Introduction
Kermes oak (Quercus coccifera L.) is a sclerophyllous evergreen shrub that is well adapted to
the Mediterranean climate (Tsiouvaras, 1987). It is the most abundant
evergreen species in Greece and constitutes one of the main shrubland types
that extends from lowlands to mountainous areas. Extensive goat farming
plays a key role in the sustainability of these ecosystems as well as
the cost-effective production of safe and high-quality animal products
(Papachristou et al., 2005; Chatzitheodoridis et al., 2007).
Worldwide, goats consume not only the leaves of the oak species but also their
acorns. Although the nutritional quality of kermes oak browse has been well
studied (Nastis, 1982; Tsiouvaras, 1987; Sallam et al., 2010; Parissi et
al., 2014; Roukos, 2016), limited research has been conducted on factors
affecting the nutritional quality of kermes oak acorns.
Acorns are the nuts of oak species and their nutritional quality is affected
by the stage of maturity (Moujahed et al., 2003), which, in turn, is
affected by climatic conditions (Merouani et al., 2003; Pons and Pausas,
2012). The maturation of kermes oak acorns is biennial. They usually reach
their maturity by the end of autumn. For example, in Epirus, northwestern
Greece, due to climatic variability (Dotsika et al., 2010) acorns are
usually harvested by goat shepherds from the end of autumn until the end
of winter and fed to goats during the winter. Acorns are a
good source of energy for small ruminants (Kayouli and Buldgen, 2001) and
they could replace 50 % of the barley in goat diets (Moujahed et al.,
2005) and 25 % in lamb diets (Al Jassim et al., 1998). Moreover, they are
a valuable feed in pig diets (Nieto et al., 2002).
In Greece, the economic crisis has forced goat producers to expand the use
of kermes oak acorns in goat diets since no alternative cheap feedstuffs are
available. Indeed, kermes oak shrublands, located in Greek mountainous and
less-favoured areas, are a valuable feed resource for goats as they cover up
to 75 % of goats' feed requirements (Zervas, 1998), which accounts for up to
50 % of goats' total production cost (Chatzitheodoridis et al., 2007).
The highly heterogeneous topography of the mountains influences local and
regional microclimates by altering the patterns of climatic variables, such
as rainfall and temperature (Holechek et al., 2010). There is limited
information available regarding the interactions between climate and
topography that lead to fluctuations in the nutritive value of kermes oak
acorns and directly affect goats' productivity. Thus, the aim of the present
study was to compare the nutritional quality of kermes oak acorns of
shrublands located at different altitudinal zones.
Materials and methods
Study area
The study was conducted at Xirovouni Mountain (longitude
20.942940∘, latitude: 39.370576∘) located in Epirus in
northwestern Greece from February 2010 to November 2011. The climate is
typically Mediterranean, characterized by cold, rainy winters and dry, warm
summers. From 1976 to 1997, the mean annual temperature was 17.2 ∘C
and the mean annual precipitation was 1085 mm (HNMS, 2009). During the
present study, an automated weather station (Onset HOBO weather station,
Onset Computer Corporation, MA, USA) was installed in each altitudinal zone
to record precipitation and temperature fluctuations throughout the 2 years of the experimental period (Fig. 1). Soils are calcareous, mainly
sandy, with a pH of 7.1–7.4, and insufficient in N, P, and K (Roukos et
al., 2011). Geologically, the plain belongs to the Ionian geotectonic zone.
The basic substrates are dolomite, Viglas limestone, and flysch.
Fluctuations of mean air temperature and monthly precipitation
during the study period in the three altitudinal zones.
Methods of sampling
Representing typical
kermes oak shrublands on Xirovouni Mountain, 15 sites of western–southwestern aspect were selected for sampling
during February 2010. Three altitudinal zones based on altitude a.s.l. were
differentiated: upper (above 1001 m), middle (501–1000 m), and lower
(0–500 m). Five experimental plots of 20 m2 were fenced in each zone
to exclude grazing. Whole kermes oak acorns were hand-clip collected from at
least four individual shrubs in each plot at the end of October in the
lower zone, in early November in the middle zone, and in mid-November in the upper
zone. A total of 100 acorns (25 acorns per shrub) per plot were randomly
selected. The selected acorns had the same morphological form and thus
represented the same stage of maturity according to Merouani et al. (2003).
The acorn samples were directly placed into individual paper bags in the
field and then spread onto a tray and air-dried gradually in shade at room
temperature for 2 weeks in the laboratory. Dry acorn samples were weighted
after drying in an oven for 48 h at 50–52 ∘C to determine
hull : whole fruit and kernel : whole fruit ratios on a dry-matter basis.
The acorns were then manually separated into kernel and hull, ground with a
Kinematica mill (model Polymix PX-MFC 90D) through a 1 mm sieve, and stored
at 4 ∘C until further analysis.
Analytical methods
Analyses were performed on the acorn part sample (kernel and hull). The N
content was determined using a Kjeldatherm KB 8 S-BS digestion unit and a
Vapodest 40 distillation unit (Gerhardt, GmbH & Co. KG, Germany) using
the AOAC (1995) method 984.13. The ANKOM Fiber Analyzer (ANKOM Technology,
Macedon, NY, USA) was used to determine the neutral detergent fibre
(aNDFom)
according to Van Soest et al. (1991) as modified by Vogel et al. (1999) for
the ANKOM system. Heat-stable α-amylase (A3306, Sigma-Aldrich, St. Louis,
MO, USA) without sodium sulfite in the neutral detergent (ND) was used in the analysis of aNDFom
content. The AOAC (1997) method was used to determine acid detergent fibre
(ADFom). ADFom and aNDFom are expressed with correction for residual ash. The
sulfuric acid procedure (AOAC, 1997; method 973.18) was used to determine
lignin(s) and ash was determined as the gravimetric residue after heating to
550 ∘C for 8 h.
An ANKOM DaisyII incubator (ANKOM Technology) was used to measure
in vitro dry-matter (DM) digestibility (IVDMD) according to Vogel et
al. (1999). Rumen fluid was obtained from eight non-lactating local breed goats
fed with kermes oak foliage, acorns, and some grain, according to their
nutrient requirements. An adiabatic bomb calorimeter (IKA C5000, IKA
Works, Inc, Wilmington, NC, USA) was used to measure the gross energy of each
sample. Post-digestion analyses were completed on the undigested residue in
order to determine the aNDFom content according to methods described above
and the gross energy of the residue was determined using a bomb calorimeter.
In vitro NDF digestibility (IVNDFD) was calculated according to Hall and
Mertens (2008) as
IVNDFD(gkg-1NDF)=(1-[post-digestion dry weightfollowing ND wash/pre-digestion dry weight of NDF]).
In addition, digestible energy was determined with the equation
DE(MJkg-1DM)=(pre-digestion gross energy-[gross energy in
residue×(1-IVDMD)]).
Concentrations of K, Ca, Mg, P, Cu, Mn, Fe, and Zn in the acorn (hull and
kernel) samples were evaluated by oxidizing each subsample with a 2 : 1
nitric / perchloric acid mixture. In separate aliquots, Ca and K were
determined by flame photometry, P by spectrophotometric methods (Khalil and
Manan 1990), and Fe, Mn, Zn, Cu, and Mg by atomic absorption spectrophotometry
(AOAC, 1999; method 968.08) (PERKIN ELMER/AA800, PerkinElmer, Inc., San
Jose, CA, USA). Each sample was analysed in triplicate. The concentrations of
macronutrients and sodium were expressed in milligram per gram dry weight, while those
of microelements are given in milligram per gram.
Statistical analyses
One-way ANOVA was carried out to determine the effect
of altitude on chemical constituents, mineral contents, and nutrient
digestibility of the acorns' hull and kernel. Acorns from each shrub (i.e.
25 acorns each) were bulked in order to ensure the quantity necessary for the
chemical analyses. Thus, five replications per zone were used to perform the
statistical analysis. The normality of the distribution was determined
through the Shapiro–Wilk test. Since the samples had a normal distribution,
the one-way ANOVA with least significant distance post hoc test was used. Differences were
considered statistically significant at the P < 0.05 level.
Correlation between nutritive value variables, climatic variables, and
altitude was measured using the Pearson correlation coefficient (Steel and Torrie,
1980). Moreover, the relationships between altitude, climatic, and nutritive
value variables were tested using linear regression over average for each
acorn part (kernel and hull). All analyses were conducted using IBM SPSS
Statistics v. 21.0.0 (New York, USA) software.
Results and discussion
Climatic conditions
Climatic conditions influence the maturation stage of acorns (Vázquez et
al., 2001; Ferraz de Oliveira, 2012) and, as a consequence, indirectly affect
their chemical composition. In the study area, the air temperature and
precipitation patterns were differentiated in the altitudinal zones (Fig. 1).
Indeed, the mean monthly precipitation was positively correlated, while air
temperature was negatively correlated with altitude (r= 0.217; P < 0.05 and r= -0.484; P < 0.01 respectively).
As a result, the mean monthly air temperature progressively reduced with the
increase in altitude during the experimental period. Thus, the
differentiation of climatic conditions in each zone might affect the
maturation of the acorns.
Proportions of kermes oak acorns components
The hull weight of the acorns ranged from 173 to 225 g kg-1 (Table 1)
and was significantly higher in the upper zone compared to the lower one. Conversely, the weight of the acorn kernels was significantly (P < 0.05) lower in the upper zone compared to the lower one
(Table 1).
Parts of kermes oak acorns (g kg-1) in the three altitudinal
zones.
Altitudinal zone
Part of acorns
Kernel
Hull
g kg-1
Lower
826a*
173b
Middle
789ab
211ab
Upper
775b
225a
SEM
21
15
* Within a column, means with different letters differ at
P < 0.05. SEM, standard error of the mean.
Crude protein and cell wall contents
In general, the Quercus coccifera acorn hulls had lower crude protein (CP) content
and higher cell wall content compared to the kernels (Table 2). The CP of the
acorn hulls did not significantly differ among the altitudinal zones
(Table 2). Inversely, the CP content of the kermes oak kernels was higher (P < 0.05) in the lower zone compared to in the middle and upper
zones (Table 2). The positive correlation between CP and air temperature (P < 0.01, r= 0.894) enhances the above-mentioned result. Due to
climatic differences between altitudinal zones, the crude protein content
declines with the decrease in air temperature, a result that is in agreement
with Saffarzadeh et al. (1999) and Parissi et al. (2014) for Quercus brantii and Quercus ithaburensis respectively.
Chemical composition of kermes oak (kernel and hull) (g kg-1 DM) according to the altitudinal zone.
Parameter
Lower
Middle
Upper
SEM
Sig.
Kernel
CP
45.6a
42.2b
39.8c
0.59
***
NDF
123.8c
147.8b
161.0a
3.77
***
ADF
30.6b
27.6b
39.0a
1.43
***
ADL
5.4b
6.0b
7.8a
0.53
*
Hull
CP
31.3a
29.4a
31.5a
0.89
NS
NDF
622.2b
671.8a
670.8a
5.91
***
ADF
456.2b
459.0b
473.8a
4.41
*
ADL
188.0b
194.2ab
201.2a
2.66
*
Within a row, means with different letters differ at P < 0.05.
M= mean. SEM = standard error of the mean. Sig. = significant level.
* P < 0.05. ** P < 0.01. *** P < 0.001. NS = not
significant.
Mineral content of kermes oak kernel and hull in the tested
altitudinal zones.
Parameter
Lower
Middle
Upper
SEM
Sig.
Kernel
Ca (g kg-1)
0.66ab
0.53b
0.80a
0.056
*
P (g kg-1)
0.94a
0.83ab
0.71b
0.052
*
Ca : P Ratio
0.72b
0.65b
1.14a
0.081
*
K (mg kg-1)
2.83a
2.91a
3.17a
0.143
NS
Mg (mg kg-1)
0.35a
0.39a
0.38a
0.040
NS
Fe (mg kg-1)
10.69a
8.97b
8.63b
0.477
*
Cu (mg kg-1)
2.80a
2.20ab
1.60b
0.252
*
Mn (mg kg-1)
17.49a
18.68a
19.78a
1.290
NS
Zn (mg kg-1)
5.30a
4.80a
2.10b
0.693
*
Hull
Ca (g kg-1)
1.41a
1.28a
1.55a
0.097
NS
P (g kg-1)
0.35a
0.38a
0.31a
0.036
NS
Ca : P Ratio
4.05ab
3.54b
5.27a
0.499
NS
K (mg kg-1)
1.43a
1.49a
1.41a
0.134
NS
Mg (mg kg-1)
0.37a
0.34a
0.32a
0.049
NS
Fe (mg kg-1)
29.36a
23.91b
22.41b
1.332
*
Cu (mg kg-1)
3.30a
3.20a
2.90a
0.283
NS
Mn (mg kg-1)
21.89a
20.85a
21.78a
1.839
NS
Zn (mg kg-1)
5.90a
3.79b
3.00b
0.694
*
Within a row, means with different letters differ at P < 0.05.
M= mean. SEM = standard error of the mean. Sig. = significant level.
* P < 0.05. ** P < 0.01. *** P < 0.001. NS = not
significant.
The CP content of acorns in the present study is in agreement with that
obtained by Moujahed et al. (2007) and Parissi et al. (2014) for acorns from
Quercus coccifera and Quercus ithaburensis respectively.
It has to be noted that the CP content of kermes oak acorns was considerably
higher than that found by Kaya and Kamalak (2012) for kermes oak.
Feed intake, dry-matter digestibility and animal performance are reduced if
dietary crude protein is below 7 % (McDonald et al., 2010), which results
in energy-protein deficiency (Van Soest, 1994). Dietary CP requirements for
maintenance of free-range goats, with a live weight of 40 kg, ranges from 62
to 77 g kg-1 (NRC, 1981, 2000; Luo et al., 2004; Moore et al., 2004).
According to this study, the average CP content in both kernel and hull were
insufficient to meet these requirements.
Altitudinal zone significantly (P < 0.05) affected the cell wall
contents of both kernel and hull (Table 2). The cell wall contents of both
acorn kernel and hull were higher in the upper zone (P < 0.05; Table 2)
compared to lower and middle zones.
The fibre content of acorns is dependent on environmental characteristics
and the stage of maturity (Moujahed et al., 2005). Thus, they decrease
significantly with the maturity stage (Moujahed et al., 2005), probably due
to an accumulation of reserves in the cotyledons (Bowersox and Ward, 1968).
According to Santini et al. (1992), the feed intake of goats can be reduced
with an ADF content over 180 g kg-1 in dry-matter forages. The average ADF
content of hull was higher at all altitudinal zones. However, as the
proportion of hull to whole acorn mass ranges from 0.17 to 0.23 (Table 1), it
seems that goats that were fed only with kermes oak acorns could maximize their
dry-matter intake (Table 4). Finally, the cell wall values were in agreement
with the results of other studies carried out on oak acorns (Al Jassim et
al., 1998; Kayouli and Buldgen, 2001; Parissi et al., 2014).
Dietary fibre plays a pivotal role in goat nutrition through its influence
in and interaction with nutrient intake and digestion (Lu et al., 2005) and
in milk-fat content (Schmidely et al., 1999). According to Buxton (1996),
ideal NDF concentration ranges from 150 to 200 g NDF kg-1 DM for fattening
ruminants. For high-production lactating dairy goats, 180–200 g ADF kg-1 DM or
410 g NDF kg-1 DM is nutritionally adequate (Lu et al., 2005).
Conversely, dietary NDF content higher than 600 g kg-1 decreased DM intake due to
rumen capacity (Mertens, 1994). In the present study, even though NDF
concentrations in the hull were greater than this threshold, it seems that the
NDF content in overall acorn mass was lower than this value due to the low
proportion of hull to overall acorn mass.
Mineral composition
Mineral concentrations depend on the part of the acorn in question. On
average, mineral concentrations tended to be higher for the hull than for
the kernel. In general, the concentrations of Ca, P, Fe, Zn, and Cu of the
kernel were affected by the altitudinal zone. In particular, the
significantly higher concentrations of P, Fe, Cu, and Zn were recorded at the
lower altitude, while a higher Ca value was recorded at the upper altitude (Table 3). Conversely, the altitudinal zone only had a significant impact (P < 0.05)
on the Fe and Zn contents of the acorn hulls, with their concentration being
significantly higher at the lower altitude (Table 3).
In order to compare the results with the nutrient requirements of a mature
40 kg live-weight goat, the chemical composition, mineral contents, and Ca : P
ratio of whole acorn fruit were estimated based on the kernel : hull ratio
(Table 4).
Estimated chemical composition and mineral contents of whole acorns
based on the hull / kernel ratio of Table 1.
Parameter
Lower
Middle
Upper
Requirement*
CP (g kg-1)
43.2
39.5
37.9
77
NDF (g kg-1)
208.5
257.8
278.3
–
ADF (g kg-1)
103.0
118.2
139.0
–
Ca (g kg-1)
0.8
0.7
1.0
2.31
P (g kg-1)
0.8
0.7
0.6
1.49
Ca : P Ratio
1.0
1.0
1.7
1 : 1–2 : 1
K (g kg-1)
2.6
2.6
2.8
2.5
Mg (mg kg-1)
0.4
0.4
0.4
0.2
Fe (mg kg-1)
13.9
12.1
11.8
50
Cu (mg kg-1)
2.9
2.4
1.9
10
Mn (mg kg-1)
18.2
19.1
20.2
50
Zn (mg kg-1)
5.4
4.6
2.3
50
* Required by mature goat with 40 kg live weight (NRC, 1981; Meschy, 2000).
Effect of altitudinal zone on nutrient and energy digestibility
(MJ kg-1) of kermes oak kernel and hull.
Lower
Middle
Upper
SEM
Sig.
Kernel
IVDMD
0.779a
0.667b
0.577c
0.012
***
IVNDFD
0.645a
0.583b
0.456c
0.005
***
DE (MJ kg-1)
12.93a
10.86b
10.34c
0.081
***
GE (MJ kg-1)
18.94b
19.11b
19.62a
0.121
**
Hull
IVDMD
0.202a
0.195b
0.183c
0.0032
**
IVNDFD
0.008a
0.006b
0.007b
0.0003
*
DE (MJ kg-1)
2.92a
2.24c
2.56b
0.076
***
GE (MJ kg-1)
18.56a
17.48a
18.03a
0.391
NS
Within a row, means with different letters differ at P < 0.05.
M= mean. SEM = standard error of the mean. Sig. = significant level.
* P < 0.05. ** P < 0.01. *** P < 0.001. NS = not
significant.
Coefficients associated with mean annual air temperature
(Tave) and altitude above sea level (Y) for significant* variance
regression analysis of IVDMD, IVNDFD, and DE (MJ kg-1 DM) as dependent
variables during growth of kermes oak acorns.
Regression equation
Adjusted
SE
R2
IVDMDkernel
= 0.354 + 0.024 Tave
0.917
0.0256
IVDMDhull
= 0.157 + 0.003 Tave - 3 × 10-6Y
0.518
0.0073
IVNDFDkernel
= 0.264 + 0.022 Tave
0.926
0.0223
IVNDFDhull
= 0.005 + 12 × 10-5 Tave - 4 × 10-8Y
0.074
0.0008
DEkernel
= 2.979 + 0.517 Tave+ 0.002 × Y
0.919
0.3346
DEhull
= -1.627 + 0.221 Tave - 0.002 × Y
0.493
0.2314
* Significant at the 0.05 probability level.
Mature, 40 kg live-weight goats require about 2.31 Ca g kg-1 DM in their
diets (NRC, 1981), although lower values were proposed by Meschy (2000). The
results of this study suggest that Ca levels in whole acorn fruits (Table 4)
would be insufficient to meet animal maintenance requirements. Phosphorus is
an essential component for both plant and animal growth. The P requirements
of mature goats are 1.49 g kg-1 in the DM of their diet (NRC, 1981). In this
study, neither whole acorns nor the acorn components had sufficient P levels
to meet the requirements. Nikolic et al. (2006) also found lower values of Ca
and P in oak (Quercus robur L.) acorns collected in Serbia.
Calcium and phosphorous are among the most abundant mineral elements for
animal growth (McDonald et al., 2010). Absorption and utilization of Ca and
P is dependent on the Ca : P ratio in the diet and the presence of adequate
amounts of vitamin D (McDonald et al., 2010; NRC, 2005). The optimum Ca : P
ratio to reduce functional disorders ranges from 1 : 1 to 2 : 1 (McDonald et
al., 2010). The Ca : P ratio of the whole acorn fruit (Table 4) was at the
optimal range in all altitudinal zones and was higher in the acorn hulls
than in the kernels (Table 3) because the hulls had higher Ca and lower P
concentrations than did the kernels, which led to higher Ca : P ratios.
Mature goats require 50 g kg-1 of Fe in the DM of their diets (Meschy, 2000).
Although Fe deficiency seldom occurs in grazing ruminants, it seems that the
acorn kernels and hulls in all altitudinal zones had insufficient Fe amounts
to meet the requirements.
The Mn content of the acorn components was also insufficient to meet the
maintenance requirements of mature goats (50 g kg-1 of DM; Meschy, 2000). This
can result in a Mn deficiency with possible effects on skeletal development
and reproductive performance, even though there is doubt regarding whether
this deficiency arises under grazing conditions (McDonald et al., 2010).
All acorn components had Zn levels that did not exceed the requirement
threshold (50 g kg-1 DM; Meschy, 2000). Conversely, the acorn kernels
and hulls had sufficient K and Mg to meet the requirements of mature goats
(2.5 and 0.2 g kg-1 respectively; Meschy, 2000).
Generally, variations in the mineral content of shrubs has been attributed
to a dilution of minerals in the shrub biomass during growth, and
subsequently to refining and resorption of minerals from the plants in
autumn (Pugnaire and Chapin, 1993) and the mobilization of minerals in the
plant tissue due to the physiological functions of development (Sabaté et
al., 1992). Thus, mineral deficiencies seldom occur in grazing goats. This is
because forages have adequate mineral concentrations and because of
soil contamination of forages and direct soil consumption, which often
provide excess quantities of dietary minerals (McDowell, 2003; McDonald et
al., 2010).
Nutrient digestibility
In vitro DM digestibility, IVNDFD, and DE significantly differed (P < 0.05)
among the altitudinal zones for both kernel and hull (Table 5). The IVDMD,
IVNDFD, and DE of kernel and hull were higher (P < 0.05) in the lower
zone than in the middle and upper zone (Table 5).
The DE values of the acorn components ranged from
10.34 to 12.93 MJ kg-1 DM
for the kernels and from 2.24 to 2.92 MJ kg-1 DM for the hulls. The recommended
daily intake of DE for goats with an average live weight of 40 kg and
moderate muscular activity amounts to 10.25 MJ kg-1 DM (NRC, 1981), although
higher recommended levels have been reported (Villena and Pfister, 1990). It
seems that only in the lower zone did the DE content of whole acorns exceed
this threshold. According to McDonald et al. (2010), the reduced DE values can
be attributed to the low IVDMD and CP contents of the acorns – as confirmed
by the results of this study (Tables 2 and 5).
The acorn kernels generally had higher IVDMD values than the hulls did. This
can be explained by the higher concentration of ADF and ADL in the hull
compared to the kernel as the dry-matter lignification degree affects its
digestibility (McDonald et al., 2010).
Furthermore, the lower digestibility of kernels in the middle and upper
zones compared to the lower zone is probably a result of the higher cell wall
concentrations. The low digestibility of NDF can dramatically impact dietary
energy content and dry-matter intake (Oba and Allen, 1999). Van Soest (1994) stated that air temperature accelerates the maturation process, with
dry-matter digestibility increasing with acorn maturity (Moujahed et al.,
2005). Consequently, the overall decrease in IVDMD for the acorn kernels and
hulls along the altitudinal zones was associated with high cell wall
content.
In the present study, acorn kernel nutrient digestibility parameters (i.e.
IVDMD, IVNDFD, DE) were positively correlated (P < 0.05) with mean
monthly air temperature (r= 0.961, r= 0.965, and r= 0.946
respectively), which was negatively correlated with altitude a.s.l. (r= -0.484;
P < 0.05). Thus, it can be stated that a higher nutritive
value of kermes oak acorns at the same stage of maturity will be found at
low-altitude growing sites.
Regression analyses quantified the relationships between the IVDMD, IVNDFD,
and DE values of each acorn component (i.e. kernel, hull), and the mean
annual air temperature and altitude above sea level were independent
variables (Table 6). For the kernels, the mean annual air temperature was
associated with IVDMD and IVNDFD. Indeed, a 1 ∘C rise in air
temperature resulted in an increase of 0.024 for IVDMD and
0.022 for IVNDFD
along the altitudinal zones.
Shrubs such as kermes oak are indispensable feed resources for goat nutrition
in southern Europe. Their nutritional quality, however, is not always
affected by their chemical composition due to the high presence of secondary
compounds, especially lignin and tannins (Sallam et al., 2010). Additionally,
it has to be noted that in vitro techniques have tended to
underestimate the digestibility of shrub species (Van Soest, 1994) due to the
presence of phenolic compounds (Rogosic et al., 2009), which reduce the
enzymatic action (Nastis and Malechek, 1988). This reduction in nutrient
digestibility parameters probably arises from increased levels of lignin or
anti-nutritional factors such as tannins, as supported by Nastis and Malechek (1988),
Van Soest (1994), and Sallam et al. (2010). Based on the results of
this study, it is strongly recommended that goat shepherds prioritize
feeding their livestock with acorns collected at the lower altitudinal zone
due to these acorns' higher nutritive value.