Intrinsic factors of acorns that influence the efficiency of their

Livestock Science 122 (2009) 281–285
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Livestock Science
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / l i v s c i
Intrinsic factors of acorns that influence the efficiency of their consumption
by Iberian pigs
V. Rodríguez-Estévez ⁎, A. García, A.G. Gómez-Castro
Departamento de Producción Animal, Facultad de Veterinaria, Universidad de Córdoba, Campus de Rabanales, Córdoba 14014, Spain
a r t i c l e
i n f o
Article history:
Received 25 July 2008
Received in revised form 15 September 2008
Accepted 16 September 2008
Keywords:
Acorn consumption
Daily intake
Efficiency
Feed selection
Grazing
Montanera
Sustained production
Waste
a b s t r a c t
This paper deals with the efficiency of Iberian pigs when shelling acorns (Quercus ilex fruits)
during the free ranging fattening period in the dehesa. The amount of kernel wasted per acorn
(18.9 ± 1.2%) presents a high degree of variation influenced by differences in the morphology
and size of the acorns. A positive correlation is observed between the weight of the waste
kernel and the weight of the whole acorn, and the diameter. However, the percentage of waste
correlates negatively with the weight of the kernel and its DM percentage, with the ratio kernel
weight/shell weight, and with the percentage of kernel. On the other hand, the percentage of
kernel wasted, correlates positively with the percentage of kernel. Oppositely to the traditional
knowledge and opinion of farmers, it has been found that the bitterness of the kernel, assessed
sensorially by humans, does not influence the amount of kernel wasted. The results indicate
that it is possible to make predictions about wastage based on two simple linear regression
models: dry kernel wasted = 0.404 × fresh shell weight (R2 = 0.898; P = 0.000) and percentage of
fresh kernel wasted = 0.893 × percentage of fresh shell (R2 = 0.875; P = 0.000). It would be
necessary to consider the kernel wastage in acorn conversion rate calculations. Also in the
processes of acorn selection for tree repopulation in dehesas for pig finishing it would be
convenient to consider the correlations between the characteristics of the acorns and the
amount of kernel waste, using the regression models defined.
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
Iberian pigs used for the production of cured products
have traditionally eaten the natural resources available in the
dehesa (pastureland with evergreen oaks and cork trees) in
the final fattening stage, which takes place during the montanera (mast-feeding) season. During this period, pigs only
eat grass and fallen acorns; Rodríguez-Estévez et al. (2008a)
suggest a daily DM intake of 3.1–3.6 kg of acorn kernel and
0.38–0.49 ± 0.04 kg of grass.
According to most authors (Carbonero et al., 2003; García
et al., 2003; López-Carrasco et al., 2004), throughout the
fruiting period, the characteristics and kernel composition for
acorns from the evergreen oak (Q. ilex rotundifolia) remain
constant for a tree. Furthermore Carbonero et al. (2002) did
⁎ Corresponding author. Tel.: +34 957218083.
E-mail address: pa2roesv@uco.es (V. Rodríguez-Estévez).
1871-1413/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.livsci.2008.09.011
not detect size differences in crown position. Acorn kernel is
very easy for the pigs to digest (Torrent et al., 1961; Varela
et al., 1965; Nieto et al., 2002), but the shell has no nutritional
value. However, Iberian pigs are capable of shelling acorns and
consuming only the kernel, discarding the shell (Aparicio
Macarro, 1964; Laguna Sanz, 1998). However, a certain amount
of the kernel is wasted. Waste is defined as the amount of
kernel that, once in the mouth of the pig, is spat out onto the
ground because it is stuck to the shell or for other reasons.
Since the pigs do not move whilst they are shelling and
chewing the acorns, the remaining shell and kernel are
dropped around the point of consumption (Rodríguez-Estévez
et al., 2007). Hence, once the ratio between the weight of the
shell and the weight of the acorn has been established, based
on the weight of the present shells, the amount of acorn (as
well as kernel) consumed can be established and the efficiency
of the pigs when shelling acorns can be calculated based on
the leftover kernel found at the point of consumption.
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V. Rodríguez-Estévez et al. / Livestock Science 122 (2009) 281–285
Table 1
Descriptive statistics of acorns and their components.
Whole Shell
acorn (g)
(g)
Wet basis 6.33 ±
0.31
(0.3)
Dry basis 3.75 ±
0.20
(0.33)
−
X ± SE (CV).
1.29 ±
0.06
(0.29)
0.75 ±
0.04
(0.33)
Kernel % Kernel/
Kernel
Kernel Shell
(g)
moisture moisture of whole shell
acorn
(%)
(%)
5.04 ±
0.27
(0.33)
3.01 ±
0.17
(0.36)
42.45 ±
1.49
(0.22)
–
40.47 ±
1.07
(0.17)
–
78.99 ±
0.79
(0.06)
79.28 ±
1.05
(0.08)
4.00 ±
0.17
(0.27)
4.36 ±
0.31
(0.45)
The aim of this paper is to quantify the kernel wastage and
to study if the characteristics of acorns could have any
influence on the peeling efficiency of the Iberian pigs.
2. Materials and methods
The experimental procedures and animal care conditions
were approved by the Animal Experimentation Ethical
Committee of the Universidad de Córdoba, Cordoba, Spain.
2.1. Animals and handling
The study was performed using a herd of 86 Iberian pigs
castrated. During 30 d, since October first, the flock grazed in an
adaptation enclosure of 10 ha of dehesa beside the experimental
paddock, receiving a daily commercial feed ration (500 g per
head and day).
Pigs aged 13 ± 0.5 mo and with an average weight of 110 ±
2.4 kg of BW when they were put out to graze on fodder (on first
November), with no supplementary feed, in a dehesa containing
evergreen oaks (Q. ilex rotundifolia) (27 ± 0.8 adult trees/ha),
located in Fuenteobejuna (Sierra Morena, Cordoba, Spain).
2.2. Sampling the oak trees and recording the acorns consumed
Sampling was done 9 d after the pigs had been put in the
experimental paddock.
Samples of acorns and waste of their consumption (wasted
kernel and shells) were gathered from beneath the canopy of
40 non-adjacent evergreen oaks (there were at least two trees
between one sampling tree and another), located on a strip of
land approximately 30 m wide.
Under each of these trees, approximately 300 g of acorns
(≈50 acorns) were randomly collected. From each tree a
subsample of 20 acorns was and subjected to the following
measurements were taken: length (L), maximum diameter
(D), L/D ratio, fresh acorn weight, fresh kernel weight and
fresh shell weight. Once these acorns were weighed, they
were dried at a temperature of 103 °C with forced ventilation
for 24 h and then weighed to obtain the dried weights of the
kernel (KD) and the shell (SD).
Using another sample of the acorns from each oak tree,
which had been conveniently peeled and chopped, a sensorial
analysis was performed in a tasting room in accordance with
the procedure defined by Costell (2002), by 4 trained tasters,
who classified the acorns into one of three categories: 0 = no
bitter (“sweet”); 1 = few bitter and 2 = bitter.
Also under each of these trees, using a 1 m × 1 m frame
placed randomly, all the shell remains (SR) and kernel remains
(KR) inside the frame were collected. These remains (SR and
KR) present were dried and weighed, obtaining the dried
weight of the shell remains (SRD) and kernel remains (KRD)
respectively, and these values were used in this study to avoid
possible variations in weight owing to a loss of moisture in the
remains that had been exposed to the elements.
The number of acorns consumed (NA) per square metre
was determined using the expression NA = SRD / SD; the
dried waste kernel per acorn consumed (WK) was calculated
as WK = KRD / NA and the percentage of dried kernel wasted
(PKW) can be determined as PKW = WK × 100 / KD.
2.3. Statistical analysis
The statistical software package SPPS© was used for
statistical analysis. The bivariate correlations between the
different factors that might be linked with the characteristics
of the acorns and their waste were calculated using Pearson's
coefficients, using the averages per tree as the input data. In
order to compare waste between the three categories of
bitterness defined, Tukey's HSD multiple comparison test was
used (α = 0.05). In order to establish a prediction model for
Fig. 1. Weight and percentage distribution of the acorn components.
V. Rodríguez-Estévez et al. / Livestock Science 122 (2009) 281–285
Table 2
Size and weight of acorns corresponding to each bitterness category.
Bitterness
category
Whole
acorn (g)
Dry
kernel (g)
Length
(mm)
Diameter
(mm)
Length/
diameter
0 = no bitter
1 = few bitter
2 = bitter
6.0 ± 0.4
6.9 ± 0.7
6.3 ± 0.5
2.7 ± 0.2
3.6 ± 0.4
2.9 ± 0.2
37.0 ± 1.0
37.5 ± 0.9
36.7 ± 0.9
16.2 ± 0.5
16.5 ± 0.6
16.3 ± 0.4
2.3 ± 0.1
2.3 ± 0.1
2.3 ± 0.1
kernel waste, the variables that had displayed a significant
Pearson coefficient with waste were used, following the
stepwise method, introducing or eliminating variables from
the model depending on the significance (probability) of the
F value or the F value itself (criterion: F probability ≤0.05 for
entry, F probability ≥0.1 for removal), until the model was
obtained with the best fit.
3. Results
On average, acorns in this experimental field measure 37 ±
0.5 mm (range: 28.8 to 41.6) in length and the greatest transversal diameter is 16.3 ± 0.3 mm (12.8 to 21.4), which gives an
average length to diameter ratio of 2.3 ± 0.1 (between 1.8 and
3.1). Besides, these acorns weigh 6.3 g (3.18 to 13.1); of which
1.3 g correspond to shell and 5.0 g to kernel (Table 1 and Fig. 1).
Classification into the pre-established categories of bitterness revealed that the proportion of no bitter to bitter acorns
was the same (37.5%) and that 25% acorns were few bitter.
Besides the test HSD of Tukey did not show any significant
difference between bitterness categories when comparing
acorn size and weight (Table 2).
Table 3 shows that, during 10 d grazing, the pigs consumed
an average of 26.1 acorns/m2 of canopy cover, with a high
variation coefficient. During this period, waste kernel accounted
for 19%, which represents 0.53 ± 0.04 g per acorn consumed. As
indicated by the variation coefficients, the amount of kernel
wasted varied a great deal, between 6.0 and 33.9%, which
represents between 0.2 and 1.2 g of kernel DM per acorn.
3.2. Relationship between the characteristics of the acorn and
the percentage of waste
Table 4 shows the Pearson coefficients for the bivariate
correlations between the characteristics of the acorns and
their waste. The variables that displayed the highest correla-
Table 3
Descriptive statistics of the kernel and shell remains (DM) found and the
number of corresponding acorns consumed.
Fresh basis
Dry basis
Shell
Number
Kernel
of acorns
available remains
2
(g/m2)
consumed/m in the
acorns
(g/m2)
26.11 ± 2.59
(0.63)
−
X ± SE (CV).
73.56 ±
6.71
(0.58)
17.76 ±
1.07
(0.38)
Kernel
Kernel
remains used
2
(g/m ) per
acorn
(g)
Kernel
wasted
per
acorn
(g)
Kernel
wasted
(%)
2.47 ±
0.16
(0.42)
0.53 ±
0.04
(0.43)
18.94 ±
1.19
(0.4)
12.6 ±
0.88
(0.44)
Table 4
Pearson's correlation matrix of the average acorn variables studied to explain
fresh or dry kernel waste.
Whole F
acorn
weight
F shell
weight
F kernel −0.272
waste %
D kernel
0.475 ⁎⁎
waste %
0.079
D shell
weight
DM % in
the shell
0.11
0.134
0.477 ⁎⁎ 0.41 ⁎⁎
0.038
DM % in
Number
F kernel
the kernel of acorns waste %
consumed/
m2
F kernel −0.438 ⁎⁎ −0.186
waste %
−0.37 ⁎
D kernel −0.071
waste %
Length/
diameter
3.1. Characteristics of the acorns
283
F kernel
0.078
waste %
D kernel −0.249
waste %
F kernel/
F shell
−0.33 ⁎
−0.426 ⁎⁎
0.439 ⁎⁎
0.381( ⁎)
D kernel
waste %
Length
Diameter
1
0.598 ⁎⁎
−0.126
−0.162
0.598 ⁎⁎
1
0.177
D kernel/ F shell %
D shell
−0.475 ⁎⁎ −0.529 ⁎⁎
0.031
F kernel D kernel
weight
weight
−0.122
0.457 ⁎⁎
F kernel %
0.496 ⁎⁎ −0.494 ⁎⁎
−0.063
0.065
Notation: F, fresh; D, dried.
⁎⁎ Correlation is significant at 0.01 (bilateral).
⁎ Correlation is significant at 0.05 (bilateral).
tion with waste were those related to the weight of the kernel
and its percentage of DM.
A significant negative correlation was found between the
amount of kernel wasted and the number of acorns
consumed/m2 (P = b0.05); in other words, less waste was
found in relation to the evergreen oaks under which more
acorns had been consumed at the start of the mast-feeding
season.
Furthermore, Tukey's HSD multiple comparison tests
(Table 5) did not reveal any differences in the weight of the
kernel waste in relation to bitterness; however, the percentage of waste was higher in no bitter (“sweet”) acorns than in
those that were few bitter.
The eight explanatory variables correlated with kernel
waste (Table 4) allow for the development of different regression models. The models that offer the best fit to explain the
weight of kernel wasted are these linear regression models:
(i) DM weight of the waste kernel = 0.404 × fresh shell
weight (R2 = 0.898; P = 0.000); (ii) DM weight of the waste
k e r ne l = 0 . 2 2 × f re sh sh el l w ei g h t + 0 . 0 2 × di a me t er
(R2 = 0.898; P = 0.000).
Table 5
Tukey's HSD multiple comparisons tests between bitterness and kernel waste.
Kernel waste
Bitterness
N
Mean ± SE (CV)
Weight (g)
0
1
2
0
1
2
15
10
15
15
10
15
0.58 ± 0.05 (0.33)
0.46 ± 0.06 (0.43)
0.54 ± 0.07 (0.52)
22.3 ± 1.35 (0.23)a
14.47 ± 2.56 (0.56)b
18.58 ± 2.03 (0.42)ab
Percentage
Bitterness: 0, no bitter; 1, few bitter; 2, bitter.
ab
Within a column, means without a common superscript letter differ
(P b 0.05).
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V. Rodríguez-Estévez et al. / Livestock Science 122 (2009) 281–285
Table 6
Linear regression models and main statistics for the percentage of waste
kernel.
Models
R2
Percentage of waste K = 0.89 ⁎ Percentage of F S
Percentage of waste K =−1.86⁎F K weight− 0.29⁎S
DM percentage−2.16⁎Length+ 6.02⁎Diameter+ 32.12⁎ Length/
diameter− 0.59⁎F K percentage
Percentage of waste kernel = − 0.38 ⁎ Shell DM + 0.91 ⁎ F shell
percentage + 0.29 ⁎ F kernel percentage
Percentage of waste K = − 0.31 ⁎ K DM + 2.74 ⁎ F K/F S + 1.26 ⁎ F S
weight
0.90 ⁎⁎⁎
0.92 ⁎⁎⁎
0.91 ⁎⁎⁎
0.91 ⁎⁎⁎
Notation: F, fresh; D, dried; DM, dry matter %; K, kernel; S, shell.
⁎⁎⁎ P b 0.001.
By other side the linear regression models that offer the
best fit to explain the percentage of kernel wasted are showed
in Table 6. Although there are models with a higher value for
R2, the simplest model was thought to be of greater interest,
in which the explanatory variable is percentage of shell.
4. Discussion
4.1. Characteristics of the acorns
According to other authors, acorns from the evergreen oak
(Q. ilex rotundifolia) come in a wide variety of shapes, sizes
and compositions among different trees (Ramírez Lozano and
Gómez Castro, 1982; Rodríguez-Estévez et al., 2008b). However, when studying acorns in a nearby area (Valle de Los
Pedroches, Cordoba), Fernández et al. (2004) found similar
average acorn weights to ones from this experimental field.
Besides, the shell percentage is in accordance with the 18–
20.3% reported by Mazuelos Vela et al. (1966).
The high variation coefficient of the average number of
consumed acorns/m2 of canopy cover can be explained by the
mentioned variety of acorns, the varying availability of acorns
under the canopy, whether they were appetising to the pigs
and by the selections made by the animals (RodríguezEstévez et al., 2008c).
In spite of the amount of kernel wasted, pigs have to peel
the acorns because the shell is not broken down either by the
enzymes in the small intestine or by fermentation in the large
intestine (Nieto et al., 2002) owing to the levels of tannin and
lignin present, which make up 8.5 and 30.4% of the shell's DM,
respectively (Morales et al., 2002). These tannins have a
negative affect on digestibility, especially of proteins and
amino acids, which according to Aparicio Macarro (1964) and
Nieto et al. (2002), are in themselves the main limiting factor
of the pigs' diet during the mast-feeding period.
4.2. Relationship between the characteristics of the acorn and
the percentage of waste
The more kernel an acorn has, the more kernel will be
wasted; but the more kernel an acorn has and the dryer that
kernel is, the less percentage of kernel is wasted. The real
consumption of kernel depends on the weight of the acorn, the
weight of the shell and the weight of the kernel wasted (Fig. 1).
It seems evident that the amount of wasted kernel that is
discarded because it is stuck to the shell (in other words, for
mechanical reasons) might be linked to the size of the acorn –
defined by its weight – or to the relative amounts of kernel
and shell or the ratio between both which, to a certain extent,
is defined by the length and diameter of the acorn, or their
ratio, which establishes the acorn's degree of sphericity.
Similarly, the amount of moisture in an acorn might affect the
ease with which the kernel comes away from the shell and
which can therefore remain in the mouth.
The bivariate correlation coefficients between the characteristics of the acorns and their waste could be low (Table 3)
because kernel waste does not appear to be the result of an
isolated variable.
In contrast, the flavour of the kernel may determine how
appetising it is to the pigs and their chewing efficiency. In this
sense the traditional understanding of pig farmers maintains
that pigs prefer larger and sweeter or no bitter acorns (Acosta et
al., 2001); in other words, its bitterness or no bitterness, from a
human's perspective (Acosta et al., 2001), might be responsible
for how appetising acorns from a certain tree are to pigs.
However the result found is contrary to expectation based on
the traditional farmer's knowledge; the level of bitterness,
assessed sensorially by humans, did not seem to have a direct
influence on the efficient use of the kernel, since the pigs make
better use of acorns with an intermediate bitterness.
Other authors have also recorded a preference towards
larger acorns (García et al., 2003). If, as Aran (1946) claims,
pigs waste acorn kernel for reasons of preference, the acorns
that have the least amount of waste should be the largest
(length and diameter), heaviest and sweetest ones. On the
contrary, there is a positive correlation between the amount
of kernel wasted and the weight of the whole acorn (P b 0.01)
and its diameter (P b 0.01). However, there is a negative
correlation between the percentage of kernel wasted and the
weight of the dried kernel (P b 0.01) and the weight of the
fresh kernel (P b 0.05) (Table 3).
Aparicio Macarro (1964) indicates that Iberian pigs leave
the kernel from the distal end of the acorn. According to this
assertion, more oval shaped acorns (long and narrow), with a
higher length/diameter ratio, should display a higher percentage of waste; however, this study found no significant
correlations in this respect (Table 3).
The variables that displayed the highest correlation with
waste were those related to the weight of the kernel and its
percentage of DM. The more kernel an acorn has, the more kernel
will be wasted; but the more kernel an acorn has and the dryer
that kernel is, the less percentage of kernel is wasted (Table 3).
The results confirm that: given the kernel is wasted when
the shell is spat out, the greater the proportion of shell, the
more kernel will stick to it; and also that the less moisture
there is in the kernel, the more easily it will come away from
the shell. Hence, the highest and most significant bivariate
correlations found correspond to the correlation between the
amount of dried kernel wasted and the ratio between dried
kernel and dried shell.
According with the obtained results, choosing the larger
acorns pigs would be more efficient when shelling acorns. This
could be enough to explain why pigs prefer acorns with higher
DM kernel percentage at the beginning of the montanera and
also why they always prefer the heaviest acorns (RodríguezEstévez et al., 2008c). Besides, this selective consumption is in
accordance with Aparicio Macarro (1964), Vázquez (1998),
Acosta et al., (2001) and García et al. (2003) and goes along
V. Rodríguez-Estévez et al. / Livestock Science 122 (2009) 281–285
with the theory of Optimal Foraging (Emlen, 1966), which
states that animals try to maximise their net rate of energy
intake per unit time through the selection of their food intake.
The negative correlation found between the amount of
kernel wasted and the number of acorns consumed/m2 might
be indicative of the relationship between waste and preference reported signalled by Aran (1946). However, bearing
in mind the theory of Optimal Foraging (Emlen, 1966),
consumption (appetising qualities) might be linked with
abundance of supply. On the other hand, according to the
perceptions of certain pig farmers, when there are lots of
acorns under the canopy of a tree, the pigs spoil and waste
many of them (Acosta et al., 2001); although this aspect has
not been studied here. Furthermore, as long as there are more
acorns available, the pigs tend to reject what has been left
over from other acorns and any split acorns (Aparicio
Macarro, 1964). However, the quantitative importance of
this waste led to the traditional practice – once all the acorns
in a plot of land had been finished and the fattened pigs had
been taken out of this area – of bringing in young pigs to use
up the leftovers and the rejected acorns (Laguna Sanz, 1998;
Acosta et al., 2001). Finishing pigs could be encouraged to
feed on the remaining kernels complementing their diet with
commercial feed when acorn mast has run out (this period of
assistance feeding is called recebo).
5. Conclusion
The quantification of kernel waste necessarily leads us to
review traditional estimations of conversion rate and daily
consumption of acorns and acorn kernel by Iberian pigs. The
real consumption of kernel depends on the weight of the
acorn, the weight of the shell and the weight of the kernel
wasted. Bearing in mind that the amount of acorns is the most
limiting factor in the montanera traditional fattening system,
grazing should be organised by plots of land, so that once the
supplies of whole or intact acorns from each plot have been
exhausted, the younger pigs could be brought in earlier to use
up the remaining kernel before it starts to deteriorate,
especially in traditional extensive and organic farming
systems, which use little supplementary feed.
Furthermore, the selection of animals used in extensive
farming should be monitored in order to ensure that it takes into
account functional characteristics, such as acorn shelling
efficiency, which is currently not included in the Iberian pig
breed standards or in the selection programmes. Moreover, it
should also be borne in mind that the aims for selection and
improvement in extensive farming systems are different to those
pursued in intensive farming systems that do not use pasture
grazing. The selection of more efficient breeds of pig could
enhance the benefits of the montanera fattening system.
From an environmental perspective, the waste kernel is an
easily accessible resource for other species of wild animals
that consume acorns.
Finally, in the process of shrub clearing of initial Mediterranean forest by man to obtain a savannah-like ecosystem and in
the process of acorn selection for tree repopulation intended for
future grazing lands, the correlations between the characteristics of the acorns and the amount of waste should be
considered, using the regression models defined in this paper.
These practices would lead to a better use of natural resources.
285
Acknowledgments
This research has been funded by the Dirección General de
la Producción Agrícola y Ganadera of the Junta de Andalucía.
Authors would like to thank the company Turcañada S.L. for
providing the necessary resources.
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