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Relationships Between Carcass Fatness and Feed Efficiency and Its Component Traits in Broiler Chickens1
Relationships Between Carcass Fatness and Feed Efficiency and Its Component Traits in Broiler Chickens1 J. R. CHAMBERS, A. FORTIN, and A. A. GRUNDER Animal Research Centre, Agriculture Canada, Ottawa, Ontario, Canada, K1A 0C6 (Received for publication January 19, 1983)
1983 Poultry Science 62:2201-2207 INTRODUCTION Feed a c c o u n t s for at least 50% and often as m u c h as 60 t o 70% of broiler chicken p r o d u c tion costs. Consequently, t h e efficiency of converting feed t o broiler m e a t is of major e c o n o m i c i m p o r t a n c e t o t h e broiler p r o d u c e r . In this paper, t h e convention p r o p o s e d b y Titus et al. ( 1 9 5 3 ) will apply, i.e., t h e ratio of gain per u n i t weight of feed will be called "feed efficiency"; feed per unit of weight gain will be called "feed conversion". Feed efficiency and weight gain are favorably related. Correlations have been r e p o r t e d : .60 b e t w e e n gain and feed efficiency from 4 t o 10 weeks of age ( F o x and Bohren, 1 9 5 4 ) ; —.52 b e t w e e n gain and feed conversion in 5- t o 10-week-old broilers (Wilson, 1 9 6 9 ) ; —.63 and —.68 b e t w e e n gain and feed conversion in 5- t o 9-week-old male and female broilers, respectively ( P y m and Nicholls, 1 9 7 9 ) . Correlations b e t w e e n feed c o n s u m p t i o n and feed conversion are smaller: —.01 and .15 in t w o replicates (Wilson, 1969) and - . 1 3 and - . 1 5 for males and females, respectively ( P y m and Nicholls,
1
Contribution No. 1136, Animal Research Centre.
1979). Gain and feed c o n s u m p t i o n are highly positively correlated: .70 and .85 in t w o replicates (Wilson, 1 9 6 9 ) and .83 and .79 in male and female broilers, respectively (Pym and Nicholls, 1 9 7 9 ) . Initial weight (weight at t h e start of test) is positively correlated with b o t h c o m p o n e n t s of efficiency w h e t h e r measured as feed efficiency or feed conversion. Correlations of initial weight w i t h gain were .37 and .50 for t w o replicates (Wilson, 1 9 6 9 ) and .51 and .40 for male and female broilers (Pym and Nicholls, 1979) and with feed c o n s u m p t i o n were .58 and .61 in t w o replicates (Wilson, 1 9 6 9 ) and .59 and .44 for male and female broilers, respectively ( P y m and Nicholls, 1979). By definition, feed conversion, feed c o n s u m p t i o n p e r u n i t weight gain, is positively related t o feed cons u m p t i o n and negatively related t o weight gain, t h e d e n o m i n a t o r of this ratio. Initial weight will be negatively correlated with t h e inverse of gain; therefore, t h e correlations b e t w e e n initial weight and t h e c o m p o n e n t s of feed conversion will t e n d t o cancel each o t h e r in correlations b e t w e e n initial weight and feed conversion: .13 and .30 for t w o replicates (Wilson, 1 9 6 9 ) and —.06 and —.10 for male and female broilers, respectively ( P y m and Nicholls, 1 9 7 9 ) .
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ABSTRACT Weight gain, feed consumption, and feed efficiency (gain/feed) from 28 to 48 days of age were measured in conjunction with initial weight (at 28 days) and percentage carcass fat of 59 male and 54 female broiler chickens from two stocks slaughtered at 49 days. Relationships among these traits were examined in terms of simple and partial correlations and of multiple regression analyses. Feed efficiency improved with rapid weight gain but declined with greater carcass fatness and initial weight. Within broiler strain, sex, and cage type subclasses, 65% of the variation in feed efficiency was accounted for by differences in these variables; gain, 36%; carcass fatness, an additional 25%; and initial weight, the final 4%. In some instances, differences between simple and corresponding partial correlations were observed. The —.48 correlation between carcass fatness and feed efficiency became —.62 after adjusting for differences in weight gain. Similarly the .40 correlation between carcass fatness and feed consumption rose to .63 after adjustment for differences in weight gain. The low correlation of .01 between initial weight and feed efficiency became —.38 after adjusting the data for differences in weight gain and carcass fatness. Hence, results from age constant feed efficiency tests should be corrected for differences in initial weight to reflect "true" differences in efficiency. (Key words: feed efficiency, growth, fatness)
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CHAMBERS ET AL.
MATERIALS AND METHODS
Hatching eggs representing two broiler stocks were obtained from a primary breeding company, incubated, and hatched. The two stocks were closely related and were being developed for commercial use. From each stock, 60 chicks, 30 of each sex, were wingbanded and brooded in batteries. For each stock-sex subclass of chicks, individuals were randomly assigned to one of three cage types. Ten were placed at 12 days of age in cages 25.4 cm wide (level floors); 10 were placed at 25 days of age in cages 20.3 cm wide (sloping floor); 10 were placed at 25 days of age in cages 25.4 cm wide (sloping floor). The wire cages used to house the 120 chicks during the test were 40.6 cm high at the rear and 40.6 cm long. They had been designed originally for laying hens. Relative performances of broilers in cages vs. pens may differ; however, testing of broilers in cages is, at present, the most practical means of measuring individual feed consumption. Appropriate brooding temperatures were maintained in the room containing the cages. The water line, with one "Hart" cup per cage was threaded through the cages at the rear and could be raised from 10 to 15 cm from the cage floor as the broilers grew. A crumbled medicated broiler starter was fed to 28 days of age; a pelleted broiler finisher ration was fed thereafter. Respective calculated quantities of crude protein (CP) and metabolizable energy (ME) contained by these rations were 23.5 and
20.8% CP and 3155 and 3210 kcal ME/kg. Each broiler was weighed at 28 days of age. From 28 to 48 days of age broilers had free access to preweighed quantities of finisher in individual plastic feed containers inserted in the trough in front of the cages. At 48 days of age each broiler and the remaining feed were weighed. These data were used to calculate 28 to 48-day weight gain, feed consumption, and feed efficiency. At 1500 hr of the 48th day all feed was removed and the broilers were fasted 19 to 20 hr until slaughter. The following morning, at 49 days of age, broilers were stunned, bled, scalded, and plucked. Evisceration was performed by hand and leaf fat was removed from each carcass. Carcass analyses methods have been described (Chambers et al, 1981). Briefly, carcasses were weighed, placed in plastic bags, and stored at —20 C until they could be ground in preparation for sampling for proximate analyses. Tissue masses were packaged in plastic containers and stored (—20 C) prior to duplicate determination of chemical carcass composition. The SAS procedures (Helwing and Council, 1979) were used for statistical analyses. Means and correlations were determined and multiple regression analyses of the data were performed. Residual sums of squares and cross products from a model including sex, strain, and cage type effects were used to estimate simple correlations for all trait pair combinations. Partial correlation coefficients were calculated from simple correlations (Snedecor and Cochran, 1967). RESULTS AND DISCUSSION
Interpretation of studies involving ratios such as feed efficiency and related traits becomes complex and results can be misleading due to part-whole relationships. For this reason Fox and Bohren (1954) stated that interpretation of correlations between gain and efficiency "warrants caution due to elements common to both variables". However, they maintain "the correlation coefficient may be satisfactorily used to demonstrate the degree of relationship existing between the variables either because of, or in spite of the common elements present". More specifically, Sutherland (1965) concluded that efficiency ratios involving weight gained and feed consumed were valid estimates of efficiency and that the automatic element in the correlation between gain and efficiency does not detract
Downloaded from http://ps.oxfordjournals.org/ at University of Alberta on April 25, 2015
In recent years, carcass and abdominal fatness in broiler carcasses have become major concerns. The percentages of carcass fat and abdominal fat increase as the broiler grows (Leeson and Summers, 1980; Tzeng and Becker, 1981). Fatness differences among broilers are large with coefficients of variation for abdominal fat measures frequently in excess of 25% (Becker et al, 1981; Chambers et al, 1981). In most instances, carcass fatness is associated with poor feed efficiency (Thomas et al, 1958; Washburn et ah, 1975; Pym and Solvyns, 1979). However, more efficient broilers were fatter in studies by Wethli and Wessels (1973). The latter concluded that more efficient broilers were larger and may have been fatter due to more advanced physiological age. The objective of this research was to examine phenotypic relationships among carcass fatness, feed efficiency, and its component traits in broiler chickens.
CARCASS FATNESS AND FEED EFFICIENCY
Carcass fat percentage and feed efficiency are negatively related, —.48. When the correlated traits were adjusted for variation in gain or gain and initial weight, respective partial correlation values of —.62 and —.66 were obtained. These values are in contrast to the values between carcass fat percentage and feed conversion reported by Pym and Solvyns
(1979) in spite of differences in mode of expressing efficiency. The correlation between carcass fat and gain, .02, is lower than the values by the same authors. The relationships of fat with feed efficiency and with weight gain appear to differ between the Australian and Canadian broiler stocks, but the correlation between carcass fat and consumption, .40, is close to values reported by Pym and Solvyns (1979). The correlation rose when adjustment was made for gain and for gain and initial weight, respectively. Feed consumption and gain were positively correlated. This value is in agreement with other reports and rose after differences in carcass fatness were removed. Faster gaining broilers consume larger quantities of feed daily; however, consumption of nutrients in excess of levels required for lean growth is a major cause of excessive fatness. The positive correlation, . 6 1 , between efficiency and gain agrees with other reports, especially the .60 reported by Fox and Bohren (1954). Similarly, the zero correlation between feed efficiency and consumption is consistent with other reports. Positive values were obtained after adjusting for carcass fatness and initial weight. A negative correlation would be expected mathematically, because feed consumption is the denominator of the feed efficiency ratio. Indeed, the observed correlation between feed efficiency and consumption after adjustment for gain was —.99. However, the strong positive correlations between feed efficiency and gain and between gain and feed consumption appear to have cancelled any negative relationship between feed consumption and feed efficiency. In this study, correlations between ef-
TABLE 1. Means, standard errors, and coefficients of variation for feed efficiency and related traits, and carcass fatness of 113 broilers tested from 28 to 48 days of age Both sexes (n = 113) Trait
X ± SE
Initial weight, g 1 Gain, g Feed consumption, g Feed efficiency Carcass fat percentage
883 1131 2472
CV
X + SE
(%)
1
± 9 + 15 ± 22 .457 ± .003 18.8 + .3
Females (n = 54)
Males (n = 59)
10.5 13.7 9.6 7.4 14.9
CV
X + SE
(%) ± 11 ± 19 + 27 .473 ± .004 17.7 + .3
929 1229 2593
Initial weight = broiler body weight at 28 days of age.
9.1 11.6 8.1 6.8 14.5
CV
(%) ± 10 + 10 ± 26 .439 ± .004 20.0 ± .3
833 1024 2339
8.7 7.4 8.3 5.9 12.7
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from the biological and economic usefulness of the correlation. Simple and Partial Correlations. Means, standard errors, and coefficients of variation for 28 to 48-day feed efficiency, feed consumption, and weight gain and for initial broiler weight (28 days) and carcass fat percentage are presented in Table 1 for all broilers and for broilers of each sex. The relationships among these traits after adjustment by multiple regression for stock, sex, and cage type differences and expressed as simple and partial correlation coefficients are shown in Table 2. Corresponding values reported by Wilson (1969), Pym and Nicholls (1979), and by Pym and Solvyns (1979) are also shown. Agreement between correlations of the current study and others reported in the literature is generally very good after adjustment is made for differences in methods of expressing efficiency. This agreement occurs in spite of differences in test age and large differences in growth rate of the broiler stocks used. Broilers tested by Wilson (1969) would grow approximately half as fast as broilers in the current study according to Marks (1979) and Chambers et al. (1981). Broilers used by Pym and coworkers weighed approximately 1.3 kg at 9 weeks of age as opposed to 2.0 kg at 7 weeks in the current study.
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-.48*
-.04
.02 .40*
.61*
.00
.01
Fat & Wl
Fat & Ga Fat & Con
FE&Ga
FE & Con
FE&W1
Simple
Fat & FE
Correlated traits 2
26** 38**
24* 28** 99**
70** 75** 64** 99**
63** 67**
26** 31**
53** 62** 66**
Partial
Ga Ga
Fat Fat Ga
Fat Fat Wl Con
Ga Ga
Con Con
Con Ga Ga
Adjusted for
Correlations, this study
Fat
Wl
Wl
Wl
Wl
.13
.15
.54
Rep 1
.64 .58
Wilson
.30
.01
.49
Rep 2
06
13
63
Ma
Correlati
TABLE 2. Simple and partial correlation coefficients among feed efficiency and related tra
ded from http://ps.oxfordjournals.org/ at University of Alberta on April 25, 2015
CARCASS FATNESS AND FEED EFFICIENCY
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ficiency and either of its components, gain and feed consumption, approached unity when the partial correlation involved correction for the other component. These results are expected, as .99 represents the relationship between gain and itself; —.99 represents the relationship between feed consumption and its inverse. However, a partial correlation of —.58 between gain and feed conversion with correction for feed consumption was reported by Wilson (1969). No explanation for the deviation of this value, the relationship between gain and its inverse, from the expected —1.0 is apparent. The correlation between feed efficiency and initial weight did not differ from zero in agreement with other reports. This appears to be a case of cancellation of the relationships between initial weight and either gain or consumption, the components of efficiency. If adjustment is made for gain the correlation between efficiency and initial weight becomes negative; however, this is equivalent to
— £• <£
t h e correlation b e t w e e n initial weight and t h e
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inverse of feed consumption. The partial correlation between initial weight and feed consumption adjusted for gain was .28. The partial correlation between efficiency and initial weight is even larger after adjustment for both gain and fatness. Again, this is similar in magnitude but opposite in sign to the correlation between consumption and initial weight adjusted for both gain and carcass fatness. Hence, broilers that are heavier at the beginning of the test appear to be less efficient. Results of tests between two ages penalize faster gaining (and more efficient) broilers by testing them at a later, less efficient stage of growth. Tests involving constant weights avoid this problem; however, this procedure is not practical for testing large numbers of broilers. Consequently, feed efficiency results obtained from constant age tests should be adjusted for differences in initial weight prior to comparison. Such adjustment raised the correlation between gain and efficiency. Compared to partial correlations, simple correlations were in some instances larger, in others smaller, and occasionally either nonexistant or even opposite in sign. Simple and corresponding partial correlations of the present study indicate that simple correlations can be misleading indicators of relationships L
.
.
.,
,
,
.
•
between trait pairs it other related traits are not considered. Multiple Regression Analyses. Multiple re-
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^ ^ g § S ^
2205
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CHAMBERS ET AL.
Fox and Bohren (1954) reported a positive regression of feed efficiency on mean body weight of chickens between 4 and 10 weeks of age and a negative regression of weekly feed efficiency on average weekly body weight within these chickens. Hence, faster growing broilers are more efficient, but efficiency declines as the broiler grows. Poultrymen wish to compare feed efficiencies measured
during the same growth interval. Hence, feed efficiencies of broilers of different size tested during the same age interval will not reflect "true" differences in efficiency. Compared to smaller broilers, large broilers will be tested during a less efficient phase of growth. This is illustrated by results of other research. Feed conversions of broilers of rapidly growing, modern stocks and of slower gaining, experimental stocks differ little; .1, when measured to the same age (Marks, 1979; Chambers et al, 1981). When these broilers are tested to the same body weight, modern broilers are much more efficient (1.9 vs. 3.2; Chambers, unpublished data). Weight differences between these stocks are larger than those normally encountered within broiler stocks of the same sex. However, the problem remains the same, and multiple regression analyses (Table 3) have shown that differences in initial weight arising due to testing procedure have a small but significant (P«.001) negative influence on feed efficiency adjusted for differences in weight gain and carcass fat percentage. Hence, results of broiler feed efficiency trials conducted between specific ages should be corrected for differences in initial weight. Substitution of traits in multiple regression models revealed that the effect on feed efficiency values of correction was similar for either initial weight or average weight during test.
ACKNOWLEDGMENTS The authors are grateful to M. Baker and M. Sauermann for technical assistance; to R. Cloutier and the Chemistry and Biology Research Institute staff, Agriculture Canada; and to A. R. Morrison and staff for care of the
TABLE 3. Influence of gain, carcass fatness, and initial weight on feed efficiency determined by multiple regression analyses Model number
Increase1 inR 2
Regression model Overall mean + strain + sex + cage Model 1 + .000153 gain Model 1 + .000158 gain - .00580 fat Model 1 + .000178 gain - .00574 fat - .000087 initial weight
.267 .532 .715 .748
.265 (.362) .183 (.250) .033 (.045)
'The increases in R2 for broilers of the same strain and sex and reared in similar cages are shown in parentheses and were determined by dividing the increase in R2 by one minus the R2 due to Model 1 (or .733).
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gression equations to predict feed efficiency based on the independent variables, strain, sex, and cage type plus gain, carcass fatness, and initial weight with the cumulative predictive value of these factors are presented (Table 3). Differences between strains, sexes, and cage types, especially the last two factors, accounted for significant portions of variation in feed efficiency. Gain, carcass fatness, and initial weight each had significant (P<.05) influences on feed efficiency. Increased gain improved feed efficiency, whereas increased carcass fatness and initial weight were associated with poorer feed efficiency. Within groups of broilers of the same sex and strain tested in the same environment, variation in gain accounted for 36% of the variation in feed efficiency. Additional variation accounted for by differences in carcass fatness and initial weight was 25 and 4.5%, respectively. Earlier research (Fox and Bohren, 1954; Wilson, 1969) suggested that selection for rapid growth would provide sufficient improvement in feed efficiency. However, these authors did not examine the influence of carcass fatness on feed efficiency. Our study indicates that selection for low carcass fatness in conjunction with rapid gain is required to achieve optimum genetic improvement of feed efficiency when it is not measured directly.
CARCASS FATNESS AND FEED EFFICIENCY
broilers during brooding and rearing and for assistance with m e a s u r e m e n t and slaughter of the broilers. Broiler hatching eggs were graciously d o n a t e d b y Shaver Poultry Breeding Farms Ltd., Cambridge, O n t a r i o .
REFERENCES
Poult. Sci. 20:73-86. Pym, R.A.E., and A. J. Solvyns, 1979. Selection for food conversion in broilers: Body composition of birds selected for increased body-weight gain, food consumption and food conversion ratio. Br. Poult. Sci. 20:87-97. Snedecor, G. W., and W. G. Cochran, 1967. Pages 400—402 in Statistical Methods. 6th ed. Iowa State Univ. Press, Ames, IA. Sutherland, T. M., 1965. The correlation between feed efficiency and rate of gain, a ratio and its denominator. Biometrics 21:739-749. Thomas, C. H., E. W. Glazener, and W. L. Blow, 1958. The relationship between feed conversion and ether extract of broilers. Poultry Sci. 37:1177— 1179. Titus, H. W., A. L. Mehring, Jr., and J. H. Brumbaugh, 1953. Variation of feed conversion. Poultry Sci. 32:1074-1077. Tzeng, R., and W. A. Becker, 1981. Growth patterns of body and abdominal fat weights in male broiler chickens. Poultry Sci. 60:1101-1106. Washburn, K. W., R. A. Guill, and H. M. Edwards, Jr., 1975. Influence of genetic differences in feed efficiency on carcass composition of young chickens. J. Nutr. 105:1311-1317. Wethli, E., and J.P.H. Wessels, 1973. The associations between body fat content and thyroid activity, feed intake, mass gain, feed conversion and final body mass in growing chickens. Agroanimalia 5:83-88. Wilson, S. P., 1969. Genetic aspects of feed efficiency in broilers. Poultry Sci. 48:487-495.
Downloaded from http://ps.oxfordjournals.org/ at University of Alberta on April 25, 2015
Becker, W. A., J. V. Spencer, L. W. Mirosh, and J. A. Verstrate, 1981. Specific gravity, carcass fat, abdominal fat, and yield data in broiler chickens. Poultry Sci. 60:2045-2052. Chambers, J. R., J. S. Gavora, and A. Fortin, 1981. Genetic changes in meat-type chickens in the last twenty years. Can. J. Anim. Sci. 61:555—563. Fox, T. W., and B. B. Bohren, 1954. An analysis of feed efficiency among breeds of chickens and its relationship to rate of growth. Poultry Sci. 33:549-561. Helwig, J. T., and K. A. Council, ed., 1979. SAS User's Guide. SAS Inst. Inc., Raleigh, NC. Leeson, S., and J. D. Summers, 1980. Production and carcass characteristics of the broiler chicken. Poultry Sci. 59:786-798. Marks, H. L., 1979. Growth rate and feed intake of selected and nonselected broilers. Growth 43: 80-90. Pym, R.A.E., and P. J. Nicholls, 1979. Selection for food conversion in broilers: Direct and correlated responses to selection for body-weight gain, food consumption and food conversion ratio. Br.
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1983 Poultry Science 62:2201-2207 INTRODUCTION Feed a c c o u n t s for at least 50% and often as m u c h as 60 t o 70% of broiler chicken p r o d u c tion costs. Consequently, t h e efficiency of converting feed t o broiler m e a t is of major e c o n o m i c i m p o r t a n c e t o t h e broiler p r o d u c e r . In this paper, t h e convention p r o p o s e d b y Titus et al. ( 1 9 5 3 ) will apply, i.e., t h e ratio of gain per u n i t weight of feed will be called "feed efficiency"; feed per unit of weight gain will be called "feed conversion". Feed efficiency and weight gain are favorably related. Correlations have been r e p o r t e d : .60 b e t w e e n gain and feed efficiency from 4 t o 10 weeks of age ( F o x and Bohren, 1 9 5 4 ) ; —.52 b e t w e e n gain and feed conversion in 5- t o 10-week-old broilers (Wilson, 1 9 6 9 ) ; —.63 and —.68 b e t w e e n gain and feed conversion in 5- t o 9-week-old male and female broilers, respectively ( P y m and Nicholls, 1 9 7 9 ) . Correlations b e t w e e n feed c o n s u m p t i o n and feed conversion are smaller: —.01 and .15 in t w o replicates (Wilson, 1969) and - . 1 3 and - . 1 5 for males and females, respectively ( P y m and Nicholls,
1
Contribution No. 1136, Animal Research Centre.
1979). Gain and feed c o n s u m p t i o n are highly positively correlated: .70 and .85 in t w o replicates (Wilson, 1 9 6 9 ) and .83 and .79 in male and female broilers, respectively (Pym and Nicholls, 1 9 7 9 ) . Initial weight (weight at t h e start of test) is positively correlated with b o t h c o m p o n e n t s of efficiency w h e t h e r measured as feed efficiency or feed conversion. Correlations of initial weight w i t h gain were .37 and .50 for t w o replicates (Wilson, 1 9 6 9 ) and .51 and .40 for male and female broilers (Pym and Nicholls, 1979) and with feed c o n s u m p t i o n were .58 and .61 in t w o replicates (Wilson, 1 9 6 9 ) and .59 and .44 for male and female broilers, respectively ( P y m and Nicholls, 1979). By definition, feed conversion, feed c o n s u m p t i o n p e r u n i t weight gain, is positively related t o feed cons u m p t i o n and negatively related t o weight gain, t h e d e n o m i n a t o r of this ratio. Initial weight will be negatively correlated with t h e inverse of gain; therefore, t h e correlations b e t w e e n initial weight and t h e c o m p o n e n t s of feed conversion will t e n d t o cancel each o t h e r in correlations b e t w e e n initial weight and feed conversion: .13 and .30 for t w o replicates (Wilson, 1 9 6 9 ) and —.06 and —.10 for male and female broilers, respectively ( P y m and Nicholls, 1 9 7 9 ) .
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Downloaded from http://ps.oxfordjournals.org/ at University of Alberta on April 25, 2015
ABSTRACT Weight gain, feed consumption, and feed efficiency (gain/feed) from 28 to 48 days of age were measured in conjunction with initial weight (at 28 days) and percentage carcass fat of 59 male and 54 female broiler chickens from two stocks slaughtered at 49 days. Relationships among these traits were examined in terms of simple and partial correlations and of multiple regression analyses. Feed efficiency improved with rapid weight gain but declined with greater carcass fatness and initial weight. Within broiler strain, sex, and cage type subclasses, 65% of the variation in feed efficiency was accounted for by differences in these variables; gain, 36%; carcass fatness, an additional 25%; and initial weight, the final 4%. In some instances, differences between simple and corresponding partial correlations were observed. The —.48 correlation between carcass fatness and feed efficiency became —.62 after adjusting for differences in weight gain. Similarly the .40 correlation between carcass fatness and feed consumption rose to .63 after adjustment for differences in weight gain. The low correlation of .01 between initial weight and feed efficiency became —.38 after adjusting the data for differences in weight gain and carcass fatness. Hence, results from age constant feed efficiency tests should be corrected for differences in initial weight to reflect "true" differences in efficiency. (Key words: feed efficiency, growth, fatness)
2202
CHAMBERS ET AL.
MATERIALS AND METHODS
Hatching eggs representing two broiler stocks were obtained from a primary breeding company, incubated, and hatched. The two stocks were closely related and were being developed for commercial use. From each stock, 60 chicks, 30 of each sex, were wingbanded and brooded in batteries. For each stock-sex subclass of chicks, individuals were randomly assigned to one of three cage types. Ten were placed at 12 days of age in cages 25.4 cm wide (level floors); 10 were placed at 25 days of age in cages 20.3 cm wide (sloping floor); 10 were placed at 25 days of age in cages 25.4 cm wide (sloping floor). The wire cages used to house the 120 chicks during the test were 40.6 cm high at the rear and 40.6 cm long. They had been designed originally for laying hens. Relative performances of broilers in cages vs. pens may differ; however, testing of broilers in cages is, at present, the most practical means of measuring individual feed consumption. Appropriate brooding temperatures were maintained in the room containing the cages. The water line, with one "Hart" cup per cage was threaded through the cages at the rear and could be raised from 10 to 15 cm from the cage floor as the broilers grew. A crumbled medicated broiler starter was fed to 28 days of age; a pelleted broiler finisher ration was fed thereafter. Respective calculated quantities of crude protein (CP) and metabolizable energy (ME) contained by these rations were 23.5 and
20.8% CP and 3155 and 3210 kcal ME/kg. Each broiler was weighed at 28 days of age. From 28 to 48 days of age broilers had free access to preweighed quantities of finisher in individual plastic feed containers inserted in the trough in front of the cages. At 48 days of age each broiler and the remaining feed were weighed. These data were used to calculate 28 to 48-day weight gain, feed consumption, and feed efficiency. At 1500 hr of the 48th day all feed was removed and the broilers were fasted 19 to 20 hr until slaughter. The following morning, at 49 days of age, broilers were stunned, bled, scalded, and plucked. Evisceration was performed by hand and leaf fat was removed from each carcass. Carcass analyses methods have been described (Chambers et al, 1981). Briefly, carcasses were weighed, placed in plastic bags, and stored at —20 C until they could be ground in preparation for sampling for proximate analyses. Tissue masses were packaged in plastic containers and stored (—20 C) prior to duplicate determination of chemical carcass composition. The SAS procedures (Helwing and Council, 1979) were used for statistical analyses. Means and correlations were determined and multiple regression analyses of the data were performed. Residual sums of squares and cross products from a model including sex, strain, and cage type effects were used to estimate simple correlations for all trait pair combinations. Partial correlation coefficients were calculated from simple correlations (Snedecor and Cochran, 1967). RESULTS AND DISCUSSION
Interpretation of studies involving ratios such as feed efficiency and related traits becomes complex and results can be misleading due to part-whole relationships. For this reason Fox and Bohren (1954) stated that interpretation of correlations between gain and efficiency "warrants caution due to elements common to both variables". However, they maintain "the correlation coefficient may be satisfactorily used to demonstrate the degree of relationship existing between the variables either because of, or in spite of the common elements present". More specifically, Sutherland (1965) concluded that efficiency ratios involving weight gained and feed consumed were valid estimates of efficiency and that the automatic element in the correlation between gain and efficiency does not detract
Downloaded from http://ps.oxfordjournals.org/ at University of Alberta on April 25, 2015
In recent years, carcass and abdominal fatness in broiler carcasses have become major concerns. The percentages of carcass fat and abdominal fat increase as the broiler grows (Leeson and Summers, 1980; Tzeng and Becker, 1981). Fatness differences among broilers are large with coefficients of variation for abdominal fat measures frequently in excess of 25% (Becker et al, 1981; Chambers et al, 1981). In most instances, carcass fatness is associated with poor feed efficiency (Thomas et al, 1958; Washburn et ah, 1975; Pym and Solvyns, 1979). However, more efficient broilers were fatter in studies by Wethli and Wessels (1973). The latter concluded that more efficient broilers were larger and may have been fatter due to more advanced physiological age. The objective of this research was to examine phenotypic relationships among carcass fatness, feed efficiency, and its component traits in broiler chickens.
CARCASS FATNESS AND FEED EFFICIENCY
Carcass fat percentage and feed efficiency are negatively related, —.48. When the correlated traits were adjusted for variation in gain or gain and initial weight, respective partial correlation values of —.62 and —.66 were obtained. These values are in contrast to the values between carcass fat percentage and feed conversion reported by Pym and Solvyns
(1979) in spite of differences in mode of expressing efficiency. The correlation between carcass fat and gain, .02, is lower than the values by the same authors. The relationships of fat with feed efficiency and with weight gain appear to differ between the Australian and Canadian broiler stocks, but the correlation between carcass fat and consumption, .40, is close to values reported by Pym and Solvyns (1979). The correlation rose when adjustment was made for gain and for gain and initial weight, respectively. Feed consumption and gain were positively correlated. This value is in agreement with other reports and rose after differences in carcass fatness were removed. Faster gaining broilers consume larger quantities of feed daily; however, consumption of nutrients in excess of levels required for lean growth is a major cause of excessive fatness. The positive correlation, . 6 1 , between efficiency and gain agrees with other reports, especially the .60 reported by Fox and Bohren (1954). Similarly, the zero correlation between feed efficiency and consumption is consistent with other reports. Positive values were obtained after adjusting for carcass fatness and initial weight. A negative correlation would be expected mathematically, because feed consumption is the denominator of the feed efficiency ratio. Indeed, the observed correlation between feed efficiency and consumption after adjustment for gain was —.99. However, the strong positive correlations between feed efficiency and gain and between gain and feed consumption appear to have cancelled any negative relationship between feed consumption and feed efficiency. In this study, correlations between ef-
TABLE 1. Means, standard errors, and coefficients of variation for feed efficiency and related traits, and carcass fatness of 113 broilers tested from 28 to 48 days of age Both sexes (n = 113) Trait
X ± SE
Initial weight, g 1 Gain, g Feed consumption, g Feed efficiency Carcass fat percentage
883 1131 2472
CV
X + SE
(%)
1
± 9 + 15 ± 22 .457 ± .003 18.8 + .3
Females (n = 54)
Males (n = 59)
10.5 13.7 9.6 7.4 14.9
CV
X + SE
(%) ± 11 ± 19 + 27 .473 ± .004 17.7 + .3
929 1229 2593
Initial weight = broiler body weight at 28 days of age.
9.1 11.6 8.1 6.8 14.5
CV
(%) ± 10 + 10 ± 26 .439 ± .004 20.0 ± .3
833 1024 2339
8.7 7.4 8.3 5.9 12.7
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from the biological and economic usefulness of the correlation. Simple and Partial Correlations. Means, standard errors, and coefficients of variation for 28 to 48-day feed efficiency, feed consumption, and weight gain and for initial broiler weight (28 days) and carcass fat percentage are presented in Table 1 for all broilers and for broilers of each sex. The relationships among these traits after adjustment by multiple regression for stock, sex, and cage type differences and expressed as simple and partial correlation coefficients are shown in Table 2. Corresponding values reported by Wilson (1969), Pym and Nicholls (1979), and by Pym and Solvyns (1979) are also shown. Agreement between correlations of the current study and others reported in the literature is generally very good after adjustment is made for differences in methods of expressing efficiency. This agreement occurs in spite of differences in test age and large differences in growth rate of the broiler stocks used. Broilers tested by Wilson (1969) would grow approximately half as fast as broilers in the current study according to Marks (1979) and Chambers et al. (1981). Broilers used by Pym and coworkers weighed approximately 1.3 kg at 9 weeks of age as opposed to 2.0 kg at 7 weeks in the current study.
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-.48*
-.04
.02 .40*
.61*
.00
.01
Fat & Wl
Fat & Ga Fat & Con
FE&Ga
FE & Con
FE&W1
Simple
Fat & FE
Correlated traits 2
26** 38**
24* 28** 99**
70** 75** 64** 99**
63** 67**
26** 31**
53** 62** 66**
Partial
Ga Ga
Fat Fat Ga
Fat Fat Wl Con
Ga Ga
Con Con
Con Ga Ga
Adjusted for
Correlations, this study
Fat
Wl
Wl
Wl
Wl
.13
.15
.54
Rep 1
.64 .58
Wilson
.30
.01
.49
Rep 2
06
13
63
Ma
Correlati
TABLE 2. Simple and partial correlation coefficients among feed efficiency and related tra
ded from http://ps.oxfordjournals.org/ at University of Alberta on April 25, 2015
CARCASS FATNESS AND FEED EFFICIENCY
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ficiency and either of its components, gain and feed consumption, approached unity when the partial correlation involved correction for the other component. These results are expected, as .99 represents the relationship between gain and itself; —.99 represents the relationship between feed consumption and its inverse. However, a partial correlation of —.58 between gain and feed conversion with correction for feed consumption was reported by Wilson (1969). No explanation for the deviation of this value, the relationship between gain and its inverse, from the expected —1.0 is apparent. The correlation between feed efficiency and initial weight did not differ from zero in agreement with other reports. This appears to be a case of cancellation of the relationships between initial weight and either gain or consumption, the components of efficiency. If adjustment is made for gain the correlation between efficiency and initial weight becomes negative; however, this is equivalent to
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inverse of feed consumption. The partial correlation between initial weight and feed consumption adjusted for gain was .28. The partial correlation between efficiency and initial weight is even larger after adjustment for both gain and fatness. Again, this is similar in magnitude but opposite in sign to the correlation between consumption and initial weight adjusted for both gain and carcass fatness. Hence, broilers that are heavier at the beginning of the test appear to be less efficient. Results of tests between two ages penalize faster gaining (and more efficient) broilers by testing them at a later, less efficient stage of growth. Tests involving constant weights avoid this problem; however, this procedure is not practical for testing large numbers of broilers. Consequently, feed efficiency results obtained from constant age tests should be adjusted for differences in initial weight prior to comparison. Such adjustment raised the correlation between gain and efficiency. Compared to partial correlations, simple correlations were in some instances larger, in others smaller, and occasionally either nonexistant or even opposite in sign. Simple and corresponding partial correlations of the present study indicate that simple correlations can be misleading indicators of relationships L
.
.
.,
,
,
.
•
between trait pairs it other related traits are not considered. Multiple Regression Analyses. Multiple re-
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^ ^ g § S ^
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CHAMBERS ET AL.
Fox and Bohren (1954) reported a positive regression of feed efficiency on mean body weight of chickens between 4 and 10 weeks of age and a negative regression of weekly feed efficiency on average weekly body weight within these chickens. Hence, faster growing broilers are more efficient, but efficiency declines as the broiler grows. Poultrymen wish to compare feed efficiencies measured
during the same growth interval. Hence, feed efficiencies of broilers of different size tested during the same age interval will not reflect "true" differences in efficiency. Compared to smaller broilers, large broilers will be tested during a less efficient phase of growth. This is illustrated by results of other research. Feed conversions of broilers of rapidly growing, modern stocks and of slower gaining, experimental stocks differ little; .1, when measured to the same age (Marks, 1979; Chambers et al, 1981). When these broilers are tested to the same body weight, modern broilers are much more efficient (1.9 vs. 3.2; Chambers, unpublished data). Weight differences between these stocks are larger than those normally encountered within broiler stocks of the same sex. However, the problem remains the same, and multiple regression analyses (Table 3) have shown that differences in initial weight arising due to testing procedure have a small but significant (P«.001) negative influence on feed efficiency adjusted for differences in weight gain and carcass fat percentage. Hence, results of broiler feed efficiency trials conducted between specific ages should be corrected for differences in initial weight. Substitution of traits in multiple regression models revealed that the effect on feed efficiency values of correction was similar for either initial weight or average weight during test.
ACKNOWLEDGMENTS The authors are grateful to M. Baker and M. Sauermann for technical assistance; to R. Cloutier and the Chemistry and Biology Research Institute staff, Agriculture Canada; and to A. R. Morrison and staff for care of the
TABLE 3. Influence of gain, carcass fatness, and initial weight on feed efficiency determined by multiple regression analyses Model number
Increase1 inR 2
Regression model Overall mean + strain + sex + cage Model 1 + .000153 gain Model 1 + .000158 gain - .00580 fat Model 1 + .000178 gain - .00574 fat - .000087 initial weight
.267 .532 .715 .748
.265 (.362) .183 (.250) .033 (.045)
'The increases in R2 for broilers of the same strain and sex and reared in similar cages are shown in parentheses and were determined by dividing the increase in R2 by one minus the R2 due to Model 1 (or .733).
Downloaded from http://ps.oxfordjournals.org/ at University of Alberta on April 25, 2015
gression equations to predict feed efficiency based on the independent variables, strain, sex, and cage type plus gain, carcass fatness, and initial weight with the cumulative predictive value of these factors are presented (Table 3). Differences between strains, sexes, and cage types, especially the last two factors, accounted for significant portions of variation in feed efficiency. Gain, carcass fatness, and initial weight each had significant (P<.05) influences on feed efficiency. Increased gain improved feed efficiency, whereas increased carcass fatness and initial weight were associated with poorer feed efficiency. Within groups of broilers of the same sex and strain tested in the same environment, variation in gain accounted for 36% of the variation in feed efficiency. Additional variation accounted for by differences in carcass fatness and initial weight was 25 and 4.5%, respectively. Earlier research (Fox and Bohren, 1954; Wilson, 1969) suggested that selection for rapid growth would provide sufficient improvement in feed efficiency. However, these authors did not examine the influence of carcass fatness on feed efficiency. Our study indicates that selection for low carcass fatness in conjunction with rapid gain is required to achieve optimum genetic improvement of feed efficiency when it is not measured directly.
CARCASS FATNESS AND FEED EFFICIENCY
broilers during brooding and rearing and for assistance with m e a s u r e m e n t and slaughter of the broilers. Broiler hatching eggs were graciously d o n a t e d b y Shaver Poultry Breeding Farms Ltd., Cambridge, O n t a r i o .
REFERENCES
Poult. Sci. 20:73-86. Pym, R.A.E., and A. J. Solvyns, 1979. Selection for food conversion in broilers: Body composition of birds selected for increased body-weight gain, food consumption and food conversion ratio. Br. Poult. Sci. 20:87-97. Snedecor, G. W., and W. G. Cochran, 1967. Pages 400—402 in Statistical Methods. 6th ed. Iowa State Univ. Press, Ames, IA. Sutherland, T. M., 1965. The correlation between feed efficiency and rate of gain, a ratio and its denominator. Biometrics 21:739-749. Thomas, C. H., E. W. Glazener, and W. L. Blow, 1958. The relationship between feed conversion and ether extract of broilers. Poultry Sci. 37:1177— 1179. Titus, H. W., A. L. Mehring, Jr., and J. H. Brumbaugh, 1953. Variation of feed conversion. Poultry Sci. 32:1074-1077. Tzeng, R., and W. A. Becker, 1981. Growth patterns of body and abdominal fat weights in male broiler chickens. Poultry Sci. 60:1101-1106. Washburn, K. W., R. A. Guill, and H. M. Edwards, Jr., 1975. Influence of genetic differences in feed efficiency on carcass composition of young chickens. J. Nutr. 105:1311-1317. Wethli, E., and J.P.H. Wessels, 1973. The associations between body fat content and thyroid activity, feed intake, mass gain, feed conversion and final body mass in growing chickens. Agroanimalia 5:83-88. Wilson, S. P., 1969. Genetic aspects of feed efficiency in broilers. Poultry Sci. 48:487-495.
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Becker, W. A., J. V. Spencer, L. W. Mirosh, and J. A. Verstrate, 1981. Specific gravity, carcass fat, abdominal fat, and yield data in broiler chickens. Poultry Sci. 60:2045-2052. Chambers, J. R., J. S. Gavora, and A. Fortin, 1981. Genetic changes in meat-type chickens in the last twenty years. Can. J. Anim. Sci. 61:555—563. Fox, T. W., and B. B. Bohren, 1954. An analysis of feed efficiency among breeds of chickens and its relationship to rate of growth. Poultry Sci. 33:549-561. Helwig, J. T., and K. A. Council, ed., 1979. SAS User's Guide. SAS Inst. Inc., Raleigh, NC. Leeson, S., and J. D. Summers, 1980. Production and carcass characteristics of the broiler chicken. Poultry Sci. 59:786-798. Marks, H. L., 1979. Growth rate and feed intake of selected and nonselected broilers. Growth 43: 80-90. Pym, R.A.E., and P. J. Nicholls, 1979. Selection for food conversion in broilers: Direct and correlated responses to selection for body-weight gain, food consumption and food conversion ratio. Br.
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