Directory UMM :Data Elmu:jurnal:A:Animal Feed Science and Technology:Vol82.Issue3-4.Dec1999:
Animal Feed Science and Technology
82 (1999) 227±241
In vitro quality assessment of tannin-containing
tropical shrub legumes: protein and fibre digestion
C.S. McSweeneya,*, B. Palmerb, R. Buncha, D.O. Krausea
a
CSIRO Tropical Agriculture, Long Pocket Laboratories, Private Bag No. 3 PO,
Indooroopilly, 4068 Qld., Australia
b
CSIRO Tropical Agriculture, Davies Laboratory, Private Mail Bag, Aitkenvale,
Townsville, 4814 Qld., Australia
Received 29 March 1999; received in revised form 9 August 1999; accepted 26 August 1999
Abstract
In vitro techniques were evaluated to determine the nutritive value of a selection of tanniniferous
tree and shrub legumes (Calliandra calothyrsus, Leucaena leucocephala L. diversifolia and L.
pallida) compared with lucerne (Medicago sativa). Polyethylene glycol (PEG) was also added to
some in vitro fermentations (10 mg PEG/50 mg plant substrate) to assess the effects of tannins on
digestion of dry matter (DM), neutral detergent fibre (NDF) and nitrogen (N).
Total tannin content was poorly correlated with digestibility of dry matter and nitrogen. Apparent
digestibility of dry matter and N were significantly different between plants and ranked in the
following order; lucerne > L. leucocephala > L. diversifolia > L. pallida > C. calothyrsus. Ammonia was not produced (net accumulation) during 72 h fermentation of C. calothyrsus, L. diversifolia
and L. pallida although apparent nitrogen digestion in these plants ranged from 36.9 to 44.3%.
Acid-pepsin digestion resulted in a further 17±22% of dry matter digestion in the shrub legumes
compared with 8% in lucerne following 72 h fermentation. The amount of acid-pepsin digestible N
available was lowest for lucerne (8.4%) and highest for L. pallida (38.9%) with the other legumes
ranging from 26.5 to 36.8%.
The PEG addition caused a significant increase in rate and extent of DM and NDF digestibility
and ammonia production for all the tannin containing shrub legumes but not for lucerne. However,
DM loss and fermentability of these plants appeared to be poorly correlated because PEG addition
resulted in an increase in volatile fatty acid production ranging from 3.7 to 202% compared with an
increase in apparent DM digestibility of 9.1±30%.
It is concluded that in vitro evaluation of apparent DM and N digestibility of tannin containing
plants provides a poor indication of true digestion (fermentability) and thus measurements of
*
Corresponding author. Tel.: 61-7-3214-2820; fax: 61-7-3214-2880
E-mail address: [email protected] (C.S. McSweeney)
0377-8401/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved.
PII: S 0 3 7 7 - 8 4 0 1 ( 9 9 ) 0 0 1 0 3 - 0
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C.S. McSweeney et al. / Animal Feed Science and Technology 82 (1999) 227±241
fermentation end-products (ammonia and short and branched chain fatty acids) should also be
undertaken to evaluate nutritive value. Also, addition of PEG to in vitro fermentations can be used
to determine the effect of tannins on N digestibility. Based on the in vitro methods of rumen
fermentation used in this study, nutritive value of the tanniniferous forages is ranked as follows: L.
leucocephala > L. diversifolia and L. pallida > C. calothyrsus. # 1999 Elsevier Science B.V. All
rights reserved.
Keywords: Tannins; Rumen; Polyethylene glycol; Tropical legumes; Nutritive value
1. Introduction
A major limitation to ruminant production in dry tropical regions is poor nutrition.
Annual growth rates of animals are restricted by low nitrogen and high fibre content of
native grasses and crop residues which form the basis of the diets in these climatic zones.
Supplementation of tropical roughages with leguminous fodder trees and shrubs (FTS) is
a promising way of alleviating nitrogen deficiencies.
Selecting plants for introduction usually involves ranking related accessions on
agronomic potential and nutritive value. Laboratory techniques are often used to
assess nutritive value and thus predict animal performance. The nylon bag technique
(Mehrez and Orskov, 1977) is used routinely to determine digestibility of substrate
in the rumen while in vitro fermentation techniques that measure gas production
(Menke et al., 1979) and apparent digestibility (Tilley and Terry, 1963) are also used
to determine nutritive value. However, FTS often contain anti-nutritive factors such as
condensed tannins which affect their nutritive value (Ahn et al., 1989). Apparent
digestibility can be confounded by the fact that soluble complexes form between
condensed tannin, protein and carbohydrate. It has been suggested that the gas production technique is more reliable than the nylon bag method for determining
nutritive value of feed containing anti-nutritive factors (Khazaal et al., 1993). However,
the two techniques are poorly correlated when determining the nutritive value of
tropical legumes of lower digestibility (Siaw et al., 1993). Furthermore, the gas
production technique is often employed to determine digestibility and infer nutritive
value without assessing nitrogen degradability even though tropical legumes are
commonly used in ruminant production systems to correct a primary nitrogen deficiency
(Siaw et al., 1993). Recent studies have also shown that the addition of polyethylene
glycol (PEG) or polyvinylpyrrolidone (PVP) to in vitro fermentations of tannin
containing forages will improve dry matter digestibility and thus provides insight
into the nutritional significance of the tannins (Makkar et al., 1995; Khazaal et al., 1996).
Curiously, the use of PEG or PVP to assess effect of tannins on in vitro N digestibility
has received little attention considering tanniniferous legumes are usually fed as
nitrogen supplements.
This paper reports on the results of a study aimed at (1) assessing the protein and
carbohydrate digestion in a range of tannin containing shrub legumes and (2) evaluating
in vitro methods that would accurately characterize the digestion of these forages thus
provide better predictions of their nutritional value.
C.S. McSweeney et al. / Animal Feed Science and Technology 82 (1999) 227±241
229
2. Materials and methods
2.1. Plant samples and preparation
The shrub legumes Calliandra calothyrsus CPI 115690, Leucaena leucocephala cv
Cunningham, L. diversifolia CPI 33820 and L. pallida CQ 3439 were grown and harvested
at Lansdown Research Station, 50 km south of Townsville, Queensland, Australia (198400 S,
1468510 E). The area was periodically grazed and then slashed to about 50 cm and allowed to
regrow to the next grazing period. After a period of regrowth, the first five fully expanded
leaves were collected as the standard sample from each plot. These tropical shrub legumes
were compared with the temperate legume Medicago sativa (lucerne). Adequate quantities
of samples were collected (approximately 1 kg) from each plant and oven dried at 658C
in a force draught oven for 48 h. Dried samples were ground in a Wiley mill fitted with a 1 mm
screen and stored in air-tight containers at ÿ708C until required. Temperature of drying
and method of drying (e.g., oven drying versus freeze drying) will affect digestibility
characteristics of tanniniferous forages but rank order of digestibility between different
plants is usually unaffected by drying technique (Palmer, unpublished data).
2.2. Anaerobic techniques and media
The anaerobic techniques of Hungate (1969) as modified by Bryant (1972) were used
for the growth of organisms and preparation of media. Finely milled oven-dried (658C)
plant material (50 mg) was weighed into sterile Balch tubes and autoclaved basal medium
(10 ml) was then dispensed under sterile conditions into the Balch tubes and immediately
stopped. Basal medium contained 15% (v/v) clarified rumen fluid, 20% (v/v) macro
mineral Solution A and 0.01% (v/v) micro mineral Solution B (Menke et al., 1979),
0.78% (w/v) NaHCO3, 0.25% (w/v) yeast extract, 0.03% (w/v) Na2S9H2O and 0.001%
(w/v) resazurin, (Caldwell and Bryant, 1966), 0.0001% (w/v) hemin and NH4Cl (3 mM).
Basal medium (pH 6.7) contained less than 0.5 mM ammonia N prior to addition of
NH4Cl. Polyethylene glycol (PEG, MW 4000; BDH Cat No. 29576 HM) was also added
in solution to culture tubes (10 mg/10 ml basal medium) as a treatment to counteract the
effect of tannins and thus determine the potential nutritive value of the plant if tannins
were not present. Fermentations were done in triplicate and uninoculated controls, with
and without PEG were routinely included. B-vitamins (Lowe et al., 1985) were added to
each tube of medium just prior to inoculation and incubations were at 398C.
2.3. In vitro digestibility
Culture tubes were inoculated with rumen fluid which was obtained from a steer fed a
diet comprising 70% rhodes grass (Chloris gayana) and 30% lucerne (Medicago sativa).
Rumen digesta was strained through muslin cloth, incubated anaerobically at 398C for
30 min in a Erlenmeyer flask so that the larger particulate matter would float to the
surface, while aliquots for inoculation were taken from the fluid phase beneath the top
layer. Culture tubes of plant substrates were then immediately inoculated with rumen
fluid (0.1 ml/10 ml culture) and sequential measurements of dry matter (DM) digestibility
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and neutral detergent fibre (NDF) digestibility were made from tubes sacrificed at varying
intervals during a period of 72 h after inoculation. Percentage DM loss at 0 h represents
the amount of DM that is soluble in uninoculated media. Tubes were also sub-sampled
for measurements of ammonia production as an indicator of the rate of protein digestion.
In a separate experiment, the two stage technique (Tilley and Terry, 1963) for in vitro
digestion of forage plants was used to determine potential digestibility of dry matter, and
nitrogen and short chain fatty acid production, as well as acid-pepsin digestible protein
that remained following fermentation for 72 h. The methods were essentially the same as
those described by Tilley and Terry (1963) except approximately 0.5 g of plant material
was used as substrate and the medium, inoculum size and ratio of plant material to culture
fluid were as described above. Triplicate assays were performed at each time point and
measurements corrected for changes in uninoculated controls. Short chain fatty acid
production was not measured for lucerne.
2.4. Chemical analyses
The indophenol method for the determination of ammonia as described by Chaney and
Marbach (1962) was used to estimate the rate and amount of ammonia production in
cultures. Volatile fatty acids (VFA) in culture fluid were analysed by high performance
liquid chromatography using a Waters System (Waters, Milford, Mass., 01757) equipped
with an Aminex HPX-87 Cation exchange column (300 mm 7.8 mm) for organic acids
and a microguard column (Bio-Rad Laboratories, Hercules, CA, 94547) with a column
heater (Waters Model 1122/WTC-120). Organic acids and ethanol were eluted using a
mobile phase of 2.5% acetonitirile in 0.2% v/v phosphoric acid at a flow rate of 0.7 ml/
min., a column temperature of 608C and UV detection at 210 nm. NDF and ADF of plant
substrates was determined by the method of Van Soest et al. (1991) using the micro
analysis technique of Pell and Schofield (1993). Total condensed tannin was determined
by the butanol-HCl method (Terrill et al., 1992). Nitrogen content of samples was
measured using the Kjeldahl method (AOAC, 1980).
2.5. Statistical analysis
Statistical analysis of dry matter disappearance, ammonia production and N digestion
was by analysis of variance with differences determined by the method of least significant
difference at the 5% level (P < 0.05). A repeated measures design was used to determine
the effect of PEG treatment on digestion and ammonia production during 72 h
incubations of substrate where a series of measurements were made during the course
of the incubation. All statistical analyses were done with Statistica 5.0 software (StatSoft,
Inc., Tulsa, OK., 74104).
3. Results
The fibre, nitrogen and condensed tannin content of lucerne, Calliandra calothyrsus,
L. pallida, L. diversifolia and L. leucocephala are given in Table 1. Lucerne had a
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Table 1
Neutral detergent fibre (NDF), acid detergent fibre (ADF), nitrogen (N) and condensed tannin content (%) of
lucerne, Calliandra calothyrsus, Leucaena leucocephala, L. diversifolia and L. pallidaa
NDF (% DM)
ADF (% DM)
N (% DM)b
N (% NDF)b
Lucerne
Calliandra
L. leucocephala
L. diversifolia
L. pallida
41.4 a
28.4 a
3.17
1.12
37.6 b
22.7 b
3.64
3.68
27.8 e
14.7 e
2.72
2.86
29.4 d
17.5 d
3.48
3.21
34.3 c
19.6 c
3.31
2.16
Condensed tannin (% DM)
Free
n.t.
Total
n.t.
4.78 b
5.71 b
3.32 c
3.84 d
6.57 a
7.70 a
4.00 c
4.72 c
Pooled sem
1.1
1.3
±
±
0.46
0.55
a
Values within rows with different letters differ significantly (P < 0.05).
Not statistically tested due to lack of replication.
Abbreviations: DM, dry matter; n.t., not tested.
b
significantly higher NDF content (P < 0.05) than the shrub legumes. Nitrogen content of
the five plants was relatively high and ranged from 2.7 to 3.6% DM. Nitrogen associated
with the NDF fraction appeared to be higher in the shrub legumes but this may have be
due to association of protein and tannin with NDF when the plant samples were originally
dried. Total condensed tannin concentrations in calliandra and the leucaenas ranged from
3.8 to 7.7% DM with greater than 84% of the tannin in a free form.
3.1. Apparent digestion of nitrogen and fermentation end-products
There were significant differences (P < 0.05) in digestion of nitrogen between the
tropical legumes tested but these were not correlated with tannin content (Table 2).
Lucerne had significantly higher water soluble N than the legumes containing tannin and
L. leucocephala had the lowest soluble N content. Apparent microbial digestion of
nitrogen was highest for lucerne followed by the shrub legumes in the following order;
Table 2
Nitrogen digestion from lucerne, Calliandra calothyrsus, Leucaena leucocephala, L. diversifolia and incubated
with rumen fluid (72 h) and followed by acid-pepsin treatmenta
b
N incubated (mg)
Water soluble N (% total)c
Microbial digested N (% total)d
Acid pepsin digested N (% total)e
Digestible N (% total)f
Lucerne
Calliandra
L. leucocephala L. diversifolia L. pallida
14.47 (0.s01)
47.9 (0.1) a
78.3 (0.7) a
8.4 (0.8) a
86.7 (0.1) a
16.52 (0.07)
20.9 (1.6) b
36.9 (0.1) e
36.8 (0.6) c
73.7 (0.3) e
12.45 (0.11)
15.3 (1.3) c
49.3 (0.1) b
26.5 (0.4) b
75.8 (0.4) c
15.87 (0.03)
22.3 (0.1) b
44.3 (0.1) c
36.1 (0.7) c
80.4 (0.2) b
15.16 (0.02)
21.3 (0.1) b
40.1 (0.1) d
38.9 (0.6)d
78.9 (0.0) d
a
Values within rows with different letters differ significantly (P < 0.05) and values in parenthesis are the
standard error of the mean.
b
mg N/500 mg plant substrate.
c
N solubilized in uninoculated control tubes.
d
N apparently digested during fermentation including water soluble N.
e
N apparently digested by acid pepsin treatment following microbial digestion.
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L. leucocephala > L. diversifolia > L. pallida > C. calothyrsus. The amount of acid
pepsin digestible N available following a 72 h fermentation was lowest for lucerne (8.4%)
and highest for L. pallida (38.9%) with the other legumes ranging from 26.5 to 36.8%.
Apparent digestible N was highest for lucerne (86.7%) although the majority of total N
(73.7±80.4%) was apparently available for digestion in the shrub legumes even though
they contained substantial amounts of condensed tannin.
Ammonia was not apparently produced but rather consumed during 72 h fermentations
of C. calothyrsus, L. diversifolia and L. pallida (Table 3). Accumulation of ammonia
occurred in fermentations of lucerne (11.5 mM) and L. leucocephala (5.44 mM) which
both had the highest apparent microbial digestion of nitrogen.
Volatile fatty acid (VFA) concentrations in uninoculated cultures were negligible (data
not shown). Production of VFA were significantly reduced by the presence of tannin in
some of the legumes (Table 4). Inclusion of PEG in fermentations of the tanniniferous
plants resulted in a significant increase in volatile acid production with all plants except
L. leucocephala which did not change significantly. The increase in VFA production for
calliandra, L. diversifolia and L. pallida were 202, 37 and 129%, respectively. The PEG
addition caused a significant increase in acetate to propionate ratio only with L.
diversifolia. Branched chain volatile fatty acids (BCVFA) were not produced from
fermentations of calliandra, L. diversifolia and L. pallida without PEG but BCVFA
increased significantly (P < 0.05) with all tanniniferous plants in the presence of PEG.
The highest concentrations (P < 0.05) of BCVFA occurred with PEG additions to L.
diversifolia and L. pallida.
3.2. Apparent digestion of dry matter
Apparent dry matter digestion was significantly different between legume species
(Table 3). A large proportion of the dry matter (approximately 0.3) was solubilized
immediately from plant material in uninoculated cultures (Table 3). Apparent dry matter
digestion was highest for lucerne (63.1%) while L. leucocephala (51%) had the highest
digestibility of the tannin containing legumes. Acid/pepsin digestion resulted in digestion
Table 3
Dry matter loss and ammonia production from lucerne, Calliandra calothyrsus, Leucaena leucocephala, L.
diversifolia L. pallida incubated (72 h) with rumen fluida
Lucerne
Calliandra
L. leucocephala L. diversifolia
L. pallida
DM loss (%)
Uninoculated
Inoculated
Acid/pepsin digest
36.1 (0.6) b
63.1 (0.4) a
71.1 (0.2) a
28.9 (0.6) d
32.2 (1.2) e
53.8 (0.2) e
38.9 (0.5) a
51.0 (0.1) b
68.8 (0.3) b
31.2 (0.2) c
38.5 (0.9) d
56.7 (0.4) d
Ammonia (mM)
Uninoculated
Inoculated
4.46 (0.03) b
11.06 (0.06) a
a
3.33 (0.03) d
2.86 (0.04) c
3.83 (0.05) c
5.44 (0.16) b
30.7 (0.5) c
43.3 (1.0) c
60.4 (0.8) c
4.70 (0.09) a
2.18 (0.09) e
4.77 (0.11) a
2.50 (0.12) d
Values within rows with different letters differ significantly (P < 0.05) and values in parenthesis are the
standard error of the mean.
Calliandra
ÿPEG
Total VFA (mM)
6.96 a (0.07)
Acetate : propionate 2.59 a (0.14)
BCVFA (mM)
0a
L. leucocephala
L. diversifolia
PEG
ÿPEG
PEG
20.99 b (0.68)
2.63 a (0.06)
0.530 b (0.021)
25.41 c (0.46)
26.34 c (0.52)
2.15 b (0.01)
2.06 b (0.03)
0.253 c (0.021) 0.433 d (0.050)
ÿPEG
L. pallida
PEG
12.57 e (0.19) 28.82 f (0.30)
2.05 b (0.02) 2.38 c (0.04)
0a
0.753 e (0.028)
ÿPEG
PEG
19.23 d (1.09) 26.39 c (0.50)
2.0 b (0.05)
2.06 b (0.05)
0a
0.722 e (0.028)
a
Values within rows with different letters differ significantly (P < 0.05) and values in parenthesis are the standard error of the mean. BCVFA, branched chain volatile
fatty acids.
C.S. McSweeney et al. / Animal Feed Science and Technology 82 (1999) 227±241
Table 4
End products of fermentation from Calliandra calothyrsus, Leucaena leucocephala, L. diversifolia and L. pallida incubated with rumen fluid, plus or minus polyethylene
glycol (PEG)a
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C.S. McSweeney et al. / Animal Feed Science and Technology 82 (1999) 227±241
of a further 17±22 units of dry matter in the shrub legumes compared with 8 units in
lucerne (Table 3).
3.3. Kinetics of fermentation of protein and dry matter in lucerne and tannin containing
legumes
Addition of PEG caused a significant and marked increase (P < 0.05) in the rate and
extent of ammonia production for all tannin containing shrub legumes (Fig. 1). A period
of about 20±30 h elapsed before there was an increase in rate and extent of ammonia
production which represents the lag phase of growth of the small inoculum (0.1 ml/10 ml
culture) used to initiate the fermentation. Significant amounts of ammonia accumulated
in fermentations of L. leucocephala without PEG but ammonia was not apparently
produced in fermentations of L. pallida, L. diversifolia and C. calothyrsus under the same
conditions. Ammonia concentration in uninoculated controls (with and without PEG) of
shrub legumes varied less than 1 mM during 96 h of incubation (data not shown).
Production of ammonia (approximately 4 mM) from lucerne was not significantly
affected by PEG (Fig. 3). Ammonia concentration peaked earlier (52 versus 64±72 h;
P < 0.05) for lucerne compared with the shrub legumes treated with PEG. Initially net
ammonia production was negative for L. leucocephala before ammonia accumulated.
PEG treatment resulted in an increase in rate and extent of DM and NDF digestibility
for all the tannin containing shrub legumes with L. leucocephala being least affected (Fig.
2). The ranking for DM matter and NDF digestibilities of the shrub legumes with PEG
was C. calothyrsus < L. diversifolia, L. pallida < L. leucocephala (P < 0.05). Similarly,
without PEG, C. calothyrsus had the lowest (P < 0.05) ranking for DM digestibility (C.
calothyrsus < L. diversifolia < L. pallida < L. leucocephala) while L leucocephala ranked
highest (P < 0.05) for NDF digestibility (C. calothyrsus < L. diversifolia < L. pallida < L.
leucocephala). Dry matter digestibility of lucerne was not significantly affected by PEG
(Fig. 3). Lucerne had the highest DM digestibility (P < 0.05) of the legumes except for L.
leucocephala PEG which was not significantly different. It was calculated that 100%
conversion of nitrogen in the shrub legumes to ammonia would have resulted in ammonia
concentrations in the culture fluid (minus ammonia in media constituents) of calliandra
(13.0 mM), L. diversifolia (12.4 mM), L. pallida (11.9 mM) and L. leucocephala
(9.7 mM).
4. Discussion
The shrub and tree legumes evaluated had high crude protein and low fibre content
which would normally indicate their potential use as protein supplements. However, the
presence of condensed tannins affected the nutritive value of these plants to a varying
degree and confounded the interpretation of conventional in vitro digestibility
measurements. Correlations between concentration of tannin fractions in the plant and
reductions in digestibility are varied (Khazaal et al., 1994; Khazaal and Orskov, 1994;
Balogun et al., 1998) and thus tannin concentration is probably unreliable in predicting
effects on nutritive value which was also the case in the current study.
C.S. McSweeney et al. / Animal Feed Science and Technology 82 (1999) 227±241
Fig. 1. Ammonia production during in vitro fermentations of calliandra (open circle), L. leucocephala (solid circle), L. diversifolia (open square) and L. pallida (solid
square) in the presence and absence of polyethylene glycol (PEG).
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C.S. McSweeney et al. / Animal Feed Science and Technology 82 (1999) 227±241
Fig. 2. Dry matter (DM) and neutral detergent fibre (NDF) digestion during in vitro fermentation of calliandra
(open circle), L. leucocephala (solid circle), L. diversifolia (open square) and L. pallida (solid square) in the
presence and absence of polyethylene glycol (PEG). NDF values are expressed as % loss relative to loss in
uninoculated controls.
The addition of tannin binding compounds such as PEG and polyvinyl pyrrolidone (PVP)
provide a better indication of the effect of tannins on digestibility of nutrients. Earlier
studies (Makkar et al., 1995; Balogun et al., 1998) have shown that PEG can be added to
in vitro fermentations of tannin containing plant material to (1) demonstrate the
nutritional significance of tannins on DM and organic matter digestibility and (2)
measure the potential nutritive value of the forage provided the effect of tannins can be
neutralized. Tannin-binding agents, PEG and PVP have also been used to assess the effect
of tannins on extent of digestion (in vitro gas production) of browse trees (Khazaal et al.,
1996). However, those studies did not consider the effect of tannins on N digestibility.
This present study demonstrates that PEG can be used to assess N digestibility in
tanniniferous plants. Inclusion of PEG at 1 mg/ml culture fluid did not significantly affect
DM and N digestion (ammonia production) of lucerne which indicates no adverse effect
of the compound at that concentration on microbial metabolism. (Fig. 3). It would be
C.S. McSweeney et al. / Animal Feed Science and Technology 82 (1999) 227±241
Fig. 3. Dry matter (DM) digestion and ammonia production during in vitro fermentation of lucerne in the presence (solid circle) and absence (open circle) of
polyethylene glycol (PEG).
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C.S. McSweeney et al. / Animal Feed Science and Technology 82 (1999) 227±241
preferable to sterilize plant material prior to inoculation with rumen microorganisms for
in vitro fermentation to remove the effect of saphrophytic bacteria but this would affect
potential digestibility of organic matter and protein particularly in tannin containing
plants. However, in the current study volatile fatty acid concentrations in uninoculated
cultures were negligible thus indicating minimal growth of residual bacteria on
unsterilized plant material.
The Tilley and Terry (1963) method and nylon bag technique measure apparent
digestion of dry matter and nitrogen but cannot account for material that is solubilized or
escapes from the nylon bag and is not digested. In conventional feeds this is probably of
minor consequence but this study shows that true digestibility of tanniniferous forages is
likely to be inaccurate based solely on measurements of apparent digestibility. In this
study, there was a 1.7-fold difference in DM digestibility between the tanninferous plants
but a 3.7-fold difference in VFA production which suggests that a proportion of the
digested DM fraction may not be fermented in some tanniniferous forages. Also, the
increase in both DM and NDF digestibility that occurred with PEG inclusion may have
been underestimated since it has been shown that the addition of tannin-binding agents
can reduce the true digestibility value by binding to NDF and thus being included as
`artefact NDF and DM' (Makkar et al., 1995). These factors make it difficult to predict
true digestibility of DM when tannins are present. The poor correlation between DM loss
and fermentability of these plants is also demonstrated by the fact that VFA increased by
3.7±202% (Table 4) in response to PEG compared with apparent increases in DM
digestibility of 9.1±30.0% (Fig. 2). This suggests that the response in digestibility to PEG
is underestimated and that estimates of short chain volatile fatty acid (SCVFA)
production may provide a better indication of fermentable carbohydrate. Furthermore,
fermentation pattern (molar proportions of SCVFA) did not change substantially in
response to PEG except for an increase in BCVFA which is indicative of increased
fermentation of protein.
Apparent microbial digestion of N in calliandra, and L. diversifolia and L. pallida
ranged from 37 to 44% but virtually no ammonia accumulated in fermentations of these
plants without PEG. By contrast, ammonia was produced from digestion of L.
leucocephala even though apparent N digestion was only marginally higher (49.3 versus
44.3%) than L. diversifolia. Nitrogen digestion in lucerne was more predictable in that
high apparent digestion (78.3%) was associated with high ammonia concentration
(11.1%, Table 3). It appears, therefore, that ammonia production and apparent microbial
digestion of N are poorly correlated in these tanniniferous forages. This could be due to
the different rates and efficiencies of incorporation of ammonia N into microbial protein
as carbohydrate is fermented. In fact it is possible that protein can be fermented without
ammonia accumulation when the rate of ammonia N assimilation is equal to or greater
than the rate of production. However, the apparent negligible amount of ammonia
produced from C. calothyrsus, L. diversifolia and L. pallida is probably a true reflection
of protein digestion because BCVFA were not apparently produced (indicative of minimal
protein digestion) from these legumes, and DM digestibility and VFA production were
also low. Therefore, estimates of apparent digestion in these forages (C. calothyrsus, L.
diversifolia and L. pallida) may be inaccurate due to the formation of indigestible soluble
complexes formed between protein and tannin. The two-stage digestion technique of
C.S. McSweeney et al. / Animal Feed Science and Technology 82 (1999) 227±241
239
Tilley and Terry (1963) showed lower potential N digestibilities (74±80%) for all tannin
containing shrub legumes. However, a large portion of this digestible N (35±50%) was
only available following the acid pepsin digest compared with 9.7% for lucerne.
The acid pepsin digestible protein could be regarded as potential rumen by-pass protein
but a limitation of this in vitro technique is that the fate of protein in the intestines
following abomasal (acid-pepsin) digestion cannot be predicted. A recent report indicates
that dietary protein complexed with tannin is made available in the abomasum and
digested in the intestines but tannin released from protein-tannin complexes may react
with non-dietary protein as it passes along the intestines thus counteracting the benefits of
digestion of by-pass dietary protein (McNeill et al., 1999). The action of different plant
tannins on digestion post-ruminally is, therefore, of critical importance in determining the
potential nutritive value of the legume. L. leucocephala should probably be used as a
standard for tanniniferous tree legumes with desirable nutritive value and fermentation
characteristic. In the present study, rate and extent of N and DM digestion of L.
leucocephala was equivalent to lucerne when the tannin effect was removed and tannins
seemed to have less influence on digestion in this legume than the others tested. The high
nutritive value of L. leucocephala is demonstrated by the exceptional liveweight gains of
1.0 kg/day in cattle grazing the plant as part of native pasture system in tropical Australia
(Shelton and Jones, 1995).
Rates of degradation of DM and evolution of ammonia were measured to predict the
synchrony of release of energy and N in the rumen. When there is a balance between
protein and carbohydrate fermentation, ammonia and peptide N are incorporated more
efficiently into microbial protein and less ammonia accumulates. However, in this study,
energy (DM and NDF) appeared to be digested sooner than protein (ammonia evolution)
for all legumes and, thus, it is unlikely that synchrony of carbohydrate and protein
digestion had much influence on the extent of ammonia accumulation for each legume.
Therefore, ammonia production was probably a reliable indicator of rate and extent of
protein fermentation. Protein, in all tannin containing plants, was fermented to ammonia
at a slower rate than lucerne. This appears to be a desirable attribute of L. leucocephala
since excessive fermentation of protein would be reduced while adequate amounts of
ammonia (5 mM) would be available for microbial growth by cellulolytic bacteria.
However, rate and extent of protein digestion in the other tanniniferous legumes was
severely restricted by tannin which could reduce ruminal ammonia concentration to levels
( L. diversifolia and L.
pallida > C. calothyrsus. However, the effect of tannins on in vitro digestibility can be
affected by plant preservation methods such as freeze or oven drying (Ahn et al., 1989;
Siaw et al., 1993; Balogun et al., 1998) and thus nutritive value of preserved material may
not accurately represent fresh forages consumed by grazing ruminants. Therefore, the
techniques described in this paper should be adapted for studying the kinetics of
fermentation of fresh tanniniferous forages as this may provide a better indication of their
true nutritive value.
240
C.S. McSweeney et al. / Animal Feed Science and Technology 82 (1999) 227±241
Acknowledgements
This work was partly supported by the Australian Centre for International Agricultural
Research (ACIAR).
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82 (1999) 227±241
In vitro quality assessment of tannin-containing
tropical shrub legumes: protein and fibre digestion
C.S. McSweeneya,*, B. Palmerb, R. Buncha, D.O. Krausea
a
CSIRO Tropical Agriculture, Long Pocket Laboratories, Private Bag No. 3 PO,
Indooroopilly, 4068 Qld., Australia
b
CSIRO Tropical Agriculture, Davies Laboratory, Private Mail Bag, Aitkenvale,
Townsville, 4814 Qld., Australia
Received 29 March 1999; received in revised form 9 August 1999; accepted 26 August 1999
Abstract
In vitro techniques were evaluated to determine the nutritive value of a selection of tanniniferous
tree and shrub legumes (Calliandra calothyrsus, Leucaena leucocephala L. diversifolia and L.
pallida) compared with lucerne (Medicago sativa). Polyethylene glycol (PEG) was also added to
some in vitro fermentations (10 mg PEG/50 mg plant substrate) to assess the effects of tannins on
digestion of dry matter (DM), neutral detergent fibre (NDF) and nitrogen (N).
Total tannin content was poorly correlated with digestibility of dry matter and nitrogen. Apparent
digestibility of dry matter and N were significantly different between plants and ranked in the
following order; lucerne > L. leucocephala > L. diversifolia > L. pallida > C. calothyrsus. Ammonia was not produced (net accumulation) during 72 h fermentation of C. calothyrsus, L. diversifolia
and L. pallida although apparent nitrogen digestion in these plants ranged from 36.9 to 44.3%.
Acid-pepsin digestion resulted in a further 17±22% of dry matter digestion in the shrub legumes
compared with 8% in lucerne following 72 h fermentation. The amount of acid-pepsin digestible N
available was lowest for lucerne (8.4%) and highest for L. pallida (38.9%) with the other legumes
ranging from 26.5 to 36.8%.
The PEG addition caused a significant increase in rate and extent of DM and NDF digestibility
and ammonia production for all the tannin containing shrub legumes but not for lucerne. However,
DM loss and fermentability of these plants appeared to be poorly correlated because PEG addition
resulted in an increase in volatile fatty acid production ranging from 3.7 to 202% compared with an
increase in apparent DM digestibility of 9.1±30%.
It is concluded that in vitro evaluation of apparent DM and N digestibility of tannin containing
plants provides a poor indication of true digestion (fermentability) and thus measurements of
*
Corresponding author. Tel.: 61-7-3214-2820; fax: 61-7-3214-2880
E-mail address: [email protected] (C.S. McSweeney)
0377-8401/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved.
PII: S 0 3 7 7 - 8 4 0 1 ( 9 9 ) 0 0 1 0 3 - 0
228
C.S. McSweeney et al. / Animal Feed Science and Technology 82 (1999) 227±241
fermentation end-products (ammonia and short and branched chain fatty acids) should also be
undertaken to evaluate nutritive value. Also, addition of PEG to in vitro fermentations can be used
to determine the effect of tannins on N digestibility. Based on the in vitro methods of rumen
fermentation used in this study, nutritive value of the tanniniferous forages is ranked as follows: L.
leucocephala > L. diversifolia and L. pallida > C. calothyrsus. # 1999 Elsevier Science B.V. All
rights reserved.
Keywords: Tannins; Rumen; Polyethylene glycol; Tropical legumes; Nutritive value
1. Introduction
A major limitation to ruminant production in dry tropical regions is poor nutrition.
Annual growth rates of animals are restricted by low nitrogen and high fibre content of
native grasses and crop residues which form the basis of the diets in these climatic zones.
Supplementation of tropical roughages with leguminous fodder trees and shrubs (FTS) is
a promising way of alleviating nitrogen deficiencies.
Selecting plants for introduction usually involves ranking related accessions on
agronomic potential and nutritive value. Laboratory techniques are often used to
assess nutritive value and thus predict animal performance. The nylon bag technique
(Mehrez and Orskov, 1977) is used routinely to determine digestibility of substrate
in the rumen while in vitro fermentation techniques that measure gas production
(Menke et al., 1979) and apparent digestibility (Tilley and Terry, 1963) are also used
to determine nutritive value. However, FTS often contain anti-nutritive factors such as
condensed tannins which affect their nutritive value (Ahn et al., 1989). Apparent
digestibility can be confounded by the fact that soluble complexes form between
condensed tannin, protein and carbohydrate. It has been suggested that the gas production technique is more reliable than the nylon bag method for determining
nutritive value of feed containing anti-nutritive factors (Khazaal et al., 1993). However,
the two techniques are poorly correlated when determining the nutritive value of
tropical legumes of lower digestibility (Siaw et al., 1993). Furthermore, the gas
production technique is often employed to determine digestibility and infer nutritive
value without assessing nitrogen degradability even though tropical legumes are
commonly used in ruminant production systems to correct a primary nitrogen deficiency
(Siaw et al., 1993). Recent studies have also shown that the addition of polyethylene
glycol (PEG) or polyvinylpyrrolidone (PVP) to in vitro fermentations of tannin
containing forages will improve dry matter digestibility and thus provides insight
into the nutritional significance of the tannins (Makkar et al., 1995; Khazaal et al., 1996).
Curiously, the use of PEG or PVP to assess effect of tannins on in vitro N digestibility
has received little attention considering tanniniferous legumes are usually fed as
nitrogen supplements.
This paper reports on the results of a study aimed at (1) assessing the protein and
carbohydrate digestion in a range of tannin containing shrub legumes and (2) evaluating
in vitro methods that would accurately characterize the digestion of these forages thus
provide better predictions of their nutritional value.
C.S. McSweeney et al. / Animal Feed Science and Technology 82 (1999) 227±241
229
2. Materials and methods
2.1. Plant samples and preparation
The shrub legumes Calliandra calothyrsus CPI 115690, Leucaena leucocephala cv
Cunningham, L. diversifolia CPI 33820 and L. pallida CQ 3439 were grown and harvested
at Lansdown Research Station, 50 km south of Townsville, Queensland, Australia (198400 S,
1468510 E). The area was periodically grazed and then slashed to about 50 cm and allowed to
regrow to the next grazing period. After a period of regrowth, the first five fully expanded
leaves were collected as the standard sample from each plot. These tropical shrub legumes
were compared with the temperate legume Medicago sativa (lucerne). Adequate quantities
of samples were collected (approximately 1 kg) from each plant and oven dried at 658C
in a force draught oven for 48 h. Dried samples were ground in a Wiley mill fitted with a 1 mm
screen and stored in air-tight containers at ÿ708C until required. Temperature of drying
and method of drying (e.g., oven drying versus freeze drying) will affect digestibility
characteristics of tanniniferous forages but rank order of digestibility between different
plants is usually unaffected by drying technique (Palmer, unpublished data).
2.2. Anaerobic techniques and media
The anaerobic techniques of Hungate (1969) as modified by Bryant (1972) were used
for the growth of organisms and preparation of media. Finely milled oven-dried (658C)
plant material (50 mg) was weighed into sterile Balch tubes and autoclaved basal medium
(10 ml) was then dispensed under sterile conditions into the Balch tubes and immediately
stopped. Basal medium contained 15% (v/v) clarified rumen fluid, 20% (v/v) macro
mineral Solution A and 0.01% (v/v) micro mineral Solution B (Menke et al., 1979),
0.78% (w/v) NaHCO3, 0.25% (w/v) yeast extract, 0.03% (w/v) Na2S9H2O and 0.001%
(w/v) resazurin, (Caldwell and Bryant, 1966), 0.0001% (w/v) hemin and NH4Cl (3 mM).
Basal medium (pH 6.7) contained less than 0.5 mM ammonia N prior to addition of
NH4Cl. Polyethylene glycol (PEG, MW 4000; BDH Cat No. 29576 HM) was also added
in solution to culture tubes (10 mg/10 ml basal medium) as a treatment to counteract the
effect of tannins and thus determine the potential nutritive value of the plant if tannins
were not present. Fermentations were done in triplicate and uninoculated controls, with
and without PEG were routinely included. B-vitamins (Lowe et al., 1985) were added to
each tube of medium just prior to inoculation and incubations were at 398C.
2.3. In vitro digestibility
Culture tubes were inoculated with rumen fluid which was obtained from a steer fed a
diet comprising 70% rhodes grass (Chloris gayana) and 30% lucerne (Medicago sativa).
Rumen digesta was strained through muslin cloth, incubated anaerobically at 398C for
30 min in a Erlenmeyer flask so that the larger particulate matter would float to the
surface, while aliquots for inoculation were taken from the fluid phase beneath the top
layer. Culture tubes of plant substrates were then immediately inoculated with rumen
fluid (0.1 ml/10 ml culture) and sequential measurements of dry matter (DM) digestibility
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C.S. McSweeney et al. / Animal Feed Science and Technology 82 (1999) 227±241
and neutral detergent fibre (NDF) digestibility were made from tubes sacrificed at varying
intervals during a period of 72 h after inoculation. Percentage DM loss at 0 h represents
the amount of DM that is soluble in uninoculated media. Tubes were also sub-sampled
for measurements of ammonia production as an indicator of the rate of protein digestion.
In a separate experiment, the two stage technique (Tilley and Terry, 1963) for in vitro
digestion of forage plants was used to determine potential digestibility of dry matter, and
nitrogen and short chain fatty acid production, as well as acid-pepsin digestible protein
that remained following fermentation for 72 h. The methods were essentially the same as
those described by Tilley and Terry (1963) except approximately 0.5 g of plant material
was used as substrate and the medium, inoculum size and ratio of plant material to culture
fluid were as described above. Triplicate assays were performed at each time point and
measurements corrected for changes in uninoculated controls. Short chain fatty acid
production was not measured for lucerne.
2.4. Chemical analyses
The indophenol method for the determination of ammonia as described by Chaney and
Marbach (1962) was used to estimate the rate and amount of ammonia production in
cultures. Volatile fatty acids (VFA) in culture fluid were analysed by high performance
liquid chromatography using a Waters System (Waters, Milford, Mass., 01757) equipped
with an Aminex HPX-87 Cation exchange column (300 mm 7.8 mm) for organic acids
and a microguard column (Bio-Rad Laboratories, Hercules, CA, 94547) with a column
heater (Waters Model 1122/WTC-120). Organic acids and ethanol were eluted using a
mobile phase of 2.5% acetonitirile in 0.2% v/v phosphoric acid at a flow rate of 0.7 ml/
min., a column temperature of 608C and UV detection at 210 nm. NDF and ADF of plant
substrates was determined by the method of Van Soest et al. (1991) using the micro
analysis technique of Pell and Schofield (1993). Total condensed tannin was determined
by the butanol-HCl method (Terrill et al., 1992). Nitrogen content of samples was
measured using the Kjeldahl method (AOAC, 1980).
2.5. Statistical analysis
Statistical analysis of dry matter disappearance, ammonia production and N digestion
was by analysis of variance with differences determined by the method of least significant
difference at the 5% level (P < 0.05). A repeated measures design was used to determine
the effect of PEG treatment on digestion and ammonia production during 72 h
incubations of substrate where a series of measurements were made during the course
of the incubation. All statistical analyses were done with Statistica 5.0 software (StatSoft,
Inc., Tulsa, OK., 74104).
3. Results
The fibre, nitrogen and condensed tannin content of lucerne, Calliandra calothyrsus,
L. pallida, L. diversifolia and L. leucocephala are given in Table 1. Lucerne had a
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C.S. McSweeney et al. / Animal Feed Science and Technology 82 (1999) 227±241
Table 1
Neutral detergent fibre (NDF), acid detergent fibre (ADF), nitrogen (N) and condensed tannin content (%) of
lucerne, Calliandra calothyrsus, Leucaena leucocephala, L. diversifolia and L. pallidaa
NDF (% DM)
ADF (% DM)
N (% DM)b
N (% NDF)b
Lucerne
Calliandra
L. leucocephala
L. diversifolia
L. pallida
41.4 a
28.4 a
3.17
1.12
37.6 b
22.7 b
3.64
3.68
27.8 e
14.7 e
2.72
2.86
29.4 d
17.5 d
3.48
3.21
34.3 c
19.6 c
3.31
2.16
Condensed tannin (% DM)
Free
n.t.
Total
n.t.
4.78 b
5.71 b
3.32 c
3.84 d
6.57 a
7.70 a
4.00 c
4.72 c
Pooled sem
1.1
1.3
±
±
0.46
0.55
a
Values within rows with different letters differ significantly (P < 0.05).
Not statistically tested due to lack of replication.
Abbreviations: DM, dry matter; n.t., not tested.
b
significantly higher NDF content (P < 0.05) than the shrub legumes. Nitrogen content of
the five plants was relatively high and ranged from 2.7 to 3.6% DM. Nitrogen associated
with the NDF fraction appeared to be higher in the shrub legumes but this may have be
due to association of protein and tannin with NDF when the plant samples were originally
dried. Total condensed tannin concentrations in calliandra and the leucaenas ranged from
3.8 to 7.7% DM with greater than 84% of the tannin in a free form.
3.1. Apparent digestion of nitrogen and fermentation end-products
There were significant differences (P < 0.05) in digestion of nitrogen between the
tropical legumes tested but these were not correlated with tannin content (Table 2).
Lucerne had significantly higher water soluble N than the legumes containing tannin and
L. leucocephala had the lowest soluble N content. Apparent microbial digestion of
nitrogen was highest for lucerne followed by the shrub legumes in the following order;
Table 2
Nitrogen digestion from lucerne, Calliandra calothyrsus, Leucaena leucocephala, L. diversifolia and incubated
with rumen fluid (72 h) and followed by acid-pepsin treatmenta
b
N incubated (mg)
Water soluble N (% total)c
Microbial digested N (% total)d
Acid pepsin digested N (% total)e
Digestible N (% total)f
Lucerne
Calliandra
L. leucocephala L. diversifolia L. pallida
14.47 (0.s01)
47.9 (0.1) a
78.3 (0.7) a
8.4 (0.8) a
86.7 (0.1) a
16.52 (0.07)
20.9 (1.6) b
36.9 (0.1) e
36.8 (0.6) c
73.7 (0.3) e
12.45 (0.11)
15.3 (1.3) c
49.3 (0.1) b
26.5 (0.4) b
75.8 (0.4) c
15.87 (0.03)
22.3 (0.1) b
44.3 (0.1) c
36.1 (0.7) c
80.4 (0.2) b
15.16 (0.02)
21.3 (0.1) b
40.1 (0.1) d
38.9 (0.6)d
78.9 (0.0) d
a
Values within rows with different letters differ significantly (P < 0.05) and values in parenthesis are the
standard error of the mean.
b
mg N/500 mg plant substrate.
c
N solubilized in uninoculated control tubes.
d
N apparently digested during fermentation including water soluble N.
e
N apparently digested by acid pepsin treatment following microbial digestion.
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C.S. McSweeney et al. / Animal Feed Science and Technology 82 (1999) 227±241
L. leucocephala > L. diversifolia > L. pallida > C. calothyrsus. The amount of acid
pepsin digestible N available following a 72 h fermentation was lowest for lucerne (8.4%)
and highest for L. pallida (38.9%) with the other legumes ranging from 26.5 to 36.8%.
Apparent digestible N was highest for lucerne (86.7%) although the majority of total N
(73.7±80.4%) was apparently available for digestion in the shrub legumes even though
they contained substantial amounts of condensed tannin.
Ammonia was not apparently produced but rather consumed during 72 h fermentations
of C. calothyrsus, L. diversifolia and L. pallida (Table 3). Accumulation of ammonia
occurred in fermentations of lucerne (11.5 mM) and L. leucocephala (5.44 mM) which
both had the highest apparent microbial digestion of nitrogen.
Volatile fatty acid (VFA) concentrations in uninoculated cultures were negligible (data
not shown). Production of VFA were significantly reduced by the presence of tannin in
some of the legumes (Table 4). Inclusion of PEG in fermentations of the tanniniferous
plants resulted in a significant increase in volatile acid production with all plants except
L. leucocephala which did not change significantly. The increase in VFA production for
calliandra, L. diversifolia and L. pallida were 202, 37 and 129%, respectively. The PEG
addition caused a significant increase in acetate to propionate ratio only with L.
diversifolia. Branched chain volatile fatty acids (BCVFA) were not produced from
fermentations of calliandra, L. diversifolia and L. pallida without PEG but BCVFA
increased significantly (P < 0.05) with all tanniniferous plants in the presence of PEG.
The highest concentrations (P < 0.05) of BCVFA occurred with PEG additions to L.
diversifolia and L. pallida.
3.2. Apparent digestion of dry matter
Apparent dry matter digestion was significantly different between legume species
(Table 3). A large proportion of the dry matter (approximately 0.3) was solubilized
immediately from plant material in uninoculated cultures (Table 3). Apparent dry matter
digestion was highest for lucerne (63.1%) while L. leucocephala (51%) had the highest
digestibility of the tannin containing legumes. Acid/pepsin digestion resulted in digestion
Table 3
Dry matter loss and ammonia production from lucerne, Calliandra calothyrsus, Leucaena leucocephala, L.
diversifolia L. pallida incubated (72 h) with rumen fluida
Lucerne
Calliandra
L. leucocephala L. diversifolia
L. pallida
DM loss (%)
Uninoculated
Inoculated
Acid/pepsin digest
36.1 (0.6) b
63.1 (0.4) a
71.1 (0.2) a
28.9 (0.6) d
32.2 (1.2) e
53.8 (0.2) e
38.9 (0.5) a
51.0 (0.1) b
68.8 (0.3) b
31.2 (0.2) c
38.5 (0.9) d
56.7 (0.4) d
Ammonia (mM)
Uninoculated
Inoculated
4.46 (0.03) b
11.06 (0.06) a
a
3.33 (0.03) d
2.86 (0.04) c
3.83 (0.05) c
5.44 (0.16) b
30.7 (0.5) c
43.3 (1.0) c
60.4 (0.8) c
4.70 (0.09) a
2.18 (0.09) e
4.77 (0.11) a
2.50 (0.12) d
Values within rows with different letters differ significantly (P < 0.05) and values in parenthesis are the
standard error of the mean.
Calliandra
ÿPEG
Total VFA (mM)
6.96 a (0.07)
Acetate : propionate 2.59 a (0.14)
BCVFA (mM)
0a
L. leucocephala
L. diversifolia
PEG
ÿPEG
PEG
20.99 b (0.68)
2.63 a (0.06)
0.530 b (0.021)
25.41 c (0.46)
26.34 c (0.52)
2.15 b (0.01)
2.06 b (0.03)
0.253 c (0.021) 0.433 d (0.050)
ÿPEG
L. pallida
PEG
12.57 e (0.19) 28.82 f (0.30)
2.05 b (0.02) 2.38 c (0.04)
0a
0.753 e (0.028)
ÿPEG
PEG
19.23 d (1.09) 26.39 c (0.50)
2.0 b (0.05)
2.06 b (0.05)
0a
0.722 e (0.028)
a
Values within rows with different letters differ significantly (P < 0.05) and values in parenthesis are the standard error of the mean. BCVFA, branched chain volatile
fatty acids.
C.S. McSweeney et al. / Animal Feed Science and Technology 82 (1999) 227±241
Table 4
End products of fermentation from Calliandra calothyrsus, Leucaena leucocephala, L. diversifolia and L. pallida incubated with rumen fluid, plus or minus polyethylene
glycol (PEG)a
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C.S. McSweeney et al. / Animal Feed Science and Technology 82 (1999) 227±241
of a further 17±22 units of dry matter in the shrub legumes compared with 8 units in
lucerne (Table 3).
3.3. Kinetics of fermentation of protein and dry matter in lucerne and tannin containing
legumes
Addition of PEG caused a significant and marked increase (P < 0.05) in the rate and
extent of ammonia production for all tannin containing shrub legumes (Fig. 1). A period
of about 20±30 h elapsed before there was an increase in rate and extent of ammonia
production which represents the lag phase of growth of the small inoculum (0.1 ml/10 ml
culture) used to initiate the fermentation. Significant amounts of ammonia accumulated
in fermentations of L. leucocephala without PEG but ammonia was not apparently
produced in fermentations of L. pallida, L. diversifolia and C. calothyrsus under the same
conditions. Ammonia concentration in uninoculated controls (with and without PEG) of
shrub legumes varied less than 1 mM during 96 h of incubation (data not shown).
Production of ammonia (approximately 4 mM) from lucerne was not significantly
affected by PEG (Fig. 3). Ammonia concentration peaked earlier (52 versus 64±72 h;
P < 0.05) for lucerne compared with the shrub legumes treated with PEG. Initially net
ammonia production was negative for L. leucocephala before ammonia accumulated.
PEG treatment resulted in an increase in rate and extent of DM and NDF digestibility
for all the tannin containing shrub legumes with L. leucocephala being least affected (Fig.
2). The ranking for DM matter and NDF digestibilities of the shrub legumes with PEG
was C. calothyrsus < L. diversifolia, L. pallida < L. leucocephala (P < 0.05). Similarly,
without PEG, C. calothyrsus had the lowest (P < 0.05) ranking for DM digestibility (C.
calothyrsus < L. diversifolia < L. pallida < L. leucocephala) while L leucocephala ranked
highest (P < 0.05) for NDF digestibility (C. calothyrsus < L. diversifolia < L. pallida < L.
leucocephala). Dry matter digestibility of lucerne was not significantly affected by PEG
(Fig. 3). Lucerne had the highest DM digestibility (P < 0.05) of the legumes except for L.
leucocephala PEG which was not significantly different. It was calculated that 100%
conversion of nitrogen in the shrub legumes to ammonia would have resulted in ammonia
concentrations in the culture fluid (minus ammonia in media constituents) of calliandra
(13.0 mM), L. diversifolia (12.4 mM), L. pallida (11.9 mM) and L. leucocephala
(9.7 mM).
4. Discussion
The shrub and tree legumes evaluated had high crude protein and low fibre content
which would normally indicate their potential use as protein supplements. However, the
presence of condensed tannins affected the nutritive value of these plants to a varying
degree and confounded the interpretation of conventional in vitro digestibility
measurements. Correlations between concentration of tannin fractions in the plant and
reductions in digestibility are varied (Khazaal et al., 1994; Khazaal and Orskov, 1994;
Balogun et al., 1998) and thus tannin concentration is probably unreliable in predicting
effects on nutritive value which was also the case in the current study.
C.S. McSweeney et al. / Animal Feed Science and Technology 82 (1999) 227±241
Fig. 1. Ammonia production during in vitro fermentations of calliandra (open circle), L. leucocephala (solid circle), L. diversifolia (open square) and L. pallida (solid
square) in the presence and absence of polyethylene glycol (PEG).
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Fig. 2. Dry matter (DM) and neutral detergent fibre (NDF) digestion during in vitro fermentation of calliandra
(open circle), L. leucocephala (solid circle), L. diversifolia (open square) and L. pallida (solid square) in the
presence and absence of polyethylene glycol (PEG). NDF values are expressed as % loss relative to loss in
uninoculated controls.
The addition of tannin binding compounds such as PEG and polyvinyl pyrrolidone (PVP)
provide a better indication of the effect of tannins on digestibility of nutrients. Earlier
studies (Makkar et al., 1995; Balogun et al., 1998) have shown that PEG can be added to
in vitro fermentations of tannin containing plant material to (1) demonstrate the
nutritional significance of tannins on DM and organic matter digestibility and (2)
measure the potential nutritive value of the forage provided the effect of tannins can be
neutralized. Tannin-binding agents, PEG and PVP have also been used to assess the effect
of tannins on extent of digestion (in vitro gas production) of browse trees (Khazaal et al.,
1996). However, those studies did not consider the effect of tannins on N digestibility.
This present study demonstrates that PEG can be used to assess N digestibility in
tanniniferous plants. Inclusion of PEG at 1 mg/ml culture fluid did not significantly affect
DM and N digestion (ammonia production) of lucerne which indicates no adverse effect
of the compound at that concentration on microbial metabolism. (Fig. 3). It would be
C.S. McSweeney et al. / Animal Feed Science and Technology 82 (1999) 227±241
Fig. 3. Dry matter (DM) digestion and ammonia production during in vitro fermentation of lucerne in the presence (solid circle) and absence (open circle) of
polyethylene glycol (PEG).
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C.S. McSweeney et al. / Animal Feed Science and Technology 82 (1999) 227±241
preferable to sterilize plant material prior to inoculation with rumen microorganisms for
in vitro fermentation to remove the effect of saphrophytic bacteria but this would affect
potential digestibility of organic matter and protein particularly in tannin containing
plants. However, in the current study volatile fatty acid concentrations in uninoculated
cultures were negligible thus indicating minimal growth of residual bacteria on
unsterilized plant material.
The Tilley and Terry (1963) method and nylon bag technique measure apparent
digestion of dry matter and nitrogen but cannot account for material that is solubilized or
escapes from the nylon bag and is not digested. In conventional feeds this is probably of
minor consequence but this study shows that true digestibility of tanniniferous forages is
likely to be inaccurate based solely on measurements of apparent digestibility. In this
study, there was a 1.7-fold difference in DM digestibility between the tanninferous plants
but a 3.7-fold difference in VFA production which suggests that a proportion of the
digested DM fraction may not be fermented in some tanniniferous forages. Also, the
increase in both DM and NDF digestibility that occurred with PEG inclusion may have
been underestimated since it has been shown that the addition of tannin-binding agents
can reduce the true digestibility value by binding to NDF and thus being included as
`artefact NDF and DM' (Makkar et al., 1995). These factors make it difficult to predict
true digestibility of DM when tannins are present. The poor correlation between DM loss
and fermentability of these plants is also demonstrated by the fact that VFA increased by
3.7±202% (Table 4) in response to PEG compared with apparent increases in DM
digestibility of 9.1±30.0% (Fig. 2). This suggests that the response in digestibility to PEG
is underestimated and that estimates of short chain volatile fatty acid (SCVFA)
production may provide a better indication of fermentable carbohydrate. Furthermore,
fermentation pattern (molar proportions of SCVFA) did not change substantially in
response to PEG except for an increase in BCVFA which is indicative of increased
fermentation of protein.
Apparent microbial digestion of N in calliandra, and L. diversifolia and L. pallida
ranged from 37 to 44% but virtually no ammonia accumulated in fermentations of these
plants without PEG. By contrast, ammonia was produced from digestion of L.
leucocephala even though apparent N digestion was only marginally higher (49.3 versus
44.3%) than L. diversifolia. Nitrogen digestion in lucerne was more predictable in that
high apparent digestion (78.3%) was associated with high ammonia concentration
(11.1%, Table 3). It appears, therefore, that ammonia production and apparent microbial
digestion of N are poorly correlated in these tanniniferous forages. This could be due to
the different rates and efficiencies of incorporation of ammonia N into microbial protein
as carbohydrate is fermented. In fact it is possible that protein can be fermented without
ammonia accumulation when the rate of ammonia N assimilation is equal to or greater
than the rate of production. However, the apparent negligible amount of ammonia
produced from C. calothyrsus, L. diversifolia and L. pallida is probably a true reflection
of protein digestion because BCVFA were not apparently produced (indicative of minimal
protein digestion) from these legumes, and DM digestibility and VFA production were
also low. Therefore, estimates of apparent digestion in these forages (C. calothyrsus, L.
diversifolia and L. pallida) may be inaccurate due to the formation of indigestible soluble
complexes formed between protein and tannin. The two-stage digestion technique of
C.S. McSweeney et al. / Animal Feed Science and Technology 82 (1999) 227±241
239
Tilley and Terry (1963) showed lower potential N digestibilities (74±80%) for all tannin
containing shrub legumes. However, a large portion of this digestible N (35±50%) was
only available following the acid pepsin digest compared with 9.7% for lucerne.
The acid pepsin digestible protein could be regarded as potential rumen by-pass protein
but a limitation of this in vitro technique is that the fate of protein in the intestines
following abomasal (acid-pepsin) digestion cannot be predicted. A recent report indicates
that dietary protein complexed with tannin is made available in the abomasum and
digested in the intestines but tannin released from protein-tannin complexes may react
with non-dietary protein as it passes along the intestines thus counteracting the benefits of
digestion of by-pass dietary protein (McNeill et al., 1999). The action of different plant
tannins on digestion post-ruminally is, therefore, of critical importance in determining the
potential nutritive value of the legume. L. leucocephala should probably be used as a
standard for tanniniferous tree legumes with desirable nutritive value and fermentation
characteristic. In the present study, rate and extent of N and DM digestion of L.
leucocephala was equivalent to lucerne when the tannin effect was removed and tannins
seemed to have less influence on digestion in this legume than the others tested. The high
nutritive value of L. leucocephala is demonstrated by the exceptional liveweight gains of
1.0 kg/day in cattle grazing the plant as part of native pasture system in tropical Australia
(Shelton and Jones, 1995).
Rates of degradation of DM and evolution of ammonia were measured to predict the
synchrony of release of energy and N in the rumen. When there is a balance between
protein and carbohydrate fermentation, ammonia and peptide N are incorporated more
efficiently into microbial protein and less ammonia accumulates. However, in this study,
energy (DM and NDF) appeared to be digested sooner than protein (ammonia evolution)
for all legumes and, thus, it is unlikely that synchrony of carbohydrate and protein
digestion had much influence on the extent of ammonia accumulation for each legume.
Therefore, ammonia production was probably a reliable indicator of rate and extent of
protein fermentation. Protein, in all tannin containing plants, was fermented to ammonia
at a slower rate than lucerne. This appears to be a desirable attribute of L. leucocephala
since excessive fermentation of protein would be reduced while adequate amounts of
ammonia (5 mM) would be available for microbial growth by cellulolytic bacteria.
However, rate and extent of protein digestion in the other tanniniferous legumes was
severely restricted by tannin which could reduce ruminal ammonia concentration to levels
( L. diversifolia and L.
pallida > C. calothyrsus. However, the effect of tannins on in vitro digestibility can be
affected by plant preservation methods such as freeze or oven drying (Ahn et al., 1989;
Siaw et al., 1993; Balogun et al., 1998) and thus nutritive value of preserved material may
not accurately represent fresh forages consumed by grazing ruminants. Therefore, the
techniques described in this paper should be adapted for studying the kinetics of
fermentation of fresh tanniniferous forages as this may provide a better indication of their
true nutritive value.
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C.S. McSweeney et al. / Animal Feed Science and Technology 82 (1999) 227±241
Acknowledgements
This work was partly supported by the Australian Centre for International Agricultural
Research (ACIAR).
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