Journal of Animal and Feed Sciences, 7, 1998, 171- 1...
Journal of Animal and Feed Sciences, 7, 1998, 171- 1 X5
In vive nutrient digestibility in sheep, and rumen dry
matter degradability in cattle fed trop
by-product based diets
Safiétou Touré Fall’, Maïmouna Cissé’, D. Ditaroh’, D. Richard’,
N’Dèye Salane Ndiaye’ and B. Diaw’
‘ISRA Senegalese ltstitute of Agricultural Research,
National Lahorutoty qfAnima1 Breeding and Veterinaty Research
BP 2057 Dakac Séuégal
‘CIRAD Internationul Center,for Agronomie Research and Development -- EMV7:
Depar?menf qf Animal Breeding and Tropical Veterinaty Medicine
BP 5035 Montpellier; Cedex 01, France
(Received Il September 1997; accepted 25 March 1998)
A B S T R A C T
Nutrient in vivo digestibility in sheep and rumen dry matter degradability in cattle fed cereal straw
and concentrate-based diets were determined in two experiments. In Experiment 1, a conventional in
vive balance tria1 was conducted with six Peul-peu1 sheep to evaluate apparent digestibility of 3 diets.
Diet I consisted of (g/kg DM): rice straw 560. molasses 115, rice bran 180, peanut cake 130 and
minera1 supplement 15; diet 2 contained (g/kg DM): millet straw 580, molasses 115. millet bran 175.
peanut cake 113 and minera1 supplement 15: diet 3 was a mixture of (gikg DM) peanut hulls 180 and
cake 50, molasses 200, maize grain 95, senal* 200, cotton seeds 250 and minera1 supplement 25. In
Experiment 2, the rumen degradation pattern of the feed ingredients was evaluated using the nylon bag
technique in three fistulated Gobra zebu bulls successively fed the same diets.
In vilv dry matter and organic matter digestibility were significantly (PcO.05) intluenced by diet
type and particularly cell wall digestion was higher (PcO.0 1) in the straw based diets (1 and 2) when
compared with the mixed feed (diet 3). The animal did not influence (PcO.05) feed degradation
profiles which were strongly (P<O.OOl) modifïed by diet and feed type. It was possible to classify
different feed types according to their theoretical DM degradability (Dt) in low (Dt < 50% DM) and
medium quality roughages (50<Dt>55 041 DM) and concentrates (Dt>55 oie DM). Treatment of straws
with molasses (17%) signifïcantly (P<O.Ol ) improved their DM degradation. The feed average de-
* Senal: manufactured concentrate made up of wheat bran. molasses, urea and minera&
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172
CROP BY-PRODUCT DIGESTIBILITY
gradation profile was influenced by tibre content, especially lignocellulose (R= - 0.76). Feed Dt
cari also be predicted by dry matter disappearance at 24 h incubation time (R=O.90) and from indi-
vidual ingredients in the case of mixed feeds.
KEY WORDS: cereal straws, in vivo digestibility, mmen degradation, cattle, sheep
INTRODUCTION
Optimization of the rumen function is of major importance for the digestion of
diets based on low quality roughages (Sundstfll and Owen, 1984; Galyean and
Goetsch, 1993). There are strong relationships between diet composition and di-
gestibility. Rumen microbial ecosystem and activity are influenced by the suppie-
ment type and level in the diet which determine the availability of minerals, nitro-
gen and glucogenic precursors. Special attention is given to ce11 wall concentra-
tion in the ration as it is related to feed efficiency (Van Soest, 1982). T~US, inter-
ingredient relationships determine the occurrence of associative effects (Frederik-
sen, 1973; Kromann, 1973; Sauvant and Giger, 1989; Berge and Dulphy, 1991;
Archimede et al., 1996). Rations should then be formulated not only to meet rumi-
nant nutrient requirements but also to stimulate positive interactions among ingre-
dients. The latter interactions may improve the feed value by enhancing feed in-
take, microbial synthesis and fibre degradation. The extent and rate of feed dry
matter intraruminal degradation are valid criteria for assessing the feed value of
fïbrous feeds for ruminants and the efficiency of the rumen (Orskov et al.. 1980;
Preston and Leng, 1987).
The main objective of this study was to assess in 11ivo dty matter, organic mat-
ter, crude protein and fibre digestibility of three diets based on rice straw (diet l),
millet straw (diet 2) and peanut hulls (diet 3) and to study rumen efficiency of zebu
cattle fed those diets by measuring dry matter degradation of different ingredients
using the nylon bag method. The influence of animal, feedstuffs and diet type on
the in sucw degradation pattern was studied by comparison of three diets with
different levels of concentrates.
MATERIAL AND METHODS
In vive digestibility trials - Experiment 1
Animal.7 and diets
Conventional in vivo balance trials were performed with six Young male Peul-
peu1 sheep for evaluation of diet dry matter digestibility. The animals of 25 kg
FALL S.T. ET AL.
173
TABLE 1
Diets composition and nutritive value, % of DM
Diets
Composition
1
2
3
Rice straw
56
-
-
Rice bran
18
-
-
Peanut cake
13
11.5
5
Peanut hulls
-
-
18
Cotton seeds
-
-
25
Millet straw
-
58
-
Molasses
11.5
11.5
20
Millet bran
-
17.5
-
Maize grain
-
-
9.5
Senal’
-
-
20
Minera1 mixture
1.5
1.5
2.5
Feed unit?/kg DM
0.64
0.6
0.80
Digestible crude protein, g/kgDM
72
67
90
Calcium
5
4.5
5
Phosphorus
3
3
3
’ Senal: manufactured concentrate made up of wheat bran (90% DM) + molasses (10% DM)
? Feed unit: Net Energy unit (INRA, 1978)
body weight were housed in individual pens equipped for measurement of feed
intake and faecal output.
Three diets were studied (Table 1). Two of them contained the test trop resi-
dues while the third contained more concentrates and served as a high quality
control diet. In straw-based diets, molassed rice straw and molassed millet straw
(17:63 w/w) were given as a basa1 diet while the other ingredients were mixed and
given as supplement 1 and 2 in diets 1 and 2, respectively. For diet 3 a11 ingredients
were mixed and given as a total compound feed.
Experimental pr-ocedures
Sheep were maintained in individual pens for 15 days of adjustment followed
by 6 days of measurement periods. During the latter period, feed offered and re-
fused as well as total faecal output were measured daily and samples were oven-
dried at 60°C for dry matter evaluation. The pooled faecal sample, each feed ingre-
dient, supplement 1, supplement 2 and mixed diet 3 were collected for chemical
analysis.
-
-
174
CROP BY-PRODUCT DIGESTIBILITY
Chemical analwis
Feed ingredients, mixtures and faecal output were analyzed for dry matter, crude
protein, phosphorus and calcium (AOAC, 1975) and ce11 wall components: NDF,
ADF and lignin (Goering and Van Soest, 1979).
IH sacco degradation trials - Experiment 2
Three Young Gobra zebu bulls fitted with rumen cannulae were successively
fed diet 1, 2 and 3 (Table 1): each during 15 days of adjustment followed by a 3
day measurement period. Feed ingredients and total rations were sampled, oven
dried (60°C) and ground through a 1 mm screen for incubation in the rumen. Bags
with an interna1 dimension of 6 x 11 cm and made of Blutex nylon material (Tripette
et Renaud, France) with 46 prn pore size were each filled with 3 g ground sample
and heat sealed. One bag of each sample was withdrawn at 2,4,24,48 and 72 h,
washed with running water until clear, then beaten with a stomacher to reduce
bacterial contamination (Michalet-Doreau and Ould-Bah, 1989), washed again and
oven dried at 60°C. For each sample six replications were run from two trials with
3 animals.
Calculations and statistical analysis
The significance of differences in dry matter and nutrient digestibilities were
assessed by variante analysis using GLM procedures. Animal and diet effects were
separated by the Duncan test (SAS, 1988).
Degradation parameters were calculated according to the mode1 of Orskov and
McDonald (1979): D = a +b (1 -ect);
where D is the DM degraded at a given time t; a represents the readily degrada-
ble fraction: b is the slowly degraded fraction and c the degradation rate of the
b fraction.
Rumen DM degradability (Dt) was calculated according to the equation:
Dt = a + (bc / c+k);
in which k represents the rumen outflow rate of 4% h-’ (Lechner-Dol1 et al.,
1992).
TO check the existence oflag time (to), the mode1 modification proposed by
Dhanoa (1988) was also applied:
D = a + b( 1 - eJc’-@) and Dt = a + (bc / c+ke-k’“)
TO calculate degradation parameters, degradation profiles were fïtted to NLIN
procedures (Marquardt Methods; SAS, 1988). Analysis of variance using the GLM
FALL S.T. ET AL.
175
procedure (SAS, 1988) was applied to examine animal, diet and feed effects on
degradation parameters. The influence of feed chemical composition on these pa-
rameters was examined by regression analysis.
DM degradability (Dt) of supplements 1 and 2 and diet 3 was evaluated from
degradability of different ingredients by the following relation:
Dt = Dt,L, + D$L_ + . . . . . . + DtXLX
where Dt,= degradability for ingredient 1; L, = ingredient 1 content in the mixture
Dtz= degradability for ingredient 2; L, = ingredient 2 content in the mixture
DtX= degradability for ingredients x; LX = ingredient x content in the mixture
Estimated Dt was then compared to that measured.
RESULTS
/II vivo digestibility - Experiment 1
Chemical composition oj”feed ingredients and diets. The composition of feed
ingredients is given in Table 2. The results are consistent with the usual ranges of
tropical feed chemical composition (Kearl, 1982; Richard et al., 1989; Pond et al.,
1995). Silica levels were high in cereal straws, peanut hulls and rice bran and also
in supplement 1 made in part of rice bran. This is due to soi1 contamination occur-
ring during processing for agro-industrial by products, or late harvesting in the
case of cereal straws.
TABLE 2
Chemical composition of feed ingredients, g/kg DM
Ingredients
Cellulose
Crude
protein NDF ADF Si P Ca
Rice straw
350
41
680
393
152
1.1
2.8
Millet straw
380
65
714
453
122
1.3
4.2
Peanut hulls
348
61
831
708 46 0.4 0.6
Rice bran
76
51
655
440
164
3.4
0.8
Millet bran
20
139
475 14 65 5.1
1.1
Senal
123
228
527
177
5.9
3.8 15.0
Peanut cake
50
485
376
202
8.6
4.6
1.2
Cotton seeds
241
240
466
327
3.9 5.0 1.3
Supplement 1
218
175
674
331
122
4.5
4.1
Supplement 2
73
232
795
121 53 5.5
7.3
Maize grain
19
88
678 47 5.5 1.3
0.9
Diet 3
184
162
607
290 23
4.9 0.6
Molasses
0
6
.-
0.7
7.3
-
176
CROP BY-PRODUCT DIGESTIBILITY
TABLE 3
Average chemical composition of feed consumed
Diets
DM intake
O M
CP
N D F
A D F
A D L
glkg B W” l5
% D M
1
64
81.6
11.6
55.3
33.3
6.4
2
59
81.0
11.2
42.3
24.6
5.5
3
88
88.3
16.0
36.6
26.4
10.5
Average chemical composition of total diet intake is shown in Table 3. OM
content was similar from one diet to another while ration 2 was higher in CP com-
pared with cereal by-product based diets which had a higher fibre concentration.
TABLE 4
In vivo digestibility of nutrients in tested rations, % of DM
Diets
DM*
OM”
CP*
NDF**
ADF**
ADL**
1
52.0”
58.7”
6X.1”
5 1.9”
49.5”
12.1”
2
56.0b
63.6b
57.3b
52.9”
53.5”
17.3”
3
56.9b
57.6”
69.8”
24.6b
17.2b
5.5h
means with different superscripts are significantly different ; * PcO.05 : ** P<O.Ol
In vivo digestibilip qfdiets. Table 4 shows the in vive apparent digestibility of
tested diets. Variations between animals were not signifïcant (PcO.05). Average
dry matter (DMD), organic matter (OMD) and crude protein digestibility (CPD)
were significantly influenced by the diet type (PcO.05). Those differences were
more pronounced in ce11 wall components which were better (PcO.01) digested in
straw based rations. NDF digestibility (NDFD) averaged 52 % in diets 1 and 2
against 2.5 % DM only in diet 3. The same tendency was observed in ADF and
lignin digestibility. Thus, the low fibre content in diet 3, appeared to have a nega-
tive effect on ce11 wall digestibility.
in sacco degradability - Experiment 2
DM degradation profiles of tested feeds are presented in Table 5 and Figures 1.
2 and 3. Table 6 shows calculated degradation parameters. The two applied mo-
dels gave similar results in concentrate feeds while degradation parameters differed
in low quality roughage. With the Dhanoa mode1 it was possible to Select the
slowly degraded feedstuffs with a Iag time varying from 3 to 4.9 h in cereal straws.
Degradation kinetics varied within a wide range from rice straw (dt=47 % DM)
to Senal (dt=70 % DM). Differences among feeds were highly significant
FALL S.T. ET AL.
177
TABLE 5
Dry matter degradability of feeds (% DM): influence of the diet type
Diet
Ingredients
1
2
3
Rice straw
44
50
47.4
Molassed rice straw
52
52.1
52.1
Millet straw
49.5
49.6
51.4
Molassed millet straw
52.1
52.1
52.1
Peanut hulls
50.4
50.4
49.6
Rice bran
50.3
Millet bran
51.4
Senal
69.8
68.8
69.7
Peanut cake
68.5
Cotton seeds
54.7
55.8
53.8
Supplement 1
52.4
Supplement 2
61.8
Maize grain
68.9
Diet 3
65.5
70
60
8
6
20
10
1
I
I
I
I
- .
.-
--
2
4
24
Incubation time ,hours
-t
Rice straw (RS)
-i- Molassed (17%) RS -+- Millet straw (MS)
+ Molassed (17%) MS + Peanut hulls
Figure 1. Degradation of basa1 diets
-
178
CROP BY-PRODUCT DIGESTIBILITY
100 ,
*05--L-"
2
4
24
4 8
--.-3--
Incubation time, hours
-M-- Millet bran
+ SBnal
S Rice bran
-B- P e a n u t c a k e -X- Maizegrain - - A - C o t t o n se-eds
Figure 2. Degradation of concentrates
TABLE 6
DM degradation parameters in diet 1: C’omparison of the two models
Dhanoa mode1
@rskov mode1
Ingredients
a
b
C
to
dt a b c dt
% D M
lh h % D M -
% D M
ih
% D M
Rice straw
27.9 22
0.259 3.9
47.1
18.8
23.8 0.398 39.9
Molassed rice straw
29.9 29.9
0.102 3
52.1 26
27.4 0.204 47.9
Millet straw
25.9 28.5
0.186 4.8
50.2
2.5 30
0.696 30.8
Molassed millet straw
30
30
0.100 3
52.1 20.3 30.9 0.232 46.3
Peanut hulls
26.2 29.2
0.156 4.7
50.1 0
49.3 0.207 38.3
Rice bran
26.4 29.1
0.152 4.6
50.3 26.4 29.1 0.152 50.3
Millet bran
32.7
28.7
0.132
1.7 55
32.7
28.7 0.132 55
Senal
45.4 30.8
0.19 0
70.5 47.3 27
0.200 69.4
Peanut cake
55.5 10.8
0.362 0
63.5 55.5
10.8 0.362 63.5
Cotton seed
34.5 29.3
0.193
0.9
58.8 32.6 30.4 0.107 54.7
Supplement 1
30
30
0.100 3
52.1 30 30
0.100 52.1
Supplement 2
40.4 26.1
0.182 0
61.8 40.4 26.1 0.182 61.8
Maize grain
4 0 . 9 2s
0.107 0
59.2 40.9 25
0.107 59.2
Diet 3
35.6 3X.8
0.168
0.2
66.9 37.7 31.9 0.276 65.5
FALL S.T. ET AL.
179
80
40 -
2
4
24
48
72
Incubation time, houn
-t Supplement
1 + Supplemant2
-Q- DM 3
Figure 3. Degradation of feed mixtures
(P<O.OOOl). Three groups could be distinguished. Low quality feeds like straws
fed alone, peanut hulls and rice bran (dt<50 %DM), medium quality forage like
molassed cereal straws (50<Dt<55 %DM) and concentrates (Dt>55 %DM). There
was a significant (PcO.01) beneficial effect when cereal straws where treated with
molasses.
Differences in feed degradability were related to fibre content. ADF and crude
fibre (CF) were the best predictors of DM degradability.
Dt = 66.8 - 0.046ADF
R= 0.76
P<O.OOl
SE = 7.1
N=l4
Dt = 63.7 - 0.057CF
R= 0.76
P<O.OOl
SE = 7.1
N=I4
Among concentrate feeds, whole cotton seeds which have the highest fibre
content, were less degraded than Senal; the same for millet straw among the fi-
brous feeds. Silica (R= - 0.65) and NDF (R= - 0.57) may also play a negative role
in DM degradation profile.
The animal did not influence the degradation profiles of different diets signifi-
cantly (PcO.05) while diet type was a major variation factor (P<O.OOl). Compared
with a high concentrate diet, straw based diets (1 and 2) offered higher dry matter
degradability at 48 h incubation time (Figure 4). The applied calculation mode1
180
CROP BY-PRODUCT DIGESTIBILITY
Influence of the diet type
80
mdassed MS
psanti hulls
Feed indgredients
Figure 4. Degradation of feedstuffs
influences the extent of between-diet variation; in the Orskov mode1 strong varia-
tion profiles were observed among diets while those differences were not impor-
tant when the Dhanoa mode1 was used (Table 5).
Measured and calculated Dt values were closely similar for Supplement 2. A
difference of 5 points was observed in supplement 1 and diet 3 (Table 7).
Dt could also be predicted from dry matter disappearance at 24 h incubation
time (Deg24h)
Dt =r 24.4 + 0563*Deg24h
R = 0.90
P<O.OOl SE = 5.0 N=14
1?z vive digestibility (DMD) was close to dry matter degradability (Dt): 56.9 vs
54.9 % DM in diet 3.
TABLE 7
Estimation of mixed feeds Dt (% DM] from individual ingredïents
Supplement 1
Supplement 2
Diet 3
Calculated
57.6
60.4
71.5
Measured
52.1
61.8
66.2
Difference
- 5.5
- 1.4
~ 5.3
FALL S.T. ET AL.
181
DISCUSSION
OMD was comparable to our previous results (Fall et al., 1989; Richard et al.,
1989) in the rice straw based diet while it was superior to the millet straw based
one. That may be due to better forage quality with good leaf preservation. The
concentrate mixture (diet 3) had low DM and OM digestibility because of low ce11
wall digestion. The NDF content of that diet was 61% DM and only 25, 17 and 5%
of NDF, ADF and lignin was digested, respectively. A lower retention time be-
cause of its fine texture might also depress total OMD of the concentrate mixture.
DM degradation profiles were quite different from one feedstuff to another. In
general, the feed classification made on the Dt basis gave the usual ranking of
feedstuffs except for rice bran which, expected to be a concentrate, was classified
among the lowest quality feeds. This may be due to soi1 contamination of rice bran
and peanut hulls during processing as indicated by high NDF and silica contents
(Table 2).
Rice and millet straw degradation profiles (a + b = 30 to 60%DM) were close to
those of Orskov (1988) for different wheat straw varieties. They differed from
those reported for Mediterranean countries in which the a+b value varied fiom 60
in triticale to 70% DM in wheat straw (Chermiti et al., 1996). They were also
lower than the barley straw DM degradability (a + b = 66% DM) described by
Bhargava et al. (1986), but higher than that reported by Hvelplund (1989)
(a + b = 24). These differences cari be explained by drought stresses and poor har-
vesting conditions leading consequently to large leaf losses, soi1 contamination
and digestibility depression. The heterogeneity of straws is well known to depend
on plant species and variety, year, harvesting period and methods (Bhargava et al.,
1988; Cappers et al., 1988; 0rskov, 1988).
As expected, the Dt values of concentrates were high in general except for that
of rice bran which was of low quality with high NDF and silica contents (Table 2).
Feed degradability was well predicted by fibre content and strong relationships
between ADF and Dt (R = - 0.76) were found. The negative effect of lignocellu-
lose (ADF) on feed digestibility in general, and particularly in low quality rougha-
ges is well known (Van Soest, 1982; Akin et al., 1990). SO processing fïbrous
feeds and supplementation strategies should be directed to improve ce11 wall di-
gestion.
Having in mind that nylon bag studies are not always easy to carry out, estima-
tion of feed degradability from chemical composition and ingredient proportions
in the diet could help simplify the method. Estimates of the mixed feed Dt from the
Dt of individual ingredients were in agreement with the tïndings of Stallings et al.
( 199 1) regarding the protein degradation profile of diets containing barley silages.
That method cari be a useful tool for estimating the degradability of mixed feed
diets.
FALL S.T. ET AL.
181
DISCUSSION
OMD was comparable to our previous results (Fall et al., 1989; Richard et al.,
1989) in the rice straw based diet while it was superior to the millet straw based
one. That may be due to better forage quality with good leaf preservation. The
concentrate mixture (diet 3) had low DM and OM digestibility because of low ce11
wall digestion. The NDF content of that diet was 61% DM and only 25, 17 and 5%
of NDF, ADF and lignin was digested, respectively. A lower retention time be-
cause of its fine texture might also depress total OMD of the concentrate mixture.
DM degradation profiles were quite different from one feedstuff to another. In
general, the feed classification made on the Dt basis gave the usual ranking of
feedstuffs except for rice bran which, expected to be a concentrate, was classifïed
among the lowest quality feeds. This may be due to soi1 contamination of rice bran
and peanut hulls during processing as indicated by high NDF and silica contents
(Table 2).
Rice and millet straw degradation profiles (a + b = 30 to 60%DM) were close to
those of Orskov (1988) for different wheat straw varieties. They differed from
those reported for Mediterranean countries in which the a+b value varied from 60
in triticale to 70% DM in wheat straw (Chermiti et al., 1996). They were also
lower than the barley straw DM degradability (a + b = 66% DM) described by
Bhargava et al. (1986), but higher than that reported by Hvelplund (1989)
(a + b = 24). These differences cari be explained by drought stresses and poor har-
vesting conditions leading consequently to large leaf losses, soi1 contamination
and digestibility depression. The heterogeneity of straws is well known to depend
on plant species and variety, year. harvesting period and methods (Bhargava et al.,
1988; Cappers et al., 1988; @rskov, 1988).
As expected, the Dt values of concentrates were high in general except for that
of rice bran which was of low quality with high NDF and silica contents (Table 2).
Feed degradability was well predicted by fibre content and strong relationships
between ADF and Dt (R = - 0.76) were found. The negative effect of lignocellu-
lose (ADF) on feed digestibility in general, and particularly in low quality rougha-
ges is well known (Van Soest, 1982; Akin et al., 1990). SO processing fibrous
feeds and supplementation strategies should be directed to improve ce11 wall di-
gestion.
Having in mind that nylon bag studies are not always easy to carry out, estima-
tion of feed degradability from chemical composition and ingredient proportions
in the diet could help simplify the method. Estimates of the mixed feed Dt from the
Dt of individual ingredients were in agreement with the findings of Stallings et al.
(199 1) regarding the protein degradation profile of diets containing barley silages.
That method cari be a useful tool for estimating the degradability of mixed feed
diets.
-
182
CROP BY-PRODUCT DIGESTIBILITY
Measurement of in vive DMD of three diets based on trop by-products availa-
ble in Senegal showed comparable value regarding DM and OM digestibility be-
tween millet and rice straw based diets compared to a high concentrate mixture.
Cereal straw based diets showed a higher tendency to digest fibre.
In vive observations were confïrmed by in sacco results which showed signi-
fïcant (P<O.OOl) influence of the ration type on DM degradation profiles for the 14
incubated feeds. In general, better fibre digestion in straw-based diets compared
with high concentrate diets was observed.
The diet influence in the rumen DM degradation profile demonstrates the
occurrence of associative effects. The different diet components, the fibrous
feedsiconcentrates ratio in particular, affect energy digestion. Such digestive inter-
actions between feeds has been widely reported (Galyean and Goetsch, 1993).
Interrelationships between rumina1 ecosystems and straw digestion have been
reviewed in detail by Durand (1989) and Galyean and Goetsch (1993). Diets 1 and
2 were characterized by high straw levels (concentrate/straw ratio = 43157) and
low readily fermentable carbohydrate content. These types of feeds have a high
buffering capacity probably linked to greater salivary secretion and higher rumen
residence time. The rumen ecosystem stimulates cellulolytic microorganisms which
degrade fibre (Chamberlain and Choung, 1995). In contrast, diet 3 had a high
concentrate proportion (82% of the diet) with a predominance of readily fermenta-
ble carbohydrate through molasses and maize grains. This diet may develop ru-
men microbial population that degrades starch and simple sugars rather than low
quality fibres. The two diet types may also be differentiated by texture. Straw
based diets were coarser than that based mainly on concentrates; this may stimu-
late intake and rumen outflow rate and thus decrease DM degradability. This is in
agreement with Orskov (1983) and Preston and Leng (1987) who reported better
degradability of high fibre content feedstuffs in straw based diets compared with
compounded concentrate ones. The latter are rich in soluble and readily degraded
carbohydrates which are preferred by microorganistns to complex sugars like cel-
lulose or hemicellulose to satisfy their energy requirements. In addition. concen-
trate diets give lower rumen pH and hence inhibit fibre digestion.
CONCLUSIONS
Marked influence of diet type on the dry matter digestion profile of feed ingre-
dients demonstrates the occurrence of negative interactions when the concentrate
proportion is greater than the level of low quality forages. The latter should be
restricted for optimal stimulation of cellulolytic bacteria. Therefore, utilization of
expensive concentrates should be minimal to avoid low rumen pH and related
fibre digestion inhibition and ensure proper utilization of fibrous feeds.
FALL S.T. ET AL.
183
ACKNOWLEDGEMENTS
This work was funded by the Canadian International Development Research
Center (IDRC), project No 3P 90 0200. Authors are indebted to P. Cheeke and A.
Skreder for helpful comments.
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Van Soest P.J., 1982. Nutritional Ecology of the Ruminant. 0. & B. BOOKS, Inc. Corvallis, OR,
pp. 374
In vive nutrient digestibility in sheep, and rumen dry
matter degradability in cattle fed trop
by-product based diets
Safiétou Touré Fall’, Maïmouna Cissé’, D. Ditaroh’, D. Richard’,
N’Dèye Salane Ndiaye’ and B. Diaw’
‘ISRA Senegalese ltstitute of Agricultural Research,
National Lahorutoty qfAnima1 Breeding and Veterinaty Research
BP 2057 Dakac Séuégal
‘CIRAD Internationul Center,for Agronomie Research and Development -- EMV7:
Depar?menf qf Animal Breeding and Tropical Veterinaty Medicine
BP 5035 Montpellier; Cedex 01, France
(Received Il September 1997; accepted 25 March 1998)
A B S T R A C T
Nutrient in vivo digestibility in sheep and rumen dry matter degradability in cattle fed cereal straw
and concentrate-based diets were determined in two experiments. In Experiment 1, a conventional in
vive balance tria1 was conducted with six Peul-peu1 sheep to evaluate apparent digestibility of 3 diets.
Diet I consisted of (g/kg DM): rice straw 560. molasses 115, rice bran 180, peanut cake 130 and
minera1 supplement 15; diet 2 contained (g/kg DM): millet straw 580, molasses 115. millet bran 175.
peanut cake 113 and minera1 supplement 15: diet 3 was a mixture of (gikg DM) peanut hulls 180 and
cake 50, molasses 200, maize grain 95, senal* 200, cotton seeds 250 and minera1 supplement 25. In
Experiment 2, the rumen degradation pattern of the feed ingredients was evaluated using the nylon bag
technique in three fistulated Gobra zebu bulls successively fed the same diets.
In vilv dry matter and organic matter digestibility were significantly (PcO.05) intluenced by diet
type and particularly cell wall digestion was higher (PcO.0 1) in the straw based diets (1 and 2) when
compared with the mixed feed (diet 3). The animal did not influence (PcO.05) feed degradation
profiles which were strongly (P<O.OOl) modifïed by diet and feed type. It was possible to classify
different feed types according to their theoretical DM degradability (Dt) in low (Dt < 50% DM) and
medium quality roughages (50<Dt>55 041 DM) and concentrates (Dt>55 oie DM). Treatment of straws
with molasses (17%) signifïcantly (P<O.Ol ) improved their DM degradation. The feed average de-
* Senal: manufactured concentrate made up of wheat bran. molasses, urea and minera&
-
172
CROP BY-PRODUCT DIGESTIBILITY
gradation profile was influenced by tibre content, especially lignocellulose (R= - 0.76). Feed Dt
cari also be predicted by dry matter disappearance at 24 h incubation time (R=O.90) and from indi-
vidual ingredients in the case of mixed feeds.
KEY WORDS: cereal straws, in vivo digestibility, mmen degradation, cattle, sheep
INTRODUCTION
Optimization of the rumen function is of major importance for the digestion of
diets based on low quality roughages (Sundstfll and Owen, 1984; Galyean and
Goetsch, 1993). There are strong relationships between diet composition and di-
gestibility. Rumen microbial ecosystem and activity are influenced by the suppie-
ment type and level in the diet which determine the availability of minerals, nitro-
gen and glucogenic precursors. Special attention is given to ce11 wall concentra-
tion in the ration as it is related to feed efficiency (Van Soest, 1982). T~US, inter-
ingredient relationships determine the occurrence of associative effects (Frederik-
sen, 1973; Kromann, 1973; Sauvant and Giger, 1989; Berge and Dulphy, 1991;
Archimede et al., 1996). Rations should then be formulated not only to meet rumi-
nant nutrient requirements but also to stimulate positive interactions among ingre-
dients. The latter interactions may improve the feed value by enhancing feed in-
take, microbial synthesis and fibre degradation. The extent and rate of feed dry
matter intraruminal degradation are valid criteria for assessing the feed value of
fïbrous feeds for ruminants and the efficiency of the rumen (Orskov et al.. 1980;
Preston and Leng, 1987).
The main objective of this study was to assess in 11ivo dty matter, organic mat-
ter, crude protein and fibre digestibility of three diets based on rice straw (diet l),
millet straw (diet 2) and peanut hulls (diet 3) and to study rumen efficiency of zebu
cattle fed those diets by measuring dry matter degradation of different ingredients
using the nylon bag method. The influence of animal, feedstuffs and diet type on
the in sucw degradation pattern was studied by comparison of three diets with
different levels of concentrates.
MATERIAL AND METHODS
In vive digestibility trials - Experiment 1
Animal.7 and diets
Conventional in vivo balance trials were performed with six Young male Peul-
peu1 sheep for evaluation of diet dry matter digestibility. The animals of 25 kg
FALL S.T. ET AL.
173
TABLE 1
Diets composition and nutritive value, % of DM
Diets
Composition
1
2
3
Rice straw
56
-
-
Rice bran
18
-
-
Peanut cake
13
11.5
5
Peanut hulls
-
-
18
Cotton seeds
-
-
25
Millet straw
-
58
-
Molasses
11.5
11.5
20
Millet bran
-
17.5
-
Maize grain
-
-
9.5
Senal’
-
-
20
Minera1 mixture
1.5
1.5
2.5
Feed unit?/kg DM
0.64
0.6
0.80
Digestible crude protein, g/kgDM
72
67
90
Calcium
5
4.5
5
Phosphorus
3
3
3
’ Senal: manufactured concentrate made up of wheat bran (90% DM) + molasses (10% DM)
? Feed unit: Net Energy unit (INRA, 1978)
body weight were housed in individual pens equipped for measurement of feed
intake and faecal output.
Three diets were studied (Table 1). Two of them contained the test trop resi-
dues while the third contained more concentrates and served as a high quality
control diet. In straw-based diets, molassed rice straw and molassed millet straw
(17:63 w/w) were given as a basa1 diet while the other ingredients were mixed and
given as supplement 1 and 2 in diets 1 and 2, respectively. For diet 3 a11 ingredients
were mixed and given as a total compound feed.
Experimental pr-ocedures
Sheep were maintained in individual pens for 15 days of adjustment followed
by 6 days of measurement periods. During the latter period, feed offered and re-
fused as well as total faecal output were measured daily and samples were oven-
dried at 60°C for dry matter evaluation. The pooled faecal sample, each feed ingre-
dient, supplement 1, supplement 2 and mixed diet 3 were collected for chemical
analysis.
-
-
174
CROP BY-PRODUCT DIGESTIBILITY
Chemical analwis
Feed ingredients, mixtures and faecal output were analyzed for dry matter, crude
protein, phosphorus and calcium (AOAC, 1975) and ce11 wall components: NDF,
ADF and lignin (Goering and Van Soest, 1979).
IH sacco degradation trials - Experiment 2
Three Young Gobra zebu bulls fitted with rumen cannulae were successively
fed diet 1, 2 and 3 (Table 1): each during 15 days of adjustment followed by a 3
day measurement period. Feed ingredients and total rations were sampled, oven
dried (60°C) and ground through a 1 mm screen for incubation in the rumen. Bags
with an interna1 dimension of 6 x 11 cm and made of Blutex nylon material (Tripette
et Renaud, France) with 46 prn pore size were each filled with 3 g ground sample
and heat sealed. One bag of each sample was withdrawn at 2,4,24,48 and 72 h,
washed with running water until clear, then beaten with a stomacher to reduce
bacterial contamination (Michalet-Doreau and Ould-Bah, 1989), washed again and
oven dried at 60°C. For each sample six replications were run from two trials with
3 animals.
Calculations and statistical analysis
The significance of differences in dry matter and nutrient digestibilities were
assessed by variante analysis using GLM procedures. Animal and diet effects were
separated by the Duncan test (SAS, 1988).
Degradation parameters were calculated according to the mode1 of Orskov and
McDonald (1979): D = a +b (1 -ect);
where D is the DM degraded at a given time t; a represents the readily degrada-
ble fraction: b is the slowly degraded fraction and c the degradation rate of the
b fraction.
Rumen DM degradability (Dt) was calculated according to the equation:
Dt = a + (bc / c+k);
in which k represents the rumen outflow rate of 4% h-’ (Lechner-Dol1 et al.,
1992).
TO check the existence oflag time (to), the mode1 modification proposed by
Dhanoa (1988) was also applied:
D = a + b( 1 - eJc’-@) and Dt = a + (bc / c+ke-k’“)
TO calculate degradation parameters, degradation profiles were fïtted to NLIN
procedures (Marquardt Methods; SAS, 1988). Analysis of variance using the GLM
FALL S.T. ET AL.
175
procedure (SAS, 1988) was applied to examine animal, diet and feed effects on
degradation parameters. The influence of feed chemical composition on these pa-
rameters was examined by regression analysis.
DM degradability (Dt) of supplements 1 and 2 and diet 3 was evaluated from
degradability of different ingredients by the following relation:
Dt = Dt,L, + D$L_ + . . . . . . + DtXLX
where Dt,= degradability for ingredient 1; L, = ingredient 1 content in the mixture
Dtz= degradability for ingredient 2; L, = ingredient 2 content in the mixture
DtX= degradability for ingredients x; LX = ingredient x content in the mixture
Estimated Dt was then compared to that measured.
RESULTS
/II vivo digestibility - Experiment 1
Chemical composition oj”feed ingredients and diets. The composition of feed
ingredients is given in Table 2. The results are consistent with the usual ranges of
tropical feed chemical composition (Kearl, 1982; Richard et al., 1989; Pond et al.,
1995). Silica levels were high in cereal straws, peanut hulls and rice bran and also
in supplement 1 made in part of rice bran. This is due to soi1 contamination occur-
ring during processing for agro-industrial by products, or late harvesting in the
case of cereal straws.
TABLE 2
Chemical composition of feed ingredients, g/kg DM
Ingredients
Cellulose
Crude
protein NDF ADF Si P Ca
Rice straw
350
41
680
393
152
1.1
2.8
Millet straw
380
65
714
453
122
1.3
4.2
Peanut hulls
348
61
831
708 46 0.4 0.6
Rice bran
76
51
655
440
164
3.4
0.8
Millet bran
20
139
475 14 65 5.1
1.1
Senal
123
228
527
177
5.9
3.8 15.0
Peanut cake
50
485
376
202
8.6
4.6
1.2
Cotton seeds
241
240
466
327
3.9 5.0 1.3
Supplement 1
218
175
674
331
122
4.5
4.1
Supplement 2
73
232
795
121 53 5.5
7.3
Maize grain
19
88
678 47 5.5 1.3
0.9
Diet 3
184
162
607
290 23
4.9 0.6
Molasses
0
6
.-
0.7
7.3
-
176
CROP BY-PRODUCT DIGESTIBILITY
TABLE 3
Average chemical composition of feed consumed
Diets
DM intake
O M
CP
N D F
A D F
A D L
glkg B W” l5
% D M
1
64
81.6
11.6
55.3
33.3
6.4
2
59
81.0
11.2
42.3
24.6
5.5
3
88
88.3
16.0
36.6
26.4
10.5
Average chemical composition of total diet intake is shown in Table 3. OM
content was similar from one diet to another while ration 2 was higher in CP com-
pared with cereal by-product based diets which had a higher fibre concentration.
TABLE 4
In vivo digestibility of nutrients in tested rations, % of DM
Diets
DM*
OM”
CP*
NDF**
ADF**
ADL**
1
52.0”
58.7”
6X.1”
5 1.9”
49.5”
12.1”
2
56.0b
63.6b
57.3b
52.9”
53.5”
17.3”
3
56.9b
57.6”
69.8”
24.6b
17.2b
5.5h
means with different superscripts are significantly different ; * PcO.05 : ** P<O.Ol
In vivo digestibilip qfdiets. Table 4 shows the in vive apparent digestibility of
tested diets. Variations between animals were not signifïcant (PcO.05). Average
dry matter (DMD), organic matter (OMD) and crude protein digestibility (CPD)
were significantly influenced by the diet type (PcO.05). Those differences were
more pronounced in ce11 wall components which were better (PcO.01) digested in
straw based rations. NDF digestibility (NDFD) averaged 52 % in diets 1 and 2
against 2.5 % DM only in diet 3. The same tendency was observed in ADF and
lignin digestibility. Thus, the low fibre content in diet 3, appeared to have a nega-
tive effect on ce11 wall digestibility.
in sacco degradability - Experiment 2
DM degradation profiles of tested feeds are presented in Table 5 and Figures 1.
2 and 3. Table 6 shows calculated degradation parameters. The two applied mo-
dels gave similar results in concentrate feeds while degradation parameters differed
in low quality roughage. With the Dhanoa mode1 it was possible to Select the
slowly degraded feedstuffs with a Iag time varying from 3 to 4.9 h in cereal straws.
Degradation kinetics varied within a wide range from rice straw (dt=47 % DM)
to Senal (dt=70 % DM). Differences among feeds were highly significant
FALL S.T. ET AL.
177
TABLE 5
Dry matter degradability of feeds (% DM): influence of the diet type
Diet
Ingredients
1
2
3
Rice straw
44
50
47.4
Molassed rice straw
52
52.1
52.1
Millet straw
49.5
49.6
51.4
Molassed millet straw
52.1
52.1
52.1
Peanut hulls
50.4
50.4
49.6
Rice bran
50.3
Millet bran
51.4
Senal
69.8
68.8
69.7
Peanut cake
68.5
Cotton seeds
54.7
55.8
53.8
Supplement 1
52.4
Supplement 2
61.8
Maize grain
68.9
Diet 3
65.5
70
60
8
6
20
10
1
I
I
I
I
- .
.-
--
2
4
24
Incubation time ,hours
-t
Rice straw (RS)
-i- Molassed (17%) RS -+- Millet straw (MS)
+ Molassed (17%) MS + Peanut hulls
Figure 1. Degradation of basa1 diets
-
178
CROP BY-PRODUCT DIGESTIBILITY
100 ,
*05--L-"
2
4
24
4 8
--.-3--
Incubation time, hours
-M-- Millet bran
+ SBnal
S Rice bran
-B- P e a n u t c a k e -X- Maizegrain - - A - C o t t o n se-eds
Figure 2. Degradation of concentrates
TABLE 6
DM degradation parameters in diet 1: C’omparison of the two models
Dhanoa mode1
@rskov mode1
Ingredients
a
b
C
to
dt a b c dt
% D M
lh h % D M -
% D M
ih
% D M
Rice straw
27.9 22
0.259 3.9
47.1
18.8
23.8 0.398 39.9
Molassed rice straw
29.9 29.9
0.102 3
52.1 26
27.4 0.204 47.9
Millet straw
25.9 28.5
0.186 4.8
50.2
2.5 30
0.696 30.8
Molassed millet straw
30
30
0.100 3
52.1 20.3 30.9 0.232 46.3
Peanut hulls
26.2 29.2
0.156 4.7
50.1 0
49.3 0.207 38.3
Rice bran
26.4 29.1
0.152 4.6
50.3 26.4 29.1 0.152 50.3
Millet bran
32.7
28.7
0.132
1.7 55
32.7
28.7 0.132 55
Senal
45.4 30.8
0.19 0
70.5 47.3 27
0.200 69.4
Peanut cake
55.5 10.8
0.362 0
63.5 55.5
10.8 0.362 63.5
Cotton seed
34.5 29.3
0.193
0.9
58.8 32.6 30.4 0.107 54.7
Supplement 1
30
30
0.100 3
52.1 30 30
0.100 52.1
Supplement 2
40.4 26.1
0.182 0
61.8 40.4 26.1 0.182 61.8
Maize grain
4 0 . 9 2s
0.107 0
59.2 40.9 25
0.107 59.2
Diet 3
35.6 3X.8
0.168
0.2
66.9 37.7 31.9 0.276 65.5
FALL S.T. ET AL.
179
80
40 -
2
4
24
48
72
Incubation time, houn
-t Supplement
1 + Supplemant2
-Q- DM 3
Figure 3. Degradation of feed mixtures
(P<O.OOOl). Three groups could be distinguished. Low quality feeds like straws
fed alone, peanut hulls and rice bran (dt<50 %DM), medium quality forage like
molassed cereal straws (50<Dt<55 %DM) and concentrates (Dt>55 %DM). There
was a significant (PcO.01) beneficial effect when cereal straws where treated with
molasses.
Differences in feed degradability were related to fibre content. ADF and crude
fibre (CF) were the best predictors of DM degradability.
Dt = 66.8 - 0.046ADF
R= 0.76
P<O.OOl
SE = 7.1
N=l4
Dt = 63.7 - 0.057CF
R= 0.76
P<O.OOl
SE = 7.1
N=I4
Among concentrate feeds, whole cotton seeds which have the highest fibre
content, were less degraded than Senal; the same for millet straw among the fi-
brous feeds. Silica (R= - 0.65) and NDF (R= - 0.57) may also play a negative role
in DM degradation profile.
The animal did not influence the degradation profiles of different diets signifi-
cantly (PcO.05) while diet type was a major variation factor (P<O.OOl). Compared
with a high concentrate diet, straw based diets (1 and 2) offered higher dry matter
degradability at 48 h incubation time (Figure 4). The applied calculation mode1
180
CROP BY-PRODUCT DIGESTIBILITY
Influence of the diet type
80
mdassed MS
psanti hulls
Feed indgredients
Figure 4. Degradation of feedstuffs
influences the extent of between-diet variation; in the Orskov mode1 strong varia-
tion profiles were observed among diets while those differences were not impor-
tant when the Dhanoa mode1 was used (Table 5).
Measured and calculated Dt values were closely similar for Supplement 2. A
difference of 5 points was observed in supplement 1 and diet 3 (Table 7).
Dt could also be predicted from dry matter disappearance at 24 h incubation
time (Deg24h)
Dt =r 24.4 + 0563*Deg24h
R = 0.90
P<O.OOl SE = 5.0 N=14
1?z vive digestibility (DMD) was close to dry matter degradability (Dt): 56.9 vs
54.9 % DM in diet 3.
TABLE 7
Estimation of mixed feeds Dt (% DM] from individual ingredïents
Supplement 1
Supplement 2
Diet 3
Calculated
57.6
60.4
71.5
Measured
52.1
61.8
66.2
Difference
- 5.5
- 1.4
~ 5.3
FALL S.T. ET AL.
181
DISCUSSION
OMD was comparable to our previous results (Fall et al., 1989; Richard et al.,
1989) in the rice straw based diet while it was superior to the millet straw based
one. That may be due to better forage quality with good leaf preservation. The
concentrate mixture (diet 3) had low DM and OM digestibility because of low ce11
wall digestion. The NDF content of that diet was 61% DM and only 25, 17 and 5%
of NDF, ADF and lignin was digested, respectively. A lower retention time be-
cause of its fine texture might also depress total OMD of the concentrate mixture.
DM degradation profiles were quite different from one feedstuff to another. In
general, the feed classification made on the Dt basis gave the usual ranking of
feedstuffs except for rice bran which, expected to be a concentrate, was classified
among the lowest quality feeds. This may be due to soi1 contamination of rice bran
and peanut hulls during processing as indicated by high NDF and silica contents
(Table 2).
Rice and millet straw degradation profiles (a + b = 30 to 60%DM) were close to
those of Orskov (1988) for different wheat straw varieties. They differed from
those reported for Mediterranean countries in which the a+b value varied fiom 60
in triticale to 70% DM in wheat straw (Chermiti et al., 1996). They were also
lower than the barley straw DM degradability (a + b = 66% DM) described by
Bhargava et al. (1986), but higher than that reported by Hvelplund (1989)
(a + b = 24). These differences cari be explained by drought stresses and poor har-
vesting conditions leading consequently to large leaf losses, soi1 contamination
and digestibility depression. The heterogeneity of straws is well known to depend
on plant species and variety, year, harvesting period and methods (Bhargava et al.,
1988; Cappers et al., 1988; 0rskov, 1988).
As expected, the Dt values of concentrates were high in general except for that
of rice bran which was of low quality with high NDF and silica contents (Table 2).
Feed degradability was well predicted by fibre content and strong relationships
between ADF and Dt (R = - 0.76) were found. The negative effect of lignocellu-
lose (ADF) on feed digestibility in general, and particularly in low quality rougha-
ges is well known (Van Soest, 1982; Akin et al., 1990). SO processing fïbrous
feeds and supplementation strategies should be directed to improve ce11 wall di-
gestion.
Having in mind that nylon bag studies are not always easy to carry out, estima-
tion of feed degradability from chemical composition and ingredient proportions
in the diet could help simplify the method. Estimates of the mixed feed Dt from the
Dt of individual ingredients were in agreement with the tïndings of Stallings et al.
( 199 1) regarding the protein degradation profile of diets containing barley silages.
That method cari be a useful tool for estimating the degradability of mixed feed
diets.
FALL S.T. ET AL.
181
DISCUSSION
OMD was comparable to our previous results (Fall et al., 1989; Richard et al.,
1989) in the rice straw based diet while it was superior to the millet straw based
one. That may be due to better forage quality with good leaf preservation. The
concentrate mixture (diet 3) had low DM and OM digestibility because of low ce11
wall digestion. The NDF content of that diet was 61% DM and only 25, 17 and 5%
of NDF, ADF and lignin was digested, respectively. A lower retention time be-
cause of its fine texture might also depress total OMD of the concentrate mixture.
DM degradation profiles were quite different from one feedstuff to another. In
general, the feed classification made on the Dt basis gave the usual ranking of
feedstuffs except for rice bran which, expected to be a concentrate, was classifïed
among the lowest quality feeds. This may be due to soi1 contamination of rice bran
and peanut hulls during processing as indicated by high NDF and silica contents
(Table 2).
Rice and millet straw degradation profiles (a + b = 30 to 60%DM) were close to
those of Orskov (1988) for different wheat straw varieties. They differed from
those reported for Mediterranean countries in which the a+b value varied from 60
in triticale to 70% DM in wheat straw (Chermiti et al., 1996). They were also
lower than the barley straw DM degradability (a + b = 66% DM) described by
Bhargava et al. (1986), but higher than that reported by Hvelplund (1989)
(a + b = 24). These differences cari be explained by drought stresses and poor har-
vesting conditions leading consequently to large leaf losses, soi1 contamination
and digestibility depression. The heterogeneity of straws is well known to depend
on plant species and variety, year. harvesting period and methods (Bhargava et al.,
1988; Cappers et al., 1988; @rskov, 1988).
As expected, the Dt values of concentrates were high in general except for that
of rice bran which was of low quality with high NDF and silica contents (Table 2).
Feed degradability was well predicted by fibre content and strong relationships
between ADF and Dt (R = - 0.76) were found. The negative effect of lignocellu-
lose (ADF) on feed digestibility in general, and particularly in low quality rougha-
ges is well known (Van Soest, 1982; Akin et al., 1990). SO processing fibrous
feeds and supplementation strategies should be directed to improve ce11 wall di-
gestion.
Having in mind that nylon bag studies are not always easy to carry out, estima-
tion of feed degradability from chemical composition and ingredient proportions
in the diet could help simplify the method. Estimates of the mixed feed Dt from the
Dt of individual ingredients were in agreement with the findings of Stallings et al.
(199 1) regarding the protein degradation profile of diets containing barley silages.
That method cari be a useful tool for estimating the degradability of mixed feed
diets.
-
182
CROP BY-PRODUCT DIGESTIBILITY
Measurement of in vive DMD of three diets based on trop by-products availa-
ble in Senegal showed comparable value regarding DM and OM digestibility be-
tween millet and rice straw based diets compared to a high concentrate mixture.
Cereal straw based diets showed a higher tendency to digest fibre.
In vive observations were confïrmed by in sacco results which showed signi-
fïcant (P<O.OOl) influence of the ration type on DM degradation profiles for the 14
incubated feeds. In general, better fibre digestion in straw-based diets compared
with high concentrate diets was observed.
The diet influence in the rumen DM degradation profile demonstrates the
occurrence of associative effects. The different diet components, the fibrous
feedsiconcentrates ratio in particular, affect energy digestion. Such digestive inter-
actions between feeds has been widely reported (Galyean and Goetsch, 1993).
Interrelationships between rumina1 ecosystems and straw digestion have been
reviewed in detail by Durand (1989) and Galyean and Goetsch (1993). Diets 1 and
2 were characterized by high straw levels (concentrate/straw ratio = 43157) and
low readily fermentable carbohydrate content. These types of feeds have a high
buffering capacity probably linked to greater salivary secretion and higher rumen
residence time. The rumen ecosystem stimulates cellulolytic microorganisms which
degrade fibre (Chamberlain and Choung, 1995). In contrast, diet 3 had a high
concentrate proportion (82% of the diet) with a predominance of readily fermenta-
ble carbohydrate through molasses and maize grains. This diet may develop ru-
men microbial population that degrades starch and simple sugars rather than low
quality fibres. The two diet types may also be differentiated by texture. Straw
based diets were coarser than that based mainly on concentrates; this may stimu-
late intake and rumen outflow rate and thus decrease DM degradability. This is in
agreement with Orskov (1983) and Preston and Leng (1987) who reported better
degradability of high fibre content feedstuffs in straw based diets compared with
compounded concentrate ones. The latter are rich in soluble and readily degraded
carbohydrates which are preferred by microorganistns to complex sugars like cel-
lulose or hemicellulose to satisfy their energy requirements. In addition. concen-
trate diets give lower rumen pH and hence inhibit fibre digestion.
CONCLUSIONS
Marked influence of diet type on the dry matter digestion profile of feed ingre-
dients demonstrates the occurrence of negative interactions when the concentrate
proportion is greater than the level of low quality forages. The latter should be
restricted for optimal stimulation of cellulolytic bacteria. Therefore, utilization of
expensive concentrates should be minimal to avoid low rumen pH and related
fibre digestion inhibition and ensure proper utilization of fibrous feeds.
FALL S.T. ET AL.
183
ACKNOWLEDGEMENTS
This work was funded by the Canadian International Development Research
Center (IDRC), project No 3P 90 0200. Authors are indebted to P. Cheeke and A.
Skreder for helpful comments.
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