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An Analysis of Feed Efficiency among Breeds of Chickens and Its Relationship to Rate of Growth1
An Analysis of Feed Efficiency among Breeds of Chickens and Its Relationship to Rate of Growth 1 THOMAS W. FOX 2 AND B. B. BOHREN Purdue University, Lafayette, Ind. (Received for publication August 28, 1953)
A
1
Journal paper No. 741 Purdue University Agricultural Experiment Station. 2 Present address—Poultry Department, University of Massachusetts, Amherst, Mass.
After adjusting the date it was indicated that some slight improvement in the physiological efficiency had been made in the early years of selection but uncontrolled environmental conditions reduced the reliability of this improvement. Dickerson and Grimes (1947) summarized the results of eight years of selection for high and low feed requirements in Duroc swine. These data afforded estimates of heritabilities and genetic correlations between traits. Because of the lower heritability of efficiency in comparison to growth rate and the high genetic correlation between the two characters, it was concluded that selection on the basis of growth alone would be equally productive in improving efficiency as selection for efficiency, and more effective in improving growth rate. The present studies were designed to determine the relationship between growth rate and feed efficiency in growing chickens and to ascertain the differences existing among breeds of fowl after correction for differences in growth rate. EXPERIMENTAL PROCEDURE
The four breeds of chickens selected for these studies were New Hampshires, Dark Cornish, Purdue Dominant Whites, and S. C. White Leghorns. On the day of hatching, random samples of 50 individuals of each of the four breeds were placed in starting batteries. At four weeks of age all birds were sexed and again a random sample consisting of twelve males and twelve females of each breed was drawn. This resulted in 96 birds for indi-
549
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GENERAL observation has been that the more rapid the growth the higher the feed efficiency or body weight increase per unit of feed. This has been explained on the basis of the slower growing individuals possessing a proportionately larger maintenance tax. There is also some evidence in poultry, although meager and conflicting, that feed efficiency per se, independent of the rate of growth is inherited. Hess and Jull (1948) reviewed much of the previous information concerned with the genetic aspects of the efficiency complex. They indicated that in general, the faster the groi th, the greater the efficiency. However, they observed some evidence that feed efficiency is inherited independently of growth. Lush (1936) described the Danish system of progeny testing swine on the basis of rate of gain and also food requirements per unit of gain. An analysis of the yearly progress from 1922 to 1929 was made and it was found that both rate and economy of gain had been improved. Improvement had occurred in the "practical efficiency," but it was opined that most of this improvement might have been associated with the improvement in growth rate and the approach to market weight at an earlier age. A correction factor was established for this difference in market age to estimate the improvement in what was termed the "physiological efficiency."
550
T. W. FOX AND B. B . BOHREN
Analysis of variance was used to compare differences in average body weight and average efficiency due to breeds, sex, trial and the possible first and second order interactions among these sources of variation. The equation E = C —kw, a derivative of the law of diminishing increment, was used to relate efficiency to average weekly body weight. The analysis of covariance provided tests of significance for differences among regression coefficients obtained from studies relating efficiency to average weekly body weight and also average efficiency to average body weight for the entire six week experimental period. The analysis of covariance was utilized for tests of significance of efficiency adjusted for differences in average body weight. RESULTS AND DISCUSSION
In order to clarify the presentation of the data derived from these experiments, a few definitions are necessary. "Total gain" or "gain" will refer to the absolute gain in body weight during the six-week period from four to ten weeks of age. It is believed that total gain provides a useful measure of growth because (1) all birds in these studies are compared during the same age interval and (2) the growth curves from four to ten weeks of age, as observed in these studies roughly approximate linearity. "Average weekly body weight" is the average body weight of any given weekly period. "Average body weight" is considered as the average body weight for the six-week period from four to ten weeks of age. "Gross efficiency" or "efficiency" is the ratio of absolute gain/feed consumed. This definition of gross efficiency is not entirely comparable with that proposed by Brody (1934) as the caloric composition of the gains and feed are not considered. As the same feed was used throughout these experiments little would be gained by transforming the
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vidual cage studies for the six-week period from four to ten weeks of age. The 96 individuals selected were stratified into four blocks of 24 birds consisting of three individuals of each sex and of each breed. The birds were then randomly assigned to cages within blocks. These 96 individuals composed a replication or trial. Four replications of this design were run during the following periods: Trials one and two, April through July, 1951; and Trials three and four, April through July, 1952. All birds were fed the Purdue Experiment Station all-mash starter ad libitum both prior to and during the test period. After a preliminary period of all-night lights to allow the birds to become familiar with the feeding devices, twelve hours of light were used. The only apparent environmental difference between trials was season. Trials two and four were conducted during the early summer months and the temperature of the laboratory became rather high for short periods of time. An exhaust fan tended to alleviate this condition and prevented overheating. Starved weekly weights and the corresponding weekly feed consumption were obtained on a Toledo gram scale. The individual feed boxes were constructed with high sides and a wide retaining lip. Also, rectangular pieces of \ inch mesh hardware cloth were placed on top of the feed to prevent mash from being billed from the feeders. The individual feeders were placed on an eight-inch apron in front of the individual cages from which spilled feed could be recovered. The data obtained for these studies were individual weekly starved weights from which weekly gains were calculated and weekly individual feed consumption corresponding to these gains. These data were then used to calculate gross efficiency (gain/feed) for each weekly period and also average gross efficiency for the entire six week period.
551
F E E D E F F I C I E N C Y AND G R O W T H
Breeds (Ave. over sex and trial) 4000
II o c
.3000 W.L. N.H. ,2000hD.C. D.W.
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E= .3003-.000234(X-6l3) E = .3399-.000145 (X-753) E = .3294-.000111 ( X - 7 5 2 ) E= .3266-.000095(X-727) 400"
600" 800" 1000 Average Weekly Body Weight (grams)
200
"400
605" 800 iOOO" Average Weekly Body Weight (grams)
1255"
1450
1200
1400
.4000c -o
2. •
.2000
.4000-
III £
.3000
UJ
.2000 200
400
600
800
1000
Average Weekly Body Weight (grams) FIG. 1. Average regressions of efficiency on average weekly body weight due to breed, sex and trial.
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"200
552
T . W . FOX AND B . B . BOHREN
feed consumed to calories as total caloric intake would be a function of the weight of the total feed consumption. However, in the case of gains, differences in carcass composition could change their caloric value and cause a discrepancy between the two measures of gross efficiency. "Adjusted efficiency" refers to the efficiency adjusted for differences in average body weight for the four-to-ten-week age period.
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lated to zero body weight the values obtained would be comparable to the " C " values proposed by Hendricks, Jull and Titus (1931) and utilized by Hess et al. (1941, 1948). It is believed that these " C " values are satisfactory when considered as a parameter of the regression equation but a questionable interpretation is involved when physiological significance is attributed to them. This is especially true in these experiments in which the regression coefficients were obThe Regression of Gross Efficiency on Aver- tained for the six-week period from four age Weekly Body Weight to ten weeks of age. The initial procedure in analyzing feed The average regressions shown in Figure efficiency data from these experiments 1 could be used to estimate the efficiency consisted of determining the regression at any body weight over the rage of the of gross efficiency on average weekly body data obtained in these studies. The diffiweight for trials one and two using the culty encountered in comparing these method proposed by Hendricks (1931) and values, e.g. at 800 grams body weight, is described by Hankins and Titus (1939). the fact that time has not been considered. As data were available for each individual Thus, if two breeds are compared at 800 studied, the 192 individual regression grams average body weight, the length coefficients for trials one and two were of time required for each to obtain this calculated. An analysis of covariance was weight is not involved. Therefore, the conducted to determine if the average group reaching this weight in the shortest breed, sex, or trial regressions differed interval of time will have the lower mainsignificantly. tenance tax. Highly significant differences (P<.01) were obtained among the average re- The Relationship of Growth to the Regression gressions of efficiency on average weekly of Gross Efficiency on Average Weekly body weight attributable to breeds and Body Weight trials while the effect of sex was nonThe three values that establish the significant. In considering these results it graphical location of the average regresmust be emphasized that they apply to sions presented in Figure 1 are the mean the experimental period of from four to body weight, mean gross efficiency and ten weeks of age. the slope of the line or "k" value relating Figure 1 is a graphical presentation of efficiency to average weekly body weight. the average breed, sex, and trial regres- Once the regression equation has been sion equations relating efficiency to aver- determined all values on the regression age weekly body weight. It will be noted line are also established. that the regression equations were plotted Differences in the slopes or "k" values only over the range of the data from which of the regression of gross efficiency on they were calculated. The origins are lo- average weekly body weight are believed cated at approximately 300 grams body to be greatly influenced by differential weight. If these equations were extrapo- growth patterns during the experimental
FEED EFFICIENCY AND GROWTH
553
TABLE 1.—Comparison of the regression of gross efficiency on average body weight associated with breed, sex, and trial and a measure of growth persistency
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sponding "k" values are accordingly decreased. In the studies of Hess and Jull (1948) evidence for genetic differences in effi"k" values Persistency Source of variation ciency independent of growth was pro(X1000) of growth 1 vided by comparisons of groups of sire Breeds progenies with similar average weights at -.234 57.8 S. C. White Leghorns -.145 60.2 New Hampshires identical ages. This procedure afforded -.111 62.2 Dark Cornish three comparisons involving the progeny -.095 64.0 Dominant White Trials of six sires. Using a derivative of the law One -.183 59.3 of diminishing increment, E = C — kw, Two -.078 63.6 Sex no significant differences in " C " values Males -.128 61.2 were found between sires. However, one Females -.151 61.3 comparison of "k" values, or the decrease relative rate of growth last thre weeks in efficiency with increasing weekly body weight, resulted in a significant difference. relative rate of growth first three weeks ( w 3 —W2) / W2 —Wi This formed the basis of the following -X100 statement, "Since body weight, or rate of 1/2 (w2—w3)/ 1/2 (wi—w2) where: Wi = average weight at 4 weeks of age growth, and time have been removed W2=average weight at 7weeks of age from the consideration, the resulting difW3 = average weight at 10 weeks of age ference must be due to an inherent differperiod. A retardation of growth during ence between these two sires." Another the latter portion of the growth curve possible explanation of this one comparirelative to the early growth will cause a son in which a significant difference in greater negative slope or " k " value. "k" values was observed could be that Evidence for this relationship is shown by the effect of rate of growth was not enTable 1 which provides a comparison of tirely eliminated. Growth rate was asthe average regressions of gross efficiency sumed to be constant because at 536 on average weekly body weight and a grams body weight the average body measure of growth persistency. The weights of the two sire progenies were absolute magnitude of this persistency equal at the same age. Up to this weight measure is irrelevant and would be de- the rate of growth was obviously compendent on the stage of growth from which parable. The "k" values however, were they were calculated. However, the rela- apparently calculated over a longer period tive magnitude of these values are be- of growth and it is possible that the lieved to illustrate the point under con- growth rate of the two groups diverged significantly after the 536 gram weight. sideration. This latent growth differential could influThis persistency measure is a comparience the observed efficiencies at the son of the relative rate of growth for the greater body weights and as the "k" last three weeks with the relative rate values were calculated by the method of of growth for the first three weeks. A least squares, they also would be affected. large value indicates less retardation of The "k" values would have an effect on growth during the latter portion of the all calculated efficiencies over the entire experimental period. Table 1 indicates range of body weights and could possibly that as this persistency measure increases explain the large differences in calculated with breed, sex, and trial the corre-
554
T . W . FOX AND B . B . BOHEEN
.4500
III « u
.3500
.2500
.1500
200
400
600
800
1000
1200
1400
Average Weekly Body Weight (grams) FIG. 2. Average lot regressions (within breed, sex and trial) of efficiency on average weekly body weight and demonstrating the positive regression of average efficiency on average body weight for the period four to ten weeks of age.
efficiency between sire progenies at the 536 gram weight. Relationship Between Mean Body Weight for the Six-Week Period and Mean Efficiency The two remaining parameters of the regression of efficiency on average weekly body weight are the mean body weight and mean efficiency for the six-week experimental period. Figure 2 also presents the graphical relationship existing among the lot regressions (within breed and sex) of efficiency on average body weight in trial one. The same relationship obtained for all four trials. In addition to the negative regressions or "k" values this graph demonstrates the important fact that there exists a positive regression of mean efficiency on mean body weight for the six-week test period. This positive regression also exists be-
tween the mean efficiency and mean body weight of individuals and suggested the possibility of using the methods of covariance as statistical control to estimate the variation in feed efficiency existing after adjustment for differences in average body weight. As the magnitude of the average body weights for the six-week period studied is largely attributable to differences in growth, adjustment of the average efficiencies to the over-all mean body weight would appear to correct for differences in growth. The problem of efficiency independent of growth has been studied by Hess et al. (1948) using pairing or experimental control. This type of experimental control is difficult to obtain. If statistical control could be utilized to alleviate the confounding effect of growth on gross efficiency, additional information on efficiency might be obtained.
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555
FEED EFFICIENCY AND GROWTH TABLE 2.—Mean body weight, mean efficiency, and adjusted efficiencies for the period four to ten weeks of age Mean body weight (gm.)
Mean efficiency
Lot
Regres- sion coef. (X1000) 2
9
Both
cP
9
Both
Trial 1 S.C. White Leghorn New Hampshire Dark Cornish Dominant White Average (Trial 1)
723 894 850 870 834
550 745 735 683 678
636 819 793 776 756
.3177 .3654 .3521 .3465 .3454
.2790 .3339 .3228 .3166 .3118
.2984 .3496 .3375 .3291 .3286
+ + + +
Trial 2 S.C. White Leghorn New Hampshire Dark Cornish Dominant White Average (Trial 2)
648 875 820 831 794
557 686 701 633 644
603 781 760 732 719
.2930 .3353 .3222 .3318 .3206
.2771 .3229 .3056 .2989 .3012
.2851 .3291 .3147 .3154 .3109
+ + + +
Trial 3 S.C. White Leghorn New Hampshire Dark Cornish Dominant White Average (Trial 3)
725 883 925 862 849
554 735 744 694 682
639 809 834 778 765
.3178 .3609 .3646 .3460 .3473
.2900 .3415 .3361 .3269 .3236
.3045 .3512 .3504 .3364 .3355
+ + + +
Trial 4 S.C. White Leghorn New Hampshire Dark Cornish Dominant White Average (Trial 4)
607 893 831 803 784
528 692 709 655 646
568 792 770 729 715
.3105 .3808 .3513 .3407 .3458
.2930 .3430 .3257 .3263 .3287
.3017 .3753 .3385 .3335 .3373
+ + + +
All Trials S.C. White Leghorn New Hampshire Dark Cornish Dominant White Average (All Trials)
676 886 857 842 815
547 714 722 666 663
612 800 789 754 739
.3098 .3606 .3476 .3412 .3398
.2848 .3420 .3226 .3159 .3163
.2973 .3513 .3351 .3286 .3281
+ + + +
.189 .251 .034 .093
.3179 .3295 .3357 .3257
—
—
.248 .135 .087 .166
.3188 .3234 .3129 .3166
—
—
.223 .171 .149 .087 —
.3268 .3392 .3362 .3330
.348 .076 .216 .106
.3612 .3713 .3318 .3346
— .248 .147 .093 .121
—
—
— .3288 .3423 .3304 .3268
—
1
Mean efficiency adjusted for differences in average body weight by use of the within lot regressions of mean efficiency on mean body weight. Adjusted to over-all body weight of 739 grams. 2 Within-lot regression coefficients of mean efficiency on mean body weight.
Before the average regression of mean efficiency on mean body weight could be legitimately used for adjustment purposes it was essential to determine if this regression differed significantly due to breed, sex or trial. An analysis of "covariance of the regression coefficients as shown in Table 2 indicated non-significant trial and sex differences. However, the differences among breeds were highly significant (P <.01) providing substantial evidence that the average breed regressions of mean efficiency on mean body weight differ significantly. Significant differences (P < .05) were obtained among the thirty-
two coefficients associated with groups (within breed, sex, and trial) and might be expected because of the significant breed differences obtained among regression coefficients. It is observed that the S.C. White Leghorn regression coefficients are larger in all trials with the exception of trial one. This provides additional evidence that the average breed regressions were actually different and the significant F-ratio obtained for breeds was not the result of chance variation. The significant differences among breed regressions questions the validity of using the average regression coefficient
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&
Mean adjusted efficiency1 •
556
T . W . FOX AND B . B . BOHREN
• 4000
New Hampshire Dark Cornish
.3500
.3000
Dominant White
-S.C.White Leghorn
.2500
W.Leg. E = N.Hamp E = D.Cornish E = D.White E=
. 2000
400
500
600
700
800
.2973.3513.3351 .3286 -
900
.000248CX-6I2) .000147(X-800) .000093 (X-789) .000121 ( X - 7 5 4 )
1000
1100
Average Body Weight (grams) FIG. 3. Average breed regressions of average efficiency on average body weight for the period four to ten weeks of age.
( + .000143 efficiency units/gram average body weight) for adjustment purposes and an analysis of covariance of adjusted efficiency. However, this significant breed difference in the regression of efficiency on average body weight may in itself be indicative of breed differences in efficiency independent of growth. It appears from Table 2 that the significant breed difference among regressions was largely attributable to the large regression coefficient associated with the S.C, White Leghorn breed. In order to test this observation an analysis of covariance of the three breed regression coefficients associated with New Hampshire, Dark Cornish and Dominant Whites was conducted. The F-ratio testing the differences among the three heavy breed regressions was less than one. This supported the conclusion that the significant differences
obtained among breed regressions was attributable to the large S.C. White Leghorn coefficient. An analysis of the twentyfour group regressions, within breed, sex, and trial with the S.C. White Leghorn groups eliminated, resulted in an F-ratio below the five percent level. These results indicate that the average regression coefficient of the heavy breeds (+.000116 efficiency units/gram increase in mean body weight) could be used legitimately for an analysis of covariance of adjusted efficiency among the heavy breeds. Figure 3 shows the four average breed regression lines. The importance of the significnatly larger Leghorn coefficient of mean efficiency on mean body weight should not be minimized. This relationship warrants further investigation in order to confirm or refute the findings of these studies.
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o
FEED EFFICIENCY AND GROWTH
557
shown to possess unusual physiological versatility in its ability to withstand nutritional deficiencies (see Hutt, 1949). In light of these previous physiological studies it appears hazardous to conclude that the large Leghorn coefficient was entirely the result of its lower over-all average body weight. Carcass composition could also be an important consideration. An eventual explanation may prove to be a combination of these factors. Although breed differences exist in the regression of mean efficiency on mean body weight, it is proposed that the use of this regression of averages will satisfactorily estimate average efficiency for a given period of time and tend to allevi-
The previous section has demonstrated the relationship between mean body weight and mean efficiency. The analysis of variance of average body weight is summarized in Table 3. For tests of significance, breeds and sex were considered fixed variables and trials random. The same classification was used in the subsequent analysis of gross efficiency and of adjusted efficiency. Highly significant differences (P<.01) in average body weight due to breeds, sex, and trials were obtained. The second-order interaction, breeds X sex X trial, was significant (P<.05). The average body weights observed in these studies are shown in Table 2.
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An explanation of the larger Leghorn re- ate the confounding effect of growth on gression coefficient is somewhat paradoxi- gross efficiency. The utilization of the cal. It is impossible at the present time to regression of efficiency on average weekly definitely attribute this large regression body weight as proposed by Hankins and coefficient to any specific characteristic Titus (1939) does not consider the time of the S.C. White Leghorn. One obvious relationships involved in obtaining idenphysical characteristic in which the Leg- tical weights. The regression of averages horn differs from the heavy meat-type will allow an estimate of the average breeds is average body weight for the sixTABLE 3.—Analysis of variance of average week period. This difference is the result body weight and average efficiency of both a smaller initial weight and lower gains during the experimental period. This Mean squares Degrees might lead one to conclude that the reof Average body Average feed variation gression of average efficiency on average weight efficiency body weight decreases as the over-all Breeds (B) 3 729,224.6f .04914730f body weight increases, indicating a curvi- Sex (S) 1 2,234,888.If .0S287978f 3 62,868.3| .013920071 linear type regression. However, in all Trials (T) XS 3 14,433.6 .00025647 cases the females have a lower average B BXT 9 4,939.7 .00133882f body weight than the males. Thus, one S X T 3 3,644.3 .00126994* BXSXT 9 8,422.5* .00024740 would expect the female to possess a Individuals 334 3,703.3 .00038794 larger regression coefficient than the male. * Significant 5 % level. Although this difference was not statistif Significant 1% level. cally significant, the average female regression (+.000137) was less than the efficiency of individual birds or groups of male (+.000146). Also, the New Hamp- birds by correcting to the same average shires have the largest average body body weights for a given period of time. weight and the largest regression coefficient among the heavy breeds. Analysis of Average Body Weight and AverThe S.C. White Leghorn has been age Gross Efficiency
558
T . W . FOX AND B . B . BOHEEN
Analysis of Covariance of Efficiency Adjusted for Differences in Average Body Weight An analysis of covariance of mean efficiency adjusted for differences in average body weight was conducted among the three heavy breeds as described by Snedecor (1946). These tests with the corresponding F-ratios are shown in Table 4. Prior to a discussion of the covariance analysis of adjusted mean efficiency, it appears necessary to clarify the interpretation of these results. The use of covari-
TABLE 4.—Analysis of covariance of average efficiency adjusted for differences in average body weight. {three heavy breeds) Residuals
Source of variation D. F.
M. S.
F-ratio
Breed+B XT Breeds Adj. BXT
7 2 5
.00348331 .00123404
2.82
Sex+SXT Sex Adj. SXT
3 1 2
.00130757 .00071489
1.83
BXS+Error BXSadj. Error
248 2 246
.00069928 .00033765
2.07
BXT+Error BXT Adj. Error
252 6 246
.00138577 .00033765
4.10f
SXT+Error SXT Adj. Error
249 3 246
.00087194 .00033765
2.58
BXSXT+Error BXSXTAdj. Error
252 6 246
.00028197 .00033765
<1
t Highly significant.
ance in these experiments is an example of the special case discussed by Cochran and Cox (1950) in which the treatments e.g., breed, sex, and trial, are influencing the x variables or average body weight. Significant breed, sex, and trial differences have been found for mean body weight and mean efficiency. Also, average body weight for the six-week period studied is largely an expression of growth. I t is postulated that differences in mean efficiency are predominantly an expression of growth. Therefore, if the F-ratios for for adjusted efficiency are no longer significant, it would be concluded that this hypothesis was tenable. However, if the F-test of adjusted values still showed significance it could be concluded that treatment differences in efficiency exist independent of the effect of growth. Because of the effect of treatment on the x variable, it must be emphasized that any statement concerning adjusted efficiency applies only to efficiency after adjustment for differences in average body weights. The analysis of covariance of adjusted average efficiency summarized in Table
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Because only three heavy breeds could be studied by an analysis of covariance of adjusted efficiency, it was necessary to conduct an analysis of variance of average body weight among these three breeds. The differences among breeds and trials were significant (P<.05) and the difference between sexes was highly significant (P<.01). The analysis of gross efficiency is also shown in Table 3. Highly significant differences (P < .01) due to breed, sex, and trial were obtained. In order to test trials the approximate F test was used as described by Cochran (1951). The breed X trial interaction was highly significant (P<.01) and the sexXtrial interaction significant (P < .05). The mean efficiencies observed in these studies are also summarized in Table 2. Again, an analysis of variance among the heavy breeds was necessary because of the covariance analysis of adjusted efficiency contemplated among these breeds. There were significant differences (P < .05) among breeds and trials and a highly significant difference between sexes in average efficiency. The breed X trial interaction was highly significant ( P < .01) paralleling the results with all four breeds. The sexX trial interaction was below the five percent level but approached significance.
559
FEED EFFICIENCY AND GROWTH
The adjusted average efficiencies of the three heavy breeds tested by the covariance analysis are shown in Table 5. If Table 5 is compared with the observed mean efficiencies shown in Table 2 (column 6), it is observed that the reduction in magnitude of the differences in average efficiency among the three heavy breeds after correction for differences in average body weight is not very great. This would be expected because of the small differences obtaining in average body weight among these meat type breeds. However, the correction for the variation attributable to regression was sufficient to lower the F-ratio well below the five percent level of significance. In the case of sex, pronounced differences in both mean body weight and mean efficiency are noted. Table 5 indicates that the difference between sexes in average efficiency is practially lost after correction for differences in body weight. This reduction in mean efficiency demonstrates that sex differences in gross
efficiency may be considered an expression of sexual dimorphism in average body weight, or growth, during the six-week period from four to ten weeks of age. As in the case of mean efficiency, the breed X trial interaction in adjusted mean efficiency was highly significant (P<.01) indicating that the magnitude of the differences among breeds varies with trials. There was very little change in rank among the breeds from trial to trial.The New Hampshire was consistently more efficient in all four trials possibly indicating some slight breed difference in adjusted efficiency. A physiological interpretation of the breed X trial interaction would be highly TABLE 5.—Average efficiency of the three heavy breeds adjusted for differences in average body weight for the period four to ten weeks of age. {Adjusted to over-all average of 781 grams body weight by use of mean regression coefficient of +.000116 Adjusted average efficiency Lot M
F
Both
Trial 1 New Hampshire Dark Cornish Dominant White Average (Trial 1)
.3523 .3441 .3362 .3443
.3381 .3281 .3172 .3298
.3452(1) .3361(2) .3297(3) .3370
Trial 2 New Hampshires Dark Cornish Dominant White Average (Trial 2)
.3243 .3177 .3260 .3227
.3339 .3149 .3161 .3217
.3291(1) .3171(3) .3211(2) .3222
Trial 3 New Hampshire Dark Cornish Dominant White Average (Trial 3)
.3491 .3479 .3366 .3445
.3468 .3404 .3370 .3414
.3480(1) .3443(2) .3368(3) .3430
New Hampshire Dark Cornish Dominant White Average (Trial 4)
.3678 .3455 .3381 .3504
.3533 .3341 .3409 .3517
.3605(1) .3398(1) .3395(3) .3511
All Trials New Hampshire Dark Cornish Dominant White Average (All Trials)
.3484 .3395 .3341 .3404
.3498 .3294 .3292 .3352
.3491(1) .3360(2) .3317(3) .3389
Trial 4
Note: ( ) = r a n k .
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4 may be compared with the results of analysis of variance of mean efficiency among the three heavy breeds. The analysis of variance among the three heavy breeds resulted in F-ratios for breeds and sex of 7.52 and 52.40 respectively. The former value was significant (P < .05) and latter highly significant (P<.01). The Fratios of adjusted efficiency for breeds and sex from the covariance analysis in Table 4 were 2.82 and 1.83 respectively. Both these values were well below the five percent significance level. This would indicate that among the heavy breeds studied in these experiments, when differences in average efficiency attributable to differences in average body weight are eliminated, breeds and sex show no significant differences. Theadjusted sex X trial interaction was below the five percent level but approached significance.
560
T . W. FOX AND B . B . BOHREN
The Relationship Between Total Gain and Mean Efficiency Average body weight for the six-week period was used as a measure of growth because it is a parameter of the equation E = C —kw or the derivative of the law of diminishing increment. A more commonly used measure of growth than mean body weight during a given period of time is absolute gain in weight. The close relationship between these two measures of growth for the four-to-ten-week age period is shown by the simple correlation coefficient of +.80 between total gain and average body weight obtained in these studies. In other classes of livestock correlations between gains and efficiency have been reported. Winters and McMahon (1933) found that in beef steers rate of gain was highly correlated (.741) with efficiency. Evvard et al. (1927) reported correlation coefficients of —.68 and —.54 between daily gains and feed requirements per 100 pounds of gain for swine grown in dry lots and forage lots respectively. Dickerson and Grimes (1947) reported the phenotypic correlation between daily gains and feed requirements in Duroc swine to be -.66. As individual chick data were available from the present studies it was desirable to investigate the relationship between total gains and average efficiency for the six week experimental period. The best estimate of this relationship was the overall correlation coefficient of +.60 between gains and efficiency obtained within breeds. An interpretation of correlations of this type warrants caution due to the elements common to both variables. In this problem gain is the independent variable and also the numerator of the dependent variable. However, regardless of the mathematical relationships existing between variables, the correlation coefficient may be satisfactorily used to
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speculative at the present time. The reasons for this are (1) the lack of information available on genetic differences in the underlying physiological processes affecting efficiency such as basal metabolism, thermo-regulation, absorption ability, and net efficiency, and (2) the limitations of the present data to distinguish between total gains and the actual caloric content of these gains. Further studies of genetic differences in the basic endocrine and physiological processes influencing food utilization may clarify these interactions. Because of the breed differences in the regression of average efficiency on average body weight, it would not be valid to conduct an analysis of covariance of adjusted efficiency among all four breeds using the average within breed regression. However, it appeared desirable to correct the average efficiencies of breeds within trials and also breeds averaged over trials using the individual regression coefficients of mean efficiency on mean body weight calculated within these groups. These adjusted mean efficiencies are provided in Table 2. A comparison of observed efficiency with adjusted efficiency indicates that the large differences existing between the S.C. White Leghorn and the heavy breeds in observed efficiency are practically removed upon adjustment for differences in average body weight or growth. The observed mean efficiency of the S.C. White Leghorn is only 84.6, 88.7, and 90.5 percent of the respective mean efficiencies of the New Hampshire, Dark Cornish, and Dominant White. The adjusted mean efficiency of the S.C. White Leghorn is 96.0 percent of the New Hampshire, 99.5 per cent of the Dark Cornish, and 100.6 percent of the Dominant White average efficiencies. Thus, Table 2 provides additional evidence that observed differences in gross efficiency are largely a reflection of average body weight or growth.
FEED EFFICIENCY AND GROWTH
561
demonstrate the degree of relationship existing between the variables either because of, or in spite of the common elements present.
that there is little justification for commercial breeders to practice selection on the basis of food requirements.
SUMMARY
The authors appreciate the assistance of Professors S. R. Miles and V. L. Anderson in the statistical analysis and interpretation of the data.
The correlation between gain and efficiency was +.60. This value is in good agreement with correlations between gains and efficiency reported in other classes of livestock. The present studies indicate
REFERENCES Brody, S., 1934. Nutrition. An. Rev. Bioch. 3: 317321. Cochran, W. G., 1951. Testing a linear relation among variance. Biometrics, 7: 17-32. Cochran, W. G., and G. M. Cox, 1950. Experimental Designs. James Wiley & Sons, New York, N. Y. Dickerson, G. E., and J. C. Grimes, 1947. Effectiveness of selection for efficiency of gain in Duroc swine. J. An. Sci. 6: 265-287. Evvard, J. M., M. G. Snell, C. C. Culbertson and G. W. Snedecor, 1927. Correlations between daily gains and feed requirements of growing and fattening swine. Proc. Am. Soc. An. Prod. 1927: 85-92. Hankins, O. G., and H. W. Titus, 1939. Growth, fattening and meat production. Yearbook of Agriculture 1939:450-468. Hendricks, W. A., 1931. Fitting the curve of diminishing increments to the feed consumption-live weight growth curves. Science, 74: 290. Hendricks, W. A., M. A. Jull and H. W. Titus, 1931. A possible physiological interpretation of the law of diminishing increment. Science, 73: 427-429. Hess, C. W., T. C. Byerly and M. A. Jull, 1941. The efficiency of feed utilization by Barred Plymouth Rock and crossbred broilers. Poultry Sci. 20: 210216. Hess, C. W., and M. A. Jull, 1948. A study of the inheritance of feed utilization efficiency in the growing domestic fowl. Poultry Sci. 27: 24-39. Hutt, F. B., 1949. Genetics of the Fowl. 590 pp. McGraw-Hill Book Co., Inc., New York. Lush, J. L., 1936. Genetic aspects of the Danish system of progeny testing swine. Iowa Agr.Exp. Sta. Res. Bull. 204: 1-109. Snedecor, G. W., 1946. Statistical Methods. Iowa State College Press, Ames, Iowa. Winters, L. M., and H. McMahon, 1933. Efficiency variation in steers. Minnesota Agr. Exp. Sta. Tech. Bull. 94: 1-28.
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Individual feed efficiency and growth studies were conducted on four breeds of chickens, the S.C. White Leghorn, New Hampshire, Dark Cornish and Purdue Dominant White. Efficiency was studied from four to ten weeks of age utilizing the derivative of the law of diminishing increment relating efficiency to weekly body weight. Breed and trial differences observed among regressions were shown to be influenced by differential growth patterns. In addition to the negative regression of efficiency on average weekly body weight, a positive regression of average efficiency on average body weight for the period was observed. This positive regression of averages provided the basis for a method of statistical control allowing for an estimate of efficiency corrected for differences in growth. These studies indicate that sex differences in gross efficiency may be considered an expression of sexual dimorphism in growth. Breed differences in gross efficiency were also predominantly a reflection of growth differences. The S.C. White Leghorn regression coefficient of mean efficiency on mean body weight was found to be significantly different from the heavy breed regressions and will necessitate further investigation to determine if this difference is due to a physiological attribute of the Leghorn or is merely the result of a metrical bias.
ACKNOWLEDGEMENT
A
1
Journal paper No. 741 Purdue University Agricultural Experiment Station. 2 Present address—Poultry Department, University of Massachusetts, Amherst, Mass.
After adjusting the date it was indicated that some slight improvement in the physiological efficiency had been made in the early years of selection but uncontrolled environmental conditions reduced the reliability of this improvement. Dickerson and Grimes (1947) summarized the results of eight years of selection for high and low feed requirements in Duroc swine. These data afforded estimates of heritabilities and genetic correlations between traits. Because of the lower heritability of efficiency in comparison to growth rate and the high genetic correlation between the two characters, it was concluded that selection on the basis of growth alone would be equally productive in improving efficiency as selection for efficiency, and more effective in improving growth rate. The present studies were designed to determine the relationship between growth rate and feed efficiency in growing chickens and to ascertain the differences existing among breeds of fowl after correction for differences in growth rate. EXPERIMENTAL PROCEDURE
The four breeds of chickens selected for these studies were New Hampshires, Dark Cornish, Purdue Dominant Whites, and S. C. White Leghorns. On the day of hatching, random samples of 50 individuals of each of the four breeds were placed in starting batteries. At four weeks of age all birds were sexed and again a random sample consisting of twelve males and twelve females of each breed was drawn. This resulted in 96 birds for indi-
549
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GENERAL observation has been that the more rapid the growth the higher the feed efficiency or body weight increase per unit of feed. This has been explained on the basis of the slower growing individuals possessing a proportionately larger maintenance tax. There is also some evidence in poultry, although meager and conflicting, that feed efficiency per se, independent of the rate of growth is inherited. Hess and Jull (1948) reviewed much of the previous information concerned with the genetic aspects of the efficiency complex. They indicated that in general, the faster the groi th, the greater the efficiency. However, they observed some evidence that feed efficiency is inherited independently of growth. Lush (1936) described the Danish system of progeny testing swine on the basis of rate of gain and also food requirements per unit of gain. An analysis of the yearly progress from 1922 to 1929 was made and it was found that both rate and economy of gain had been improved. Improvement had occurred in the "practical efficiency," but it was opined that most of this improvement might have been associated with the improvement in growth rate and the approach to market weight at an earlier age. A correction factor was established for this difference in market age to estimate the improvement in what was termed the "physiological efficiency."
550
T. W. FOX AND B. B . BOHREN
Analysis of variance was used to compare differences in average body weight and average efficiency due to breeds, sex, trial and the possible first and second order interactions among these sources of variation. The equation E = C —kw, a derivative of the law of diminishing increment, was used to relate efficiency to average weekly body weight. The analysis of covariance provided tests of significance for differences among regression coefficients obtained from studies relating efficiency to average weekly body weight and also average efficiency to average body weight for the entire six week experimental period. The analysis of covariance was utilized for tests of significance of efficiency adjusted for differences in average body weight. RESULTS AND DISCUSSION
In order to clarify the presentation of the data derived from these experiments, a few definitions are necessary. "Total gain" or "gain" will refer to the absolute gain in body weight during the six-week period from four to ten weeks of age. It is believed that total gain provides a useful measure of growth because (1) all birds in these studies are compared during the same age interval and (2) the growth curves from four to ten weeks of age, as observed in these studies roughly approximate linearity. "Average weekly body weight" is the average body weight of any given weekly period. "Average body weight" is considered as the average body weight for the six-week period from four to ten weeks of age. "Gross efficiency" or "efficiency" is the ratio of absolute gain/feed consumed. This definition of gross efficiency is not entirely comparable with that proposed by Brody (1934) as the caloric composition of the gains and feed are not considered. As the same feed was used throughout these experiments little would be gained by transforming the
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vidual cage studies for the six-week period from four to ten weeks of age. The 96 individuals selected were stratified into four blocks of 24 birds consisting of three individuals of each sex and of each breed. The birds were then randomly assigned to cages within blocks. These 96 individuals composed a replication or trial. Four replications of this design were run during the following periods: Trials one and two, April through July, 1951; and Trials three and four, April through July, 1952. All birds were fed the Purdue Experiment Station all-mash starter ad libitum both prior to and during the test period. After a preliminary period of all-night lights to allow the birds to become familiar with the feeding devices, twelve hours of light were used. The only apparent environmental difference between trials was season. Trials two and four were conducted during the early summer months and the temperature of the laboratory became rather high for short periods of time. An exhaust fan tended to alleviate this condition and prevented overheating. Starved weekly weights and the corresponding weekly feed consumption were obtained on a Toledo gram scale. The individual feed boxes were constructed with high sides and a wide retaining lip. Also, rectangular pieces of \ inch mesh hardware cloth were placed on top of the feed to prevent mash from being billed from the feeders. The individual feeders were placed on an eight-inch apron in front of the individual cages from which spilled feed could be recovered. The data obtained for these studies were individual weekly starved weights from which weekly gains were calculated and weekly individual feed consumption corresponding to these gains. These data were then used to calculate gross efficiency (gain/feed) for each weekly period and also average gross efficiency for the entire six week period.
551
F E E D E F F I C I E N C Y AND G R O W T H
Breeds (Ave. over sex and trial) 4000
II o c
.3000 W.L. N.H. ,2000hD.C. D.W.
UJ
E= .3003-.000234(X-6l3) E = .3399-.000145 (X-753) E = .3294-.000111 ( X - 7 5 2 ) E= .3266-.000095(X-727) 400"
600" 800" 1000 Average Weekly Body Weight (grams)
200
"400
605" 800 iOOO" Average Weekly Body Weight (grams)
1255"
1450
1200
1400
.4000c -o
2. •
.2000
.4000-
III £
.3000
UJ
.2000 200
400
600
800
1000
Average Weekly Body Weight (grams) FIG. 1. Average regressions of efficiency on average weekly body weight due to breed, sex and trial.
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"200
552
T . W . FOX AND B . B . BOHREN
feed consumed to calories as total caloric intake would be a function of the weight of the total feed consumption. However, in the case of gains, differences in carcass composition could change their caloric value and cause a discrepancy between the two measures of gross efficiency. "Adjusted efficiency" refers to the efficiency adjusted for differences in average body weight for the four-to-ten-week age period.
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lated to zero body weight the values obtained would be comparable to the " C " values proposed by Hendricks, Jull and Titus (1931) and utilized by Hess et al. (1941, 1948). It is believed that these " C " values are satisfactory when considered as a parameter of the regression equation but a questionable interpretation is involved when physiological significance is attributed to them. This is especially true in these experiments in which the regression coefficients were obThe Regression of Gross Efficiency on Aver- tained for the six-week period from four age Weekly Body Weight to ten weeks of age. The initial procedure in analyzing feed The average regressions shown in Figure efficiency data from these experiments 1 could be used to estimate the efficiency consisted of determining the regression at any body weight over the rage of the of gross efficiency on average weekly body data obtained in these studies. The diffiweight for trials one and two using the culty encountered in comparing these method proposed by Hendricks (1931) and values, e.g. at 800 grams body weight, is described by Hankins and Titus (1939). the fact that time has not been considered. As data were available for each individual Thus, if two breeds are compared at 800 studied, the 192 individual regression grams average body weight, the length coefficients for trials one and two were of time required for each to obtain this calculated. An analysis of covariance was weight is not involved. Therefore, the conducted to determine if the average group reaching this weight in the shortest breed, sex, or trial regressions differed interval of time will have the lower mainsignificantly. tenance tax. Highly significant differences (P<.01) were obtained among the average re- The Relationship of Growth to the Regression gressions of efficiency on average weekly of Gross Efficiency on Average Weekly body weight attributable to breeds and Body Weight trials while the effect of sex was nonThe three values that establish the significant. In considering these results it graphical location of the average regresmust be emphasized that they apply to sions presented in Figure 1 are the mean the experimental period of from four to body weight, mean gross efficiency and ten weeks of age. the slope of the line or "k" value relating Figure 1 is a graphical presentation of efficiency to average weekly body weight. the average breed, sex, and trial regres- Once the regression equation has been sion equations relating efficiency to aver- determined all values on the regression age weekly body weight. It will be noted line are also established. that the regression equations were plotted Differences in the slopes or "k" values only over the range of the data from which of the regression of gross efficiency on they were calculated. The origins are lo- average weekly body weight are believed cated at approximately 300 grams body to be greatly influenced by differential weight. If these equations were extrapo- growth patterns during the experimental
FEED EFFICIENCY AND GROWTH
553
TABLE 1.—Comparison of the regression of gross efficiency on average body weight associated with breed, sex, and trial and a measure of growth persistency
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sponding "k" values are accordingly decreased. In the studies of Hess and Jull (1948) evidence for genetic differences in effi"k" values Persistency Source of variation ciency independent of growth was pro(X1000) of growth 1 vided by comparisons of groups of sire Breeds progenies with similar average weights at -.234 57.8 S. C. White Leghorns -.145 60.2 New Hampshires identical ages. This procedure afforded -.111 62.2 Dark Cornish three comparisons involving the progeny -.095 64.0 Dominant White Trials of six sires. Using a derivative of the law One -.183 59.3 of diminishing increment, E = C — kw, Two -.078 63.6 Sex no significant differences in " C " values Males -.128 61.2 were found between sires. However, one Females -.151 61.3 comparison of "k" values, or the decrease relative rate of growth last thre weeks in efficiency with increasing weekly body weight, resulted in a significant difference. relative rate of growth first three weeks ( w 3 —W2) / W2 —Wi This formed the basis of the following -X100 statement, "Since body weight, or rate of 1/2 (w2—w3)/ 1/2 (wi—w2) where: Wi = average weight at 4 weeks of age growth, and time have been removed W2=average weight at 7weeks of age from the consideration, the resulting difW3 = average weight at 10 weeks of age ference must be due to an inherent differperiod. A retardation of growth during ence between these two sires." Another the latter portion of the growth curve possible explanation of this one comparirelative to the early growth will cause a son in which a significant difference in greater negative slope or " k " value. "k" values was observed could be that Evidence for this relationship is shown by the effect of rate of growth was not enTable 1 which provides a comparison of tirely eliminated. Growth rate was asthe average regressions of gross efficiency sumed to be constant because at 536 on average weekly body weight and a grams body weight the average body measure of growth persistency. The weights of the two sire progenies were absolute magnitude of this persistency equal at the same age. Up to this weight measure is irrelevant and would be de- the rate of growth was obviously compendent on the stage of growth from which parable. The "k" values however, were they were calculated. However, the rela- apparently calculated over a longer period tive magnitude of these values are be- of growth and it is possible that the lieved to illustrate the point under con- growth rate of the two groups diverged significantly after the 536 gram weight. sideration. This latent growth differential could influThis persistency measure is a comparience the observed efficiencies at the son of the relative rate of growth for the greater body weights and as the "k" last three weeks with the relative rate values were calculated by the method of of growth for the first three weeks. A least squares, they also would be affected. large value indicates less retardation of The "k" values would have an effect on growth during the latter portion of the all calculated efficiencies over the entire experimental period. Table 1 indicates range of body weights and could possibly that as this persistency measure increases explain the large differences in calculated with breed, sex, and trial the corre-
554
T . W . FOX AND B . B . BOHEEN
.4500
III « u
.3500
.2500
.1500
200
400
600
800
1000
1200
1400
Average Weekly Body Weight (grams) FIG. 2. Average lot regressions (within breed, sex and trial) of efficiency on average weekly body weight and demonstrating the positive regression of average efficiency on average body weight for the period four to ten weeks of age.
efficiency between sire progenies at the 536 gram weight. Relationship Between Mean Body Weight for the Six-Week Period and Mean Efficiency The two remaining parameters of the regression of efficiency on average weekly body weight are the mean body weight and mean efficiency for the six-week experimental period. Figure 2 also presents the graphical relationship existing among the lot regressions (within breed and sex) of efficiency on average body weight in trial one. The same relationship obtained for all four trials. In addition to the negative regressions or "k" values this graph demonstrates the important fact that there exists a positive regression of mean efficiency on mean body weight for the six-week test period. This positive regression also exists be-
tween the mean efficiency and mean body weight of individuals and suggested the possibility of using the methods of covariance as statistical control to estimate the variation in feed efficiency existing after adjustment for differences in average body weight. As the magnitude of the average body weights for the six-week period studied is largely attributable to differences in growth, adjustment of the average efficiencies to the over-all mean body weight would appear to correct for differences in growth. The problem of efficiency independent of growth has been studied by Hess et al. (1948) using pairing or experimental control. This type of experimental control is difficult to obtain. If statistical control could be utilized to alleviate the confounding effect of growth on gross efficiency, additional information on efficiency might be obtained.
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UJ
555
FEED EFFICIENCY AND GROWTH TABLE 2.—Mean body weight, mean efficiency, and adjusted efficiencies for the period four to ten weeks of age Mean body weight (gm.)
Mean efficiency
Lot
Regres- sion coef. (X1000) 2
9
Both
cP
9
Both
Trial 1 S.C. White Leghorn New Hampshire Dark Cornish Dominant White Average (Trial 1)
723 894 850 870 834
550 745 735 683 678
636 819 793 776 756
.3177 .3654 .3521 .3465 .3454
.2790 .3339 .3228 .3166 .3118
.2984 .3496 .3375 .3291 .3286
+ + + +
Trial 2 S.C. White Leghorn New Hampshire Dark Cornish Dominant White Average (Trial 2)
648 875 820 831 794
557 686 701 633 644
603 781 760 732 719
.2930 .3353 .3222 .3318 .3206
.2771 .3229 .3056 .2989 .3012
.2851 .3291 .3147 .3154 .3109
+ + + +
Trial 3 S.C. White Leghorn New Hampshire Dark Cornish Dominant White Average (Trial 3)
725 883 925 862 849
554 735 744 694 682
639 809 834 778 765
.3178 .3609 .3646 .3460 .3473
.2900 .3415 .3361 .3269 .3236
.3045 .3512 .3504 .3364 .3355
+ + + +
Trial 4 S.C. White Leghorn New Hampshire Dark Cornish Dominant White Average (Trial 4)
607 893 831 803 784
528 692 709 655 646
568 792 770 729 715
.3105 .3808 .3513 .3407 .3458
.2930 .3430 .3257 .3263 .3287
.3017 .3753 .3385 .3335 .3373
+ + + +
All Trials S.C. White Leghorn New Hampshire Dark Cornish Dominant White Average (All Trials)
676 886 857 842 815
547 714 722 666 663
612 800 789 754 739
.3098 .3606 .3476 .3412 .3398
.2848 .3420 .3226 .3159 .3163
.2973 .3513 .3351 .3286 .3281
+ + + +
.189 .251 .034 .093
.3179 .3295 .3357 .3257
—
—
.248 .135 .087 .166
.3188 .3234 .3129 .3166
—
—
.223 .171 .149 .087 —
.3268 .3392 .3362 .3330
.348 .076 .216 .106
.3612 .3713 .3318 .3346
— .248 .147 .093 .121
—
—
— .3288 .3423 .3304 .3268
—
1
Mean efficiency adjusted for differences in average body weight by use of the within lot regressions of mean efficiency on mean body weight. Adjusted to over-all body weight of 739 grams. 2 Within-lot regression coefficients of mean efficiency on mean body weight.
Before the average regression of mean efficiency on mean body weight could be legitimately used for adjustment purposes it was essential to determine if this regression differed significantly due to breed, sex or trial. An analysis of "covariance of the regression coefficients as shown in Table 2 indicated non-significant trial and sex differences. However, the differences among breeds were highly significant (P <.01) providing substantial evidence that the average breed regressions of mean efficiency on mean body weight differ significantly. Significant differences (P < .05) were obtained among the thirty-
two coefficients associated with groups (within breed, sex, and trial) and might be expected because of the significant breed differences obtained among regression coefficients. It is observed that the S.C. White Leghorn regression coefficients are larger in all trials with the exception of trial one. This provides additional evidence that the average breed regressions were actually different and the significant F-ratio obtained for breeds was not the result of chance variation. The significant differences among breed regressions questions the validity of using the average regression coefficient
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&
Mean adjusted efficiency1 •
556
T . W . FOX AND B . B . BOHREN
• 4000
New Hampshire Dark Cornish
.3500
.3000
Dominant White
-S.C.White Leghorn
.2500
W.Leg. E = N.Hamp E = D.Cornish E = D.White E=
. 2000
400
500
600
700
800
.2973.3513.3351 .3286 -
900
.000248CX-6I2) .000147(X-800) .000093 (X-789) .000121 ( X - 7 5 4 )
1000
1100
Average Body Weight (grams) FIG. 3. Average breed regressions of average efficiency on average body weight for the period four to ten weeks of age.
( + .000143 efficiency units/gram average body weight) for adjustment purposes and an analysis of covariance of adjusted efficiency. However, this significant breed difference in the regression of efficiency on average body weight may in itself be indicative of breed differences in efficiency independent of growth. It appears from Table 2 that the significant breed difference among regressions was largely attributable to the large regression coefficient associated with the S.C, White Leghorn breed. In order to test this observation an analysis of covariance of the three breed regression coefficients associated with New Hampshire, Dark Cornish and Dominant Whites was conducted. The F-ratio testing the differences among the three heavy breed regressions was less than one. This supported the conclusion that the significant differences
obtained among breed regressions was attributable to the large S.C. White Leghorn coefficient. An analysis of the twentyfour group regressions, within breed, sex, and trial with the S.C. White Leghorn groups eliminated, resulted in an F-ratio below the five percent level. These results indicate that the average regression coefficient of the heavy breeds (+.000116 efficiency units/gram increase in mean body weight) could be used legitimately for an analysis of covariance of adjusted efficiency among the heavy breeds. Figure 3 shows the four average breed regression lines. The importance of the significnatly larger Leghorn coefficient of mean efficiency on mean body weight should not be minimized. This relationship warrants further investigation in order to confirm or refute the findings of these studies.
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o
FEED EFFICIENCY AND GROWTH
557
shown to possess unusual physiological versatility in its ability to withstand nutritional deficiencies (see Hutt, 1949). In light of these previous physiological studies it appears hazardous to conclude that the large Leghorn coefficient was entirely the result of its lower over-all average body weight. Carcass composition could also be an important consideration. An eventual explanation may prove to be a combination of these factors. Although breed differences exist in the regression of mean efficiency on mean body weight, it is proposed that the use of this regression of averages will satisfactorily estimate average efficiency for a given period of time and tend to allevi-
The previous section has demonstrated the relationship between mean body weight and mean efficiency. The analysis of variance of average body weight is summarized in Table 3. For tests of significance, breeds and sex were considered fixed variables and trials random. The same classification was used in the subsequent analysis of gross efficiency and of adjusted efficiency. Highly significant differences (P<.01) in average body weight due to breeds, sex, and trials were obtained. The second-order interaction, breeds X sex X trial, was significant (P<.05). The average body weights observed in these studies are shown in Table 2.
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An explanation of the larger Leghorn re- ate the confounding effect of growth on gression coefficient is somewhat paradoxi- gross efficiency. The utilization of the cal. It is impossible at the present time to regression of efficiency on average weekly definitely attribute this large regression body weight as proposed by Hankins and coefficient to any specific characteristic Titus (1939) does not consider the time of the S.C. White Leghorn. One obvious relationships involved in obtaining idenphysical characteristic in which the Leg- tical weights. The regression of averages horn differs from the heavy meat-type will allow an estimate of the average breeds is average body weight for the sixTABLE 3.—Analysis of variance of average week period. This difference is the result body weight and average efficiency of both a smaller initial weight and lower gains during the experimental period. This Mean squares Degrees might lead one to conclude that the reof Average body Average feed variation gression of average efficiency on average weight efficiency body weight decreases as the over-all Breeds (B) 3 729,224.6f .04914730f body weight increases, indicating a curvi- Sex (S) 1 2,234,888.If .0S287978f 3 62,868.3| .013920071 linear type regression. However, in all Trials (T) XS 3 14,433.6 .00025647 cases the females have a lower average B BXT 9 4,939.7 .00133882f body weight than the males. Thus, one S X T 3 3,644.3 .00126994* BXSXT 9 8,422.5* .00024740 would expect the female to possess a Individuals 334 3,703.3 .00038794 larger regression coefficient than the male. * Significant 5 % level. Although this difference was not statistif Significant 1% level. cally significant, the average female regression (+.000137) was less than the efficiency of individual birds or groups of male (+.000146). Also, the New Hamp- birds by correcting to the same average shires have the largest average body body weights for a given period of time. weight and the largest regression coefficient among the heavy breeds. Analysis of Average Body Weight and AverThe S.C. White Leghorn has been age Gross Efficiency
558
T . W . FOX AND B . B . BOHEEN
Analysis of Covariance of Efficiency Adjusted for Differences in Average Body Weight An analysis of covariance of mean efficiency adjusted for differences in average body weight was conducted among the three heavy breeds as described by Snedecor (1946). These tests with the corresponding F-ratios are shown in Table 4. Prior to a discussion of the covariance analysis of adjusted mean efficiency, it appears necessary to clarify the interpretation of these results. The use of covari-
TABLE 4.—Analysis of covariance of average efficiency adjusted for differences in average body weight. {three heavy breeds) Residuals
Source of variation D. F.
M. S.
F-ratio
Breed+B XT Breeds Adj. BXT
7 2 5
.00348331 .00123404
2.82
Sex+SXT Sex Adj. SXT
3 1 2
.00130757 .00071489
1.83
BXS+Error BXSadj. Error
248 2 246
.00069928 .00033765
2.07
BXT+Error BXT Adj. Error
252 6 246
.00138577 .00033765
4.10f
SXT+Error SXT Adj. Error
249 3 246
.00087194 .00033765
2.58
BXSXT+Error BXSXTAdj. Error
252 6 246
.00028197 .00033765
<1
t Highly significant.
ance in these experiments is an example of the special case discussed by Cochran and Cox (1950) in which the treatments e.g., breed, sex, and trial, are influencing the x variables or average body weight. Significant breed, sex, and trial differences have been found for mean body weight and mean efficiency. Also, average body weight for the six-week period studied is largely an expression of growth. I t is postulated that differences in mean efficiency are predominantly an expression of growth. Therefore, if the F-ratios for for adjusted efficiency are no longer significant, it would be concluded that this hypothesis was tenable. However, if the F-test of adjusted values still showed significance it could be concluded that treatment differences in efficiency exist independent of the effect of growth. Because of the effect of treatment on the x variable, it must be emphasized that any statement concerning adjusted efficiency applies only to efficiency after adjustment for differences in average body weights. The analysis of covariance of adjusted average efficiency summarized in Table
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Because only three heavy breeds could be studied by an analysis of covariance of adjusted efficiency, it was necessary to conduct an analysis of variance of average body weight among these three breeds. The differences among breeds and trials were significant (P<.05) and the difference between sexes was highly significant (P<.01). The analysis of gross efficiency is also shown in Table 3. Highly significant differences (P < .01) due to breed, sex, and trial were obtained. In order to test trials the approximate F test was used as described by Cochran (1951). The breed X trial interaction was highly significant (P<.01) and the sexXtrial interaction significant (P < .05). The mean efficiencies observed in these studies are also summarized in Table 2. Again, an analysis of variance among the heavy breeds was necessary because of the covariance analysis of adjusted efficiency contemplated among these breeds. There were significant differences (P < .05) among breeds and trials and a highly significant difference between sexes in average efficiency. The breed X trial interaction was highly significant ( P < .01) paralleling the results with all four breeds. The sexX trial interaction was below the five percent level but approached significance.
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FEED EFFICIENCY AND GROWTH
The adjusted average efficiencies of the three heavy breeds tested by the covariance analysis are shown in Table 5. If Table 5 is compared with the observed mean efficiencies shown in Table 2 (column 6), it is observed that the reduction in magnitude of the differences in average efficiency among the three heavy breeds after correction for differences in average body weight is not very great. This would be expected because of the small differences obtaining in average body weight among these meat type breeds. However, the correction for the variation attributable to regression was sufficient to lower the F-ratio well below the five percent level of significance. In the case of sex, pronounced differences in both mean body weight and mean efficiency are noted. Table 5 indicates that the difference between sexes in average efficiency is practially lost after correction for differences in body weight. This reduction in mean efficiency demonstrates that sex differences in gross
efficiency may be considered an expression of sexual dimorphism in average body weight, or growth, during the six-week period from four to ten weeks of age. As in the case of mean efficiency, the breed X trial interaction in adjusted mean efficiency was highly significant (P<.01) indicating that the magnitude of the differences among breeds varies with trials. There was very little change in rank among the breeds from trial to trial.The New Hampshire was consistently more efficient in all four trials possibly indicating some slight breed difference in adjusted efficiency. A physiological interpretation of the breed X trial interaction would be highly TABLE 5.—Average efficiency of the three heavy breeds adjusted for differences in average body weight for the period four to ten weeks of age. {Adjusted to over-all average of 781 grams body weight by use of mean regression coefficient of +.000116 Adjusted average efficiency Lot M
F
Both
Trial 1 New Hampshire Dark Cornish Dominant White Average (Trial 1)
.3523 .3441 .3362 .3443
.3381 .3281 .3172 .3298
.3452(1) .3361(2) .3297(3) .3370
Trial 2 New Hampshires Dark Cornish Dominant White Average (Trial 2)
.3243 .3177 .3260 .3227
.3339 .3149 .3161 .3217
.3291(1) .3171(3) .3211(2) .3222
Trial 3 New Hampshire Dark Cornish Dominant White Average (Trial 3)
.3491 .3479 .3366 .3445
.3468 .3404 .3370 .3414
.3480(1) .3443(2) .3368(3) .3430
New Hampshire Dark Cornish Dominant White Average (Trial 4)
.3678 .3455 .3381 .3504
.3533 .3341 .3409 .3517
.3605(1) .3398(1) .3395(3) .3511
All Trials New Hampshire Dark Cornish Dominant White Average (All Trials)
.3484 .3395 .3341 .3404
.3498 .3294 .3292 .3352
.3491(1) .3360(2) .3317(3) .3389
Trial 4
Note: ( ) = r a n k .
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4 may be compared with the results of analysis of variance of mean efficiency among the three heavy breeds. The analysis of variance among the three heavy breeds resulted in F-ratios for breeds and sex of 7.52 and 52.40 respectively. The former value was significant (P < .05) and latter highly significant (P<.01). The Fratios of adjusted efficiency for breeds and sex from the covariance analysis in Table 4 were 2.82 and 1.83 respectively. Both these values were well below the five percent significance level. This would indicate that among the heavy breeds studied in these experiments, when differences in average efficiency attributable to differences in average body weight are eliminated, breeds and sex show no significant differences. Theadjusted sex X trial interaction was below the five percent level but approached significance.
560
T . W. FOX AND B . B . BOHREN
The Relationship Between Total Gain and Mean Efficiency Average body weight for the six-week period was used as a measure of growth because it is a parameter of the equation E = C —kw or the derivative of the law of diminishing increment. A more commonly used measure of growth than mean body weight during a given period of time is absolute gain in weight. The close relationship between these two measures of growth for the four-to-ten-week age period is shown by the simple correlation coefficient of +.80 between total gain and average body weight obtained in these studies. In other classes of livestock correlations between gains and efficiency have been reported. Winters and McMahon (1933) found that in beef steers rate of gain was highly correlated (.741) with efficiency. Evvard et al. (1927) reported correlation coefficients of —.68 and —.54 between daily gains and feed requirements per 100 pounds of gain for swine grown in dry lots and forage lots respectively. Dickerson and Grimes (1947) reported the phenotypic correlation between daily gains and feed requirements in Duroc swine to be -.66. As individual chick data were available from the present studies it was desirable to investigate the relationship between total gains and average efficiency for the six week experimental period. The best estimate of this relationship was the overall correlation coefficient of +.60 between gains and efficiency obtained within breeds. An interpretation of correlations of this type warrants caution due to the elements common to both variables. In this problem gain is the independent variable and also the numerator of the dependent variable. However, regardless of the mathematical relationships existing between variables, the correlation coefficient may be satisfactorily used to
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speculative at the present time. The reasons for this are (1) the lack of information available on genetic differences in the underlying physiological processes affecting efficiency such as basal metabolism, thermo-regulation, absorption ability, and net efficiency, and (2) the limitations of the present data to distinguish between total gains and the actual caloric content of these gains. Further studies of genetic differences in the basic endocrine and physiological processes influencing food utilization may clarify these interactions. Because of the breed differences in the regression of average efficiency on average body weight, it would not be valid to conduct an analysis of covariance of adjusted efficiency among all four breeds using the average within breed regression. However, it appeared desirable to correct the average efficiencies of breeds within trials and also breeds averaged over trials using the individual regression coefficients of mean efficiency on mean body weight calculated within these groups. These adjusted mean efficiencies are provided in Table 2. A comparison of observed efficiency with adjusted efficiency indicates that the large differences existing between the S.C. White Leghorn and the heavy breeds in observed efficiency are practically removed upon adjustment for differences in average body weight or growth. The observed mean efficiency of the S.C. White Leghorn is only 84.6, 88.7, and 90.5 percent of the respective mean efficiencies of the New Hampshire, Dark Cornish, and Dominant White. The adjusted mean efficiency of the S.C. White Leghorn is 96.0 percent of the New Hampshire, 99.5 per cent of the Dark Cornish, and 100.6 percent of the Dominant White average efficiencies. Thus, Table 2 provides additional evidence that observed differences in gross efficiency are largely a reflection of average body weight or growth.
FEED EFFICIENCY AND GROWTH
561
demonstrate the degree of relationship existing between the variables either because of, or in spite of the common elements present.
that there is little justification for commercial breeders to practice selection on the basis of food requirements.
SUMMARY
The authors appreciate the assistance of Professors S. R. Miles and V. L. Anderson in the statistical analysis and interpretation of the data.
The correlation between gain and efficiency was +.60. This value is in good agreement with correlations between gains and efficiency reported in other classes of livestock. The present studies indicate
REFERENCES Brody, S., 1934. Nutrition. An. Rev. Bioch. 3: 317321. Cochran, W. G., 1951. Testing a linear relation among variance. Biometrics, 7: 17-32. Cochran, W. G., and G. M. Cox, 1950. Experimental Designs. James Wiley & Sons, New York, N. Y. Dickerson, G. E., and J. C. Grimes, 1947. Effectiveness of selection for efficiency of gain in Duroc swine. J. An. Sci. 6: 265-287. Evvard, J. M., M. G. Snell, C. C. Culbertson and G. W. Snedecor, 1927. Correlations between daily gains and feed requirements of growing and fattening swine. Proc. Am. Soc. An. Prod. 1927: 85-92. Hankins, O. G., and H. W. Titus, 1939. Growth, fattening and meat production. Yearbook of Agriculture 1939:450-468. Hendricks, W. A., 1931. Fitting the curve of diminishing increments to the feed consumption-live weight growth curves. Science, 74: 290. Hendricks, W. A., M. A. Jull and H. W. Titus, 1931. A possible physiological interpretation of the law of diminishing increment. Science, 73: 427-429. Hess, C. W., T. C. Byerly and M. A. Jull, 1941. The efficiency of feed utilization by Barred Plymouth Rock and crossbred broilers. Poultry Sci. 20: 210216. Hess, C. W., and M. A. Jull, 1948. A study of the inheritance of feed utilization efficiency in the growing domestic fowl. Poultry Sci. 27: 24-39. Hutt, F. B., 1949. Genetics of the Fowl. 590 pp. McGraw-Hill Book Co., Inc., New York. Lush, J. L., 1936. Genetic aspects of the Danish system of progeny testing swine. Iowa Agr.Exp. Sta. Res. Bull. 204: 1-109. Snedecor, G. W., 1946. Statistical Methods. Iowa State College Press, Ames, Iowa. Winters, L. M., and H. McMahon, 1933. Efficiency variation in steers. Minnesota Agr. Exp. Sta. Tech. Bull. 94: 1-28.
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Individual feed efficiency and growth studies were conducted on four breeds of chickens, the S.C. White Leghorn, New Hampshire, Dark Cornish and Purdue Dominant White. Efficiency was studied from four to ten weeks of age utilizing the derivative of the law of diminishing increment relating efficiency to weekly body weight. Breed and trial differences observed among regressions were shown to be influenced by differential growth patterns. In addition to the negative regression of efficiency on average weekly body weight, a positive regression of average efficiency on average body weight for the period was observed. This positive regression of averages provided the basis for a method of statistical control allowing for an estimate of efficiency corrected for differences in growth. These studies indicate that sex differences in gross efficiency may be considered an expression of sexual dimorphism in growth. Breed differences in gross efficiency were also predominantly a reflection of growth differences. The S.C. White Leghorn regression coefficient of mean efficiency on mean body weight was found to be significantly different from the heavy breed regressions and will necessitate further investigation to determine if this difference is due to a physiological attribute of the Leghorn or is merely the result of a metrical bias.
ACKNOWLEDGEMENT