Further Studies on the Influence of Dietary Calorie: Protein Ratios on the Weight Gain and Feed Efficiency of Growing Chicks

626

L. E. DAWSON, C. W. HALL, E. H. FARMER AND W. L. MALLMANN

Crawford, A. E., 1955. Ultrasonic Engineering with Particular Reference to High Power Applications. Butterworths Scientific Publications, London, England. Murdock, A., Jr., 1956. Ultrasonics. How it works and what it does. American Machine, 100: 97-104 Thornley, M. J., 1955. Influence of ultrasonic waves

on biological materials. British Electrical and Allied Industries Research Association Technical Report, pp. 2-21. Sabet, T. Y., 1955. Studies on egg washing and preservation. Ph.D. thesis, Michigan State University.

I. R. SIBBALD, S. J. SLINGER AND G. C. ASHTON Departments of Nutrition, Poultry Science and Physics and Mathematics, Ontario Agricultural College, Guelph, Ontario, Canada (Received for publication August 4, 1961)

T

HE chick, like many mammalian species, attempts to consume sufficient feed to satisfy its energy requirements (Dansky and Hill, 1951; Hill and Dansky 1950, 1954; Peterson et al, 1954; Sibbald et al, 1960). Consequently, unless nutrient allowances take account of available energy they are relatively meaningless. General awareness of this problem is illustrated by the adoption of calorie: protein (C/P) ratios in feed formulation. It is generally accepted that for each type of bird in each stage of production there is an optimal dietary C/P ratio. Many experiments designed to determine such ratios have been reported (Berg and Bearse, 1958; Berg, 1959; Creger et al., 1960; Donaldson et al, 1955, 1956, 1958; Douglas and Harms, 1960; Frank and Waibel, 1960; Leong et al, 1955, 1959; Lockhart and Thayer, 1955; Matterson et al, 1955; McDaniel et al, 1957, 1959; Marz et al, 1958; Scott et al, 1959; Sunde, 1956; Thornton and Whittet, 1960; Vondell and Ringrose, 1958). Al-

though these reports contain much useful information they are open to two major criticisms: 1) the findings are based on calculated energy values, and 2) it is assumed that the optimal C/P ratio is that which allows maximum production. The use of calculated energy values may well introduce a major source of error and conclusions based on such data may be only approximations to the truth. The importance of this source of error is magnified when one considers that feed formulators, in applying information derived in this manner, also use calculated energy values. It would therefore appear desirable to use determined energy values at least to establish optimal C/P ratios. The assumption that maximum production is desirable is not necessarily tenable. Many considerations such as the cost of feed ingredients and the price of the product must be taken into account in determining the optimal level of production. Rather than determining a single C/P ratio which allows maximum production it would be preferable to measure

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Further Studies on the Influence of Dietary Calorie: Protein Ratios on the Weight Gain and Feed Efficiency of Growing Chicks

DIETARY CALORIE : PROTEIN RATIOS FOR CHICKS

The present report concerns an attempt to relate dietary protein and M.E. values to a number of biological functions in chicks.

MATERIALS AND METHODS

Source of the data Two forms of data were employed in this study. Theoretical data were used to demonstrate the results of certain statistical procedures. The preponderance of the data were collected from two experiments designed primarily to study the effects of level of dietary inclusion on the M.E. values of wheat, barley and two samples of corn. The M.E. values were determined by the "chromic oxide" technique and form the subject of an earlier report (Sibbald et al., 1962). Each experiment involved a basal diet consisting primarily of soybean oil meal and cornstarch, 10 diets in which succeeding 10% increments of basal were replaced with ground yellow corn and 10 diets, similar to those described previously, in which wheat (exp. 1) or barley (exp. 2) replaced the corn. To all diets were added constant amounts of minerals, vitamins and fat. The first experiment involved 3-week old chicks of the Ames 505 strain, 7 males and 7 females being assigned to each pen. Each diet was allotted to 3 pens and the birds were fed ad libitum for two weeks. Weight change and feed consumption data were recorded. The second experiment was similar to the first but 6-week old, male, strain cross, White Leghorn chicks were used. Protein values for the diets were obtained by the method of Kjeldahl (A.O.A.C., 1955), the conversion factor for nitrogen to protein of 6.25 being employed. In one phase of the study the more accurate conversion factors of Sahyun (1948) were employed. Treatment of the data The results obtained by the feeding of each grain were treated separately. Several statistical procedures were employed

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the relationship between level of production and dietary C/P ratio. Knowledge of such a relationship would allow the feed formulator to select the optimal C/P ratio based on all economic considerations. An attempt to describe such relationships by means of regression equations has been made by Sibbald et al. (1961) who used data collected during a study of the metabolizable energy (M.E.) content of fats; many of the rations contained high levels of fats and consequently the relationships observed might not occur under more practical conditions. Almquist (1958) has criticized the use of the C/P ratio on the basis that he was unable to demonstrate a consistent relationship between the ratio and productive performance. Further, the ratio was criticized because the feed protein appears in both terms of the ratio. Correction of the ratio for the energy donated by the protein reduces the ratio by a constant amount and therefore cannot improve the relationship between the C/P ratio and biological production. It was therefore proposed that the function, nonprotein calorie intake X protein intake, should be used instead of the C/P ratio. Even if this function were more closely correlated with biological responses than the more widely adopted C/P ratio it too would have its disadvantages. In order to measure non-protein calories one must correct for protein by assuming a calculated constant for protein energy; it is doubtful if the total, and highly improbable that the available, energy content of all proteins is the same under all conditions of feeding. Further, it would be difficult to apply the function in ration formulation.

627

628

I . R . SlBBALD, S. J . SLINGER AND G. C. ASHTON

with the main emphasis on regression analysis. The collected data were initially organized to yield 4 dependent variables, the response criteria, and 6 independent variables. The 10 variables were as follows: Dependent Fi = the weight gain per chick during the experiment (gm.) F j = t h e weight gain per 100 gm. of feed consumed (gm.) Fa = the weight gain per 100 Cal. of M.E. consumed (gm.) F 4 = the weight gain per gm. of protein (NX6.25) consumed (gm.) 20

.X\ = the Calorie:protein ratio of the diet, defined as the Calories of M.E. per 1 lb. of feed divided by the percentage of protein therein. Xi=the M.E. per gm. of feed (Cal.) X 3 = the M.E. consumed per bird (Cal.) Xi = the protein per 100 gm. feed (gm.) X5 = the protein consumed per bird (gm.) X 6 = the average weight of the bird (gm.), defined as the initial weight+final weight+ (2X midexperiment weight) divided by 4.

Specific data processing methods will be described in the appropriate sections of this report. RESULTS AND DISCUSSION Weight gains T h e relationship between weight gains (Fi) and dietary C / P ratios (Xi) is demonstrated by the 4 regression lines and equations of Figure 1 while other pertinent statistics are presented in Table 1. T h e diets containing either wheat or TABLE 1.—The relationship between weight gain and dietary C/P ratio Maxima Grain 1

Wheat Corn (a) Barley Corn (b) 1

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r

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X1

222 218 249 260

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Number of observations for each grain was 33.

to

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FIG. 1. The relationships between chick weight gains and dietary calorie:protein ratios.

corn (a) were fed to equal numbers of male and female chicks of the Ames 505 strain between the ages of 3 and 5 weeks whereas the rations containing either barley or corn (b) were fed to male White Leghorn chicks between the ages of 6 and 8 weeks. This difference in age and genetic background helps to explain the differences in the maximum weight gains made by the chicks receiving the 4 grains. I t is noticeable t h a t the weight gain curves for the older birds tend to be horizontal at low C / P ratios whereas the curves for the younger birds have more clearly defined peaks and tend to be lower a t low ratios; this suggests t h a t high protein intakes m a y limit weight gains by young birds but have little or no deleterious effect upon older chicks. The squared multiple correlation coefficients (i?2) presented in Table 1 were consistently and markedly greater than the squared simple r values demonstrating t h a t the distribution of the data was curvilinear rather t h a n linear. This may explain why Almquist (1958) detected only a negligible correlation between

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Independent

629

D I E T A R Y C A L O R I E : P R O T E I N R A T I O S TOR CHICKS

The loci of the maxima of the fitted weight gain curves are indicated in Table 1. I t is remarkable t h a t the C / P ratios which allowed maximum gains are higher for the young birds t h a n for the older chicks; this is contrary to accepted theory and may be explainable on a genetic basis. Further, it is notable that between grains fed to similar chicks there was a marked difference in the C / P ratios which allowed maximum gains. A possible explanation of some of the peculiarities may lie in the nature of the curve-fitting. I t is true that when polynomials as high as the fifth power were fitted to the data no significant improvement was noted in the multiple correlation coefficients; however, two theoretical examples will be used to demonstrate a possible source of misinterpretation. In Figure 2a an a t t e m p t has been made to illustrate the type of weight gain curve

which might occur if birds were able to catabolize excess protein and to use it as a source of energy with no reduction in gain. Weight gains would be maximal until a C / P ratio was reached at which energy needs were fulfilled before an adequate amount of protein was consumed; a distinct breaking point ( C / P 50) might then be observed in the curve. Figure 2a illustrates the results of fitting both linear and quadratic regressions to the data. The simple correlation coefficient is highly significant but it is immediately apparent t h a t the linear regression line does not correctly describe the theoretical data. T h e multiple correlation

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C / P ratios and growth functions. T h e high degree of association between the variables illustrated by the R2 values indicates t h a t predictions of chick weight gains from a knowledge of dietary C / P ratios may have a relatively high degree of precision. The superiority for growth of corn to both wheat and barley at relatively high C / P ratios (80-100) is demonstrated quite clearly by Figure 1. The superiority must have resulted from some extracaloric property of the grain possibly amino acid balance or nutrient density. When the protein values for the diets were recalculated using the conversion factors of Sahyun (1948) instead of the constant 6.25 the locations of the curves were changed slightly b u t the relative positions of the curves remained relatively constant. I t was therefore decided t h a t all future calculations would involve the 6.25 conversion factor as this is widely accepted and more simple to apply.

630

I. R. SlBBALD, S. J. SLINGER AND G. C. ASHTON

theoretical outline of Figure 2b whereas the older birds behaved in a manner similar to that described in Figure 2a. The graphs of Fig. 3 indicate that a relatively wide range of dietary C/P ratios allowed maximum weight gains and that, in consequence, visual interpretation, guided by the positions and shapes of the curves, is called for. It would appear that chicks between the ages of 3 and 5 weeks (Fig. 3a and b) cannot make maximum weight gains if the dietary C/P ratio is less than 45; the maximum C/P ratio to allow full expression of weight gain potential appears to lie between 65 and 80. For chicks aged 6 to 8 weeks low dietary C/P ratios appeared to have no limiting effects while the highest C/P ratio to allow maximum gains was between 50 and 60 for the barley diets and 60 and 70 for the corn (b) diets. Differences in average chick weights (X6) during the experimental periods might have influenced responses to dissimilar dietary C/P ratios; consequently, this additional variable was included in the regression analysis. The sums of squares due to regression associated with X6 were found to be relatively small and thus the multiple correlation coefficients were not noticeably raised (Table 2). When the appropriate mean values were substituted for X6 in each of the 4 regression equations it was found that the shapes and locations of the curves described thereby were almost identical with the regression lines of Figure 3. Thus the inclusion of the additional variable added little of practical value to the previous findings. When the regressions of weight gain (Fi) on dietary C/P ratios (Xi) were adjusted for variations in dietary M.E. (X^ or protein (X4) concentration the squares of the multiple correlation coefficients were only slightly higher than

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coefficient is extremely high but again it is apparent that the maximum of the curve which occurs at a C/P ratio of 34.5 is misleading. The interpretation of the statistical curve suggests that maximum weight gains occur at only one C/P ratio and that at ratios above or below this, weight gains would be sub-maximal. From the theoretical data it may be seen that maximum gains occur at C/P ratios up to 50; the difference in cost between diets with C/P ratios of 34.5 and 50.0 is generally quite substantial. Figure 2b illustrates an alternative. The theoretical data are similar to those in Figure 2a but indicate a situation in which the excess protein consumed, when diets of low C/P are fed, proves to be a weight gain depressor. The theoretical data suggest that a slight excess of protein is not deleterious consequently the curve has a distinct plateau. Again the linear regression line is obviously misleading. The quadratic curve fits the data extremely well but the position of the maximum (C/P 39.9) is again misleading since a diet of C/P 50 would allow gains as great as those of a ratio of C/P 40. From the curves of Figures 2a and 2b it is apparent that the basic need is to determine the location of the breaking point of the weight gain curve. Examination of the raw data suggests that plotting both the data and the quadratic curves on a semi-logarithmic scale would more clearly reveal the breaking points. The results of treating the data in this manner are illustrated in Figures 3a, b, c and d. The 4 graphs of Figure 3, based on actual data, are more indicative of the actual responses of chicks to rations of varying C/P ratios than are either the graphs of Figure 1 or the data of Table 1. The curves and raw data suggest that the younger chicks (wheat and corn (a)) responded in a manner similar to the

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Correction of the weight gain curves to -RV*i*il (Table 2). The shapes of the regression lines were however changed indi- either a constant M.E. (X3) or protein cating that nutrient density in the feed TABLE 2.—Squares of the multiple correlation coefficients pertaining to weight gains may influence chick responses to variations in C/P ratios. The derived regresCorrelation1 D.F. Wheat Corn (a) Barley Corn (b) sion lines are presented in Figure 4. For 30 0.912 0.789 0.960 0.919 each set of data the mean dietary M.E. XiXu 29 XiXnXt 0.938 0.854 0.949 0.970 29 0.926 0.844 0.920 0.968 or protein value was employed as a con- XiXnXt 29 X1X11X4 0.912 0.792 0.920 0.967 28 stant; differences between means par- XiXnXzXi 0.942 0.916 0.953 0.977 XIXUXIXB 28 0.938 0.857 0.949 0.975 28 tially explain variable changes occurring XiXnXjXtt 0.959 0.900 0.924 0.967 27 XiXiiXtXuXe 0.960 0.900 0.952 0.976 28 when either X2 or X4 was held constant. XiXnXiXw 0.956 0.924 0.923 0.972 27 XiXuXiXiiXe 0.957 0.924 0.952 0.980 29 The incorporation of X6 into these analy- XiXviXt 0.696 0.764 0.841 0.876 29 0.884 0.777 0.897 0.899 ses had little effect on the precision of XtXuXa 27 X2XisXiXuXi 0.899 0.787 0.920 0.901 Y\ or the shape of these curves. 1 Use of double subscript represents the squared term.

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I. R. SIBBALD, S. J. SLINGER AND G. C. ASHTON TABLE 3.—Regression equations and constants used to derive the curves of Figure 5 Constant 1 substituted Grain

Equation 2

2

X3

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1,496 —

— 140

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Fi=142.5+2.793Xi-0.03424Xi -0.07303X 3 +0.00005680X 3 Fi=-306.6+6.686Xi-0.04439X l 2 +3.220X 6 -0.007496X s 2

Corn (a)

F 1 = -101.3+0.2159X 1 -0.009076X 1 2 +0.3017X 3 -0.00005647X 3 2 Fi=-433.6+5.353Xi-O.O228OX 1 2 +4.822X 6 -0.O1183X 6 2

1,574 —

— 130

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F 1 = 723.8+2.323X 1 -0.O3O36X 1 2 -0.5528X 3 +O.0001444X 3 2 F 1 =-65.72+4.858Xi-0.03909X, 2 +1.233X 6 -0.001904X s 2

2,078 —

— 202

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2,390 —

— 194

1

(X6) intake caused a number of interesting changes to occur. The regression equations are presented in Table 3 while the curves derived therefrom are described graphically in Figure 5. When the M.E. intakes of all treatment groups receiving a particular grain were held constant (statistically) the peaks of the curves moved to the left {i.e., towards lower dietary C/P ratios) suggesting either that energy obtained by the catabolism of protein is not a complete substitute for nonprotein energy or that a high protein intake may depress feed consumption. This may be explained by the stress resulting from the need to excrete large quantities of nitrogenous materials via the kidneys. The effect of holding X3 constant is particularly noticable when Figures 3a and 3b are compared with the appropriate curves of Figures 5a and 5b. Whereas the former regression lines show evidence of growth retardation at low C/P ratios the latter provide little evidence of such a condition. Figures 3c and 3d correspond closely with the appropriate curves of figures 5c and 5d. The effects of holding protein intakes constant, at the mean for the data concerned, are more noticeable. All adjusted curves (Figures 5a, b, c and d) show dis-

tinct peaks which are markedly to the right {i.e., at higher dietary C/P ratios) of the breaking points revealed in Figure 3. It is apparent that the major factor limiting the weight gains of chicks fed rations with high C/P ratios is a deficiency of protein resulting from the fact that such birds tend to eat to satisfy their energy requirements and therefore cease eating before they have consumed sufficient protein to allow them to fully express their weight gain potential. The curvilinear form of the regression lines may reflect a change in protein quality with changes in C/P ratio. In order to increase the C/P ratios grain was used to replace a basal diet containing soybean oil meal and methionine; a change in the proprotions of basal and cereal would undoubtedly result in a change in the biological value of the total protein mixture. The regulation of variability in the average body weights (Xe) of the chicks in the analyses concerning Xi and X3 or Xi and X$ had little effect upon the shapes or locations of the regression lines; consequently, no equations or curves are presented. The appropriate R2 values are reported in Table 2. Regression analysis conducted to measure the association between weight gain

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The constant selected was the mean for the data involved in the calculations.

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(Fi) and dietary M.E. content (X2) revealed that predictions of Y\ based on a knowledge of X2 would not be as precise as when Xi was employed. Similarly predictions based on a knowledge of dietary protein content (Z 4 ), though superior to those based on X2, were less precise than those employing Xx. When both X2 and Xi were employed the precision of the Yi values predicted was similar to that obtained when the C/P ratio was employed (Table 2); however, the lack of super-

iority suggests that the ratio (X{) is as satisfactory as the two individual variables. Feed efficiency The efficiency of feed utilization was measured in three ways, viz.: gain per 100 gm. of feed (F 2 ), gain per 100 Cal. M.E. (F3) and gain per gm. of protein (Y^. Although all 3 variables will be considered the major emphasis of this section will be on F 2 .

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Before considering the influences of the X variables on F 2 , F 3 and F 4 it is of interest to note the high degree of association between F 2 and Y\; this is demonstrated in Figure 6. It is apparent that, the greater the gain in a given period of time, the less the amount of feed necessary to produce each unit of the weight increase. This is quite logical if one assumes that the nature of the weight gain is relatively constant and can be explained by the fact the proportion of the ingested feed employed for body maintenance de-

creases as feed intake and rate of gain increase. In view of the high degree of association between F 2 and Y\ it was anticipated that the influences of many of the X variables on F 2 would be similar to those described in the preceding section. Two other points are revealed by Figure 6. The slopes of the wheat and corn (a) lines are very similar as are the barley and corn (b) lines; however, the heights and slopes of the two pairs of regression lines are different. This suggests that young, dual purpose strain birds (wheat and

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corn (a)) make greater gains on a unit quantity of feed than do older birds of an egg laying strain (barley and corn (b)); evidence which supports the earlier discussion concerning maintenance requirements. The greater feed efficiency at high weight gains has a special economic significance. From the information of Figure 3 it might be decided that the C/P ratio which allows maximum weight gains is not economic when the cost of feed ingredients and the prices of the products are considered; however, Figure 6 provides an argument in favour of maximum gains. The selection of dietary C/P ratios should be based on many factors. Figure 7 demonstrates the relationship between F 2 and Xt for each of the 4 grains; as anticipated the curves bear a

close resemblance to those of Figure 3. The similarity between the two sets of curves coupled with the high degree of association between the Y variables (Fig. 6) suggests that from a purely practical standpoint sufficient information is contained in figures 3 and 6 to make Figure 7 unnecessary. In interpreting Figure 7 cognizance must be taken of the scatter of the raw data as well as of the locations of the regression curves because the latter may be misleading when considered alone (see Fig. 2). It is of interest to note the apparent depression in feed efficiency at low C/P ratios when young birds were employed (wheat and corn (a)); this parallels the observation made with the weight gain data. Comparisons of feed efficiencies when measured in terms of gain per unit weight

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of feed can be extremely misleading if the nutrient density of the feeds being considered should vary. In the present study an illustration of the problem was provided by Figure 8. If the protein or M.E. content of the feeds is held constant (statistically) the shapes and locations of the regression curves relating F 2 to Xi are changed considerably; however, the corrected curves are also misleading for if the dietary M.E. (X2) is held constant then the dietary protein content (X4) must vary widely if the observed range in Xi is to occur; the same is true if X4 is held constant. Naturally when the C/P ratio is being considered it is impossible to hold both protein and M.E. constant for this would yield only 1 C/P value. This set of circumstances suggests that

an impasse has been reached unless the components of the ratio are considered separately and a surface (more than 1 dimension) is fitted to the data. In view of the high degree of precision (R2) with which weight gains and gain-feed ratios may be predicted from C/P data and bearing in mind the high degree of association between Y\ and F 2 there seems to be little practical value in taking this course. This is especially true when the errors associated with changes in the biological values of the proteins of the feeds are considered. Fitting a surface of the type described to data derived from feeding diets of constant protein quality might reveal very useful information; however, it would of necessity involve feeding diets containing ingredients not

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40

D I E T A R Y CALORIE : P R O T E I N R A T I O S FOR CHICKS

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FIG. 8. The relationships between gain:feed ratios and C/P ratios when dietary M.E. or protein concentration is held constant.

generally employed under practical conditions. Further, the application of such a surface in feed formulation might be questioned unless the quality of the protein remained constant. The relationship between the gain per 100 Cal. M . E . and the dietary C / P ratio is shown in Figure 9. I t is apparent t h a t at C / P ratios which allow maximum weight gains (Fig. 3) the gain per unit of M . E . is relatively constant within a particular type of grain. When gains are limited by high dietary C / P ratios gain per unit of energy tends to decrease in a similar manner. The regression curves and raw data relating gain per gm. of protein to Xx are presented in Figure 10. As C / P ratios in-

creased, gain per gm. of protein increased in an apparently linear manner, suggesting t h a t the excess protein consumed at low C / P ratios was being catabolized as a source of energy. Above certain C / P ratios the efficiency of protein utilization decreased rapidly because the amount of protein consumed decreased relative to the amount required for body maintenance. SUMMARY

D a t a collected from two experiments were analysed, primarily b y regression analyses, in an a t t e m p t to relate dietary protein and metabolizable energy values to a number of biological functions in chicks. Although open to a number of

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REFERENCES Almquist, H. J., 19S8. Calory and protein intake as related to growth. Proc. Soc. Exp. Biol. Med. 97:353-355. Berg, L. R., 1959. Protein, energy and method of feeding as factors in the nutrition of developing White Leghorn pullets. Poultry Sci. 38: 158-165. Berg, L. R., and G. E. Bearse, 1958. Protein and energy studies with developing White Leghorns pullets. Poultry Sci. 37: 1340-1346. Creger, C. R., R. H. Mitchell, R. L. Atkinson and J. R. Couch, 1960. The effects of different protein and energy levels on the growth of turkey poults. Poultry Sci. 39: 1350-1354. Dansky, L. M., and F. W. Hill, 1951. The effect of energy level and physical nature of the diet on growth and body composition of chicks. Poultry Sci. 30: 910. Donaldson, W. E., G. F. Combs, G. L. Romoser and W. C. Supplee, 1955. Body composition, energy intake, feed efficiency, growth rate and feather condition of growing chickens as influenced by the calorie-protein ratio of the ration. Poultry Sci. 34: 1190. Donaldson, W. E., G. F. Combs and G. L. Romoser, 1956. Studies on energy levels in poultry rations. 1. The effect of calorie-protein ratio of the ration on growth, nutrient utilization and body composition of chicks. Poultry Sci. 35:1100-1105. Donaldson, W. E., G. F. Combs and G. L. Romoser,

1958. Studies on energy levels in poultry rations. 3. Effect of calorie-protein ratio of the ration on growth, nutrient utilization and body composition of poults. Poultry Sci. 37: 614-619. Douglas, C. R., and R. H. Harms, 1960. Effects of varying protein and energy levels of broiler diets during the finishing period. Poultry Sci. 39: 1003-1008. Frank, F. R., and P. E. Waibel, 1960. Effect of dietary energy and protein levels and energy source on White Leghorn hens in cages. Poultry Sci. 39: 1049-1056. Hill, F. W., and L. M. Dansky, 1950. Studies of the protein requirement of chicks and its relation to dietary energy level. Poultry Sci. 29: 763. Hill, F. W., and L. M. Dansky, 1954. Studies of the energy requirements of chickens. 1. The effect of dietary energy level on growth and feed consumption. Poultry Sci. 33: 112-119. Leong, K. C , M. L. Sunde, H. R. Bird and C. A. Elvehjem, 1955. Effect of energy: protein ratio on growth rate, efficiency, feathering and fat deposition in chickens. Poultry Sci. 34: 1206. Leong, K. C , M. L. Sunde, H. R. Bird and C. A. Elvehjem, 1959. Interrelationships among dietary energy, protein and amino acids for chickens. Poultry Sci. 38: 1267-1285. Lockhart, W. C , and R. H. Thayer, 1955. Energyprotein relationships in poult turkey starters. Poultry Sci. 34: 1208. Matterson, L. D., L. M. Potter, L. D. Stinson and E. P. Singsen, 1955. Studies on the effect of varying protein and energy levels in poultry rations on growth and feed efficiency. Poultry Sci. 34: 1210. McDaniel, A. H., J. D. Price, J. H. Quisenberry, B. L. Reid and J. R. Couch, 1957. Effects of energy and protein level on cage layers. Poultry Sci. 36: 850-854. McDaniel, A. H., J. H. Quisenberry, B. L. Reid and J. R. Couch, 1959. The effect of dietary fat, caloric intake and protein level on caged layers. Poultry Sci. 38: 213-219. Mraz, F. R., R. V. Boucher and M. G. McCartney, 1958. The influence of dietary energy and protein • on growth response in chickens. Poultry Sci. 37: 1308-1313. Peterson, D. W., C. R. Grau and N. F. Peek, 1954. Growth and food consumption in relation to dietary levels of protein and fibrous bulk. J. Nutrition, 52: 241-257. Sahyun, M., 1948. Proteins and Amino Acids in Nutrition. Reinhold Publishing Corporation, New York, p. 136. Scott, M. L., F. W. Hill, E. H. Parsons, Jr. and

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criticisms it was found that a knowlege of dietary calorie: protein (C/P) ratios could be used to predict weight gains, gain:feed ratios, gain: energy ratios and gain p r o tein ratios. Correction for variations in the body weights of the chicks did little to improve the precision of predictions. Fitting parabolic curves to data may yield misleading information and it is suggest that visual interpretation, based on an examination of the raw data and the fitted curve, should be adopted. Dietary nutrient densities were found to exert profound influences upon response curves. A high degree of association between gain:feed ratios and chick weight gains was apparent. The weight gains of young chicks were restricted when rations of low C/P were fed; however, older chicks were able to tolerate high protein diets.

639

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I . R . SlBBALD, S. J . SLINGER AND G. C. ASHTON

J. H. Bruckner, 1959. Studies on duck nutrition. 7. Effect of dietary energy:protein relationships upon growth, feed utilization and carcass composition in market ducklings. Poultry Sci. 38: 497-507. 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. Sibbald, I. R., S. J. Slinger and G. C. Ashton, 1961. The influence of dietary calorie: protein ratios on the weight gain and feed efficiency of growing chicks. Poultry Sci. 40: 308-313. Sibbald, I. R., S. J. Slinger and G. C. Ashton, 1962. Factors affecting the metabolizable energy con-

tent of poultry feeds. 5. The level of protein and of test material in the diet. 6. A note on the relationship between digestible and metabolizable energy values. Poultry Sci. 41: 107-116. Sunde, M. L., 1956. A relationship between protein level and energy level in chick rations. Poultry Sci. 35:350-354. Thornton, P. A., and W. A. Whittet, 1960. Protein requirement for egg production as influenced by management, genetic background and dietary energy level. Poultry Sci. 39: 916-921. Vondell, R. M., and R. C. Ringrose, 1958. The effect of protein and fat levels and calorie to protein ratio upon performance of broilers. Poultry Sci. 37: 147-151.

TILL M. HUSTON, HARDY M. EDWARDS, JR. AND JOANNA J. WILLIAMS Poultry Division, University of Georgia, Athens, Ga. (Received for publication August 8. 1961)

T

HERE is considerable evidence that the thyroid gland of birds responds to environmental temperature changes (Hoffman and Shaffner, 1950; Joiner and Huston, 1957; Premachandra et al., 1959; Heninger et al., 1960; Stahl and Turner, 1961). Studies have shown a relationship between thyroxine secretion rate and growth (Schultze and Turner, 1945; Glazener and Shaffner, 1948; Glazener et al., 1949; Hoffmann, 1950; Premachandra et al., 1958). Since growth rate and egg production are retarded by high environmental temperatures, an experiment was designed to measure the thyroid secretion rate of fowl of different ages exposed to different environmental temperatures. 1

Journal Series Paper Number 189. College Experiment Station, University of Georgia, Athens, Georgia.

MATERIALS AND METHODS

The birds used in all three trials were held from day of hatch in a room with a constant environmental temperature of either 66°F. or 88°F. All birds were grown in battery brooders until four weeks of age. Brooder temperatures were maintained at 95°F. the first week and dropped 5°F. each week. At the end of four weeks, all birds were transferred to growing batteries and no heat was provided. After intraperitoneal dosing with I1S1 measurements of thyroid radioactivity were taken at three different ages in order to establish the effects of age and acclimation upon thyroxine secretion. Trial 1. In this trial, forty White Plymouth Rock males, twelve weeks of age, were used. Twenty of these birds had been grown from day of hatch in an environmental temperature of 88°F. and

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The Effects of High Environmental Temperature on Thyroid Secretion Rate of Domestic Fowl1