Reprinted from ANIMAL FEED SCIENCE AND...
Reprinted from
ANIMAL FEED SCIENCE
AND TECHNOLOGY
Animal Feed Science and Technology 74 (199X) 63-78
Occurrence of digestive interactions in tree
forage-based diets for sheep
S. Fall Touréa7*, B. Michalet-Doreaub,
E. Traoré”, D. Friotc,
D. Richarde
“ISRA-LNERV
BP 2057, Dakar; Senegal
‘INRA SNRH Centre de Clermont Ferrand, Theix 63122 Sainr-Genès,
Champanelle,
France
cCIRAD-EMVT
BP 5035, Montpellier Cedex 01, France
Accepted 2 December 1997
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ANIMAL FEED
SCIENCE AND
TECHNOLOGY
ELSEVIER
Animal Feed Science and Technology 74 (1998) 63-78
Occurrence of digestive interactions in tree
forage-based diets for sheep
S. Fall Touréa3*, B. Michalet-Doreaub,
E. Traoréa, D. Friot”,
D. Richarde
“ISRA-LNERV
BP 2057, Dakac Senegal
hINRA SNRH Centre de Clermont Ferrand, Theix 63122 Saint-Genès, Champanelle,
France
“CIRAD-EMVI:
BP 5035, Montpellier Cedex 01, France
Accepted 2 December 1997
--
Abstract
The effect of browse level in the diet on the in vivo dry matter digestibility (DMD) in sheep and
the DM degradation of peanut hay in the rumen of cattle-fed tree forage-based diets were
investigated in order to detect the occurrence of digestive interactions between diet components.
Selected browse species Acacia albida pods, Pithecellobium dulce, Adansonia digitata and
Calotropis procera leaf samples were collected in the central regions of Senegal, sundtied and
stored in LNERV animal bams for in vivo trials. Classical in vivo balance trials were performed for
each tree forage sample. The proportion of browse in the applied diet varied between 0 and 75% of
DM. Regression and difference procedures were both tested to estimate the DMD of the browse
component of the diet. DMD capacity in the rumen of three Young Gobra bulls fed the browse-
based diets was estimated by measurement of the in sacco dry matter degradation profile of a
standard sample, peanut hay. For each sample, large variations were observed when the browse
DMD was calculated by difference. Both total ration DMD and rumen DM degradation capacity
were significantly (~~0.001) influenced by browse level in the diet. However, non-linear response
of total diet DMD to increasing levels of browse was observed only in A. digitata and P. dulce
indicating occurrence of digestive interactions in those species. Rumen DM degradation capacity
varied according to plant species which played a major role in the observed digestion profile.
Results suggest that the digestion of tree forages-based diets may be influenced by digestive
8-
interactions but the large variations observed in plant species show their importance. Optimal DM
degradation occurred at 1530% of browse level in the diet for both A. albida and A. digitata while
for P dulce it was at 50%. Corresponding browse digestibility was of 50%, 47.1%, 51.3% and
60.7% DM for A. albida, A. digitata, l? dulce and C. procera respectively. Fnrther work using the
* Corresponding author.
0377~8401/98/$19.0 oc 1998 Elsevier Science B.V. All rights reserved
PIISO377-8401(98)00127-8
regression method in a wider range of browse species could help confirm between-species
variations. If., 1998 Elsevier Science B.V. Al1 rights reserved
Keywords: Rrowse plants; In vivo dry matter digestihility: Rumen DM degradation; Shecp; Digestive
interactions; Tropical regions
1. Introduction
In vivo dry matter digestibility is a basic measurement for the evaluation of
energy utilisation of animal feeds. This method may be labour-intensive and time-
consuming, but the importance of its application is often emphasised by many research
workers.
The application of the in vivo method to evaluate browse digestibility needs some
precautions as their chemical composition is characterised by the occurrence of
secondary compounds which may be toxic for ruminants. Condensed tannins are well
known for their negative effect on rumen microbial activity and consequently, on energy
and nitrogen metabolism (McLeod, 1974; D’Mello, 1992; Bernays et al., 1989;
Leinmuller et al., 1991; Reed et al., 1990). Toxic amino acids including mimosine are
one of the major constraints to legumes incorporation in ruminant diet (Lowry. 1989).
Other toxic compounds include cyanogenetic glycosides and alkaloids which may cause
deleterious effects in livestock (Conn, 1973; Culvenor, 1973; James et al., 1992).
Therefore, the occurrence of these chemical compounds in tree forages limits their ad
libitum use by ruminants. This mises the crucial question of their restricted incorporation
in ruminant diet. Thus, the selection of appropriate browse level in the diet is problematic.
Previous results have shown marked effects of browse level on in vivo DMD of tree
forage-based rations (Fall, 1993; Fall et al., 1996; Miranda, 1989; Dick and Urness.
1981). Linear and non-linear relationships were found between browse proportion and in
vivo DMD of diets involving different browse species. A linear relationship shows
additivity of different diets components while a curvilinear relationship indicates
non-additivity of the different ingredients which is also visualised by differences in
browse digestibility when calculated by difference. This implies the occurrence of
digestive interactions or associative effects as described in rations involving graded levels
of concentrates (Wainmann et al., 1981; Sauvant and Giger, 1989; Berge and Dulphy,
1991).
The aim of the present study was to investigate the occurrence of digestive interactions
in tree forage-based diets.
2. Material and methods
2.1. Experiment 1: Measurements of in vivo DMD of tree forages-huwd diets
In vivo triais were conducted in the ISRA (Senegalcsc Institute of Agricultural
Research) station of LNERV (National Laboratory for livestock and Veterinary Research)
S.E Tour6 et al. /Animal Feed Science and Technology 74 (1998) 63-78
61
appreciated by an analysis of variante. The significance of differences was appreciated
by an independent analysis of variante for each incubation time in each plant species.
3. Results
3.1. Estimation of BDMD by difference
In vivo DMD of studied browse species is presented in Table 2.
Average ration dry matter intake was 54.9, 53.0, 53.4, 53.3 gikg BW”.75 for A. albida
pods, A. digitata leaves, l? dulce and C. procera leaves-based diets respectively.
Therefore, feed restriction allowed similar dry matter intake for a11 diets.
Total crude protein content of the diet (12.5% DM in average) was not a limiting factor
for studied diets. It was similar between diets and browse species except for J? dulce
leaves (CP= 19.1% DM) which has higher nitrogen content than the other browse species.
TRDMD averaged 49.6, 47.7, 51.0 and 55.7 respectively for A. albida pods, A.
digitata, II dulce and C. procera leaves-based diets. There was little between animal
variations (2>rsd>0.4).
Calculated by difference, BDMD varied markedly between 3 1% to 68%, 40% to lOO%,
24% to 65% and 0% to 51% DM for A. albida pods, C. procera, l? dulce and A. digitata
leaves respectively. The highest standard deviations (>lO) were observed in the lowest
browse proportion (15% DM) in the diet. Between-animal variations decreased as the
proportion of browse level increased. Within a browse species, there were marked
Table 2
Influence of the browse proportion in the diet (% DM) on the total diet and browse in vivo digestibility (% DM)
in sheep
Tria1
1
2
3
4
5
Browse level % DM
0
15
3 0
5 0
7 5
Diet composition
Browse
0 (0)
15 (0)
30 (0)
50 (0)
75 (0)
Peanut cake
15.2 (0.5)
12.9 (0.1)
10.5 (0)
7.5 (0)
3.1 (0.0)
Rice straw
84.7 (0.5)
72.1 (0.1)
59.5 (0)
42.5 (0)
21.3 (0)
Total CP content % DM
12.1 (0)
12.0 (0.9)
12.3 (1.3)
12.8 (2.1)
13.5 (3.2)
Intake g DIWkg MBW
52.5 (0.3)
53.4 (2.3)
55.2 (1.0)
54.4 (1.2)
52.8 (2.1)
Digestibility 8 DM**
Total diet
A. albida
46.4 (1.9)
49.5 (0.7)
47.1 (1.1)
50.2 (0.7)
51.2 (1.4)
A. digitata
45.3 (0.9)
49.1 (0.7)
47.5 (0.7)
49.0 (0.6)
I? dulce
47.8 (0.7)
52.2 (0.8)
52.7 (1.3)
51.4 (0.4)
C. procera
53.1 (1.6)
50.0 (0.5)
58.0 (0.5)
61.1 (1.1)
Browse
A. albida
48.01 (0.1)
43.0 (7.1)
50.7 (2.9)
51.6 (3.8)
A. digitata
19.6 (12.8)
47.7 (4.7)
45.0 (3.0)
48.6 (1.7)
l? dulce
35.6 (9.5)
57.9 (5.5)
55.6 (5.1)
52.0 (1.2)
C. procera
70.3 (22.6)
52.6 (3.7)
67.6 (2.3)
65.3 (2.8)
( ) Standard deviation.
* MBW: Metabolic Body weight.
---
6 X
S. I;: To~ur! et al. /Animal Fe& Science und Technology 74 (1998) 63-78
Table 3
Influence of the browse proportion on the diet digestibility: Analysis of variante
Browse species
P
R2
c v
SME
S i g n i f i c a n c e N
AU species
0.000 I
0 . 3 8
6.1
**i*rr
3
128
Acacia albida
0.24
0.36
4 . 7
2 . 3
N S
32
Adartsunia digitczm
0.0009
0.66
3 . 4
1.6
**
3 2
Pithecelobium dulce
0.0013
0 . 6 3
3 . 7
1.9
*
3 2
Culotropis procera
0.0001
0 . 8 7
4.1
2 . 2
***
3 2
Significance: ‘/x0.05; “1><0.01 ; ***~<O.OOl ; **‘$<O.OOOI,
NS: not signifïcant.
SME: Standard error of mean.
between-animals and between-diets variations owing to a poor accuracy of the calculation
method.
3.2. Estimation of hrowse DMD by regression
The relationships between browse level in the diet and TRDMD are described in
Table 3, and Fig. 1. TRDMD fluctuates significantly according to browse proportion
(p<O.OOOl) and species (p<O.OOOl) in the diet. An independent analysis of variante for
each species shows significant intluence of browse level on TRDMD for A. digituta
@<O.OOl), P dulce (~~0.01) and C. procera (‘~~0.0001) while A. alhida-based diets did
not give a significant response (~00.05).
Table 4
Relationships between browse level and in vivo DMD: Stepwise regressions
Btowse species Step
Stepwise regressions*
S
SEM**
A. albidu
1 linear
y=48.94+0.02~~
N S
2 . 3
2 quadratic
p=49.58-0.054.to.Oo1x~
N S
3 cubic
)'=49.78-0.13X-+-0.004x2-0.003,~"
N S
4 quarctic
y=49.58-~0.26Xx-0.0281x~+0.0007x" -0.000005x4 N S
A. digitutu
1 linear
y=48.09$-0.002~
N S
1.6
*
2 quadratic
y=48.82-0.086x-$-0.001x2
3 cubic
y=49.20-0.241x+0.007n2-0.00005.x'
N S
4 quarctic
y=49.58-- 1 .OS 1x+0.071x2-o.0051x”+0.00001x1
***
*1(
l? dulce
1. linear
y=49.32+0.042x
1.9
2 quadratic
y=48.74$-0.1 11.x-0.0009x*
N S
*
3 cubic
y=49.33-0.130x+0.008x2-0.00008x'
4 quarctic
y=49.59-0.675.ï+0.o5lX~-o.ooh~-mooOoo7x~
'*I*I
C. procera
1 linear
y=49.29t0.152.n
2 . 2
2 quadratic
y=49.86+0.084.r+0.0009x2
N S
3 cubic
y=50.05$0.008,c+0.004x2-0.00003x"
N S
1-1
4 quarctic
y=49.59+0.99&0.074n2+0.002~~-0.00001x4
y=ln vive total ration DMD; x=browse proportion in the diet.
**Standard error of mean.
XE i'imré et al./Aninml Fercl S&nw urrd Techrdog~ 74 (iY98) 63-7X
69
60
+ A . a l b i d a
+ A . d i g i t a t a
-A-P. dulce
* C. procera
0
15
3 0
5 0
7 5
Browse level in the diet (% D M )
Fig. 1. Influence of browse level on the total diet in vive DMD.
10
S.F: RIUV~ et al. /Animal Feed Science and Teclznolo~y 74 (1998) 63-78
Table 5
In situ dry matter disappearance of peanut hay (% DM) in zebu cattles fed different hrowse proportions in the
diet
Browse species
Incubation time (h)
Browse level in the diet (% DM)
0
15
30
50
15
A. albidu
4
3 2 . 6 ”
3 2 . 4 ”
3 2 . 5 ”
30.3”
3 0 . 4 ”
0 . 5
1 . 5
0 . 3
0 . 6
0.8
24
5 7 . 6 ”
5 7 . 9 ”
5 5 . 2 ”
52.Th
53.lb
3 . 8
2 . 6
3 . 3
5 . 4
5 . 8
48
6 4 . 4 ”
6 3 . 5 ”
6 4 . 5 ”
62.P
61.3’
0 . 8
0 . 2
0 . 4
0 . 6
1.5
72
6 5 . 7 ”
6 4 . 6 ”
65.6ÿ
6 2 . 3 ’
6 3 . 1 ”
1.4
0. I
0 . 5
0 . 6
0 . 6
A. digitata
4
3 2 . 8 ”
3 3 . 2 ”
3 0 . 9 ”
0 . 3
1 . 5
0.7
24
4X.2”
5 3 . 6 ”
5 0 . 4 ”
1.8
5 . 3
5 . 1
4 8
6 0 . 2 ’
6 3 . 1 ”
5 9 . 4 ”
2 . 3
2.5
2.7
7 2
6 5 . 6 ”
6 6 . 9 ”
63.1”
0.7
I
0 . 6
P: dulce
4
3 2 . 6 ”
34.2b
35.9b
3 3 . 2 ”
0 . 4
0 . 6
1 . 4
1.2
2 4
5 7 . 6 ”
49.lh
5 6 . 9 ”
6 1 . 2 ”
3 . 3
3.7
3.9
0 . 9
48
6 4 . 4 ”
6 3 . 1 ”
61.5;’
6 4 . 5 ”
0.6
1.2
3 . 1
0 . 3
12
65.7ÿ
6 3 . 7 ”
6 6 . 5 ”
6 4 . 5 ”
1.1
1.1
0 . 6
0 . 4
Standard deviation.
Mean values followed by the different superscripts within the same line are significantly different 07<0.05).
Stepwise regressions are presented in Table 4. A significant non-linear quadratic
relationship was observed for A. digitutu ~~0.05). In r) dulce both linear (~~0.01) and
non-linear ($0.02) relationships were significant while for C. procera, the linear
regression was most adequate (~~0.0001) to describe the relationship between TRDMD
and browse proportion in the diet. For A. ulbida, regressions between browse level and
TRDMD were not significant @0.05).
Optimal browse level corresponding to maximum diet digestibility was of 15%, 30%,
50% and 15% DM (Fig. 1) while corresponding browse digestibility was 50%, 47.1%,
51.3% and 60.7% for A. albida pods, A. digitata, P. dulce and C. procera leaves
respectively.
3.3. DMD prnflle of peanut hay in the rumen of Gobra bulls fed graded levels of tree
forages
The degradation
profile of pcanut hay incubated in the rumen of zebu bulls fed graded
level of tree forages is presented in Table 5. Average peanut hay dry matter disappearance
60
5 0
40
30
10
C. procera
P. dulce
A. albida
A. digitata
Fig. 2. Tannin content and in vivo digestibility of tree forages
---
_.
12
S.F Touré rt al. /Animal Fwd S&wce and Trchnolog~ 74 (1998) 63-78
was 52.370, 53.37G, 54.3%. 55.0% and 51,070 DM for Oc/, 15%, 30%, 5070 and 75% DM
browse level respectively. It was signifkantly influenced by browse level in the diet
@<O.OOl), animal (p<O.O5) and incubation time Q~O.0001). Peanut hay DM
disappearance was higher in the control as compared to browse-based diets. At 24 h
incubation time, maximum peanut hay DM disappearance was observed at the browse
proportion of 15% both for A. albida pods and A digitata leaves and 50% for I? n’uice
leaves. Results are in agreement with in vivo observation regarding optimal level for A.
albida and P. dulce while a higher value (50%) was identified for A. digitata
3.4. Tannin concentration in tree forages
Condensed tannin content in browse were 7.8%, 1 1.3%, 6.5% and 3.1% DM
respectively for A. albida pods, A. digituta, t? dulce and C. procera leaves. Tannin
influence on in vivo DMD of diets was highly significant. Tannin concentration allowed a
classification of studied species as the highest tannin content corresponded to the lowest
digestibility (Fig. 2).
4. Discussion
In vivo DMD of browse-based diets showed marked variation when browse
digestibility was calculated by the difference method. These variations raise the question
of the browse level that cari be considered to assess browse-specific digestibility.
Optimal rumen DM degradation capacity and diets digestibility at low browse
proportions (15-30% DM) would suggest its adoption as a standard level of browse in
ruminant diets. However, a poor DMD accuracy has been observed with high standard
deviations (SD>IO), when BDMD was evaluated by the difference from low browse level
diets. This result is in agreement with that reported by Sauvant and Giger (1989) for
concentrate-based diets and by Fall (1993) for browse-based rations. This suggests that
the difference method to determine browse DMD may not be appropriate for diet with a
low level of browse. That result is confirmed by the significant influence of browse
proportion in the diet on the rumen DM degradation capacity measured as dry matter
disappearance of peanut hay in the rumen of zebu cattle fed graded levels of tree forages.
However, a11 browse species do not show the same variation trends. A. albida and C.
procera showed linear relationships between browse level and TRDMD. Digestibility
response was curvilinear for A. digitata while P. dulce showed both linear and non-linear
responses. Therefore, animal response to browse-based diet seems tu be complex and the
influence of browse level in the diet may be variable according to plant species.
The highest tannin concentration in A. digituta (11.3% DM), more than double of the
critical level (5% DM) reported by McLeod (1974), may explain the occurrence of
digestive interactions in that species. Specificity of A. digitata may also be due to the
occurrence of an emollient which may cause transit acceleration in the ruminant digestive
tract as observed in human food containing A digitata leaves commonly consumed in
West Africa.
Although the occurrence of digestive interaction is not demonstrated for a11 studied
species, large variations were observed in browse DMD within the same sample when the
difference
method was applied. That method, assuming the additivity of different
ingredients in the diet, may trot be relevant for a11 browse species. It suggests the
application of the regression method to test the occurrence of digestive interaction.
Browse proportions were different in the preliminaty study (Fall, 1993) but there is a
confirmation of the non-linear response of TRDMD to increasing browse level in the diet
for A. digitata and l? dulce leaves although linear response was more significant (p<O.Ol)
than non-linear one (p<O.O2) in that last species.
The influence of browse proportion is not often taken into account in the evaluation of
the digestibility of tree forages-based diets. Single and high browse proportions are often
applied in in vivo tria1 studies (Table 6). However, potential toxicity of many browse
species would recommend to check the role of browse level in the diet. Nastis and
Malechek ( 198 1), Villena and Pfister (1990) and Dick and Urness (1981) studying
digestibility of rations based on Quercus gamhelii, Quercus havardii and Quercus
gambelii leaves respectively, a found negative and linear effect of browse level on the
ration DMD. Those observations do not support the occurrence of digestive interactions
in browse-based diets. Interactions between the diet components may have been masked
by the high initial level (40%) of browse as curvilinearity is often observed at low browse
proportion in the diet (3O>L<lS%; Fall. 1993). In contrast, digestive interactions have
been evidenced by Miranda (1989), Preston and Leng (1987) and Traoré et al. (1995)
when assessing in vivo DMD and performances of animals fed with rations based on
Prosopis sp pods, Gliricidia sepium and Leucaena leucocephala leaves respectively. The
non-linear regression equation was calculated and optimal browse proportions of 40%,
30% and 30% DM were identified respectively for Prosopis pods, Leucaena and
Glyricidia leaves.
The occurrence of digestive interactions or associative effects between forage basa1
diet and concentrates has been widely reported (Frederiksen, 1973; Kromann, 1973;
Giger and Sauvant, 1983; Sauvant and Giger, 1989; Berge and Dulphy, 1991). They are
attributed to ce11 wall components (Berge and Dulphy, 1991). Their digestibility is
decreased by soluble carbohydrates leading to a drop in rumen pH and cellulolytic
activity for diets with high levels of concentrate. Concentrate physical form cari also be
involved in the occurrence of digestive interactions. Their fine consistency may cause
transit acceleration
in the digestive tract reducing particle rumen residence time and thus,
ce11 wall degradation in the rumen.
Studied browse species are rather coarse and often not subject to physical treatment;
thus, transit modifications cannot be held responsible for the occurrence of digestive
interactions except for A. digitata leaves in which an emollient has been identified and
used as human food. Cell wall components are more important sources of energy as
compared to soluble carbohydrates in browses (Fall and Michalet-Doreau,
1995). SO the
ce11 wall fraction may have a negative influence in browse digestibility as generally
described in conventional feestuffs (Van Soest, 1982), particularly in browse plants
(Wilson, 1977; Craig et al.. 199 1; Rafique et ai., 1992). Potentially toxic compounds may
also be involved in the occurrence of digestive interactions in browse-based diets. A.
digitata. higher than the other species in tannin, was the most affected by digestive
interactions. Condensed tannins reduce the digestibility of carbohydrates. Their negative
effect has been widely reported (McLeod, 1974; Mangan, 1988; Lowry, 1989; Leinmuller
Table 6
In iivo digestibility of tropical tree forages: A review
Browse species
Plant part
stage
Animal
Browse level C P
RDMD*
ROMD*’
Authors
% DM
8DM
%DM
%DM
c. IILontanus
Dry Laves
Goat
30-60
X-12
-
54.2
Boutouba et al.. 1990
A. (‘anPS(‘CRS
30-60
-
52.2
Mixture of species
Cattle
40-60
11
54
-
Arthun et al., 1992
A. hreviscipcr
Wet season
Sheep
22-38
9-11
44
-
Coppock and Reed, 1992
A. rortilis
Fruits
22-38
50
A. tridentnta
Goat
30
8
-
5 9 . 1
Nunez-Hemandez et al.. 1989
Q. grisea
30
8
-
45.6
c. I?Lotlîanus
30
8
-
51.8
J. rn”nosperma
30
8
-
58.2
c. ldnaltz
30
8
-
5 1 . 1
A. cunata
30
8
-
54.1
A. qnnophylla
Dry leaves
Vegetative
Sheep
80
13
64
-
Bhattacharya, 1989
H. persicum
Vegetative
8 0
1 0
52
A. halimus
Vegetative
50
18
54
S. mccineo +
Dried and
ground leaves
C. coty~bulosu.s 50: 50
42
10.5
44
Rafïque et al., 1992
c. >lio?Ltallus +
A. cnnesrens 50: 50
42
10.5
43
-
Rabique et al., 1991
M. indica
Fresh leaves
Goat
85
6 . 5
5 5
Akbar an3 Alam, 199 1
s. uspeu
8 5
6 . 5
61.9
Akbar and Alam, 1991
* RDMD: ration dry matter digestibility
**ROMD: ration organic matter digestibility
XI? Touré et al. /Animal Feed Science ami Technology 74 (1998) 63-78
7 5
et al., 199 1; Bernays et al., 1989). Tannins cari kil1 microbes and depress rumen microbial
activity. They cari also inhibit forage digestion through the formation of insoluble
complexes with long chain ce11 wall carbohydrates such as cellulose and hemicellulose
(Bernays et al., 1989). They may act as enzyme inhibitors by inactivation of protein
enzymes responsible for ce11 wall degradation (McLeod, 1974; Lohan et al., 1981). They
cari also bind muco proteins in the digestive tract reducing gut wall permeability and
nutrients absorption consequently.
In browse-based diets, the negative effect in rumen DMD capacity has been identified
as two mechanisms that cari be involved in digestive interactions for studied species.
Although the in vitro effect of plant tannins on browse digestibility has been in
evidence, demonstration of the mechanism in vivo is not often reproducible and
conflicting results are reported. Barnes et al. (1991) have established a negative
relationship between ration digestibility and tannin content in Acacia berlandieri while
Dick and Urness (1981) were not able to show a similar relationship in Quercus gambelii.
There is a large diversity in animal response to high tannin diets. The animal response
is determined by the plant species and the type of tannins, its concentration in the diet and
the physiological capacity of animal species to adapt to high tannin levels in the diet
(McLeod, 1974; Hagennann et al., 1992). Tannin-resistant animal species are reputed to
secrete saliva rich in hydroxyproline which cari inactivate tannins by precipitation during
chewing (Burrit et al., 1987; Robbins et al., 1987). This adaptation capacity cari offset the
depressing effect of tannin up to a certain level corresponding to the tannin-binding
capacity of secreted proline-rich saliva. Beyond that level, one cari observe tannin-
binding activity on proteins in general, digestive enzyme in particular and also
carbohydrates. This thesis is supported by anatomie observations which showed larger
salivary glands in browsers as compared to grazers.
5. Conclusion
The in vivo DMD of diets based on selected tree forages in the present study varied
according to the diet composition and particularly to browse proportion in the diet.
However, the non-linear response of total diet DMD to graded level of browse in the diet
was clearly demonstrated only for one species A. digitatu. The digestion profile of P.
dulce-based diets was significantly non-linear but explained more by a linear relationship.
Results suggest that digestive interactions may occur in browse-based diets but the
browse species is a major variation factor.
The in vivo DMD of browse-based diets appears to be a relative value. It cari be
evaluated taking into account the browse level and species which seems to be a major
source of variation. In consequence
the regression method, including graded browse
levels in the diet, seems to be more appropriate than the difference method to test
occurrence of digestive interactions.
TO improve the use of tree forages in tropical feeding systems, there is a need to
evaluate their nutritive value taking into account the ration type in which they are
included. Their digestibility, markedly influenced by the occurrence of secondary
compounds, raises the question of browse limitation in ruminants diets. Low levels of
16
S.F Kwré et al./Animul
Ferd Scrence and Technology 74 (1998) 63-78
incorporation varying from 15-5096 of the diet give a higher ration digestibility and
therefore avoid ruminants intoxication.
Further work with a wider range of browse species is needed to confirm the influence
of plant species and to elucidate the role of intake and basa1 diet type in the occurrence of
digestive interactions in browse-based diet for sheep.
Acknowledgements
The authors are indebted to N’Dèye Salane N’Diaye and Massamba Diop for their
valuable assistance in chemical analysis and in vivo trials. They express their gratitude to
Mohamadou Cissoko and Dave Thomas for helpful comments on the statistical analysis
and P. Cheeke and A. Skrede for a critical review of the text.
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ANIMAL FEED SCIENCE
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Animal Feed Science and Technology 74 (199X) 63-78
Occurrence of digestive interactions in tree
forage-based diets for sheep
S. Fall Touréa7*, B. Michalet-Doreaub,
E. Traoré”, D. Friotc,
D. Richarde
“ISRA-LNERV
BP 2057, Dakar; Senegal
‘INRA SNRH Centre de Clermont Ferrand, Theix 63122 Sainr-Genès,
Champanelle,
France
cCIRAD-EMVT
BP 5035, Montpellier Cedex 01, France
Accepted 2 December 1997
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ANIMAL FEED
SCIENCE AND
TECHNOLOGY
ELSEVIER
Animal Feed Science and Technology 74 (1998) 63-78
Occurrence of digestive interactions in tree
forage-based diets for sheep
S. Fall Touréa3*, B. Michalet-Doreaub,
E. Traoréa, D. Friot”,
D. Richarde
“ISRA-LNERV
BP 2057, Dakac Senegal
hINRA SNRH Centre de Clermont Ferrand, Theix 63122 Saint-Genès, Champanelle,
France
“CIRAD-EMVI:
BP 5035, Montpellier Cedex 01, France
Accepted 2 December 1997
--
Abstract
The effect of browse level in the diet on the in vivo dry matter digestibility (DMD) in sheep and
the DM degradation of peanut hay in the rumen of cattle-fed tree forage-based diets were
investigated in order to detect the occurrence of digestive interactions between diet components.
Selected browse species Acacia albida pods, Pithecellobium dulce, Adansonia digitata and
Calotropis procera leaf samples were collected in the central regions of Senegal, sundtied and
stored in LNERV animal bams for in vivo trials. Classical in vivo balance trials were performed for
each tree forage sample. The proportion of browse in the applied diet varied between 0 and 75% of
DM. Regression and difference procedures were both tested to estimate the DMD of the browse
component of the diet. DMD capacity in the rumen of three Young Gobra bulls fed the browse-
based diets was estimated by measurement of the in sacco dry matter degradation profile of a
standard sample, peanut hay. For each sample, large variations were observed when the browse
DMD was calculated by difference. Both total ration DMD and rumen DM degradation capacity
were significantly (~~0.001) influenced by browse level in the diet. However, non-linear response
of total diet DMD to increasing levels of browse was observed only in A. digitata and P. dulce
indicating occurrence of digestive interactions in those species. Rumen DM degradation capacity
varied according to plant species which played a major role in the observed digestion profile.
Results suggest that the digestion of tree forages-based diets may be influenced by digestive
8-
interactions but the large variations observed in plant species show their importance. Optimal DM
degradation occurred at 1530% of browse level in the diet for both A. albida and A. digitata while
for P dulce it was at 50%. Corresponding browse digestibility was of 50%, 47.1%, 51.3% and
60.7% DM for A. albida, A. digitata, l? dulce and C. procera respectively. Fnrther work using the
* Corresponding author.
0377~8401/98/$19.0 oc 1998 Elsevier Science B.V. All rights reserved
PIISO377-8401(98)00127-8
regression method in a wider range of browse species could help confirm between-species
variations. If., 1998 Elsevier Science B.V. Al1 rights reserved
Keywords: Rrowse plants; In vivo dry matter digestihility: Rumen DM degradation; Shecp; Digestive
interactions; Tropical regions
1. Introduction
In vivo dry matter digestibility is a basic measurement for the evaluation of
energy utilisation of animal feeds. This method may be labour-intensive and time-
consuming, but the importance of its application is often emphasised by many research
workers.
The application of the in vivo method to evaluate browse digestibility needs some
precautions as their chemical composition is characterised by the occurrence of
secondary compounds which may be toxic for ruminants. Condensed tannins are well
known for their negative effect on rumen microbial activity and consequently, on energy
and nitrogen metabolism (McLeod, 1974; D’Mello, 1992; Bernays et al., 1989;
Leinmuller et al., 1991; Reed et al., 1990). Toxic amino acids including mimosine are
one of the major constraints to legumes incorporation in ruminant diet (Lowry. 1989).
Other toxic compounds include cyanogenetic glycosides and alkaloids which may cause
deleterious effects in livestock (Conn, 1973; Culvenor, 1973; James et al., 1992).
Therefore, the occurrence of these chemical compounds in tree forages limits their ad
libitum use by ruminants. This mises the crucial question of their restricted incorporation
in ruminant diet. Thus, the selection of appropriate browse level in the diet is problematic.
Previous results have shown marked effects of browse level on in vivo DMD of tree
forage-based rations (Fall, 1993; Fall et al., 1996; Miranda, 1989; Dick and Urness.
1981). Linear and non-linear relationships were found between browse proportion and in
vivo DMD of diets involving different browse species. A linear relationship shows
additivity of different diets components while a curvilinear relationship indicates
non-additivity of the different ingredients which is also visualised by differences in
browse digestibility when calculated by difference. This implies the occurrence of
digestive interactions or associative effects as described in rations involving graded levels
of concentrates (Wainmann et al., 1981; Sauvant and Giger, 1989; Berge and Dulphy,
1991).
The aim of the present study was to investigate the occurrence of digestive interactions
in tree forage-based diets.
2. Material and methods
2.1. Experiment 1: Measurements of in vivo DMD of tree forages-huwd diets
In vivo triais were conducted in the ISRA (Senegalcsc Institute of Agricultural
Research) station of LNERV (National Laboratory for livestock and Veterinary Research)
S.E Tour6 et al. /Animal Feed Science and Technology 74 (1998) 63-78
61
appreciated by an analysis of variante. The significance of differences was appreciated
by an independent analysis of variante for each incubation time in each plant species.
3. Results
3.1. Estimation of BDMD by difference
In vivo DMD of studied browse species is presented in Table 2.
Average ration dry matter intake was 54.9, 53.0, 53.4, 53.3 gikg BW”.75 for A. albida
pods, A. digitata leaves, l? dulce and C. procera leaves-based diets respectively.
Therefore, feed restriction allowed similar dry matter intake for a11 diets.
Total crude protein content of the diet (12.5% DM in average) was not a limiting factor
for studied diets. It was similar between diets and browse species except for J? dulce
leaves (CP= 19.1% DM) which has higher nitrogen content than the other browse species.
TRDMD averaged 49.6, 47.7, 51.0 and 55.7 respectively for A. albida pods, A.
digitata, II dulce and C. procera leaves-based diets. There was little between animal
variations (2>rsd>0.4).
Calculated by difference, BDMD varied markedly between 3 1% to 68%, 40% to lOO%,
24% to 65% and 0% to 51% DM for A. albida pods, C. procera, l? dulce and A. digitata
leaves respectively. The highest standard deviations (>lO) were observed in the lowest
browse proportion (15% DM) in the diet. Between-animal variations decreased as the
proportion of browse level increased. Within a browse species, there were marked
Table 2
Influence of the browse proportion in the diet (% DM) on the total diet and browse in vivo digestibility (% DM)
in sheep
Tria1
1
2
3
4
5
Browse level % DM
0
15
3 0
5 0
7 5
Diet composition
Browse
0 (0)
15 (0)
30 (0)
50 (0)
75 (0)
Peanut cake
15.2 (0.5)
12.9 (0.1)
10.5 (0)
7.5 (0)
3.1 (0.0)
Rice straw
84.7 (0.5)
72.1 (0.1)
59.5 (0)
42.5 (0)
21.3 (0)
Total CP content % DM
12.1 (0)
12.0 (0.9)
12.3 (1.3)
12.8 (2.1)
13.5 (3.2)
Intake g DIWkg MBW
52.5 (0.3)
53.4 (2.3)
55.2 (1.0)
54.4 (1.2)
52.8 (2.1)
Digestibility 8 DM**
Total diet
A. albida
46.4 (1.9)
49.5 (0.7)
47.1 (1.1)
50.2 (0.7)
51.2 (1.4)
A. digitata
45.3 (0.9)
49.1 (0.7)
47.5 (0.7)
49.0 (0.6)
I? dulce
47.8 (0.7)
52.2 (0.8)
52.7 (1.3)
51.4 (0.4)
C. procera
53.1 (1.6)
50.0 (0.5)
58.0 (0.5)
61.1 (1.1)
Browse
A. albida
48.01 (0.1)
43.0 (7.1)
50.7 (2.9)
51.6 (3.8)
A. digitata
19.6 (12.8)
47.7 (4.7)
45.0 (3.0)
48.6 (1.7)
l? dulce
35.6 (9.5)
57.9 (5.5)
55.6 (5.1)
52.0 (1.2)
C. procera
70.3 (22.6)
52.6 (3.7)
67.6 (2.3)
65.3 (2.8)
( ) Standard deviation.
* MBW: Metabolic Body weight.
---
6 X
S. I;: To~ur! et al. /Animal Fe& Science und Technology 74 (1998) 63-78
Table 3
Influence of the browse proportion on the diet digestibility: Analysis of variante
Browse species
P
R2
c v
SME
S i g n i f i c a n c e N
AU species
0.000 I
0 . 3 8
6.1
**i*rr
3
128
Acacia albida
0.24
0.36
4 . 7
2 . 3
N S
32
Adartsunia digitczm
0.0009
0.66
3 . 4
1.6
**
3 2
Pithecelobium dulce
0.0013
0 . 6 3
3 . 7
1.9
*
3 2
Culotropis procera
0.0001
0 . 8 7
4.1
2 . 2
***
3 2
Significance: ‘/x0.05; “1><0.01 ; ***~<O.OOl ; **‘$<O.OOOI,
NS: not signifïcant.
SME: Standard error of mean.
between-animals and between-diets variations owing to a poor accuracy of the calculation
method.
3.2. Estimation of hrowse DMD by regression
The relationships between browse level in the diet and TRDMD are described in
Table 3, and Fig. 1. TRDMD fluctuates significantly according to browse proportion
(p<O.OOOl) and species (p<O.OOOl) in the diet. An independent analysis of variante for
each species shows significant intluence of browse level on TRDMD for A. digituta
@<O.OOl), P dulce (~~0.01) and C. procera (‘~~0.0001) while A. alhida-based diets did
not give a significant response (~00.05).
Table 4
Relationships between browse level and in vivo DMD: Stepwise regressions
Btowse species Step
Stepwise regressions*
S
SEM**
A. albidu
1 linear
y=48.94+0.02~~
N S
2 . 3
2 quadratic
p=49.58-0.054.to.Oo1x~
N S
3 cubic
)'=49.78-0.13X-+-0.004x2-0.003,~"
N S
4 quarctic
y=49.58-~0.26Xx-0.0281x~+0.0007x" -0.000005x4 N S
A. digitutu
1 linear
y=48.09$-0.002~
N S
1.6
*
2 quadratic
y=48.82-0.086x-$-0.001x2
3 cubic
y=49.20-0.241x+0.007n2-0.00005.x'
N S
4 quarctic
y=49.58-- 1 .OS 1x+0.071x2-o.0051x”+0.00001x1
***
*1(
l? dulce
1. linear
y=49.32+0.042x
1.9
2 quadratic
y=48.74$-0.1 11.x-0.0009x*
N S
*
3 cubic
y=49.33-0.130x+0.008x2-0.00008x'
4 quarctic
y=49.59-0.675.ï+0.o5lX~-o.ooh~-mooOoo7x~
'*I*I
C. procera
1 linear
y=49.29t0.152.n
2 . 2
2 quadratic
y=49.86+0.084.r+0.0009x2
N S
3 cubic
y=50.05$0.008,c+0.004x2-0.00003x"
N S
1-1
4 quarctic
y=49.59+0.99&0.074n2+0.002~~-0.00001x4
y=ln vive total ration DMD; x=browse proportion in the diet.
**Standard error of mean.
XE i'imré et al./Aninml Fercl S&nw urrd Techrdog~ 74 (iY98) 63-7X
69
60
+ A . a l b i d a
+ A . d i g i t a t a
-A-P. dulce
* C. procera
0
15
3 0
5 0
7 5
Browse level in the diet (% D M )
Fig. 1. Influence of browse level on the total diet in vive DMD.
10
S.F: RIUV~ et al. /Animal Feed Science and Teclznolo~y 74 (1998) 63-78
Table 5
In situ dry matter disappearance of peanut hay (% DM) in zebu cattles fed different hrowse proportions in the
diet
Browse species
Incubation time (h)
Browse level in the diet (% DM)
0
15
30
50
15
A. albidu
4
3 2 . 6 ”
3 2 . 4 ”
3 2 . 5 ”
30.3”
3 0 . 4 ”
0 . 5
1 . 5
0 . 3
0 . 6
0.8
24
5 7 . 6 ”
5 7 . 9 ”
5 5 . 2 ”
52.Th
53.lb
3 . 8
2 . 6
3 . 3
5 . 4
5 . 8
48
6 4 . 4 ”
6 3 . 5 ”
6 4 . 5 ”
62.P
61.3’
0 . 8
0 . 2
0 . 4
0 . 6
1.5
72
6 5 . 7 ”
6 4 . 6 ”
65.6ÿ
6 2 . 3 ’
6 3 . 1 ”
1.4
0. I
0 . 5
0 . 6
0 . 6
A. digitata
4
3 2 . 8 ”
3 3 . 2 ”
3 0 . 9 ”
0 . 3
1 . 5
0.7
24
4X.2”
5 3 . 6 ”
5 0 . 4 ”
1.8
5 . 3
5 . 1
4 8
6 0 . 2 ’
6 3 . 1 ”
5 9 . 4 ”
2 . 3
2.5
2.7
7 2
6 5 . 6 ”
6 6 . 9 ”
63.1”
0.7
I
0 . 6
P: dulce
4
3 2 . 6 ”
34.2b
35.9b
3 3 . 2 ”
0 . 4
0 . 6
1 . 4
1.2
2 4
5 7 . 6 ”
49.lh
5 6 . 9 ”
6 1 . 2 ”
3 . 3
3.7
3.9
0 . 9
48
6 4 . 4 ”
6 3 . 1 ”
61.5;’
6 4 . 5 ”
0.6
1.2
3 . 1
0 . 3
12
65.7ÿ
6 3 . 7 ”
6 6 . 5 ”
6 4 . 5 ”
1.1
1.1
0 . 6
0 . 4
Standard deviation.
Mean values followed by the different superscripts within the same line are significantly different 07<0.05).
Stepwise regressions are presented in Table 4. A significant non-linear quadratic
relationship was observed for A. digitutu ~~0.05). In r) dulce both linear (~~0.01) and
non-linear ($0.02) relationships were significant while for C. procera, the linear
regression was most adequate (~~0.0001) to describe the relationship between TRDMD
and browse proportion in the diet. For A. ulbida, regressions between browse level and
TRDMD were not significant @0.05).
Optimal browse level corresponding to maximum diet digestibility was of 15%, 30%,
50% and 15% DM (Fig. 1) while corresponding browse digestibility was 50%, 47.1%,
51.3% and 60.7% for A. albida pods, A. digitata, P. dulce and C. procera leaves
respectively.
3.3. DMD prnflle of peanut hay in the rumen of Gobra bulls fed graded levels of tree
forages
The degradation
profile of pcanut hay incubated in the rumen of zebu bulls fed graded
level of tree forages is presented in Table 5. Average peanut hay dry matter disappearance
60
5 0
40
30
10
C. procera
P. dulce
A. albida
A. digitata
Fig. 2. Tannin content and in vivo digestibility of tree forages
---
_.
12
S.F Touré rt al. /Animal Fwd S&wce and Trchnolog~ 74 (1998) 63-78
was 52.370, 53.37G, 54.3%. 55.0% and 51,070 DM for Oc/, 15%, 30%, 5070 and 75% DM
browse level respectively. It was signifkantly influenced by browse level in the diet
@<O.OOl), animal (p<O.O5) and incubation time Q~O.0001). Peanut hay DM
disappearance was higher in the control as compared to browse-based diets. At 24 h
incubation time, maximum peanut hay DM disappearance was observed at the browse
proportion of 15% both for A. albida pods and A digitata leaves and 50% for I? n’uice
leaves. Results are in agreement with in vivo observation regarding optimal level for A.
albida and P. dulce while a higher value (50%) was identified for A. digitata
3.4. Tannin concentration in tree forages
Condensed tannin content in browse were 7.8%, 1 1.3%, 6.5% and 3.1% DM
respectively for A. albida pods, A. digituta, t? dulce and C. procera leaves. Tannin
influence on in vivo DMD of diets was highly significant. Tannin concentration allowed a
classification of studied species as the highest tannin content corresponded to the lowest
digestibility (Fig. 2).
4. Discussion
In vivo DMD of browse-based diets showed marked variation when browse
digestibility was calculated by the difference method. These variations raise the question
of the browse level that cari be considered to assess browse-specific digestibility.
Optimal rumen DM degradation capacity and diets digestibility at low browse
proportions (15-30% DM) would suggest its adoption as a standard level of browse in
ruminant diets. However, a poor DMD accuracy has been observed with high standard
deviations (SD>IO), when BDMD was evaluated by the difference from low browse level
diets. This result is in agreement with that reported by Sauvant and Giger (1989) for
concentrate-based diets and by Fall (1993) for browse-based rations. This suggests that
the difference method to determine browse DMD may not be appropriate for diet with a
low level of browse. That result is confirmed by the significant influence of browse
proportion in the diet on the rumen DM degradation capacity measured as dry matter
disappearance of peanut hay in the rumen of zebu cattle fed graded levels of tree forages.
However, a11 browse species do not show the same variation trends. A. albida and C.
procera showed linear relationships between browse level and TRDMD. Digestibility
response was curvilinear for A. digitata while P. dulce showed both linear and non-linear
responses. Therefore, animal response to browse-based diet seems tu be complex and the
influence of browse level in the diet may be variable according to plant species.
The highest tannin concentration in A. digituta (11.3% DM), more than double of the
critical level (5% DM) reported by McLeod (1974), may explain the occurrence of
digestive interactions in that species. Specificity of A. digitata may also be due to the
occurrence of an emollient which may cause transit acceleration in the ruminant digestive
tract as observed in human food containing A digitata leaves commonly consumed in
West Africa.
Although the occurrence of digestive interaction is not demonstrated for a11 studied
species, large variations were observed in browse DMD within the same sample when the
difference
method was applied. That method, assuming the additivity of different
ingredients in the diet, may trot be relevant for a11 browse species. It suggests the
application of the regression method to test the occurrence of digestive interaction.
Browse proportions were different in the preliminaty study (Fall, 1993) but there is a
confirmation of the non-linear response of TRDMD to increasing browse level in the diet
for A. digitata and l? dulce leaves although linear response was more significant (p<O.Ol)
than non-linear one (p<O.O2) in that last species.
The influence of browse proportion is not often taken into account in the evaluation of
the digestibility of tree forages-based diets. Single and high browse proportions are often
applied in in vivo tria1 studies (Table 6). However, potential toxicity of many browse
species would recommend to check the role of browse level in the diet. Nastis and
Malechek ( 198 1), Villena and Pfister (1990) and Dick and Urness (1981) studying
digestibility of rations based on Quercus gamhelii, Quercus havardii and Quercus
gambelii leaves respectively, a found negative and linear effect of browse level on the
ration DMD. Those observations do not support the occurrence of digestive interactions
in browse-based diets. Interactions between the diet components may have been masked
by the high initial level (40%) of browse as curvilinearity is often observed at low browse
proportion in the diet (3O>L<lS%; Fall. 1993). In contrast, digestive interactions have
been evidenced by Miranda (1989), Preston and Leng (1987) and Traoré et al. (1995)
when assessing in vivo DMD and performances of animals fed with rations based on
Prosopis sp pods, Gliricidia sepium and Leucaena leucocephala leaves respectively. The
non-linear regression equation was calculated and optimal browse proportions of 40%,
30% and 30% DM were identified respectively for Prosopis pods, Leucaena and
Glyricidia leaves.
The occurrence of digestive interactions or associative effects between forage basa1
diet and concentrates has been widely reported (Frederiksen, 1973; Kromann, 1973;
Giger and Sauvant, 1983; Sauvant and Giger, 1989; Berge and Dulphy, 1991). They are
attributed to ce11 wall components (Berge and Dulphy, 1991). Their digestibility is
decreased by soluble carbohydrates leading to a drop in rumen pH and cellulolytic
activity for diets with high levels of concentrate. Concentrate physical form cari also be
involved in the occurrence of digestive interactions. Their fine consistency may cause
transit acceleration
in the digestive tract reducing particle rumen residence time and thus,
ce11 wall degradation in the rumen.
Studied browse species are rather coarse and often not subject to physical treatment;
thus, transit modifications cannot be held responsible for the occurrence of digestive
interactions except for A. digitata leaves in which an emollient has been identified and
used as human food. Cell wall components are more important sources of energy as
compared to soluble carbohydrates in browses (Fall and Michalet-Doreau,
1995). SO the
ce11 wall fraction may have a negative influence in browse digestibility as generally
described in conventional feestuffs (Van Soest, 1982), particularly in browse plants
(Wilson, 1977; Craig et al.. 199 1; Rafique et ai., 1992). Potentially toxic compounds may
also be involved in the occurrence of digestive interactions in browse-based diets. A.
digitata. higher than the other species in tannin, was the most affected by digestive
interactions. Condensed tannins reduce the digestibility of carbohydrates. Their negative
effect has been widely reported (McLeod, 1974; Mangan, 1988; Lowry, 1989; Leinmuller
Table 6
In iivo digestibility of tropical tree forages: A review
Browse species
Plant part
stage
Animal
Browse level C P
RDMD*
ROMD*’
Authors
% DM
8DM
%DM
%DM
c. IILontanus
Dry Laves
Goat
30-60
X-12
-
54.2
Boutouba et al.. 1990
A. (‘anPS(‘CRS
30-60
-
52.2
Mixture of species
Cattle
40-60
11
54
-
Arthun et al., 1992
A. hreviscipcr
Wet season
Sheep
22-38
9-11
44
-
Coppock and Reed, 1992
A. rortilis
Fruits
22-38
50
A. tridentnta
Goat
30
8
-
5 9 . 1
Nunez-Hemandez et al.. 1989
Q. grisea
30
8
-
45.6
c. I?Lotlîanus
30
8
-
51.8
J. rn”nosperma
30
8
-
58.2
c. ldnaltz
30
8
-
5 1 . 1
A. cunata
30
8
-
54.1
A. qnnophylla
Dry leaves
Vegetative
Sheep
80
13
64
-
Bhattacharya, 1989
H. persicum
Vegetative
8 0
1 0
52
A. halimus
Vegetative
50
18
54
S. mccineo +
Dried and
ground leaves
C. coty~bulosu.s 50: 50
42
10.5
44
Rafïque et al., 1992
c. >lio?Ltallus +
A. cnnesrens 50: 50
42
10.5
43
-
Rabique et al., 1991
M. indica
Fresh leaves
Goat
85
6 . 5
5 5
Akbar an3 Alam, 199 1
s. uspeu
8 5
6 . 5
61.9
Akbar and Alam, 1991
* RDMD: ration dry matter digestibility
**ROMD: ration organic matter digestibility
XI? Touré et al. /Animal Feed Science ami Technology 74 (1998) 63-78
7 5
et al., 199 1; Bernays et al., 1989). Tannins cari kil1 microbes and depress rumen microbial
activity. They cari also inhibit forage digestion through the formation of insoluble
complexes with long chain ce11 wall carbohydrates such as cellulose and hemicellulose
(Bernays et al., 1989). They may act as enzyme inhibitors by inactivation of protein
enzymes responsible for ce11 wall degradation (McLeod, 1974; Lohan et al., 1981). They
cari also bind muco proteins in the digestive tract reducing gut wall permeability and
nutrients absorption consequently.
In browse-based diets, the negative effect in rumen DMD capacity has been identified
as two mechanisms that cari be involved in digestive interactions for studied species.
Although the in vitro effect of plant tannins on browse digestibility has been in
evidence, demonstration of the mechanism in vivo is not often reproducible and
conflicting results are reported. Barnes et al. (1991) have established a negative
relationship between ration digestibility and tannin content in Acacia berlandieri while
Dick and Urness (1981) were not able to show a similar relationship in Quercus gambelii.
There is a large diversity in animal response to high tannin diets. The animal response
is determined by the plant species and the type of tannins, its concentration in the diet and
the physiological capacity of animal species to adapt to high tannin levels in the diet
(McLeod, 1974; Hagennann et al., 1992). Tannin-resistant animal species are reputed to
secrete saliva rich in hydroxyproline which cari inactivate tannins by precipitation during
chewing (Burrit et al., 1987; Robbins et al., 1987). This adaptation capacity cari offset the
depressing effect of tannin up to a certain level corresponding to the tannin-binding
capacity of secreted proline-rich saliva. Beyond that level, one cari observe tannin-
binding activity on proteins in general, digestive enzyme in particular and also
carbohydrates. This thesis is supported by anatomie observations which showed larger
salivary glands in browsers as compared to grazers.
5. Conclusion
The in vivo DMD of diets based on selected tree forages in the present study varied
according to the diet composition and particularly to browse proportion in the diet.
However, the non-linear response of total diet DMD to graded level of browse in the diet
was clearly demonstrated only for one species A. digitatu. The digestion profile of P.
dulce-based diets was significantly non-linear but explained more by a linear relationship.
Results suggest that digestive interactions may occur in browse-based diets but the
browse species is a major variation factor.
The in vivo DMD of browse-based diets appears to be a relative value. It cari be
evaluated taking into account the browse level and species which seems to be a major
source of variation. In consequence
the regression method, including graded browse
levels in the diet, seems to be more appropriate than the difference method to test
occurrence of digestive interactions.
TO improve the use of tree forages in tropical feeding systems, there is a need to
evaluate their nutritive value taking into account the ration type in which they are
included. Their digestibility, markedly influenced by the occurrence of secondary
compounds, raises the question of browse limitation in ruminants diets. Low levels of
16
S.F Kwré et al./Animul
Ferd Scrence and Technology 74 (1998) 63-78
incorporation varying from 15-5096 of the diet give a higher ration digestibility and
therefore avoid ruminants intoxication.
Further work with a wider range of browse species is needed to confirm the influence
of plant species and to elucidate the role of intake and basa1 diet type in the occurrence of
digestive interactions in browse-based diet for sheep.
Acknowledgements
The authors are indebted to N’Dèye Salane N’Diaye and Massamba Diop for their
valuable assistance in chemical analysis and in vivo trials. They express their gratitude to
Mohamadou Cissoko and Dave Thomas for helpful comments on the statistical analysis
and P. Cheeke and A. Skrede for a critical review of the text.
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