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Deactivation of condensed tannins in Acacia cyanophylla Lindl.
foliage by polyethylene glycol in feed blocks
Effect on feed intake, diet digestibility, nitrogen balance,
microbial synthesis and growth by sheep
a ,
*
a b cH. Ben Salem
, A. Nefzaoui , Lamia Ben Salem , J.L. Tisserand
a
´
INRA-Tunisie, Laboratoire de Nutrition Animale, rue Hedi Karray, 2049 Ariana, Tunisia b
ˆ
Office de l’Elevage et des Paturages, rue Alain Savary, 1002 Tunis, Tunisia c
´
ENESAD-Unite de Formation des I.T.A. Formation Initiale, BP 1607, 21036 Dijon, France
Abstract
Unmolassed feed blocks were used as supplement of Acacia cyanophylla Lindl. (acacia)-based diets. They were made, on the basis of crude weight, with olive cake (42.2%), wheat bran (26.7%), wheat flour (10.7%), quicklime (10.7%), urea (4.4%), salt (4.4%) and a mineral and vitamin supplement (0.9%). Polyethylene glycol (PEG, mol. wt. 4000) was introduced in these feed blocks at five increasing levels (0 (control), 6, 12, 18 and 24% on the basis of ingredient mixture before moulding) to deactivate acacia condensed tannins. The five dietary treatments (levels of PEG) were evaluated simultaneously
and each diet was tested on 6 yearling male Barbarine sheep (average initial weight 1961.5 kg) and five Barbarine rams
(average initial weight 4763.0 kg). Animals were held in individual pens and metabolism cages, respectively. They were
adapted to experimental conditions for 10 and 21 days, respectively before the commencement of the measurement periods. Freshly cut foliage of acacia was distributed ad libitum and the feed block was permanently available in the trough. Feed intake and daily gain were measured on yearlings during a 60-day period. Feed intake, in vivo diet digestibilities, nitrogen balance and microbial nitrogen synthesis using urinary excretion of allantoin were measured on rams during 10 consecutive
0.75
days. Dry matter intake (DMI) of feed blocks ranged from 13.7 and 16.5 g / kg W for yearling sheep and from 15.5 and
0.75
24.3 g / kg W for rams. PEG had no effect on feed block intake. Acacia DMI by sheep given feed blocks without PEG
0.75
was low (37.7 and 39.9 g / kg W , respectively for yearlings and rams). Supplementation with feed blocks containing either
6, 12, 18 or 24% of PEG increased acacia DMI by 25, 50, 56 and 45%, respectively for yearling sheep and 22, 25, 56 and 71%, respectively for rams. Providing PEG in feed blocks at a rate of 6, 12, 18 or 24% increased digestible organic matter
0.75
intake (DOMi) by 3.4, 4.3, 9.4 and 18.2 g / kg W and digestible crude protein intake by 17, 12, 23 and 36 g / day,
respectively. The higher the rate of PEG introduced in feed blocks the greater N retention and urinary excretion of allantoin. Estimated microbial N (g / kg DOMi) was increased by 37, 94, 135 and 153% with feed blocks containing 6, 12, 18 and 24% of PEG, respectively. The clear-cut improvement of the nutritive value of acacia-based diets following PEG supply resulted in a linear increase of the growth rate of sheep. The optimum responses of acacia intake, nitrogen retention, microbial N yield and daily gain were obtained in sheep given feed blocks with 18% of PEG which corresponded to a PEG consumption of about 23 g / day. It was concluded that feed blocks may be used as carrier of PEG for improving the nutritive value of
tannin-rich diets. 2000 Elsevier Science B.V. All rights reserved.
*Corresponding author. Tel.: 1216-1-230-024; fax: 1216-1-752-897. E-mail address: bensalem.hichem@iresa.agrinet.tn (H. Ben Salem)
Keywords: Acacia cyanophylla; Condensed tannins; Polyethylene glycol; Feed blocks; Nutritive value; Sheep
1. Introduction block making is an easy technique which is adapted
to small farmer conditions. Owing to these advan-Acacia cyanophylla Lindl. (syn. A. saligna) tages, including PEG in feed blocks may be an other (acacia) a multipurpose tree widespread in Tunisia way of improving the nutritive value of tannin-mainly in arid and semi-arid zones. It is used either containing shrubs. PEG-containing feed blocks will for soil fixation, as a fuel, an ornamental plant or as a be nibbled periodically by the animal and their use valuable source of forage available throughout the may in this case synchronise the consumption of year. The large biomass produced by this plant PEG and shrub tannins. A question arose as to species may alleviate forage shortage in harsh con- whether such synchronisation could lead or not to a ditions. However, national (Ben Salem et al., further improvement of acacia nutritive value. In the 1997a,b) and international studies (Degen et al., current paper we assessed the effect of supplying 1995, 1997) showed that the nutritive value of acacia increasing amounts of PEG in feed blocks on feed foliage is low. Crude protein and condensed tannins intake, diet digestibility, nitrogen balance, microbial contents in acacia foliage are relatively high. These synthesis and growth by sheep given Acacia secondary compounds form complexes with acacia cyanophylla Lindl. foliage.
proteins rendering them unavailable for rumen micro-organisms. Performance of sheep and goats
receiving acacia foliage are, therefore, often low. 2. Material and methods
Ben Salem et al. (1999a) showed that polyethylene
glycol (PEG, mol. wt. 4000) inactivated acacia 2.1. Study area condensed tannins, thus improved microbial protein
synthesis and growth of sheep. Recently, Ben Salem The experiment was carried out during the spring et al. (1999b) compared different means of supplying months (March–May) of 1996 at the ‘Office de
ˆ
PEG for sheep given acacia-based diets and con- l’Elevage et des Paturages’ (OEP) farm at Jebibina cluded that including PEG in concentrate improved (Central Tunisia). The region is semi-arid with an acacia intake and digestion more than inclusion in average annual rainfall of about 390 mm. Tempera-drinking water or spraying it as a solution on acacia ture in the barn recorded during the experimental foliage. In arid and semi-arid zones of Tunisia, period ranged between 20 and 298C, the highest farmers are obliged to use large quantities of concen- values being obtained in May.
trate feeds to supplement sheep and goat flocks on
low quality roughages and pastures. Their aim is to 2.2. Plant material and feed block making reduce the use of these expensive supplements by
Table 1
Proportion, role and main nutrients supplied by feed block ingredients
a a
Ingredients CW (%) DM (%) Role Main nutrients supplied
b
Olive cake 42.2 38 – Fibre
Wheat bran 26.7 28 Absorbent agent Energy, nitrogen, phosphorus
Wheat flour 10.7 11 Binder Energy
Quicklime 10.7 12 Binder Calcium
Urea 4.4 5 Preservative Nitrogen
Salt 4.4 5 Preservative Sodium
c
MVS 0.9 1 – Minerals and vitamins
a
Proportions of block ingredients are expressed as percentage of crude weight of ingredient mixture (CW (%)) or percentage of DM (DM (%)) of ingredient mixture before water addition and block moulding.
b
Solvent extracted olive cake (centrifuge system). c
MVS, commercial mineral and vitamin supplement.
produced by hand-mixing. The nature, proportion, made. The first steel disk was put in the bottom of and main role of each ingredient are given in Table the mould, which was then filled with the block 1. Five rates of PEG (mol. wt. 4000) were included mixture. The second disk was put on top of the in feed blocks: 0 (control), 6, 12, 18 or 24% of mixture and then pressed using a manual pressing ingredient mixture before water supply and mould- tool. The upper disk was removed and the block ing. The mixing procedure used was as follows: tipped from the mould. These blocks were allowed to dry in a shady site for few days, 7–10 days in spring • weigh the amounts of ingredients, and summer seasons. The final weight of the blocks • dissolve the quicklime, urea, salt and PEG in ranged from 4.0 to 4.5 kg.
water in separate vessels, The blocks were only offered to the sheep when • mix these solutions together (final solution), they met the characteristics of hardness, resistance, • mix olive cake, wheat bran and wheat flour absence of urea and lime lumps defined by Ben
residue using a shovel, Salem et al. (1997c).
• while mixing solid ingredients, add gradually the The rates of PEG (6, 12, 18 and 24%) used in this solution of quicklime, urea, salt and PEG, experiment were chosen on the assumption that the • add from time to time a small volume of water average feed block and acacia intakes would be and continue mixing, approximately 150 and 350 g of dry matter / day per • test from time to time if the mixture is ready for head, respectively. The 6, 12, 18 and 24% rates of moulding. PEG corresponded to ratios of PEG:condensed tan-nins of 0.5, 1.0, 2.0 and 3.0, respectively. Earlier This was done by taking a small amount of the studies (Ben Salem et al., 1997a,b) showed that the mixture and press it in the fingers. If the compressed methanol extractable condensed tannins content of mixture maintains its compacted form and a very acacia foliage would be approximately 4.0 g equiva-small amount of liquid leaks between fingers, it is lent catechin / 100 g DM.
ready for moulding. The volume of water needed
was approximately 40% (volume / weight of ingredi- 2.3. Animals and diets ent mixture before moulding).
feed blocks were continuously available in the urine were collected daily from each animal and stored at 258C. At the end of the measurement troughs.
period, feed refusals faeces and urine samples were Thirty Barbarine yearling male sheep aged ca. 4
pooled for each animal and representative samples months were used in a 70-day growth trial in which
were taken for analyses. Feeds offered, refusals and the first 10 days were used for adaptation to the
faeces samples were dried in a forced air-oven (508C experiment conditions. The initial mean liveweight
for 48 h) and ground to pass a 1 mm-screen and of the sheep was 1961.5 kg. Twenty five Barbarine
stored until needed. Urine samples were frozen at rams of 4763.0 kg and aged 3 years were used in a
2158C until analysed. total faecal collection trial with 21 days for
adapta-tion followed by 10 days for measurements. Animals
2.5. Laboratory analyses used in each trial were divided into five homogenous
groups. Those used in the growth trial were housed
The feeds offered and individual refusals and in individual pens and those used in the digestibility
faeces were analysed for DM, ash, crude protein trial were housed in metabolism cages. Each group
(CP) (AOAC, 1975), neutral detergent fibre (NDF) of animals received each morning (9:00 h) fresh
and non sequential acid detergent fibre (ADF) (Goer-foliage of acacia ad libitum and feed blocks
con-ing and Van Soest, 1970) contents. Feeds were also taining either 0 (control), 6, 12, 18 or 24% of PEG.
analysed for acid detergent lignin (ADL) content The amount of acacia foliage offered to sheep was
according to Goering and Van Soest (1970). A adjusted to be about 20% in excess of the amount
sample of acacia used in the digestibility trial was consumed on the previous day. Acacia and feed
air-dried then ground to pass a 1 mm-screen. Trip-blocks were distributed into two separated troughs.
licate samples weighing 200 mg were extracted with Animals had free access to clean water.
10 ml of 50% aqueous methanol. The solution was stored at 48C overnight then centrifuged at 11003g 2.4. Measurements
for 15 min at 48C. The extract was collected and used to estimate condensed tannin (CT) content of Over the 60-day period of the growth trial, feeds
acacia using the vanillin-HCl method of Broadhurst offered and refusals left by individual animals were
and Jones (1978) using (1)-Catechin (Sigma, lot weighed and samples were collected for dry matter
100H0586) as a standard. Nitrogen in urine was (DM) determination by drying at 1058C in a
forced-determined by the Kjeldahl N method (AOAC, air oven for 24 h. The feed block of each animal was
1975). Urinary excretion of allantoin (Y, mmol / day) weighed before acacia distribution then given to the
was used to calculate microbial purines absorbed (X, same animal approximately 0.5 h later. Whenever it
mmol / day) from the equation: Y50.84X1(0.1503 was entirely consumed by the animal, a new feed
0.75 0.25X 0.75
block was offered. Samples of acacia foliage and W 3e ) where W is the metabolic weight feed blocks were collected weekly, dried at 508C (kg) and e is an exponential function of X. Microbial then ground to pass a 1 mm-screen for subsequent nitrogen supply (g / day) was calculated from the analyses. Animals were weighed for 2 consecutive relationship: 70X /(0.8330.11631000) where 70 is days at the commencement and the end of the trial. the nitrogen content of purines (mg N / mmol), X is Water and food were removed from troughs approxi- as defined above, 0.83 is the assumed digestibility of mately 15 h before the animals were weighed. microbial purines, 0.116 is the ratio of purine N / total In the digestibility trial, feed refusals, faeces N in mixed rumen microbes and 1000 converts voided and urine excreted by each animal were milligrams to grams (Chen et al., 1990; Chen and recorded daily at 8:00 h. Urine was collected in Gomes, 1992).
bucket containing 100 ml of 10% sulphuric acid
(urine pH,3) (Chen and Gomes, 1992). DM con- 2.6. Statistical analysis tents of feed offered, individual refusals and faeces
variance using the SAS general linear models pro- blocks containing 6, 12, 18 and 24% of PEG cedure of the SAS (1987) for a completely random- respectively. The highest increase in acacia intake ised design. The linear and quadratic effects of PEG was obtained with the PEG rate of 18% as the rate in feed blocks were detected using polynomial highest rate of PEG did not lead to a further increase contrasts. in acacia consumption.
The positive effect of PEG in feed blocks on acacia intake was reflected in sheep growth. Sheep
3. Results given feed blocks containing PEG gained more than
those on feed blocks without PEG. The response was 3.1. Feed composition linear and significant (P,0.001) although the gains
with the rates 18 and 24% of PEG were similar. Nutrient contents of acacia foliage and feed blocks
are given in Table 2. Acacia foliage had a relative 3.3. Feed intake by adult sheep and diet high content of CP, lignin and CT. Including PEG in digestibility
feed blocks had no effect on their chemical
com-position. CP contents of the five types of feed blocks Feed intake and digestibility values are given in were quite similar and somewhat high, ca. 21 g / 100 Tables 4 and 5, respectively. There was no effect of g of DM. In contrast, all the feed blocks had low PEG inclusion on feed block intake (P.0.05). The contents of NDF and ADF. positive effect of PEG on acacia intake by growing sheep was observed also on adult sheep. DM intake 3.2. Feed intake during the growth trial and daily of acacia foliage increased linearly with the rate of gain PEG in feed blocks (P,0.001) with increases of 22, 25, 56 and 71% with the rates 6, 12, 18 and 24% of DM intake of feed blocks by growing sheep PEG respectively. Feed block intake by adult sheep ranged from 130 to 150 g / day and was not affected was greater than that measured on yearling sheep by PEG supply (P.0.05) (Table 3). Sheep receiv- ranging from 150 and 210 g DM / day or 15.5 and
0.75
ing feed blocks without PEG consumed a small 24.3 g / kg W .
0.75
amount of acacia DM (37.7 g / kg W ). There was The digestibilities of DM, organic matter, CP and a linear increase of acacia intake with the rate of NDF of acacia supplemented with feed block without PEG in the feed blocks (P,0.001). Increases of PEG was low. They were substantially increased by about 25, 35, 56 and 46% were obtained with feed feeding PEG. Sheep offered blocks containing 18 or
Table 2
Chemical composition of acacia foliage and feed blocks (% of DM)
a
DM (%) OM CP NDF ADF ADL CT
Acacia 27.1 90.9 15.2 50.3 27.7 11.9 3.2
Feed block ingredients
Olive cake 75.3 95.0 5.4 78.7 56.8 29.0 –
Wheat bran 86.5 94.4 18.1 45.5 12.4 2.1 –
Wheat flour 84.7 98.0 15.3 17.8 9.1 1.9 –
Feed blocks containing
0% PEG 69.1 75.1 21.4 20.6 13.3 3.6 –
6% PEG 71.0 73.8 21.2 21.2 13.0 4.5 –
12% PEG 68.2 74.0 20.8 20.2 12.6 3.8 –
18% PEG 70.5 75.5 21.0 20.9 12.1 3.3 –
24% PEG 68.9 76.4 20.9 21.0 13.5 4.2 –
a
Table 3
Feed intake and sheep growth (growth trial)
a
PEG in feed blocks (%) Contrasts
0 6 12 18 24 S.E. Linear effect
DM intake ( g /day)
Acacia 357 429 461 539 516 13.4 ***
Feed block 129 150 145 128 145 23.7 ns
Diet 486 578 606 667 660 29.2 ***
0.75 DM intake ( g /kg W )
Acacia 37.7 47.1 50.9 58.8 54.8 1.47 ***
Feed block 13.7 16.5 16.0 14.0 15.2 2.61 ns
Diet 51.4 63.5 67.0 72.8 70.0 3.30 ***
PEG intake (g / day) 0 7.1 13.0 19.5 28.0 – –
b
(PEGi / CTi) 0 0.52 0.88 1.13 1.70 – –
Daily gain (g / day) 14 39 50 61 63 3.2 ***
a
Whenever significant, quadratic effects are reported in the text. b
Amount of PEG consumed to the amount of condensed tannins consumed ratio. ***P,0.001, ns not significant (P.0.05).
24% of PEG increased DM digestibility of the diet ty and negative values of ADF digestibility indicated by approximately 13 and 21 percentage units, respec- an analytical problem which is discussed below. tively. The rate 6% of PEG was too low to make any The positive effect of including increasing significant improvement of diet DM digestibility. In amounts of PEG in feed blocks on acacia intake and contrast, even with the lowest rate of PEG there was diet digestibility resulted in an improvement of the a significant increase (P,0.001) of diet CP di- energy and nitrogen value of diets. The amounts of gestibility of 17.4 percentage units. Higher rates of digestible OM intake (DOMI) and digestible CP PEG (12, 18 and 24%) led to further improvement in intake (DCPI) were significantly increased. The CP digestibility but the rate of increase was lower. highest increase was recorded with the 24% PEG-PEG supply improved cell wall digestibility of the rate, 89 and 78%, respectively. It should be noted diet. Nevertheless, the low values of NDF digestibili- that the lowest rate of PEG used in this experiment
Table 4
Feed intake by rams (digestibility trial)
PEG in feed blocks (%) Contrasts
0 6 12 18 24 S.E. Linear effect
DM intake ( g /day)
Acacia 351 468 473 609 697 41.7 ***
Feed block 207 189 194 151 180 26.8 ns
Diet 558 657 667 760 877 54.4 ***
0.75 DM intake ( g /kg W )
Acacia 39.9 48.5 50.0 62.4 68.2 3.80 ***
Feed block 24.3 19.6 20.3 15.5 17.7 2.96 ns
Diet 64.2 68.1 70.3 77.8 85.9 5.06 **
PEG intake (g / day) 0 9.0 17.4 23.0 34.8 – –
a
(PEGi / CTi) 0 0.62 1.17 1.21 1.56 – –
a
Table 5
Effect of the level of PEG in feed blocks on nutrient digestibility of acacia-based diets fed to sheep
PEG in feed blocks (%) Contrasts
0 6 12 18 24 S.E. Linear effect
Diet digestibility(% )
DM 28.8 36.8 37.7 42.2 50.2 2.95 ***
OM 33.3 39.5 40.3 43.2 51.2 2.67 ***
CP 43.5 50.9 54.9 55.1 57.2 2.31 **
NDF 28.4 5.8 14.8 17.5 26.3 6.79 **
ADF 286.5 252.0 267.3 265.8 241.0 8.69 *
a Feeding value of diets
DOMi 20.5 23.9 24.8 29.9 38.7 3.20 ***
DCPi 46 63 58 69 82 7.7 **
NDCP 104.2 82.1 75.3 73.2 69.8 3.42 ***
a 0.75
DOMi, digestible organic matter intake expressed as g / kg W ; DCPi, digestible crude protein intake expressed as g / day; NDCP, non digestible crude protein expressed as g / kg dry matter intake.
*P,0.05, **P,0.01, ***P,0.001.
(6%) was sufficient to increase intake of the acacia percent of N intake, when increasing amounts of supplemented with unmolassed feed blocks to cover PEG were included in feed blocks (P,0.001). The maintenance requirements of sheep. increase of N retention resulted from the positive effect of PEG on N intake which was associated to a 3.4. Nitrogen balance and microbial synthesis decrease of faecal (P,0.05) and urinary (P,0.01)
N excretions.
Excretion and retention of N, urinary excretion of The amount of allantoin excreted in urine was allantoin and estimated microbial synthesis are given higher in sheep receiving PEG-containing feed in Table 6. N retention was low for sheep receiving blocks than in those on the feed block without PEG feed blocks without PEG. There was a linear in- (P,0.01). The rates 12, 18 and 24% of PEG crease of N retention, expressed as g / day or as increased allantoin excretion by 19, 17 and 27 mg /
Table 6
Nitrogen balance and microbial sytnthesis by sheep given acacia foliage and graded levels of PEG in feed blocks
PEG in feed blocks (%) Contrasts
0 6 12 18 24 S.E. Linear effect
( g /day)
N intake 16.7 18.4 17.9 19.8 22.9 1.60 *
Faecal N 12.0 10.6 11.1 11.6 12.7 0.62 ns
Urinary N 6.2 6.6 5.1 5.7 5.1 0.69 ns
N retention 21.5 0.8 1.7 2.5 5.1 0.99 ***
Percentage of N intake
Faecal N 72.8 57.8 63.3 59.7 56.0 3.49 *
Urinary N 37.0 37.1 29.4 28.3 22.4 3.60 **
N retention 29.7 2.3 7.6 12.2 21.8 4.63 ***
Allantoin in urine 0.75
(mg / kg W ) 35.8 49.8 54.5 59.1 62.9 2.27 **
a
Microbial N 4.9 6.7 9.5 11.5 12.4 1.04 **
(g / kg DOMi)
a
0.75
kg W , respectively, as compared with the control increasing amounts of PEG in feed blocks signifi-diet with blocks without PEG. The optimal responses cantly increased acacia DM intake by sheep. Similar of N retention as well as allantoin excretion were findings were reported by Silanikove et al. (1996) obtained in sheep receiving feed blocks containing who distributed tannin-containing leaves of Quercus 12% of PEG. Animals on acacia foliage sup- calliprinos, Pistacia lentiscus and Ceratonia siliqua plemented with PEG-containing feed blocks ex- to goats. Estimated amounts of PEG consumed by hibited higher microbial synthesis as compared to adult sheep averaged 9.0, 17.4, 23.0 and 34.8 g / day those given feed blocks without PEG. The optimal and ratios of PEG:condensed tannins of 0.62, 1.17, response in this case was also obtained with the PEG 1.21 and 1.56 with feed blocks containing 6, 12, 18 rate of 12%. The further increase of the amount of and 24% of PEG, respectively. The optimal acacia microbial nitrogen which was obtained with the two intake was obtained with the 18% PEG rate corre-highest rates of PEG (18 and 24%) was not signifi- sponding to the PEG consumed:condensed tannins cant as compared to that obtained with the 12% PEG consumed ratio of 1.13 or 1.21, respectively for rate. growing and adult sheep (Tables 3 and 4). According to Silanikove et al. (1996), the amount of PEG needed to produce a maximal increase in feed intake
4. Discussion by goat varied with shrub species. Highest DM
intake was obtained after PEG supplementation at 10 4.1. Effect on feed intake g / day to goats fed carob and oak, and at 20 g / day to
goats fed pistacia. DM intake of acacia foliage by sheep supplied
with feed blocks without PEG ranged from 37.7 to 4.2. Effect on diet digestibility
0.75
39.9 g / kg W . Lower intakes of acacia (28 et 34
0.75
g / kg W ) were obtained on sheep supplied with Our experiment suggested that administering PEG 300 (Ben Salem et al., 1999b) or 400 (Ben Salem et in feed blocks is a successful practice to improve al., 1999a) g / day of barley. The higher intake of acacia digestibility. Even using a low rate of PEG acacia by sheep supplemented with feed blocks may (6%), DM digestibility of acacia foliage was im-have been probably due to the presence of large proved by approximately 8 percentage units. A quantities of rumen degradable proteins mainly from further important increase in digestibility occurred wheat bran and wheat flour residue. Similar effects when the rate of PEG in feed blocks was increased to were observed by Silanikove et al. (1996) when oak 24%. According to our previous findings (Ben Salem leaves were supplemented with PEG. Further im- et al., 1999b) this increase of diet DM digestibility provement in oak intake was observed when soybean may be ascribed to an improvement of rumen meal was added with PEG. Additionally, hard feed fermentation and, therefore, to an increase of acacia block, which is nibbled periodically, might allowed a digestion as a result of PEG application.
balanced and a spaced out supply of energy and The problem of the negative values of cell wall protein nitrogen mainly from wheat flour and wheat (NDF) and lignocellulose (ADF) digestibilities of the bran, non protein nitrogen from urea, minerals and experimental diets has been discussed in previous vitamins from the other ingredients, resulting in an studies (Makkar et al., 1995; Ben Salem et al., increase of acacia consumption by sheep. The benefi- 1999b) on the same plant material (Acacia cial effect of a synchronous supply of energy, cyanophylla Lindl.). Briefly, this problem may be nitrogen and phosphorus to sheep fed on condensed ascribed to the artefacts arising due to condensed tannin-rich diet was confirmed by McMeniman tannins in acacia.
(1976).
There are only few data on the influence of PEG 4.3. Effect on nitrogen utilisation level upon the nutritive value of tannin-rich shrubs,
CP digestibility increased substantially with the rate 4.4. Effect on the feeding value of diets of PEG in feed blocks. Such trend suggested that
condensed tannins were the main causative factor of The positive effect of PEG on acacia intake and the poor utilisation of acacia nitrogen by the animal. diet digestibility resulted in an improvement of Such increase was significant even when a low rate energy and nitrogen values of experimental diets as of PEG was used (6%). The linear increase of diet indicated by the increased amount of digestible CP digestibility with the rate of PEG in feed blocks organic matter intake (DOMi) and digestible crude confirmed the findings of Larwence et al. (1984) on protein intake (DCPi), respectively. PEG supply deactivation of grape marc tannins with PEG offered resulted in a quadrically decrease (P,0.01) of the to sheep. Similar trend was observed on goats fed amount of non digestible crude protein from acacia-tannin-rich shrub species and supplemented with based diets. Overall, we can state that administering increasing amounts of PEG (Silanikove et al., 1996). PEG in feed blocks at a rate of 18%, led to the In order to take into account the differences of the optimal improvement of the feeding value of acacia-amounts of nitrogen intake among dietary treatments, based diets.
amounts of faecal and urinary nitrogen losses were
expressed as percentage of N intake (Table 6). N 4.5. Effect on sheep performance losses were significantly reduced following PEG
supply resulting in an increase of N retention. The beneficial effect of supplying PEG on acacia ˜
Nunez-Hernandez et al. (1989) ascribed the increase intake, nitrogen utilisation and microbial synthesis of faecal N excretion in goat given tannin-rich shrubs was reflected on the animal performance. Pritchard et to the interaction between these secondary com- al. (1992) claimed that the increase of growth rate pounds and dietary proteins. Tannins form complex- and wool production following PEG supply for sheep es with proteins rendering them unavailable for fed on Acacia aneura, another condensed tannin-rich rumen micro-organisms and the animal host. shrub, was the result of the improvement of sulphur Overall, the increase in digestibility of acacia- containing amino-acid availability. In our study, based diet is likely to result from the positive effect there was a linear increase of the daily gain of sheep of PEG addition on microbial activity in the rumen. given PEG-containing feed blocks. As for the nutri-Ben Salem et al. (1999b) found that administering tive value of experimental diets, the optimal growth PEG in concentrate, drinking water or as solution rate was obtained with the PEG rate of 18%. The sprayed on acacia foliage increased ammonia nitro- further increase of daily gain when a higher rate of gen concentration in the rumen and microbial protein PEG was used (24%) was not significant when synthesis as indicated by the increased urinary compared to that obtained with the rate 18% of PEG. excretion of allantoin. Recently, Makkar (pers. The PEG rate of 24% increased considerably the comm.) showed that the slow release of PEG in the amount of nitrogen available for rumen microflora in vitro incubation system (gas method coupled with and the animal host but in absence of sufficient determination of microbial protein) containing tan- quantity of digestible OM, the excess of N was not nin-rich feed produced higher microbial protein in utilised by the animal.
the rumen as compared to one delivery of the same amount of PEG in the system. Additionally,
accord-ing to this author, the requirements of PEG is 5. Conclusion
decreased substantially when PEG is used in in vitro
Chen, X.B., Hovell, F., Orskov, E.R., Brown, D.S., 1990. Excre-nitrogen utilisation and daily gain by Barbarine
tion of purine derivatives by ruminants: effect of exogenous sheep fed acacia foliage occurred when the feed
nucleic acid supply on purine derivative excretion by sheep. Br. blocks contained 18% of PEG. J. Nutr. 63, 131–142.
Degen, A.A., Becker, K., Makkar, H.P.S., Borowy, N., 1995. Acacia saligna as a fodder tree for desert livestock and the interaction of its tannins with fibre fractions. J. Sci. Food
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