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Genetic Parameter Estimates for Wing Bone Strength Measurements of Cage-Reared Broilers1
Genetic Parameter Estimates for Wing Bone Strength Measurements of Cage-Reared Broilers 1 M. A. MANDOUR, K. E. NESTOR,2 R. E. SACCO, C. R. POLLEY, and G. B. HAVENSTEIN Department of Poultry Science, Ohio State University, Ohio Agricultural Research and Development Center, Wooster, Ohio 44691 (Received for publication December 12, 1988) ABSTRACT Estimates of genetic parameters were calculated for wing bone measurements of broilers. Traits examined were elastic force, elastic energy, length, stress, and moment of inertia of the humerus, radius clastic force and elastic energy, and ulna elastic force and elastic energy. Records were taken from a shortterm selection experiment (three generations) in which selection was based on increased humerus strength. The Athens-Canadian randombred broiler line served as the base population. Hentability estimates for humerus elastic force, stress, and length exceeded .50. Moderate (>.2) heritability estimates were observed for humerus elastic energy and radius elastic energy. Phenotypic and genetic correlations among measurements of humerus, radius, and ulna elastic force were positive. The genetic correlation between humerus elastic force and humerus stress was .98 ± . 17, suggesting mat these two measurements are genetically the same character. Results of this study indicate it should be possible to increase humerus strength through selection. Selection should be equally effective whether based on humerus elastic force or humerus stress, both of which are measures of humerus strength. In addition, selection for increased humerus strength should result in an increase in strength of the other wing bones. (Key words: heritabilities, phenotypic correlations, genetic correlations, wing bone strength, broilers) 1989 Poultry Science 68:1174-1178 INTRODUCTION
A higher incidence of broken bones during processing of cage-reared broilers than floorraised broilers has been a deterrent to the acceptance of cage rearing by the commercial industry. The higher incidence of broken wings may be the result of reduced humerus strength of caged broilers (Wabeck and Littlefield, 1972; Andrews and Goodwin, 1973; Merkley and Wabeck, 1975; May et al., 1981; Travis et al., 1983). A short-term selection experiment by Mandour et al. (1989) indicated that it is possible to increase humerus strength of cagereared broilers by selection. Design of effective selection programs to increase wing bone strength of caged broilers requires estimates of genetic parameters for wing bone strength measures.
'Salaries and research support provided by State and Federal Funds appropriated to the Ohio Agricultural Research and Development Center, the Ohio State University. Manuscript number 315-88. T"o whom correspondence should be addressed.
The authors are not aware of any estimates in the literature of heritabilities of, or genetic and phenotypic correlations among, wing bone strength measurements on cage-reared broilers. Therefore, the objective of this study was to estimate genetic parameters for wing bone strength measurements of cage-reared broilers. MATERIALS AND METHODS
A sample of birds representing 36 sire families from the Athens-Canadian randombred control population of broilers (Hess, 1962) was used as the base population for a selection experiment designed to increase humerus strength of cage-reared broilers. Eight randomly chosen chicks from each of the sire families were assigned to one of 24 Lohmann cage rearing units (Lohmann Export GMBH, Altenwalder Chaussee, P.O. Box 468, 219 Cuxhaven, Federal Republic of Germany). Chicks were provided a corn-soybean meal diet (22% protein, 2,970 kcal ME/kg, 1.0% Ca, and .8% P) fed ad libitum until slaughter at 9 wk of age. Body weight and sex of each bird were recorded prior to slaughter.
1174
GENETIC PARAMETERS OF BONE STRENGTH MEASURES
Mechanical properties of humeri, radii, and a sample of ulnae (from each sire family) of cage-reared broilers in the base generation were determined by a flexure test (Baker and Haugh, 1979) using an Instron Universal Testing Machine (Model TM-M, Metric Standard Speed, Instron Corporation, Canton, MA 02021). Bones were supported at each end by a breaking bar and a force was applied at the midspan by the downward movement of a crosshead at constant rate of 5 cm/min. The distance between the two fulcra points supporting the bones was 3.5 cm. Bones were placed on the breaking bars with the thinnest convex side of the bone parallel to the direction of loading. A force-deflection curve was recorded for each bone. Bones were loaded until a peak or maximum force was recorded on the forcedeflection curve. The elastic energy was recorded for each bone on the Instron. Humerus length was measured from the farthest point of the proximal head to the end of the trochlea using a vernier caliper. Inside and outside diameters of the bones perpendicular and parallel to the direction of applied force were also measured with a vernier caliper. Area moment of inertia (MOI) was calculated as:
1175
an ellipse, C = 1/2 the diameter (1/2 x B in the present study). A strength index (SI) was calculated for each potential sire, based on progeny performance, according to the following equation: SI = (Eo/Fo)F-IE-Eol
where Eo is the overall average humerus elastic energy; Fo is the overall average humerus elastic force; F is the average humerus elastic force for a sire's progeny; E is the average humerus elastic energy for each individual; and E—Eo is the average absolute value of the deviation of individual elastic energy from the overall average elastic energy for each sire. This index was developed to discriminate among strong bones having high elastic force and high elastic energy, rubbery bones having low force and high energy, and weak bones having low force and low energy. The index would have the greatest value when elastic force is high and elastic energy is near average. The ratio of (Eo/Fo) was a constant for all birds and was used to convert units of elastic force to the same units as elastic energy in the SI. The nine highest ranking sire families from the 36 families sampled were selected to produce the first generation. One Area MOI = .0491 (BD3 - bd 3 ) sire family was excluded from the ranking due where the constant .0491 = rc/64 in the to low hatchability and high mortality. The calculation of area MOI for an ellipse, B and D Southeastern Poultry Research Laboratory, are the outside diameters (centimeters), and b Athens, GA, resupplied pedigreed eggs from and d are the inside diameters (centimeters) at the nine sire families. point of applied force. B and b are diameters Chicks were wingbanded, vaccinated for perpendicular to the direction of loading, Marek's disease; single comb chicks were whereas D and d are diameters parallel to the dubbed at hatching. Birds were vaccinated for direction of applied force. Newcastle disease and bronchitis at 1, 5, and Bone stress, defined as force per unit area 16 wk of age. At 9 wk of age, sex was of bone, takes into account the geometrical recorded and birds were vaccinated against shape of the area over which the force is pox. Five males and eighteen females from applied (Crenshaw et al., 1981). In this study it each sire family were retained for potential was assumed that the cross section of the breeding stock. The breeders were moved to bones was elliptical in shape. Bone stress was individual cages at 5 mo of age where they were maintained under 14 h of light and calculated as: provided a corn-soybean meal diet (15% protein, 2,860 kcal ME/kg, 3.6% Ca, and .53% Force x Length x C P). Four males per sire family were actually used for breeding. Four hens were randomly where stress is in kilograms per square assigned to be mated by artificial insemination centimeter, force is in kilograms, length (AI) to each sire, with sib matings avoided. (centimeters) is the distance between the two The first hatch of chicks was used for fulcra points, and C is the distance from the measuring progeny performance. Eight chicks neutral axis to the extreme outside fiber. For were randomly chosen from each of 36 sire
1176
MANDOUR ET AI.
families and randomly assigned to a cage rearing unit on the day of hatching. A restriction used was that no more than one chick from a sire family could be assigned to a particular cage. Methods of management of birds and collection of bone measurements were similar to those described above for the base generation. Sires were ranked within sire family on the basis of the average strength index of their progeny and the highest ranked male from each family was selected and mated by AI to four hens to produce the second generation. To ensure that there would be 5 males and 18 females from each family, approximately 40 chicks per mating were hatched each generation. The third generation was produced in a similar manner, except that each sire family was represented by four males and eight females. Selected males (one sire per family) were mated by AI to two females, with sib matings avoided. Progeny of the third generation were produced by mating two hens, using AI, to a single male. Four chicks per sire were randomly assigned to a cage rearing unit. Methods of management of chicks and collection of bone measurements were similar to those described for earlier generations, except that the diet was modified. The diet fed contained 19% protein, 3,000 kcal ME/kg, .92% Ca, and .65% P. Data were analyzed using least squares procedures for unequal subclass numbers (Harvey, 1985). Two sets of data were analyzed. Humerus elastic force and elastic energy and radius elastic force and elastic energy measurements were collected on all birds. Humerus stress, humerus length, humerus MOI and ulna elastic force and elastic energy measurements were collected on a sample of the birds. The following mixed model was assumed in the analysis of bone measures: yijk = H + Gi + Sji + Xk + P wyk + eijk where yyk. = the observed dependent variable; \i = the overall mean; Gj = the fixed effect of the ith generation; Sji = the random effect of the jth sire within the ith generation [the s are assumed to be normally and independently distributed (NID) (0, c^)]; X k = the fixed effect of the kth sex; P wyk = the linear regression of the dependent variable on body weight (wyk); and ejjk = the random residual
error [the e are assumed to be NID (0,a )]. Preliminary analyses indicated that the interaction of generation and sex was nonsignificant; it was excluded from the final model. Heritabilities and phenotypic and genetic correlations for the bone measures were calculated from the paternal half-sib analysis of variance (Harvey, 1985). There were 138 df for sires in both analyses. RESULTS AND DISCUSSION
Generation of selection was a significant source of variation for all characters examined. Random effect of sire was significant for humerus elastic force and elastic energy, stress, and length, and for radius elastic energy. Radius elastic force of females was significantly greater than that of males, whereas humerus stress of males was significantly greater than that of females. In analyses of base and third generations from the same population as the present study, Mandour et al. (1989) reported no significant difference between males and females for humerus stress. Ruff (1984) also observed no significant difference between males and females for humerus stress. Heritability Estimates. Heritability estimates and associated standard errors are presented in Table 1. Heritability estimates for humerus elastic force, stress, and length were high (>.50) whereas those for humerus elastic energy and radius elastic energy were moderate (>.2), and those for humerus MOI and radius elastic force were low (<.l). Therefore, the additive genetic variance for humerus elastic force, stress, and length was large in
TABLE 1. Heritability estimates ± SE for bone measurements based on paternal half-sib analysis Trait Humerus elastic force, kg Humerus elastic energy, cm-kg Humerus stress, kg/cm2 Humerus moment of inertia, cm 4 Humerus length, mm Radius elastic force, kg Radius elastic energy, cm-kg Ulna elastic force, kg Ulna elastic energy, cm-kg
Heritability .67 ± .13 .25 ± .11 .80 ± .26 .09 ± .25 .51 ± .26 .08 ± .10 .26 ± .11 .17 ± .25
NC1
*NC = Not calculated due to negative additive genetic variance component.
GENETIC PARAMETERS OF BONE STRENGTH MEASURES
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proportion to the total variance and provides an explanation for the progress made in increasing humerus strength of a selected line compared with a control line from short-term selection in the study of Mandour et al. (1989). Phenotypic and Genetic Correlations. Most of the phenotypic correlations among the bone measurements were positive (Table 2). Phenotypic correlations among measures of elastic force for the humerus, radius, and ulna exceeded .40. The large negative phenotypic correlation (-.54) and the negative genetic correlation observed for humerus stress and MOI likely result from use of MOI in the denominator of the equation for stress. The generic correlation between humerus elastic force and humerus stress was .98 ± .17, suggesting these two measures reflect what is genetically the same character. Genetic correlations among humerus, radius, and ulna elastic force were positive, The heritability estimates for humerus elastic force and stress, both of which are measures of humerus strength, indicate that it should be possible to increase humerus strength through selection. Because humerus elastic force and stress reflect what is genetically the same character, selection should be based on humerus elastic force, as the calculation of humerus stress requires measurements of bone diameters in addition to humerus force. Further, because humerus elastic force was positively correlated, both phenotypically and genetically, with radius and ulna elastic force, increases in these characteristics would be expected as correlated responses to selection for increased humerus elastic force.
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Andrews, L. D., and T. L. Goodwin, 1973. Performance of broilers in cages. Poultry Sci. 52:723-728. Baker, J. L., and C. G. Haugh, 1979. Mechanical properties of bone: A review. Trans. ASAE 22:678-687. Crenshaw, T. D., E. R. Peo, Jr., A. J. Lewis, and B. D. Moser, 1981. Bone strength as a trait for assessing mineralization in swine: A critical review of techniques involved. Anim. Sci. 53:827-835. Harvey, W. R., 1985. User's guide for LSMLMW. The Ohio State Univ., Columbus, OH. Hess, C. W., 1962. Randombred populations of the Southern Regional Poultry Breeding Project. World's Poult. Sci. J. 18:147-152. Mandour, M. A., K. E. Nestor, R. E. Sacco, C. R. Polley, and G. B. Havenstein, 1989. Selection for increased humerus strength of cage-reared broilers. Poultry Sci. 68: 1168-1173.
1178
MANDOUR ET AL.
May, J. D., J. W. Merkley, G. W. Malone, and G. W. Cbaloupka, 1981. Relationship of pen height to bone strength of broilers. Poultry Sci. 60:546-549. Merkley, J. W., and C. J. Wabeck, 1975. Cage density and frozen storage effect on bone strength of broilers. Poultry Sci. 54:1624-1627. Ruff, C. R., 1984. Dietary regimes to improve bone strength of broilers grown in cages. M.S. Thesis. Clemson
Univ., Clemson, SC. Travis, Jr., D. S., D. R. Sloan, and B. L. Hughes, 1983. Bone fragility in broilers as affected by pen height, sex, and a comparison of left and right humeri. Poultry Sci. 62: 2117-2119. Wabeck, C. J., and L. H. Littlefield, 1972. Bone strength of broilers reared in floor pens and in cages having different bottoms. Poultry Sci. 51:897-899.
A higher incidence of broken bones during processing of cage-reared broilers than floorraised broilers has been a deterrent to the acceptance of cage rearing by the commercial industry. The higher incidence of broken wings may be the result of reduced humerus strength of caged broilers (Wabeck and Littlefield, 1972; Andrews and Goodwin, 1973; Merkley and Wabeck, 1975; May et al., 1981; Travis et al., 1983). A short-term selection experiment by Mandour et al. (1989) indicated that it is possible to increase humerus strength of cagereared broilers by selection. Design of effective selection programs to increase wing bone strength of caged broilers requires estimates of genetic parameters for wing bone strength measures.
'Salaries and research support provided by State and Federal Funds appropriated to the Ohio Agricultural Research and Development Center, the Ohio State University. Manuscript number 315-88. T"o whom correspondence should be addressed.
The authors are not aware of any estimates in the literature of heritabilities of, or genetic and phenotypic correlations among, wing bone strength measurements on cage-reared broilers. Therefore, the objective of this study was to estimate genetic parameters for wing bone strength measurements of cage-reared broilers. MATERIALS AND METHODS
A sample of birds representing 36 sire families from the Athens-Canadian randombred control population of broilers (Hess, 1962) was used as the base population for a selection experiment designed to increase humerus strength of cage-reared broilers. Eight randomly chosen chicks from each of the sire families were assigned to one of 24 Lohmann cage rearing units (Lohmann Export GMBH, Altenwalder Chaussee, P.O. Box 468, 219 Cuxhaven, Federal Republic of Germany). Chicks were provided a corn-soybean meal diet (22% protein, 2,970 kcal ME/kg, 1.0% Ca, and .8% P) fed ad libitum until slaughter at 9 wk of age. Body weight and sex of each bird were recorded prior to slaughter.
1174
GENETIC PARAMETERS OF BONE STRENGTH MEASURES
Mechanical properties of humeri, radii, and a sample of ulnae (from each sire family) of cage-reared broilers in the base generation were determined by a flexure test (Baker and Haugh, 1979) using an Instron Universal Testing Machine (Model TM-M, Metric Standard Speed, Instron Corporation, Canton, MA 02021). Bones were supported at each end by a breaking bar and a force was applied at the midspan by the downward movement of a crosshead at constant rate of 5 cm/min. The distance between the two fulcra points supporting the bones was 3.5 cm. Bones were placed on the breaking bars with the thinnest convex side of the bone parallel to the direction of loading. A force-deflection curve was recorded for each bone. Bones were loaded until a peak or maximum force was recorded on the forcedeflection curve. The elastic energy was recorded for each bone on the Instron. Humerus length was measured from the farthest point of the proximal head to the end of the trochlea using a vernier caliper. Inside and outside diameters of the bones perpendicular and parallel to the direction of applied force were also measured with a vernier caliper. Area moment of inertia (MOI) was calculated as:
1175
an ellipse, C = 1/2 the diameter (1/2 x B in the present study). A strength index (SI) was calculated for each potential sire, based on progeny performance, according to the following equation: SI = (Eo/Fo)F-IE-Eol
where Eo is the overall average humerus elastic energy; Fo is the overall average humerus elastic force; F is the average humerus elastic force for a sire's progeny; E is the average humerus elastic energy for each individual; and E—Eo is the average absolute value of the deviation of individual elastic energy from the overall average elastic energy for each sire. This index was developed to discriminate among strong bones having high elastic force and high elastic energy, rubbery bones having low force and high energy, and weak bones having low force and low energy. The index would have the greatest value when elastic force is high and elastic energy is near average. The ratio of (Eo/Fo) was a constant for all birds and was used to convert units of elastic force to the same units as elastic energy in the SI. The nine highest ranking sire families from the 36 families sampled were selected to produce the first generation. One Area MOI = .0491 (BD3 - bd 3 ) sire family was excluded from the ranking due where the constant .0491 = rc/64 in the to low hatchability and high mortality. The calculation of area MOI for an ellipse, B and D Southeastern Poultry Research Laboratory, are the outside diameters (centimeters), and b Athens, GA, resupplied pedigreed eggs from and d are the inside diameters (centimeters) at the nine sire families. point of applied force. B and b are diameters Chicks were wingbanded, vaccinated for perpendicular to the direction of loading, Marek's disease; single comb chicks were whereas D and d are diameters parallel to the dubbed at hatching. Birds were vaccinated for direction of applied force. Newcastle disease and bronchitis at 1, 5, and Bone stress, defined as force per unit area 16 wk of age. At 9 wk of age, sex was of bone, takes into account the geometrical recorded and birds were vaccinated against shape of the area over which the force is pox. Five males and eighteen females from applied (Crenshaw et al., 1981). In this study it each sire family were retained for potential was assumed that the cross section of the breeding stock. The breeders were moved to bones was elliptical in shape. Bone stress was individual cages at 5 mo of age where they were maintained under 14 h of light and calculated as: provided a corn-soybean meal diet (15% protein, 2,860 kcal ME/kg, 3.6% Ca, and .53% Force x Length x C P). Four males per sire family were actually used for breeding. Four hens were randomly where stress is in kilograms per square assigned to be mated by artificial insemination centimeter, force is in kilograms, length (AI) to each sire, with sib matings avoided. (centimeters) is the distance between the two The first hatch of chicks was used for fulcra points, and C is the distance from the measuring progeny performance. Eight chicks neutral axis to the extreme outside fiber. For were randomly chosen from each of 36 sire
1176
MANDOUR ET AI.
families and randomly assigned to a cage rearing unit on the day of hatching. A restriction used was that no more than one chick from a sire family could be assigned to a particular cage. Methods of management of birds and collection of bone measurements were similar to those described above for the base generation. Sires were ranked within sire family on the basis of the average strength index of their progeny and the highest ranked male from each family was selected and mated by AI to four hens to produce the second generation. To ensure that there would be 5 males and 18 females from each family, approximately 40 chicks per mating were hatched each generation. The third generation was produced in a similar manner, except that each sire family was represented by four males and eight females. Selected males (one sire per family) were mated by AI to two females, with sib matings avoided. Progeny of the third generation were produced by mating two hens, using AI, to a single male. Four chicks per sire were randomly assigned to a cage rearing unit. Methods of management of chicks and collection of bone measurements were similar to those described for earlier generations, except that the diet was modified. The diet fed contained 19% protein, 3,000 kcal ME/kg, .92% Ca, and .65% P. Data were analyzed using least squares procedures for unequal subclass numbers (Harvey, 1985). Two sets of data were analyzed. Humerus elastic force and elastic energy and radius elastic force and elastic energy measurements were collected on all birds. Humerus stress, humerus length, humerus MOI and ulna elastic force and elastic energy measurements were collected on a sample of the birds. The following mixed model was assumed in the analysis of bone measures: yijk = H + Gi + Sji + Xk + P wyk + eijk where yyk. = the observed dependent variable; \i = the overall mean; Gj = the fixed effect of the ith generation; Sji = the random effect of the jth sire within the ith generation [the s are assumed to be normally and independently distributed (NID) (0, c^)]; X k = the fixed effect of the kth sex; P wyk = the linear regression of the dependent variable on body weight (wyk); and ejjk = the random residual
error [the e are assumed to be NID (0,a )]. Preliminary analyses indicated that the interaction of generation and sex was nonsignificant; it was excluded from the final model. Heritabilities and phenotypic and genetic correlations for the bone measures were calculated from the paternal half-sib analysis of variance (Harvey, 1985). There were 138 df for sires in both analyses. RESULTS AND DISCUSSION
Generation of selection was a significant source of variation for all characters examined. Random effect of sire was significant for humerus elastic force and elastic energy, stress, and length, and for radius elastic energy. Radius elastic force of females was significantly greater than that of males, whereas humerus stress of males was significantly greater than that of females. In analyses of base and third generations from the same population as the present study, Mandour et al. (1989) reported no significant difference between males and females for humerus stress. Ruff (1984) also observed no significant difference between males and females for humerus stress. Heritability Estimates. Heritability estimates and associated standard errors are presented in Table 1. Heritability estimates for humerus elastic force, stress, and length were high (>.50) whereas those for humerus elastic energy and radius elastic energy were moderate (>.2), and those for humerus MOI and radius elastic force were low (<.l). Therefore, the additive genetic variance for humerus elastic force, stress, and length was large in
TABLE 1. Heritability estimates ± SE for bone measurements based on paternal half-sib analysis Trait Humerus elastic force, kg Humerus elastic energy, cm-kg Humerus stress, kg/cm2 Humerus moment of inertia, cm 4 Humerus length, mm Radius elastic force, kg Radius elastic energy, cm-kg Ulna elastic force, kg Ulna elastic energy, cm-kg
Heritability .67 ± .13 .25 ± .11 .80 ± .26 .09 ± .25 .51 ± .26 .08 ± .10 .26 ± .11 .17 ± .25
NC1
*NC = Not calculated due to negative additive genetic variance component.
GENETIC PARAMETERS OF BONE STRENGTH MEASURES
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.+1.+1 +1 +1 +1 +\ — M-
-f«^+ r-
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proportion to the total variance and provides an explanation for the progress made in increasing humerus strength of a selected line compared with a control line from short-term selection in the study of Mandour et al. (1989). Phenotypic and Genetic Correlations. Most of the phenotypic correlations among the bone measurements were positive (Table 2). Phenotypic correlations among measures of elastic force for the humerus, radius, and ulna exceeded .40. The large negative phenotypic correlation (-.54) and the negative genetic correlation observed for humerus stress and MOI likely result from use of MOI in the denominator of the equation for stress. The generic correlation between humerus elastic force and humerus stress was .98 ± .17, suggesting these two measures reflect what is genetically the same character. Genetic correlations among humerus, radius, and ulna elastic force were positive, The heritability estimates for humerus elastic force and stress, both of which are measures of humerus strength, indicate that it should be possible to increase humerus strength through selection. Because humerus elastic force and stress reflect what is genetically the same character, selection should be based on humerus elastic force, as the calculation of humerus stress requires measurements of bone diameters in addition to humerus force. Further, because humerus elastic force was positively correlated, both phenotypically and genetically, with radius and ulna elastic force, increases in these characteristics would be expected as correlated responses to selection for increased humerus elastic force.
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Andrews, L. D., and T. L. Goodwin, 1973. Performance of broilers in cages. Poultry Sci. 52:723-728. Baker, J. L., and C. G. Haugh, 1979. Mechanical properties of bone: A review. Trans. ASAE 22:678-687. Crenshaw, T. D., E. R. Peo, Jr., A. J. Lewis, and B. D. Moser, 1981. Bone strength as a trait for assessing mineralization in swine: A critical review of techniques involved. Anim. Sci. 53:827-835. Harvey, W. R., 1985. User's guide for LSMLMW. The Ohio State Univ., Columbus, OH. Hess, C. W., 1962. Randombred populations of the Southern Regional Poultry Breeding Project. World's Poult. Sci. J. 18:147-152. Mandour, M. A., K. E. Nestor, R. E. Sacco, C. R. Polley, and G. B. Havenstein, 1989. Selection for increased humerus strength of cage-reared broilers. Poultry Sci. 68: 1168-1173.
1178
MANDOUR ET AL.
May, J. D., J. W. Merkley, G. W. Malone, and G. W. Cbaloupka, 1981. Relationship of pen height to bone strength of broilers. Poultry Sci. 60:546-549. Merkley, J. W., and C. J. Wabeck, 1975. Cage density and frozen storage effect on bone strength of broilers. Poultry Sci. 54:1624-1627. Ruff, C. R., 1984. Dietary regimes to improve bone strength of broilers grown in cages. M.S. Thesis. Clemson
Univ., Clemson, SC. Travis, Jr., D. S., D. R. Sloan, and B. L. Hughes, 1983. Bone fragility in broilers as affected by pen height, sex, and a comparison of left and right humeri. Poultry Sci. 62: 2117-2119. Wabeck, C. J., and L. H. Littlefield, 1972. Bone strength of broilers reared in floor pens and in cages having different bottoms. Poultry Sci. 51:897-899.