The use of polyethylene glycol to reduce the
anti-nutritional effects of tannins in goats fed woody species
M. Decandia
a,*, M. Sitzia
a, A. Cabiddu
a, D. Kababya
b, G. Molle
a aIstituto Zootecnico e Caseario per la Sardegna, 07040 Olmedo, ItalybSheep and Goats Department, Extension Service, Ministry of Agriculture, P.O. Box 7054, Tel Aviv 61070, Israel
Received 1 October 1999; accepted 21 March 2000
Abstract
The effect of polyethylene glycol (PEG) on intake and digestibility in Sarda goats either fed in metabolic crates or browsing a Mediterranean scrubland, featured by tannin-rich species, was studied. In metabolic cages (Experiment 1) nine dry goats were fed ad libitum with foliage of lentisk (Pistacia lentiscusL.) supplemented with 200 g per day of a concentrate and either 0, 25 or 50 g per day of PEG. In the scrubland (Experiment 2), with lentisk as main species, 20 goats at the end of lactation, were supplemented with hay and concentrate, receiving 0 or 50 g per day of PEG. In both experiments the PEG did not affect the diet intake. The in vivo CP digestibility of the diet raised from 37% without PEG to 71% with 50 g of PEG in Experiment 1 (P<0.05) and from 40 to 53% in Experiment 2 (P<0.01). In the latter experiment the PEG supplemented goats had a higher proportion of lentisk in the diet than the controls (P<0.05). The milk production and the milk urea content increased as a consequence of PEG supplementation. The PEG showed its capacity to neutralize the tannins allowing a better utilisation of tanniferous species.#2000 Elsevier Science B.V. All rights reserved.
Keywords:Goats; Tannins; Polyethylene glycol; Feeding behaviour; Alkanes
1. Introduction
Goats of Mediterranean regions are often fed on scrubland consisting of woody species characterised by a medium-low level of crude protein (CP) and high tannin content. The level of CP in the foliage of Mediterranean bushes is often lower than 10% on dry matter (DM) basis (Leclerc, 1984; Cabiddu et al., 2000) with a decreasing trend from spring up to the following autumn. On the contrary the concen-tration of tannins in particular condensed tannins (CT) is very high (>10% DM), mainly in some species like
Pistacia lentiscus L. (Silanikove et al., 1996a; Cabiddu et al., 2000) and tends to increase during the dry season (Cabiddu et al., 2000). Condensed tannins form complexes by hydrogen-binding with proteins, in particular at neutral pH (McLeod, 1974). Following CT ingestion feed intake and digest-ibility of woody (Kumar and Vaithiyanathan, 1990; Silanikove et al., 1994; Miller et al., 1997) and herbaceous species (Barry and Duncan, 1984; Terrill et al., 1992; Waghorn and Shelton, 1997) are reduced. Goats are able to select plants or parts of plants with higher proportion of CP and lower proportion of tannins than the vegetation on offer as shown by Kababya et al. (1998). These authors also pointed out that the concentration of CP and CT in goats' diet is almost steady when they select diets in a *Corresponding author. Tel.:39-79-387235;
fax:39-79-389450.
E-mail address: izcszoo@tin.it (M. Decandia).
Mediterranean scrubland. In some areas around the Mediterranean basin, tannin-rich species (P. lentiscus
L.) are widespread and although they are less preferred their contribution to the diet is important (Decandia et al., 1997), probably due to the ability of goats to consume higher amounts of tannins than other rumi-nants (Silanikove et al., 1996b).
Polyethylene glycol (PEG) is an inert and unab-sorbed molecule that can form a stable complex with tannins, preventing the binding between tannins and proteins (Badran and Jones, 1965). Therefore, it has been used to alleviate negative effects of CT in sheep (Silanikove et al., 1994) and goats kept at maintenance level (Silanikove et al., 1996a). At our best knowl-edge, the effects of PEG on intake, digestibility and production in freely grazing Mediterranean dairy goats have not been reported. The aim of this study was to assess the effect of PEG on intake and digest-ibility in Sarda goats either fed in metabolic crates or browsing a Mediterranean scrubland featured by tan-nin-rich species.
2. Materials and methods
The experiments were performed at the beginning of summer in the Bonassai experimental farm (NW Sardinia, 418N, 33 m a.s.l., mean annual rainfall 569 mm).
2.1. Experiment 1: effects of PEG on dry goat intake and digestibility in metabolic cages
Nine dry non-pregnant Sarda goats (mean live-weightS.E.M. 31.32.43 kg) were placed in meta-bolic cages designed according to Meuret (1989). Branches of lentisk (P. lentiscus L.) were cut daily at 8:00 hours clamped to the cages in a `tree-like' frame (Meuret, 1989) and offered ad libitum. After a 5-week period of adaptation, the animals were blocked by liveweight (LW) and body condition (BC) score (Santucci et al., 1991) and allotted to three homo-geneous groups. These groups received, during a 10-day period, three levels of PEG: 0 (PEG0), 25 (PEG25) and 50 g per goat per day (PEG50). The PEG (MW 4000) was dissolved in water (1:1) and dosed once daily at 8:00 hours by a dosing gun. All goats were supplemented with 200 g per day of
con-centrate (a mixture of sugarbeet pulp, 66.5% DM and soybean meal, 33.5% of DM). Fresh matter intake, total faecal and urinary output were measured daily at 9:00 hours. Samples of the feeds offered, faeces and urine were collected every day. K2Cr2O7was added to
urine to prevent loss of N. These samples were imme-diately frozen. Daily samples of foliage, faeces and urine were bulked across the experimental period. The samples of foliage, concentrate and faeces were freeze-dried and analysed to determine the chemical composition: dry matter (DM), organic matter (OM), crude protein (CP), neutral detergent ®bre (NDF), acid detergent ®bre (ADF), acid detergent lignin (ADL) (Goering and Van Soest, 1970). The level of CT in lentisk offered to the goats was determined using a procedure for measuring extractable CT and protein-bound CT as described by Terrill et al. (1992). CT for preparation of standards were extracted from fresh-frozen lentisk while extracts of white clover (Trifolium repens L.) were used as blank for the spectrophotometric colour development. The nitrogen (N) content of urine was also measured (Kjeldahl). Individual intake of lentisk and concentrate was mea-sured daily and averaged upon the 10-day experimen-tal period. The in vivo digestibility of DM (DMD), OM (OMD), CP (CPD), NDF (NDFD) and ADF (ADFD) and the N retention (N retainedN intakeÿ(N in faecesN in urine)) were also calcu-lated. The amount of PEG given was deducted from the DM excreted in the faeces. Goats were weighed and scored at the beginning and the end of the experiment. All individual data were analysed by one-way model of analysis of variance with the level of PEG as ®xed effect. Regression analysis was performed to study the relationships between level of PEG and coef®cients of digestibility. Treat-ment means were separated by the t-test for pre-planned comparisons.
2.2. Experiment 2: effects of PEG on browsing lactating goats
day (from 8:00 to 15:00 hours). Feeding area was featured by a high proportion of lentisk 57.5%, 15.5% of herbaceous species and 5.9% of Quercus
sp. (Decandia et al., 2000). At the beginning of the experiment the goats were blocked by LW, BC, milk yield and milk composition and allotted to two homo-geneous groups: PEG50, supplemented with 50 g of PEG mixed in 50 g of sugar beet pulp and control (C), receiving no PEG supplementation. All goats received 200 g per head of ryegrass hay. In order to provide daily the same amount of energy as concentrate to both groups, PEG50 received 100 g and C 150 g of a commercial concentrate (metabolisable energy 2.49 Mcal/kg DM, CP: 16% DM). The goats were machine-milked twice daily.
Feeding behaviour was studied once a week by direct observation of bites (Bourbouze, 1980). During the observation, focal animals were monitored con-tinuously through the grazing period by one observer as detailed by Kababya et al. (1998). This observer tape-recorded the feeding activity, the species and, within the species, the part of the plants eaten or refused. In the meantime another observer measured bite frequency (number of prehension bites per minute of feeding) of the different species. The quality of the pasture consumed by goats was evaluated by analysis of hand plucked bite-like samples of vegetation (Meuret et al., 1985) every 3 weeks. On each occasion 300 bite-like samples were collected for each species. The weight of the bites was calculated by dividing the sample weight of each species by 300. Concentrate, hay and the hand plucked samples were immediately frozen at ÿ208C, until freeze-dried and analysed to determine DM, OM, CP, NDF, ADF, ADL (Goering and Van Soest, 1970) and CT by Folin Ciocalteau method (Martillotti et al., 1987). The intake of DM and of the other nutrients (as group average per observation day) was calculated according to the following equation:
I X
i 1ÿn
IRiGTi (1)
where IRi(intake rate for theith species) is the product of number of bites in a minute and the weight of a bite and GTi(grazing time of theith species) is calculated as follows:
GTiRGTiGT (2)
where RGTi is the relative grazing time de®ned as the total time spent grazing the ith species divided bythe total grazing time for all species, G the pro-portion of the observation time spent grazing all species and Tis the total time spent by the goats in the pasture.
To assess daily faecal output of the goats on two occasions during the experiment each goat was orally dosed for 10 days once daily (after morning milking at 7:30 hours) with a paper bung containing 65 mg of hexatriocantane alkane (C36) used as external marker.
During the last 5 days after the beginning of the alkane administration, grab-samples were collected twice daily immediately after dosing and after the afternoon milking (15:30 hours). These samples were frozen at
ÿ208C, bulked across days to generate one ®nal sample per goat. These samples were freeze-dried to determine the DM content and analysed for N (Kjeldahl) and alkane concentration (Mayes et al., 1986).
The faecal output (FO) of DM and CP was then calculated according to the following equation:
FODC36 FC36
rC36 (3)
where DC36 and FC36, are respectively, the dose rate (mg per day), the faecal concentration (mg/kg DM) ofthe dosed alkane andrC36 its recovery rate (0.947; Dove et al., 1989). The average group digestibility (D)of DM and CP was then calculated using the average group intake (I) from Eq. (1) and faecal out-put (FO) from Eq. (3), equally averaged for each group
D IÿFO I
100 (4)
3. Results
3.1. Experiment 1
The chemical composition of lentisk leaves and of concentrate are in Table 1. Lentisk showed low CP and high extractable CT and ADL content. PEG supple-mentation did not affect DM intake of lentisk either total or per kg of metabolic weight (MW) and the coef®cients of in vivo digestibility of DM, OM, NDF
and ADF (Table 2). However, PEG signi®cantly affected the in vivo digestibility of crude protein (CPD) that raised from 37.5% with PEG0 to 71.2% with PEG50 (Table 2). The regression of PEG level (X, g) on the CPD (Y, %) wasY40.4%0.68X;R20.52;
P0.02.
PEG increased the intake of digestible N and tended to increase N retention. A tendency towards a higher loss of N in the urine was found in the goats supple-mented with PEG50 (P0.09).
No differences were found between groups in LW and BC changes through the experiment that averaged
ÿ1 g per day andÿ0.22 BC unit per period.
3.2. Experiment 2
Supplements offered were well accepted by the goats. The feeding behaviour of the two grazing groups was similar. The grazing time, expressed as percentage of the total observation time, was 53 and 57% in C and PEG50, respectively. The PEG-supple-mented goats ate a higher proportion of lentisk (P<0.05) and a lower proportion of herbaceous species (NS) than the controls (Table 3).
The chemical composition of different species grazed or browsed by goats is in Table 4. Almost all species had low CP, with the exception of Rubus
Table 1
Chemical composition (% DM) of the lentisk leaves (n10 samples) and of the concentrate offered to goatsa
Parameters Lentisk Concentrateb
Dry matter 45.90.3
Organic matter 95.60.3 94.3 Crude protein 8.50.3 21.9 NDF 42.10.9 45.9 ADF 31.40.9 21.1 ADL 20.40.3 3.1 Extractable CTc 21.73.4 0.0
Protein bound CT 2.61.0 0.0
aMeansS.E.
bThe concentrate consisted of a mixture of sugarbeet pulp
(66.5% DM) and soybean meal (33.5% DM).
cCT condensed tannins.
Table 2
Dry matter intake, in vivo digestibility coef®cients, nitrogen intake, digestible N intake, daily output of N in the urine and retained N in goats fed lentisk either without (PEG0) or with polyethylene glycol supplementation at a level of 25 (PEG25) or 50 g per day (PEG50)a
PEG0 PEG25 PEG50 Probability level
Daily dry matter intakes
Lentisk (g) 448.698 490.698 478.698 NS Lentisk (g/kg LW0.75) 33.75 36.75 36.05 NS
Total diet (g) 620.3103 662.0103 634.3103 NS Total diet (g/kg LW0.75) 46.64 50.34 48.34 NS
Digestibility coefficients(%)
DMD 66.16 68.36 68.56 NS
OMD 67.86 70.16 70.26 NS
CPD 37.59 a 63.39 b 71.59 b *
NDFD 45.414 51.514 55.114 NS
ADFD 34.516 45.616 47.716 NS
Nitrogen balance(g)
Intake of N 12.22 12.62 11.92 NS Intake of digestible N 4.51 a 7.61 b 8.31 b *
Urinary N 2.51 3.81 5.21 NS
Retained N 2.11 3.81 3.11 NS
ulmifoliusSchott. and Rhamnus alaternus L.P. len-tiscusL. was the species with the highest level of CT. As a consequence the PEG50 goats ingested diets richer (P0.02) in CT than C (Table 4). No signi®cant difference was found in the average total DM intake
at pasture between groups (1187 C and 1366 g DM PEG50). However, a trend towards higher intake in PEG50 than C goats was noticeable through almost all the experimental period (Fig. 1). Moreover the PEG50 goats ingested more lentisk than the controls (321, PEG50 versus 139 g DM C,P<0.02) and less herbac-eous species (146, PEG50 versus 213 g DM, C, NS). No differences were found in the faecal DM output averaged over the two periods (74629 versus 72729 g per day in PEG50 and C groups, respec-tively), whereas PEG50 had lower average N faecal output than C goats (12.30.6 versus 15.90.6 g per day, P0.001).
In vivo DMD did not differ in the two groups (54% C and 56% PEG50). The PEG50 group had, however, higher in vivo CPD (53 versus 40%, P0.01). Milk yield and urea were relatively low in both groups but with higher levels in PEG50 than C goats: 755 versus 645 ml,P0.01; and 19.8 versus 16.5 mg/dl,P0.01 (Fig. 2). In the group PEG50 the milk urea content was positively correlated with the total CP intake (R20.77; P0.02) and the digestible CP intake (R20.78; P0.01), whereas no such relation was found in the controls. No differences were found in the milk fat and protein percentage (5.70.13 and 3.30.06 versus 5.40.13 and 3.50.06, respectively in PEG50 and C). Live weight decreased in all goats
Table 3
Botanical composition as proportion of the main species or class of species included in the diet of goats allowed to graze a Mediterranean scrubland for 7 h daily with (PEG50) or without (C) the supplementation of 50 g per day of polyethylene glycola
Species Diet composition (% of DM intake)
C PEG50
Herbaceous species 21.0 11.0
Chamaerops humilisL. 5.3 15.0
Lonicera implexaAiton 6.8 a 2.3 b
Myrtus communisL. 3.3 5.2
Pistacia lentiscusL. 11.2 a 23.7 b
Rhamnus alaternusL. 3.2 4.2
Rubia peregrinaL. 8.8 3.3
Rubus ulmifoliusSchott. 12.0 10.3
Smilax asperaL. 4.2 1.8
Quercus ilexL.,Q. suberL. 11.3 11.8 Other species 12.0 12.0
aLeast square means. Values in rows with different letters differ
signi®cantly (P<0.05).
Table 4
Chemical composition of species or class of species and of the diet at the pasture of goats allowed to graze a Mediterranean scrubland for 7 h daily with (PEG50) or without (C) the supplementation of 50 g per day of polyethylene glycola
Chemical composition (% of DM)
CP NDF ADF ADL CT
Species or class of species
Herbaceous species 6.6 67.1 33.1 4.2 0.7
Chamaerops humilisL. 9.5 61.9 38.5 7.7 2.3
Lonicera implexaAiton 6.3 28.1 18.9 8.1 3.2
Myrtus communisL. 7.7 35.1 25.2 9.5 15.0
Pistacia lentiscusL. 9.8 37.2 31.4 22.1 23.4
Rhamnus alaternusL. 11.5 21.4 14.4 5.9 6.5
Rubia peregrinaL. 8.3 44.0 32.4 6.9 2.5
Rubus ulmifoliusSchott. 11.9 35.4 19.6 6.4 15.3
Smilax asperaL. 9.4 43.6 29.9 14.4 8.3
Quercus ilexL.,Q. suberL. 9.0 55.3 37.3 14.9 9.4
Goats diet
C 8.2 43.8 27.0 9.1 7.6 a
PEG50 8.6 41.8 27.4 10.9 10.3 b
without any PEG effect. However, PEG supplemen-tation was associated with small gain in BC during the experiment (PEG50 0.160.05 versus C
ÿ0.060.05,P0.05).
4. Discussion
Overall this study con®rms some observations on goat selection at pasture. Goats showed low preference for lentisk (57.5% in the scrubland compared with 17.5% in the diet) as already found by Leclerc (1984) and Perevolotsky et al. (1998). In contrast, they tend to prefer other species like Quercus sp. (5.9% in the scrubland and 11.6% in the diet) as also shown by Perevolotsky et al. (1998) and Kababya et al. (1998). However, this study points out that PEG affects the selection of browsing goats, increasing the proportion of lentisk and hence of CT in the diet, in agreement
with preliminary data on cattle browsing Mediterra-nean scrubland (Silanikove, personal communica-tion).
In both the trials there was no signi®cant effect of PEG on goat total intake. Other authors found an increase of DM intake in sheep and goats supplemen-ted with PEG. This effect was higher in sheep than goats, probably because sheep are less adapted to digest tannin-rich plants (Kumar and Vaithiyanathan, 1990). Silanikove et al. (1996b) found an increase in the intake of goats dosed with PEG and fed with tannin-containing leaves. However, their goats were fed only foliage from tanniferous species, without supplementation of concentrate, in contrast with the Sarda goats in our study. Even though the supplemen-tation level was low, it could have been suf®cient to smooth the PEG effect on intake, bearing also in mind that these goats were either dry or at the end of the lactation in the Experiments 1 and 2, respectively. The
Fig. 1. Estimated group dry matter intake of pasture in goats allowed to graze a Mediterranean scrubland for 7 h daily with (PEG50) or without (C) the supplementation of 50 g per day of polyethylene glycol.
allowance of metabolisable energy was slightly higher than metabolisable energy requirements according to NRC (1981) in both Experiments 1 (0.01,0.13 and
0.11 Mcal ME for groups PEG0, PEG25 and PEG50) and 2 (0.16 and 0.58 Mcal ME in C and PEG50, respectively). This can explain why goats supplemented with PEG were not aimed at increasing their total intake of DM and hence of energy. The feeding drive associated with lactation (Experiment 2), was however suf®cient to exert at least a tendency towards a higher intake at pasture in PEG-supplemen-ted than in unsupplemenPEG-supplemen-ted goats as shown in Fig. 1. Moreover, in this experiment, the intake of lentisk (i.e. the species with the highest CT content) was actually enhanced by the PEG and this is in line with the data of Silanikove et al. (1996a).
The effect of PEG on the digestibility was much more important than that on intake, particularly as far as protein nutrition is concerned. The digestibility of CP, either in metabolic cages or at pasture, was markedly increased by PEG supplementation, with a bigger effect in the indoor as compared with the outdoor experiment (90 versus30%, respectively). The goats of Experiment 1, with a diet basically consisting of leaves of lentisk had a proportion of tannins in the diet probably suf®cient for complexing all the protein coming from the supplements (Table 1). In fact the in vivo CP digestibility in goats fed exclusively with lentisk is markedly negative (Silani-kove et al., 1996a). Therefore, without any supple-mentation the effect of PEG at the same levels of dosing could have been much more evident. More-over, this study shows that under tannin-rich feeding regimen even dry goats receiving supplements can experience a negative protein balance, unless the PEG is added to the concentrate. Indeed, the digestible protein (DP) requirements in Experiment 1 (NRC, 1981) are satis®ed only in the goats receiving PEG (ÿ8, PEG0; 12, PEG25; 16 g DP, PEG50). As a consequence the PEG supplemented groups exhibited a higher N loss in the urine (Table 2). This waste of nitrogen could be lower in lactating goats, due to their higher requirements for metabolisable protein. The positive N retention level observed in the PEG0 group could be reasonably explained by the N recycling. In Experiment 2 the goats showed a lower positive effect of PEG on CP digestibility probably due to (i) the lower percentage of tannins in the diet, (ii) the
dif-ference between tannin concentration in PEG50 and C diet (PEG50>C). In this experiment, even with a lower CT intake, all the lactating goats probably experienced a negative DP balance with a smaller de®cit in the PEG50 than in the C group (ÿ17 versusÿ34 g DP). In lactating goats, the most important consequence of improved CP digestibility was the higher milk yield in the PEG supplemented goats, that suggests the use of this substance when the protein availability is reduced by the tannins. Moreover, PEG does not reduce the proportion of solids (protein and fat) in the milk that can be useful for cheese manufacturing as compared with some standard concentrate mixtures (Landau et al., 1993). PEG increased milk urea, in agreement with the increase in blood plasma found by Silanikove et al. (1996b) considering that urea levels in milk and plasma are strictly correlated (Ropstad et al., 1989). The extremely low level of urea in the C goats supports the hypothesis that CP availability is the main limiting nutrient for goats fed a lentisk-based scrubland. This urea level is also much lower than that measured by Cabiddu et al. (1999) in goats raised on a mountainous Mediterranean scrubland, probably fea-tured by a lower proportion of tanniferous species. A positive correlation has been found between the CP intake and the level of milk urea in dairy ewes (Cannas et al., 1998). Under our feeding conditions a similar relationship was found only in the PEG-supplemented goats. The lack of response in the C goats could have been linked to the detrimental effect of tannins that was not offset by the PEG in these animals.
5. Conclusions
Acknowledgements
The authors gratefully thank Mr. G. Scanu for running the chemical analysis and Dr. S. Landau for the helpful comments on the manuscript.
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