58 H
. Ben Salem et al. Livestock Production Science 64 2000 51 –60
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, mainly that of Mediterranean species Silanikove et
Administering PEG in feed blocks seems to al., 1994, 1996. In our experiment the addition of
improve the acacia nitrogen utilisation by sheep. Diet
H . Ben Salem et al. Livestock Production Science 64 2000 51 –60
59
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
