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The Effect of Dietary Energy and Protein on Carcass Composition with a Note on a Method for Estimating Carcass Composition
DIETARY VOLUME MEASUREMENTS
tical ration formulation, introduce an error which is evident when the calculated value and the measured value of the dietary volume of a mixed ration are compared. In order to minimize this error, dietary volume will need to be measured to more decimal places, and this will involve further refinements in laboratory equipment and techniques. ACKNOWLED GMENT
This work was supported from the National Science (GE-997) under auspices of graduate Science Education
by a grant Foundation the UnderProgram.
REFERENCES Berg, B. N., 1960. Nutrition and longevity in the rat. I. Food intake in relation to size, health and fertility. J. Nutrition, 71: 242-254. Berg, B. N., and H. S. Simms, 1960. Nutrition and longevity in the rat. II. Longevity and onset of disease with different levels of food intake. J. Nutrition, 71:255-263. Cohn, C , R. Pick and L. N. Katz, 1961. Effect of meal eating compared to nibbling upon athero-
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sclerosis in chickens. Circulation Res. 9: 139-145. Gleaves, E. W., L. V. Tonkinson, K. E. Dunkelgod, R. H. Thayer, R. J. Sirny and R. D. Morrison, 1963a. Regulating nutrient intake in laying hens with diets fed ad libitum. Poultry Sci. 42: 363376. Hollifield, G., and W. Parson, 1962. Metabolic adaptations to a "stuff and starve" feeding program. II. Obesity and the persistence of adaptive changes in adipose tissue and liver occurring in rats limited to a short daily feeding period. J. Clin. Invest. 41: 250-253. Janowitz, H. D., and M. I. Grossman, 1949. Some factors affecting the food intake of normal dogs and dogs with esophagostomy and gastric fistula. Amer. J. Physiol. 159: 143-148. Richardson, C. E., A. B. Watts and E. A. Epps, 1958. The effect of added fiber with and without fat in a practical broiler ration. Poultry Sci. 37: 1278-1283. Sibbald, I. R., S. J. Slinger and G. C. Ashton, 1960. The weight gain and feed intake of chicks fed a ration diluted with cellulose or kaolin. J. Nutrition, 72: 441-446. Singsen, E. P., L. D. Matterson, J. Tlustohowicz and L. M. Potter, 1958. The effect of controlled feeding, energy intake and type of diet on the performance of heavy-type laying hens. Poultry Sci. 37: 1243-1244. Abstract.
The Effect of Dietary Energy and Protein on Carcass Composition with a Note on a Method for Estimating Carcass Composition J. D. SUMMERS, S. J. SLINGER AND G. C. ASHTON Departments of Poultry Science and Physics and Mathematics, Ontario Agricultural College, Guelph, Ontario, Canada (Received for publication September 4, 1964)
T
HE importance of considering carcass composition of animals reared for meat purposes has long been appreciated (Lawes and Gilbert, 1859; Haecker, 1920; McMeekan, 1940). In large animals, especially swine, a great deal of work has been done in developing rations and methods of feeding which would produce carcasses of desired composition at the
time of slaughter (Crampton et ah, 1954; Hill and O'Carroll, 1962; and others). Although much work has been done in developing rations to finish chickens, there is little information available on the influence of diet on carcass composition at time of marketing except for some studies on the effect of hormones (Donovan and Sherman, 1960; and Bogdonoff et al.,
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J. D. SUMMERS, S. J. SLINGER AND G. C. ASHTON
1961) and antibiotics (Jukes et al., 1957). It is becoming increasingly important to consider not only weight gain and feed efficiency of meat animals but also their carcass composition. The increased demands of the processor for broilers of uniform size and composition has stimulated interest in the formulation of rations to give specific weights and body composition at time of marketing. Public awareness of the possible health hazards and wastefulness of excess fat in meat is also increasing and thus the demand for carcasses with specific amounts of lean and fat will continue to grow. Summers and Fisher (1961) and Combs and Robel (1962) have shown that the method of Bender and Miller (1953b), to estimate carcass nitrogen from carcass water content, may be used in the chicken with a high degree of accuracy. This method eliminates much of the work involved in measuring carcass protein. The availability of a similar simplified method to estimate carcass fat would assist materially in encouraging carcass composition measurements of market type poultry. The present study was designed to investigate the effect of various dietary protein and energy levels on weight gain and carcass composition of chickens of two breeds and ages, two sexes and different types of diets. EXPERIMENTAL
Experiment 1 was arranged factorially in a randomized complete block design and involved 5 levels of crude protein (10, 14, 18, 22 and 26 percent) and 4 levels of energy: 1,135, 1,260, 1,385 and 1,510 Kcal. of metabolizable energy (M.E.) per lb. Soybean meal (50% protein) supplemented with 0.5% DL-methionine was used as the sole source of protein in order to achieve the same balance of amino acids in all diets. An equal parts mixture
of corn starch and dextrose served as the main carbohydrate source for all rations. Four replicate groups of 10 birds each were used for each treatment. Male, one day old White Leghorn chicks were used in the experiment. The diets employed and the procedures used in preparing the carcasses for analysis are outlined in the report of Summers et al. (1964). All birds were used for carcass analysis, the 10 birds per replicate being pooled into a composite sample. Moisture determinations were conducted by drying the various pooled samples in a freeze-drier and calculating the difference between wet and dry weight. For fat determinations, a sample of the ground, pooled carcasses was placed in a chloroform, methanol solution (2:1 V-V) and shaken for a 24 hour period. The sample was then filtered and the resulting filtrate evaporated to dryness on a steam bath. The dried residue was further extracted with petroleum ether, filtered, and the filtrate evaporated to dryness. Percentage fat was calculated by expressing the weight of lipid material remaining after the last extraction as a percentage of the original weight. Nitrogen determinations were conducted in triplicate on the pooled, ground samples by a semi-micro Kjeldahl technique employing copper sulfate as a catalyst. Experiment 2 was a factorial arrangement in a randomized complete block design involving two sexes, two levels of added animal tallow (2.5 and 5 percent) and 4 levels of crude protein (20, 22, 24 and 26 percent). Practical broiler type rations were employed and the basal diet used is shown in Table 1. To increase the level of protein, soybean meal was substituted for corn with no attempt being made to keep the diets isocaloric. Vantress & cf X Pilch White Rock 9 9 chicks were used with two replicate groups of 14 birds
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CARCASS COMPOSITION TABLE 1.—Composition of basal ration Ingredient
%
Ground wheat Ground yellow corn Soybean meal (50% protein) Dicalcium phosphate Pulverized limestone Tallow (stabilized) Iodized salt (0.015% KI) Vitamin, mineral premix 1 1
by the procedure outlined in the previous experiment. All data were analyzed by the method of analysis of variance, the total treatment variation being partitioned into portions appropriate to individual degrees of freedom. The resulting mean squares were tested sequentially for significance using the tables of the studentized form of the extreme mean square test due to Nair (1948) as indicated by Hartley (1955).
10.0 56.35 27.25 2.00 1.00 2.50 0.50 0.40
Vitamin, mineral premix Ingredient
gm./lOO lb. of diet
Vitamin A(10,000 I.U./gm.) Vitamin D s (1,650 I.C.U./gm.) Riboflavin (24 gm./lb.) Calcium D pantothenate (2 gm./oz.) Niacin Vitamin Bi2 (9 mg./lb.) Choline chloride (25%) Manganous oxide (56% Mn) Zinc Oxide (80%o Zn) Ethoxyquin (50%) Menadione sodium bisulfite (4 gm./lb.) DL methionine Vitamin E (20,000 I.U./lb.) Folic acid (3%)
22.7 22.7 3.8 3.0 1.0 30.3 22.7 5.0 3.0 10.0 11.4 22.7 22.7 1.0
RESULTS AND DISCUSSION
each per treatment. The chicks were started at one day of age and remained in battery brooders for the 6 week experimental period. Carcass analyses were conducted at the conclusion of the experiment
Experiment 1. Body weight and feed efficiency for this experiment were discussed in an earlier report (Summers et al., 1964) and will not be considered here. The unsummarized data on carcass composition, Table 2, suggested that the increments of dietary protein effected similar increases in percent carcass protein for all levels of dietary fat. The data also indicated a similar but negative relation between dietary energy level and per cent carcass protein for all levels of dietary protein. In the process of statistical analysis of these data, consideration of a transformation of the percentage figures lead to the probit transform (Finney, 1962) since a
TABLE 2.—A comparison of the determined (D) and estimated (E) protein and fat contents of the carcasses from experiment 1 {dry weight basis) Dietary protein (%) Energy level of diet
14
10
22
18
D
E
D
E
M.E./Kcal./lb. 1,135 1,260 1,385 1,510
53.4 49.9 48.5 48.0
53.9 49.9 48.6 47.5
56.6 53.7 52.2 50.5
56.6 52.2 50.5 49.3
1,135 1,260 1,385 1,510
28.3 31.9 33.2 33.9
28.0 32.1 33.3 34.5
25.6 29.0 31.4 32.3
25.4 29.7 31.5 32.8
D
E
D
Percent carcass protein 60.5 60.4 64.4 57.5 57.3 61.0 55.3 55.1 58.6 52.8 52.6 56.2 Percent carcass fat 21.6 21.5 25.4 24.7 27.6 26.9 26.4 29.5
Data are means of four observations and are decoded probit values.
19.3 22.3 24.7 21.0
26 E
D
E
64.9 60.2 59.0 56.2
63.4 62.6 61.5 58.6
65.5 63.5 60.9 59.6
17.1 21.9 22.9 25.8
18.2 20.5 23.0 19.7
16.5 18.5 21.0 22.4
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J. D. SUMMERS, S. J. SLINGER AND G. C. ASHTON
TABLE 3.—Percent carcass protein {dry weight basis) as influenced by energy and protein level of the diet Carcass protein Observed
Protein, % 10 14 18 22 26 M.E. (Kcal./lb.) 1,135 1,260 1,385 1,510
Adjusted to equal N intak
%
%
49.9(4.999) 53.3(5.082) 56.5(5.164) 60.1 (5.255) 61.5(5.292)
45.0(4.875) 51.9(5.049) 57.5(5.188) 62.1(5.308) 64.5(5.372)
59.7(5.245) 57.0(5.176) 55.3(5.132) 53.3(5.081)
60.2(5.258) 57.3(5.183) 55.1(5.127) 52.6(5.065)
( ) = Probit Values. Standard deviation of individual unadjusted probit values=0.039, adjusted = 0.029. Degrees of freedom for experimental error for unadjusted data = 57, adjusted = 56.
plot of the probit of the percent carcass protein against the logarithm of the nitrogen intake showed essentially a linear relation. An analysis of variance confirmed (1) the independence of the responses to energy and protein levels of the diets and (2) that these dietary nutrient levels exerted significant effects on the protein content of the carcasses. A covariance analysis of the probit of percent carcass protein and the logarithm of nitrogen intake showed the latter variable accounted for 13.5 percent of the variability in the protein content of the carcasses. Adjustment of the mean responses to equal nitrogen intakes as outlined by Steel and Torrie (1962) (Table 3), still leaves one with the conclusion that the protein content of the carcass was affected independently by dietary levels of energy and protein. A plot of the data for percent fat in the carcass, Figure 1A suggested an interdependence of protein and energy level in the diet by the failure of the 1,510 Kcal.
line to be parallel to the other three. This protein by energy interaction proved to be statistically significant on analysis of the probit transformation of the percent carcass fat values. The only explanation that can be offered is that some difficulty was encountered with the fat determinations on carcasses from the three highest levels of dietary protein at this energy level. In an attempt to present what is considered a more realistic picture a multiple regression analysis was performed with the logarithms of nitrogen and calorie intake as independent variable and the probit of percent carcass fat as the dependent variable. The determined linear regression equation accounted for approximately 84 percent of the variation in percent carcass fat and so fitted the data reasonably well. Substitution of the logarithms of the known nitrogen and calorie intakes for the 20 protein-energy combinations yield estimates of per cent carcass fat indicated in Figure IB. The plot indicates that the carcass fat of the chicks decreased with increasing levels of dietary protein and increased with increasing levels of dietary energy. The apparent parallelism of the lines in Figure IB suggest that dietary level of protein and energy affect the carcass fat independently over the levels employed in this trial. Experiment 2. The response criteria of body weight gain and feed consumption gave little evidence of interdependence between the factors under study and this was confirmed by statistical analysis. Main effect comparisons, Table 4, yielded significant weight gain differences for sex and fat levels while for feed consumption only the difference between sexes proved to be significant. Because of this latter condition only the gains for dietary pro-
505
CARCASS COMPOSITION
Kcsl. M.E./lb.
• Kcal. M.E./lb. • Kcal. M.E./lb. • Kcal. M.E./lb.
1510 1385 1260 1135
18
-
Kcal. Kcal. Kcal. Kcal.
M.E./lb. M.E./lb. M.E./lb. M.E./lb.
22
Dietary Crude Protein, ;
FIG. 1. Percent carcass fat (dry weight basis) of 3 week old birds as influenced by protein and energy levels of the diet.
tein and fat levels were adjusted to equal increased the carcass fat there was insufficient evidence to conclude it affected the feed intake. The carcass composition data, Table 5, protein content of the carcass. Carcasses showed some evidence of interdependence of the male birds had significantly less fat among the factors in the case of carcass and more protein than did those from fat content but not for carcass protein female birds. The results presented in Experiment 1 content. Since the statistical analysis inare in agreement with those of Donaldson dicated the interaction for the fat content et al. (1956) and indicate that marked of the carcass to be borderline, the data changes in body composition can be were summarized by main effects only, achieved by altering protein and energy Table 6. Increased levels of dietary protein effected relatively uniform increases levels in a ration. The present study emin carcass protein and decreases in carcass ployed a much wider range of calorie :profat. While dietary fat level significantly tein ratios than were used in the work of
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J. D. SUMMERS, S. J. SLINGER AND G. C. ASHTON TABLE 6.—Average percent protein and fat (dry weight basis) in chicken carcasses fed varying levels of protein and fat
T A B L E 4 . — A v e r a g e body weight gain and feed consumption of 6-week old chickens fed diets varying in protein and fat Body weight gain Treatments Observed
Carcass protein
Adjusted to equal feed intake
Protein level, % 20 22 24 26 Fal level, % 2.5 5.0 Sex Female Male
gm.
Protein level, % 20 22 24 26
905 921 919 912
903 920 920 915
1,781 1,770 1,757 1,731
Fat level, % 2.5 5.0
899 930
900 929
1,749 1,770
Sex Male Female
980 849
%
Probit
50.0 52.4 53.8 55.6
(5.001*1) (5.064) (5.096) (5.141)
26.6 25.5 24.2 21.5
(4.376") (4.342) (4.299) (4.209)
53.6 52.4
(5.091) (5.060)
23.8 25.0
(4.286*) (4.327)
51.6 54.5
(5.039*) (5.112)
26.6 22.3
(4.376*) (4.237)
Probit
( ) = P r o b i t Values. Standard deviation of individual probit values for protein =0.054, fat =0.044 Degrees of freedom for experimental error = 15. *i Means form significantly linear response P = > . 0 5 . * Means are significantly different P = > . 0 5 .
1,847 1,672
Standard deviation of individual values Degrees of freedom for experimental
Carcass fat
Treatment
Feed consumption
32.6 15
Means joined by vertical lines are not significantly different. P>.05.
Donaldson et al. (1956). However, the same general conclusion can be drawn, namely, that the level of energy in a ration influences the protein requirement for maximum weight gain, feed efficiency and nitrogen content of the carcass. Care must
be taken in extrapolating the above results to practical type diets, where it is often not possible to keep the protein quality constant as the level of protein in the diet is increased. Experiment 2, a practical type experiment, was designed to cover the range of protein and energy levels which would normally be encountered in practice. Under the conditions employed no signifi-
TABLE 5.—A comparison of the determined (D) and estimated (E) and fat contents of the carcasses from experiment 2 (dry weight basis) Dietary protein (%) Added tallow
M.E.
22
20 D
E
D
26
24 E
D
E
D
E
56.2 56.9
57.9 57.6
%
Kcal./lb.
2.5 5.0
1,371 1,419
52.5 50.4
53.3 48.6
Percent carcass protein Males 55.6 54.6 57.7 54.4 56.4 54.1 52.4 56.2
2.5 5.0
1,354 1,402
49.1 48.2
49.9 46.9
52.7 48.9
Females 53.8 53.2 46.3 51.3
53.8 48.4
56.2 52.9
55.3 53.8
18.8 20.3
19.8 20.9
18.8 19.1
24.6 29.9
21.7 23.5
23.0 24.6
2.5 5.0
1,336 1,384
23.9 23.5
23.4 26.0
22.6 23.4
Percent carcass fat Males 21.0 22.4 24.4 21.8
2.5 5.0
1,320 1,368
28.4 31.1
28.4 31.4
26.6 29.9
Females 24.6 25.4 32.0 27.4
Data are means of two observations and are decoded probit values.
CARCASS COMPOSITION
cant gain in weight was obtained by increasing the level of protein from 20 to 26 percent by increments of two units percent. This agrees with the work of Spring and Wilkinson (1957) who showed no increased gain in weight of eight week old broilers when the protein content of the diet was raised from 22 to 28 percent with M.E. values of 1,200, 1,350 and 1,500 Kcal. per pound. The results of Experiment 2 are of interest in view of the growing demand for "junior" broilers which are normally marketed at weights of about 2.50 to 2.75 pounds and are used for TV dinners and cut-up or whole ready-cooked specialties. While diets containing 20-22 percent protein gave essentially maximum weights, the relatively high fat and low protein content of these carcasses may limit their acceptance for certain uses. The present results indicate that the sexes should be reared separately and fed different diets if more precise control of carcass composition is desired. To achieve similar carcass composition with the energy levels used here it would appear necessary to feed the males a 20 to 22 percent protein ration while the females would require a diet containing about 26 percent protein. On the other hand, where a high fat, low protein carcass is desired, females fed a diet containing about 20 percent of protein may be used to advantage. A METHOD FOR ESTIMATING CARCASS COMPOSITION
As mentioned previously, the method of Bender and Miller (1953b) has been employed to estimate carcass nitrogen from carcass moisture [Summers and Fisher (1961) and Combs and Robel (1962)] in the chicken. Using the average water to nitrogen ratios found in Experiments 1 and 2, the percentage of carcass protein has been estimated from moisture
507
determinations for the various treatment groups. The estimated and determined values for Experiments 1 and 2 are shown in Tables 2 and 5, respectively. If the combined quantity of fat and protein in the carcass is expressed as a ratio of dry carcass weight, a relatively constant value is obtained. One can then use this relationship to estimate percentage carcass fat from the following equation: kW-P Percent carcass fat = X100: W where k = a determined constant (fat+ protein as a ratio of dry weight), P = gm. of carcass protein and W = dry weight of carcass. Using the above equation the fat contents of the carcasses have been estimated for Experiments 1 and 2 using the estimated protein values; these data are also shown in Tables 2 and 5 along with the determined values. The results indicate that the carcass protein values calculated from carcass moisture agree very closely with determined values. This is further confirmation of the reliability of using the simplified method of Bender and Miller (1953b) for estimating carcass protein. In general, the estimated fat values agreed well with the determined values and indicate the feasibility of using such a simplified procedure in obtaining gross carcass fat estimates of poultry. The correlation between the determined and the estimated values was determined for both protein and fat in both experiments. The four correlation coefficients calculated without regard to treatment or replication groupings ranged from 0.86 to 0.97. It must be kept in mind, however, that as with the water to nitrogen ration, the "k" value used to estimate fat may vary with the age and perhaps the strain of bird. Thus a value may have to be calculated from actual
508
J. D. SUMMERS, S. J. SLINGER AND G. C. ASHTON TABLE 7.—Percentage carcass fat {dry weight basis)1 Carcass fat (%)
Dietary protein (%)
Determined
Estimated
12 16 18 21 27
34.6 29.4 25.5 23.0 21.8
34.4 29.7 25.7 22.9 19.1
1
Data from Table 3 of Summers and Fisher 1961.
determinations for the age and type of bird one wishes to work with. Using the data of Summers and Fisher (1961, Table 3), with birds of the same age as those used in Experiment 1 of the present study, and employing the same "k" value, the percent carcass fat has been estimated. The results are shown in Table 7 and demonstrate that the estimated fat values agree quite well with the determined values; this is further evidence that the method can be used with a reasonable degree of reliability.
content of the diet exerted a greater effect in lowering the fat and increasing the protein content of the female than the male carcass. The results indicate that the sexes should perhaps be reared separately and fed different diets if carcasses of constant composition are to be obtained. A method is presented that enables an estimate to be made of carcass composition from a simple moisture determination. Carcass protein is calculated from moisture by taking advantage of the constancy of the water :nitrogen ratio in the carcass. Because of the constancy of the ratio of total protein and fat to carcass dry matter it is then possible to estimate the fat content of the carcass with a high degree of precision. ACKNOWLEDGEMENT
Grateful acknowledgement is made to Dr. J. Singh, Physics and Mathematics Department for advice on statistical analyses. REFERENCES
SUMMARY
Experiments were conducted to determine the influence of a wide range of protein and energy levels on weight gain, feed efficiency and carcass composition using both semi-purified and practical diets. In general, carcass protein was increased and carcass fat was decreased in a linear manner with increasing levels of dietary protein. Conversely, increasing levels of dietary energy resulted in decreased carcass protein and increased carcass fat. Practical diets were used for the production of "junior" broilers and the observations recorded are of commercial significance. While little or no improvement in weight gain was achieved by increasing the protein level beyond 20 percent, marked changes in carcass composition were noted. Raising the protein
Bender, A. E., and D. S. Miller, 1953b. Constancy of the N/H2O ratios of the rat and its use in the determination of the net protein value. Biochem. J. 5 3 : V H - V n i . Bogdonoff, P. D., J. N. Hensen and G. W. Thrasher, 1961. The effect of oral administration of hormones known to affect carcass composition. Poultry Sci. 40: 1637-1644. Combs, G. F., and E. J. Robel, 1962. Effect of energy intake on body composition and sulfur amino acid requirements for maintenance and growth of chicks. Poultry Sci. 41: 1636. Crampton, E. W., G. C. Ashton and L. E. Lloyd, 1954. Improvement of bacon carcass quality by the introduction of fibrous feeds into the hogfinishing ration. J. Animal Sci. 13: 327-331. Donaldson, W. E., G. F. Combs and G. L. Romoser, 1956. Studies on energy level in poultry rations. 1. The effect of calorie-to-protein ratio of the ration on growth, nutrient utilization and body composition of chicks. Poultry Sci. 35:1100-1105. Donovan, G. A., and W. C. Sherman, 1960. Analysis of the growth pattern and the body composition of chickens implanted with diethylstitbestrol. Poultry Sci. 39: 757-765.
CARCASS COMPOSITION Finney, D. J., 1962. Probit Analysis. Cambridge University Press, London. Haecker, T. L., 1920. Investigations in beef production. Minnesota Agr. Exp. Stat. Bui., 193. Hartley, H. 0., 1955. The sequential F test for multiple discussion on significance in an analysis of variance table. Mimeographed Lecture Notes. Iowa State College. Hill, F., and F. M. O'Carroll, 1962. The chemical composition of pig carcass at pork, bacon, and manufacturing weights. Irish J. Agr. Res. 1: 115, 130. Jukes, H. G., D. C. Hill and H. D. Branion, 1957. Effect of penicillin on the carcass composition of the chicken. Poultry Sci. 36: 423-425. Lawes, J. B., and F. H. Gilbert, 1859. Experimental inquiry into the composition of the animals fed and slaughtered as human food. Trans. Roy. Soc. (London) 2: 493. McMeekan, C. P., 1940. Growth and development
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in the pig with special reference to carcass quality characters. Parts I, II and III. J. Agr. Sci. 30: 276-343, 387^35, 511-569. Nair, K. R., 1948. The studentized form of the extreme mean square test in the analysis of variance. Biometrika, 35: 16-31. Spring, J. L., and W. S. Wilkinson, 1957. The influence of dietary protein and energy level on body composition of broilers. Poultry Sci. 36:1159. Steel, R. G. D., and J. H. Torrie, 1960. Principles and Procedures of Statistics. McGraw-Hill Book Co., New York, N. Y. Summers, J. D., and H. Fisher, 1961. Net protein values for the growing chicken as determined by carcass analysis: Exploration of the method. J. Nutrition, 75: 435-442. Summers, J. D., S. J. Slinger, I. R. Sibbald and W. F. Pepper, 1964. The influence of protein and energy on growth and protein utilization in the growing chicken. J. Nutrition, 82: 463-468.
Duration of Immunity to Infectious Laryngotracheitis 1 L. G. RAGGI AND G. G. L E E Department of Avian Medicine, University of California, Davis, California (Received for publication September 14, 1964)
T
HE duration of immunity to infectious laryngotracheitis (LT) of chickens has not been established conclusively. Beach et al. (1934) demonstrated that immunity to intratracheal challenge lasted for 364 days, the longest period tested. Hofstad (1959) stated: "The duration of immunity has not been demonstrated, although results indicate that birds are protected for at least a year." Hitchner and Winterfield (1960) found that 36 percent of birds vaccinated 1
This work was supported by funds supplied by USDA, CSRS Regional W-5 Project, Regional Research Fund 1770, American Scientific Labs. (Schering Corp.), Madison, Wisconsin, and American Cyanamid Company, Pearl River, New York.
at 8 weeks of age and 100 percent at one week of age became susceptible when challenged at 16 weeks of age. In Australia, Hunt (1962) vaccinated day-old broiler chickens by the feather follicle method and challenged 9 weeks later by contact exposure or by the intratracheal route. Thirteen of 20 birds challenged by contact exposure were susceptible, whereas 9 of 20 were susceptible by the intratracheal route. Sinkovic (1962) noted a significant reduction in immunity from 12| to 14| weeks after vaccination of 10| week-old birds. Shibley et al. (1963) standardized the 146 strain originally isolated by Cover and Benton (1957), and administered one drop of the vaccine onto the conjunctival sac of 13-week-old
tical ration formulation, introduce an error which is evident when the calculated value and the measured value of the dietary volume of a mixed ration are compared. In order to minimize this error, dietary volume will need to be measured to more decimal places, and this will involve further refinements in laboratory equipment and techniques. ACKNOWLED GMENT
This work was supported from the National Science (GE-997) under auspices of graduate Science Education
by a grant Foundation the UnderProgram.
REFERENCES Berg, B. N., 1960. Nutrition and longevity in the rat. I. Food intake in relation to size, health and fertility. J. Nutrition, 71: 242-254. Berg, B. N., and H. S. Simms, 1960. Nutrition and longevity in the rat. II. Longevity and onset of disease with different levels of food intake. J. Nutrition, 71:255-263. Cohn, C , R. Pick and L. N. Katz, 1961. Effect of meal eating compared to nibbling upon athero-
501
sclerosis in chickens. Circulation Res. 9: 139-145. Gleaves, E. W., L. V. Tonkinson, K. E. Dunkelgod, R. H. Thayer, R. J. Sirny and R. D. Morrison, 1963a. Regulating nutrient intake in laying hens with diets fed ad libitum. Poultry Sci. 42: 363376. Hollifield, G., and W. Parson, 1962. Metabolic adaptations to a "stuff and starve" feeding program. II. Obesity and the persistence of adaptive changes in adipose tissue and liver occurring in rats limited to a short daily feeding period. J. Clin. Invest. 41: 250-253. Janowitz, H. D., and M. I. Grossman, 1949. Some factors affecting the food intake of normal dogs and dogs with esophagostomy and gastric fistula. Amer. J. Physiol. 159: 143-148. Richardson, C. E., A. B. Watts and E. A. Epps, 1958. The effect of added fiber with and without fat in a practical broiler ration. Poultry Sci. 37: 1278-1283. Sibbald, I. R., S. J. Slinger and G. C. Ashton, 1960. The weight gain and feed intake of chicks fed a ration diluted with cellulose or kaolin. J. Nutrition, 72: 441-446. Singsen, E. P., L. D. Matterson, J. Tlustohowicz and L. M. Potter, 1958. The effect of controlled feeding, energy intake and type of diet on the performance of heavy-type laying hens. Poultry Sci. 37: 1243-1244. Abstract.
The Effect of Dietary Energy and Protein on Carcass Composition with a Note on a Method for Estimating Carcass Composition J. D. SUMMERS, S. J. SLINGER AND G. C. ASHTON Departments of Poultry Science and Physics and Mathematics, Ontario Agricultural College, Guelph, Ontario, Canada (Received for publication September 4, 1964)
T
HE importance of considering carcass composition of animals reared for meat purposes has long been appreciated (Lawes and Gilbert, 1859; Haecker, 1920; McMeekan, 1940). In large animals, especially swine, a great deal of work has been done in developing rations and methods of feeding which would produce carcasses of desired composition at the
time of slaughter (Crampton et ah, 1954; Hill and O'Carroll, 1962; and others). Although much work has been done in developing rations to finish chickens, there is little information available on the influence of diet on carcass composition at time of marketing except for some studies on the effect of hormones (Donovan and Sherman, 1960; and Bogdonoff et al.,
502
J. D. SUMMERS, S. J. SLINGER AND G. C. ASHTON
1961) and antibiotics (Jukes et al., 1957). It is becoming increasingly important to consider not only weight gain and feed efficiency of meat animals but also their carcass composition. The increased demands of the processor for broilers of uniform size and composition has stimulated interest in the formulation of rations to give specific weights and body composition at time of marketing. Public awareness of the possible health hazards and wastefulness of excess fat in meat is also increasing and thus the demand for carcasses with specific amounts of lean and fat will continue to grow. Summers and Fisher (1961) and Combs and Robel (1962) have shown that the method of Bender and Miller (1953b), to estimate carcass nitrogen from carcass water content, may be used in the chicken with a high degree of accuracy. This method eliminates much of the work involved in measuring carcass protein. The availability of a similar simplified method to estimate carcass fat would assist materially in encouraging carcass composition measurements of market type poultry. The present study was designed to investigate the effect of various dietary protein and energy levels on weight gain and carcass composition of chickens of two breeds and ages, two sexes and different types of diets. EXPERIMENTAL
Experiment 1 was arranged factorially in a randomized complete block design and involved 5 levels of crude protein (10, 14, 18, 22 and 26 percent) and 4 levels of energy: 1,135, 1,260, 1,385 and 1,510 Kcal. of metabolizable energy (M.E.) per lb. Soybean meal (50% protein) supplemented with 0.5% DL-methionine was used as the sole source of protein in order to achieve the same balance of amino acids in all diets. An equal parts mixture
of corn starch and dextrose served as the main carbohydrate source for all rations. Four replicate groups of 10 birds each were used for each treatment. Male, one day old White Leghorn chicks were used in the experiment. The diets employed and the procedures used in preparing the carcasses for analysis are outlined in the report of Summers et al. (1964). All birds were used for carcass analysis, the 10 birds per replicate being pooled into a composite sample. Moisture determinations were conducted by drying the various pooled samples in a freeze-drier and calculating the difference between wet and dry weight. For fat determinations, a sample of the ground, pooled carcasses was placed in a chloroform, methanol solution (2:1 V-V) and shaken for a 24 hour period. The sample was then filtered and the resulting filtrate evaporated to dryness on a steam bath. The dried residue was further extracted with petroleum ether, filtered, and the filtrate evaporated to dryness. Percentage fat was calculated by expressing the weight of lipid material remaining after the last extraction as a percentage of the original weight. Nitrogen determinations were conducted in triplicate on the pooled, ground samples by a semi-micro Kjeldahl technique employing copper sulfate as a catalyst. Experiment 2 was a factorial arrangement in a randomized complete block design involving two sexes, two levels of added animal tallow (2.5 and 5 percent) and 4 levels of crude protein (20, 22, 24 and 26 percent). Practical broiler type rations were employed and the basal diet used is shown in Table 1. To increase the level of protein, soybean meal was substituted for corn with no attempt being made to keep the diets isocaloric. Vantress & cf X Pilch White Rock 9 9 chicks were used with two replicate groups of 14 birds
503
CARCASS COMPOSITION TABLE 1.—Composition of basal ration Ingredient
%
Ground wheat Ground yellow corn Soybean meal (50% protein) Dicalcium phosphate Pulverized limestone Tallow (stabilized) Iodized salt (0.015% KI) Vitamin, mineral premix 1 1
by the procedure outlined in the previous experiment. All data were analyzed by the method of analysis of variance, the total treatment variation being partitioned into portions appropriate to individual degrees of freedom. The resulting mean squares were tested sequentially for significance using the tables of the studentized form of the extreme mean square test due to Nair (1948) as indicated by Hartley (1955).
10.0 56.35 27.25 2.00 1.00 2.50 0.50 0.40
Vitamin, mineral premix Ingredient
gm./lOO lb. of diet
Vitamin A(10,000 I.U./gm.) Vitamin D s (1,650 I.C.U./gm.) Riboflavin (24 gm./lb.) Calcium D pantothenate (2 gm./oz.) Niacin Vitamin Bi2 (9 mg./lb.) Choline chloride (25%) Manganous oxide (56% Mn) Zinc Oxide (80%o Zn) Ethoxyquin (50%) Menadione sodium bisulfite (4 gm./lb.) DL methionine Vitamin E (20,000 I.U./lb.) Folic acid (3%)
22.7 22.7 3.8 3.0 1.0 30.3 22.7 5.0 3.0 10.0 11.4 22.7 22.7 1.0
RESULTS AND DISCUSSION
each per treatment. The chicks were started at one day of age and remained in battery brooders for the 6 week experimental period. Carcass analyses were conducted at the conclusion of the experiment
Experiment 1. Body weight and feed efficiency for this experiment were discussed in an earlier report (Summers et al., 1964) and will not be considered here. The unsummarized data on carcass composition, Table 2, suggested that the increments of dietary protein effected similar increases in percent carcass protein for all levels of dietary fat. The data also indicated a similar but negative relation between dietary energy level and per cent carcass protein for all levels of dietary protein. In the process of statistical analysis of these data, consideration of a transformation of the percentage figures lead to the probit transform (Finney, 1962) since a
TABLE 2.—A comparison of the determined (D) and estimated (E) protein and fat contents of the carcasses from experiment 1 {dry weight basis) Dietary protein (%) Energy level of diet
14
10
22
18
D
E
D
E
M.E./Kcal./lb. 1,135 1,260 1,385 1,510
53.4 49.9 48.5 48.0
53.9 49.9 48.6 47.5
56.6 53.7 52.2 50.5
56.6 52.2 50.5 49.3
1,135 1,260 1,385 1,510
28.3 31.9 33.2 33.9
28.0 32.1 33.3 34.5
25.6 29.0 31.4 32.3
25.4 29.7 31.5 32.8
D
E
D
Percent carcass protein 60.5 60.4 64.4 57.5 57.3 61.0 55.3 55.1 58.6 52.8 52.6 56.2 Percent carcass fat 21.6 21.5 25.4 24.7 27.6 26.9 26.4 29.5
Data are means of four observations and are decoded probit values.
19.3 22.3 24.7 21.0
26 E
D
E
64.9 60.2 59.0 56.2
63.4 62.6 61.5 58.6
65.5 63.5 60.9 59.6
17.1 21.9 22.9 25.8
18.2 20.5 23.0 19.7
16.5 18.5 21.0 22.4
504
J. D. SUMMERS, S. J. SLINGER AND G. C. ASHTON
TABLE 3.—Percent carcass protein {dry weight basis) as influenced by energy and protein level of the diet Carcass protein Observed
Protein, % 10 14 18 22 26 M.E. (Kcal./lb.) 1,135 1,260 1,385 1,510
Adjusted to equal N intak
%
%
49.9(4.999) 53.3(5.082) 56.5(5.164) 60.1 (5.255) 61.5(5.292)
45.0(4.875) 51.9(5.049) 57.5(5.188) 62.1(5.308) 64.5(5.372)
59.7(5.245) 57.0(5.176) 55.3(5.132) 53.3(5.081)
60.2(5.258) 57.3(5.183) 55.1(5.127) 52.6(5.065)
( ) = Probit Values. Standard deviation of individual unadjusted probit values=0.039, adjusted = 0.029. Degrees of freedom for experimental error for unadjusted data = 57, adjusted = 56.
plot of the probit of the percent carcass protein against the logarithm of the nitrogen intake showed essentially a linear relation. An analysis of variance confirmed (1) the independence of the responses to energy and protein levels of the diets and (2) that these dietary nutrient levels exerted significant effects on the protein content of the carcasses. A covariance analysis of the probit of percent carcass protein and the logarithm of nitrogen intake showed the latter variable accounted for 13.5 percent of the variability in the protein content of the carcasses. Adjustment of the mean responses to equal nitrogen intakes as outlined by Steel and Torrie (1962) (Table 3), still leaves one with the conclusion that the protein content of the carcass was affected independently by dietary levels of energy and protein. A plot of the data for percent fat in the carcass, Figure 1A suggested an interdependence of protein and energy level in the diet by the failure of the 1,510 Kcal.
line to be parallel to the other three. This protein by energy interaction proved to be statistically significant on analysis of the probit transformation of the percent carcass fat values. The only explanation that can be offered is that some difficulty was encountered with the fat determinations on carcasses from the three highest levels of dietary protein at this energy level. In an attempt to present what is considered a more realistic picture a multiple regression analysis was performed with the logarithms of nitrogen and calorie intake as independent variable and the probit of percent carcass fat as the dependent variable. The determined linear regression equation accounted for approximately 84 percent of the variation in percent carcass fat and so fitted the data reasonably well. Substitution of the logarithms of the known nitrogen and calorie intakes for the 20 protein-energy combinations yield estimates of per cent carcass fat indicated in Figure IB. The plot indicates that the carcass fat of the chicks decreased with increasing levels of dietary protein and increased with increasing levels of dietary energy. The apparent parallelism of the lines in Figure IB suggest that dietary level of protein and energy affect the carcass fat independently over the levels employed in this trial. Experiment 2. The response criteria of body weight gain and feed consumption gave little evidence of interdependence between the factors under study and this was confirmed by statistical analysis. Main effect comparisons, Table 4, yielded significant weight gain differences for sex and fat levels while for feed consumption only the difference between sexes proved to be significant. Because of this latter condition only the gains for dietary pro-
505
CARCASS COMPOSITION
Kcsl. M.E./lb.
• Kcal. M.E./lb. • Kcal. M.E./lb. • Kcal. M.E./lb.
1510 1385 1260 1135
18
-
Kcal. Kcal. Kcal. Kcal.
M.E./lb. M.E./lb. M.E./lb. M.E./lb.
22
Dietary Crude Protein, ;
FIG. 1. Percent carcass fat (dry weight basis) of 3 week old birds as influenced by protein and energy levels of the diet.
tein and fat levels were adjusted to equal increased the carcass fat there was insufficient evidence to conclude it affected the feed intake. The carcass composition data, Table 5, protein content of the carcass. Carcasses showed some evidence of interdependence of the male birds had significantly less fat among the factors in the case of carcass and more protein than did those from fat content but not for carcass protein female birds. The results presented in Experiment 1 content. Since the statistical analysis inare in agreement with those of Donaldson dicated the interaction for the fat content et al. (1956) and indicate that marked of the carcass to be borderline, the data changes in body composition can be were summarized by main effects only, achieved by altering protein and energy Table 6. Increased levels of dietary protein effected relatively uniform increases levels in a ration. The present study emin carcass protein and decreases in carcass ployed a much wider range of calorie :profat. While dietary fat level significantly tein ratios than were used in the work of
506
J. D. SUMMERS, S. J. SLINGER AND G. C. ASHTON TABLE 6.—Average percent protein and fat (dry weight basis) in chicken carcasses fed varying levels of protein and fat
T A B L E 4 . — A v e r a g e body weight gain and feed consumption of 6-week old chickens fed diets varying in protein and fat Body weight gain Treatments Observed
Carcass protein
Adjusted to equal feed intake
Protein level, % 20 22 24 26 Fal level, % 2.5 5.0 Sex Female Male
gm.
Protein level, % 20 22 24 26
905 921 919 912
903 920 920 915
1,781 1,770 1,757 1,731
Fat level, % 2.5 5.0
899 930
900 929
1,749 1,770
Sex Male Female
980 849
%
Probit
50.0 52.4 53.8 55.6
(5.001*1) (5.064) (5.096) (5.141)
26.6 25.5 24.2 21.5
(4.376") (4.342) (4.299) (4.209)
53.6 52.4
(5.091) (5.060)
23.8 25.0
(4.286*) (4.327)
51.6 54.5
(5.039*) (5.112)
26.6 22.3
(4.376*) (4.237)
Probit
( ) = P r o b i t Values. Standard deviation of individual probit values for protein =0.054, fat =0.044 Degrees of freedom for experimental error = 15. *i Means form significantly linear response P = > . 0 5 . * Means are significantly different P = > . 0 5 .
1,847 1,672
Standard deviation of individual values Degrees of freedom for experimental
Carcass fat
Treatment
Feed consumption
32.6 15
Means joined by vertical lines are not significantly different. P>.05.
Donaldson et al. (1956). However, the same general conclusion can be drawn, namely, that the level of energy in a ration influences the protein requirement for maximum weight gain, feed efficiency and nitrogen content of the carcass. Care must
be taken in extrapolating the above results to practical type diets, where it is often not possible to keep the protein quality constant as the level of protein in the diet is increased. Experiment 2, a practical type experiment, was designed to cover the range of protein and energy levels which would normally be encountered in practice. Under the conditions employed no signifi-
TABLE 5.—A comparison of the determined (D) and estimated (E) and fat contents of the carcasses from experiment 2 (dry weight basis) Dietary protein (%) Added tallow
M.E.
22
20 D
E
D
26
24 E
D
E
D
E
56.2 56.9
57.9 57.6
%
Kcal./lb.
2.5 5.0
1,371 1,419
52.5 50.4
53.3 48.6
Percent carcass protein Males 55.6 54.6 57.7 54.4 56.4 54.1 52.4 56.2
2.5 5.0
1,354 1,402
49.1 48.2
49.9 46.9
52.7 48.9
Females 53.8 53.2 46.3 51.3
53.8 48.4
56.2 52.9
55.3 53.8
18.8 20.3
19.8 20.9
18.8 19.1
24.6 29.9
21.7 23.5
23.0 24.6
2.5 5.0
1,336 1,384
23.9 23.5
23.4 26.0
22.6 23.4
Percent carcass fat Males 21.0 22.4 24.4 21.8
2.5 5.0
1,320 1,368
28.4 31.1
28.4 31.4
26.6 29.9
Females 24.6 25.4 32.0 27.4
Data are means of two observations and are decoded probit values.
CARCASS COMPOSITION
cant gain in weight was obtained by increasing the level of protein from 20 to 26 percent by increments of two units percent. This agrees with the work of Spring and Wilkinson (1957) who showed no increased gain in weight of eight week old broilers when the protein content of the diet was raised from 22 to 28 percent with M.E. values of 1,200, 1,350 and 1,500 Kcal. per pound. The results of Experiment 2 are of interest in view of the growing demand for "junior" broilers which are normally marketed at weights of about 2.50 to 2.75 pounds and are used for TV dinners and cut-up or whole ready-cooked specialties. While diets containing 20-22 percent protein gave essentially maximum weights, the relatively high fat and low protein content of these carcasses may limit their acceptance for certain uses. The present results indicate that the sexes should be reared separately and fed different diets if more precise control of carcass composition is desired. To achieve similar carcass composition with the energy levels used here it would appear necessary to feed the males a 20 to 22 percent protein ration while the females would require a diet containing about 26 percent protein. On the other hand, where a high fat, low protein carcass is desired, females fed a diet containing about 20 percent of protein may be used to advantage. A METHOD FOR ESTIMATING CARCASS COMPOSITION
As mentioned previously, the method of Bender and Miller (1953b) has been employed to estimate carcass nitrogen from carcass moisture [Summers and Fisher (1961) and Combs and Robel (1962)] in the chicken. Using the average water to nitrogen ratios found in Experiments 1 and 2, the percentage of carcass protein has been estimated from moisture
507
determinations for the various treatment groups. The estimated and determined values for Experiments 1 and 2 are shown in Tables 2 and 5, respectively. If the combined quantity of fat and protein in the carcass is expressed as a ratio of dry carcass weight, a relatively constant value is obtained. One can then use this relationship to estimate percentage carcass fat from the following equation: kW-P Percent carcass fat = X100: W where k = a determined constant (fat+ protein as a ratio of dry weight), P = gm. of carcass protein and W = dry weight of carcass. Using the above equation the fat contents of the carcasses have been estimated for Experiments 1 and 2 using the estimated protein values; these data are also shown in Tables 2 and 5 along with the determined values. The results indicate that the carcass protein values calculated from carcass moisture agree very closely with determined values. This is further confirmation of the reliability of using the simplified method of Bender and Miller (1953b) for estimating carcass protein. In general, the estimated fat values agreed well with the determined values and indicate the feasibility of using such a simplified procedure in obtaining gross carcass fat estimates of poultry. The correlation between the determined and the estimated values was determined for both protein and fat in both experiments. The four correlation coefficients calculated without regard to treatment or replication groupings ranged from 0.86 to 0.97. It must be kept in mind, however, that as with the water to nitrogen ration, the "k" value used to estimate fat may vary with the age and perhaps the strain of bird. Thus a value may have to be calculated from actual
508
J. D. SUMMERS, S. J. SLINGER AND G. C. ASHTON TABLE 7.—Percentage carcass fat {dry weight basis)1 Carcass fat (%)
Dietary protein (%)
Determined
Estimated
12 16 18 21 27
34.6 29.4 25.5 23.0 21.8
34.4 29.7 25.7 22.9 19.1
1
Data from Table 3 of Summers and Fisher 1961.
determinations for the age and type of bird one wishes to work with. Using the data of Summers and Fisher (1961, Table 3), with birds of the same age as those used in Experiment 1 of the present study, and employing the same "k" value, the percent carcass fat has been estimated. The results are shown in Table 7 and demonstrate that the estimated fat values agree quite well with the determined values; this is further evidence that the method can be used with a reasonable degree of reliability.
content of the diet exerted a greater effect in lowering the fat and increasing the protein content of the female than the male carcass. The results indicate that the sexes should perhaps be reared separately and fed different diets if carcasses of constant composition are to be obtained. A method is presented that enables an estimate to be made of carcass composition from a simple moisture determination. Carcass protein is calculated from moisture by taking advantage of the constancy of the water :nitrogen ratio in the carcass. Because of the constancy of the ratio of total protein and fat to carcass dry matter it is then possible to estimate the fat content of the carcass with a high degree of precision. ACKNOWLEDGEMENT
Grateful acknowledgement is made to Dr. J. Singh, Physics and Mathematics Department for advice on statistical analyses. REFERENCES
SUMMARY
Experiments were conducted to determine the influence of a wide range of protein and energy levels on weight gain, feed efficiency and carcass composition using both semi-purified and practical diets. In general, carcass protein was increased and carcass fat was decreased in a linear manner with increasing levels of dietary protein. Conversely, increasing levels of dietary energy resulted in decreased carcass protein and increased carcass fat. Practical diets were used for the production of "junior" broilers and the observations recorded are of commercial significance. While little or no improvement in weight gain was achieved by increasing the protein level beyond 20 percent, marked changes in carcass composition were noted. Raising the protein
Bender, A. E., and D. S. Miller, 1953b. Constancy of the N/H2O ratios of the rat and its use in the determination of the net protein value. Biochem. J. 5 3 : V H - V n i . Bogdonoff, P. D., J. N. Hensen and G. W. Thrasher, 1961. The effect of oral administration of hormones known to affect carcass composition. Poultry Sci. 40: 1637-1644. Combs, G. F., and E. J. Robel, 1962. Effect of energy intake on body composition and sulfur amino acid requirements for maintenance and growth of chicks. Poultry Sci. 41: 1636. Crampton, E. W., G. C. Ashton and L. E. Lloyd, 1954. Improvement of bacon carcass quality by the introduction of fibrous feeds into the hogfinishing ration. J. Animal Sci. 13: 327-331. Donaldson, W. E., G. F. Combs and G. L. Romoser, 1956. Studies on energy level in poultry rations. 1. The effect of calorie-to-protein ratio of the ration on growth, nutrient utilization and body composition of chicks. Poultry Sci. 35:1100-1105. Donovan, G. A., and W. C. Sherman, 1960. Analysis of the growth pattern and the body composition of chickens implanted with diethylstitbestrol. Poultry Sci. 39: 757-765.
CARCASS COMPOSITION Finney, D. J., 1962. Probit Analysis. Cambridge University Press, London. Haecker, T. L., 1920. Investigations in beef production. Minnesota Agr. Exp. Stat. Bui., 193. Hartley, H. 0., 1955. The sequential F test for multiple discussion on significance in an analysis of variance table. Mimeographed Lecture Notes. Iowa State College. Hill, F., and F. M. O'Carroll, 1962. The chemical composition of pig carcass at pork, bacon, and manufacturing weights. Irish J. Agr. Res. 1: 115, 130. Jukes, H. G., D. C. Hill and H. D. Branion, 1957. Effect of penicillin on the carcass composition of the chicken. Poultry Sci. 36: 423-425. Lawes, J. B., and F. H. Gilbert, 1859. Experimental inquiry into the composition of the animals fed and slaughtered as human food. Trans. Roy. Soc. (London) 2: 493. McMeekan, C. P., 1940. Growth and development
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in the pig with special reference to carcass quality characters. Parts I, II and III. J. Agr. Sci. 30: 276-343, 387^35, 511-569. Nair, K. R., 1948. The studentized form of the extreme mean square test in the analysis of variance. Biometrika, 35: 16-31. Spring, J. L., and W. S. Wilkinson, 1957. The influence of dietary protein and energy level on body composition of broilers. Poultry Sci. 36:1159. Steel, R. G. D., and J. H. Torrie, 1960. Principles and Procedures of Statistics. McGraw-Hill Book Co., New York, N. Y. Summers, J. D., and H. Fisher, 1961. Net protein values for the growing chicken as determined by carcass analysis: Exploration of the method. J. Nutrition, 75: 435-442. Summers, J. D., S. J. Slinger, I. R. Sibbald and W. F. Pepper, 1964. The influence of protein and energy on growth and protein utilization in the growing chicken. J. Nutrition, 82: 463-468.
Duration of Immunity to Infectious Laryngotracheitis 1 L. G. RAGGI AND G. G. L E E Department of Avian Medicine, University of California, Davis, California (Received for publication September 14, 1964)
T
HE duration of immunity to infectious laryngotracheitis (LT) of chickens has not been established conclusively. Beach et al. (1934) demonstrated that immunity to intratracheal challenge lasted for 364 days, the longest period tested. Hofstad (1959) stated: "The duration of immunity has not been demonstrated, although results indicate that birds are protected for at least a year." Hitchner and Winterfield (1960) found that 36 percent of birds vaccinated 1
This work was supported by funds supplied by USDA, CSRS Regional W-5 Project, Regional Research Fund 1770, American Scientific Labs. (Schering Corp.), Madison, Wisconsin, and American Cyanamid Company, Pearl River, New York.
at 8 weeks of age and 100 percent at one week of age became susceptible when challenged at 16 weeks of age. In Australia, Hunt (1962) vaccinated day-old broiler chickens by the feather follicle method and challenged 9 weeks later by contact exposure or by the intratracheal route. Thirteen of 20 birds challenged by contact exposure were susceptible, whereas 9 of 20 were susceptible by the intratracheal route. Sinkovic (1962) noted a significant reduction in immunity from 12| to 14| weeks after vaccination of 10| week-old birds. Shibley et al. (1963) standardized the 146 strain originally isolated by Cover and Benton (1957), and administered one drop of the vaccine onto the conjunctival sac of 13-week-old