. The situation in the ru- In studies in which ru-
tion of cells. However, in the ruminant, minant differs due to the action of mi- minants are switched abruptly from for-
ruminal fermentation transforms most of croorganisms in the rumen. Digestion of age-based diets to grain based rations an
the cell wall polysaccharides and all of starch to glucose requires the action of acute ruminal lactic acidosis occurs, the
the intracellular carbohydrates present in several enzymes produced by the sali- numbers of Streptococcus sp. increase by
the forage into short-chain volatile fatty vary glands, the rumen microorganisms 2-3 orders of magnitude within hours af-
acids, which are then absorbed by the ru- or the pancreas and small intestine. ter feeding, protozoa populations are
men epithelium (Bird et al., 1996). Amylase secreted by the nasolabial eliminated and lactobacilli become domi-
Plant tissues contain glands is found at relatively high levels nant within 24h (Krogh, 1963; Mann,
about 75% carbohydrates, providing the in the saliva of some ruminants, such as 1970). Ciliated protozoa are found in
primary sources of energy for both the the buffalo (Church, 1979). Alpha-amy- large quantities in grain-fed ruminants.
rumen organisms and the host animal lase is secreted by the pancreas, while Low ruminal pH occurring during all or
(Morrison, 1959; Church, 1979). The car- isomaltase, maltase-glucoamylase, tre- part of the daily feeding cycle is thought
bohydrates found in plant tissues are pri- halase and lactase are secreted by the to limit protozoa populations (Eadie and
marily polysaccharides, cellulose, hemi- intestinal mucosa (Harmon, 1993). Al- Hobson, 1962; Eadie et al., 1967), be-
cellulose, pectins, fructans and starches, pha-amylase, beta-amylase, R-enzyme, cause many are unable to survive below
with minor amounts of other compounds pullulanase, iso-amylase or alpha-limit pH 6.0 (Hino et al., 1973).
(Theander, 1981). Cellulose is the most dextrinase are produced by the rumen In grain-fed animals,
abundant. However, grains are widely microorganisms. protozoa can exert an influence on rumi-
used in diets used in intensive production Several species of rumi- nal starch hydrolysis rates in at least two
systems with highly productive animals nal bacteria are able to digest starch. respects: 1) by ingesting bacteria in num-
(Waldo, 1973; Theurer, 1986), providing Amylolitic organisms are found in larger bers sufficient to decrease ruminal fer-
an appreciable amount of starch for rumi- percentages of the total microbial popula- mentation rates (Eadie and Hobson, 1962;
nal and intestinal digestion (Armstrong tion when rations high in starch are fed. Clark and Bauchop, 1977; Kurihara et
and Smithard, 1979; Sutton, 1979). The Important species that have been enumer- al., 1978), and 2) by ingesting starch
purpose of this review is to summarize ated in cattle fed high grain diets are granules and soluble sugars, thus decreas-
the present knowledge on starch digestion Bacteroides amylophilus, Butyrivibrio fi- ing the accessibility of these substrates to
in the ruminant, as well as glucose me- brisolvens, Bacteroides ruminocola, Sele- fermentation by the faster growing bacte-
tabolism in the rumen, post-ruminal ab- nomona lactylitica, Streptococcus bovis, ria (Coleman, 1986; Coleman, 1992).
sorption of starch and glucose require- Prevotella ruminocola, Eubacterium ru- The presence of ciliates
ments of the ruminant. minantium, Ruminobacter amylophilus, influences the site of starch digestion. It
KEYWORDS / Glucose Metabolism / Starch Digestion / Ruminants /
Received: 10/24/2002. Modified: 06/16/2003. Accepted: 06/18/2003
M. Esther Ortega Cerrilla. Veterinary Zootechnician. Master and Doctor in Ruminant Nu-
trition. Professor, Colegio de Postgraduados, Mexico. Address: Colegio de Postgraduados. Carretera México-Texcoco km
36.5, 56230 Montecillo, Estado de México, México. e-mail: meoc@colpos.colpos.mx
Germán Mendoza Martínez. Veterinary Zootechnician. Master and Doctor in Ruminant nu-
trition. Professor, Colegio de Postgraduados, Montecillo, Mexico.
380 0378-1844/03/07/380-07 $ 3. 00/0 JUL 2003, VOL. 28 Nº 7
has been reported that protozoa reduce in the rumen and enter the small intestine of starch or starchy feedstuffs, mainly
the rate of starch digestion and ruminal (Weeks, 1979). due to competition for essential nutrients
starch digestibility, shifting the site of It has been observed by amylolytic and cellulolytic microor-
starch digestion to the small intestine that the degree of processing is an impor- ganisms within the rumen, resulting in
(Mendoza et al., 1993). tant factor which influences the degree of proliferation of starch digesting bacteria.
Most amilolytic microor- fermentation of grains in the rumen and Rapid fermentation of starch leads to a
ganisms possess extracellular amylases, their post-ruminal digestibility. Xiong et decrease in rumen pH which encourages
usually of the alpha-type, which is an en- al. (1991) observed that processing of amylolytic microorganisms to proliferate
doenzyme acting randomly in the interior sorghum by steam-flaking increased and compete successfully against cellu-
parts of the starch chain. The fragmenta- starch digestion in the rumen, there being lolytic bacteria, which grow at a higher
tion by alpha-amylase initially leads to a less starch available for fermentation in pH (Mould and Orskov, 1983; Mould et
rapid reduction in the molecular size of the lower gastrointestinal tract. Ruminal al., 1983).
the starch with formation of water retention time is another important factor Both cattle and sheep
soluble dextrins and oligosaccharides. which determines the degree of ruminal are able to digest completely the starch
The final products from amylose are mal- starch fermentation. It has been demon- contained in certain cereal based diets
tose, maltotriose and sometimes small strated that steam-rolling causes a greater (Armstrong and Beever, 1969). The re-
amounts of free glucose. Maltotriose is ruminal retention time than dry-rolling sults obtained by MacRae and Armstrong
generally stable to the action of both al- (Zinn, 1993, 1994). (1969) for sheep fed whole or rolled bar-
pha and beta-amylases, unless massive The inclusion of iono- ley suggest that its digestive tract can
quantities of enzyme are added. The final phores or Na bicarbonate cause minor handle with equal effectiveness either
products from amylopectin are maltose, changes in the site and extent of starch form of grain. However, the same might
maltotriose, a little glucose and a mixture digestion, as observed by Zinn and not be true for cattle fed whole or rolled
of alpha-limit dextrins. These latter oli- Borques (1993) and Zinn (1987). Iono- barley, and there is evidence that in dairy
gosaccharides consist of 4-8 glucose moi- phores usually reduce intake, which re- cows fed whole shelled maize, 18-35% of
eties and still contain the alpha-(1-6) sults in less starch being fermented in the the grain passes undigested through the
linkage(s) which cannot be hydrolyzed by rumen, reducing incidence of acidosis in entire digestive tract (Morrison, 1959).
amylases. Debranching enzymes (R-en- feedlot diets. Combination of slow (25- The method of grain
zyme, pullulanase. iso-amylase, or alpha- 33%) and fast (75-66%) digesting grains processing affects the site of digestion of
limit dextrinase) are necessary to break improve gain and feed efficiency (Stock starch in ruminants. Wu et al. (1994),
these bonds (Clark and Bauchop, 1977). et al., 1987), presumably because those found in cows fed steam flaked sorghum
Starch digestion in the combinations stimulate protozoal numbers that the main site of starch digestion was
total digestive tract of ruminants exceeds (Mendoza et al., 1998; Mendoza et al. the rumen, while in cows fed dry rolled
95% (Tucker et al., 1968). With roughage 1999) reducing ruminal starch digestion sorghum it was the intestine. Type and
diets only small quantities of alpha-linked and acidosis. variety of grain also affect the site of
glucose polymers pass to the abomasums Manipulation of starch starch digestion. Hatfield et al. (1993)
(Heald, 1951) and it is very likely that fermentation in the rumen is important found differences in ruminal starch diges-
such material, which does leave the ru- when slow digested grains such as sor- tion in wethers fed different barley variet-
men, is mostly of microbial origin. Both ghum are fed. The use of exogenous ies. Herrera-Saldaña et al. (1990a) found
rumen protozoa and bacteria store alpha- amyolytic enzymes from Bacillus licheni- that starch degradability in vitro was the
linked glucose polymers when available formis increased ruminal starch digestion highest for oats (98%), followed by
energy is in excess of growth require- and feed efficiency in sorghum based di- wheat, barley, maize and sorghum (95,
ments (Hungate, 1966; Walker and Nader, ets (Rojo et al., 2001). Therefore, exog- 90, 62 and 49%, respectively). Wilcox et
1970; Cheng et al., 1973; McAllan and enous enzymes could be considered as an al. (1994) compared two varieties of
Smith, 1974). alternative treatment to improve ruminal maize, Sugary-Brawn2 and dent maize,
With roughage diets this starch digestion when diets with a high being the highest total starch digestion
would occur shortly after feeding, due to grain content are fed to ruminants. that of Sugary Brawn2. McCarthy et al.
the rapid fermentation of the soluble sug- It has been calculated (1989) observed in lactating cows that
ars present in the higher quality rough- that when rolled barley or ground maize passage of starch to the duodenum was
ages. McAllan and Smith (1974) reported is fed to sheep the total starch digestibil- greater for corn based diets than barley
values of 17-30g alpha-dextran per kg ity was 99.9% and the proportion of based diets. Streeter et al. (1990) fed
bacterial DM passing to the duodenum of starch disappearance before the small in- steers with four sorghum grain hybrids
hay-fed sheep and cows. Calculations testine was 91.8%, whilst in cattle fed and maize; ruminal starch digestibility,
based on these estimates yield a value of ground corn the total starch digestibility pre-cecal starch digestibility and total
3-6g alpha-dextran per day and per kg was 98.5% with 68.0% of the starch dis- starch digestibility were higher for maize
hay consumed, which is close to reported appearing before the small intestine than for sorghum grains.
-1 (Armstrong and Beever, 1969). Other au- It has also been ob-
values of 5g·day with sheep (Armstrong
and Beever, 1969). Thus, on hay diets the thors (Nocek and Tamminga, 1991) indi- served that when different grain process-
quantity of glucose available for absorp- cate that rumen degradable starch as per- ing methods are compared, some of them
tion in the small intestine would be of centage of total starch varies from 73.2 have a greater effect than others in im-
minimal importance. to 90.3 for rolled barley, whilst whole proving starch digestibility in the rumen
When diets containing and ground maize range from 58.9 to of cattle. Cheng et al. (1994) studied the
grains are fed, depending on the type of 75.0 and 51.4 to 93.0, when fed to sheep effect of steam flaking of corn and sor-
the grain, the extent of processing prior and cattle, respectively. ghum grains on performance of lactating
feeding, and the species of animal fed, an El-Shazly et al. (1961) cows. They found that compared to roll-
appreciable amount of starch and proto- noticed that dry matter (DM) digestibility ing, flaking of both grains increased
zoal glycogen may escape fermentation of roughages is lowered in the presence yields of milk, milk protein and fat, due
JUL 2003, VOL. 28 Nº 7 381
to a higher rumen digestibility of starch microorganism which carried out the ini- Post-ruminal Digestion and Starch
which increased ruminal volatile fatty ac- tial fermentation, followed by the con- Absorption
ids (VFA) concentration, with more VFA version of the free lactate to the VFA
absorption from the rumen and greater and other products by a second microor- The residence time of a
flow of bacterial protein to the duode- ganism. The other process simply in- feed particle in the rumen is a major de-
num. Oliveira et al. (1992) and Poore et volves a direct conversion of a 3-carbon terminant of the extent it will be fer-
al. (1993) also observed that steam flaked intermediate to acetate and formate or mented. However, with regard to starch
sorghum grain had a higher ruminal and acetate, CO , and a reduced product digestion, the rate of starch digestion in
2 the rumen is an important factor because
total tract starch digestibility than dry such as H , succinate, propionate, or bu-
2 of competition of passage and digestion
rolled sorghum grain when fed to dairy tyrate, by the same microorganism that
cows. carried out the initial degradation (Allen and Mertens, 1988). It is evident
The differences in ap- (Baldwin, 1965). that a decrease in starch digestion in the
parent digestibility of grains between Phosphoroclastic reac- rumen can be accomplished by increas-
species may be related to the physical tions are the major reactions involved in ing the fluid dilution rate. The dilution
size of the reticulo-omasal orifice, which acetate synthesis. Two of them appear to rate of rumen fluid is higher with long
is considerably greater in cattle than in be prominent in the rumen, the clostridial than ground roughage (Hodgeson and
sheep or goats. In cattle unmasticated phosphoroclastic type and the coliaero- Thomas, 1975) and is probably related
whole grains which are those still resis- genes phosphoroclastic (formate phospho- to the greater amount of time spent ru-
tant to enzyme attack are able to pass roclastic) type. The requirements for both minating. This could explain the twofold
from the reticulo-rumen into the aboma- systems include thiamine pyrophosphate, increase in ground maize starch passing
sum, whereas in sheep or goats similar coenzyme A and phosphate. The clostri- to the doudenum of sheep when ground
grains are retained within the reticulo- dia system also requires ferredoxin (Bald- straw was replaced with long straw
rumen and subjected to further mastica- win, 1965). (Thompson and Lamming, 1972; Thomp-
tion during rumination, which results in Groups of organisms son, 1973). Orskov et al. (1969) ob-
rupture of the seed coat of most of the differ in the fate of the pairs of electrons served a larger proportion of dietary
cereal grains, so enzyme degradation can removed in this reaction. Clostridia trans- starch passing to the duodenum when
occur (Nordin and Campling, 1976). fer them to protons which are then liber- hay and ground barley was fed to lambs
However, excessive ated as molecular H2. Other bacteria in comparison to an all ground barley
amounts of readily fermented carbohy- transfer them to CO2 and produce for- diet. A higher amount of dietary starch
drates might occur when diets rich in mate. Formate is oxidized rapidly in the may escape ruminal fermentation and
concentrate are fed to ruminants, causing rumen, probably by the ferredoxin-depen- thus become available post-ruminally for
marked acidosis as acids and glucose ac- dent formic dehydrogenase, with forma- possible absorption as glucose. However,
cumulate. These compounds damage the tion of H and CO (Baldwin, 1965; other researchers (Topps et al., 1968a, b;
ruminal and intestinal walls, decrease 2 2 Nicholson and Sutton, 1969) did not find
Leng, 1970a).
blood pH, and cause dehydration that There are two mecha- any significant increase in the concentra-
proves fatal. Feeding higher amounts of nisms known for the conversion of lactate tion of starch escaping rumen fermenta-
dietary roughage, processing grains less or pyruvate to propionate. The first path- tion with animals on high-concentrate
thoroughly and limiting the quantity of way involves the formation of oxaloac- diets.
feed should reduce the incidence of acido- etate and succinate, and the second in- Starch and N2 metabo-
sis, although these practices may depress volves the formation of acrylate. Defi- lism are closely linked in the rumen since
performance and economic efficiency. ciencies of sulphur may change the energy released from starch degradation
Therefore, it is necessary to continue re- routes of propionate production which is is required for the incorporation of N2
search on grain processing, dietary cation- probably due to a change in microbial into microbial cells but, conversely, insuf-
anion balance, narrow spectrum antibiot- population where the acrylate pathway ficient N may limit microbial growth and
ics, glucose or lactate utilizing microbes assumes a more important role. The acry- 2
enzyme production (Herrera-Saldaña et
and feeding management in order to re- late pathway may be more important in al., 1990b). In support of this latter state-
duce the incidence of acute and chronic the rumen of animals given grain rations ment Orskov et al. (1972) found the
acidosis (Owens et al., 1998). (Baldwin et al., 1963; Leng, 1970a). starch flow at the duodenum of growing
Butyrate synthesis may lambs to be 14.2% of intake with a 10%
Glucose Metabolism in the Rumen occur in the rumen from acetate, or from CP rolled barley diet; this decreased to
compounds giving rise to acetyl-CoA, 6.8, 3.4 and 3.4% when urea was added
The carbohydrate com- such as pyruvate or glutamate. Two path- to bring the diets to 12.4, 16.6 and
ponents in ruminant diets are mainly ways may be available for butyrate synthe- 16.45% CP respectively, suggesting that
hexose polymers like cellulose, starch, sis from acetate in anaerobic ruminal or- N2 was limiting microbial growth when
fructans, and pentose polymers, mostly ganisms. The most likely pathway is the the non-supplemented diet was fed.
xylan; (Walker, 1965; Martin, 1994). reversal of beta-oxidation. The synthesis of The capacity of the ru-
The most important pathway of hexose long chain and branched-chain fatty acids minant small intestine to digest large
fermentation in the rumen is the Embden of the bacterial cells of rumen organisms amounts of starch has been questioned
Meyerhof pathway, resulting in the deg- suggests the possibility of butyrate synthe- (Croome et al., 1992; Waldo, 1973), as a
radation of glucose to pyruvate (Bald- sis via a second pathway involving malo- consequence of the low levels of pancre-
win, 1965). The 3-carbon intermediates nyl-CoA. In the malonyl pathway 2 moles atic amylase, intestinal maltase and
arising as a result of hexose and pentose of ATP are required for the formation of 1 isomaltase (Keller et al., 1958; Siddons,
degradation may be utilized via, at least, mole of butyrate from 2 moles of acetate, 1968; Coombe and Siddons, 1973;
two alternative pathways. One involves as compared with 1 mole of ATP for the Coombe and Smith, 1974) and also be-
the release of lactate or another 3-car- synthesis of butyrate by the reversal of cause low glucose absorption (Orskov,
bon intermediate into the medium by the beta-oxidation (Leng, 1970a). 1986; Kreikemeier et al., 1991; Tanigushi
382 JUL 2003, VOL. 28 Nº 7
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