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Breaking Strength and Composition of the Skin of Broiler Chicks: Response to Dietary Calorie-Protein Ratios
Breaking Strength and Composition of the Skin of Broiler Chicks: Response to Dietary Calorie-Protein Ratios I. KAFRI, J. A. CHERRY, 1 D. E. JONES, and P. B. SIEGEL Poultry Science Department, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061 (Received for publication October 29, 1984)
1985 Poultry Science 64:2143-2149 INTRODUCTION Because fragile skin increases t h e incidence of downgrades, skin strength is i m p o r t a n t in p o u l t r y processing. F a c t o r s c o n t r i b u t i n g t o skin strength, however, are largely u n k n o w n . Edwards et al. ( 1 9 7 3 ) observed m o r e skin tears in females t h a n in males and in older t h a n in y o u n g e r broilers. S m i t h et al. ( 1 9 7 7 ) also observed a greater incidence of t o r n skin in females t h a n in males and p o s t u l a t e d t h a t higher levels of skin fat, accompanied b y a r e d u c t i o n in t o t a l collagen c o n c e n t r a t i o n , m a d e t h e skin of females m o r e susceptible t o tearing. In t h e e x p e r i m e n t r e p o r t e d here, skin breaking strength was related t o its chemical composition. M e a s u r e m e n t s were o b t a i n e d a t several ages and a t different b o d y sites in male and female broilers fed diets differing in t h e ratio of calories t o p r o t e i n (C:P). MATERIALS AND METHODS A t 1 day of age, 4 0 male and 4 0 female commercial broiler chicks were w i n g b a n d e d a n d assigned to t r e a t m e n t g r o u p s at r a n d o m . T h e chicks were reared in electrically h e a t e d b a t t e r y b r o o d e r s in g r o u p s of 10 chicks of t h e same sex. All chicks were initially fed Diet A (Table
1
To whom correspondence should be addressed.
1) which contained a C:P ratio of 1 4 0 kcal t o 1% p r o t e i n . A t 14 d a y s of age, 2 feeding regimes, designated as either wide o r n a r r o w , d e p e n d i n g o n their relative C:P ratio, w e r e each imposed o n 2 g r o u p s of each sex. T h e n a r r o w C:P regimes consisted of C:P ratios of 1 4 0 : 1 (A), 1 6 0 : 1 (B), and 1 8 0 : 1 (C), fed 1 4 t o 2 8 , 2 8 t o 4 2 , a n d 4 2 t o 5 6 d a y s o f age, respectively. T h e wide C:P regime consisted of diets containing C:P ratios of 1 6 0 : 1 (B), 1 8 0 : 1 (C), and 2 0 0 : 1 (D) fed during t h e same age p e r i o d s . All diets were f o r m u l a t e d using ingredient c o m p o sition values provided b y t h e National Research Council ( 1 9 7 7 ) . F e e d and water w e r e provided ad libitum, and lighting was c o n t i n u o u s . A t 2 8 , 4 2 , and 56 d a y s of age, t h e birds were weighed individually to t h e nearest g r a m , and 5 individuals from each sex-diet t r e a t m e n t were killed b y cervical dislocation. T h e y w e r e t h e n defeathered (dry picking), and u n i f o r m sections of skin were removed from t h e lateral b o d y a p t e r i u m (breast) and from t h e areas delineated b y t h e dorsopelvic ( b a c k ) and femoral (thigh) feather tracts for measuring breaking strength (Kafri et al, 1 9 8 4 ) . Chemical analyses for fat ( F o l c h - J o r d i and Sloane-Stanley, 1 9 6 5 ) , p r o t e i n (Nir et al., 1 9 7 4 ) , and water (Association of Official Analytical Chemists, 1 9 7 5 ) were performed o n all skin samples. T o t a l skin collagen w a s estimated from h y d r o x y p r o l i n e c o n c e n t r a t i o n s as described b y L o g a n et al. (1950).
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ABSTRACT Skin breaking strength and protein, fat moisture, and total collagen concentrations of skin from the breast, thigh, and back of male and female commercial broilers were measured in response to diets containing relatively wide or narrow ratios of calories to protein (C:P). Comparisons were made at 28, 42, and 56 days of age. Chicks fed diets containing relatively wide C:P ratios had weaker skin than those fed diets with narrower C:P ratios, with differences being greater at 56 days than at younger ages. Regardless of diet, males had stronger skin than females. Although the magnitude of the differences varied with age, breast skin was stronger than thigh skin, with skin from the back being intermediate in strength. The observed differences in breaking strength were not consistently associated with fat, protein, moisture, or collagen concentrations of the skin. (Key words: broilers, breaking-strength, skin-composition, calorie-protein)
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KAFRI ET AL. TABLE 1. Co,mposition of diets Diet A
B
C •
Calculated analysis Crude protein Metabolizable energy, kcal/kg Calories: protein ratio
65.38 24.10 7.50 .80 .55 1.00 .17 .25 .25
21.4 3016 140
62.23 17.90 7.60 8.10 .55 3.00 .12 .25 .25
19.4 3080 160
( % )
•
72.53 13.90 7.90 3.60 .50 1.00 .07 .25 .25
17.3 3130 180
78.89 13.90 4.10 1.10 .50 1.00 .01 .25 .25
16.0 3267 200
1 Vitamin and trace mineral premix provided per kilogram of diet: vitamin A, 8000 IU; vitamin D 3 , 3000 IU; vitamin E, 10 IU; vitamin K, 2 mg; riboflavin, 4 mg; d-pantothenic acid, 10 mg; thiamine, 1 mg; niacin, 35 mg; choline chloride, 350 mg; vitamin B, 2 , 8 Mg; folic acid, .5 mg; ethoxyquin, 250 mg; pyridoxine, 5 mg; biotin, .4 mg; manganese, 80 mg; zinc, 112 mg; iron, 62 mg; copper, 16 mg; iodine, .1 mg; selenium, .1 mg.
RESULTS Main Effects. Means and standard errors for skin breaking strength and skin composition traits of male broilers are presented in Table 2. li + Aj + Sj + D k + Lj + (AS)^ Y Comparable data for females are reported in ijklm Table 3. Body weights of both males and + ( A D ) ^ + (AL) n + (SD)j k females are contained in Table 4. + (SL)j! + (DL) k l + (ASD) i j k When main effects were examined (Table 5), the males were heavier and had stronger skin + ( A S D J J J + (SDL)j k l + (ADDjk! than females. Skin from females contained more fat and less protein and collagen then skin + ASDL + e < >ijkl ijklm from males. No sexual dimorphism in skin where i = 1, 2, 3 ages; j = 1,2 sexes; k = 1, 2 moisture was observed. Chicks fed diets with comparatively narrow dietary regimes; 1 = 1, 2, 3 sites; and m = 1, 2...n individuals. Prior to analysis, breaking C:P ratios were heavier and had stronger skin strength values were transformed to natural than those fed diets with wider C:P ratios. logarithms because means and variances were Feeding the wide C:P diets resulted in an correlated. Percentages were transformed to arc increase in skin fat and decreases in the moissine square root for analyses. When age and site ture, protein, and collagen contents of the skin. differences were significant, the means were Skin breaking strength increased between 28 separated by Duncan's multiple range test. and 56 days of age, with the value at 42 days of When significant interactions among main age being intermediate. Comparatively more fat variables occurred, the data were analyzed and less moisture, protein, and collagen were within pairs of variables. Product-moment contained in the skin at 56 days of age than at correlation coefficients between breaking 28 days of age. Skin composition values at 42 strength and skin composition were also calcu- days of age were intermediate to values oblated within each site (Sokal and Rohlf, 1969). tained at 28 and 56 days of age. In addition, the data were subjected to multiple Breast skin was stronger than skin from the regression analysis using step-wise procedures. thigh. Skin from the back exhibited inter-
The data were analyzed by analysis of variance with age, sex, site, and diet considered as fixed effects. The statistical model was:
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Ingredient Yellow corn Dehulled soybean meal Meat and bone meal Wheat bran Ground limestone Hydrolyzed fat DL-Methionine Salt Vitamin and trace mineral mix 1
D
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SKIN COMPOSITION AND STRENGTH TABLE 2. Means and standard errors for skin breaking strength and skin composition of male broilers fed diets with comparatively wide (W) and narrow (N) ratios of calories to protein Skin site
Breast
Thigh
(days) 28
Diet
Breaking strength —
Fat
Water
Protein
(% protein)
(%)
(g)
Collagen
W N
984 ± 46 995 ± 30
37.7 + 2.4 22.7 ± 1.5
50.2 ± 2.4 58.5 + 1.4
12.1 ± .2 12.7 + .3
71.7 ± 1.1 78.5 + 3.3
42
W N
1038 ± 42 1144 ± 51
38.5 ± .6 33.9 ± 1.0
48.7 ± 1.4 51.7 + 1.4
12.3 + .6 12.9 + .2
70.6 ±1.5 77.1 ± 1.5
56
W N
945 ± 14 1049 + 20
44.1 ± .7 44.4 ± 2.6
43.9 ± 1.0
44.9 ± 1.9
12.1 + .3 12.4 + .3
67.6 + 2.2 74.4 ± 2.3
28
W N
736 ± 46 847 ± 52
53.6 ± 1.6 40.4 ±1.2
38.2 ± 1.7 49.1 ± 1.3
9.2 + .3 10.6 + .3
68.7 ± .5 70.3 ± 1.3
42
W N
850 ± 36 869 ± 55
51.1 ±2.2 43.6 ± 1.4
37.6 ± 1.2 44.1 + .8
8.7 + .2 10.1 + .3
57.6 ± 1.4 64.6 ±1.0
56
W N
789 ± 70 952 ± 52
50.3 ± 1.7 53.3 ±1.2
39.0 + 1.6 36.6 ± .7
9.6 + .2 9.4 + .5
62.1 ± 1.3 68.5 ± 3.0
28
W N
701 ± 30 737 ± 36
27.4 ±2.1 24.7 ±1.2
57.2 ± 6.7 58.9 ± 1.0
15.4 + .4 16.4 + .3
75.4 ± 4.4 83.1 + 2.1
42
W N
695 ± 8 560 ± 65
26.8 ± 2.5 20.3 ± .4
56.1 ± 2.3 60.9 ± .8
15.8 + .3 15.4 ± .3
78.6 ± 2.6 88.2 ± 1.9
56
W N
738 ± 25 821 ± 36
27.9 ± 1.0 29.9 ± 2.1
55.2 ± 1.0 54.1 ± 1.2
16.4 + .2 15.9 ± .3
81.3 + 2.1 86.4 ± 1.4
TABLE 3. Means and standard errors for skin breaking strength and skin composition of female broilers fed diets with comparatively wide (W) and narrow (N) ratios of calories to protein Skin site
Breast
Back
Thigh
Diet
Breaking strength
Fat
Water
W N
— ( g ) — 877 ± 24 938 ± 39
34.9 ±2.1 27.2 ± 1.3
53.4 ±2.1 61.0 ± 1.2
11.1 ± .2 11.8 ± .3
75.6 ± 2.6 77.9 ± 1.4
42
W N
1081 ± 38 1097 ± 42
37.7 ± 1.1 34.8 + .7
48.9 ± 1.2 50.1 ± 1.2
11.3 ± .1 12.4 + .2
74.8 ±1.8 70.6 ± 2.1
56
W N
831 ± 33 1003 ± 25
42.6 ± 2.0 38.0 ±2.3
46.1 ± 2.4 51.8 + 2.7
10.1 ± .4 11.1 ± .3
65.9 ± 2.2 61.3 ±2.2
28
W N
673 ± 21 743 ± 33
50.6 + 2.0 45.0 ± 1.8
39.8 ± 1.9 44.9 ± 1.3
9.4 ± .2 10.1 ± .3
65.9 ± 2.4 70.1 ± 1.5
42
W N
746 ± 46 7 7 0 + 15
51.7 ±2.3 45.0 + .8
37.6 ± 2.3 43.3 ± .7
7.1 ±.1 7.9 ± .1
57.9 ± 2.4 61.0 ± 1.0
56
W N
753 ± 4 3 800 ± 27
51.7 ±2.6 55.4 ± 1.2
34.5 ± 2.6 36.4 ± 1.2
7.6 ± .3 8.2 ± .4
59.9 ± 1.1 56.4 + 1.5
28
W N
591 ± 27 536 ± 38
35.0 ± 1.5 26.2 ±1.2
51.3 ± 1.5 60.4 ±1.5
13.7 ± .2 14.4 + .1
82.8 ± 1.4 79.9 ± 1.5
42
W N
473 + 9 551 + 2 7
32.8 ± 1.5 24.2 ± .9
53.2 ± 1.3 59.1 ± .7
12.9 ± .3 13.6 ± .3
78.8 ± .8 77.2 ± 1.8
56
W N
576 ± 3 3 761 ± 4 0
37.8 ±1.8 38.6 + 1.6
48.1 ± 1.1 48.7 ± .9
13.1 + .6 13.5 + .1
70.2 ± 2.4 70.5 + 3.3
Age (days) 28
Protein
Collagen (% protein)
(%)
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Back
Age
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KAFRI ET AL. TABLE 4. Means and standard errors of body weights by sex, within age and diet Males
Females
Age
Diet
28
W N
709 ± 42 783 ± 16
665 ± 22 617 + 40
42
W N
1302 ± 44 1321± 21
1049 ± 43 1202 + 49
56
W N
1515 ± 1 2 1 1884± 50
1621 ± 79 1689 ± 5 2
sji>
1
W = Wide, n = narrow.
strength was 94, 89, and 82% for breast, back, and thigh, respectively. Percentage fat in thigh skin was higher in males than in females, but no significant differences in skin fat due to sex were obtained in breast or back skin. No differences in moisture were obtained between sexes for breast or back skin, but thigh skin from the males contained more moisture than that from females. Skin from males was consistently higher in protein than skin from females, but the difference was most pronounced in thigh skin. Collagen concentrations of back and thigh skin were greater in males than in females, but the collagen content of
TABLE 5. Body weights, breaking strength, and chemical composition of skin by sex, diet, age, and site with main effects pooled Main effect
Breaking strength
Body weight
Fat
Moisture
(g/
Protein
Collagen (% protein)
(">)
Sex Male Female
1069 a 989 b
860 a 763b
38b 40a
49a 48a
12.6 b 11. l a
74a 70b
Diet W N
981b 1078 a
779b 843a
41a 36 b
47b 5ia
11.5b 12.2 a
70b 73a
Age 28 42 56
702 c 1196 b 1677 a
779b 82iab 835 a
36b 37b 43a
52a 49b 45 c
12.3a 11.7b 11.6 b
75a 71b 69c
996b 794a 644c
37b 50 a 29c
5lb 40c 55a
11.9b 8.9 C 14.2 a
72b 64c 79a
Site Breast Back Thigh
' 'cWithin main effects, means in a column having the same superscript were not significantly different (P>.05). 1 W = Wide, N = narrow.
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mediate values for breaking strength; back skin also contained more fat and less moisture, protein, and collagen than skin from the breast or thigh. Skin from the breast had more fat and less moisture, protein, and collagen than thigh skin. Interactions Among Main Variables. Significant sex by site interactions for skin breaking strength and for the percentages of protein, collagen, moisture, and fat in skin were obtained (Table 6). Although females had weaker skin than males regardless of site, the magnitude of the sexual dimorphism differed among sites. The ratio of female to male skin breaking
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SKIN COMPOSITION AND STRENGTH TABLE 6. Sex by site interactions for skin breaking strength and fat, moisture, protein, and collagen concentrations of the skin
Site
Sex
Breaking strength
Breast
Male Female
1026 a 966°
Male Female
a
Male Female
Fat
Moisture
37.9 s 35.9a
49.7a 52.0a
841 748b
48.7a
a
Sl.oa
40.6 39.4a
9.6 a 8.4°
709 a 581b
26.3 b 32.4a
57.1 a 53.4 b
15.9 a 13.6°
(g)
Back Thigh
Protein
(%) 12.4a 11.3D
Collagen (% protein) 73.3a 71.0 a 65.3 a 61.9 b 82.2a 76.6 b
breast skin did not differ significantly due to sex. Age by diet interactions for breaking strength and for fat, moisture, and protein concentrations of the skin are presented in Table 7. Broilers fed diets containing narrower C:P ratios had stronger skin than those fed the wide C:P diets at all ages examined, with the difference being significant at 56 days. Differences in skin composition due to diet, while pronounced at younger ages, disappeared by 56 days of age. None of the significant age by site interactions (Table 8) were of the crossover type. Regardless of age, thigh skin exhibited the highest values for protein, collagen, and moisture; back skin exhibited the lowest values for moisture, protein, and collagen, the highest value for percentage fat, and was intermediate in breaking strength; breast skin, while strong-
est, exhibited intermediate values for fat, water, protein, and collagen. Correlations and Regressions. At all skin sites examined, correlations between breaking strength and the skin composition traits measured were low and not significant. Step-wise regression analysis failed to reveal any consistent cause-effect relationships between breaking strength and skin composition. DISCUSSION The tendency of males to have stronger skin than females (Table 5) was consistent with previous observations that the incidence of torn skin occurring during processing was greater in female than in male broilers (Edwards et al., 1973; Smith et al., 1977). Smith et al. (1977) proposed that the propensity for female skin to
TABLE 7. Age by diet interactions for skin breaking strength and for fat, moisture, and protein concentrations of the skin
Age
Diet
Breaking strength
Fat
Moisture
28
W N
759* 799a
39.9a 32.0 b
48.2 a 55.4 b
11.8 a 12.7 b
42
W N
809 a 832 a
39.8 a 33.8 b
47.1a 51.5 b
11.3a 12.1b
56
W N
772 a 899b
43.4a 43.3a
44.5 a 45.4a
11.5a 11.7a
Protein
(%)
ab ' Within sites, means in a column having the same superscript were not significantly different (P>.05). 1
W = Wide, N = narrow.
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a,b,Within sites, means in a column having the same superscript were not significantly different (P>.05).
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KAFRI ET AL. TABLE 8. Age by site interactions for skin breaking strength and for fat, moisture, protein, and collagen concentrations of the skin Breaking strength
Collagen
Fat
Moisture
Protein
946 a 749b 641c
32.1 b 47.4 a 28.3 C
11.9b 9.8 C 15.0 a
75.9 b 68.8 C 80.3 a
Breast Back Thigh
1090 a 809 b 563c
30.2 b 48 .Oa 26.2 C
55.8 a 42.7b 56.9 a 50.1b 40.6 C 57.3 a
12.2 b 8.5 C 14.4 a
73.2 b 60.3 c 80.7 a
Breast Back Thigh
95 7 a 825b 724 c
42.3b 54.2 a 33.5 C
46.7 b 36.6 C 51.5 a
11.4b 8.7C 14.7 a
67.3 b 62.0 C 77.la
Site
28
Breast Back Thigh
42
56
(% protein)
' 'cWithin sites, means in a column having the same superscript were not significantly different (P>.05).
be weaker than that of males was related to increased concentrations of fat or decreased collagen concentrations. In our study, skin from females tended to contain more fat and less protein and collagen than skin from males (Table 5). Similar observations were reported in broiler chicks by Grey et al. (1983) and in goslings by Nitsan et al. (1981). The significant sex by site interactions obtained in our study, however, were not entirely consistent with the theory that increased fat and decreased protein or collagen contents of the skin are associated with weaker skin. Although breaking strength of skin from males was greater than that of females at all ages examined (Table 6), skin fat was significantly greater in females only for thigh skin. Numerically, percentage fat in breast skin was higher for males than for females. Protein and collagen concentrations of the skin, however, were consistently greater in males than in females, irrespective of site. If increases in skin fat result in weaker skin, feeding diets with wide C:P ratios would be expected to reduce skin breaking strength; Bartov et al. (1974) reported that decreased dietary protein resulted in an increased deposition of fat in the skin of broilers. Feeding the wide C:P diets did result in weaker skin that was associated with a general increase in skin fat and decreases in the moisture, protein, and collagen concentrations of the skin (Table 5). It may be of importance, however, that differences in breaking strength due to diet were more pronounced at 56 days of age, while differences in skin composition were more pronounced at younger ages (Table 7). No
significant dietary differences in skin composition were observed at 56 days of age when the age by diet interaction was examined. Although there were inconsistencies, differences in breaking strength and skin composition between males and females, and between chicks fed diets differing in C:P ratios, tended to support the conclusion that weak skin is associated with increased fat and decreased protein or collagen concentrations of the skin (Smith et al., 1977). Different patterns, however, emerged when site and age differences were examined. Edwards et al. (1973) reported that the incidence of torn skin was higher in broilers at 9 weeks than at younger ages, inferring that the skin of chickens becomes weaker with age. Our results were inconsistent with this inference. Breaking strength tended to increase with age (Table 5), although the patterns varied between diets (Table 7) and among sites (Table 8). For example, a decrease in breaking strength of the skin occurred between 42 and 56 days of age when the diets containing wide C:P ratios were fed, but breaking strength of skin increased during this period in response to the narrow C:P diets (Table 7). Moreover, thigh and back skin tended to become stronger with age while breast skin did not (Table 8). Changes in skin composition with age indicated that skin strength was not solely associated with increased fat and decreased protein and collagen concentrations. Increases in breaking strength with age were accompanied by increases in skin fat and decreases in moisture and collagen concentrations. Skin protein content increased from 28 to 42 days of age
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Age
SKIN COMPOSITION AND STRENGTH
Similar t o t h e age effects, differences in t h e c o m p o s i t i o n and strength of skin a t different sites did n o t indicate t h a t breaking strength was associated with increased fat, decreased p r o t e i n , or collagen c o n c e n t r a t i o n s . Skin from t h e back contained m o r e fat and less p r o t e i n and collagen t h a n skin from t h e o t h e r sites b u t exhibited intermediate values for breaking strength (Table 5). A l t h o u g h breast skin contained m o r e fat and less p r o t e i n and collagen t h a n t h a t from t h e thigh, it was considerably stronger t h a n thigh skin. Skin from t h e thigh, which was weakest a m o n g t h e sites e x a m i n e d , had t h e least fat and t h e m o s t p r o t e i n and collagen. These site differences were generally consistent at t h e different ages (Table 8) and b e t w e e n sexes (Table 6 ) ; t h e significant interactions were n o t of t h e crossover t y p e . Differences in skin c o m p o s i t i o n and skin strength due t o sex, diet, age, and skin site revealed n o consistent relationship b e t w e e n t h e strength and chemical c o m p o s i t i o n of t h e skin. Correlation and multiple regression analyses were also ineffective in relating skin strength t o any of t h e chemical characteristics measured. Perhaps a m o r e detailed chemical o r histological e x a m i n a t i o n of specific skin layers w o u l d be beneficial in elucidating physical o r chemical factors determining skin strength. Classification of t o t a l skin collagen into t h e distinct t y p e s of
tropocollagens m i g h t be especially beneficial.
REFERENCES Association of Official Analytical Chemists, 1975. Official Methods of Analysis, 12th ed., Assoc. Offic. Anal. Chem., Washington, DC. Bartov, I., S. Bornstein, and B. Lipstein, 1974. Effect of calorie to protein ratio on the degree of fatness in broilers fed on practical diets. Br. Poult. Sci. 15:107-117. Edwards, H. M., F. Denman, A. Abou-Ashour, and D. Nugara, 1973. Carcass composition studies. 1. Influence of age, sex, and type of dietary fat supplementation on total carcass and fatty acid composition. Poultry Sci. 52:934-943. Folch-Jordi, M. L., and G. H. Sloane-Stanley, 1965. Simple method for the isolation and purification of total lipids from animal tissues. J. Biol. Chem. 226:497-509. Grey, T. C , D. Robinson, J. H. Jones, S. W. Stock, and N. L. Thomas, 1983. Effect of age and sex on the composition of muscle and skin from a commercial broiler strain. Br. Poult. Sci. 24: 219-231. Kafri, I., D. J. Zelenka, J. A. Cherry, and P. B. Siegel, 1984. Skin breaking strength in chickens: Comparisons among genetic combinations. Poultry Sci. 63:1279-1280. Logan, A. M., and E. R. Newman, 1950. The determination of hydroxyproline. J. Biol. Chem. 184:299-306. National Research Council, 1977. Nutrient Requirements of Poultry. 7th ed. Natl. Acad. Sci., Washington, DC. Nir, I., Z. Nitsan, and Y. Dror, 1974. Force-feeding effects on growth, carcass and blood composition in the young chick. Br. J. Nutr. 32:229-239. Nitsan, Z., A. Dvorin, and I. Nir, 1981. Composition and amino acid content of carcass, skin, and feathers of the growing gosling. Br. Poult. Sci. 22:79-84. Smith, T. W., Jr., J. R. Couch, R. L. Garret, and C. R. Creger, 1977. The effect of sex, dietary energy, meat protein, ascorbic acid, and iron on broiler skin collagen. Poultry Sci. 56:1216—1220. Sokal, R. R., and F. J. Rohlf, 1969. Biochemistry: The Principles and Practices of Statistics in Biological Research. W. H. Freeman and Co., San Francisco, CA. Suderman, D. R., and F. E. Cunningham, 1980. The effect of age of bird and method of chilling on composition of broiler skin. Poultry Sci. 59: 2247-2249.
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b u t decreased from 4 2 t o 56 days of age. R e p o r t s in t h e literature concerning age effects o n skin p r o t e i n are also ambiguous. S u d e r m a n et al. ( 1 9 8 0 ) reported t h a t skin p r o t e i n remained fairly c o n s t a n t at different ages while o t h e r investigators (Nitsan et al., 1 9 8 1 , Grey et al, 1983) observed a general decline in protein c o n c e n t r a t i o n s in t h e skin with age. N o n e t h e less, changes in t h e p r o t e i n c o n t e n t of t h e skin failed to parallel those in breaking strength. Neither were t h e significant age b y diet (Table 7) nor age b y site (Table 8) interactions indicative of a consistent relationship b e t w e e n skin composition and skin strength.
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1985 Poultry Science 64:2143-2149 INTRODUCTION Because fragile skin increases t h e incidence of downgrades, skin strength is i m p o r t a n t in p o u l t r y processing. F a c t o r s c o n t r i b u t i n g t o skin strength, however, are largely u n k n o w n . Edwards et al. ( 1 9 7 3 ) observed m o r e skin tears in females t h a n in males and in older t h a n in y o u n g e r broilers. S m i t h et al. ( 1 9 7 7 ) also observed a greater incidence of t o r n skin in females t h a n in males and p o s t u l a t e d t h a t higher levels of skin fat, accompanied b y a r e d u c t i o n in t o t a l collagen c o n c e n t r a t i o n , m a d e t h e skin of females m o r e susceptible t o tearing. In t h e e x p e r i m e n t r e p o r t e d here, skin breaking strength was related t o its chemical composition. M e a s u r e m e n t s were o b t a i n e d a t several ages and a t different b o d y sites in male and female broilers fed diets differing in t h e ratio of calories t o p r o t e i n (C:P). MATERIALS AND METHODS A t 1 day of age, 4 0 male and 4 0 female commercial broiler chicks were w i n g b a n d e d a n d assigned to t r e a t m e n t g r o u p s at r a n d o m . T h e chicks were reared in electrically h e a t e d b a t t e r y b r o o d e r s in g r o u p s of 10 chicks of t h e same sex. All chicks were initially fed Diet A (Table
1
To whom correspondence should be addressed.
1) which contained a C:P ratio of 1 4 0 kcal t o 1% p r o t e i n . A t 14 d a y s of age, 2 feeding regimes, designated as either wide o r n a r r o w , d e p e n d i n g o n their relative C:P ratio, w e r e each imposed o n 2 g r o u p s of each sex. T h e n a r r o w C:P regimes consisted of C:P ratios of 1 4 0 : 1 (A), 1 6 0 : 1 (B), and 1 8 0 : 1 (C), fed 1 4 t o 2 8 , 2 8 t o 4 2 , a n d 4 2 t o 5 6 d a y s o f age, respectively. T h e wide C:P regime consisted of diets containing C:P ratios of 1 6 0 : 1 (B), 1 8 0 : 1 (C), and 2 0 0 : 1 (D) fed during t h e same age p e r i o d s . All diets were f o r m u l a t e d using ingredient c o m p o sition values provided b y t h e National Research Council ( 1 9 7 7 ) . F e e d and water w e r e provided ad libitum, and lighting was c o n t i n u o u s . A t 2 8 , 4 2 , and 56 d a y s of age, t h e birds were weighed individually to t h e nearest g r a m , and 5 individuals from each sex-diet t r e a t m e n t were killed b y cervical dislocation. T h e y w e r e t h e n defeathered (dry picking), and u n i f o r m sections of skin were removed from t h e lateral b o d y a p t e r i u m (breast) and from t h e areas delineated b y t h e dorsopelvic ( b a c k ) and femoral (thigh) feather tracts for measuring breaking strength (Kafri et al, 1 9 8 4 ) . Chemical analyses for fat ( F o l c h - J o r d i and Sloane-Stanley, 1 9 6 5 ) , p r o t e i n (Nir et al., 1 9 7 4 ) , and water (Association of Official Analytical Chemists, 1 9 7 5 ) were performed o n all skin samples. T o t a l skin collagen w a s estimated from h y d r o x y p r o l i n e c o n c e n t r a t i o n s as described b y L o g a n et al. (1950).
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ABSTRACT Skin breaking strength and protein, fat moisture, and total collagen concentrations of skin from the breast, thigh, and back of male and female commercial broilers were measured in response to diets containing relatively wide or narrow ratios of calories to protein (C:P). Comparisons were made at 28, 42, and 56 days of age. Chicks fed diets containing relatively wide C:P ratios had weaker skin than those fed diets with narrower C:P ratios, with differences being greater at 56 days than at younger ages. Regardless of diet, males had stronger skin than females. Although the magnitude of the differences varied with age, breast skin was stronger than thigh skin, with skin from the back being intermediate in strength. The observed differences in breaking strength were not consistently associated with fat, protein, moisture, or collagen concentrations of the skin. (Key words: broilers, breaking-strength, skin-composition, calorie-protein)
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KAFRI ET AL. TABLE 1. Co,mposition of diets Diet A
B
C •
Calculated analysis Crude protein Metabolizable energy, kcal/kg Calories: protein ratio
65.38 24.10 7.50 .80 .55 1.00 .17 .25 .25
21.4 3016 140
62.23 17.90 7.60 8.10 .55 3.00 .12 .25 .25
19.4 3080 160
( % )
•
72.53 13.90 7.90 3.60 .50 1.00 .07 .25 .25
17.3 3130 180
78.89 13.90 4.10 1.10 .50 1.00 .01 .25 .25
16.0 3267 200
1 Vitamin and trace mineral premix provided per kilogram of diet: vitamin A, 8000 IU; vitamin D 3 , 3000 IU; vitamin E, 10 IU; vitamin K, 2 mg; riboflavin, 4 mg; d-pantothenic acid, 10 mg; thiamine, 1 mg; niacin, 35 mg; choline chloride, 350 mg; vitamin B, 2 , 8 Mg; folic acid, .5 mg; ethoxyquin, 250 mg; pyridoxine, 5 mg; biotin, .4 mg; manganese, 80 mg; zinc, 112 mg; iron, 62 mg; copper, 16 mg; iodine, .1 mg; selenium, .1 mg.
RESULTS Main Effects. Means and standard errors for skin breaking strength and skin composition traits of male broilers are presented in Table 2. li + Aj + Sj + D k + Lj + (AS)^ Y Comparable data for females are reported in ijklm Table 3. Body weights of both males and + ( A D ) ^ + (AL) n + (SD)j k females are contained in Table 4. + (SL)j! + (DL) k l + (ASD) i j k When main effects were examined (Table 5), the males were heavier and had stronger skin + ( A S D J J J + (SDL)j k l + (ADDjk! than females. Skin from females contained more fat and less protein and collagen then skin + ASDL + e < >ijkl ijklm from males. No sexual dimorphism in skin where i = 1, 2, 3 ages; j = 1,2 sexes; k = 1, 2 moisture was observed. Chicks fed diets with comparatively narrow dietary regimes; 1 = 1, 2, 3 sites; and m = 1, 2...n individuals. Prior to analysis, breaking C:P ratios were heavier and had stronger skin strength values were transformed to natural than those fed diets with wider C:P ratios. logarithms because means and variances were Feeding the wide C:P diets resulted in an correlated. Percentages were transformed to arc increase in skin fat and decreases in the moissine square root for analyses. When age and site ture, protein, and collagen contents of the skin. differences were significant, the means were Skin breaking strength increased between 28 separated by Duncan's multiple range test. and 56 days of age, with the value at 42 days of When significant interactions among main age being intermediate. Comparatively more fat variables occurred, the data were analyzed and less moisture, protein, and collagen were within pairs of variables. Product-moment contained in the skin at 56 days of age than at correlation coefficients between breaking 28 days of age. Skin composition values at 42 strength and skin composition were also calcu- days of age were intermediate to values oblated within each site (Sokal and Rohlf, 1969). tained at 28 and 56 days of age. In addition, the data were subjected to multiple Breast skin was stronger than skin from the regression analysis using step-wise procedures. thigh. Skin from the back exhibited inter-
The data were analyzed by analysis of variance with age, sex, site, and diet considered as fixed effects. The statistical model was:
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Ingredient Yellow corn Dehulled soybean meal Meat and bone meal Wheat bran Ground limestone Hydrolyzed fat DL-Methionine Salt Vitamin and trace mineral mix 1
D
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SKIN COMPOSITION AND STRENGTH TABLE 2. Means and standard errors for skin breaking strength and skin composition of male broilers fed diets with comparatively wide (W) and narrow (N) ratios of calories to protein Skin site
Breast
Thigh
(days) 28
Diet
Breaking strength —
Fat
Water
Protein
(% protein)
(%)
(g)
Collagen
W N
984 ± 46 995 ± 30
37.7 + 2.4 22.7 ± 1.5
50.2 ± 2.4 58.5 + 1.4
12.1 ± .2 12.7 + .3
71.7 ± 1.1 78.5 + 3.3
42
W N
1038 ± 42 1144 ± 51
38.5 ± .6 33.9 ± 1.0
48.7 ± 1.4 51.7 + 1.4
12.3 + .6 12.9 + .2
70.6 ±1.5 77.1 ± 1.5
56
W N
945 ± 14 1049 + 20
44.1 ± .7 44.4 ± 2.6
43.9 ± 1.0
44.9 ± 1.9
12.1 + .3 12.4 + .3
67.6 + 2.2 74.4 ± 2.3
28
W N
736 ± 46 847 ± 52
53.6 ± 1.6 40.4 ±1.2
38.2 ± 1.7 49.1 ± 1.3
9.2 + .3 10.6 + .3
68.7 ± .5 70.3 ± 1.3
42
W N
850 ± 36 869 ± 55
51.1 ±2.2 43.6 ± 1.4
37.6 ± 1.2 44.1 + .8
8.7 + .2 10.1 + .3
57.6 ± 1.4 64.6 ±1.0
56
W N
789 ± 70 952 ± 52
50.3 ± 1.7 53.3 ±1.2
39.0 + 1.6 36.6 ± .7
9.6 + .2 9.4 + .5
62.1 ± 1.3 68.5 ± 3.0
28
W N
701 ± 30 737 ± 36
27.4 ±2.1 24.7 ±1.2
57.2 ± 6.7 58.9 ± 1.0
15.4 + .4 16.4 + .3
75.4 ± 4.4 83.1 + 2.1
42
W N
695 ± 8 560 ± 65
26.8 ± 2.5 20.3 ± .4
56.1 ± 2.3 60.9 ± .8
15.8 + .3 15.4 ± .3
78.6 ± 2.6 88.2 ± 1.9
56
W N
738 ± 25 821 ± 36
27.9 ± 1.0 29.9 ± 2.1
55.2 ± 1.0 54.1 ± 1.2
16.4 + .2 15.9 ± .3
81.3 + 2.1 86.4 ± 1.4
TABLE 3. Means and standard errors for skin breaking strength and skin composition of female broilers fed diets with comparatively wide (W) and narrow (N) ratios of calories to protein Skin site
Breast
Back
Thigh
Diet
Breaking strength
Fat
Water
W N
— ( g ) — 877 ± 24 938 ± 39
34.9 ±2.1 27.2 ± 1.3
53.4 ±2.1 61.0 ± 1.2
11.1 ± .2 11.8 ± .3
75.6 ± 2.6 77.9 ± 1.4
42
W N
1081 ± 38 1097 ± 42
37.7 ± 1.1 34.8 + .7
48.9 ± 1.2 50.1 ± 1.2
11.3 ± .1 12.4 + .2
74.8 ±1.8 70.6 ± 2.1
56
W N
831 ± 33 1003 ± 25
42.6 ± 2.0 38.0 ±2.3
46.1 ± 2.4 51.8 + 2.7
10.1 ± .4 11.1 ± .3
65.9 ± 2.2 61.3 ±2.2
28
W N
673 ± 21 743 ± 33
50.6 + 2.0 45.0 ± 1.8
39.8 ± 1.9 44.9 ± 1.3
9.4 ± .2 10.1 ± .3
65.9 ± 2.4 70.1 ± 1.5
42
W N
746 ± 46 7 7 0 + 15
51.7 ±2.3 45.0 + .8
37.6 ± 2.3 43.3 ± .7
7.1 ±.1 7.9 ± .1
57.9 ± 2.4 61.0 ± 1.0
56
W N
753 ± 4 3 800 ± 27
51.7 ±2.6 55.4 ± 1.2
34.5 ± 2.6 36.4 ± 1.2
7.6 ± .3 8.2 ± .4
59.9 ± 1.1 56.4 + 1.5
28
W N
591 ± 27 536 ± 38
35.0 ± 1.5 26.2 ±1.2
51.3 ± 1.5 60.4 ±1.5
13.7 ± .2 14.4 + .1
82.8 ± 1.4 79.9 ± 1.5
42
W N
473 + 9 551 + 2 7
32.8 ± 1.5 24.2 ± .9
53.2 ± 1.3 59.1 ± .7
12.9 ± .3 13.6 ± .3
78.8 ± .8 77.2 ± 1.8
56
W N
576 ± 3 3 761 ± 4 0
37.8 ±1.8 38.6 + 1.6
48.1 ± 1.1 48.7 ± .9
13.1 + .6 13.5 + .1
70.2 ± 2.4 70.5 + 3.3
Age (days) 28
Protein
Collagen (% protein)
(%)
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Back
Age
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KAFRI ET AL. TABLE 4. Means and standard errors of body weights by sex, within age and diet Males
Females
Age
Diet
28
W N
709 ± 42 783 ± 16
665 ± 22 617 + 40
42
W N
1302 ± 44 1321± 21
1049 ± 43 1202 + 49
56
W N
1515 ± 1 2 1 1884± 50
1621 ± 79 1689 ± 5 2
sji>
1
W = Wide, n = narrow.
strength was 94, 89, and 82% for breast, back, and thigh, respectively. Percentage fat in thigh skin was higher in males than in females, but no significant differences in skin fat due to sex were obtained in breast or back skin. No differences in moisture were obtained between sexes for breast or back skin, but thigh skin from the males contained more moisture than that from females. Skin from males was consistently higher in protein than skin from females, but the difference was most pronounced in thigh skin. Collagen concentrations of back and thigh skin were greater in males than in females, but the collagen content of
TABLE 5. Body weights, breaking strength, and chemical composition of skin by sex, diet, age, and site with main effects pooled Main effect
Breaking strength
Body weight
Fat
Moisture
(g/
Protein
Collagen (% protein)
(">)
Sex Male Female
1069 a 989 b
860 a 763b
38b 40a
49a 48a
12.6 b 11. l a
74a 70b
Diet W N
981b 1078 a
779b 843a
41a 36 b
47b 5ia
11.5b 12.2 a
70b 73a
Age 28 42 56
702 c 1196 b 1677 a
779b 82iab 835 a
36b 37b 43a
52a 49b 45 c
12.3a 11.7b 11.6 b
75a 71b 69c
996b 794a 644c
37b 50 a 29c
5lb 40c 55a
11.9b 8.9 C 14.2 a
72b 64c 79a
Site Breast Back Thigh
' 'cWithin main effects, means in a column having the same superscript were not significantly different (P>.05). 1 W = Wide, N = narrow.
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mediate values for breaking strength; back skin also contained more fat and less moisture, protein, and collagen than skin from the breast or thigh. Skin from the breast had more fat and less moisture, protein, and collagen than thigh skin. Interactions Among Main Variables. Significant sex by site interactions for skin breaking strength and for the percentages of protein, collagen, moisture, and fat in skin were obtained (Table 6). Although females had weaker skin than males regardless of site, the magnitude of the sexual dimorphism differed among sites. The ratio of female to male skin breaking
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SKIN COMPOSITION AND STRENGTH TABLE 6. Sex by site interactions for skin breaking strength and fat, moisture, protein, and collagen concentrations of the skin
Site
Sex
Breaking strength
Breast
Male Female
1026 a 966°
Male Female
a
Male Female
Fat
Moisture
37.9 s 35.9a
49.7a 52.0a
841 748b
48.7a
a
Sl.oa
40.6 39.4a
9.6 a 8.4°
709 a 581b
26.3 b 32.4a
57.1 a 53.4 b
15.9 a 13.6°
(g)
Back Thigh
Protein
(%) 12.4a 11.3D
Collagen (% protein) 73.3a 71.0 a 65.3 a 61.9 b 82.2a 76.6 b
breast skin did not differ significantly due to sex. Age by diet interactions for breaking strength and for fat, moisture, and protein concentrations of the skin are presented in Table 7. Broilers fed diets containing narrower C:P ratios had stronger skin than those fed the wide C:P diets at all ages examined, with the difference being significant at 56 days. Differences in skin composition due to diet, while pronounced at younger ages, disappeared by 56 days of age. None of the significant age by site interactions (Table 8) were of the crossover type. Regardless of age, thigh skin exhibited the highest values for protein, collagen, and moisture; back skin exhibited the lowest values for moisture, protein, and collagen, the highest value for percentage fat, and was intermediate in breaking strength; breast skin, while strong-
est, exhibited intermediate values for fat, water, protein, and collagen. Correlations and Regressions. At all skin sites examined, correlations between breaking strength and the skin composition traits measured were low and not significant. Step-wise regression analysis failed to reveal any consistent cause-effect relationships between breaking strength and skin composition. DISCUSSION The tendency of males to have stronger skin than females (Table 5) was consistent with previous observations that the incidence of torn skin occurring during processing was greater in female than in male broilers (Edwards et al., 1973; Smith et al., 1977). Smith et al. (1977) proposed that the propensity for female skin to
TABLE 7. Age by diet interactions for skin breaking strength and for fat, moisture, and protein concentrations of the skin
Age
Diet
Breaking strength
Fat
Moisture
28
W N
39.9a 32.0 b
48.2 a 55.4 b
11.8 a 12.7 b
42
W N
809 a 832 a
39.8 a 33.8 b
47.1a 51.5 b
11.3a 12.1b
56
W N
772 a 899b
43.4a 43.3a
44.5 a 45.4a
11.5a 11.7a
Protein
(%)
ab ' Within sites, means in a column having the same superscript were not significantly different (P>.05). 1
W = Wide, N = narrow.
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a,b,Within sites, means in a column having the same superscript were not significantly different (P>.05).
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KAFRI ET AL. TABLE 8. Age by site interactions for skin breaking strength and for fat, moisture, protein, and collagen concentrations of the skin Breaking strength
Collagen
Fat
Moisture
Protein
32.1 b 47.4 a 28.3 C
11.9b 9.8 C 15.0 a
75.9 b 68.8 C 80.3 a
Breast Back Thigh
1090 a 809 b 563c
30.2 b 48 .Oa 26.2 C
55.8 a 42.7b 56.9 a 50.1b 40.6 C 57.3 a
12.2 b 8.5 C 14.4 a
73.2 b 60.3 c 80.7 a
Breast Back Thigh
95 7 a 825b 724 c
42.3b 54.2 a 33.5 C
46.7 b 36.6 C 51.5 a
11.4b 8.7C 14.7 a
67.3 b 62.0 C 77.la
Site
28
Breast Back Thigh
42
56
(% protein)
' 'cWithin sites, means in a column having the same superscript were not significantly different (P>.05).
be weaker than that of males was related to increased concentrations of fat or decreased collagen concentrations. In our study, skin from females tended to contain more fat and less protein and collagen than skin from males (Table 5). Similar observations were reported in broiler chicks by Grey et al. (1983) and in goslings by Nitsan et al. (1981). The significant sex by site interactions obtained in our study, however, were not entirely consistent with the theory that increased fat and decreased protein or collagen contents of the skin are associated with weaker skin. Although breaking strength of skin from males was greater than that of females at all ages examined (Table 6), skin fat was significantly greater in females only for thigh skin. Numerically, percentage fat in breast skin was higher for males than for females. Protein and collagen concentrations of the skin, however, were consistently greater in males than in females, irrespective of site. If increases in skin fat result in weaker skin, feeding diets with wide C:P ratios would be expected to reduce skin breaking strength; Bartov et al. (1974) reported that decreased dietary protein resulted in an increased deposition of fat in the skin of broilers. Feeding the wide C:P diets did result in weaker skin that was associated with a general increase in skin fat and decreases in the moisture, protein, and collagen concentrations of the skin (Table 5). It may be of importance, however, that differences in breaking strength due to diet were more pronounced at 56 days of age, while differences in skin composition were more pronounced at younger ages (Table 7). No
significant dietary differences in skin composition were observed at 56 days of age when the age by diet interaction was examined. Although there were inconsistencies, differences in breaking strength and skin composition between males and females, and between chicks fed diets differing in C:P ratios, tended to support the conclusion that weak skin is associated with increased fat and decreased protein or collagen concentrations of the skin (Smith et al., 1977). Different patterns, however, emerged when site and age differences were examined. Edwards et al. (1973) reported that the incidence of torn skin was higher in broilers at 9 weeks than at younger ages, inferring that the skin of chickens becomes weaker with age. Our results were inconsistent with this inference. Breaking strength tended to increase with age (Table 5), although the patterns varied between diets (Table 7) and among sites (Table 8). For example, a decrease in breaking strength of the skin occurred between 42 and 56 days of age when the diets containing wide C:P ratios were fed, but breaking strength of skin increased during this period in response to the narrow C:P diets (Table 7). Moreover, thigh and back skin tended to become stronger with age while breast skin did not (Table 8). Changes in skin composition with age indicated that skin strength was not solely associated with increased fat and decreased protein and collagen concentrations. Increases in breaking strength with age were accompanied by increases in skin fat and decreases in moisture and collagen concentrations. Skin protein content increased from 28 to 42 days of age
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Age
SKIN COMPOSITION AND STRENGTH
Similar t o t h e age effects, differences in t h e c o m p o s i t i o n and strength of skin a t different sites did n o t indicate t h a t breaking strength was associated with increased fat, decreased p r o t e i n , or collagen c o n c e n t r a t i o n s . Skin from t h e back contained m o r e fat and less p r o t e i n and collagen t h a n skin from t h e o t h e r sites b u t exhibited intermediate values for breaking strength (Table 5). A l t h o u g h breast skin contained m o r e fat and less p r o t e i n and collagen t h a n t h a t from t h e thigh, it was considerably stronger t h a n thigh skin. Skin from t h e thigh, which was weakest a m o n g t h e sites e x a m i n e d , had t h e least fat and t h e m o s t p r o t e i n and collagen. These site differences were generally consistent at t h e different ages (Table 8) and b e t w e e n sexes (Table 6 ) ; t h e significant interactions were n o t of t h e crossover t y p e . Differences in skin c o m p o s i t i o n and skin strength due t o sex, diet, age, and skin site revealed n o consistent relationship b e t w e e n t h e strength and chemical c o m p o s i t i o n of t h e skin. Correlation and multiple regression analyses were also ineffective in relating skin strength t o any of t h e chemical characteristics measured. Perhaps a m o r e detailed chemical o r histological e x a m i n a t i o n of specific skin layers w o u l d be beneficial in elucidating physical o r chemical factors determining skin strength. Classification of t o t a l skin collagen into t h e distinct t y p e s of
tropocollagens m i g h t be especially beneficial.
REFERENCES Association of Official Analytical Chemists, 1975. Official Methods of Analysis, 12th ed., Assoc. Offic. Anal. Chem., Washington, DC. Bartov, I., S. Bornstein, and B. Lipstein, 1974. Effect of calorie to protein ratio on the degree of fatness in broilers fed on practical diets. Br. Poult. Sci. 15:107-117. Edwards, H. M., F. Denman, A. Abou-Ashour, and D. Nugara, 1973. Carcass composition studies. 1. Influence of age, sex, and type of dietary fat supplementation on total carcass and fatty acid composition. Poultry Sci. 52:934-943. Folch-Jordi, M. L., and G. H. Sloane-Stanley, 1965. Simple method for the isolation and purification of total lipids from animal tissues. J. Biol. Chem. 226:497-509. Grey, T. C , D. Robinson, J. H. Jones, S. W. Stock, and N. L. Thomas, 1983. Effect of age and sex on the composition of muscle and skin from a commercial broiler strain. Br. Poult. Sci. 24: 219-231. Kafri, I., D. J. Zelenka, J. A. Cherry, and P. B. Siegel, 1984. Skin breaking strength in chickens: Comparisons among genetic combinations. Poultry Sci. 63:1279-1280. Logan, A. M., and E. R. Newman, 1950. The determination of hydroxyproline. J. Biol. Chem. 184:299-306. National Research Council, 1977. Nutrient Requirements of Poultry. 7th ed. Natl. Acad. Sci., Washington, DC. Nir, I., Z. Nitsan, and Y. Dror, 1974. Force-feeding effects on growth, carcass and blood composition in the young chick. Br. J. Nutr. 32:229-239. Nitsan, Z., A. Dvorin, and I. Nir, 1981. Composition and amino acid content of carcass, skin, and feathers of the growing gosling. Br. Poult. Sci. 22:79-84. Smith, T. W., Jr., J. R. Couch, R. L. Garret, and C. R. Creger, 1977. The effect of sex, dietary energy, meat protein, ascorbic acid, and iron on broiler skin collagen. Poultry Sci. 56:1216—1220. Sokal, R. R., and F. J. Rohlf, 1969. Biochemistry: The Principles and Practices of Statistics in Biological Research. W. H. Freeman and Co., San Francisco, CA. Suderman, D. R., and F. E. Cunningham, 1980. The effect of age of bird and method of chilling on composition of broiler skin. Poultry Sci. 59: 2247-2249.
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b u t decreased from 4 2 t o 56 days of age. R e p o r t s in t h e literature concerning age effects o n skin p r o t e i n are also ambiguous. S u d e r m a n et al. ( 1 9 8 0 ) reported t h a t skin p r o t e i n remained fairly c o n s t a n t at different ages while o t h e r investigators (Nitsan et al., 1 9 8 1 , Grey et al, 1983) observed a general decline in protein c o n c e n t r a t i o n s in t h e skin with age. N o n e t h e less, changes in t h e p r o t e i n c o n t e n t of t h e skin failed to parallel those in breaking strength. Neither were t h e significant age b y diet (Table 7) nor age b y site (Table 8) interactions indicative of a consistent relationship b e t w e e n skin composition and skin strength.
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