Variability in bone mineralization among purebred lines of meat-type chickens
P. N. Talaty,* M. N. Katanbaf,† and P. Y. Hester*1
*Department of Animal Sciences, Purdue University, West Lafayette, IN 47907;
and †Cobb-Vantress Inc., Monticello, KY 42633
ABSTRACT The variability of bone traits was assessed
in purebred lines of meat-type chickens using dualenergy x-ray absorptiometry. Experiment 1 evaluated
changes in bone mineralization and size traits of the
tibia and humerus in 4 purebred lines from 6 to 24 wk
of age. Experiment 2 compared the same traits of the
tibia, radius, and ulna of 9 purebred lines at 6 wk of
age. Differences in bone traits among purebred lines
were apparent in both experiments. Of the 4 purebred
lines compared in experiment 1, line C demonstrated
the best phenotypic traits relative to bone quality.
Even though line C had the longest tibia, one of the
largest bone areas, one of the heaviest BW, and one
of the highest bone mineral content (BMC) at 24 wk
of age, the tibia of line C did not become less dense
in mineral as this line of chickens approached sexual
maturity as did the tibia of the other purebred lines
of chickens. Specifically, its tibial bone mineral density
(BMD) showed age-related increases unlike the other
purebred lines of chickens, which showed little change
in tibial BMD from 6 to 24 wk of age. In experiment
2, all bone traits as well as BW were different among
purebred lines (P < 0.001). The 2 purebred lines (7
and 8) with the lightest 6-wk-old BW (2,033 and 2,055
g, respectively) had diverse skeletal traits. Birds of line
7 had the lowest BMD (0.1131 g/cm2), BMC (1.05 g),
shortest bone length (69.2 mm), and smallest bone area
(8.0 cm2); however, the other purebred line low in BW
(line 8) showed the opposite trend in that bones from
these birds were the highest in BMD (0.1276 g/cm2),
BMC (1.38 g), bone length (74.6 mm), and area (9.2
cm2) when compared with all of the other lines. In conclusion, several purebred lines of meat-type chickens
expressed large differences in bone traits, suggesting
the potential to genetically select birds for increased
BMD.
Key words: bone mineralization, bone size trait, dual-energy x-ray absorptiometry,
purebred line, meat-type chicken
2009 Poultry Science 88:1963–1974
doi:10.3382/ps.2008-00478
INTRODUCTION
Meeting the US consumer demand for white meat
and the desire of industry to reach market weights in
as short a life cycle as possible have made meat-type
fowl more susceptible to metabolic disorders such as leg
abnormalities. Skeletal integrity problems are a major
welfare issue because lameness is a source of bird discomfort (Danbury et al., 2000). It also results in economic losses for the meat industry (Cook, 2000). Condemnations and downgrades at slaughtering due to leg
problems range from 1.2 to 5.6% (Morris, 1993).
There is a genetic foundation for the incidence of
leg abnormalities. A comparison of 4 different crosses
of commercial broilers demonstrated different vulnerabilities in the incidence of tibial dyschondroplasia and
©2009 Poultry Science Association Inc.
Received November 3, 2008.
Accepted April 22, 2009.
1
Corresponding author: [email protected]
walking ability implicating genotype as a contributing
factor (Kestin et al., 1999). In addition, the walking
ability of turkey toms can be improved by selecting
turkeys with wider shanks (Nestor et al., 1985; Nestor
and Emmerson, 1990).
Growth rate is also considered a major factor affecting leg abnormalities among meat-type fowl. Feed restricting a fast-growing line of broilers improved mineralization and porosity to the level observed in the tibia
of the slow-growing broilers, strongly suggesting that
growth rate was a major contributor to poorer bone
quality (Williams et al., 2003).
Bone mineralization is an important factor reflecting the status of skeletal health. A study conducted by
Mazzuco and Hester (2005) reported that as the bone
mineral density (BMD) of the excised tibia decreased
in White Leghorns, the incidence of bone breakage increased (r = −0.54, P < 0.05). Calcium is an important
factor in maintaining bone mineralization (Rath et al.,
2000). Broilers fed increasing levels of dietary Ca and
P showed linear and quadratic increases in BMD and
bone mineral content (BMC) with correlation coeffi-
1963
1964
Talaty et al.
cients between dietary Ca and BMD, BMC, shear force,
and ash of 0.91, 0.88, 0.48, or 0.89, respectively (Onyango et al., 2003).
Dual-energy x-ray absorptiometry (DEXA) is a noninvasive technique that allows for in vivo measurements
of bone in which its mineralization and growth can be
monitored over time as a bird ages. Because of its high
correlation with live scans (Schreiweis et al., 2005) and
bone ash (Onyango et al., 2003), excised bones can also
be scanned using DEXA to assess bone quality. The objective of the current study was to determine the variability of BMD, BMC, and bone size traits of the tibia
and wing bones in purebred lines of meat-type chickens.
This study was conducted to determine if differences
in skeletal traits exist among several purebred lines of
meat-type chickens and to identify genetic lines that
could possibly be used in the development of progeny
with improved bone mineralization.
MATERIALS AND METHODS
Experiments were conducted under guidelines approved by the Purdue University Animal Care and Use
Committee.
Experiment 1
Male and female chicks of 4 purebred lines of meattype chickens (A, B, C, and D) were hatched at Purdue
University. Birds were sexed, banded in the right wing,
and housed in littered floor pens at Purdue University
Poultry Research Farm. Males were raised separately
from females. Each of the 4 commercial lines of male
and female birds was assigned randomly to 3 replicate
pens/line of chicken per sex for a total of 24 pens. During brooding, 22 chicks were placed in each pen, resulting in a stocking density of 1,681 cm2/bird. Bird
numbers within a pen were reduced at 4, 6, and 13
wk of age by weighing each bird individually and culling the lightest birds or any birds with obvious defects
such as leg problems. At 4 wk of age, bird numbers
were reduced to 19 chickens/pen, allowing for an equal
density among pens of 1,960 cm2/bird. One feeder was
provided for each pen with a circumference of 135 cm,
allowing for 7.1 cm of feeder space/bird. At 6 wk of age,
bird numbers were reduced to 14 chickens (9.4 cm of
feeder space/bird), allowing for an equal density among
pens of 2,861 cm2/bird. At 13 wk of age, bird numbers
were reduced to 11 chickens/pen, allowing for 15 cm of
feeder space/bird using feeders with a wider circumference (165 cm).
One-day-old chicks were injected s.c. in the neck with
Marek’s disease virus and viral arthritis vaccine and
spray-vaccinated with Newcastle disease and infectious
bronchitis. At 2 wk of age, vaccines for Newcastle disease, infectious bronchitis, and infectious bursal disease
were administered in the water. A bursal booster was
administered in the water at 21 d of age. Coccidiosis
vaccine was given at 6 wk of age, followed by Newcastle
disease, infectious bronchitis, and bursal vaccines at 10
wk of age, all administered through the water. At 13 wk
of age, female chickens were administered chicken anemia virus by wing web. Females were given s.c. boosters of Newcastle disease, infectious bronchitis, viral arthritis, and infectious bursal disease at 13 and 18 wk
of age. Avian encephalomyelitis and fowl pox vaccines
were administered by wing web and Salmonella given
s.c. at 13 wk of age. Vaccines for Newcastle disease and
infectious bronchitis were administered at 15 wk of age
via the water and Salmonella vaccine was administered
s.c. at 18 wk of age.
A photoperiod of 23L:1D at 60 lx was used the first 3
d of age. A light schedule of 20L:4D at 60 lx was used
from 4 to 42 d of age followed by 8L:16D from 43 to 139
d of age. Light intensity was maintained at 20 lx from
43 to 86 d of age and at 2.5 lx from 87 to 139 d of age.
At 140 d of age, light hours were stepped up to 11L:13D
at 40 lx with weekly 1-h increases until termination of
the study at 24 wk of age.
Ambient room brooding was used with temperature
maintained at approximately 35°C the first week of the
chick’s life. Subsequently, the temperature was reduced
by 1.7 to 2.8°C each week until a temperature of approximately 21°C was reached; this temperature was
maintained until termination of the experiment.
Feed was formulated to meet or exceed nutrient recommendations of NRC (1994). Birds were fed a crumbled starter diet from 1 to 21 d of age and a pelleted
finisher diet from 22 to 42 d of age. Feed was provided
for ad libitum consumption up to 6 wk of age. At 43 d
of age, the diet was switched to a pullet grower diet and
feed restriction was initiated. A breeder diet was fed
from 141 d of age until the end of the experiment. The
amount of feed restriction differed among the 4 lines of
birds as well as between males and females. Individual
BW were collected on the same day and the same time
of day each week to ensure that the correct amount of
feed was given. Each week, 5 birds/pen were selected
randomly and individual BW was determined. The average BW for each line and sex of birds (n = 15 birds/
line per sex), CV, and uniformity (percentage of birds
within ±10% of average BW) were calculated. Feed levels were adjusted each week to ensure that the BW targets were achieved as recommended by Cobb-Vantress
management. When the birds met their target BW,
the recommended daily feed allocations were continued.
If the weekly average BW was below the target BW,
the daily feed allocations were slightly increased. Water
was provided for ad libitum consumption throughout
the study.
Starting at 6 wk of age, 3 birds without any defects
and a gait score of 0 (Garner et al., 2002) were selected
from each pen and the entire left humerus and tibia
were scanned for BMD and BMC using DEXA (Norland
Medical Systems, Fort Atkinson, WI). Scanning began
at the proximal end of the bone and took approximate-
BONE MINERALIZATION IN PUREBRED LINES OF MEAT-TYPE CHICKENS
ly 10 min for each bone. Birds (6 and 15 wk of age) or
parts of birds (wing and drum at 24 wk of age) were
positioned in such a manner to allow similar orientation
of the respective bone for each scan. The DEXA unit
used a moving x-ray generator that produced photons
at 2 energy levels. A collimated scintillation detector
moved simultaneously on the opposite side of the bone
measuring flux. As the beam passed through the limb,
photon output was filtered to produce 2 distinct peaks
that distinguished soft tissue from bone, generating
BMD and BMC values. From each scan, bone length,
width, and area were also determined. Bone area was
indicative of the periosteal surface of the bone. Individual BW was also determined after each scan. Blue
livestock paint was applied to the back feathers to allow
for easy identification of birds 9 wk later so as not to be
used in subsequent scans at 15 and 24 wk of age. Use of
blue paint was discontinued after the first scan because
of cannibalism problems; therefore, scanned birds were
identified with plastic leg rings placed on the shank. To
avoid selecting birds that could later develop skeletal
defects, repeated scans of the same bird throughout
the study were not done. Instead, different birds were
scanned at each age. At 6 and 15 wk of age, live scans
were collected. Chickens were restrained without anesthesia and their bones were scanned in vivo as described previously (Schreiweis et al., 2003). At 24 wk of
age, the birds were too big for the scanner, so they were
killed using carbon dioxide gas, and the left drum and
humerus were scanned with muscle, skin, and feathers
intact. At each of the 3 ages of 6, 15, and 24 wk, 72
birds (9 chickens/strain per sex) were scanned.
Bone mineralization and bone size traits were analyzed as an analysis of covariance with BW as the covariate (Steel et al., 1997) with the exception of bone
length in which an ANOVA was used because BW was
NS as a covariate. The mixed model procedure of SAS
Institute (2003) was used. Line, sex, and age of the
bird as well as bone within bird (used as a subplot)
were considered fixed effects. The Tukey-Kramer test
partitioned differences among means (Oehlert, 2000).
An ANOVA was employed for BW without use of bone
as a subplot.
1965
Experiment 2
Male and female birds of 3 purebred commercial lines
(3, 11, and 13) and 6 experimental purebred lines (7,
8, 4, 12, 9, and 6) were raised by Cobb according to
standard management practices to 6 wk of age. The
current gene pools used for propagation of grandparent
and parent stock are the purebred commercial lines and
the 6 experimental lines are the gene pools of perhaps
future commercial pedigree lines. Excised tibia, ulna,
and radius with muscles, skin, and feathers intact were
frozen and express mailed to Purdue University. Bones
were thawed and mineralization and bone size traits
were determined using DEXA. Scans totaled 486 (9
chickens/strain × 9 strains × 2 sexes × 3 bones).
Using the mixed model of SAS Institute (2003), data
for BMD, BMC, bone length, and bone area were analyzed using an analysis of covariance with BW as the
covariate (Steel et al., 1997). An ANOVA was used for
bone width as BW was NS as a covariate. Bone as a
subplot was removed from the statistical model in the
ANOVA for BW. Bone within bird (used as a subplot),
line, and sex of the bird were considered fixed effects.
The Tukey-Kramer test partitioned differences among
means (Oehlert, 2000).
RESULTS
Experiment 1
As a main effect, the BMD was similar among purebred lines, whereas the BMC of line C was greater than
lines A and D, but not B (P < 0.003). The respective
CV for BMD and BMC were 12 and 19%. Bone length
of line C was greater than the other purebred lines of
chickens, whereas bone area was greater for lines B and
C as compared with lines A and D. Bone width of line
D was less than that of lines A, B, and C (P < 0.0001;
Table 1).
The bone mineralization of the purebred line of
chicken interacted with bone and age (P < 0.01). Due
to the complexity of the 3-way interaction, the BMD
of the humerus and tibia were separated into Figures
1a and 1b, respectively. The BMD of the humerus of
Table 1. Bone mineralization and size traits (adjusted for BW) of 4 purebred lines of chickens (experiment 1)
Line
A
B
C
D
SEM
P-value
a,b
Bone mineral
density1 (g/cm2)
0.235
0.235
0.238
0.241
0.003
0.36
CV (%)
11.2
12.0
11.6
11.8
Bone mineral
content1 (g)
4.07b
4.25ab
4.48a
4.12b
0.08
0.003
CV (%)
18.3
18.3
18.6
18.8
Bone length1,2
(mm)
Bone width1
(mm)
Bone area1
(cm2)
BW3 (kg)
93.5b
94.0b
98.1a
92.1b
0.7
0.0001
9.59a
9.51a
9.36a
9.04b
0.08
0.0001
16.8b
17.7a
18.5a
16.7b
0.2
0.0001
3.02ab
3.18a
3.21a
2.88b
0.06
0.001
Means within a column with no common superscript differ significantly (P < 0.05).
Values represent the least squares means averaged across bone (humerus and tibia), sex, and age (6, 15, and 24 wk) of the chicken. The number of
observations/mean ranged from 100 to 108.
2
For bone length, BW was not used as a covariate.
3
Values represent the least squares means averaged across the sex and age of the chicken.
1
1966
Talaty et al.
Figure 1. The effect of purebred lines of chickens and age on bone mineral density of the (a) humerus and (b) tibia (experiment 1). Values,
adjusted for BW, represent the least squares mean ± SEM averaged across the sex of the bird. a,bMeans for the bone mineral density of the (a)
humerus of line D and (b) tibia of line C with no common letter are significantly different (P < 0.05).
lines A, B, and C did not change as the birds aged;
however, the BMD of the humerus of line D peaked at
15 wk of age (Figure 1a). The tibial BMD of lines A,
B, and D did not change as the birds aged. In contrast,
the tibial BMD of line C increased as the birds aged
(Figure 1b).
The BMC was similar among purebred lines of chickens at 6 and 15 wk of age; however, at 24 wk of age,
line C had higher BMC than lines A and D, but did
not differ from line B, resulting in a line × age interaction (P < 0.01; Figure 2a). The bone area was similar
among purebred lines of chickens at 6 and 15 wk of
BONE MINERALIZATION IN PUREBRED LINES OF MEAT-TYPE CHICKENS
age; however, at 24 wk of age, lines B and C had higher
bone areas than lines A and D, resulting in a line × age
interaction (P < 0.003; Figure 2b).
When averaged across sex and age, the BW of line
D was lighter than lines B and C, but not line A (P <
0.001; Table 1). The interaction for BW between line,
sex, and age was NS (P = 0.37); nevertheless, the data
are presented (Figure 3) to show growth patterns over
time among purebred lines of male and female chickens
whose feed consumption was restricted after 6 wk of
age. Without feed restriction, chickens of lines A and
1967
D birds were the lightest at 6 wk of age. The BW was
maintained between 6 and 15 wk of age, whereas a controlled increase in BW occurred between 15 and 24 wk
of age. Males were heavier than females (P < 0.0001).
Final 24-wk-old BW of scanned birds (9 birds/line per
sex) met targeted BW within a 15% margin or less.
An age-related increase (P < 0.001) in BMD occurred
between 6 wk (mean = 0.224 g/cm2, CV = 14%) and
15 wk (mean = 0.244 g/cm2, CV = 13%) with no further increase noted at 24 wk of age (mean = 0.244 g/
cm2, CV = 17%); however, bones from male and female
Figure 2. The effect of purebred lines of chickens on (a) bone mineral content and (b) bone area (experiment 1). Each value, averaged across
the sex of the bird and type of bone, represents the least squares mean ± SEM of 28 to 36 observations. The bone mineral content and bone area
values were adjusted for BW. a,bMeans within an age (24 wk) with no common letter are significantly different (P < 0.05).
1968
Talaty et al.
Figure 3. The BW of 4 purebred lines of male and female chickens at 6, 15, and 24 wk of age (experiment 1). Each value represents the mean
of 9 observations. SEM = 0.2.
chickens responded differently to age, resulting in a sex
× bone × age interaction (Figure 4; P < 0.03). From
6 to 15 wk of age, an age-related increase in BMD occurred with all bones with the exception of the female
tibia, whose BMD remained unchanged. There was no
change in BMD with any of the bones from 15 to 24
wk of age (Figure 4). Within an age, the BMD values
did not differ among groups until 24 wk of age (sex
× bone × age interaction; P < 0.03). The male tibia
had higher BMD than the humerus of both sexes, but
Figure 4. The effect of sex on the bone mineral density of the tibia and humerus of purebred lines of chickens at 6, 15, and 24 wk of age (experiment 1). Values, adjusted for BW, represent the least squares mean ± SEM averaged across the purebred line of the bird. a,bMeans within an
age (24 wk) with no common letter are significantly different (P < 0.05). Each value represents the least squares mean of 36 observations/sex.
BONE MINERALIZATION IN PUREBRED LINES OF MEAT-TYPE CHICKENS
1969
Figure 5. (a) The length of the humerus and tibia of 4 purebred lines of chickens. Values represent the least squares mean ± SEM averaged
across the sex and age of the bird. (b) The length of humerus and tibia at 6, 15, and 24 wk of age. Values represent the least squares mean ±
SEM averaged across purebred lines and the sex of the bird. Values for both figures were not adjusted for BW. a–cMeans within a bone with no
common letter are significantly different (experiment 1, P < 0.05).
was not different from the female tibia at 24 wk of age
(Figure 4).
A line × bone interaction (P < 0.0005) for bone
length was due to the greater length of the tibia of line
C as compared with the tibia of the other purebred
lines of chickens. Although the humerus was also longer
for line C than Line D, its length did not differ from
lines A and B (Figure 5a). The length of the humerus
and tibia increased as the birds aged, with the tibia
growing faster than the humerus (Figure 5b, bone ×
age; P < 0.0001).
Bone width, adjusted for BW, was greater in males
(9.7 mm) than females (9.0 mm, P < 0.0001, SEM =
0.06). An age effect (P < 0.0001; SEM = 0.1) was due
to the increase in bone width from 6 wk (8.6 mm) to
15 wk of age (9.7 mm) with no subsequent increase at
24 wk of age (9.8 mm). However, not all of the bones
of purebred lines of chickens responded to age in the
same manner, causing a significant line × age × bone
interaction (P < 0.02) for bone width (Figure 6). The
width of the humerus of lines A, B, and C, but not line
D, increased from 6 to 15 wk of age. The tibial width of
1970
Talaty et al.
Table 2. The effect of purebred line of meat-type chicken on BW and bone traits at 6 wk of age (experiment 2)
Purebred line
BW1 (g)
7
8
4
12
13
9
11
6
3
SEM
CV (%)
P-value
2,033c
2,055c
2,157bc
2,186bc
2,271ab
2,278ab
2,304ab
2,388a
2,455a
44
12.6
<0.0001
Bone mineral
density2 (g/cm2)
Bone mineral
content2 (g)
Bone
length2 (mm)
Bone
width2 (mm)
Bone
area2 (cm2)
0.1131b
0.1276a
0.1192b
0.1276a
0.1215ab
0.1218a
0.1216ab
0.1273a
0.1208ab
0.0019
30.4
<0.0001
1.05c
1.38a
1.18b
1.25b
1.19b
1.26b
1.20b
1.28ab
1.23b
0.02
80.1
<0.0001
69.2d
74.6a
72.3abc
72.0bc
69.8cd
72.8ab
72.3abc
71.0bcd
72.5ab
0.6
24.7
<0.0001
5.38cd
5.72abc
5.49bcd
5.37d
5.64abcd
5.86a
5.50bcd
5.74ab
5.56abcd
0.08
28.7
<0.0001
8.0e
9.2a
8.5cd
8.5cde
8.4de
9.0ab
8.6bcd
8.9abc
9.1ab
0.1
53.6
<0.0001
a–e
Means within a column with no common superscript differ significantly (P < 0.05).
Values represent least squares means of 18 birds/purebred line.
2
Values represent the least squares means of 54 scans/purebred line (2 sexes × 3 bones × 9 chickens/purebred line). The means for bone mineral
density, bone mineral content, bone length, and bone area were adjusted for BW.
1
lines C and D increased from 6 to 15 wk of age, whereas
lines A (P = 0.08) and B had tibial widths that were
similar between 6 to 15 wk of age. There was no change
in the width of either the humerus or tibia from 15 to
24 wk of age. Within an age, bone width did not differ
among lines at 6 and 24 wk of age, but at 15 wk of age,
the width of line D humerus (9.0 mm) was less than the
width of lines A (10.1 mm) and B (10.0 mm), but not
line C (9.9 mm) humerus (Figure 6).
Experiment 2
All bone traits as well as BW were different among
genetic lines (P < 0.0001; Table 2). The 2 purebred
lines (lines 7 and 8) with the lightest BW had diverse
skeletal traits. Birds of line 7 had the lowest BMD
(0.1131 g/cm2), BMC (1.05 g), shortest bone length
(69.2 mm), and smallest bone area (8.0 cm2); however,
the other line low in BW (line 8) showed the opposite
trend in that bones from these birds were the highest
in BMD (0.1276 g/cm2), BMC (1.38 g), bone length
(74.6 mm), and area (9.2 cm2) when compared with all
of the other lines.
It was specifically the tibia of line 7 that had the lowest BMD when compared with all of the other genetic
lines of chickens (line × bone interaction, P < 0.0001;
Figure 7a). The BMD of the radius was similar among
genetic lines. The BMD of the ulna of line 7 was similar
to the other genetic lines with the exception of line 12,
which had higher BMD.
It was the BMC of the tibia and not the radius and
ulna that caused the differences in BMC among purebred lines of chickens (line × bone interaction, P <
0.0001; Figure 7b). Specifically, the tibia of line 8 had
Figure 6. The width of the humerus and tibia of 4 purebred lines of chickens at 6, 15, and 24 wk of age (experiment 1). Values, adjusted for
BW, represent the least squares mean averaged across the sex of the bird. SEM = 0.02.
BONE MINERALIZATION IN PUREBRED LINES OF MEAT-TYPE CHICKENS
1971
Figure 7. The (a) bone mineral density and (b) bone mineral content of the radius, tibia, and ulna of 9 purebred lines of meat-type chickens
(experiment 2). a–dWithin the bone, values with no common letter are significantly different at P < 0.05. Each value ± SEM is the least squares
mean of 18 observations (2 sexes × 9 observations/sex).
the highest BMC and line 7 had the lowest tibial BMC.
The tibial BMC among all other purebred lines (3, 6, 9,
12, 4, 11, and 13) of chickens did not differ from one another, with tibial BMC values in between lines 7 and 8.
The BMC of the radius and ulna did not differ among
purebred lines of chickens.
Line 8 had the longest tibia, ulna, and radius, whereas line 7 had the shortest tibia. The length of the radius
and ulna of line 7 was similar to all lines except line 8
(line × bone interaction; P < 0.02, Figure 8a). The line
× bone interaction (P < 0.0001) for bone width was
due in part to the wider tibia of line 8 as compared with
the other 8 purebred lines of chickens (Figure 8b). The
radius of lines 8 and 12 was narrower than the radius of
line 9, but did not differ from the radius of chickens of
the remaining purebred lines. The width of the ulna did
not differ among purebred lines of chickens.
The line × bone interaction (P < 0.0001) for bone
area was due in part to the large tibial area of line 8;
however, line 8 did not differ from the tibial areas of
1972
Talaty et al.
lines 11 and 13 (Figure 8c). There were also differences
among purebred lines in the area of the radius and
ulna. The radius of line 9 had the greatest bone area,
but it did not differ from the radial areas of lines 3, 6,
and 8. Chickens of line 13 had the smallest ulna area,
but it did not differ from the ulna area of lines 11, 7,
4, and 12.
Males were heavier than females in all purebred lines
except for lines 8, 4, 7, and 11, in which BW were NS
between sexes (line × sex interaction, P < 0.02; Figure
9).
2 types of chickens were unexpected even after bone
mineralization was adjusted for BW. However, as expected, from 35 to 65 wk of age, the tibial BMD of the
Cobb female was much greater than the tibial BMD of
the Leghorn female, most likely due to the huge BW
and egg production differences between the 2 types of
chickens. Medullary bone development contributed to
the increases in tibial BMD in both the Cobb and Leghorn female after 25 wk of age (Schreiweis et al., 2005).
Medullary bone deposition in the tibia of the 24-wk-old
female chickens of experiment 2 had not yet occurred
DISCUSSION
This is the first study reporting bone mineralization
and bone size traits of purebred lines of meat-type chickens. Previous studies have focused on the morphological
characteristics of bones from commercial broilers (Bond
et al., 1991; for a review, see Lilburn, 1994; Yalcin et
al., 2001; Williams et al., 2003; Schreiweis et al., 2005;
Talaty et al., 2009). Although most of these previous
studies showed little differences in bone mineralization
among commercial lines of broilers, the present study
showed significant differences in bone mineralization
among purebred lines of chickens (Figures 1, 2a, and 7;
Tables 1 and 2). In fact, the 2 purebred lines of chickens
that were the lightest in BW (lines 7 and 8 of experiment 2) were on the opposite ends of the scale relative
to bone mineralization and bone size traits (Table 2),
demonstrating the diversity among purebred lines of
chickens in the expression of bone phenotypic traits.
The significant differences in bone mineralization and
morphological traits among the 9 purebred lines of
chickens of experiment 2 (Table 2) were of particular
interest because the age at the time of sampling was 6
wk, similar to the market age of commercial broilers.
The 4 purebred lines of chickens of experiment 1, while
differing in BMC and bone size traits (Table 1), did
not show main effect differences due to genetic line in
BMD (Table 1), but the 3-way interaction indicated
that the mineralization of the humerus and tibia did
not always respond the same among lines as the birds
aged (Figure 1).
Relative to the life cycle of the commercial broiler,
BMD peaked at 4 wk of age, with no further increases
noted up to 7 or 8 wk of age (Talaty et al., 2009). In
experiment 1 of the current study, life cycle changes
in bone morphological traits were studied up to the
age when birds approach sexual maturity (24 wk of
age). For most bones and purebred lines of chickens,
increases in BMD occurred between 6 to 15 wk of age,
with no further increases at 24 wk of age (Figures 1 and
4). Our previous work (Schreiweis et al., 2005) with the
feed-restricted commercial Cobb female showed that
the BMD values of the tibia (adjusted for BW) were
not unlike the tibial BMD of a full-fed purebred line of
White Leghorn at 15 and 25 wk of age. Given that the
Cobb broiler is a much larger chicken than the Leghorn,
similar BMD values at 15 and 25 wk of age between the
Figure 8. The bone (a) length, (b) width, and (c) area of the
radius, tibia, and ulna of 9 purebred lines of meat-type chickens (experiment 2). a–cWithin a bone, values with no common letter are significantly different (P < 0.05). Each value ± SEM is the least squares
mean of 18 observations (2 sexes × 9 observations/sex).
BONE MINERALIZATION IN PUREBRED LINES OF MEAT-TYPE CHICKENS
due to the fact that there were no increases in tibial
BMD at 24 wk of age and hens had not yet initiated
egg production.
Our previous results with commercial broilers have
shown that as the tibia continued to grow as the birds
aged up to 7 or 8 wk of age as indicated by bone length,
width, and BMC, it did not become denser in mineral
after 4 wk of age as its surface area increased (Talaty
et al., 2009). Similar results were noted in experiment
1 with the purebred lines of chickens from 15 to 24 wk
of age. The BMC (Figure 2a), bone length (Figure 5b),
and bone area (Figure 2b) increased from 6 to 24 wk of
age, but with most bones and lines of birds, the BMD
did not increase from 15 to 24 wk of age (Figures 1 and
4). None of the birds of experiment 1 that were used
for the collection of bone morphological traits were
lame or had broken bones. Our subjective evaluations
noted that bird activity and aggressiveness increased
after the initiation of feed restriction at 6 wk of age;
therefore, the lack of increase in BMD from 15 to 24
wk of age among most purebred lines of chickens cannot be attributed solely to lack of exercise as suggested
for full-fed broilers who spend a greater proportion of
their time lying and less time walking as they approach
market age (Weeks et al., 2000).
Rapid weight gains have been suggested as a cause
of lower mineralization in fast-growing broilers. Feed
restricting a fast-growing line of commercial broiler
improved mineralization and porosity to the level observed in the tibia of the slow-growing broilers, strongly
suggesting that growth rate and not genotype was the
main contributor to poorer bone quality (Williams et
1973
al., 2003). Our data of experiment 1 support this observation by Williams et al. (2003). Between 6 and 15
wk of age, the BW of male and female purebred lines
of chickens were maintained with no increases in rate
of gain allowed through controlled feeding. At the same
time that birds were not gaining weight (Figure 3), the
BMD increased (Figures 1 and 4). When controlled increases in BW were once again allowed by increasing
feed allocation between 15 and 24 wk of age (Figure 3),
increases in BMD between 15 and 24 wk of age stopped
for most bones (Figure 4). The 1 exception was for line
C tibia in which BMD increased as BW increased between 15 and 24 wk of age (Figure 1b, line × bone ×
age interaction, P < 0.01). As juvenile purebred lines
of birds approached sexual maturity, their bones continued to grow in length (Figure 5b) and area (Figure
2b) and increased in BMC (Figure 2a) from 6 to 24 wk
of age regardless of whether or not weight gains were
occurring. In contrast, during periods of the juvenile
life cycle when BW were increasing (15 to 24 wk of age,
Figure 3), the BMD was unable to keep up with bone
linear growth, leading to bones that were less dense in
mineral and perhaps more porous. If, however, growth
was restricted so as to prevent weight gains, then the
BMD was able to keep up with linear growth of bones.
Of the 4 purebred lines compared in experiment 1,
line C demonstrated the best phenotypic traits relative
to bone quality. Even though line C had the longest tibia (Figure 5a), one of the largest bone areas at 24 wk of
age (Figure 2b), one of the heaviest BW (Table 1), and
one of the highest BMC at 24 wk of age (Figure 2a), the
tibia of line C did not become less dense in mineral as
Figure 9. The 6-wk-old BW of 9 purebred lines of male and female meat-type chickens (experiment 2). a,bWithin a purebred line of chicken,
values between sexes with no common letter are significantly different (P < 0.05). Each value ± SEM is the least squares mean of 9 observations.
1974
Talaty et al.
the chickens approached sexual maturity. Specifically,
its tibial BMD showed incremental increases from 6 to
24 wk of age, unlike the other purebred lines of chickens, which showed little change in tibial BMD from 6 to
24 wk of age (Figure 1b). Thus, the birds of line C were
better able to at least maintain and perhaps increase
tibial BMD as the bones grew even though they had
a larger bone surface area to mineralize as compared
with the other purebred line of chickens. However, it
is unknown if line C traits of increased BMD, BMC,
bone length, and bone area improve skeletal integrity
(i.e., less broken bones and reduced lameness), or vice
versa, if reduced BMC, bone length, and bone area of
lines A and D result in more broken bones and lameness problems, especially for breeding stock. Evidence
to suggest that there is a correlation between bone fragility and mineralization was reported by Mazzuco and
Hester (2005), who showed that as the BMD of the excised tibia decreased in White Leghorns, the incidence
of bone breakage increased (r = −0.54, P < 0.05). In
addition, Lilburn (1994) suggested that the increase in
biomechanical problems such as long bone distortions
in broilers could be attributed to relatively smaller tibial bone length and width when compared with tibial
bone length and width of ducks and turkeys. Both experiments of the current study showed genetic differences in bone width among purebred lines of chickens.
Although the bones of birds of line C of experiment 1
did not excel in bone width, its width was not different
from lines A and B. It was the bones of line D that were
narrower (Table 1). It is also worth noting that between
15 and 24 wk of age when most bones were increasing
in length (Figure 5b), area (Figure 2b), and BMC (Figure 2a) with little to no change in BMD (Figures 1 and
4), these same bones were not growing appositionally
(Figure 6). Wider shanks in male turkeys improve walking ability (Nestor et al., 1985; Nestor and Emmerson,
1990) and perhaps meat-type chickens would respond
similarly if genetically selected for wider leg bones.
In conclusion, several purebred lines of chickens phenotypically expressed large differences in BMD, BMC,
and bone size traits. The respective CV for BMD and
BMC were 12 and 19% in experiment 1 (Table 1) and
30 and 80% in experiment 2 (Table 2). These results
suggest that the potential exists to genetically select
birds for increased bone mineralization.
ACKNOWLEDGMENTS
This research was supported by Cobb-Vantress Inc.
(Monticello, KY). We thank F. A. Haan and O. M. Van
Dame of Purdue University Poultry Research Farm
(West Lafayette, IN) for management and care of the
birds and M. E. Einstein, Purdue University, for providing statistical advice and assistance.
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