Effect of selection for growth rate on relative growth in rabbits1,2
M. Pascual,3 M. Pla, and A. Blasco
Instituto de Ciencia y Tecnología Animal, Universidad Politécnica de Valencia,
PO Box 22012, 46071 Valencia, Spain
ABSTRACT: The effect of selection for growth rate
on relative growth of the rabbit body components was
studied. Animals from the 18th generation of a line selected for growth rate were compared with a contemporary control group formed with offspring of embryos that
were frozen at the seventh generation of selection of the
same line. A total of 313 animals were slaughtered at 4,
9, 13, 20, and 40 wk old. The offal, organs, tissues, and
retail cuts were weighed, and several carcass linear measurements were recorded. Huxley’s allometric equations
relating the weights of the components with respect
to BW were fitted. Butterfield’s quadratic equations
relating the degree of maturity of the components and
the degree of maturity of BW were also fitted. In most
of the components studied, both models lead to similar
patterns of growth. Blood was isometric or early maturing and skin was late maturing or isometric depending
on the use of Huxley’s or Butterfield’s model. Full gas-
trointestinal tract, liver, kidneys, thoracic viscera, and
head were early maturing, and the chilled carcass and
reference carcass were late maturing. The retail cuts
of the reference carcass showed isometry (forelegs) or
late maturing growth (breast and ribs, loin, hind legs,
and abdominal walls). Dissectible fat of the carcass and
meat of the hind leg had a late development, whereas
bone of the hind leg was early maturing. Lumbar circumference length was later maturing than the carcass
length and thigh length. Sex did not affect the relative
growth of most of the components. Butterfield’s model
showed that males had an earlier development of full
gastrointestinal tract and later growth of kidneys than
females. No effect of selection on the relative growth of
any of the components studied was found, leading to
similar patterns of growth and similar carcass composition at a given degree of maturity after 11 generations
of selection for growth rate.
Key words: allometry, carcass composition, growth rate, rabbit, selection
©2008 American Society of Animal Science. All rights reserved.
INTRODUCTION
Rabbit meat is produced by a 3-way cross in which
crossbred females from lines selected for reproductive
components are mated with males from parental lines
selected for growth rate. The selection of the latter has
increased the BW of the rabbits along the whole growth
curve (Blasco et al., 2003), but differences disappear
when representing the growth curves in the metabolic
scale proposed by Taylor (1980), showing that these
differences are due to a scale effect. However, selection for growth rate in rabbits may change the relative
growth of the different body components.
1
This research was supported by the Comisión Interministerial de
Ciencia y Tecnología (CICYT-AGL2002-04383-02).
2
The authors gratefully acknowledge St. Clair Taylor (Roslin Institute, Midlothian, Scotland) for his comments and advice.
3
Corresponding author: [email protected]
Received February 22, 2008.
Accepted July 3, 2008.
J. Anim. Sci. 2008. 86:3409–3417
doi:10.2527/jas.2008-0976
Relative growth of the different body components
is usually studied applying the allometric equation of
Huxley (1932), but Butterfield et al. (1983a) proposed
relating the degree of maturity of the components with
respect to the degree of maturity of the animal by a
quadratic equation. This equation leads to a better
goodness of fit in some components where Huxley’s allometric equation does not fit properly.
Studies about the effect of selection for growth rate
on relative growth in rabbits have used indirect approaches. Deltoro and Lopez (1985) compared Huxley’s
allometric coefficients of 2 lines of rabbits selected for
growth rate and litter size respectively, but the differences they found could be due to the different genetic origin of the lines. Blasco et al. (1990) estimated
Huxley’s and Butterfield’s allometric coefficients in 2
groups of animals from the same line differing in 10
generations of selection for growth rate, but the groups
were not contemporary. In this work, we study the effect of selection for growth rate on relative growth by
comparing 2 contemporary groups of rabbits with the
same genetic origin and differing in 11 generations of
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3410
Pascual et al.
selection for growth rate. Both Huxley’s and Butterfield’s models were fitted.
MATERIALS AND METHODS
All experimental procedures involving animals were
approved by the Universidad Politécnica de Valencia
Research Ethics Committee.
Animals
The experiment was carried out with animals from a
synthetic line (R) reared at the Universidad Politécnica
de Valencia. Line R is selected for growth rate between
the fourth and the ninth week of age, using individual
selection (Baselga, 2002), and it is currently used as a
terminal sire in many breeding schemes. At the seventh generation of selection of the line for growth rate,
embryos were recovered and kept frozen. After some
generations of selection, the embryos were thawed and
transferred to mature does, and offspring of the animals
obtained from these embryos formed group C. Group
S was formed with animals from the 18th generation
of selection. Both groups were reared contemporarily
under the same conditions.
Young rabbits were weaned at 4 wk old and placed in
flat-deck cages, 8 rabbits per cage, and fed ad libitum
with a commercial diet (crude protein, 16.1%; crude fiber, 16.5%; ether extract, 4.4%; ash, 8.1% as-fed basis;
NANTA S.A., Valencia, Spain). At 9 wk old, rabbits
were placed in individual flat-deck cages and fed ad
libitum with a commercial diet (crude protein, 17.5%;
crude fiber, 15.5%; ether extract, 5.4%; ash, 8.1% asfed basis; NANTA S.A.).
A total of 313 animals from both groups and sexes,
kept without reproductive activity, were weighed and
slaughtered via exsanguination at 4, 9, 13, 20, and 40
wk old (approximately 15 rabbits per group, sex, and
slaughter age). Moreover, animals slaughtered at 40 wk
old were weighed weekly from 1 to 40 wk old. After exsanguination, each rabbit was weighed, and the blood
weight was calculated as the difference between BW
and weight after exsanguination. Skin and full gastrointestinal tract were removed and weighed. Carcasses
were stored at 3°C for 24 h.
dorsal length and thigh length. Lumbar circumference
length was measured as the carcass circumference at
the level of the seventh lumbar vertebra. Perirenal and
scapular fat were separated and weighted. Dissectible
fat weight was calculated as the sum of both fat depots.
The carcass obtained was divided according to the dissection used by Deltoro and Lopez (1985), obtaining
forelegs, including the insertion muscles; breast and
ribs, by cutting at the joint between the last thoracic
and the first lumbar vertebra; loin, including sacral vertebrae and excluding the abdominal walls; abdominal
walls; and hind legs, including the coxal bone. The left
hind leg from each rabbit was dissected, and meat and
bone of the hind leg were weighed. All the weights were
measured in grams and the carcass linear measurements
in millimeters.
Statistical Analysis
As the animals were slaughtered to obtain the data,
records came from different animals along time (crosssectional studies). Data obtained from animals of the
same group-sex (selected males, selected females, control males, control females) and the same slaughter age
(4, 9, 13, 20, and 40 wk old) were considered repeated
measurements of an ideal animal and were averaged.
Huxley’s Allometric Equation. Each component was related to BW by the allometric equation proposed by Huxley (1932): y = bx k , where b is a parameter
relating the scale of measure of BW (x) and the weight
of the component (y), and k is the allometric coefficient. According to this equation, when k < 1 the component is early maturing, when k > 1 the component is
late maturing, and when k = 1 there is isometry, maturing the component and the animal BW at the same
rate.
The model fitted was
log yij = log bi + ki log x ij + eij [1]
where log yij is the logarithm of the average weight of
the data of all the rabbits of group-sex i at age j (logarithms in base 10), logbi is the value of logb for
group-sex
i, ki is Huxley’s allometric coefficient of
Carcass Dissection
group-sex i, log x ij is the logarithm of the average BW
At 24 h postmortem, carcasses were weighed to obtain the chilled carcass weight (Blasco and Ouhayoun,
1996). Liver, kidneys, thoracic viscera (the set of lungs,
thymus, esophagus, and heart), and head were removed
and weighed. The carcass obtained after removing these
parts (reference carcass; Blasco and Ouhayoun, 1996)
was weighed. Dorsal length was measured as the interval between the atlas vertebra and the seventh lumbar
vertebra. Thigh length was measured as the interval between the seventh lumbar vertebra and the distal part
of os Ischii. Carcass length was calculated as the sum of
of all the rabbits of group-sex i at age j, and eij is the
residual. When relating carcass linear measurements to
BW, yij was the cubic value of the average measurement of the data of all the rabbits of group-sex i at age
j, to relate variables of first and third order (Pézard,
1918). The GLM procedure (SAS Inst. Inc., Cary, NC)
was used.
Obtaining Mature BW. Butterfield’s equation requires estimates of the mature BW and mature weight
or length of the components. Mature BW were estimat-
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Relative growth in rabbits
Table 1. Mature BW estimated from the Gompertz model and mature BW and carcass linear measurements of the components estimated from Huxley’s allometric equation, except for full gastrointestinal tract, liver, and kidneys, where mature BW were
obtained as the mean value of the rabbits 40 wk old
Group and sex1
Component
BW, g
Blood weight, g
Skin weight, g
Full gastrointestinal tract weight, g
Chilled carcass weight, g
Liver weight, g
Kidney weight, g
Thoracic viscera weight, g
Head weight, g
Reference carcass weight, g
Forelegs, g
Abdominal wall, g
Breast and ribs, g
Loin, g
Hind legs, g
Dissectible fat, g
Meat of the hind leg, g
Bone of the hind leg, g
Dorsal length, mm
Thigh length, mm
Carcass length, mm
Lumbar circumference, mm
1
SF
CM
CF
4,639
151
767
564
2,820
81
20
49.0
206
2,422
255
176
616
497
808
75
350
52
322
105
427
227
5,082
173
722
706
3,151
78
19
54.1
204
2,777
262
202
712
592
906
111
405
49
338
111
449
243
4,410
140
725
559
2,657
88
20
46.4
195
2,296
238
160
592
458
753
91
328
47
318
101
418
223
4,953
161
735
680
3,072
81
18
51.3
195
2,691
252
190
699
564
855
137
378
50
331
106
438
240
SM = selected males; SF = selected females; CM = control males; CF = control females.
ed by fitting the Gompertz’s growth curve to the rabbits weighed weekly from 1 to 40 wk old by a nonlinear
regression using the NLIN procedure (SAS Inst. Inc.):
SM
x ijm = Aim exp[ −bim exp(−kim t )] + eijm , where x ijm is the BW of animal m from group-sex i at
age j; Aim, bim, and kim are the Gompertz’s growth curve
parameters of animal m from group-sex i; t is the age
(wk); and eijm is the residual. The average mature BW
CFi = exp(2.303 ´ SEEi 2 / 2), where SEEi is the SE of the estimate of the regression
for group-sex i. The SEEi was estimated as
SEEi =
n
å (log yij − log yˆij )2 / (n − 2), j =1
of each group-sex i (x Ai ) was calculated as the average
where yij is the average value of the component of all
from this group-sex.
The average mature weights of the components (except for the mature weights of the full gastrointestinal
tract, liver, and kidneys) for each group-sex (yAi ) were
of yij estimated after fitting equation Eq. [1].
of the estimated mature BW (Aim ) of all the rabbits
predicted by setting x i = x Ai in the previously fitted
Huxley’s equation (Eq. [1]):
k
yAi = bi x Ai i CFi , where CFi was a correction factor for group-sex i. The
correction factor was proposed by Sprugel (1983) to
correct the bias when the Huxley’s equation is fitted in
logarithm scale:
the rabbits of group-sex i at age j, and ŷij is the value
Huxley’s allometric equation did not fit properly in
the cases of full gastrointestinal tract, liver, and kidneys; therefore, their mature weight for each group-sex
i ( yAi ) was calculated as the average weight of rabbits
of group-sex i and 40 wk old.
The estimated mature weights x Ai and yAi are
shown in Table 1.
Butterfield’s Allometric Equation. Butterfield
et al. (1983a) proposed studying relative growth by relating the degree of maturity of the component (v) and
the degree of maturity of BW (u) by a quadratic equation. As v = 0 when u = 0 and v = 1 when u = 1, the
equation remains
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Pascual et al.
of k if relating degrees of maturity instead of weights (
v = u k ). A disadvantage of Butterfield’s model is that
v = qu + (1 − q )u 2 . The interpretation of q (Butterfield’s allometric coefficient) is opposite to the interpretation of k. When
q < 1 the component is late maturing with respect to
BW, when q > 1, the component is early maturing with
respect to the whole, and when q = 1, the component
matures at the same rate.
The equation was transformed to a linear form and
the model fitted was
(vij − uij2 ) = qi (uij − uij2 ) + eij , [2]
where vij is the average value of v for all the rabbits of
group-sex i at age j
(vij = yij / yAi ),
uij is the average
value of u for all the rabbits of group-sex i at age j
(uij = xij / xAi ), qi is Butterfield’s allometric coefficient for group-sex i, and eij is the residual. In the
case of the carcass linear measurements, it was observed
that Butterfield’s equation did not fit properly, due to
the adjustment of a linear measurement with respect to
a variable of third order. Therefore, vij was calculated
as
vij = (yij / yAi )3 . The GLM procedure (SAS Inst.
Inc.) was used.
RESULTS AND DISCUSSION
Huxley’s model has been widely used in allometric
studies in different species (in pig, Walstra, 1980; in
sheep, Thonney et al., 1987; in mice, Eisen, 1986; and
in rabbits, Cantier et al., 1969). However, Huxley’s
equation cannot describe properly the evolution of tissues or organs achieving a greater weight than its mature weight in previous stages of development, as is the
case of liver, which regresses after having had a maximum weight before raising maturity. Butterfield et al.
(1983a) proposed a new allometric equation, which fitted better in these cases and let defining the allometric
growth by only 1 coefficient (q) instead of 2 (b and k).
Moreover, the transformation of Huxley’s equation to
logarithm scale implies that it is not possible to obtain
the standard errors of the coefficients in the original
scale but only approximate standard errors. Both k and
q coefficients from both models define the relative
growth of the component with respect to BW, independently of the scale used. In the case of Huxley’s model,
the scale would be reflected in b, whereas in Butterfield’s model the scale is not reflected in any coefficient,
due to the use of degrees of maturity instead of weights.
However, according to St. Clair Taylor (Roslin Institute, Midlothian, Scotland, personal communication),
Huxley’s model can be also expressed only in function
it needs the estimation of mature weights of the components and the mature BW, but it fits better in some
components where Huxley’s model does not have a
good fit. In the present study, Huxley’s model did not
fit properly in the case of full gastrointestinal, kidneys,
and liver, which achieved a greater weight than the
weight at maturity and then decreased to raise their
mature weight. This lack of fit was observed also by
Butterfield (1988) when fitting Huxley’s equation to
data from some parts of the gastrointestinal tract of
sheep. According to Butterfield (1988) this pattern of
growth corresponds to q values greater than 2. Butterfield’s model fitted properly for these 3 components,
but their model was not appropriate in the case of fat
(r2 < 0.01). Butterfield et al. (1983a) did not indicate
the r2 values when fitting their model, but Blasco et al.
(1990) also found a low r2 for Butterfield’s allometric
coefficient of this tissue. There are 2 reasons for this
lack of fit. First, in Eq. [2] the coefficient q is close to
zero for the later maturing components, which is the
case of fat. Second, this low r2 might be caused by the
low percentage of this tissue in rabbit, which makes the
analysis of fat content inaccurate in this species, as discussed by Blasco et al. (1990).
Most of the k and q values were different from 1
(Tables 2 to 9), which indicates no isometry of the components studied with respect to BW. Although in Huxley’s model the weight of the component is related to
BW and Butterfield’s model relates degrees of maturity
of both components, in most of the components both
analyses were consistent obtaining k < 1 when q > 1
and k > 1 when q < 1.
We considered that the relative growth of the components had a single Huxley’s allometric coefficient (k).
Some previous works in rabbit considered changes in k
values during growth (Cantier et al., 1969; Deltoro and
Lopez, 1985), and fitted more than one straight line
with different k values instead of a single one. However,
the point of allometric change was calculated by using
statistical tools and these points had not always a physiological meaning. To avoid this problem, Deltoro and
Lopez (1988) and Vicente et al. (1989) fitted a quadratic curve, considering that the change in allometry did
not appear at a determined point. However, although
more complex models always fit better, the interpretation of the coefficients is more difficult than in simple
models. Because of that, when possible, more parsimonious models are preferred. Moreover, the graphic
representation of our data in logarithmic scale did not
show changes in Huxley’s allometric coefficient, and the
coefficients of determination were always greater than
0.94; thus only one straight line was fitted.
The weight of reference in rabbit has been usually
the empty BW (Cantier et al., 1969; Deltoro and Lopez, 1985) because it would correct variations due to
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Relative growth in rabbits
Table 2. Mean values and SEM of Huxley’s logb and allometric coefficients k for offal, organs, and chilled carcass
with respect to BW, coefficient of determination (r2), and difference, SED, and P-values between k values of selected (S) and control (C) groups and males (M) and females (F)
Response to selection
Effect of sex
Component1
logb
SEM
k
SEM
r2
S−C
SED
P-value
M−F
SED
P-value
Bl
Sk2
FGT2
CC2
Lv2
Ki2
ThV2
−1.25
−0.99
0.07
−0.50
−0.57
−0.89
−1.46
0.10
0.08
0.11
0.02
0.23
0.10
0.11
0.94
1.05
0.75
1.08
0.70
0.60
0.86
0.03
0.02
0.03
0.01
0.07
0.03
0.03
0.98
0.99
0.97
0.99
0.87
0.97
0.98
0.02
−0.02
0.02
0.00
−0.03
−0.01
−0.02
0.07
0.04
0.07
0.01
0.16
0.07
0.07
0.81
0.63
0.80
0.94
0.85
0.87
0.78
−0.06
0.08
−0.04
−0.01
0.07
0.05
0.00
0.07
0.04
0.07
0.01
0.16
0.07
0.07
0.40
0.07
0.60
0.70
0.68
0.48
0.93
1
2
Bl = blood; Sk = skin; FGT = full gastrointestinal tract; CC = chilled carcass; Lv = liver; Ki = kidneys; ThV = thoracic viscera.
k was significantly different from 1 (P < 0.05).
the repletion of the gastrointestinal tract of the animal.
However, Butterfield et al. (1983a) defended the use
of BW as the weight of reference because according to
these authors the empty BW would represent an artificial situation that is not found in real life. In addition,
using BW as the weight of reference avoids the necessity of emptying the gastrointestinal tract. According
to Butterfield et al. (1983a), variations due to the different gastrointestinal content can be avoided by weighing and slaughtering the animals at a similar hour of
the day. The use of BW instead of empty BW has been
applied in rabbits (Blasco et al., 1990), mice (Eisen,
1986), and beef (Keane and Allen, 2002).
Changes in the coefficient b from Huxley’s equation
could lead to a different percentage of the component
along the whole growth of the animal even if no change
in the k value is observed, but the selection for growth
rate and sex did not affect the b coefficient in any of the
components studied (results not shown).
those referred to the gastrointestinal tract, which represents the greater proportion at early ages (Ouhayoun,
1989). Moreover, food intake is greater in group S than
in group C (Sánchez et al., 2004). However, the relative
growth of these components was not affected by selection for growth rate (P > 0.05, Tables 2 and 3), nor was
there an effect of selection on the relative growth of the
chilled carcass (P > 0.05, Tables 2 and 3). Deltoro et
al. (1984) did not find differences either in k values of
blood between 2 lines of rabbits selected for growth rate
and litter size, respectively. Moreover, Butterfield et al.
(1983b) did not find differences in q values of blood,
skin, alimentary tract, and alimentary tract contents
between 2 different strains of rams after selecting one of
them for BW at 1 yr old for 4 generations. The relative
growth of blood, skin, and chilled carcass did not differ
between sexes either (P > 0.05). No differences between
sexes in relative growth of blood were found in rabbits
(Deltoro et al., 1984) and sheep (Thonney et al., 1987),
but Walstra (1980) observed later development of skin
of boars compared with sows. Although the k value of
full gastrointestinal tract did not differ between sexes
(P > 0.05), the q value indicated an earlier development in males. Relative growth of alimentary tract and
its contents did not differ between sexes in sheep (Butterfield et al., 1983b).
Offal and Organs
Changes in the relative growth of blood, skin, and
full gastrointestinal tract, which are the components
removed from the animal to obtain the chilled carcass,
would lead to changes in dressing percentage, especially
Table 3. Mean values and SEM of Butterfield’s allometric coefficients q for offal, organs, and chilled carcass with
respect to BW, coefficient of determination and difference, SED, and P-values between q values of selected (S) and
control (C) groups and males (M) and females (F)
Response to selection
Component1
Bl2
Sk
FGT2
CC2
Lv2
Ki2
ThV2
Effect of sex
q
SEM
r2
S−C
SED
P-value
M−F
SED
P-value
1.24
0.87
1.92
0.89
3.18
2.28
1.33
0.09
0.07
0.14
0.02
0.13
0.06
0.09
0.86
0.71
0.82
0.97
0.97
0.99
0.84
−0.10
0.04
−0.10
0.01
0.31
−0.15
−0.03
0.19
0.14
0.24
0.04
0.25
0.09
0.20
0.61
0.79
0.68
0.84
0.23
0.12
0.89
0.13
−0.28
0.72
0.00
0.09
−0.27
0.02
0.19
0.14
0.23
0.04
0.25
0.09
0.20
0.49
0.07
0.01
0.91
0.73
0.01
0.93
1
Bl = blood; Sk = skin; FGT = full gastrointestinal tract; CC = chilled carcass; Lv = liver; Ki = kidneys; ThV = thoracic viscera.
q was significantly different from 1 (P < 0.05).
2
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Table 4. Mean values and SEM of Huxley’s logb and allometric coefficients k for the different retail cuts of the
carcass with respect to BW, coefficient of determination and difference, SED, and P-values between k values of
selected (S) and control (C) groups and males (M) and females (F)
Response to selection
Component1
2
H
RC2
FL
BR2
L2
AW2
HL2
Effect of sex
logb
SEM
k
SEM
r2
S−C
SED
P-value
M−F
SED
P-value
−0.38
−0.85
−1.23
−1.33
−1.85
−2.53
−1.26
0.06
0.03
0.04
0.06
0.05
0.05
0.02
0.73
1.16
0.99
1.13
1.24
1.30
1.14
0.02
0.01
0.01
0.02
0.02
0.01
0.01
0.99
0.99
0.99
0.99
0.99
0.99
0.99
0.02
0.00
0.02
−0.03
0.02
0.00
0.01
0.04
0.02
0.02
0.04
0.03
0.03
0.01
0.63
0.90
0.38
0.54
0.43
0.95
0.44
0.05
−0.01
0.03
−0.01
−0.01
−0.03
0.01
0.04
0.02
0.02
0.04
0.03
0.03
0.01
0.18
0.67
0.08
0.79
0.82
0.30
0.39
1
H = head; RC = reference carcass; FL = forelegs; BR = breast and ribs; L = loin; AW = abdominal walls; HL = hind legs.
k was significantly different from 1 (P < 0.05).
2
The early maturing pattern of liver (Tables 2 and 3)
was not in agreement with results found by Deltoro et
al. (1984), where liver had isometric growth with respect
to the empty BW. It seems that the pattern of growth
of this organ depends on the range of ages considered,
due mainly to the poor fit of Huxley’s equation in components that achieve a greater weight than its mature
weight in previous stages, as is the case of liver. The
early maturing pattern of kidneys and thoracic viscera
(Tables 2 and 3) has been observed in several species
(in rabbit, Deltoro et al., 1984; in pig, Doornenbal and
Tong, 1981; Tess et al., 1986; Landgraf et al., 2006; in
beef, Kim et al., 2003; and in sheep, Butterfield et al.,
1983b, 1984). Mice seem to be an exception, showing
late maturing patterns in kidneys (Eisen, 1986; Shea
et al., 1987; Siddiqui et al., 1992). The relative growth
of liver, kidneys, and thoracic viscera was not affected
by selection for growth rate. No differences were found
either when comparing giant transgenic with respect
to nontransgenic mice (Shea et al., 1987), 2 different
strains of rams after selecting one of them for yearling
weight (Butterfield et al., 1983b), or rabbits selected
for growth rate with respect to rabbits selected for litter size (Deltoro et al., 1984). However, Eisen (1986),
when comparing mice selected for rapid postweaning
growth with the control line, found q values that in-
dicated a later maturing pattern of liver and k and
q values indicating earlier maturing of kidneys in the
selected line. Siddiqui et al. (1992) found, however, a
later maturing of kidneys in mice selected for increased
IGF with respect to the low line.
The relative growth of liver and thoracic viscera did
not differ between sexes (P > 0.05), agreeing with results found by Deltoro et al. (1984) in rabbit. Sex did
not affect relative growth of thoracic viscera in pigs
(Rook et al., 1987) and liver in pigs (Rook et al., 1987)
and mice (Siddiqui et al., 1992). Although the k value
of kidneys was similar for both sexes (P > 0.05), the q
value indicated a later development of these organs in
males than in females, in agreement with results found
in pig (Rook et al., 1987) and mice (Siddiqui et al.,
1992).
Retail Cuts
No effect of selection for growth rate was found on
the relative growth of the head and the reference carcass (P > 0.05, Tables 4 and 5). Deltoro et al. (1984)
did not find differences in k values of head between rabbit lines selected for growth rate or litter size. However,
an earlier development of the head of rams selected
for BW at 1 yr old than in unselected rams was found
Table 5. Mean values and SEM of Butterfield’s allometric coefficients q for the different retail cuts of the carcass
with respect to BW, coefficient of determination and difference, SED, and P-values between q values of selected
(S) and control (C) groups and males (M) and females (F)
Response to selection
Component1
2
H
RC2
FL
BR2
L2
AW2
HL2
Effect of sex
q
SEM
r2
S−C
SED
P-value
M−F
SED
P-value
1.34
0.77
0.99
0.74
0.74
0.67
0.84
0.07
0.02
0.03
0.05
0.04
0.05
0.03
0.84
0.95
0.96
0.58
0.88
0.84
0.96
0.00
0.03
−0.04
0.08
−0.01
0.07
−0.01
0.14
0.05
0.05
0.11
0.10
0.11
0.05
0.99
0.51
0.48
0.51
0.88
0.53
0.80
−0.22
0.00
−0.09
0.05
0.00
0.05
−0.05
0.14
0.05
0.05
0.11
0.10
0.11
0.05
0.13
0.97
0.11
0.66
0.99
0.64
0.33
1
H = head; RC = reference carcass; FL = forelegs; BR = breast and ribs; L = loin; AW = abdominal walls; HL = hind legs.
q was significantly different from 1 (P < 0.05).
2
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3415
Relative growth in rabbits
Table 6. Mean values and SEM of Huxley’s logb and allometric coefficients k for dissectible fat of the carcass and
meat and bone of the hind leg with respect to BW, coefficient of determination and difference, SED, and P-values
between k values of selected (S) and control (C) groups and males (M) and females (F)
Response to selection
Component1
2
DFa
MHL2
BHL2
1
2
Effect of sex
logb
SEM
k
SEM
r2
S−C
SED
P-value
M−F
SED
P-value
−3.35
−1.98
−1.04
0.25
0.04
0.09
1.45
1.24
0.74
0.07
0.01
0.03
0.97
0.99
0.99
−0.06
−0.02
0.09
0.14
0.02
0.05
0.69
0.33
0.08
−0.15
0.01
0.05
0.14
0.02
0.05
0.29
0.68
0.33
DFa = dissectible fat of the carcass; MHL = meat of the hind leg; BHL = bone of the hind leg.
k was significantly different from 1 (P < 0.05).
(Butterfield et al., 1983b, 1984). Sex did not affect the
relative growth of head and reference carcass, as observed in pigs (Rook et al., 1987) and sheep (Thonney
et al., 1987).
No effect of selection for growth rate on relative
growth was found for the different retail cuts of the
carcass (P > 0.05, Tables 4 and 5). Deltoro et al. (1984)
did not find differences in relative growth when comparing 2 lines selected for growth rate and litter size,
respectively. No differences were found between sexes
(P > 0.05), agreeing with results found in rabbits (Deltoro et al., 1984) and pigs (Rook et al., 1987).
The patterns of growth of the retail cuts are in agreement with the waves of growth defined by Hammond
(1932): first, forelegs had an earlier growth than the
breast and ribs, and hind legs had an earlier growth
than the loin and abdominal walls, which is in concordance with the distal to proximal limb wave of growth.
Second, head had an earlier growth than the breast and
ribs and loin, in concordance with the second wave of
growth from the head to the lumbar part.
The isometric growth obtained for forelegs (Tables 4
and 5) was in agreement with results obtained by Deltoro et al. (1984) in rabbits. The late maturing patterns
of breast and ribs, loin, abdominal wall, and hind legs
were also found in rabbits (Deltoro et al., 1984), in pigs
(Evans and Kempster, 1979; Rook et al., 1987; Fisher
et al., 2003), and sheep (Thonney et al., 1987).
hind leg are shown in Tables 6 and 7. Dissectible fat of
the carcass and meat of the hind leg were late maturing (Tables 6 and 7) with respect to BW, whereas bone
of the hind leg was early maturing. Results found for
fat and bone agree with those found for fat and bone
of the carcass in rabbit (Cantier et al., 1969; Deltoro
et al., 1984), in sheep (Butterfield et al., 1983a, 1984;
Thompson et al., 1985; Taylor et al., 1989), and in pig
(Evans and Kempster, 1979; Fortin et al., 1987; Wagner
et al., 1999; Fisher et al., 2003). However, the muscle
of carcass seems to follow a different pattern of growth
in other species. Although we found that meat of the
hind leg was late maturing in rabbit, muscle of the
carcass was early maturing in pig (Evans and Kempster, 1979; Fortin et al., 1987; Fisher et al., 2003) and
in sheep (Butterfield et al., 1983a, 1984; Taylor et al.,
1989). This difference in pattern of growth of muscle
is due to the little amount of dissectible fat in rabbit
compared with other species, as discussed by Blasco et
al. (1990).
A greater food intake in group S than in group C was
observed by Sánchez et al. (2004). In pigs, it has been
shown that as feed intake increases, fat and protein deposition increase, but over a particular feed intake level, deposition of protein stops and the extra-feed consumption goes into the production of fat (Whittemore,
1993). However, it seems that the selection for growth
rate in the line used in the present study has not led to
this increase of fat deposition because coefficient b for
fat in Huxley’s equation has not changed after selection (results not shown) and k and q allometric coefficients for fat did not differ between groups (P > 0.05,
Tables 6 and 7). No effect of selection for growth rate
Carcass Tissues
Values of the allometric coefficients k and q for dissectible fat of the carcass and meat and bone of the
Table 7. Mean values and SEM of Butterfield’s allometric coefficients q for dissectible fat of the carcass and meat
and bone of the hind leg with respect to BW, coefficient of determination and difference, SED, and P-values between q values of selected (S) and control (C) groups and males (M) and females (F)
Response to selection
Component1
2
DFa
MHL2
BHL2
Effect of sex
q
SEM
r2
S−C
SED
P-value
M−F
SED
P-value
0.20
0.73
1.48
0.17
0.03
0.08
<0.01
0.94
0.88
0.18
0.02
−0.17
0.36
0.06
0.17
0.62
0.73
0.34
0.17
−0.06
−0.08
0.36
0.06
0.17
0.64
0.37
0.64
1
DFa = dissectible fat of the carcass; MHL = meat of the hind leg; BHL = bone of the hind leg.
q was significantly different from 1 (P < 0.05).
2
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3416
Pascual et al.
Table 8. Mean values and SEM of Huxley’s logb and allometric coefficients k for the carcass linear measurements
of the carcass with respect to BW, coefficient of determination and difference, SED, and P-values between k values
of selected (S) and control (C) groups and males (M) and females (F)
Response to selection
Component1
2
DL
TL2
CL2
LCL2
Effect of sex
logb
SEM
k
SEM
r2
S−C
SED
P
M−F
SED
P
3.80
1.69
4.01
2.46
0.04
0.07
0.04
0.06
1.02
1.19
1.06
1.26
0.01
0.02
0.01
0.02
0.99
0.99
0.99
0.99
0.02
0.02
0.02
0.02
0.02
0.04
0.02
0.03
0.32
0.69
0.33
0.62
0.00
0.00
0.00
−0.03
0.02
0.04
0.02
0.03
0.92
0.90
0.90
0.37
1
DL = dorsal length; TL = thigh length; CL = carcass length; LCL = lumbar circumference length.
k was significantly different from 1 (P < 0.05).
2
on the patterns of growth of the meat and bone of the
hind leg was found either (Tables 6 and 7), as Deltoro
et al. (1984) found when comparing a line of rabbits
selected for litter size and Butterfield et al. (1983a) observed when comparing sheep for yearling weight with
an unselected strain. According to the results obtained
by Thompson et al. (1985), the relative growth of fat
and muscle of the carcass did not differ between sheep
selected for weaning weight and a random group, but
selected animals presented a later development of bone.
However, studies in mice show that total fat in the carcass is later maturing in mice selected for postweaning
gain (Allen and McCarthy, 1980; Eisen, 1987).
We did not find differences between sexes in relative growth of dissectible fat and meat and bone of the
hind leg (P > 0.05). Similarly, Deltoro et al. (1984) did
not find differences in muscle and bone growth in rabbit carcass between males and females, but they found
a later development of dissectible fat in females. Sex
seems to not affect relative growth of the 3 tissues in
pig (Fortin et al., 1987) and sheep (Butterfield et al.,
1984). Thompson et al. (1985), however, found a later
development of fat depots and earlier development of
muscle and bone of the carcass in sheep females than
in males.
al. (1984) found also a late maturing of carcass length
and lumbar circumference length in rabbits and Siddiqui et al. (1992) observed a late maturing pattern of
nose-anus length (thus, including head) in mice. Pugliese et al. (2003) found, however, early growth of body
length with respect to BW in pigs. Lumbar circumference length was later maturing than dorsal length,
which would lead to an increase of conformation as age
increases.
We did not find any effect of the selection for growth
rate on any of the carcass linear measurements studied
(P > 0.05), agreeing with results found by Deltoro et al.
(1984) when comparing k values of the carcass and lumbar circumference length of rabbits selected for growth
with a line selected for reproductive components. Siddiqui et al. (1992) found an earlier growth of nose-anus
measurement with respect to BW in mice females selected for increased IGF with respect to the low line,
although no differences were found between males. Any
of the k and q values of carcass length measurements
differed between sexes (P > 0.05). Allometric growth
of carcass length and lumbar circumference length has
been seen to be similar for both sexes in rabbits (Deltoro et al., 1984).
Conclusions
Carcass Linear Measurements
All the carcass linear measurements were late maturing with respect to BW (Tables 8 and 9). Deltoro et
Selection for growth rate has not affected the relative
growth of the different parts of the carcass, carcass tissues, and carcass linear measurements. Sex did not af-
Table 9. Mean values and SEM of Butterfield’s allometric coefficients q for the carcass
linear measurements of the carcass with respect to BW, coefficient of determination
and difference, SED, and P-values between q values of selected (S) and control (C)
groups and males (M) and females (F)
Response to selection
Component1
DL
TL2
CL2
LCL2
Effect of sex
q
SEM
r2
S−C
SED
P
M−F
SED
P
0.98
0.70
0.91
0.62
0.03
0.05
0.03
0.04
0.94
0.75
0.94
0.78
0.02
0.08
0.03
−0.07
0.07
0.10
0.07
0.08
0.82
0.45
0.64
0.37
−0.05
−0.08
−0.06
0.04
0.07
0.10
0.07
0.08
0.46
0.43
0.36
0.59
1
DL = dorsal length; TL = thigh length; CL = carcass length; LCL = lumbar circumference length.
q was significantly different from 1 (P < 0.05).
2
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Relative growth in rabbits
fect the relative growth of most of the components, but
males had Butterfield’s allometric coefficients showing
an earlier development of full gastrointestinal tract and
later growth of kidneys than females.
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