Directory UMM :Data Elmu:jurnal:S:Small Ruminant Research:Vol36.Issue3.2000:
Small Ruminant Research 36 (2000) 261±268
Dry matter intake, apparent digestibility and excretion of purine
derivatives in sheep fed tropical legume hay
J.F. Mupangwaa,*, N.T. Ngongonib, J.H. Toppsc, T. Acamovicd,
H. Hamudikuwandab, L.R. Ndlovub
a
Department of Agritex, Ministry of Agriculture, P.O. Box CY 639, Causeway, Harare, Zimbabwe
Department of Animal Science, University of Zimbabwe, P.O. Box MP 167, Mt. Pleasant, Harare, Zimbabwe
c
Department of Agriculture, University of Aberdeen, 581 King Street, Aberdeen, AB24 5UA, Scotland, UK
d
Department of Biochemistry and Nutrition, Scottish Agricultural College, Auchincruive, Ayr, KA6 5HW, Scotland, UK
b
Accepted 27 September 1999
Abstract
Four ruminally cannulated wethers (31 1.3 kg) were used in an experiment with a 4 4 Latin square design to estimate the
DM intake, apparent digestibility, nitrogen balance, rumen ammonia and microbial protein production. The sheep had ad libitum
access to either Cassia rotundifolia (Cassia), Lablab purpureus (Lablab), Macroptilium atropurpureum (Siratro) or Stylosanthes
guianensis (Stylo). Dry matter intake of cassia was lower (P < 0.001) than that of lablab, siratro and stylo hays. Organic matter
intake was greater (P < 0.001) for lablab, siratro and stylo hays than that of cassia. Dry matter digestibility was higher (P < 0.05)
for lablab hay, than that of cassia, siratro and stylo hays. The organic matter digestibility ranged from 0.579 for cassia hay to
0.617 for stylo hay and there were no differences (P > 0.05) among the legume hays. Nitrogen intake was highest (P < 0.05) in
sheep given stylo hay and least in sheep fed cassia hay. Animals given lablab, siratro and stylo hays had higher (P < 0.05) faecal
and urinary N compared to those on cassia hay. Rumen ammonia N concentration was highest (P < 0.05) in sheep given lablab
while sheep offered siratro and stylo had intermediate values, and least in animals fed cassia hay. The ammonia levels were
above the recommended optimal level of 50 mg N/l. The total purine derivative excretion in the urine and microbial N supply
was not different (P > 0.05) among treatments. From the presented ®ndings it is concluded that the intake and digestibility in
sheep of the four legume hays are variable and provide adequate rumen ammonia N for maximum rumen microbial growth making
then ideal protein supplements to ruminants fed low quality roughages. # 2000 Elsevier Science B.V. All rights reserved.
Keywords: Legumes; DM intake; Digestibility; Purine derivatives
1. Introduction
Tropical forages are low in protein and have high
cell wall contents resulting in low digestibilities
*
Corresponding author.
E-mail address: agriani@africaonline.co.zw (J.F. Mupangwa)
(Leng, 1990). Supplementation of tropical grasses
with legumes has been reported to result in increased
DM intake and to improve DM (Ndlovu and Buchanan-Smith, 1985). Herbaceous tropical forage
legumes such as Cassia rotundifolia(Cassia), Lablab
purpureus (Lablab), Macroptilium atropurpureum
(Siratro) and Stylosanthes guianensis (Stylo) have
0921-4488/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 9 2 1 - 4 4 8 8 ( 9 9 ) 0 0 1 2 5 - X
262
J.F. Mupangwa et al. / Small Ruminant Research 36 (2000) 261±268
great potential as protein supplements to low quality
roughages (Topps and Oliver, 1993; D'Mello and
Devendra, 1995). Studies on DM intake and digestibility of tropical legumes offered as sole diets are few
(Vera et al., 1989). Very little is known about cassia,
lablab, siratro and stylo regarding their intakes by
ruminants and the rumen microbial protein production
and levels of animal response they can elicit when
given to ruminants as sole diets.
The objective of this experiment was to investigate
the effect of feeding four legume hays as sole diets to
mature sheep on DM intake, apparent digestibility,
rumen ammonia levels, urinary excretion of purine
derivatives and rumen microbial protein production.
2. Materials and methods
2.1. Animals and diets
Four mature Dorper wethers (31 1.3 kg) were
used in this experiment. Each animal was surgically
®tted with a rubber rumen cannula (3 cm internal
diameter; Piggot and Maskew, Bulawayo, Zimbabwe).
The animals were housed in individual metabolism
crates, measuring 0.6 0.75 1.0 m and raised
0.5 m above the ¯oor in the Bioassay Laboratory,
Department of Animal Science at the University of
Zimbabwe. Each crate was equipped with a feed
trough and water was available all the time.The
legumes were grown each in rows 0.45 m apart in
plots measuring 15 50 m in the Marirangwe Small
Scale Commercial farming area near Harare, Zimbabwe, on sandy soils (pH 5.5 on CaCl2 scale),
receiving a rainfall of 750±1000 mm per year. The
legumes were harvested at 20 weeks after germination
and sun dried in the ®eld. During sun drying the hay
was turned twice a day for 4 days to ensure even
drying. Four diets comprising of hays only of cassia,
lablab, siratro and stylo were used in the experiment.
The animals were randomly allocated to the four
dietary treatments in a 4 4 Latin square design.
2.2. DM intake measurement
Before each experimental period lasted 21 days, the
animals were weighed and then adapted to the diets for
14 d followed by a 7-day of total collection period.
Animals had ad libitum access to legume hays. The
daily allocation of hay was offered in two equal
proportions at 08:00 and 16:00 h. A mineral and
vitamin mix (Hamish Cameron, Harare, Zimbabwe)
was added to the diet to ensure an adequate supply of
all minerals and vitamins. Water was always available.
Feed refusals were collected, weighed every morning
and bulked per treatment in each period before being
sub-sampled for laboratory analysis. Faeces were
collected daily, weighed and stored frozen while a
representative sub-sample was taken for dry matter
determination. Urine was collected daily in plastic
buckets that had 10 ml of concentrated sulphuric acid
as a preservative (Chen and Gomez, 1992) to maintain
a ®nal pH of 3 or below. A 10% sample of urine was
diluted by a factor of ®ve with distilled water to
prevent precipitation of purine derivatives (PD) and
was stored at ÿ208C for analysis of PD. A second
sample of urine was collected and kept frozen pending
total N analysis.
2.3. Rumen ammonia concentration
On the last day of each measurement period, 200±
250 ml of rumen liquor were collected through the
rumen cannula by a suction pump. The rumen liquor
samples were taken before feeding at 08:00 h and at
09:00, 11:00, 13:00, 15:00 and 17:00 h post-feeding.
Rumen liquor was strained through two layers of
cheese cloth and 200 ml acidi®ed with 1 ml of 25%
sulphuric acid and stored at ÿ208C for ammonia
analysis.
3. Laboratory analysis
The feed samples were analysed for neutral detergent ®bre (NDF), acid detergent ®bre (ADF) and acid
detergent lignin (ADL) according to the procedures of
Goering and Van Soest (1970) and for neutral detergent insoluble nitrogen (NDIN) and acid detergent
insoluble nitrogen (ADIN) according to standard procedures (AOAC, 1984). Feed, refusals and faeces were
analysed for nitrogen according to the procedures
described by the AOAC (1984). Feed samples were
analysed for condensed tannin content by the butanolHCl method as described by Makkar, 1995. Ash was
determined by ignition of a dry sample in a muf¯e
263
J.F. Mupangwa et al. / Small Ruminant Research 36 (2000) 261±268
furnace at 5508C for 24 h. Urine was analysed for N
(AOAC, 1984) and purine derivatives according to the
method of Chen and Gomez (1992).
4. Calculations and statistical analysis
From the PD excreted, the corresponding amount of
microbial purines (P mmol/day) absorbed by the
animal was estimated using the model described by
Chen and Gomez (1992). The supply of microbial N
was then calculated from P using the following factors: digestibility of microbial purines 0.83 and purine-N:total microbial N ratio of 0.116 : 1.00 (Chen
and Gomez, 1992):
P 70
0:83 0:116 1000
0:727 P
Microbial N supply g=day
The model (1) for DM, OM, N intakes and microbial
protein production tested the effects of period, animal
and legume type. The model (2) for rumen ammonia
concentration tested the effects of period, animal,
sampling time and legume type. Animal and period
were used as blocks. The models used for the analysis
were:
Yijk m Li Pj Ak eijk
(1)
Yijkl m Li Pj Ak T1 eijkl
(2)
Where: Yijk or Yijkl is the independent variable (e.g.
DM intake or ammonia concentration, respectively);
Li (i 1, 2, 3,4) is the ®xed effect of legume species,
Table 1
Chemical composition (g/kg DM) of forage legume hays
DM
OM
CP
NDF
ADF
ADL
NDIN (g/kgN)
ADIN (g/kg N)
Ca
P
Condensed tannin
Cassia
Lablab
Siratro
Stylo
956
889
182
419
275
75.7
91.6
72.5
16.8
1.40
29.5
918
841
162
473
294
64.2
146
48.7
20.8
1.10
16.9
930
800
229
512
355
72.3
139
59.5
22.2
1.30
12.4
924
860
253
593
416
116
246
102
22.1
1.30
15.6
Pj (j 1, 2, 3,4) is the effect of period; Ak is the effect
of the animal (k 1, 2, 3, 4), Tl is the effect of
sampling time (l 1, 2, 3, 4, 5, 6) and eijk is the
random error. The differences between treatment
means were assessed by the Tukey Studentized Range
test (SAS, 1990).
5. Results
5.1. Intake and apparent digestibility
The chemical composition of the four legume hays
used in the experiment is shown in Table 1. Among the
hays, siratro and stylo were higher in CP, NDF and
ADF than cassia and lablab. The intake and apparent
digestibility of the legume hays are given in Table 2.
Table 2
The intake and apparent digestibility of legume hays given to sheep as sole diets
DM intake
g/kgW0.75
Organic matter intake
g/kgW0.75
DOM intake (g/day)
Apparent Digestibility
DM
OM
a,b
Cassia
Lablab
Siratro
12.1b
48.2a
52.6a
50.9a
2.94
10.9b
82.9b
44.1a
377a
43.3a
329a
47.8a
384a
7.11
30.8
0.550b
0.579
0.638a
0.653
Means in the same row with different letters differ (P < 0.001).
SED±Standard error of difference.
0.581ab
0.588
Stylo
0.577ab
0.617
SED
0.034
0.037
264
J.F. Mupangwa et al. / Small Ruminant Research 36 (2000) 261±268
The DM and organic matter (OM) intakes of lablab,
siratro and stylo hays were greater (P < 0.001) than
that of cassia hay. The digestible organic matter intake
(DOMI) followed a trend similar to that of OM intake
but OM apparent digestibility was not (P > 0.05)
different among the four hays. There were signi®cant
differences in apparent digestibility of DM. Lablab
hay had a higher (P < 0.05) DM digestibility than
cassia hay. Siratro and stylo had DM digestibilities
that were not statistically different from either cassia
or lablab.
5.2. Nitrogen balance and purine derivative
excretion
Table 3 shows the results of nitrogen balance,
daily excretion of purine derivatives and microbial
N supply measurements. Nitrogen intake was highest
(P < 0.001) in animals given stylo hay followed by
animals given siratro hay then lablab hay and least for
cassia hay. Although there were signi®cant differences
in N intake between animals given lablab, siratro and
stylo diets, the faecal nitrogen outputs were similar
between the three diets and higher (P < 0.001) than
that of animals on cassia hay. As a percentage of total
N intake, faecal N excretion was greater (P < 0.05) in
animals offered cassia (26%) and lablab (29%) hays
than in those given siratro (19%) and stylo (15%)
hays.Animals given lablab, siratro and stylo hays had a
greater (P < 0.01) urinary nitrogen excretion than
those on cassia hay. Urinary N excretion as a multiple
of N intake was greater (P < 0.05) in animals offered
cassia (2.55), lablab (1.41) and siratro (1.15) hays
compared to those on stylo hay which had a value of
0.863. Animals on all treatments exhibited negative
nitrogen balance with those on lablab hay having a
greater (P < 0.05) negative nitrogen balance than
sheep on the other treatments (Table 3). Apparent N
digestibility of stylo hay was higher (P < 0.05) than
that of the other hays and the difference reached
signi®cance with cassia and lablab hays but not with
siratro hay.
Rumen ammonia-N concentration was highest
(P < 0.01) in rumen of sheep given lablab, intermediate (P < 0.05) in sheep offered siratro and stylo hays
and least in animals on cassia hay. The daily PD
excretion, the estimated purine absorption and the
calculated microbial N supply (g/d) were higher in
animals which had high DOMI but there was no
signi®cant (P > 0.05) difference between the four
Table 3
N balance (g/day), rumen ammonia (mg N/l), excretion of purine derivatives (mmol/day) and microbial nitrogen supply in sheep given forage
legume hays as sole dietsa
N intake
Faecal N
Urinary N
N retention
Apparent N digestibility
Rumen NH3-N
Purine derivatives:
Allantoin
Uric acid
Xanthine plus Hypoxanthine
Total PD
Absorbed purine
Microbial N supply
g/day
g/kg DOMRe
a,b,c,d
Cassia
Lablab
Siratro
Stylo
SED
5.75d
1.08b
10.2b
ÿ5.53ab
0.735bc
150b
16.5c
4.77a
23.1a
ÿ11.4b
0.706c
225a
25.1b
4.81a
29.1a
ÿ8.68ab
0.809ab
159b
27.7a
4.20a
23.9a
ÿ0.53a
0.848a
161b
0.83
0.55
3.54
4.04
0.05
15.9
2.89
0.04
0.23
3.12
3.71
3.29
0.12
0.29
3.64
4.33
3.58
0.07
0.33
3.96
4.72
4.73
0.25
0.24
5.23
6.22
1.16
0.13
0.06
1.14
1.36
2.70
50.1a
3.15
12.9b
3.43
16.0b
4.53
18.1b
0.98
8.69
Values in the same row with different superscripts differ (P < 0.05).
SED±Standard error of difference.
e
Digestible organic matter in the rumen (DOMR) 0.65 DOMI (g/day).
265
J.F. Mupangwa et al. / Small Ruminant Research 36 (2000) 261±268
legume hay diets. When expressed on the basis of
organic matter apparently digested in the rumen
(DOMR 0.65 DOMI), the calculated microbial
N supply ranged from 12.9 to 50.1 g N/kg DOMR
with no signi®cant differences between lablab, siratro
and stylo hay diets all of which gave lower (p < 0.001)
values than that for cassia hay diet.
6. Discussion
Siratro and stylo hays used in this experiment were
higher in CP and had lower NDF and ADF content
compared with the legume hays used by Chikumba
(1990). He reported CP content of 159 and 114 g/kg
DM, NDF values of 640 and 630 g/kg DM and ADF
content of 550 and 540 g/kg DM for siratro and stylo,
respectively. Bengaly (1996) reported CP content of
122 g/kg DM for lablab hay. The NDF and ADF
content of lablab hay used in this study were lower
than values reported by Bengaly (1996). The cassia
hay had higher CP and lower ADF content compared
to the cassia hay used by Ahn et al. (1988) in their
study. The difference between the hays used in this
study and those reported in other studies could be due
to variation in leaf content of the hays and stage of
growth at which the hay was harvested. In this study
the legumes were harvested at 20 weeks of growth and
®eld cured which could have resulted in some loss of
leaf material from the hay.
The DM intakes of lablab, siratro and stylo hays by
sheep were higher than for cassia hay. This difference
could be attributed to higher apparent DM digestibility
which may have resulted in lower rumen retention
time and greater turnover rate of particulate matter
from the rumen of sheep offered the three legume hays
compared to cassia hay. Cassia has been reported to
have a relatively low acceptability compared to that of
siratro in earlier studies (Clements, 1989). The low
intake of cassia in this study could be due to the
presence of anti-nutritive factors such as condensed
tannins. The hays used in this study had total condensed tannin content of 29.5 for cassia, 16.9 for
lablab, 12.4 for siratro and 15.6 g/kg DM for stylo.
Condensed tannins are reported to reduce voluntary
intake through astringency, an unpleasant puckering
sensation in the mouth brought about by complexing
of tannins with salivary glycoproteins (Kumar and
D'Mello, 1995). The extractable CT content of the
cassia hay was lower than values of 50±100 g/kg DM
(Barry and Duncan, 1984) and 65±90 g/kg DM (Barry
and Manley, 1986) reported to cause a reduction in
voluntary intake in sheep. With the reduced cassia hay
intake a reduced ruminal turnover and rate of digestion
are likely as suggested by Manyuchi, 1994. Any
reduction in ruminal turnover and rate of digestion
may be caused by an inhibition in microbial activity
(Salawu, 1997) and inhibition of microbial enzymes
(Muhammed et al., 1994) brought about by tannins in
the feed.
The intake of lablab hay of 0.624 kg/day was lower
than intake levels of 0.91 and 1.57 kg/day of leaf and
stem fractions, respectively, for L. purpureus cv Rongai reported by Hendricksen et al. (1981) with sheep
of 47 kg live mass. However, when expressed per
metabolic body mass, the intakes of lablab from this
study are similar to those obtained by Hendricksen
et al. (1981). The intake of cassia hay was lower than
values reported in other studies (Ahn et al., 1988) of
1.09 kg/day using sheep of similar weight to those
used in this study. The intakes of siratro and stylo hays
used in this study were lower than values reported for
similar hays made from younger material (Wanapat,
1987). The intakes of lablab, siratro and stylo were
similar to that of Neonotonia wightii hay of 50.2
g/kgW0.75/day reported by Vera et al. (1989) and
Minson (1977) reported a general, relationship
between intake and digestibility of a range of tropical
legumes to be as;
DM intake g=kg W0:75 =day 1:76 DM digestibility
ÿ 44:5 r 0:86
Using this relationship, the potential intake of the
legume hays used in this study would be 52.3, 68.1,
58.0 and 57.7 g/kg W0.75/day for cassia, lablab, siratro
and stylo hays. The values obtained in this study are
below these intakes. Also the intakes of the hays fell
below the standard intake of 80 g/kgW0.75/day suggested by Crampton et al. (1960) for forages with 0.70
digestibility possibly due to the effect of condensed
tannins.
The apparent DM digestibility coef®cients of the
four legume hay diets are comparable to that of N.
wightii hay which had an apparent DM digestibility of
0.53 (Vera et al., 1989). Siratro had a higher DM
266
J.F. Mupangwa et al. / Small Ruminant Research 36 (2000) 261±268
digestibility than that reported in earlier studies of
0.504±0.535 (Skerman et al., 1988) while that of stylo
fell within the range of reported values (Wanapat,
1987; Skerman et al., 1988). The DM digestibility
of cassia , 0.596, is consistent with the result of 0.555
reported by Ahn et al. (1988) and by Ahn, 1990 as
cited by Norton and Poppi, (1995) of 0.555±0.640.
The DM digestibility of lablab, 0.638, is higher than
values reported in previous studies (Hendricksen et al.,
1981; Skerman et al., 1988). Hendricksen et al. (1981)
reported DM digestibilities of 0.558 and 0.495,
respectively, for leaf and stem fractions of L. purpureus cv Rongai. Similarly apparent OM digestibility of
lablab was higher than reported in the literature (Hendricksen et al., 1981). The lack of signi®cant differences between cassia and the other hays in OM
digestibility may indicate that the high condensed
tannin content in cassia did not depress microbial
activity in the rumen and therefore its observed lower
intake could be due to an astringency effect in the
mouth.
According to the metabolisable energy (ME MJ/
day) requirements published by the MAFF (1984)
the maintenance requirements for sheep used in this
study, 30 kg live mass, is 5.1 MJ/day. Calculations
based on the AFRC (1993) equation (ME intake
MJ/day 0.0157 DOMI), the metabolisable energy
intakes of the sheep given the legume hays were,
respectively, 1.30, 5.92, 5.17 and 6.03 MJ/day for
animals on the cassia, lablab, siratro and stylo hay
diets. Animals on the cassia hay diet were clearly
in negative energy balance and needed to meet the
de®cit by mobilising body reserves. Animals on the
other treatments had suf®ciently high intakes to
meet ME requirements for maintenance and some
gain but showed negative N balance. This observation
can be attributed to the type of protein and its degradability and also possibly due to lack of synchrony
between N release through protein degradation and
energy availability as reported by Stern et al. (1994).
This results in reduced ef®ciency of rumen ammonia
utilisation by the microbes leading to excess ammonia
being absorbed into the blood stream and lost in urine
as urea (Robinson, 1997) which is consistent with
the increased urinary N excretions by animals on
these diets.
The rumen ammonia N concentrations were above
the recommended optimal level of 50 mg N/l (Satter
and Slyter, 1974) for maximum microbial growth.
Animals in this study given cassia hay although having
lower intakes than those reported by Ahn et al. (1988)
had similar rumen ammonia levels of 150 mg N/l to
those reported by Ahn et al. (1988) which had rumen
ammonia levels of 140 mg N/l. Although lablab hay
had a lower CP content than the other legume hays, it
resulted in the highest rumen ammonia concentration
indicating that its protein was more degradable than
that of the other legumes. The ®ndings from this study
are in agreement with those of Higgins et al. (1992)
and of Ahn et al. (1988) who reported rumen ammonia
levels of 307 and 140 mg N/l in rumen of sheep given
Aeschynomene americana and C. rotundifolia as sole
diets. The high rumen ammonia concentrations in the
absence of readily fermentable energy source, which
is likely with these diets, could result in energy limiting microbial growth and a signi®cant loss of legume
protein N in net transfer to the small intestines. Losses
of up to 43% of A. americana protein N in the rumen
have been reported and were found to be associated
with a decline in non-ammonia-nitrogen ¯ow to the
intestines (Higgins et al., 1992). Results from this
study indicate that similar losses might occur when the
legume hays are used as sole diets resulting in
increased urinary N excretion and reduced N retention
by the animals.
The PD excretions indicate that microbial protein
supply increased when dietary protein intake was
increased from 5.75 to 27.7 g/day for sheep on cassia
and siratro hays. This was probably due to an
increased intake of degradable N (Kolade and Ternouth, 1996) and more fermentable organic matter
being made available for microbial fermentation in the
rumen of sheep on the other hay diets compared to
those on the cassia hay diet. Increased OM intake and
of OM apparently digested in the rumen have been
reported to result in an increase in microbial production (Clark et al, 1992). The increased microbial
production can be attributed at least partially to the
larger amount of energy supplied by the larger quantity of OM fermented in the rumen (Chen et al., 1992;
Clark et al., 1992).
When expressed on the basis of DOMR, the ef®ciency of microbial nitrogen synthesis was suprisingly
greater in animals given cassia hay (47.9 g/kg DOMR)
than those given lablab, siratro and stylo hays for
which values of 12.9, 16.0 and 18.1 g/kg DOMR,
J.F. Mupangwa et al. / Small Ruminant Research 36 (2000) 261±268
respectively, were obtained. This result was unexpected and could indicate a variable contribution of
endogenous tissue nucleic acids. Animals on this diet
had on average total PD excretion of 0.49 mmol/kg
W0.75/day which was lower than the value of
0.6 mmol/kg W0.75/day below which large net endogenous contribution are evident (Dewhurst and Webster, 1992). Since DM and N intake were low on the
cassia hay diet, animals on this treatment would have
been mobilising tissue protein, as shown by a large
negative N retention, to meet their energy and protein
requirements. This may have resulted in an increased
endogenous purine contribution in the urine and an
overestimation of rumen microbial protein production. In view of the likelihood of an increased endogenous nucleic acid contribution in animals given
cassia hay, the derivation of microbial protein production based on purine derivatives described by Chen
and Gomez (1992) has to include endogenous contributions especially in animals given similar tropical
feeds.
The microbial N supply was lower while ef®ciency
of microbial N production was higher from cassia hay,
2.70 g/day and 47.9 g/kg DOMR, than the values of
12.0 g/day and 40 g/kg DOMR reported by Ahn et al.
(1988) and AFRC (1993) has reviewed data on the
ef®ciency of microbial protein production of different
feeds. The values for the ef®ciency vary widely ranging from 14 to 49 g/kg DOMR. The legume hays
other than cassia gave an ef®ciency at the lower end of
the range and this could be due to lack of suf®cient
energy to support microbial protein synthesis. However, caution should be used when microbial growth is
expressed on an ef®ciency basis because interpretation
of the data could be much different when total microbial N supply to the duodenum is evaluated (Stern
et al., 1994).
Using the ARC (1984) relationship between ME
intake (MJ/day) and microbial N production (Microbial N (g/day) ME intake 1.34), the calculated
microbial N supply (g/day), from the diets was 1.74,
7.93, 6.93 and 8.08 g/day for cassia, lablab, siratro and
stylo hays, respectively. The values obtained in this
study for lablab, siratro and stylo hays are lower, while
that for cassia hay is higher than these predicted
values. The true amino acid N in microbial N is
80% since about 15% consists of nucleic acid N
and 5% is composed of other non-protein nitrogen
267
forms (érskov and Miller, 1988). Microbial protein
has a digestibility value of 0.85 (AFRC, 1993), so the
microbial protein digested and absorbed from the
intestines was 11.5, 13.4, 14.6 and 19.3 g/day for
cassia, lablab, siratro and stylo diets, respectively.
Based on these calculations, lablab, siratro and stylo
contribute greater amounts of microbial amino acids
for tissue protein synthesis than does cassia.
7. Conclusions
This study demonstrates that an extensive loss
of legume protein as ammonia in the rumen occurs
in the absence of a readily fermentable energy source,
which can result in a reduction of undegraded
dietary protein ¯owing to post-ruminal sites. The
ef®ciency of microbial production from rations based
on forage legumes can be limited by a lack of readily
available energy source and the addition of such
energy sources may assist in giving increased ammonia utilisation and microbial protein production. The
legumes can be fed as protein supplements to ruminants consuming low quality tropical grasses and
crop residues.
Acknowledgements
The authors are indebted to the European Union
(Project Number ERBTS*CT930211) for funding this
research. We also wish to thank the laboratory staff
of the Department of Animal Science, University of
Zimbabwe for their technical assistance.
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Dry matter intake, apparent digestibility and excretion of purine
derivatives in sheep fed tropical legume hay
J.F. Mupangwaa,*, N.T. Ngongonib, J.H. Toppsc, T. Acamovicd,
H. Hamudikuwandab, L.R. Ndlovub
a
Department of Agritex, Ministry of Agriculture, P.O. Box CY 639, Causeway, Harare, Zimbabwe
Department of Animal Science, University of Zimbabwe, P.O. Box MP 167, Mt. Pleasant, Harare, Zimbabwe
c
Department of Agriculture, University of Aberdeen, 581 King Street, Aberdeen, AB24 5UA, Scotland, UK
d
Department of Biochemistry and Nutrition, Scottish Agricultural College, Auchincruive, Ayr, KA6 5HW, Scotland, UK
b
Accepted 27 September 1999
Abstract
Four ruminally cannulated wethers (31 1.3 kg) were used in an experiment with a 4 4 Latin square design to estimate the
DM intake, apparent digestibility, nitrogen balance, rumen ammonia and microbial protein production. The sheep had ad libitum
access to either Cassia rotundifolia (Cassia), Lablab purpureus (Lablab), Macroptilium atropurpureum (Siratro) or Stylosanthes
guianensis (Stylo). Dry matter intake of cassia was lower (P < 0.001) than that of lablab, siratro and stylo hays. Organic matter
intake was greater (P < 0.001) for lablab, siratro and stylo hays than that of cassia. Dry matter digestibility was higher (P < 0.05)
for lablab hay, than that of cassia, siratro and stylo hays. The organic matter digestibility ranged from 0.579 for cassia hay to
0.617 for stylo hay and there were no differences (P > 0.05) among the legume hays. Nitrogen intake was highest (P < 0.05) in
sheep given stylo hay and least in sheep fed cassia hay. Animals given lablab, siratro and stylo hays had higher (P < 0.05) faecal
and urinary N compared to those on cassia hay. Rumen ammonia N concentration was highest (P < 0.05) in sheep given lablab
while sheep offered siratro and stylo had intermediate values, and least in animals fed cassia hay. The ammonia levels were
above the recommended optimal level of 50 mg N/l. The total purine derivative excretion in the urine and microbial N supply
was not different (P > 0.05) among treatments. From the presented ®ndings it is concluded that the intake and digestibility in
sheep of the four legume hays are variable and provide adequate rumen ammonia N for maximum rumen microbial growth making
then ideal protein supplements to ruminants fed low quality roughages. # 2000 Elsevier Science B.V. All rights reserved.
Keywords: Legumes; DM intake; Digestibility; Purine derivatives
1. Introduction
Tropical forages are low in protein and have high
cell wall contents resulting in low digestibilities
*
Corresponding author.
E-mail address: agriani@africaonline.co.zw (J.F. Mupangwa)
(Leng, 1990). Supplementation of tropical grasses
with legumes has been reported to result in increased
DM intake and to improve DM (Ndlovu and Buchanan-Smith, 1985). Herbaceous tropical forage
legumes such as Cassia rotundifolia(Cassia), Lablab
purpureus (Lablab), Macroptilium atropurpureum
(Siratro) and Stylosanthes guianensis (Stylo) have
0921-4488/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 9 2 1 - 4 4 8 8 ( 9 9 ) 0 0 1 2 5 - X
262
J.F. Mupangwa et al. / Small Ruminant Research 36 (2000) 261±268
great potential as protein supplements to low quality
roughages (Topps and Oliver, 1993; D'Mello and
Devendra, 1995). Studies on DM intake and digestibility of tropical legumes offered as sole diets are few
(Vera et al., 1989). Very little is known about cassia,
lablab, siratro and stylo regarding their intakes by
ruminants and the rumen microbial protein production
and levels of animal response they can elicit when
given to ruminants as sole diets.
The objective of this experiment was to investigate
the effect of feeding four legume hays as sole diets to
mature sheep on DM intake, apparent digestibility,
rumen ammonia levels, urinary excretion of purine
derivatives and rumen microbial protein production.
2. Materials and methods
2.1. Animals and diets
Four mature Dorper wethers (31 1.3 kg) were
used in this experiment. Each animal was surgically
®tted with a rubber rumen cannula (3 cm internal
diameter; Piggot and Maskew, Bulawayo, Zimbabwe).
The animals were housed in individual metabolism
crates, measuring 0.6 0.75 1.0 m and raised
0.5 m above the ¯oor in the Bioassay Laboratory,
Department of Animal Science at the University of
Zimbabwe. Each crate was equipped with a feed
trough and water was available all the time.The
legumes were grown each in rows 0.45 m apart in
plots measuring 15 50 m in the Marirangwe Small
Scale Commercial farming area near Harare, Zimbabwe, on sandy soils (pH 5.5 on CaCl2 scale),
receiving a rainfall of 750±1000 mm per year. The
legumes were harvested at 20 weeks after germination
and sun dried in the ®eld. During sun drying the hay
was turned twice a day for 4 days to ensure even
drying. Four diets comprising of hays only of cassia,
lablab, siratro and stylo were used in the experiment.
The animals were randomly allocated to the four
dietary treatments in a 4 4 Latin square design.
2.2. DM intake measurement
Before each experimental period lasted 21 days, the
animals were weighed and then adapted to the diets for
14 d followed by a 7-day of total collection period.
Animals had ad libitum access to legume hays. The
daily allocation of hay was offered in two equal
proportions at 08:00 and 16:00 h. A mineral and
vitamin mix (Hamish Cameron, Harare, Zimbabwe)
was added to the diet to ensure an adequate supply of
all minerals and vitamins. Water was always available.
Feed refusals were collected, weighed every morning
and bulked per treatment in each period before being
sub-sampled for laboratory analysis. Faeces were
collected daily, weighed and stored frozen while a
representative sub-sample was taken for dry matter
determination. Urine was collected daily in plastic
buckets that had 10 ml of concentrated sulphuric acid
as a preservative (Chen and Gomez, 1992) to maintain
a ®nal pH of 3 or below. A 10% sample of urine was
diluted by a factor of ®ve with distilled water to
prevent precipitation of purine derivatives (PD) and
was stored at ÿ208C for analysis of PD. A second
sample of urine was collected and kept frozen pending
total N analysis.
2.3. Rumen ammonia concentration
On the last day of each measurement period, 200±
250 ml of rumen liquor were collected through the
rumen cannula by a suction pump. The rumen liquor
samples were taken before feeding at 08:00 h and at
09:00, 11:00, 13:00, 15:00 and 17:00 h post-feeding.
Rumen liquor was strained through two layers of
cheese cloth and 200 ml acidi®ed with 1 ml of 25%
sulphuric acid and stored at ÿ208C for ammonia
analysis.
3. Laboratory analysis
The feed samples were analysed for neutral detergent ®bre (NDF), acid detergent ®bre (ADF) and acid
detergent lignin (ADL) according to the procedures of
Goering and Van Soest (1970) and for neutral detergent insoluble nitrogen (NDIN) and acid detergent
insoluble nitrogen (ADIN) according to standard procedures (AOAC, 1984). Feed, refusals and faeces were
analysed for nitrogen according to the procedures
described by the AOAC (1984). Feed samples were
analysed for condensed tannin content by the butanolHCl method as described by Makkar, 1995. Ash was
determined by ignition of a dry sample in a muf¯e
263
J.F. Mupangwa et al. / Small Ruminant Research 36 (2000) 261±268
furnace at 5508C for 24 h. Urine was analysed for N
(AOAC, 1984) and purine derivatives according to the
method of Chen and Gomez (1992).
4. Calculations and statistical analysis
From the PD excreted, the corresponding amount of
microbial purines (P mmol/day) absorbed by the
animal was estimated using the model described by
Chen and Gomez (1992). The supply of microbial N
was then calculated from P using the following factors: digestibility of microbial purines 0.83 and purine-N:total microbial N ratio of 0.116 : 1.00 (Chen
and Gomez, 1992):
P 70
0:83 0:116 1000
0:727 P
Microbial N supply g=day
The model (1) for DM, OM, N intakes and microbial
protein production tested the effects of period, animal
and legume type. The model (2) for rumen ammonia
concentration tested the effects of period, animal,
sampling time and legume type. Animal and period
were used as blocks. The models used for the analysis
were:
Yijk m Li Pj Ak eijk
(1)
Yijkl m Li Pj Ak T1 eijkl
(2)
Where: Yijk or Yijkl is the independent variable (e.g.
DM intake or ammonia concentration, respectively);
Li (i 1, 2, 3,4) is the ®xed effect of legume species,
Table 1
Chemical composition (g/kg DM) of forage legume hays
DM
OM
CP
NDF
ADF
ADL
NDIN (g/kgN)
ADIN (g/kg N)
Ca
P
Condensed tannin
Cassia
Lablab
Siratro
Stylo
956
889
182
419
275
75.7
91.6
72.5
16.8
1.40
29.5
918
841
162
473
294
64.2
146
48.7
20.8
1.10
16.9
930
800
229
512
355
72.3
139
59.5
22.2
1.30
12.4
924
860
253
593
416
116
246
102
22.1
1.30
15.6
Pj (j 1, 2, 3,4) is the effect of period; Ak is the effect
of the animal (k 1, 2, 3, 4), Tl is the effect of
sampling time (l 1, 2, 3, 4, 5, 6) and eijk is the
random error. The differences between treatment
means were assessed by the Tukey Studentized Range
test (SAS, 1990).
5. Results
5.1. Intake and apparent digestibility
The chemical composition of the four legume hays
used in the experiment is shown in Table 1. Among the
hays, siratro and stylo were higher in CP, NDF and
ADF than cassia and lablab. The intake and apparent
digestibility of the legume hays are given in Table 2.
Table 2
The intake and apparent digestibility of legume hays given to sheep as sole diets
DM intake
g/kgW0.75
Organic matter intake
g/kgW0.75
DOM intake (g/day)
Apparent Digestibility
DM
OM
a,b
Cassia
Lablab
Siratro
12.1b
48.2a
52.6a
50.9a
2.94
10.9b
82.9b
44.1a
377a
43.3a
329a
47.8a
384a
7.11
30.8
0.550b
0.579
0.638a
0.653
Means in the same row with different letters differ (P < 0.001).
SED±Standard error of difference.
0.581ab
0.588
Stylo
0.577ab
0.617
SED
0.034
0.037
264
J.F. Mupangwa et al. / Small Ruminant Research 36 (2000) 261±268
The DM and organic matter (OM) intakes of lablab,
siratro and stylo hays were greater (P < 0.001) than
that of cassia hay. The digestible organic matter intake
(DOMI) followed a trend similar to that of OM intake
but OM apparent digestibility was not (P > 0.05)
different among the four hays. There were signi®cant
differences in apparent digestibility of DM. Lablab
hay had a higher (P < 0.05) DM digestibility than
cassia hay. Siratro and stylo had DM digestibilities
that were not statistically different from either cassia
or lablab.
5.2. Nitrogen balance and purine derivative
excretion
Table 3 shows the results of nitrogen balance,
daily excretion of purine derivatives and microbial
N supply measurements. Nitrogen intake was highest
(P < 0.001) in animals given stylo hay followed by
animals given siratro hay then lablab hay and least for
cassia hay. Although there were signi®cant differences
in N intake between animals given lablab, siratro and
stylo diets, the faecal nitrogen outputs were similar
between the three diets and higher (P < 0.001) than
that of animals on cassia hay. As a percentage of total
N intake, faecal N excretion was greater (P < 0.05) in
animals offered cassia (26%) and lablab (29%) hays
than in those given siratro (19%) and stylo (15%)
hays.Animals given lablab, siratro and stylo hays had a
greater (P < 0.01) urinary nitrogen excretion than
those on cassia hay. Urinary N excretion as a multiple
of N intake was greater (P < 0.05) in animals offered
cassia (2.55), lablab (1.41) and siratro (1.15) hays
compared to those on stylo hay which had a value of
0.863. Animals on all treatments exhibited negative
nitrogen balance with those on lablab hay having a
greater (P < 0.05) negative nitrogen balance than
sheep on the other treatments (Table 3). Apparent N
digestibility of stylo hay was higher (P < 0.05) than
that of the other hays and the difference reached
signi®cance with cassia and lablab hays but not with
siratro hay.
Rumen ammonia-N concentration was highest
(P < 0.01) in rumen of sheep given lablab, intermediate (P < 0.05) in sheep offered siratro and stylo hays
and least in animals on cassia hay. The daily PD
excretion, the estimated purine absorption and the
calculated microbial N supply (g/d) were higher in
animals which had high DOMI but there was no
signi®cant (P > 0.05) difference between the four
Table 3
N balance (g/day), rumen ammonia (mg N/l), excretion of purine derivatives (mmol/day) and microbial nitrogen supply in sheep given forage
legume hays as sole dietsa
N intake
Faecal N
Urinary N
N retention
Apparent N digestibility
Rumen NH3-N
Purine derivatives:
Allantoin
Uric acid
Xanthine plus Hypoxanthine
Total PD
Absorbed purine
Microbial N supply
g/day
g/kg DOMRe
a,b,c,d
Cassia
Lablab
Siratro
Stylo
SED
5.75d
1.08b
10.2b
ÿ5.53ab
0.735bc
150b
16.5c
4.77a
23.1a
ÿ11.4b
0.706c
225a
25.1b
4.81a
29.1a
ÿ8.68ab
0.809ab
159b
27.7a
4.20a
23.9a
ÿ0.53a
0.848a
161b
0.83
0.55
3.54
4.04
0.05
15.9
2.89
0.04
0.23
3.12
3.71
3.29
0.12
0.29
3.64
4.33
3.58
0.07
0.33
3.96
4.72
4.73
0.25
0.24
5.23
6.22
1.16
0.13
0.06
1.14
1.36
2.70
50.1a
3.15
12.9b
3.43
16.0b
4.53
18.1b
0.98
8.69
Values in the same row with different superscripts differ (P < 0.05).
SED±Standard error of difference.
e
Digestible organic matter in the rumen (DOMR) 0.65 DOMI (g/day).
265
J.F. Mupangwa et al. / Small Ruminant Research 36 (2000) 261±268
legume hay diets. When expressed on the basis of
organic matter apparently digested in the rumen
(DOMR 0.65 DOMI), the calculated microbial
N supply ranged from 12.9 to 50.1 g N/kg DOMR
with no signi®cant differences between lablab, siratro
and stylo hay diets all of which gave lower (p < 0.001)
values than that for cassia hay diet.
6. Discussion
Siratro and stylo hays used in this experiment were
higher in CP and had lower NDF and ADF content
compared with the legume hays used by Chikumba
(1990). He reported CP content of 159 and 114 g/kg
DM, NDF values of 640 and 630 g/kg DM and ADF
content of 550 and 540 g/kg DM for siratro and stylo,
respectively. Bengaly (1996) reported CP content of
122 g/kg DM for lablab hay. The NDF and ADF
content of lablab hay used in this study were lower
than values reported by Bengaly (1996). The cassia
hay had higher CP and lower ADF content compared
to the cassia hay used by Ahn et al. (1988) in their
study. The difference between the hays used in this
study and those reported in other studies could be due
to variation in leaf content of the hays and stage of
growth at which the hay was harvested. In this study
the legumes were harvested at 20 weeks of growth and
®eld cured which could have resulted in some loss of
leaf material from the hay.
The DM intakes of lablab, siratro and stylo hays by
sheep were higher than for cassia hay. This difference
could be attributed to higher apparent DM digestibility
which may have resulted in lower rumen retention
time and greater turnover rate of particulate matter
from the rumen of sheep offered the three legume hays
compared to cassia hay. Cassia has been reported to
have a relatively low acceptability compared to that of
siratro in earlier studies (Clements, 1989). The low
intake of cassia in this study could be due to the
presence of anti-nutritive factors such as condensed
tannins. The hays used in this study had total condensed tannin content of 29.5 for cassia, 16.9 for
lablab, 12.4 for siratro and 15.6 g/kg DM for stylo.
Condensed tannins are reported to reduce voluntary
intake through astringency, an unpleasant puckering
sensation in the mouth brought about by complexing
of tannins with salivary glycoproteins (Kumar and
D'Mello, 1995). The extractable CT content of the
cassia hay was lower than values of 50±100 g/kg DM
(Barry and Duncan, 1984) and 65±90 g/kg DM (Barry
and Manley, 1986) reported to cause a reduction in
voluntary intake in sheep. With the reduced cassia hay
intake a reduced ruminal turnover and rate of digestion
are likely as suggested by Manyuchi, 1994. Any
reduction in ruminal turnover and rate of digestion
may be caused by an inhibition in microbial activity
(Salawu, 1997) and inhibition of microbial enzymes
(Muhammed et al., 1994) brought about by tannins in
the feed.
The intake of lablab hay of 0.624 kg/day was lower
than intake levels of 0.91 and 1.57 kg/day of leaf and
stem fractions, respectively, for L. purpureus cv Rongai reported by Hendricksen et al. (1981) with sheep
of 47 kg live mass. However, when expressed per
metabolic body mass, the intakes of lablab from this
study are similar to those obtained by Hendricksen
et al. (1981). The intake of cassia hay was lower than
values reported in other studies (Ahn et al., 1988) of
1.09 kg/day using sheep of similar weight to those
used in this study. The intakes of siratro and stylo hays
used in this study were lower than values reported for
similar hays made from younger material (Wanapat,
1987). The intakes of lablab, siratro and stylo were
similar to that of Neonotonia wightii hay of 50.2
g/kgW0.75/day reported by Vera et al. (1989) and
Minson (1977) reported a general, relationship
between intake and digestibility of a range of tropical
legumes to be as;
DM intake g=kg W0:75 =day 1:76 DM digestibility
ÿ 44:5 r 0:86
Using this relationship, the potential intake of the
legume hays used in this study would be 52.3, 68.1,
58.0 and 57.7 g/kg W0.75/day for cassia, lablab, siratro
and stylo hays. The values obtained in this study are
below these intakes. Also the intakes of the hays fell
below the standard intake of 80 g/kgW0.75/day suggested by Crampton et al. (1960) for forages with 0.70
digestibility possibly due to the effect of condensed
tannins.
The apparent DM digestibility coef®cients of the
four legume hay diets are comparable to that of N.
wightii hay which had an apparent DM digestibility of
0.53 (Vera et al., 1989). Siratro had a higher DM
266
J.F. Mupangwa et al. / Small Ruminant Research 36 (2000) 261±268
digestibility than that reported in earlier studies of
0.504±0.535 (Skerman et al., 1988) while that of stylo
fell within the range of reported values (Wanapat,
1987; Skerman et al., 1988). The DM digestibility
of cassia , 0.596, is consistent with the result of 0.555
reported by Ahn et al. (1988) and by Ahn, 1990 as
cited by Norton and Poppi, (1995) of 0.555±0.640.
The DM digestibility of lablab, 0.638, is higher than
values reported in previous studies (Hendricksen et al.,
1981; Skerman et al., 1988). Hendricksen et al. (1981)
reported DM digestibilities of 0.558 and 0.495,
respectively, for leaf and stem fractions of L. purpureus cv Rongai. Similarly apparent OM digestibility of
lablab was higher than reported in the literature (Hendricksen et al., 1981). The lack of signi®cant differences between cassia and the other hays in OM
digestibility may indicate that the high condensed
tannin content in cassia did not depress microbial
activity in the rumen and therefore its observed lower
intake could be due to an astringency effect in the
mouth.
According to the metabolisable energy (ME MJ/
day) requirements published by the MAFF (1984)
the maintenance requirements for sheep used in this
study, 30 kg live mass, is 5.1 MJ/day. Calculations
based on the AFRC (1993) equation (ME intake
MJ/day 0.0157 DOMI), the metabolisable energy
intakes of the sheep given the legume hays were,
respectively, 1.30, 5.92, 5.17 and 6.03 MJ/day for
animals on the cassia, lablab, siratro and stylo hay
diets. Animals on the cassia hay diet were clearly
in negative energy balance and needed to meet the
de®cit by mobilising body reserves. Animals on the
other treatments had suf®ciently high intakes to
meet ME requirements for maintenance and some
gain but showed negative N balance. This observation
can be attributed to the type of protein and its degradability and also possibly due to lack of synchrony
between N release through protein degradation and
energy availability as reported by Stern et al. (1994).
This results in reduced ef®ciency of rumen ammonia
utilisation by the microbes leading to excess ammonia
being absorbed into the blood stream and lost in urine
as urea (Robinson, 1997) which is consistent with
the increased urinary N excretions by animals on
these diets.
The rumen ammonia N concentrations were above
the recommended optimal level of 50 mg N/l (Satter
and Slyter, 1974) for maximum microbial growth.
Animals in this study given cassia hay although having
lower intakes than those reported by Ahn et al. (1988)
had similar rumen ammonia levels of 150 mg N/l to
those reported by Ahn et al. (1988) which had rumen
ammonia levels of 140 mg N/l. Although lablab hay
had a lower CP content than the other legume hays, it
resulted in the highest rumen ammonia concentration
indicating that its protein was more degradable than
that of the other legumes. The ®ndings from this study
are in agreement with those of Higgins et al. (1992)
and of Ahn et al. (1988) who reported rumen ammonia
levels of 307 and 140 mg N/l in rumen of sheep given
Aeschynomene americana and C. rotundifolia as sole
diets. The high rumen ammonia concentrations in the
absence of readily fermentable energy source, which
is likely with these diets, could result in energy limiting microbial growth and a signi®cant loss of legume
protein N in net transfer to the small intestines. Losses
of up to 43% of A. americana protein N in the rumen
have been reported and were found to be associated
with a decline in non-ammonia-nitrogen ¯ow to the
intestines (Higgins et al., 1992). Results from this
study indicate that similar losses might occur when the
legume hays are used as sole diets resulting in
increased urinary N excretion and reduced N retention
by the animals.
The PD excretions indicate that microbial protein
supply increased when dietary protein intake was
increased from 5.75 to 27.7 g/day for sheep on cassia
and siratro hays. This was probably due to an
increased intake of degradable N (Kolade and Ternouth, 1996) and more fermentable organic matter
being made available for microbial fermentation in the
rumen of sheep on the other hay diets compared to
those on the cassia hay diet. Increased OM intake and
of OM apparently digested in the rumen have been
reported to result in an increase in microbial production (Clark et al, 1992). The increased microbial
production can be attributed at least partially to the
larger amount of energy supplied by the larger quantity of OM fermented in the rumen (Chen et al., 1992;
Clark et al., 1992).
When expressed on the basis of DOMR, the ef®ciency of microbial nitrogen synthesis was suprisingly
greater in animals given cassia hay (47.9 g/kg DOMR)
than those given lablab, siratro and stylo hays for
which values of 12.9, 16.0 and 18.1 g/kg DOMR,
J.F. Mupangwa et al. / Small Ruminant Research 36 (2000) 261±268
respectively, were obtained. This result was unexpected and could indicate a variable contribution of
endogenous tissue nucleic acids. Animals on this diet
had on average total PD excretion of 0.49 mmol/kg
W0.75/day which was lower than the value of
0.6 mmol/kg W0.75/day below which large net endogenous contribution are evident (Dewhurst and Webster, 1992). Since DM and N intake were low on the
cassia hay diet, animals on this treatment would have
been mobilising tissue protein, as shown by a large
negative N retention, to meet their energy and protein
requirements. This may have resulted in an increased
endogenous purine contribution in the urine and an
overestimation of rumen microbial protein production. In view of the likelihood of an increased endogenous nucleic acid contribution in animals given
cassia hay, the derivation of microbial protein production based on purine derivatives described by Chen
and Gomez (1992) has to include endogenous contributions especially in animals given similar tropical
feeds.
The microbial N supply was lower while ef®ciency
of microbial N production was higher from cassia hay,
2.70 g/day and 47.9 g/kg DOMR, than the values of
12.0 g/day and 40 g/kg DOMR reported by Ahn et al.
(1988) and AFRC (1993) has reviewed data on the
ef®ciency of microbial protein production of different
feeds. The values for the ef®ciency vary widely ranging from 14 to 49 g/kg DOMR. The legume hays
other than cassia gave an ef®ciency at the lower end of
the range and this could be due to lack of suf®cient
energy to support microbial protein synthesis. However, caution should be used when microbial growth is
expressed on an ef®ciency basis because interpretation
of the data could be much different when total microbial N supply to the duodenum is evaluated (Stern
et al., 1994).
Using the ARC (1984) relationship between ME
intake (MJ/day) and microbial N production (Microbial N (g/day) ME intake 1.34), the calculated
microbial N supply (g/day), from the diets was 1.74,
7.93, 6.93 and 8.08 g/day for cassia, lablab, siratro and
stylo hays, respectively. The values obtained in this
study for lablab, siratro and stylo hays are lower, while
that for cassia hay is higher than these predicted
values. The true amino acid N in microbial N is
80% since about 15% consists of nucleic acid N
and 5% is composed of other non-protein nitrogen
267
forms (érskov and Miller, 1988). Microbial protein
has a digestibility value of 0.85 (AFRC, 1993), so the
microbial protein digested and absorbed from the
intestines was 11.5, 13.4, 14.6 and 19.3 g/day for
cassia, lablab, siratro and stylo diets, respectively.
Based on these calculations, lablab, siratro and stylo
contribute greater amounts of microbial amino acids
for tissue protein synthesis than does cassia.
7. Conclusions
This study demonstrates that an extensive loss
of legume protein as ammonia in the rumen occurs
in the absence of a readily fermentable energy source,
which can result in a reduction of undegraded
dietary protein ¯owing to post-ruminal sites. The
ef®ciency of microbial production from rations based
on forage legumes can be limited by a lack of readily
available energy source and the addition of such
energy sources may assist in giving increased ammonia utilisation and microbial protein production. The
legumes can be fed as protein supplements to ruminants consuming low quality tropical grasses and
crop residues.
Acknowledgements
The authors are indebted to the European Union
(Project Number ERBTS*CT930211) for funding this
research. We also wish to thank the laboratory staff
of the Department of Animal Science, University of
Zimbabwe for their technical assistance.
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