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Genetic Parameters of Unadjusted and Adjusted Age-Constant Feed Consumption and Efficiency of Meat Type Chickens1
Genetic Parameters of Unadjusted and Adjusted Age-Constant Feed Consumption and Efficiency of Meat Type Chickens 1 D. E. BERNON and J. R. CHAMBERS Animal Research Centre, Research Branch, Agriculture Canada, Ottawa, Ontario K1A 0C6 (Received for publication January 19, 1988)
1988 Poultry Science 67:1497-1504 INTRODUCTION
Breeding programs for meat type chickens have emphasized body weight and rate of gain. Efficiency of feed conversion is important but has been neglected in many broiler selection programs. Recently, two factors have enhanced the value of this trait in breeding programs: Increases in feed costs have become persistent, plus a negative relationship, both phenotypic and genetic, has been reported between feed efficiency and fatness (Washburn et al., 1975; Pym and Solvyns, 1979; Leclercq et al., 1980; Chambers et al., 1983, 1984; Leenstra et al., 1986; Whitehead and Griffin, 1986). The law of diminishing increment has been applied to describe the relationship between feed intake and liveweight with respect to time for the growing chicken (Jull and Titus, 1928; Hendricks, 1931; Hendricks et al., 1931; Fox and Bohren, 1954). Using differential equation techniques, the equation can be modified to express
Contribution Number 1295, Animal Research Centre.
feed efficiency as a function of the amount of feed required for maintenance. The modified equation reveals an increase in the proportion of feed intake used for maintenance as the chicken grows. Consequently, feed efficiency declines with growth, attaining zero at maturity. Differences in maintenance requirements in ageconstant feed efficiency tests give rise to an error that is negatively correlated with growth rate and hence, with feed efficiency (Chambers and Lin, 1988). In comparisons based on specific weights, a slow growing chicken has a higher maintenance requirement than a fast growing bird; however, in comparisons based on specific ages, faster growing chickens are penalized, because they have heavier body weights and thus are tested at a less efficient phase of growth. The increase in feed required for maintenance due to increased body weight almost nullifies any beneficial effect of rapid growth on feed efficiency (gain/consumption) or its reciprocal, feed conversion (Marks, 1979; Chambers et al., 1981). Broilers from rapid and slow growing strains differed little in feed conversion (1.9 vs. 2.0) when tested to the same age (47 days, Chambers
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ABSTRACT Body weights at 28 and 42 days (BW28 and BW42) and weight gain (G), feed consumption (FC), and efficiency (FE, gain/feed) for the intervening period were either measured or calculated for pedigreed chickens from synthetic sire and dam populations. Additional variables were calculated: feed consumed to meet maintenance requirements (FCM); feed consumed if growth rates were similar (FCA); feed consumed for weight-constant test periods (FCW); and efficiency of feed used for growth (FEG). Heritabilities of and correlations among these traits were estimated for each population. Heritabilities of most traits were similar to corresponding estimates available in the literature. Heritability values from the sire and dam populations for BW28, BW42, G, FC, FCM, FE, and FEG were similar. Exceptions were FCA and FCW in the sire population. Genetic and phenotypic correlations for the two populations were similar, with exceptions involving FE, FEG, FCA, and FCW most of the time. Results indicate that weight-constant measures of feed consumption and efficiency or age-constant measures of these traits corrected for differences in test body weights may have lower heritabilities than age-constant feed consumption and efficiency in the sire population. However, these measures of consumption may be better indicators of efficiency due to their smaller positive or negative correlations, both phenotypic and genetic, with body weight and weight gain, and their large negative correlations with efficiency measures, especially efficiency of feed used for growth. Based on partial correlations, consumption or efficiency measures corrected for both beginning and concluding test body weights are better predictors of efficiency than uncorrected measures. (Key words: feed efficiency, heritabilities, body weight, age constant, weight constant)
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BERNON AND CHAMBERS
MATERIALS AND METHODS
Chickens. Pedigreed chickens from broiler sire and dam populations synthesized from nine broiler sire and seven broiler dam stocks (Chambers et al., 1984) were tested. Parents of the sire population were selected for body weight (28 days) and either feed efficiency (28 to 42 days) or low percentage abdominal fat of sibs (47 days), or both. In the dam population parents had been selected for body weight (28 days), feed efficiency (28 to 42 days), and hatching egg production of females to 40 wk of age. For both populations, this was the first generation of genetic selection following population synthesis. In the sire population, progeny were obtained from 181 sires and 510 dams (2 to 4 dams per sire). In the dam population, progeny were obtained from 115 sires and 361 dams. Most parents were represented by progeny of both sexes. Management Procedures. Pedigreed chicks were vent sexed and wingbanded at hatch, vaccinated to prevent Marek's disease, and placed in pens for brooding to 25 days of age. From
25 days of age to end of test at 42 days of age, chickens were confined individually in wire cages (20.3 cm x 40.7 cm) normally used for laying hens. Cages were adapted for the meat type chickens by inserting level wire mesh floors and water lines with drippers approximately 15 cm above the floor. Plastic feed containers were inserted in the feed trough to permit measurement of individual feed intake. A crumbled broiler starter diet (3,155 kcal ME/kg, with 23.5% CP) was fed to 28 days of age. Thereafter a crumbled broiler grower diet (3,210 kcal ME/kg; 20.8% CP) was fed. The amount of feed provided each chicken was recorded and the amount of feed remaining at the completion of the test was weighed to permit calculation of feed consumed. From hatch to the end of test, 23.5 h of light were provided daily. Red incandescent light was used for the first 25 days; white incandescent light was used after 25 days while chickens were in cages. Traits Measured. Traits measured in the test included 28-day body weight (BW28), 42-day body weight (BW42), and feed consumption (FC) from 28 to 42 days. Gain (G) from 28 to 42 days was calculated as the difference between body weights (BW42 - BW28), and used with feed consumption to calculate feed efficiency (FE = G/FC). A maintenance requirement of 131.4 kcal ME/BW75kg per day was assumed based on published estimates (Shannon and Brown, 1969, 1970; Johnson and Crownover, 1976). Using this value, energy required for maintenance (EM) during the test period (14 days) was estimated as: EM = 14 (131.4 [(BW28 + BW42)/ 2] 75) kcal ME and feed intake used for maintenance (FCM) measured in grams as FCM = [EM(1,000)]/3,210. Therefore, feed used for growth (FCG) is FCG = FC - FCM and the feed efficiency of growth (FEG) is FEG = G/ FCG. Body weight in all EM calculations was expressed in kilograms. Differences among chickens in maintenance requirements due to factors other than size will not be reflected by these estimates. Similar calculations were performed to adjust feed intake for 28 and 42-day body weights. FCA = FC -b!(BW28-BW28) -b2(BW42-BW42), where b, and b 2 are the partial regression coefficients of FC on BW28 and BW42, respectively. Consumption based on constant weights (FCW) was calculated from FC by means of a two-phase procedure within each sex using intercept (i) and regression coefficient (b) estimates
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et al., 1981). However, tests of these strains to similar body weights revealed the true difference in feed conversion to be 1.8 vs. 3.2 (Chambers et al., 1983). Differences between age-constant feed consumption and efficiency values currently used, and either weight-constant, weightcorrected, or age-constant-weight-corrected (by regression) measures have been demonstrated by Chambers and Lin (1988). Unfortunately, testing feed efficiency of birds based on fixed weights is not practical for breeders due to repeated daily weighing required to determine the starting and finishing target weight for each individual. Correction of age-constant tests for differences in test body weights appears to be the best alternative. Heritability estimates for feed efficiency based on age-constant tests may be lower than those based on tests between fixed weights. The purpose of the current study was to estimate and compare heritabilities and correlations of feed consumption and efficiency before and after adjustments for differences in either maintenance requirements or initial and final test body weights. Such information will aid in the determination of appropriate adjustment procedures for estimating weight constant or true feed efficiency using age-constant feed efficiency test results.
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Interactions were nonsignificant, hence these obtained in a separate study as described for weight-corrected consumption by Chambers and effects were omitted from the model. HeritabilLin (1988). In the first phase, the regression ity estimates and genetic correlations based on coefficient (b) of body weight on age in days paternal half-sibs and phenotypic correlations (average daily gain) was divided into the differ- were obtained (Harvey, 1977). Partial correlaences between individual and average body tion coefficients were calculated from simple weights for that sex at 28 and 42 days. Resultant correlations (Snedecor and Cochran, 1967). values represented the number of days needed to adjust feed consumption at each age. In the RESULTS second phase, the coefficient for regression of feed consumption on age (average daily feed Trait means for each population-sex subclass consumption increase), and the intercept, both are presented in Table 1. Differences between derived from the separate study, were used to sexes and between populations of the same sex calculate daily feed consumption values. Hence were significant (P<.01) for all traits. The sire estimated feed consumption on day x = i + population grows so much faster than the dam bx. The resultant amount of feed consumption population that sire population pullets have adjustment is the accumulated daily feed con- weights, G and FCM comparable to those for sumption for Days 28, 27, etc. (to be added) if dam population males. Standard errors of poputhe chicken is heavier than average for that sex lation-sex means for FC, FCA, and FCW were at 4 wk; or consumption for Days 29, 30 etc. similar within traits for each subclass but dif(to be subtracted) if the chicken is below average fered among traits. Values for FCA were about weight at 28 days. The adjustment procedure is half the size of those values for FC and FCW. similar for deviations at 42 days, except feed is Heritabilities of unadjusted and adjusted traits subtracted for body weights above average and estimated within populations are presented in added for body weights below average. Table 2. Adjustment of FC for differences in Parameters used were obtained from dam body weight tended to reduce estimates of heritapopulation chickens (used in this study) tested bility within the sire population. Heritabilities in the same cages and building with similar ra- for each population were similar with the exceptions. Any error of estimation in feed consump- tion of FCA and FCW, which were lower in the tion adjustment is reduced due to the high sire population. phenotypic correlation between feed consumpGenetic and phenotypic correlations are listed tion and rate of gain and their inverse relation- in Table 3. Correlations, both phenotypic and ship in the calculation of adjustment. genetic, among body weights, G, and measures Statistical Analyses. Procedures of SAS of consumption were positive and generally (Goodnight, 1979) were used for statistical large (^.5) with the exception of FCW. The analyses; means and correlations were deter- traits with the exception of FCA generally had mined and multiple regression analyses were large (5=.5) negative correlations with FCW. performed. Differences among means of the Correlations of FCW and FCA with measures various population-sex subclasses were com- of efficiency (FE and FEG) were large and negative; correlations between FE and FEG were pared using Duncan's multiple range test. Within each population, variance and large and positive, probably due to the partcovariance components for sire, dam, and re- whole relationship. The two populations had similar phenotypic sidual effects were estimated (Harvey, 1977). correlations (differences < . l ) (Table 3). ExcepThe model used was: tions included smaller coefficients in the sire yijkim = M- + St; + Sy + dijk + X, + eijklm populations for FCA correlated with body weights, G, and FCM, and BW28 correlated with FEG. There were fewer similarities bewhere yyvim w a s t n e observation of the ith strain, j * sire, k dam, and 1th sex, \x was the population tween the populations in terms of genetic corremean, St; was the fixed effect of the i strain, lations. The FCA, and both FE and FEG were similar Sy was the random effect of the j t h sire of the i strain, dyk was the random effect of the kth in that their correlations with all other traits difdam mated to the j t h sire of the ith strain, X, was fered in the two populations, often by at least the fixed effect of the 1 sex, and eyk]m was the .5 (Table 3). In contrast, genetic correlations between FCA and the efficiency traits did not random error effect.
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BERNON AND CHAMBERS TABLE 1. Means ± SE for feed efficiency and related traits for broilers from 28 to 42 days of age Trait 1
Male
Sire
N BW28, g BW42, g G,g FQg FCM, g FCA,g FCW.g FE FEG
930 902 1,771 869 1,730 712 1,729 1,740
N BW28, g BW42, g
660 759 1,457 698 1,546 618 1,546 1,553
Dam
G,g FC, g FCM, g FCA.g FCW, g FE FEG
Female 902 770 1,458 688 1,474 621 1,474 1,481
± 3.2a ±5.5a ±3.5a ±5.7a ±1.7a ± 2.9a ±6.5a 502 + . 0 0 1 2 a 858 ± .0024a
673 686 1,271 585 1,328 563 1,328 1,333
± 3.3b ±5.3b ± 3.3b ±6.0b ± 1.7 b + 3.1b ±5.9b 451 ± . 0 0 1 3 C 756 + . 0 0 2 6 d
± 3.3b ±4.6b ± 2.6b ±5.1c ± 1.6 b + 3.0 C ±5.1c .467 ± . 0 0 1 2 b .813 ± . 0 0 2 8 b ± 3.1c ± 4.7 c ± 2.7C ± 5.3d ±1.6C ± 2.7d + 4.8d .441 ± . 0 0 1 2 d .771 ± . 0 0 2 8 c
a—d Means for the same trait with no common superscripts differ significantly (P<.01). 'BW28, BW42 = Body weight at 28 and 42 days, respectively; G = weight gain; FC = feed consumption; FCM = feed used for maintenance; FCA = feed consumption adjusted for body weight; FCW = feed consumption estimated for weight-constant test periods; FE = feed efficiency; FEG = feed efficiency of growth.
differ in the two populations. In most cases genetic correlations involving FCA in the dam population were less positive or more negative than coefficients involving efficiency traits, which were more positive or less negative. Hence, FCA is a better indicator of feed efficiency than FC. For FCW only genetic correlations with BW42 and G failed to differ in the two populations. Phenotypic and genetic partial correlations between consumption and efficiency traits when either BW28, or BW42, or G, or both BW28 and BW42, or FCM is held constant are presented in Table 4. Most correlations varied as the trait held constant changed. Holding body weights constant at the beginning and conclusion of the test tended to either eliminate correlation (values JUDY zero) or give rise to perfect (values JUDY 1.0) positive or negative correlation. These patterns were less apparent when individual traits were held constant. Phenotypic partial correlations from the two populations were quite similar (differences < . l ) . Genetic partial correlations from the two populations were similar less frequently, probably due to the larger errors associated with the genetic (vs. phenotypic) correlations used to estimate partial correlations.
DISCUSSION
Stocks tested in this study were derived from several commercial broiler parent stocks obtained in 1978. Hence these populations are ex-
TABLE 2. Heritability ± SE estimates for efficiencyrelated traits of sire and dam populations of meat type chickens
Trait 1
Sire population
Dam population
BW28 BW42 G FC FCM FCA FCW FE FEG
.35 .32 .26 .33 .33 .10 .13 .20 .16
.37 ± .44 ± .33 ± .33 ± .43 ± .31 ± .42+ .35 ± .28 +
± ± ± + ± ± ± ± ±
.08 .08 .07 .08 .08 .06 .06 .07 .06
.10 .10 .09 .09 .10 .09 .10 .09 .09
1 BW28, BW42 = Body weight at 28 and 42 days, respectively; G = weight gain; FC = feed consumption; FCM = feed used for maintenance; FCA = feed consumption adjusted for body weight; FCW = feed consumption estimated for weight-constant test periods; FE = feed efficiency; FEG = feed efficiency of growth.
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Population
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TABLE 3. Simple phenotypic (rp, above diagonals) and genetic (below diagonals) correlations between feed efficiency and broiler traits for the sire and dam populations Traits 1 ' 2
BW28
Sire
BW28 BW42 G FC FCM FCA FCW FE FEG
.86 .40 .80 .95 .81 -.28 -.69 -.80
BW28 BW42 G FC FCM FCA FCW FE FEG
.92 .64 .86 .96 .23 -.52 -.12 -.17
Dam
BW42 .81 .81 .98 .98 .70 -.70 -.28 -.63 .82 .89 .91 .99 .02 -.78 .23 .13
G
FC .32 .82
.84 .67 .32 -.94 .29 -.20 .30 .79 .78 .82 -.22 -.92 .59 .44
.52 .81 .80 .94 .76 -.66 -.27 -.68 .56 .84 .80 .91 .41 -.51 -.05 -.23
FCM
FCA
FCW
FE
FEG
.92 .97 .66 .73
.01 .01 .00 .53 .01
-.41 -.77 -.85 -.46 -.66 .49
-.19 .22 .55 -.05 .07 -.73 -.77
-.05 .06 .15 -.42 .02 -.96 -.59 .84
.77 -.55 -.46 -.72 .93 .97 .63 .76 .10 -.70 .10 .02
.01 -.78 -.99 .12 .14 .11 .61 .14 .57 -.84 -.98
-.50 -.10 -.38 -.74 -.83 -.45 -.63 .42 -.78 -.73
.85 -.28 .14 .54 -.06 -.02 -.66 -.76
-.16 -.02 .13 -.43 -.08 -.93 -.58 .84
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'BW28, BW42 = Body weight at 28 and 42 days, respectively; G = weight gain; FC = feed consumption; FCM = feed used for maintenance; FCA = feed consumption adjusted for body weight; FCW = feed consumption estimated for weight-constant test periods; FE = feed efficiency; FEG = feed efficiency of growth. 2 Minimum values of rp required to be significantly different from 0 are: for sire population, P<.05, .046; P«.01, .061; for dam population, P«.05, .054;P«.01, .071.
pected to represent genetically modern commercial broiler stocks (Chambers et al., 1984). Adjustment of feed consumption for differences in body weight by regression was based on a linear growth curve between 28 and 42 days of age. The growth curve is rarely perfectly linear during 2-wk intervals (Chambers and Lin, 1988); however, the effect of deviations from true values for the traits being estimated and estimates of these traits based on the assumption that growth is linear on the rank of a flock of chickens is believed to be negligible. Means for FC and FCA are similar because FCA values of each subclass were adjusted to average BW28 and BW42 values. Hence, means did not change. The reduced standard errors for FCA suggest that much of the variation in FC is due to differences in body weight during the test period. Changes in FCW are not based on linear relationships between FC and BW with age. Therefore, these correlations for body weight do alter the average FCS value. Heritabilities estimated in the present study are comparable to those reported by Chambers et al. (1984) for identical traits (BW28, BW42)
or similar traits (G28-42 vs. G28^19; FC28^2 vs. FC28^19; FE28^12 vs. FE28^19), and comparable to similar traits (FE vs. FC21—42; BW42) for values based on sire or dam variance components presented by Leenstra et al. (1986). Much higher heritabilities for similar traits (35day weight vs. BW28 and BW42; 5 to 9-wk gain vs. 4 to 6-wk gain; and consumption) were found by Pym and Nicholls (1979). Results observed for body weights by Leenstra etal. (1986) were similar to results of the present research; however, lower estimates were obtained for gain. Corresponding heritability estimates from the two studies mentioned above differed less than one standard deviation. The low phenotypic correlations (Table 3) between FCA and BW28 and BW42 in both populations indicate that FCA reflects differences in feed consumption due to effects other than size or growth rate. Differences between FCW correlations and other consumption traits correlated with body weight reveal the effects of initial and final body weight differences. These effects were also noted by Chambers and Lin (1988).
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Population
FC-FCM FC-FCA FC-FCW FC-FE FC-FEG FCM-FCA FCM-FCW FCM-FE FCM-FEG FCA-FCW FCA-FE FCA-FEG FCW-FE FCW-FEG FE-FEG
FC-FCM FC-FCA FC-FCW FC-FE FC-FEG FCM-FCA FCM-FCW FCM-FE FCM-FEG FCA-FCW FCA-FE FCA-FEG FCW-FE FCW-FEG FE-FEG
Trait 1
-.99 -.90 .00
.91 .46
-.34 -.76
.01 .59
-.71 -.25
.12
.00
-.98 -.96 -.97 -.95 .98
.95
-.82 -.95 -.66 -.85 .92
.78
-.69 -.94 -.98 -.88
.85
.83
.08 .98
-.72 -.95 -.97 -.69
-.98 -.97 -.00 -.09 -.04
.98
1.00
.68 .18 .57
-.82
-.42
-.31
-.24 -.56 -.21
.56 .83 .66
-.41 .05
.99
.92
.85
.85
.80 .60
-.97 -.97 -.96 -.95
.93
-.87 -.97 -.70 -.88
.78
-.75 -.96 -.97 -.85
-.74 -.96 -.95 -.66
.02 .01 .97
.01
.01 .58
-.64 -.16
-.98 -.98 -.01 -.11
-.98 -.91
-.41 -.81 -.27 -.46 -.10
.03 .99 .96
BW(28+42) .45 .89 .70
G
.90 .45
-.41
BW42
Phenotypic partial correlations adjusted for:
.65 .17 .54
-.46 -.01 -.83
.05
-.32
.78 .61
BW28
68 69 94 99 80 84
76 05 06 57
66 .73 96 98 .76 84
.77 .05 15 .64
FCM
.97
-.84 -.98 -1.00 -.97
.87 .71 .83
-.17 -.48 -.88
.12
-.14
.58 .43
.68
-.51 -.98 -1.01 -.56
.86 .20 .42
-.92
.01
-.12
.65
-.76
.95 .33
BW28
.9
-1.0
.2 .9 .7 -.6 -.8 .5 .8 -.9 -.7 .9 -.8 -.9 -.9
-.9 .9
-1.0
.5 .9 -.9 -.6 .9 -.8 -.9
-.
-.4 .5 .1 .0
BW4
Gene
2
Values in parentheses are based on calculations using .999 value for correlations > 1 .
1 BW28, BW42 = Body weight at 28 and 42 days, respectively; G = weight gain; FC = feed consumption; FCM = feed adjusted for body weight; FCW = feed consumption estimated for weight-constant test periods; FE = feed efficiency; FEG
Population
TABLE 4. Phenotypic and genetic partial correlations for the sire and dam populations adjusted fo
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FEED EFFICIENCY ADJUSTMENT
± 1, demonstrates the importance of removing differences due to growth rate (or body weights) from measures of feed consumption when attempting to evaluate feed efficiency. This is supported by differences between correlations (Table 3) of weight-constant feed consumption (FCW) with body weights and gain and age-constant feed consumption (FC) with body weights and gain. These points further support the conclusion of Chambers and Lin (1988) that ageconstant measures of feed consumption and efficiency must be corrected for differences in body weights for more accurate evaluation of feed efficiency differences among chickens. The current study presents statistical support for the argument of Chambers et al. (1983) and Chambers and Lin (1988) that feed efficiency tests require adjustment for differences in test body weights. Based on the current results, it is advisable to adjust feed consumption and efficiency for differences in initial and final body weights in order to represent efficiency tests based on given weights.
ACKNOWLEDGMENTS
The authors wish to thank M. E. Reid for technical assistance and A. R. Morrison and his farm crew for care of the birds. REFERENCES Chambers, J. R., D. E. Bernon, and J. S. Gavora, 1984. Synthesis and parameters in new populations of meattype chickens. Theor. Appl. Genet. 69:23-30. Chambers, J. R., A. Fortin, and A. A. Grunder, 1983. Relationships between carcass fatness and feed efficiency and its component traits in broiler chickens. Poultry Sci. 62:2201-2207. Chambers, J. R., J. S. Gavora, and A. Fortin, 1981. Genetic changes in meat-type chickens in the last twenty years. Can. J. Anim. Sci. 61:555-563. Chambers, J. R., and C. Y. Lin, 1988. Age-constant versus weight-constant feed consumption and efficiency in broiler chickens. Poultry Sci. 67:565-576. Fox, T. W., and B. B. Bohren, 1954. An analysis of feed efficiency among breeds of chickens and its relationship to rate of growth. Poultry Sci. 33:549-561. Goodnight, J. H., 1979. SAS User's Guide. SAS Inst. Inc., Raleigh, NC. Harvey, W. R., 1977. User's guide for LSML76. Monograph, Ohio State Univ., Columbus, OH. Hendricks, W. A., 1931. Fitting the curve of the diminishing increment to feed consumption-live weight growth curves. Science 74:290-291. Hendricks, W. A., M. A. Jull, and H. W. Titus, 1931. A possible physiological interpretation of the law of the diminishing increment. Science 73:427-429. Johnson, D. E., and J. C. Crownover, 1976. Maintenance energy requirements of lean vs. obese growing chicks
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Correction for both BW28 and BW42 with consumption and efficiency traits (Table 4) results in larger values or values closer to zero; it thus appears that correction for both body weights results in better prediction of other traits than correction for only one weight or weight gain. Correction of feed consumption for differences in test body weights (FCA) reduced the variation of this trait (i.e., FC). In the sire population, heritability was also reduced by this correction. The correction removes variation due to differences in growth rate. However, feed consumption corrected for differences in body weights (FCA) is highly negatively correlated with feed efficiency measures and weakly correlated with body weights (except the genetic correlation in the sire population). Weight-constant feed consumption (FCW) showed similar patterns, but had higher heritability in the dam population. Compared with FCW, FC of rapidly growing chickens will be greater due not only to greater gains during test but also to larger body size during the test (resulting in greater maintenance requirements). This relationship is believed to be partly, if not totally, responsible for the vast differences between FC and FCW in correlations with BW42: e.g., .52 to .84 for FC vs. -.38 to -.77 for FCW. The FCA, which estimates FC of chickens of the same growth rate and age, had very small phenotypic correlations with BW42(.01 to .14). Correlations, both phenotypic and genetic, between FC and FCW were negative (-.45 to -.46 and -.51 to -.66, respectively); however, corresponding correlations between FC and FCA (.53 to .61 and .41 to .76) and between FCA and FCW (.42 to .49 and .01 to .57) were positive. The FCW represents a proper evaluation of FC, including differences in growth rate. The FC is not, however, a proper evaluation of feed consumption, as it includes an error due to differences in size and amount of gain due to variation in growth rate during test. Hence, selection against either of these traits should be an effective method for joint genetic improvement of feed efficiency and growth rate. It would certainly be more effective than selection against either age-constant feed consumption or feed consumed for maintenance. The tendency for partial correlations (Table 4) of feed efficiency measures in which body weights at the beginning and the conclusion of the feed trial are held constant to approach either zero or, where high correlations are desired,
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BERNON AND CHAMBERS sumption and food conversion ratio. Br. Poult. Sci. 20:87-97. Shannon, D.W.F., and W. O. Brown, 1969. Calorimetric studies on the effect of dietary energy source and environmental temperature on the metabolic efficiency of energy utilization by mature Light Sussex cockerels. J. Agric. Sci. 72:479-489. Shannon, D.W.F., andW. O. Brown, 1970. A calorimetric estimate of the efficiency of utilization of dietary energy by the growing cockerel. Br. Poult. Sci. 11:16. Snedecor, G. W., and W. G. Cochran, 1967. Statistical Methods. 6th ed. Iowa State Univ. Press, Ames, IA. Washburn, K. W., R. A. Guill, and H. M. Edwards, Jr., 1975. Influence of genetic differences in feed efficiency on carcass composition of young chickens. J. Nutr. 105:1311-1317. Whitehead, C. C , and H. D. Griffin, 1986. Development of divergent lines of lean and fat broilers using plasma very low density lipoprotein concentration as selection criterion: results over the fourth generation and lack of effect of dietary fat on performance and carcass fat content. Br. Poult. Sci. 27:317-324.
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at equal age and body energy. Pages 121-124 in: Energy Metabolism of Farm Animals. M. Vermorel, ed. G. de Bussac, Clermont-Ferrand, France. Jull, M. A., and H. W. Titus, 1928. Growth of chickens in relation to feed consumption. J. Agric. Res. 36:541550. Leclercq, B., J. C. Blum, and J. P. Boyer, 1980. Selecting broilers for low or high abdominal fat: Initial observations. Br. Poult. Sci. 21:107-113. Leenstra, F. R., P.F.G. Vereijken, and R. Pit, 1986. Fat deposition in a broiler sire strain. 1. Phenotypic and genetic variation in, and correlations between, abdominal fat, body weight, and feed conversion. Poultry Sci. 65:1225-1235. Marks, H. L., 1979. Growth rate and feed intake of selected and nonselected broilers. Growth 43:80-90. Pym, R.A.E., and P. J. Nicholls, 1979. Selection for food conversion in broilers: Direct and correlated responses to selection for body-weight gain, food consumption and food conversion ratio. Br. Poult. Sci. 20:73-86. Pym, R.A.E., and A. J. Solvyns, 1979. Selection for food conversion in broilers: Body composition of birds selected for increased body-weight gain, food con-
1988 Poultry Science 67:1497-1504 INTRODUCTION
Breeding programs for meat type chickens have emphasized body weight and rate of gain. Efficiency of feed conversion is important but has been neglected in many broiler selection programs. Recently, two factors have enhanced the value of this trait in breeding programs: Increases in feed costs have become persistent, plus a negative relationship, both phenotypic and genetic, has been reported between feed efficiency and fatness (Washburn et al., 1975; Pym and Solvyns, 1979; Leclercq et al., 1980; Chambers et al., 1983, 1984; Leenstra et al., 1986; Whitehead and Griffin, 1986). The law of diminishing increment has been applied to describe the relationship between feed intake and liveweight with respect to time for the growing chicken (Jull and Titus, 1928; Hendricks, 1931; Hendricks et al., 1931; Fox and Bohren, 1954). Using differential equation techniques, the equation can be modified to express
Contribution Number 1295, Animal Research Centre.
feed efficiency as a function of the amount of feed required for maintenance. The modified equation reveals an increase in the proportion of feed intake used for maintenance as the chicken grows. Consequently, feed efficiency declines with growth, attaining zero at maturity. Differences in maintenance requirements in ageconstant feed efficiency tests give rise to an error that is negatively correlated with growth rate and hence, with feed efficiency (Chambers and Lin, 1988). In comparisons based on specific weights, a slow growing chicken has a higher maintenance requirement than a fast growing bird; however, in comparisons based on specific ages, faster growing chickens are penalized, because they have heavier body weights and thus are tested at a less efficient phase of growth. The increase in feed required for maintenance due to increased body weight almost nullifies any beneficial effect of rapid growth on feed efficiency (gain/consumption) or its reciprocal, feed conversion (Marks, 1979; Chambers et al., 1981). Broilers from rapid and slow growing strains differed little in feed conversion (1.9 vs. 2.0) when tested to the same age (47 days, Chambers
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ABSTRACT Body weights at 28 and 42 days (BW28 and BW42) and weight gain (G), feed consumption (FC), and efficiency (FE, gain/feed) for the intervening period were either measured or calculated for pedigreed chickens from synthetic sire and dam populations. Additional variables were calculated: feed consumed to meet maintenance requirements (FCM); feed consumed if growth rates were similar (FCA); feed consumed for weight-constant test periods (FCW); and efficiency of feed used for growth (FEG). Heritabilities of and correlations among these traits were estimated for each population. Heritabilities of most traits were similar to corresponding estimates available in the literature. Heritability values from the sire and dam populations for BW28, BW42, G, FC, FCM, FE, and FEG were similar. Exceptions were FCA and FCW in the sire population. Genetic and phenotypic correlations for the two populations were similar, with exceptions involving FE, FEG, FCA, and FCW most of the time. Results indicate that weight-constant measures of feed consumption and efficiency or age-constant measures of these traits corrected for differences in test body weights may have lower heritabilities than age-constant feed consumption and efficiency in the sire population. However, these measures of consumption may be better indicators of efficiency due to their smaller positive or negative correlations, both phenotypic and genetic, with body weight and weight gain, and their large negative correlations with efficiency measures, especially efficiency of feed used for growth. Based on partial correlations, consumption or efficiency measures corrected for both beginning and concluding test body weights are better predictors of efficiency than uncorrected measures. (Key words: feed efficiency, heritabilities, body weight, age constant, weight constant)
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MATERIALS AND METHODS
Chickens. Pedigreed chickens from broiler sire and dam populations synthesized from nine broiler sire and seven broiler dam stocks (Chambers et al., 1984) were tested. Parents of the sire population were selected for body weight (28 days) and either feed efficiency (28 to 42 days) or low percentage abdominal fat of sibs (47 days), or both. In the dam population parents had been selected for body weight (28 days), feed efficiency (28 to 42 days), and hatching egg production of females to 40 wk of age. For both populations, this was the first generation of genetic selection following population synthesis. In the sire population, progeny were obtained from 181 sires and 510 dams (2 to 4 dams per sire). In the dam population, progeny were obtained from 115 sires and 361 dams. Most parents were represented by progeny of both sexes. Management Procedures. Pedigreed chicks were vent sexed and wingbanded at hatch, vaccinated to prevent Marek's disease, and placed in pens for brooding to 25 days of age. From
25 days of age to end of test at 42 days of age, chickens were confined individually in wire cages (20.3 cm x 40.7 cm) normally used for laying hens. Cages were adapted for the meat type chickens by inserting level wire mesh floors and water lines with drippers approximately 15 cm above the floor. Plastic feed containers were inserted in the feed trough to permit measurement of individual feed intake. A crumbled broiler starter diet (3,155 kcal ME/kg, with 23.5% CP) was fed to 28 days of age. Thereafter a crumbled broiler grower diet (3,210 kcal ME/kg; 20.8% CP) was fed. The amount of feed provided each chicken was recorded and the amount of feed remaining at the completion of the test was weighed to permit calculation of feed consumed. From hatch to the end of test, 23.5 h of light were provided daily. Red incandescent light was used for the first 25 days; white incandescent light was used after 25 days while chickens were in cages. Traits Measured. Traits measured in the test included 28-day body weight (BW28), 42-day body weight (BW42), and feed consumption (FC) from 28 to 42 days. Gain (G) from 28 to 42 days was calculated as the difference between body weights (BW42 - BW28), and used with feed consumption to calculate feed efficiency (FE = G/FC). A maintenance requirement of 131.4 kcal ME/BW75kg per day was assumed based on published estimates (Shannon and Brown, 1969, 1970; Johnson and Crownover, 1976). Using this value, energy required for maintenance (EM) during the test period (14 days) was estimated as: EM = 14 (131.4 [(BW28 + BW42)/ 2] 75) kcal ME and feed intake used for maintenance (FCM) measured in grams as FCM = [EM(1,000)]/3,210. Therefore, feed used for growth (FCG) is FCG = FC - FCM and the feed efficiency of growth (FEG) is FEG = G/ FCG. Body weight in all EM calculations was expressed in kilograms. Differences among chickens in maintenance requirements due to factors other than size will not be reflected by these estimates. Similar calculations were performed to adjust feed intake for 28 and 42-day body weights. FCA = FC -b!(BW28-BW28) -b2(BW42-BW42), where b, and b 2 are the partial regression coefficients of FC on BW28 and BW42, respectively. Consumption based on constant weights (FCW) was calculated from FC by means of a two-phase procedure within each sex using intercept (i) and regression coefficient (b) estimates
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et al., 1981). However, tests of these strains to similar body weights revealed the true difference in feed conversion to be 1.8 vs. 3.2 (Chambers et al., 1983). Differences between age-constant feed consumption and efficiency values currently used, and either weight-constant, weightcorrected, or age-constant-weight-corrected (by regression) measures have been demonstrated by Chambers and Lin (1988). Unfortunately, testing feed efficiency of birds based on fixed weights is not practical for breeders due to repeated daily weighing required to determine the starting and finishing target weight for each individual. Correction of age-constant tests for differences in test body weights appears to be the best alternative. Heritability estimates for feed efficiency based on age-constant tests may be lower than those based on tests between fixed weights. The purpose of the current study was to estimate and compare heritabilities and correlations of feed consumption and efficiency before and after adjustments for differences in either maintenance requirements or initial and final test body weights. Such information will aid in the determination of appropriate adjustment procedures for estimating weight constant or true feed efficiency using age-constant feed efficiency test results.
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Interactions were nonsignificant, hence these obtained in a separate study as described for weight-corrected consumption by Chambers and effects were omitted from the model. HeritabilLin (1988). In the first phase, the regression ity estimates and genetic correlations based on coefficient (b) of body weight on age in days paternal half-sibs and phenotypic correlations (average daily gain) was divided into the differ- were obtained (Harvey, 1977). Partial correlaences between individual and average body tion coefficients were calculated from simple weights for that sex at 28 and 42 days. Resultant correlations (Snedecor and Cochran, 1967). values represented the number of days needed to adjust feed consumption at each age. In the RESULTS second phase, the coefficient for regression of feed consumption on age (average daily feed Trait means for each population-sex subclass consumption increase), and the intercept, both are presented in Table 1. Differences between derived from the separate study, were used to sexes and between populations of the same sex calculate daily feed consumption values. Hence were significant (P<.01) for all traits. The sire estimated feed consumption on day x = i + population grows so much faster than the dam bx. The resultant amount of feed consumption population that sire population pullets have adjustment is the accumulated daily feed con- weights, G and FCM comparable to those for sumption for Days 28, 27, etc. (to be added) if dam population males. Standard errors of poputhe chicken is heavier than average for that sex lation-sex means for FC, FCA, and FCW were at 4 wk; or consumption for Days 29, 30 etc. similar within traits for each subclass but dif(to be subtracted) if the chicken is below average fered among traits. Values for FCA were about weight at 28 days. The adjustment procedure is half the size of those values for FC and FCW. similar for deviations at 42 days, except feed is Heritabilities of unadjusted and adjusted traits subtracted for body weights above average and estimated within populations are presented in added for body weights below average. Table 2. Adjustment of FC for differences in Parameters used were obtained from dam body weight tended to reduce estimates of heritapopulation chickens (used in this study) tested bility within the sire population. Heritabilities in the same cages and building with similar ra- for each population were similar with the exceptions. Any error of estimation in feed consump- tion of FCA and FCW, which were lower in the tion adjustment is reduced due to the high sire population. phenotypic correlation between feed consumpGenetic and phenotypic correlations are listed tion and rate of gain and their inverse relation- in Table 3. Correlations, both phenotypic and ship in the calculation of adjustment. genetic, among body weights, G, and measures Statistical Analyses. Procedures of SAS of consumption were positive and generally (Goodnight, 1979) were used for statistical large (^.5) with the exception of FCW. The analyses; means and correlations were deter- traits with the exception of FCA generally had mined and multiple regression analyses were large (5=.5) negative correlations with FCW. performed. Differences among means of the Correlations of FCW and FCA with measures various population-sex subclasses were com- of efficiency (FE and FEG) were large and negative; correlations between FE and FEG were pared using Duncan's multiple range test. Within each population, variance and large and positive, probably due to the partcovariance components for sire, dam, and re- whole relationship. The two populations had similar phenotypic sidual effects were estimated (Harvey, 1977). correlations (differences < . l ) (Table 3). ExcepThe model used was: tions included smaller coefficients in the sire yijkim = M- + St; + Sy + dijk + X, + eijklm populations for FCA correlated with body weights, G, and FCM, and BW28 correlated with FEG. There were fewer similarities bewhere yyvim w a s t n e observation of the ith strain, j * sire, k dam, and 1th sex, \x was the population tween the populations in terms of genetic corremean, St; was the fixed effect of the i strain, lations. The FCA, and both FE and FEG were similar Sy was the random effect of the j t h sire of the i strain, dyk was the random effect of the kth in that their correlations with all other traits difdam mated to the j t h sire of the ith strain, X, was fered in the two populations, often by at least the fixed effect of the 1 sex, and eyk]m was the .5 (Table 3). In contrast, genetic correlations between FCA and the efficiency traits did not random error effect.
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BERNON AND CHAMBERS TABLE 1. Means ± SE for feed efficiency and related traits for broilers from 28 to 42 days of age Trait 1
Male
Sire
N BW28, g BW42, g G,g FQg FCM, g FCA,g FCW.g FE FEG
930 902 1,771 869 1,730 712 1,729 1,740
N BW28, g BW42, g
660 759 1,457 698 1,546 618 1,546 1,553
Dam
G,g FC, g FCM, g FCA.g FCW, g FE FEG
Female 902 770 1,458 688 1,474 621 1,474 1,481
± 3.2a ±5.5a ±3.5a ±5.7a ±1.7a ± 2.9a ±6.5a 502 + . 0 0 1 2 a 858 ± .0024a
673 686 1,271 585 1,328 563 1,328 1,333
± 3.3b ±5.3b ± 3.3b ±6.0b ± 1.7 b + 3.1b ±5.9b 451 ± . 0 0 1 3 C 756 + . 0 0 2 6 d
± 3.3b ±4.6b ± 2.6b ±5.1c ± 1.6 b + 3.0 C ±5.1c .467 ± . 0 0 1 2 b .813 ± . 0 0 2 8 b ± 3.1c ± 4.7 c ± 2.7C ± 5.3d ±1.6C ± 2.7d + 4.8d .441 ± . 0 0 1 2 d .771 ± . 0 0 2 8 c
a—d Means for the same trait with no common superscripts differ significantly (P<.01). 'BW28, BW42 = Body weight at 28 and 42 days, respectively; G = weight gain; FC = feed consumption; FCM = feed used for maintenance; FCA = feed consumption adjusted for body weight; FCW = feed consumption estimated for weight-constant test periods; FE = feed efficiency; FEG = feed efficiency of growth.
differ in the two populations. In most cases genetic correlations involving FCA in the dam population were less positive or more negative than coefficients involving efficiency traits, which were more positive or less negative. Hence, FCA is a better indicator of feed efficiency than FC. For FCW only genetic correlations with BW42 and G failed to differ in the two populations. Phenotypic and genetic partial correlations between consumption and efficiency traits when either BW28, or BW42, or G, or both BW28 and BW42, or FCM is held constant are presented in Table 4. Most correlations varied as the trait held constant changed. Holding body weights constant at the beginning and conclusion of the test tended to either eliminate correlation (values JUDY zero) or give rise to perfect (values JUDY 1.0) positive or negative correlation. These patterns were less apparent when individual traits were held constant. Phenotypic partial correlations from the two populations were quite similar (differences < . l ) . Genetic partial correlations from the two populations were similar less frequently, probably due to the larger errors associated with the genetic (vs. phenotypic) correlations used to estimate partial correlations.
DISCUSSION
Stocks tested in this study were derived from several commercial broiler parent stocks obtained in 1978. Hence these populations are ex-
TABLE 2. Heritability ± SE estimates for efficiencyrelated traits of sire and dam populations of meat type chickens
Trait 1
Sire population
Dam population
BW28 BW42 G FC FCM FCA FCW FE FEG
.35 .32 .26 .33 .33 .10 .13 .20 .16
.37 ± .44 ± .33 ± .33 ± .43 ± .31 ± .42+ .35 ± .28 +
± ± ± + ± ± ± ± ±
.08 .08 .07 .08 .08 .06 .06 .07 .06
.10 .10 .09 .09 .10 .09 .10 .09 .09
1 BW28, BW42 = Body weight at 28 and 42 days, respectively; G = weight gain; FC = feed consumption; FCM = feed used for maintenance; FCA = feed consumption adjusted for body weight; FCW = feed consumption estimated for weight-constant test periods; FE = feed efficiency; FEG = feed efficiency of growth.
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Population
FEED EFFICIENCY ADJUSTMENT
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TABLE 3. Simple phenotypic (rp, above diagonals) and genetic (below diagonals) correlations between feed efficiency and broiler traits for the sire and dam populations Traits 1 ' 2
BW28
Sire
BW28 BW42 G FC FCM FCA FCW FE FEG
.86 .40 .80 .95 .81 -.28 -.69 -.80
BW28 BW42 G FC FCM FCA FCW FE FEG
.92 .64 .86 .96 .23 -.52 -.12 -.17
Dam
BW42 .81 .81 .98 .98 .70 -.70 -.28 -.63 .82 .89 .91 .99 .02 -.78 .23 .13
G
FC .32 .82
.84 .67 .32 -.94 .29 -.20 .30 .79 .78 .82 -.22 -.92 .59 .44
.52 .81 .80 .94 .76 -.66 -.27 -.68 .56 .84 .80 .91 .41 -.51 -.05 -.23
FCM
FCA
FCW
FE
FEG
.92 .97 .66 .73
.01 .01 .00 .53 .01
-.41 -.77 -.85 -.46 -.66 .49
-.19 .22 .55 -.05 .07 -.73 -.77
-.05 .06 .15 -.42 .02 -.96 -.59 .84
.77 -.55 -.46 -.72 .93 .97 .63 .76 .10 -.70 .10 .02
.01 -.78 -.99 .12 .14 .11 .61 .14 .57 -.84 -.98
-.50 -.10 -.38 -.74 -.83 -.45 -.63 .42 -.78 -.73
.85 -.28 .14 .54 -.06 -.02 -.66 -.76
-.16 -.02 .13 -.43 -.08 -.93 -.58 .84
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'BW28, BW42 = Body weight at 28 and 42 days, respectively; G = weight gain; FC = feed consumption; FCM = feed used for maintenance; FCA = feed consumption adjusted for body weight; FCW = feed consumption estimated for weight-constant test periods; FE = feed efficiency; FEG = feed efficiency of growth. 2 Minimum values of rp required to be significantly different from 0 are: for sire population, P<.05, .046; P«.01, .061; for dam population, P«.05, .054;P«.01, .071.
pected to represent genetically modern commercial broiler stocks (Chambers et al., 1984). Adjustment of feed consumption for differences in body weight by regression was based on a linear growth curve between 28 and 42 days of age. The growth curve is rarely perfectly linear during 2-wk intervals (Chambers and Lin, 1988); however, the effect of deviations from true values for the traits being estimated and estimates of these traits based on the assumption that growth is linear on the rank of a flock of chickens is believed to be negligible. Means for FC and FCA are similar because FCA values of each subclass were adjusted to average BW28 and BW42 values. Hence, means did not change. The reduced standard errors for FCA suggest that much of the variation in FC is due to differences in body weight during the test period. Changes in FCW are not based on linear relationships between FC and BW with age. Therefore, these correlations for body weight do alter the average FCS value. Heritabilities estimated in the present study are comparable to those reported by Chambers et al. (1984) for identical traits (BW28, BW42)
or similar traits (G28-42 vs. G28^19; FC28^2 vs. FC28^19; FE28^12 vs. FE28^19), and comparable to similar traits (FE vs. FC21—42; BW42) for values based on sire or dam variance components presented by Leenstra et al. (1986). Much higher heritabilities for similar traits (35day weight vs. BW28 and BW42; 5 to 9-wk gain vs. 4 to 6-wk gain; and consumption) were found by Pym and Nicholls (1979). Results observed for body weights by Leenstra etal. (1986) were similar to results of the present research; however, lower estimates were obtained for gain. Corresponding heritability estimates from the two studies mentioned above differed less than one standard deviation. The low phenotypic correlations (Table 3) between FCA and BW28 and BW42 in both populations indicate that FCA reflects differences in feed consumption due to effects other than size or growth rate. Differences between FCW correlations and other consumption traits correlated with body weight reveal the effects of initial and final body weight differences. These effects were also noted by Chambers and Lin (1988).
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Population
FC-FCM FC-FCA FC-FCW FC-FE FC-FEG FCM-FCA FCM-FCW FCM-FE FCM-FEG FCA-FCW FCA-FE FCA-FEG FCW-FE FCW-FEG FE-FEG
FC-FCM FC-FCA FC-FCW FC-FE FC-FEG FCM-FCA FCM-FCW FCM-FE FCM-FEG FCA-FCW FCA-FE FCA-FEG FCW-FE FCW-FEG FE-FEG
Trait 1
-.99 -.90 .00
.91 .46
-.34 -.76
.01 .59
-.71 -.25
.12
.00
-.98 -.96 -.97 -.95 .98
.95
-.82 -.95 -.66 -.85 .92
.78
-.69 -.94 -.98 -.88
.85
.83
.08 .98
-.72 -.95 -.97 -.69
-.98 -.97 -.00 -.09 -.04
.98
1.00
.68 .18 .57
-.82
-.42
-.31
-.24 -.56 -.21
.56 .83 .66
-.41 .05
.99
.92
.85
.85
.80 .60
-.97 -.97 -.96 -.95
.93
-.87 -.97 -.70 -.88
.78
-.75 -.96 -.97 -.85
-.74 -.96 -.95 -.66
.02 .01 .97
.01
.01 .58
-.64 -.16
-.98 -.98 -.01 -.11
-.98 -.91
-.41 -.81 -.27 -.46 -.10
.03 .99 .96
BW(28+42) .45 .89 .70
G
.90 .45
-.41
BW42
Phenotypic partial correlations adjusted for:
.65 .17 .54
-.46 -.01 -.83
.05
-.32
.78 .61
BW28
68 69 94 99 80 84
76 05 06 57
66 .73 96 98 .76 84
.77 .05 15 .64
FCM
.97
-.84 -.98 -1.00 -.97
.87 .71 .83
-.17 -.48 -.88
.12
-.14
.58 .43
.68
-.51 -.98 -1.01 -.56
.86 .20 .42
-.92
.01
-.12
.65
-.76
.95 .33
BW28
.9
-1.0
.2 .9 .7 -.6 -.8 .5 .8 -.9 -.7 .9 -.8 -.9 -.9
-.9 .9
-1.0
.5 .9 -.9 -.6 .9 -.8 -.9
-.
-.4 .5 .1 .0
BW4
Gene
2
Values in parentheses are based on calculations using .999 value for correlations > 1 .
1 BW28, BW42 = Body weight at 28 and 42 days, respectively; G = weight gain; FC = feed consumption; FCM = feed adjusted for body weight; FCW = feed consumption estimated for weight-constant test periods; FE = feed efficiency; FEG
Population
TABLE 4. Phenotypic and genetic partial correlations for the sire and dam populations adjusted fo
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FEED EFFICIENCY ADJUSTMENT
± 1, demonstrates the importance of removing differences due to growth rate (or body weights) from measures of feed consumption when attempting to evaluate feed efficiency. This is supported by differences between correlations (Table 3) of weight-constant feed consumption (FCW) with body weights and gain and age-constant feed consumption (FC) with body weights and gain. These points further support the conclusion of Chambers and Lin (1988) that ageconstant measures of feed consumption and efficiency must be corrected for differences in body weights for more accurate evaluation of feed efficiency differences among chickens. The current study presents statistical support for the argument of Chambers et al. (1983) and Chambers and Lin (1988) that feed efficiency tests require adjustment for differences in test body weights. Based on the current results, it is advisable to adjust feed consumption and efficiency for differences in initial and final body weights in order to represent efficiency tests based on given weights.
ACKNOWLEDGMENTS
The authors wish to thank M. E. Reid for technical assistance and A. R. Morrison and his farm crew for care of the birds. REFERENCES Chambers, J. R., D. E. Bernon, and J. S. Gavora, 1984. Synthesis and parameters in new populations of meattype chickens. Theor. Appl. Genet. 69:23-30. Chambers, J. R., A. Fortin, and A. A. Grunder, 1983. Relationships between carcass fatness and feed efficiency and its component traits in broiler chickens. Poultry Sci. 62:2201-2207. Chambers, J. R., J. S. Gavora, and A. Fortin, 1981. Genetic changes in meat-type chickens in the last twenty years. Can. J. Anim. Sci. 61:555-563. Chambers, J. R., and C. Y. Lin, 1988. Age-constant versus weight-constant feed consumption and efficiency in broiler chickens. Poultry Sci. 67:565-576. Fox, T. W., and B. B. Bohren, 1954. An analysis of feed efficiency among breeds of chickens and its relationship to rate of growth. Poultry Sci. 33:549-561. Goodnight, J. H., 1979. SAS User's Guide. SAS Inst. Inc., Raleigh, NC. Harvey, W. R., 1977. User's guide for LSML76. Monograph, Ohio State Univ., Columbus, OH. Hendricks, W. A., 1931. Fitting the curve of the diminishing increment to feed consumption-live weight growth curves. Science 74:290-291. Hendricks, W. A., M. A. Jull, and H. W. Titus, 1931. A possible physiological interpretation of the law of the diminishing increment. Science 73:427-429. Johnson, D. E., and J. C. Crownover, 1976. Maintenance energy requirements of lean vs. obese growing chicks
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Correction for both BW28 and BW42 with consumption and efficiency traits (Table 4) results in larger values or values closer to zero; it thus appears that correction for both body weights results in better prediction of other traits than correction for only one weight or weight gain. Correction of feed consumption for differences in test body weights (FCA) reduced the variation of this trait (i.e., FC). In the sire population, heritability was also reduced by this correction. The correction removes variation due to differences in growth rate. However, feed consumption corrected for differences in body weights (FCA) is highly negatively correlated with feed efficiency measures and weakly correlated with body weights (except the genetic correlation in the sire population). Weight-constant feed consumption (FCW) showed similar patterns, but had higher heritability in the dam population. Compared with FCW, FC of rapidly growing chickens will be greater due not only to greater gains during test but also to larger body size during the test (resulting in greater maintenance requirements). This relationship is believed to be partly, if not totally, responsible for the vast differences between FC and FCW in correlations with BW42: e.g., .52 to .84 for FC vs. -.38 to -.77 for FCW. The FCA, which estimates FC of chickens of the same growth rate and age, had very small phenotypic correlations with BW42(.01 to .14). Correlations, both phenotypic and genetic, between FC and FCW were negative (-.45 to -.46 and -.51 to -.66, respectively); however, corresponding correlations between FC and FCA (.53 to .61 and .41 to .76) and between FCA and FCW (.42 to .49 and .01 to .57) were positive. The FCW represents a proper evaluation of FC, including differences in growth rate. The FC is not, however, a proper evaluation of feed consumption, as it includes an error due to differences in size and amount of gain due to variation in growth rate during test. Hence, selection against either of these traits should be an effective method for joint genetic improvement of feed efficiency and growth rate. It would certainly be more effective than selection against either age-constant feed consumption or feed consumed for maintenance. The tendency for partial correlations (Table 4) of feed efficiency measures in which body weights at the beginning and the conclusion of the feed trial are held constant to approach either zero or, where high correlations are desired,
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BERNON AND CHAMBERS sumption and food conversion ratio. Br. Poult. Sci. 20:87-97. Shannon, D.W.F., and W. O. Brown, 1969. Calorimetric studies on the effect of dietary energy source and environmental temperature on the metabolic efficiency of energy utilization by mature Light Sussex cockerels. J. Agric. Sci. 72:479-489. Shannon, D.W.F., andW. O. Brown, 1970. A calorimetric estimate of the efficiency of utilization of dietary energy by the growing cockerel. Br. Poult. Sci. 11:16. Snedecor, G. W., and W. G. Cochran, 1967. Statistical Methods. 6th ed. Iowa State Univ. Press, Ames, IA. Washburn, K. W., R. A. Guill, and H. M. Edwards, Jr., 1975. Influence of genetic differences in feed efficiency on carcass composition of young chickens. J. Nutr. 105:1311-1317. Whitehead, C. C , and H. D. Griffin, 1986. Development of divergent lines of lean and fat broilers using plasma very low density lipoprotein concentration as selection criterion: results over the fourth generation and lack of effect of dietary fat on performance and carcass fat content. Br. Poult. Sci. 27:317-324.
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at equal age and body energy. Pages 121-124 in: Energy Metabolism of Farm Animals. M. Vermorel, ed. G. de Bussac, Clermont-Ferrand, France. Jull, M. A., and H. W. Titus, 1928. Growth of chickens in relation to feed consumption. J. Agric. Res. 36:541550. Leclercq, B., J. C. Blum, and J. P. Boyer, 1980. Selecting broilers for low or high abdominal fat: Initial observations. Br. Poult. Sci. 21:107-113. Leenstra, F. R., P.F.G. Vereijken, and R. Pit, 1986. Fat deposition in a broiler sire strain. 1. Phenotypic and genetic variation in, and correlations between, abdominal fat, body weight, and feed conversion. Poultry Sci. 65:1225-1235. Marks, H. L., 1979. Growth rate and feed intake of selected and nonselected broilers. Growth 43:80-90. Pym, R.A.E., and P. J. Nicholls, 1979. Selection for food conversion in broilers: Direct and correlated responses to selection for body-weight gain, food consumption and food conversion ratio. Br. Poult. Sci. 20:73-86. Pym, R.A.E., and A. J. Solvyns, 1979. Selection for food conversion in broilers: Body composition of birds selected for increased body-weight gain, food con-