52 H
. Ben Salem et al. Livestock Production Science 64 2000 51 –60 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 29 8C, 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
replacing them with alternative feed sources. Feed Acacia cyanophylla foliage leaves and twigs was
blocks, solidified mixture of inexpensive and locally hand-harvested daily early each morning from a
available agro-industrial by-products, appear to be an plantation at the OEP farm. This plantation was
interesting alternative to feeding supplements. Feed established in 1989 and is cut every 2 years. Thus,
blocks have been developed for housed or grazing acacia foliage used in this experiment was a mixture
ruminants. They are considered to be a good supple- of growth in the current year and 2-year-old material.
ment for poor quality diets, as they allow a balanced, Owing to the small quantities of molasses pro-
synchronous and fractionated supply of nutrients i.e. duced by the national sugar factories, unmolassed
energy, nitrogen, minerals and vitamins to rumin- feed blocks were used in this experiment. Feed
ants Sansoucy, 1986; Sudana and Leng, 1986. In blocks was formulated using ingredients available to
addition, feed blocks are a useful method of preserv- small farmers in the arid and semi-arid zones of
ing some agro-industrial by-products. Finally, feed Tunisia. Feed blocks used in this experiment were
H . Ben Salem et al. Livestock Production Science 64 2000 51 –60
53 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. Moulding was carried out using inexpensive
In order to avoid any toxicity problem, the ex- moulds made from PVC tube, diameter 18 cm, cut
perimental animals were gradually adapted to eating into 20 cm-length portions. Thus, dimensions of the
the feed blocks. Two weeks before the start of the mould used in this experiment were 18
3 20 cm experiment, they received oat hay ad libitum and
diameter 3 length. Approximately, at 3 cm from the
feed blocks. Feed blocks were offered to animals bottom of the mould three equidistant screw-bolts
only for 1 h during the first 3 days and for 5 h during were fixed. Two steel disks 17.5 cm-diameter were
the following 3 days. During the remaining period,
54 H
. Ben Salem et al. Livestock Production Science 64 2000 51 –60
feed blocks were continuously available in the urine were collected daily from each animal and
stored at 2 58C. 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 50
8C experiment conditions. The initial mean liveweight
for 48 h and ground to pass a 1 mm-screen and of the sheep was 19
61.5 kg. Twenty five Barbarine stored until needed. Urine samples were frozen at
rams of 47 63.0 kg and aged 3 years were used in a
2 158C 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 4
8C overnight then centrifuged at 1100 3 g 2.4. Measurements
for 15 min at 4 8C. 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 105 8C 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: Y 5 0.84X 1 0.150 3
was entirely consumed by the animal, a new feed
0.75 0.25X
0.75
block was offered. Samples of acacia foliage and W
3 e where W
is the metabolic weight feed blocks were collected weekly, dried at 50
8C 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 0.83
3 0.116 3 1000 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
were estimated after drying samples at 105 8C for 24
Within each trial growth or digestibility, the h. Samples of feed offered and refusals, faeces and
effects of treatments were compared by analysis of
H . Ben Salem et al. Livestock Production Science 64 2000 51 –60
55
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
