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Prediction of Initial Carcass Composition in Comparative Slaughter Experiments1
Prediction of Initial Carcass Composition in Comparative Slaughter Experiments1 M. S. WOLYNETZ2 and I. R. SIBBALD3 Agriculture Canada, Ottawa, Ontario, Canada .K1A QC6 (Received for publication July 10, 1984)
1985 Poultry Science 6 4 : 6 8 1 - 6 8 7 INTRODUCTION
MATERIALS AND METHODS
Comparative slaughter is the classical method for estimating changes in the body composition of domestic animals. Representative animals, selected at the start of an experiment, are slaughtered and their carcasses are analyzed. The resulting data are used, in conjunction with live weight measurements of initial slaughter and experimental animals, to predict the initial carcass composition of the experimental animals. The procedure may be criticized on the grounds that animals of the same live weight can have different compositions, but Just et al. (1982) state that most errors of this type can be eliminated by use of a proper experimental design. Data from experiments with broiler chicks and adult Single Comb White Leghorn (SCWL) cockerels provided an opportunity to investigate the ability to predict carcass composition. The findings form the basis of this report.
Experiment 1. Sixty-five male and 109 female broiler chicks of a single strain, which were reared to 18 days of age under identical conditions, were fasted for 18 hr to reduce the amount of feed residues in their alimentary canals. Five birds of each sex having extreme body weights or which were crippled were removed from the populations. In addition, 20 of the remaining 60 males and 24 of the remaining 104 females were selected at random. The 10 discarded birds and the 44 selected birds were weighed, slaughtered by carbon dioxide inhalation, reweighed, placed in labeled plastic bags, and frozen to await analysis. Experiment 2. Eight hundred male broiler chicks of a single strain were reared to 10 days of age. After division into weight classes, with a class interval of 5 g, the heaviest and lightest classes of birds were discarded. The remaining birds were distributed four to a pen among 120 pens contained in five battery brooders, each brooder was comprised of six tiers with four pens per tier. The order of filling the tiers was randomized. Starting with birds of the heaviest weight class, the next heaviest, and so on, a bird was placed in each pen of the first tier, a second bird was added in the same sequence, then a third, and then a fourth. Once housing was completed, all birds were wingbanded and
1 Contribution Numbers 1-624 Engineering and Statistical Research Institute and 1260 Animal Research Centre. 2 Engineering and Statistical Research Institute. 3 Animal Research Centre. To whom correspondence should be addressed.
681
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ABSTRACT The comparative slaughter technique for estimating changes in body composition relies on the ability to predict accurately initial carcass composition; in many experiments the only independent variable is live body weight (LW). The relationships of total carcass energy (TCE), nitrogen (TCN), fat (TCF), ash (TCA), and water (TCW) with LW were examined in two sets of broiler chick data and one set of data from adult, Single Comb White Leghorn (SCWL) cockerels. For chicks, the linear relationships between TCW, TCN, and to a lesser extent, TCE and TCA with LW were strong [multiple coefficients of determination (R 2 ) = .766 to .984], but the association between TCF and LW was generally weaker (R 2 = .531 to .700). For adult birds, the R2 values involving LW were smaller, particularly for TCN (.386) and TCW (.365). The statistical analyses indicated it was possible to use LW to predict accurately the body composition of young broiler chicks (14 to 18 days of age), particularly males, but LW was unable to provide an accurate estimate of the body composition of mature SCWL cockerels. The failure to make satisfactory predictions from adults reflected the great variation in body composition among birds of similar LW. (Key words: carcass composition, comparative slaughter, assay methodology)
682
WOLYNETZ AND SIBBALD
RESULTS AND DISCUSSION
Experiment I. The results reported are based on the entire data set of 25 male and 29 female
chicks and are not significantly different (P> .05) from those obtained after exclusion of the five atypical birds of each sex. The body weight and carcass composition data are summarized in Table 1. There were no significant sex differences (P>.05) in live weight (LW), dead weight, total carcass dry matter (TCDM), water (TCW), nitrogen (TCN), or ash (TCA); the female birds contained more total carcass energy (TCE) (P<.05) and total carcass fat (TCF) (P<.01) than the males. The total carcass residual (TCR), defined by: TCR = TCDM - (6.25 TCN + TCF + TCA) was similar for both sexes and was approximately .5% of LW or 1.6% of TCDM. The regression analyses of TCE, TCN, TCF, TCA, TCR, and TCW on LW are summarized in Table 2. There was no evidence (P>.05) of a curvilinear relationship between any dependent variable and LW. Although the analyses indicated that a common line could be fitted to both sexes, the results for each sex are presented separately. In all 12 regression equations, the intercept was not different from zero (P>.05); consequently, the analyses were repeated forcing the regression equations to pass through the origin. The results are also summarized in Table 2. The LW sum of squares corrected for the mean, S(XJ-X) 2 , were substantially less than the uncorrected sum of squares, 2x 2 j, and therefore, the standard errors of the slopes in the second analysis were much smaller. There were significant differences (P<.01) between the sexes for the slopes of TCE, TCF, and TCW on LW for the regression lines through the origin. For regression through the origin, the estimated slope of the line can be written as:
where x; and y; are the values of the independent (LW) and dependent (TCE, TCN, TCF, TCA, TCR, TCW) variables, respectively, for the i t n bird. An alternative estimator of carcass composition is:
n = I 2 II n
XJ
where n is the number of birds and yj/xj is the value of the dependent variable per unit of LW.
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group weighed. All birds in the fifth brooder (i.e., 24 pens of 4 chicks) were group weighed, killed by carbon dioxide inhalation, reweighed, placed in labeled plastic bags, and frozen to await analysis. Experiment 3. Approximately 90 2-year-old adult SCWL cockerels of each of two strains (Strains 5 and 10; Gowe and Fairfull, 1980) were housed in individual wire cages in each of two windowless rooms where they received 12 hr of light daily and had continuous access to feed and fresh water. Two weeks prior to a metabolism experiment, 48 birds of each strain in each room that had body weights closest to the strain-by-room mean were chosen; the remaining birds were discarded. Two days prior to the experiment, the birds were reweighed, and 36 birds of each strain in each room that had the narrowest range of body weights were chosen, the remaining birds were discarded. The four groups of 36 birds were fasted for 48 hr to clear feed residues from their alimentary canals. Twelve birds from each group were chosen at random, weighed, killed by carbon dioxide inhalation, reweighed, placed in labeled plastic bags, and frozen to await analysis. Carcass Analysis. The individual chicks of Experiment 1 and the groups of 4 chicks of Experiment 2 were reduced to free-flowing homogeneous powders by the method of Sibbald and Fortin (1982). The adult cockerels were treated in a similar manner, except that aliquots comprising more than 10% of each wet homogenate were dried and reduced to powder (Sibbald and Wolynetz, 1984). The air-dry carcass homogenates were assayed for dry matter, nitrogen, crude fat, and ash using methods of the Association of Official Analytical Chemists (AOAC, 1980). Gross energy was measured with an adiabatic oxygen bomb calorimeter. Statistical Analysis. The data were examined using analysis of variance and multiple linear regression. In Experiment 1, the analyses were performed on both the entire data set and on the set excluding the five birds of each sex that were crippled or which had extreme body weights. The data of Experiment 1 were examined for sex differences, those of Experiment 2 for tier effects not due to body weight, and those of Experiment 3 for strain and room effects.
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2x; Values of r 2 with their standard errors are also shown in Table 2. The three estimators /?i, r i , and r 2 produced very similar results. In a comparative slaughter experiment, the estimates of initial body composition using any of the four estimators (ordinary regression, |3i, r x , and r 2 ) would be very similar when body composition is proportional to LW. There was no evidence that the variances of the dependent variables increased with LW, perhaps due to the relatively narrow LW range: .32 to .57 kg for males and .29 to .52 kg for females. Consequently, the least squares regression analysis through the origin, leading to |3i, is appropriate. However, when the variance of the dependent variable increases with LW, weighted regression analysis may be appropriate. If the variance of the dependent variable increases in proportion to LW, r 2 is the best estimator; however, if the standard deviation of the dependent variable increases in proportion to LW, ri is preferred. For a more complete discussion see Snedecor and Cochran (1967). For all dependent variables, but particularly TCF, the multiple coefficients of determination (R 2 ) were larger for males than for females. The sexual discrepancy may be associated with the well known greater fatness of female broilers at market weight. Dansky (1952), cited by Hill et al. (1957), observed sex differences in carcass fat content of birds fed the same diet at approximately 3 weeks of age, which is in agreement with the present results. However, Yoshida and Morimoto (1970) found no sex differences among 3 to 4-weekold chicks and the extensive data of Leeson and Summers (1980) suggest that differences did not appear until after 5 weeks of age. The lack of consistency among these findings might be expected because, in the first of two experiments, Edwards and Denham (1975) found females to be fatter than males at 4 weeks of age, but the sex effect was not consistent among either genotypes or diet groups. In the second experiment, there were no sex related differences at 4 weeks of age. Sexual dimorphism is unlikely to occur at the same time among all birds of a population. If sex differ-
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Values of rt with their standard errors are shown in Table 2. A third estimator, which is even easier to compute, is:
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1
.000846
2.39846
Female
Female
3.39963
Male
.001689
1.6339
Female
Male
2.6966
Female
Male
95.3027
115.3442
Male
1.5575
Female
Standard error of parameter estimate directly above.
TCW, kg
TCR.g
TCA.g
TCF.g
2.6296
Female
Male
.3114
Male
TCE, MJ
TCN.g
.3465
Sex
25.542 LW (1.127) 26.134 LW (1.508)
.012 (.008) .016 (.012)
-.966 (2.753) -.921 (2.885)
.991 (.705) .711 (.995)
-10.711 (8.216) -18.934 (14.048)
7.059 LW (6.012) 7.420 LW (6.357)
+
+
+
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.656 LW (.018) .622 LW (.025)
25.385 LW (1.540) 25.519 LW (2.191)
+
+
+ 131.522 LW (17.942) + 171.040 LW (30.953)
+
+
.63 3 (.516) .461 (.684)
8.855 LW (.704) 10.675 LW (1.136)
+ -.315 (.323)' + -.748 (.516)
.984 .957
.000038
.048
2.36781 .000028
.057
3.34686
.2813
.834
.922
.531
.2195
.700
56.1347
.918
29.8077
.1332
.957
.766
.0757 .1175
.873
R2
.681 LW (.002) .657 LW (.003)
4.966 LW (.784) 5.401 LW (.619)
27.531 LW (.209) 27.078 LW (.215)
108.339 LW (2.419) 129.526 LW (3.110)
26.913 LW (.151) 27.144 LW (.158)
8.172 LW (.094) 9.034 LW (.115)
Slope (J,
Regressio
residual (TCR), and water (I'CW) on kil
.0459
Residual mean square
Linear regression
(TCS), fat (TCE), ash (TCA),
Equation <-x +(3 LW
carcass energy (TCE), nitrogen
Total corrected mean square
oftotal
Dependent variable
TABLE 2, Regression
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PREDICTION OF CARCASS COMPOSITION
through the origin, and the r t and r 2 estimators were not computed. For each of the dependent variables, the estimated slopes of the regression lines for the male birds in the first experiment and second experiment were not significantly different (P> .05). The negative intercepts for TCE, TCN, TCF, and TCA may be due to TCR not being related to LW (P>.05) and TCR being a relatively larger percentage of TCDM than in the first experiment (2.7 vs. 1.6%). Experiment 3. Data were available from only 36 of the 48 birds due to a malfunction of the autoclave. There were no significant room effects (P>.05), nor were there any significant differences between the two strains (P>.05). The body weight and carcass composition data are summarized in Table 1. The composition of the adult SCWL carcasses was different from those of the two groups of broiler chicks. The TCR was approximately 1.7% of LW and was 3.5 and 4.3% of TCDM for strains 5 and 10, respectively; these values are considerably greater than those observed in the chick experiments. The TCW comprised 57% of adult LW but 67 and 72% of chick LW in the first two experiments, respectively. The regression analyses on LW revealed that, for each of the dependent variables, there were no significant differences (P>.05) between strains in either intercept or slope. There was no indication (P>.05) of a curvilinear relation-
TABLE 3. Regression of total carcass energy (TCE), nitrogen (TCN), fat (TCF), ash (TCA), residual (TCR), and water (TCW) on kilograms live body weight (LW) — Experiment 2 Dependent variable TCE, MJ
Regression equation a + f3LW
Residual mean square
+
8.351 LW (.534)
.034272
.917
.224582
.952
-.758 (.394)'
TCN.g
-2.401 (1.009)
+
28.596 LW (1.367)
TCF.g
-16.858 (11.706)
+
112.007 LW (15.866)
TCA,g
-3.188 (.911)
+
27.236 LW (1.235)
TCR.g
7.767 (5.792)
TCW, kg
1
.026 (.011)
—
.003 LW (.008)
+
.683 LW (.015)
Standard error of parameter estimate directly above.
30.2429 .183168 7.4058 .000026
R2
.694 .957 .007 .990
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ences begin to appear at about 3 weeks of age with females becoming fatter, then the carcass composition of females may be more heterogeneous than that of males. Greater heterogeneity could explain the aforementioned smaller R 2 values and the much larger residual mean square for females (Table 2). The data indicate that TCW, TCN, and to a lesser extent, TCE and TCA could be predicted from LW. The TCF was not as closely related to LW as were the other dependent variables, and there seemed to be no relationship between TCR and LW. Experiment 2. The body weight and carcass composition data are summarized in Table 1. The mean TCR was .8% of mean LW or 2.7% of TCDM; this was about 60% higher than the corresponding values in the first experiment. The regression analyses using LW as the independent variable are summarized in Table 3. Only for TCW was there a significant (P<.05) quadratic effect with an R 2 of .992. However, because R 2 = .990 for the linear equation, and because this would provide a good estimate of initial TCW, only the parameter estimates of the linear equation are shown in Table 3. The R 2 values of the second experiment are similar to those of the male birds in the first experiment. Unlike the first experiment, except for TCF and TCR, the intercepts are significantly different (P<.05) from zero. Consequently, no attempt was made to force the regressions
685
686
WOLYNETZ AND SIBBALD
TABLE 4. Regression of total carcass energy (TCE), nitrogen (TCN), fat (TCF), ash (TCA), residual (TCR), and water (TCW) on kilograms live body weight (LW) — Experiment 3 Dependent variable
Regressi o n equation
Residual mean square
a 4• £ L W
R2
+
24.225 LW (2.985)
21.997
.660
TCN.g
50.460 (11.551)
+
19.140 LW (4.136)
42.240
.386
TCF.g
1136.128 (270.551)
+
567.875 LW (96.880)
23171.406
.503
TCA,g
95.523 (22.592)
+
.098 LW (8.090)
161.568
.000
TCR.g
29.257 (25.752)
+
6.003 LW (9.221)
209.925
.012
.756 (.186)
+
.295 LW (.067)
TCW, kg
1
.010995
.365
Standard error of parameter estimate directly above.
ship between any dependent variable and LW. The regression equations are summarized in Table 4. Except for TCR, the R 2 values were much smaller and the residual mean square values were considerably larger than in either of the chick experiments; this suggests that in this population of adult birds accurate prediction of initial body composition from LW was not possible. For example, for a bird of average weight (2.78 kg), the predicted TCN is 103.7 g; the associated 95% confidence interval of 90.5 to 116.9 g is not much narrower than the range of observed values, 83.9 to 117.9 g. For both TCA and TCR, there was no linear relationship with LW and their means provide alternate estimates of initial composition. Except for TCR, the intercepts of the regression equations were significantly different from zero (P<.01); hence, regression through the origin was not appropriate and the ri and r2 estimates were not computed. Unlike Experiment 2, the intercept for TCN was positive for the adult birds. General Discussion. The R 2 value obtained by regressing TCW on LW was only .365 for the adult cockerels, whereas it ranged from .957 to .990 for the chicks. The relatively weak relationship for the adults probably reflects greater heterogeneity, in composition. Examination of the raw data revealed a high degree of uniformity in composition among chicks of similar LW, but this was not so for the adults.
For example, for 8 adults with LW between 2.8 and 2.9 kg, the TCDM ranged from 38.2 to 48.5% of the LW. In face of such variation within a narrow weight class, it is not surprising that LW was an inadequate predictor of body composition. The relatively large TCR for the adult birds, and the lack of a relationship between TCR and LW, further contributed to the inability to predict adult body composition from LW. Increasing the number of birds slaughtered and analyzed would not have improved the ability to predict initial body composition. In all three experiments, the R 2 value relating TCF to LW was much smaller than those relating TCE and TCN to LW; this may reflect a weakness in the assay for carcass fat and, therefore, a series of calculations was made to investigate this possibility. Assuming that the carbohydrate content of the carcass is negligible, the TCE comprises the energy present as fat and protein (N x 6.25). Then an indirect estimate of total carcass fat (TCFI) can be taken as: TCFI(k) = TCE - k X 6.25 X TCN
where K is the energy value (MJ/kg) of protein. Published estimates are in the range 23 to 25 MJ/kg (Znaniecka, 1969; Sibbald and Morse, 1984). Calculation of TCFI(k) using a range of values of k from 22 to
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-34.709 (8.336) 1
TCE, MJ
PREDICTION OF CARCASS COMPOSITION
ACKNOWLEDGMENTS The authors thank S. Tobin for her able technical assistance. The fat and ash analyses were made by the Chemical and Biological Research Institute of Agriculture Canada. REFERENCES Association of Official Analytical Chemists, 1980. Official Methods of Analysis. 11th ed. Assoc.
Offic. Anal. Chem., Washington, D.C. Dansky, L. M., 1952. Thesis, Cornell Univ., Ithaca, NY. (Cited by Hill et al., 1957.) Edwards, Jr., H. M., and F. Denhan, 1975. Carcass composition studies. 2. Influences of breed, sex and diet on gross composition of the carcass and fatty acid composition of the adipose tissue. Poultry Sci. 54:1230-1238. Gowe, R. S., and R. W. Fairfull, 1980. Performance of six long-term multi-trait selected leghorn strains and three control strains, and a strain cross evaluation of the selected strains. Pages 141 — 162 in Proc. South Pacific Poult. Sci. Conv. Hill, F. W., M. L. Scott, and L. B. Carew, Jr., 1975. The effect of nutrition on the fat content of poultry. Pages 99—109 in Proc. Cornell Nutr. Conf. Just, A., J. A. Fernandez, and H. Jorgensen, 1982. Nitrogen balance studies and nitrogen retention. Pages 111—122 in Physiologie Digestive chez le Pore, les Colloques de I'lnra. J. P. Laplace, T. Corring and A. Rerat, ed. Jouy-en-Josas, France. Leeson, S., and J. D. Summers, 1980. Production and carcass characteristics of the broiler chicken. Poultry Sci. 59:786-798. Sibbald, I. R., and A. Fortin, 1982. Preparation of dry homogenates from whole and eviscerated chickens. Poultry Sci. 61:589-590. Sibbald, I. R., and P. M. Morse, 1984. A preliminary investigation of the utilization of true metabolizable energy by chicks. Poultry Sci. 63:954—971. Sibbald, I. R., and M. S. Wolynetz, 1984. Variation among aliquots of entire-chicken homogenates. Poultry Sci. 63:1446-1448. Snedecor, G. W., and W. G. Cochran, 1967. Statistical Methods. 6th ed. Iowa State Univ. Press, Ames, IA. Yoshida, M., and H. Morimoto, 1970. Interrelationships between dietary protein level and carcass composition of chicks. Agric. Biol. Chem. 34: 414-422. Znaniecka, G., 1969. Calorific value of protein and fat of the chickens body. Pages 407—408 in Energy Metabolism of Farm Animals. K. L. Blaxter, J. Kielanoswki, and G. Thorbek, ed. Oriel Press Ltd., London.
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27 and then regressing TCFI(k) on LW resulted in R 2 values very similar to those for TCF in all three data sets. Furthermore, the correlation coefficient between TCFI(k) and TCF exceeded .950 in all three experiments. These findings suggest that the relatively low correlation between TCF and LW is not due to problems in the assay for carcass fat. A more probable cause is the greater heterogeneity of TCF values relative to TCN, which results in TCE being of intermediate variability. For both chick experiments, there were large R 2 values between TCN and TCW with LW ( R 2 > . 9 1 ) . For adult birds they were much smaller ( R 2 < . 4 0 ) . Thus, it appears that the TCN and TCW of experimental chicks, but not adult birds, can be accurately predicted from LW. The statistical analyses have related observed carcass characteristics to measured LW in the same birds and have also identified prediction equations which best fit the data. The R 2 values of such equations overstate the ability to predict the body composition of other birds from the same population, for example, the initial body composition of birds slaughtered at the end of the experiment.
687
1985 Poultry Science 6 4 : 6 8 1 - 6 8 7 INTRODUCTION
MATERIALS AND METHODS
Comparative slaughter is the classical method for estimating changes in the body composition of domestic animals. Representative animals, selected at the start of an experiment, are slaughtered and their carcasses are analyzed. The resulting data are used, in conjunction with live weight measurements of initial slaughter and experimental animals, to predict the initial carcass composition of the experimental animals. The procedure may be criticized on the grounds that animals of the same live weight can have different compositions, but Just et al. (1982) state that most errors of this type can be eliminated by use of a proper experimental design. Data from experiments with broiler chicks and adult Single Comb White Leghorn (SCWL) cockerels provided an opportunity to investigate the ability to predict carcass composition. The findings form the basis of this report.
Experiment 1. Sixty-five male and 109 female broiler chicks of a single strain, which were reared to 18 days of age under identical conditions, were fasted for 18 hr to reduce the amount of feed residues in their alimentary canals. Five birds of each sex having extreme body weights or which were crippled were removed from the populations. In addition, 20 of the remaining 60 males and 24 of the remaining 104 females were selected at random. The 10 discarded birds and the 44 selected birds were weighed, slaughtered by carbon dioxide inhalation, reweighed, placed in labeled plastic bags, and frozen to await analysis. Experiment 2. Eight hundred male broiler chicks of a single strain were reared to 10 days of age. After division into weight classes, with a class interval of 5 g, the heaviest and lightest classes of birds were discarded. The remaining birds were distributed four to a pen among 120 pens contained in five battery brooders, each brooder was comprised of six tiers with four pens per tier. The order of filling the tiers was randomized. Starting with birds of the heaviest weight class, the next heaviest, and so on, a bird was placed in each pen of the first tier, a second bird was added in the same sequence, then a third, and then a fourth. Once housing was completed, all birds were wingbanded and
1 Contribution Numbers 1-624 Engineering and Statistical Research Institute and 1260 Animal Research Centre. 2 Engineering and Statistical Research Institute. 3 Animal Research Centre. To whom correspondence should be addressed.
681
Downloaded from http://ps.oxfordjournals.org/ at University of Michigan on June 20, 2015
ABSTRACT The comparative slaughter technique for estimating changes in body composition relies on the ability to predict accurately initial carcass composition; in many experiments the only independent variable is live body weight (LW). The relationships of total carcass energy (TCE), nitrogen (TCN), fat (TCF), ash (TCA), and water (TCW) with LW were examined in two sets of broiler chick data and one set of data from adult, Single Comb White Leghorn (SCWL) cockerels. For chicks, the linear relationships between TCW, TCN, and to a lesser extent, TCE and TCA with LW were strong [multiple coefficients of determination (R 2 ) = .766 to .984], but the association between TCF and LW was generally weaker (R 2 = .531 to .700). For adult birds, the R2 values involving LW were smaller, particularly for TCN (.386) and TCW (.365). The statistical analyses indicated it was possible to use LW to predict accurately the body composition of young broiler chicks (14 to 18 days of age), particularly males, but LW was unable to provide an accurate estimate of the body composition of mature SCWL cockerels. The failure to make satisfactory predictions from adults reflected the great variation in body composition among birds of similar LW. (Key words: carcass composition, comparative slaughter, assay methodology)
682
WOLYNETZ AND SIBBALD
RESULTS AND DISCUSSION
Experiment I. The results reported are based on the entire data set of 25 male and 29 female
chicks and are not significantly different (P> .05) from those obtained after exclusion of the five atypical birds of each sex. The body weight and carcass composition data are summarized in Table 1. There were no significant sex differences (P>.05) in live weight (LW), dead weight, total carcass dry matter (TCDM), water (TCW), nitrogen (TCN), or ash (TCA); the female birds contained more total carcass energy (TCE) (P<.05) and total carcass fat (TCF) (P<.01) than the males. The total carcass residual (TCR), defined by: TCR = TCDM - (6.25 TCN + TCF + TCA) was similar for both sexes and was approximately .5% of LW or 1.6% of TCDM. The regression analyses of TCE, TCN, TCF, TCA, TCR, and TCW on LW are summarized in Table 2. There was no evidence (P>.05) of a curvilinear relationship between any dependent variable and LW. Although the analyses indicated that a common line could be fitted to both sexes, the results for each sex are presented separately. In all 12 regression equations, the intercept was not different from zero (P>.05); consequently, the analyses were repeated forcing the regression equations to pass through the origin. The results are also summarized in Table 2. The LW sum of squares corrected for the mean, S(XJ-X) 2 , were substantially less than the uncorrected sum of squares, 2x 2 j, and therefore, the standard errors of the slopes in the second analysis were much smaller. There were significant differences (P<.01) between the sexes for the slopes of TCE, TCF, and TCW on LW for the regression lines through the origin. For regression through the origin, the estimated slope of the line can be written as:
where x; and y; are the values of the independent (LW) and dependent (TCE, TCN, TCF, TCA, TCR, TCW) variables, respectively, for the i t n bird. An alternative estimator of carcass composition is:
n = I 2 II n
XJ
where n is the number of birds and yj/xj is the value of the dependent variable per unit of LW.
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group weighed. All birds in the fifth brooder (i.e., 24 pens of 4 chicks) were group weighed, killed by carbon dioxide inhalation, reweighed, placed in labeled plastic bags, and frozen to await analysis. Experiment 3. Approximately 90 2-year-old adult SCWL cockerels of each of two strains (Strains 5 and 10; Gowe and Fairfull, 1980) were housed in individual wire cages in each of two windowless rooms where they received 12 hr of light daily and had continuous access to feed and fresh water. Two weeks prior to a metabolism experiment, 48 birds of each strain in each room that had body weights closest to the strain-by-room mean were chosen; the remaining birds were discarded. Two days prior to the experiment, the birds were reweighed, and 36 birds of each strain in each room that had the narrowest range of body weights were chosen, the remaining birds were discarded. The four groups of 36 birds were fasted for 48 hr to clear feed residues from their alimentary canals. Twelve birds from each group were chosen at random, weighed, killed by carbon dioxide inhalation, reweighed, placed in labeled plastic bags, and frozen to await analysis. Carcass Analysis. The individual chicks of Experiment 1 and the groups of 4 chicks of Experiment 2 were reduced to free-flowing homogeneous powders by the method of Sibbald and Fortin (1982). The adult cockerels were treated in a similar manner, except that aliquots comprising more than 10% of each wet homogenate were dried and reduced to powder (Sibbald and Wolynetz, 1984). The air-dry carcass homogenates were assayed for dry matter, nitrogen, crude fat, and ash using methods of the Association of Official Analytical Chemists (AOAC, 1980). Gross energy was measured with an adiabatic oxygen bomb calorimeter. Statistical Analysis. The data were examined using analysis of variance and multiple linear regression. In Experiment 1, the analyses were performed on both the entire data set and on the set excluding the five birds of each sex that were crippled or which had extreme body weights. The data of Experiment 1 were examined for sex differences, those of Experiment 2 for tier effects not due to body weight, and those of Experiment 3 for strain and room effects.
PREDICTION OF CARCASS COMPOSITION Os ON t"- fS '4-Tt-t*-rn\0(Nf^O\'lO O O O 0 0 ( N C 0 f s ) m r-H (N c-i m rri m t-- t-»
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2x; Values of r 2 with their standard errors are also shown in Table 2. The three estimators /?i, r i , and r 2 produced very similar results. In a comparative slaughter experiment, the estimates of initial body composition using any of the four estimators (ordinary regression, |3i, r x , and r 2 ) would be very similar when body composition is proportional to LW. There was no evidence that the variances of the dependent variables increased with LW, perhaps due to the relatively narrow LW range: .32 to .57 kg for males and .29 to .52 kg for females. Consequently, the least squares regression analysis through the origin, leading to |3i, is appropriate. However, when the variance of the dependent variable increases with LW, weighted regression analysis may be appropriate. If the variance of the dependent variable increases in proportion to LW, r 2 is the best estimator; however, if the standard deviation of the dependent variable increases in proportion to LW, ri is preferred. For a more complete discussion see Snedecor and Cochran (1967). For all dependent variables, but particularly TCF, the multiple coefficients of determination (R 2 ) were larger for males than for females. The sexual discrepancy may be associated with the well known greater fatness of female broilers at market weight. Dansky (1952), cited by Hill et al. (1957), observed sex differences in carcass fat content of birds fed the same diet at approximately 3 weeks of age, which is in agreement with the present results. However, Yoshida and Morimoto (1970) found no sex differences among 3 to 4-weekold chicks and the extensive data of Leeson and Summers (1980) suggest that differences did not appear until after 5 weeks of age. The lack of consistency among these findings might be expected because, in the first of two experiments, Edwards and Denham (1975) found females to be fatter than males at 4 weeks of age, but the sex effect was not consistent among either genotypes or diet groups. In the second experiment, there were no sex related differences at 4 weeks of age. Sexual dimorphism is unlikely to occur at the same time among all birds of a population. If sex differ-
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.824
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Values of rt with their standard errors are shown in Table 2. A third estimator, which is even easier to compute, is:
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1
.000846
2.39846
Female
Female
3.39963
Male
.001689
1.6339
Female
Male
2.6966
Female
Male
95.3027
115.3442
Male
1.5575
Female
Standard error of parameter estimate directly above.
TCW, kg
TCR.g
TCA.g
TCF.g
2.6296
Female
Male
.3114
Male
TCE, MJ
TCN.g
.3465
Sex
25.542 LW (1.127) 26.134 LW (1.508)
.012 (.008) .016 (.012)
-.966 (2.753) -.921 (2.885)
.991 (.705) .711 (.995)
-10.711 (8.216) -18.934 (14.048)
7.059 LW (6.012) 7.420 LW (6.357)
+
+
+
-t-
.656 LW (.018) .622 LW (.025)
25.385 LW (1.540) 25.519 LW (2.191)
+
+
+ 131.522 LW (17.942) + 171.040 LW (30.953)
+
+
.63 3 (.516) .461 (.684)
8.855 LW (.704) 10.675 LW (1.136)
+ -.315 (.323)' + -.748 (.516)
.984 .957
.000038
.048
2.36781 .000028
.057
3.34686
.2813
.834
.922
.531
.2195
.700
56.1347
.918
29.8077
.1332
.957
.766
.0757 .1175
.873
R2
.681 LW (.002) .657 LW (.003)
4.966 LW (.784) 5.401 LW (.619)
27.531 LW (.209) 27.078 LW (.215)
108.339 LW (2.419) 129.526 LW (3.110)
26.913 LW (.151) 27.144 LW (.158)
8.172 LW (.094) 9.034 LW (.115)
Slope (J,
Regressio
residual (TCR), and water (I'CW) on kil
.0459
Residual mean square
Linear regression
(TCS), fat (TCE), ash (TCA),
Equation <-x +(3 LW
carcass energy (TCE), nitrogen
Total corrected mean square
oftotal
Dependent variable
TABLE 2, Regression
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PREDICTION OF CARCASS COMPOSITION
through the origin, and the r t and r 2 estimators were not computed. For each of the dependent variables, the estimated slopes of the regression lines for the male birds in the first experiment and second experiment were not significantly different (P> .05). The negative intercepts for TCE, TCN, TCF, and TCA may be due to TCR not being related to LW (P>.05) and TCR being a relatively larger percentage of TCDM than in the first experiment (2.7 vs. 1.6%). Experiment 3. Data were available from only 36 of the 48 birds due to a malfunction of the autoclave. There were no significant room effects (P>.05), nor were there any significant differences between the two strains (P>.05). The body weight and carcass composition data are summarized in Table 1. The composition of the adult SCWL carcasses was different from those of the two groups of broiler chicks. The TCR was approximately 1.7% of LW and was 3.5 and 4.3% of TCDM for strains 5 and 10, respectively; these values are considerably greater than those observed in the chick experiments. The TCW comprised 57% of adult LW but 67 and 72% of chick LW in the first two experiments, respectively. The regression analyses on LW revealed that, for each of the dependent variables, there were no significant differences (P>.05) between strains in either intercept or slope. There was no indication (P>.05) of a curvilinear relation-
TABLE 3. Regression of total carcass energy (TCE), nitrogen (TCN), fat (TCF), ash (TCA), residual (TCR), and water (TCW) on kilograms live body weight (LW) — Experiment 2 Dependent variable TCE, MJ
Regression equation a + f3LW
Residual mean square
+
8.351 LW (.534)
.034272
.917
.224582
.952
-.758 (.394)'
TCN.g
-2.401 (1.009)
+
28.596 LW (1.367)
TCF.g
-16.858 (11.706)
+
112.007 LW (15.866)
TCA,g
-3.188 (.911)
+
27.236 LW (1.235)
TCR.g
7.767 (5.792)
TCW, kg
1
.026 (.011)
—
.003 LW (.008)
+
.683 LW (.015)
Standard error of parameter estimate directly above.
30.2429 .183168 7.4058 .000026
R2
.694 .957 .007 .990
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ences begin to appear at about 3 weeks of age with females becoming fatter, then the carcass composition of females may be more heterogeneous than that of males. Greater heterogeneity could explain the aforementioned smaller R 2 values and the much larger residual mean square for females (Table 2). The data indicate that TCW, TCN, and to a lesser extent, TCE and TCA could be predicted from LW. The TCF was not as closely related to LW as were the other dependent variables, and there seemed to be no relationship between TCR and LW. Experiment 2. The body weight and carcass composition data are summarized in Table 1. The mean TCR was .8% of mean LW or 2.7% of TCDM; this was about 60% higher than the corresponding values in the first experiment. The regression analyses using LW as the independent variable are summarized in Table 3. Only for TCW was there a significant (P<.05) quadratic effect with an R 2 of .992. However, because R 2 = .990 for the linear equation, and because this would provide a good estimate of initial TCW, only the parameter estimates of the linear equation are shown in Table 3. The R 2 values of the second experiment are similar to those of the male birds in the first experiment. Unlike the first experiment, except for TCF and TCR, the intercepts are significantly different (P<.05) from zero. Consequently, no attempt was made to force the regressions
685
686
WOLYNETZ AND SIBBALD
TABLE 4. Regression of total carcass energy (TCE), nitrogen (TCN), fat (TCF), ash (TCA), residual (TCR), and water (TCW) on kilograms live body weight (LW) — Experiment 3 Dependent variable
Regressi o n equation
Residual mean square
a 4• £ L W
R2
+
24.225 LW (2.985)
21.997
.660
TCN.g
50.460 (11.551)
+
19.140 LW (4.136)
42.240
.386
TCF.g
1136.128 (270.551)
+
567.875 LW (96.880)
23171.406
.503
TCA,g
95.523 (22.592)
+
.098 LW (8.090)
161.568
.000
TCR.g
29.257 (25.752)
+
6.003 LW (9.221)
209.925
.012
.756 (.186)
+
.295 LW (.067)
TCW, kg
1
.010995
.365
Standard error of parameter estimate directly above.
ship between any dependent variable and LW. The regression equations are summarized in Table 4. Except for TCR, the R 2 values were much smaller and the residual mean square values were considerably larger than in either of the chick experiments; this suggests that in this population of adult birds accurate prediction of initial body composition from LW was not possible. For example, for a bird of average weight (2.78 kg), the predicted TCN is 103.7 g; the associated 95% confidence interval of 90.5 to 116.9 g is not much narrower than the range of observed values, 83.9 to 117.9 g. For both TCA and TCR, there was no linear relationship with LW and their means provide alternate estimates of initial composition. Except for TCR, the intercepts of the regression equations were significantly different from zero (P<.01); hence, regression through the origin was not appropriate and the ri and r2 estimates were not computed. Unlike Experiment 2, the intercept for TCN was positive for the adult birds. General Discussion. The R 2 value obtained by regressing TCW on LW was only .365 for the adult cockerels, whereas it ranged from .957 to .990 for the chicks. The relatively weak relationship for the adults probably reflects greater heterogeneity, in composition. Examination of the raw data revealed a high degree of uniformity in composition among chicks of similar LW, but this was not so for the adults.
For example, for 8 adults with LW between 2.8 and 2.9 kg, the TCDM ranged from 38.2 to 48.5% of the LW. In face of such variation within a narrow weight class, it is not surprising that LW was an inadequate predictor of body composition. The relatively large TCR for the adult birds, and the lack of a relationship between TCR and LW, further contributed to the inability to predict adult body composition from LW. Increasing the number of birds slaughtered and analyzed would not have improved the ability to predict initial body composition. In all three experiments, the R 2 value relating TCF to LW was much smaller than those relating TCE and TCN to LW; this may reflect a weakness in the assay for carcass fat and, therefore, a series of calculations was made to investigate this possibility. Assuming that the carbohydrate content of the carcass is negligible, the TCE comprises the energy present as fat and protein (N x 6.25). Then an indirect estimate of total carcass fat (TCFI) can be taken as: TCFI(k) = TCE - k X 6.25 X TCN
where K is the energy value (MJ/kg) of protein. Published estimates are in the range 23 to 25 MJ/kg (Znaniecka, 1969; Sibbald and Morse, 1984). Calculation of TCFI(k) using a range of values of k from 22 to
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-34.709 (8.336) 1
TCE, MJ
PREDICTION OF CARCASS COMPOSITION
ACKNOWLEDGMENTS The authors thank S. Tobin for her able technical assistance. The fat and ash analyses were made by the Chemical and Biological Research Institute of Agriculture Canada. REFERENCES Association of Official Analytical Chemists, 1980. Official Methods of Analysis. 11th ed. Assoc.
Offic. Anal. Chem., Washington, D.C. Dansky, L. M., 1952. Thesis, Cornell Univ., Ithaca, NY. (Cited by Hill et al., 1957.) Edwards, Jr., H. M., and F. Denhan, 1975. Carcass composition studies. 2. Influences of breed, sex and diet on gross composition of the carcass and fatty acid composition of the adipose tissue. Poultry Sci. 54:1230-1238. Gowe, R. S., and R. W. Fairfull, 1980. Performance of six long-term multi-trait selected leghorn strains and three control strains, and a strain cross evaluation of the selected strains. Pages 141 — 162 in Proc. South Pacific Poult. Sci. Conv. Hill, F. W., M. L. Scott, and L. B. Carew, Jr., 1975. The effect of nutrition on the fat content of poultry. Pages 99—109 in Proc. Cornell Nutr. Conf. Just, A., J. A. Fernandez, and H. Jorgensen, 1982. Nitrogen balance studies and nitrogen retention. Pages 111—122 in Physiologie Digestive chez le Pore, les Colloques de I'lnra. J. P. Laplace, T. Corring and A. Rerat, ed. Jouy-en-Josas, France. Leeson, S., and J. D. Summers, 1980. Production and carcass characteristics of the broiler chicken. Poultry Sci. 59:786-798. Sibbald, I. R., and A. Fortin, 1982. Preparation of dry homogenates from whole and eviscerated chickens. Poultry Sci. 61:589-590. Sibbald, I. R., and P. M. Morse, 1984. A preliminary investigation of the utilization of true metabolizable energy by chicks. Poultry Sci. 63:954—971. Sibbald, I. R., and M. S. Wolynetz, 1984. Variation among aliquots of entire-chicken homogenates. Poultry Sci. 63:1446-1448. Snedecor, G. W., and W. G. Cochran, 1967. Statistical Methods. 6th ed. Iowa State Univ. Press, Ames, IA. Yoshida, M., and H. Morimoto, 1970. Interrelationships between dietary protein level and carcass composition of chicks. Agric. Biol. Chem. 34: 414-422. Znaniecka, G., 1969. Calorific value of protein and fat of the chickens body. Pages 407—408 in Energy Metabolism of Farm Animals. K. L. Blaxter, J. Kielanoswki, and G. Thorbek, ed. Oriel Press Ltd., London.
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27 and then regressing TCFI(k) on LW resulted in R 2 values very similar to those for TCF in all three data sets. Furthermore, the correlation coefficient between TCFI(k) and TCF exceeded .950 in all three experiments. These findings suggest that the relatively low correlation between TCF and LW is not due to problems in the assay for carcass fat. A more probable cause is the greater heterogeneity of TCF values relative to TCN, which results in TCE being of intermediate variability. For both chick experiments, there were large R 2 values between TCN and TCW with LW ( R 2 > . 9 1 ) . For adult birds they were much smaller ( R 2 < . 4 0 ) . Thus, it appears that the TCN and TCW of experimental chicks, but not adult birds, can be accurately predicted from LW. The statistical analyses have related observed carcass characteristics to measured LW in the same birds and have also identified prediction equations which best fit the data. The R 2 values of such equations overstate the ability to predict the body composition of other birds from the same population, for example, the initial body composition of birds slaughtered at the end of the experiment.
687