The effect of dent versus flint maize genotype on site and the extent of starch and protein digestion, ruminal fermentation and microbial protein synthesis in the rumen of sheep
The effect of dent versus flint maize genotype on site and the extent of starch and protein digestion, ruminal fermentation and microbial protein synthesis in the rumen of sheep
go BABNIK 0
ŽNIDARŠI CŠ-PONGRAC 0
0 Agricultural Institute of Slovenia , Hacquetova 17, 1000 Ljubljana , Slovenia
- Ruminal and whole tract digestibility of protein and starch, microbial protein yield in the rumen and molar proportions of volatile fatty acids (VFA) in the rumen fluid and faeces were determined in sheep that were given either maize silage derived from the dent (DTS) or flint type hybrid (FTS). Degradabilities in the rumen were determined by means of the in sacco method using three sheep and microbial protein yield in the rumen by means of urinary purine derivative excretion using four sheep. The extent of starch and protein degradation was significantly (P < 0.001) lower in FTS than in DTS (718 vs. 913 and 704 vs. 767 g·kg-1 respectively). Differences in whole tract starch and protein digestibility were small (986 vs. 997, P < 0.01 and 939 vs. 931 g·kg-1, P < 0.05 in FTS and DTS respectively). The concentration of VFA in the rumen fluid was not affected by the type of silage (94.5 and 94.7 mmol·L-1); however, FTS induced a higher molar proportion of propionate (22.6% vs. 20.7%, P < 0.05) than DTS. The proportion of butyrate was higher in DTS (13.5% vs. 10.8%, P < 0.05). Microbial protein supply in FTS was significantly higher than in DTS (109.7 vs. 96.0 g·kg-1 DM intake, P < 0.05). When expressed in relation to fermentable organic matter (FOM) intake, the differences between hybrids were greater (215.6 vs. 158.6 g of microbial protein per kg FOM, P < 0.01). Lower efficiency of microbial protein synthesis in DTS was probably due to a lower pH value of rumen liquor (6.21 vs. 6.33, P < 0.05). FTS offered better conditions for the degradation of the fibre fraction in the rumen than DTS. The faeces of sheep that were given FTS contained less DM (304 vs. 371 g·kg-1, P < 0.05) and tended to have a higher concentration of VFA (383 and 235 mmol·kg-1 DM, P < 0.1) indicating that in FTS at least one part of the starch which escaped digestion in the rumen passed through the small intestine undigested and was fermented in the large intestine. It was concluded that FTS could provide about three times more postruminally digested starch and about 20% more metabolisable protein than DTS.
Résumé – Effet du génotype de maïs (denté vs. corné) sur le site et le volume de la digestion de
l’amidon et des protéines, sur la fermentation ruminale, et sur la synthèse des protéines
microbiennes dans le rumen chez les ovins. La digestibilité des protéines, de l’amidon et des
constituants pariétaux dans l’ensemble du tractus digestif, le rendement de synthèse des protéines
microbiennes dans le rumen et les proportions molaires des acides gras volatils (AGV) dans le jus
de rumen et dans les fèces ont été déterminées sur des ovins qui ont reçu de l’ensilage de maïs de
type denté (DTS) ou corné (FTS). Les dégradabilités dans le rumen ont été mesurées par la méthode
in sacco sur trois ovins alors que le rendement de synthèse des protéines microbiennes a été déterminé
à partir de l’excrétion urinaire des dérivés de purines en utilisant quatre ovins. L’importance de la
dégradation de l’amidon et des protéines a été significativement (P < 0,001) plus faible pour FTS que
pour DTS (respectivement, 718 vs. 913 et 704 vs. 767 g·kg–1). Les différences entre les digestibilités
de l’amidon et des protéines dans l’ensemble du tractus digestif ont été faibles (respectivement,
986 vs. 997 g·kg–1, P < 0,01 et 939 vs. 931 g·kg–1, P < 0,05 pour FTS et DTS). La concentration en
AGV totaux dans le jus de rumen n’a pas été affectée par le type d’ensilage (94,5 et 94,7 mmol·L–1),
pourtant, FTS a induit des proportions molaires plus élevées de propionate (22,6 vs. 20,7 %, P < 0,05)
et DTS des proportions de butyrate supérieures (13,5 vs. 10,8 %, P < 0,05). La synthèse des protéines
microbiennes pour FTS a été significativement plus élevée que pour DTS (109,7 vs. 96,0 g·kg–1 de
MS ingérée, P < 0,05). Quand elles sont exprimées par rapport à l’ingestion de la matière organique
fermentescible (MOF), les différences entre les hybrides sont plus importantes (215,6 vs. 158,6 g
de protéines microbiennes par kg MOF, P < 0,01). L’efficacité plus faible de la synthèse de protéines
microbiennes pour DTS est probablement due à la valeur plus faible du pH ruminal (6,21 vs. 6,33,
P < 0,05). Les FTS ont présenté les conditions les plus favorables pour la dégradation de la fraction
fibreuse dans le rumen. Les fèces des ovins qui ont reçu FTS étaient moins riches en MS (304 vs.
371 g·kg–1, P < 0,05) et tendaient à être plus riche en AGV (383 et 235 mmol·kg–1 DM, P < 0,1) ;
ce qui laisse supposer que, pour FTS, une partie de l’amidon, non dégradé dans le rumen, n’a pas
été digéré dans l’intestin grêle et a été fermenté dans le gros intestin. En conclusion, FTS semble
fournir approximativement trois fois plus d’amidon digéré dans l’intestin et 20 % de protéines
métabolisables en plus que DTS.
ensilage de maïs / digestion de l’amidon / digestion de la protéine / protéine microbienne /
Maize silage is an important component
of winter diets for cattle. In many countries
the estimated production of maize silage
exceeds that of grass silage [
]. In several
European countries, special approval
schemes for hybrids suitable for silage
production have been developed [
dry matter yields, the quality criteria has
become an important factor which is taken
into account in such approval schemes.
Quality is usually assessed on the basis of
in vitro organic matter digestibility. In
comparison to stover, organic matter digestibility
of ears is relatively high but considerably
less variable [
]. Therefore, it is not
surprising that until now relatively little attention
was paid to the quality of the grain.
However, there are some data indicating that,
despite relatively constant whole tract
digestibility, maize hybrids can vary widely
as to the site of grain digestion. Flachowsky
et al. [
] reported that dry matter
degradability of the maize grain after six-hour
incubation in the rumen varied between
35.2% and 56.9%. A few years later,
differences between hybrids were attributed to
the type of grain, with the dent grain being
degraded in the rumen more extensively
than the flint type [
]. Shifting starch
digestion from the rumen to the small
intestine may have potential benefits, although
the available information is not consistent.
Based on a review of experiments with
growing cattle, Owens et al.  reported
that starch digested in the small intestine
provides about 40% more energy to the
animal than starch digested in the rumen. On
the contrary, in experiments with dairy
cows there was no evidence to show a
benefit of shifting starch digestion from the
rumen to the intestine [
]. In the case
of manipulating the ruminal starch
digestion of dried grains by the means of less
intensive processing, milk production can
even be decreased . Knowlton et al. [
reported that starch infused to the
abomasum of early lactating cows resulted in a
better glucose supply than starch infused to
the rumen. However, several studies
indicate that in usual diets, the reduction of
starch digestion in the rumen increases the
large intestinal fermentation [
decreases starch total tract digestibility [
27, 39, 45
]. It is likely that the
reasonableness of manipulating starch digestion in
ruminants depends on the type of the diet.
Since the ensiling process increases starch
degradability in the rumen [
manipulation of starch degradability in maize silage
may be of greater importance than in dried
Philippeau et al. [
] reported that
variation in ruminal starch degradation of
maize grains was closely related to the
protein distribution in the endosperm. It is
surprising that, to our knowledge, there are no
reports in the literature on the variability
among hybrids in ruminal degradability of
protein. The reason may lie in a relatively
low protein concentration in maize silage,
which makes the research in this field
unattractive. However, in regions where the
conditions for maize growing are
favourable, maize silage is included in diets for
cattle in such quantities that protein derived
from it cannot be neglected. Besides protein
per se, maize silage provides a considerable
amount of energy for microbial protein
synthesis in the rumen. The Dutch protein
] takes into account the fact that
starch which escapes degradation in the
rumen does not supply any energy for
microbial protein synthesis. Therefore,
lower microbial protein yield may be
expected in diets containing silages made
from hybrids which are characterised by
low starch degradability in the rumen.
Studies reporting variability among hybrids
in ruminal degradability of starch were
done mainly on dried and ground samples
of maize grain. Since it has been shown that
degradability of starch in the rumen is
affected by drying [
], grinding [
], we decided to design a study
on undried, unground maize silage
samples. The purpose of this work was also to
examine whether the possible differences in
degradation of starch affected ruminal pH
value, ruminal fermentation pattern,
digestion of non-grain parts of maize and
microbial protein yield in the rumen. Protein
degradability was also examined.
2. MATERIALS AND METHODS
Two maize hybrids differing in their
endosperm texture were sown in alternate
row arrangement at maize population
density of 94 700 plants per ha. Maize was
harvested with a precision-chop harvester with
a theoretical chop length of 6.5 mm and
ensiled in 6 experimental silos containing
0.785 m3 of silage each. The silages were
prepared within the same day. Fifteen
plants of each hybrid from the same
experiment were also harvested for determination
of the degradation of the non-grain part
fraction of forage maize in the rumen. The
plants were separated into five fractions
consisting of stalks, leaves, husks, cobs and
grain, dried at 60 °C and ground through a
5 mm screen in a laboratory hammer mill.
2.2. Rumen study
2.2.1. Animals and feeding
Three adult Jezersko-Solcšavska sheep
(74.3 ± 1.2 kg), each fitted with a ruminal
cannula, were used. During a three week
pre-experimental period they were
gradually adapted to experimental diets. The
sheep were kept in individual pens and had
free access to water. During the experiment,
they were offered 3200 g (1180 g DM) of
dent type hybrid silage (DTS) or 3100 g
(1160 g DM) of flint type hybrid silage
(FTS) per day in the first and second period
respectively. Each period lasted for 29 days.
The diets were given to animals in two equal
portions at 6.30 a.m. and 6.30 p.m. They
were supplemented with urea (14.8 g
N·day–1) and mineral vitamin mix which
provided 3.95 g Ca, 0.73 g P, 1.35 g Na,
1.62 g S, 140 mg Mn, 40 mg Zn, 0.24 mg J
and 0.12 mg Se, 300 i.u. D3 and 3000 i.u.
vitamin A per sheep per day. Together with
minerals and vitamins from silages, the
mineral vitamin mix should cover both the
needs of the animals [
] as well as the
needs of rumen microorganisms .
2.2.2. In situ measurements
The degradabilities of starch and protein
were determined by the in sacco method as
described by Ørskov et al. [
]. About 11 g
of fresh silage sample which was equivalent
to about 4 g DM were weighed into nylon
bags with an internal size of 100 × 75 mm
and stored at –20 °C before incubation in
the rumen. The bags were made from nylon
filter cloth LT 075 (Locker Wire Weawers,
Warrington, England) with a pore size of
45–55 µm. Average sample size to bag
surface ratio was 27 mg DM·cm–2. Bags in
triplicates were introduced into the rumen
of each sheep 1.5 h after feeding and
removed after 3, 6, 12, 24, 48, and 72 h of
incubation. All incubations were repeated
twice during the days 21 to 28 of each
experimental period. Eighteen
measurements were made for each incubation time
(three sheep × three replicates × two time
repetitions). After removal from the rumen,
the bags were first thoroughly rinsed under
running tap water and then washed in a
domestic washing machine (4 rinses × 3 min
and 1 rinse × 5 min). The bags with
undegraded samples were then dried at 60 °C and
weighed. Undegraded residues were ground
and homogenised in a laboratory mill and
then analysed for ash, crude protein and
starch. The whole procedure was repeated
twice, first in the rumen of sheep that were
given DTS (first experimental period) and
then in the rumen of sheep that were given
FTS (second experimental period).
Starch and protein losses from the bags
were expressed as proportions of individual
compounds which disappeared from nylon
bags during the incubation in the rumen.
Washing losses of starch and the protein
fraction (AS and AP) were determined by
soaking the bags with samples in hot water
(39 °C) for 1 hour. Then they were washed
as described previously. Data on starch and
protein losses from the bags at different
incubation times (t) were fitted to the
equation p = a + b (1–e–ct) where coefficient a
represents the rapidly degradable fraction,
coefficient b the slowly degradable fraction
and coefficient c the degradation rate of
fraction b [
]. The coefficients of the
exponential equation (a, b, c) were obtained
with the aid of STATGRAPHICS PLUS
computer program [
] using a non-linear
regression procedure. The potential
degradabilities of starch and protein (PDGS and
PDGP) were defined as (a + b). Effective
degradabilities (EDGS and EDGP) were
calculated as EDG = a + ----------- [
c + k
measured rumen outflow rate (k) was used
in the calculations. It was not affected by the
type of silage and averaged k = 0.043 h–1.
Conditions for cellulolysis in the rumen
of sheep given either FTS or DTS were
tested by incubating samples of non-grain
parts of both hybrids in the rumen. Samples
of stalks, leaves, husks and cobs were
pooled taking into account their
proportions in DM yields and incubated in the
rumen using a similar procedure as
described previously. About 3 g of sample
were weighed into nylon bags and
incubated in the rumen for 24 hours.
Measurements in 6 repetitions (6 bags per sample
and sheep) were done on day 29 of each
2.2.3. Rumen fluid measurements
About 60 mL of rumen fluid was
withdrawn through the rumen cannula just
before and 2, 4, 6, 8 and 12 h after the
morning feeding on days 18 and 19 of each
experimental period. Within 3 minutes after
withdrawal from the rumen, the pH value of
the fluid was measured. After
centrifugation at 1250 × g, the supernatant was
acidified with concentrated H2SO4 to give a
final pH < 4 and stored frozen until volatile
fatty acids and ammonia determinations.
2.3. In vivo study
2.3.1. Animals and feeding
Four Jezersko-Solcvavska sheep
weighing 57.3 kg on average (SEM 1.9 kg) were
used. They were adapted to diets containing
urea supplemented maize silage during the
three week pre-experimental period.
During the experiment, the animals were kept
in metabolism cages with free access to
fresh water. Each silage was offered to each
sheep for 28 days using a cross-over design.
They were given 3750 g (1335 g DM) of
DTS or 3530 g (1300 g DM) of FTS per day.
The diets were formulated to contain equal
amounts of DM and were given in two equal
portions at 07.00 and 19.00. They were
supplemented with urea (16.4 g N·day–1) and
the same amount of mineral vitamin mix as
in the rumen study.
2.3.2. Measurement of digestibility, microbial protein supply and rumen outflow rates
Digestibility was determined using the
total faeces collection method. The faeces
were collected daily from days 22–28 and
stored at –20 °C until the end of the
collection period. The samples were then bulked,
weighed, sampled and dried at 60 °C for
determination of organic matter, starch, crude
protein, neutral detergent fibre (NDF), acid
detergent fibre (ADF) and acid detergent
insoluble nitrogen (ADIN). In addition,
fresh samples for the determination of
volatile fatty acids were taken. True protein
digestibility was calculated by taking into
account the fact that acid detergent
insoluble protein was the only truly undigested
feed protein fraction which appeared in the
Microbial protein supply was
determined on the basis of the urinary purine
derivative excretion using the model
described by Chen et al. [
]. Urine was
collected daily from days 22–28. The
collection was performed under 1 M H2SO4 to
maintain a pH value below 3. Daily urine
amounts were diluted to 5.5 L, mixed,
sampled and stored at –20 °C. Daily samples
were bulked before purine derivative
The rumen particle outflow rate was
determined using Cr-mordanted hay as a
]. Sixty g of Cr-mordanted hay
was mixed into a morning diet on d 15.
Faecal samples for determination of Cr were
collected 6, 12, 18, 30, 36, 48, 60, 72, 84,
96 and 120 h after administration of the
marker into the diet. The outflow rates were
calculated on the basis of Cr concentrations
in faeces using the model G2G1 according
to Moore et al. [
2.4. Analytical procedures
Dry matter of silages was determined by
the distillation method as described by
Dewar and McDonald [
]. Crude protein
was analysed according to the Kjeldahl
]. Neutral detergent fibre (NDF),
acid detergent fibre (ADF), acid detergent
lignin (ADL) and acid detergent insoluble
nitrogen (ADIN) were determined
according to Goering and Van Soest [
]. Prior to
NDF determination, starch was degraded
by a heat stable α-amylase [
sugars were determined according to the
LuffShoorl method [
]. For determination of
starch, an enzymatic method was used [
Purine derivatives (allantoin, uric acid,
xanthine and hypoxanthine) in urine were
analysed by high performance liquid
chromatography with the modified method of
Diez et al. [
]. Concentrations of Cr in
faecal samples were determined by atomic
FTS: flint type silage, DTS: dent type silage, DM: dry matter, NDF: neutral detergent fibre, ADF: acid
detergent fibre, ADL: acid detergent lignin, SEM: standard error of the mean, NS: P > 0.10.
absorption spectrophotometry on a Perkin
Elmer 2380 instrument at 357.9 nm using a
nitrous oxide-acetylene flame. The
concentrations of volatile fatty acids in the silage,
rumen fluid and faeces were determined
using gas chromatography according to
Holdemann and Moore [
concentration in rumen liquid was analysed
by an automated photometric technique
using the Bran + Luebbe, AA II method
2.5. Statistical analyses
Data were subjected to analysis of
variance with the aid of STATGRAPHICS
]. Data on degradation
characteristics of starch, protein and non-grain parts
were analysed using the model Yijkl = µ +
Ti + Dj + Ak + TDij + eijkl, where Yijkl =
dependent variable, µ = overall mean, Ti =
direct effect of hybrid (i = 1 to 2), Dj = dietary
effect of hybrid (j = 1 to 2), Ak = effect of
experimental animal (k = 1 to 3), TDij =
interaction of T by D, and eijkl = residual
error. With an in vivo study (digestibilities,
microbial protein supply, outflow rates and
nitrogen balance, composition of faeces)
only the minor part of total variation was
explained by the effect of experimental
animal. Therefore, the model was reduced to
Yij = µ + Ti + eij, where Ti = effect of grain
texture (i = 1 to 2), and eij = residual error.
Ruminal variables were analysed as repeated
measures by the MIXED Model procedure
of SAS [
] assuming a compound
symmetry variance-covariance structure. The
model used was Yijkl = µ + Ti + eij + Pk +
TPik + eijkl, where Yijkl = dependent
variable, µ = overall mean, Ti = effect of hybrid
(i = 1 to 2), eij = whole plot error, Pk = effect
of time (k = 1 to 6), TPik = interaction of T
and P, and eijkl = repeated subplot error.
3.1. Chemical composition
The chemical composition of silages is
presented in Table I. Silages of similar DM
concentration were prepared from both
hybrids. FTS had higher starch
concentration and lower NDF concentration
compared to DTS. Higher concentration of
starch in FTS was a consequence of a
considerably higher proportion of grain in total
DM yield (0.515 vs. 0.454, P < 0.05).
Silages did not differ significantly in the
A: washing loss, a: rapidly degradable fraction, b: slowly degradable fraction, c: degradation rate of
slowly degradable fraction, PDG: potential degradability, EDG: effective degradability.
For abbreviations see also Table I.
concentrations of crude protein, ADF,
ADL, fermentation acids and total sugars
(P > 0.10).
3.2. Degradability of starch, protein and non-grain parts in the rumen
Degradation characteristics of starch in
the rumen are presented in Table II.
Effective starch degradability in FTS was lower
than in DTS (740 vs. 912 g·kg–1, P < 0.001).
Differences in effective starch
degradabilities were mainly due to differences in the
rapidly degradable fraction a. Across both
diets, flint and dent, the FTS sample was
also characterised by a lower degradation
rate of starch (0.049 vs. 0.108 h–1, P < 0.01)
than FTS. The diet did not significantly
affect the effective starch degradability and
sample × diet interaction was not significant.
Characteristics of protein degradation in
the rumen are presented in Table III. Across
both diets, flint and dent, FTS had a lower
effective protein degradability than DTS
(702 vs. 766 g·kg–1, P < 0.001). Differences
in the effective degradability were due to
differences in the rapidly degradable
protein fraction (550 vs. 633 g·kg–1, P < 0.01)
and to a lower degradation rate (0.041 vs.
0.058 h–1, P < 0.05).
Values illustrating the disappearance of
the samples of non-grain parts from nylon
bags inserted in the rumen are presented in
Table IV. Hybrid per se did not affect the
degradability of non-grain parts, however,
a significant effect due to diet was noted.
For abbreviations see Table I.
Mean rumen pH
Total VFA (mmol·L–1)
Molar proportion of VFAs (%)
(Acetate + Butyrate):Propionate ratio
For abbreviations see Table I.
For both samples, a 24-hour DM
degradability of non-grain parts in the FTS diet was
higher than in the DTS diet (467 vs.
397 g·kg–1, P < 0.01). Compared to DTS,
FTS seemed to have a beneficial effect on
conditions for the degradation of fibre in the
3.3. Ruminal fermentation
Ruminal fermentation characteristics
are presented in Table V and Figure 1. The
mean pH value was significantly lower in
sheep given DTS than in sheep given FTS
(6.21 vs. 6.33, P < 0.05). The diurnal pattern
for rumen pH differed among diets (Fig. 1).
The minimal pH value was similar for both
silages, whereas the period during which
the rumen pH was below 6.2 was
considerably longer in animals receiving DTS
(about 5.8 h) than in animals fed with FTS
(about 4.5 h). The concentrations of
ammonia and total VFA in the rumen fluid were
not affected by the type of silage (Tab. V).
FTS induced a higher molar proportion of
propionate while the proportion of butyrate
was higher in sheep given DTS (P < 0.05).
The acetate to propionate ratio and acetate
plus butyrate to propionate ratio tended to
be lower in FTS than in DTS (P < 0.10,
For abbreviations see Table I.
a Calculated by taking into account that acid detergent insoluble protein was the only truly undigested feed
protein that appeared in the faeces. Value refers only to silage protein, urea supplement was not taken into
account in the calculation of digestibility.
3.4. Total tract digestion of nutrients
The digestibilities of nutrients in the total
digestive tract of sheep are presented in
Table VI. In comparison to FTS, DTS had
significantly higher digestibilities of organic
matter, starch and ADF. In contrast, true
protein digestibility was higher in FTS.
Although significant (P < 0.05), the
differences in starch and true protein digestibility
were numerically small (11 and 8 g·kg–1
respectively). The most pronounced
differences between silages were observed in the
digestibility of ADF.
3.5. Microbial protein synthesis in the rumen and nitrogen retention
Microbial protein yield in the rumen was
greater for FTS compared to that of DTS
(109.7 vs. 96.0 g·kg–1 DMI, P < 0.05,
Tab. VII). When expressed in relation to
DMI: dry matter intake, DOM: digestible organic matter, FOM: fermentable organic matter.
For abbreviations see also Table I.
digestible or fermentable organic matter
intake, the differences between hybrids
were even greater (153.5 vs. 131.6 and
215.6 vs. 158.6 g·kg–1 in FTS and DTS
respectively, Tab. VII). In comparison to
DTS, FTS also improved nitrogen retention
3.6. Faecal composition
Relative to sheep fed DTS, the faeces of
sheep fed FTS contained less dry matter
(304 vs. 371 g·kg–1, P < 0.05, Tab. VIII).
The faeces of sheep fed FTS were also
characterised by a higher concentration of VFA
(383 vs. 235 mmol·kg–1 DM, P < 0.10)
indicating more extensive fermentation in the
large intestine of sheep given FTS in
comparison to sheep given DTS. The type of
silage did not affect molar proportions of
volatile fatty acids in the faeces (Tab. VIII).
4.1. Digestion of maize starch and the effect of grain type on rumen fermentation
The results of the present experiment
have indicated a pronounced effect of grain
type on ruminal degradability of starch
from maize silage (Tab. II). The results are
consistent with those of Philippeau and
] who reported lower
starch degradability for ensiled flint maize
grain in comparison to dent. Absolute
values (740 and 912 g·kg–1 for FTS and DTS
respectively) were higher than those
reported for chopped and ensiled flint and
dent maize grain by Philippeau and
] (664 and 750 g·kg–1). The
results obtained by the in sacco method
depend largely on the method of sample
preparation and therefore the results derived
from different studies may not be
comparable. Degradation of maize starch increases
with grinding fineness, which affects first
of all the rapidly degradable fraction [
An important factor which affects the in
sacco results is also drying which leads to
a reduction in the degradability of maize
starch in the rumen [
]. To eliminate the
effect of sample preparation, undried,
unground samples of maize silage were
used in the present experiment. This may be
of special importance since it was found
that in maize silage, the distribution of
starch in particles of various sizes varied
widely between hybrids [
]. The grinding
of samples might minimise the differences,
which are related to particle size distribution.
Genotype also affected whole tract
digestibility of starch which was higher in
DTS (Tab. VI). In agreement with Mills
et al. [
], total tract digestion of starch was
positively related to its digestion in the
rumen. However, in the present experiment
the difference in the whole tract
digestibility among hybrids was small (997 vs.
986 g·kg–1) and, although significant, of no
The estimated amount of starch digested
postruminally was more than three times
higher in the flint type hybrid than in the
dent (92 vs. 24 g·kg–1 DM intake, Tab. IX).
It should be emphasised that a combination
of in vivo and in sacco methods in two
separate experiments was used to assess the
amount of postruminally digested starch.
The latter approach may not give the
absolute values; however, we believe that it is
suitable for a direct comparison of two
From the present results, we were not
able to locate the postruminal starch
digestion. Knowlton et al. [
] reported that by
lowering ruminal starch degradability by
means of drying (high moisture grain vs.
dried grain), starch digestion was shifted to
the large intestine. On the contrary, when
the extent of starch digestion in the rumen
was manipulated by the grain texture it was
found that low ruminal digestibility was
accompanied by both higher small
intestinal and higher large intestinal starch
]. The faeces of sheep which were
given FTS contained less DM and had a
higher concentration of VFA (Tab. VIII),
indicating that a higher amount of starch
was fermented in the colon and caecum
]. The results of the present study
suggest that in the FTS diet, at least one part of
starch escaping ruminal fermentation
passed through the small intestine
undigested and was fermented in the large
intestine. Extensive starch fermentation in the
large intestine is expected to increase the
proportion of butyrate to the detriment of
propionate in the faeces [
]. However, it
seems that in the present experiment the
difference in the amount of starch fermented
in the large intestine between diets was not
For abbreviations see Table I.
a The values presented in the table were calculated on the basis of the results from in vivo and in sacco
experiments. Starch and protein digested postruminally was estimated from the difference between
amounts digested in the total tract and those digested in the rumen. Digestible microbial true protein was
estimated as the microbial protein yield × 0.75 × 0.85, AFRC (1992).
b The values refer only to silage protein, urea supplement was not taken into account.
large enough to affect the proportions of
VFA in the faeces (Tab. VIII).
Data on ruminal and postruminal starch
digestion suggest that FTS support more
glucogenic energy than DTS. An additional
important source of glucogenic energy is
propionic acid produced in the rumen and
it seems that FTS may be beneficial also
from this point of view. The molar
proportion of non-glucogenic acetate and butyrate
to glucogenic propionate tended to be
higher in sheep which were given DTS than
in sheep which were fed with FTS (Tab. V).
Silages did not differ in concentrations of
fermentation end-products (Tab. I) and
therefore the differences are considered to
be a result of fermentation in the rumen. Our
results are in disagreement with data
reported by [
12, 14, 20, 22, 37, 40
]. In these
trials, the acetate to propionate ratio in the
rumen tended to decrease with the
increased rate and extent of starch
degradation in the rumen. Such disagreements may
be due to the amount of starch digested in
the rumen. In the above-mentioned studies
the effect of degradation characteristics of
starch was confounded with the higher
amount of rumen degradable starch.
However, in the present experiment, the diets
differed only in the extent and rate of starch
digestion (Tab. II) while the estimated
amount of starch digested in the rumen was
similar in both diets (Tab. IX). It seems that
the characteristics of ruminal starch
degradation per se turn the fermentation pattern
in the rumen in the opposite direction to that
of the amount of rumen degradable starch.
The results from the present experiment
(Fig. 1) suggest that FTS may maintain
ruminal pH at a higher level than DTS. The
data were not consistent with the findings
of Martin et al. [
] who observed no
difference in ruminal pH value of steers
given either flint or dent corn. In diets
containing large amounts of maize silage,
animal performance may be adversely affected
by subacute rumen acidosis [
possibility of manipulating rumen pH value by
choosing an adequate genotype, is therefore
of great practical importance. However, as
already mentioned, the results concerning
faeces composition (Tab. VIII) indicate that
the maize genotype can also affect the
intensity of fermentation in the large intestine.
Žust et al. [
] reported that infusion of
large amounts of maize-based concentrates
into the caecum caused pronounced lactic
acid production in the large intestine,
resulting in acute ruminant lactic acidosis.
From this point of view, dent type maize
might have a comparative advantage.
4.2. Digestion of non-grain parts of the maize plant
The results regarding the degradation of
non-grain parts (Tab. IV) suggest that the
FTS diet offers better conditions for
cellulolysis in the rumen than the DTS diet. The
better conditions in a diet containing FTS
were probably due mainly to a higher
ruminal pH value. For optimal cellulose
degradation, rumen pH should be maintained
above 6.2 [
]. In the present experiment,
the period during which the rumen pH was
below the critical value was considerably
longer in animals receiving DTS than in
animals which were given FTS (Fig.1). A
difference in digestion of non-grain parts of
the maize plant in the rumen may also be a
consequence of a direct negative effect of
starch on the degradation of cell walls.
Using an in vitro buffer system that can
maintain pH at 5.8, 6.2 or 6.8, Grant and
] observed that the addition of
maize starch accentuated the negative
effect of low pH on fibre digestion. In
practice, maize silage is usually combined with
other feeds. In the case of a high roughage
diet, the positive effect of FTS on the rumen
environment may fail while in the case of
high concentrate diet it may be accelerated.
Although the in sacco degradability of
non-grain parts of flint hybrid in the rumen
of sheep given the FTS diet was higher than
the degradability of non-grain parts of the
dent hybrid in the rumen of sheep which
were given a DTS diet (Tab. IV), the whole
tract digestibility of NDF and ADF was
higher in DTS than in FTS (Tab. VI). The
diet did not affect the fractional outflow rate
(Tab. VI). Therefore, a higher digestibility
of NDF and ADF in DTS could only be
explained by the compensatory digestion of
the fibre fraction in the large intestine. The
observation that increased starch
availability in the rumen may lead to a shift of NDF
digestion from the rumen to the large
intestine has already been reported [
4.3. Protein value of silages made from dent and flint type hybrids
The results of the current study suggest
that grain texture substantially affects the
protein value of maize silage. FTS was
characterised by lower ruminal protein
degradability than DTS (Tab. III).
Differences may be due to a different protein
distribution of maize endosperm. A proportion
of (α, β, δ)-zeins is reported to be higher in
flint maize than in dent [
] and the latest
(δ-zein) is highly resistant to degradation in
the rumen [
]. Lower protein
degradability in FTS did not adversely affect true
protein digestibility in the total tract (Tab. VI)
and therefore the estimated amount of
postruminally digested protein was about 40%
higher in FTS than in DTS (Tab. IX). The
results in regard to protein degradability are
in agreement with the results of Babnik,
Verbicš, Michalet-Doreau and
ŽnidaršicšPongrac (unpublished results) who reported
higher protein degradability for ensiled dent
type maize grain in comparison to the
ensiled flint type grain. Besides protein
degradability, the type of hybrid also
affected the synthesis of microbial protein
in the rumen. The efficiency of microbial
protein synthesis when expressed in
relation to dry matter intake was about 15%
higher in sheep given FTS than in sheep
given DTS (Tab. VII). We believe that the
probable reason for higher microbial
protein yield in FTS was its beneficial effect on
the rumen environment. Low rumen pH
value, measured for the DTS diet, is
expected to divert the available energy in
the rumen to nongrowth functions, i.e.
maintaining neutral pH in bacterial cells
]. A tendency for a higher efficiency of
microbial protein synthesis in flint type
maize when compared to dent type has also
been reported for dried grain [
]. As a
result of lower protein degradability and
higher microbial protein synthesis in the
rumen, FTS had a higher concentration of
metabolisable protein (87 vs. 73 g·kg–1 DM
intake, Tab. IX) and higher nitrogen
retention (146 vs. 120 g N per kg of N intake,
P < 0.05; Tab. VII) than DTS. Extensive
fermentation in the large intestine (Tab. VIII)
is expected to increase faecal N excretion
]. However, in the present experiment,
faecal N excretion was similar among diets
(Tab. VII) while urinary N excretion tended
to be higher in DTS (Tab. VII). The most
probable reason for lower urinary N
excretion in FTS was a more balanced protein
and energy supply for the growth of the
microbial population in the rumen.
The results suggest that the type of maize
grain (dent vs. flint) markedly affects the
site of starch and protein digestion along the
gastro-intestinal tract of sheep. In silage
made from FTS, ruminal degradability of
starch and protein was lower than in the
silage made from DTS. Flint type hybrid
silage enabled more favourable conditions
for cellulolysis and more efficient
microbial protein synthesis in the rumen than the
dent type silage. As a result of lower protein
degradability and higher microbial protein
yield in the rumen, FTS provides about 20%
more metabolisable protein than DTS.
Research was supported by the Slovenian
Ministry of Higher Education, Science and
Technology and by the Ministry of Agriculture,
Forestry and Food.
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