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Genetic Association of Selection for Increased Leg Muscle and Increased Shank Diameter with Body Composition and Walking Ability1
Genetic Association of Selection for Increased Leg Muscle and Increased Shank Diameter with Body Composition and Walking Ability1 D. A. EMMERSON, N. B. ANTHONY,2 K. E. NESTOR, and Y. M SAIF Department of Poultry Science, The Ohio State University, Ohio Agricultural Research and Development Center, Wooster, Ohio 44691 (Received for publication September 14, 1990)
1991 Poultry Science 70:739-745 INTRODUCTION
Leg weakness in broilers and turkeys is a problem of increasing importance for the poultry industry. Leg abnormalities are responsible for poor growth performance, increased downgrading, and mortality, all of which reduce the efficiency of commercial production. There is evidence for a strong genetic component to leg weakness. In broilers, most types of leg weakness have been shown to be highly heritable (Sheridan et al., 191A; Burton et al, 1981; Mercer and Hill, 1984) or to be under the influence of major genes (Somes, 1969; Sheridan et al, 1978). Consequently, leg abnormalities are very responsive to selection (Serfontein and Payne, 1934; Leach and
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 258-90. 'Present address: Department of Animal and Poultry Sciences, University of Arkansas, Fayetteville, AR 72701.
Nesheim, 1965, 1972; Riddell, 1976; Leenstra et al, 1984). Although not as extensively studied, line differences for the incidence of leg problems have been reported in turkeys (Walser et al, 1982; Nestor, 1984). Similar contributory factors and types of leg problems are observed in broiler chickens and turkeys (Nestor and Emmerson, 1990). Therefore, leg weakness in turkeys would also be expected to respond to selection. Indeed, selection for shank width has been shown to improve walking ability in large-bodied turkeys (Nestor et al, 1985). A close relationship exists between body size and the relative proportion of body parts. In general, as BW increases, there is an increase in the proportion of breast muscle and a decline in the proportion of leg muscle and bone. These changes in proportion accompany both developmental (Harshaw and Rector, 1940; Clayton et al, 1978; Peng et al, 1985; Larsen et al, 1986) and genetic (Marsden, 1940; Miller, 1968; Clayton et al, 1978; Walser et al, 1982; Nestor, 1984; Nestor et
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ABSTRACT Body compositions of a randombred control population (RBC2), a line selected for increased 16-wk BW (F), a line selected for increased shank diameter (EL), and a line selected for increased leg muscle mass (FM) were compared at 16 wk of age. The F line originated from the RBC2 population and the FL and FM lines were developed as sublines of the F line. The F and EL lines were developed through mass selection for BW and shank width, respectively. The FM line was developed through family selection for leg muscle mass. Selection for B W in the F line has resulted in an increase in the proportion of breast muscle and a decrease in the relative amount of leg bone in comparison with the RBC2 population. Selection for increased shank width in the FL line has increased not only shank weight but has produced concomitant increases in the weight of the tibiotarsal and femur bones. Although the FL line has also shown a correlated increase in BW in association with increases in shank width, the increase in leg bone weight was relatively greater than the increase in BW. Therefore, the relative weights of leg bones in the FL line increased such that they were more similar to the RBC2 population than to the F line. Although the amount of leg muscle increased slightly in the FM line, the relative proportion of leg muscle did not increase significantly. The selection program used was probably ineffective in increasing the proportion of leg muscle because of the high percentage (low selection intensity) of families selected and small family size for estimating family means. There were no differences in the walking ability of F, FL, and FM lines. These selected lines did have poorer walking ability than RBC2 as indicated by higher leg scores for these lines. (Key words: body composition, leg bones, leg muscles, leg weakness, turkeys)
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EMMERSON ET AL.
MATERIALS AND METHODS
Four lines of turkeys (RBC2, F, FL, FM) were surveyed for carcass traits in each of six generations. The RBC2 control line was established through the crossing of two commercial strains of large white turkeys and has been maintained for 23 generations under random mating (Nestor et al, 1969). The F line was derived from the RBC2 line by mass selection for increased 16-wk BW (Nestor, 1977b). The FL line is a subline of the F line that has been mass selected for increased shank width at the dewclaw (Nestor et al., 1985). The FM line, also a subline of F, was developed through family selection for increased leg muscle mass. Assuming the genetic correlation between leg muscle mass and BW is less than 1, direct selection for leg muscle mass would be expected to increase leg muscle mass to a greater extent than the correlated trait BW. Thus, selection for increase muscle mass would also be expected to increase the relative proportion of leg muscle. This selection program was initiated after 13 generations of selection for 16-wk BW in the F line. In the
base generation of subline FM, two groups of 36 F-line females were mated to a single group of F-line males. These groups of hens were assembled such that the average selection differentials for 16-wk BW were similar for the two groups. One group of females was used to reproduce the F line and the other group served as the base population for the FM line. Each generation, family selection of the FM line was based on thigh and drum muscle weights from two male and two female offspring. Fifty percent of the families were then selected based on the total family weight of thigh and drum muscle. Leg muscle weights for families with fewer than four offspring were appropriately weighted to avoid unnecessary loss of families. Due to poor reproduction in the F and FM lines, it was impossible to sample more than four offspring per family and still adequately reproduce the line. All lines were maintained under a paired mating system with 36 parental pairs in each line (Nestor, 1977a). In each of six generations, lines were surveyed for carcass composition. Approximately 150 birds per line and 300 birds per sex were used for the determination of carcass component weights in each generation. Poults were grown to 16 wk of age in floor pens in an enclosed building under standard management practices (Nestor et al., 1988). All lines were fed a six-ration declining protein system throughout the growout period (Naber and Touchburn, 1970). At 16 wk of age, birds were weighed and subjectively scored for walking ability as described by Nestor (1984). Birds were fasted overnight, killed by severing the jugular vein, scalded at 68 C, and defeathered. Carcasses were then split lengthwise and onehalf of the carcass was chilled overnight in cold water for later dissection. Legs were removed from the carcass halves and separated into drumsticks and thighs by cutting through the knee joint. Thigh, drum, and breast muscles were dissected away from the bone and weighed. The femur, tibiotarsus, and tarsometatarsus (skin and scales included) were then individually weighed. Data were analyzed using ANOVA in a 4 x 2 factorial with line and sex as main effects. When appropriate, means were separated using Tukey's test (Steel and Torrie, 1960). Percentage data represent the weight of individual body parts relative to live BW. Weight
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al, 1985) increases in BW. Changes in the relative proportions of body parts are further compounded as a result of selection for increased breast development in commercial poultry stocks. It has been suggested that changes in the relative proportions of body component parts may be partially responsible for the increased incidence of leg problems in modern commercial turkeys. Nestor et al. (1985) hypothesized that increases in BW and breast muscle development without concomitant increases in leg muscle and bone is biologically incompatible with the maintenance of normal walking ability. As a result, two selection experiments have been initiated with the goal of increasing the relative proportions of leg muscle and bone in turkeys which have been previously selected for increased BW. The development of a subline (FL) selected for increased leg bone development has been previously reported (Nestor et al, 1985). A second subline (FM) has been selected for increased leg muscle. The objective of the present study is to report the establishment of the FM subline and to investigate the effect of these selection programs on carcass composition traits and walking ability over six generations of selection.
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LEG MUSCLE SELECTION AND BODY COMPOSITION
TABLE 1. Body, muscle, and bone weights of a randombred control population (RBC2), a line selected for increased 16-wk body weight (F), a line selected for increased shank width (FL), and a line selected for increased leg muscle weight (FM) in the fourth generation of selection in FM Line RBC2
Measurement
EL
F 9.37"
6.24°
FM
9.08"
8.73b
Male 9.46"
Female 7.30b
524.1° 16.8b 296.6° 9.6° 212.1° 6.8 b 508.8° 16.4b
818.8" 17.4" 482.1" 10.4" 338.7" 7.2" 820.9" 17.5"
733.8 b 16.2° 438.5 b 9.$* 317.0b 7.0"b 755.6b 17.3"
757.2b 17.4" 439. l b 10.0"b 314.9b 7.2" 754.0b 16.7b
802.9" 17.0" 464.4" 9.8 b 334.4" 7.0 b 798.9" 16.8b
620.9 b 17.0" 368. l b 10.0" 260.0b 7.2" 628.1 b 17.2"
62.9d 2.0 b 91.6° 2.9" 57.7° 1.8" 212.2d 6.7" 1.64b
87.6 b 1.8° 127.0" 2.7 b 78.5" 1.6b 293.1 b 6.2b 2.73"
97.0" 2.2" 131.7" 2.9" 81.3" 1.8" 310.0" 6.8" 2.70"
80.8° 1.8° 115.6b 2.6 b 73.0 b 1.6b 269.9° 6.1 b
102.2" 2.2" 144.3" 3.1" 89.8" 2.0" 336.3" 7.2" 3.05"
61.0 b 1.6b 87.2 b 2.4 b 54.6 b 1.6b 202.7 b 5.6b 1.87b
2.71"
""^Means within lines and sex subgroups with no common superscripts are significantly different (P<05). Absolute weights measured on one-half of each carcass. Absolute weights were multiplied by 2 prior to calculation of percentages. 2 Subjective rating of 1 (no difficulty walking) through 5 (extreme lateral deviation of legs and extreme difficulty walking).
measures were log transformed and percentage data were arc sine square root transformed prior to analysis. RESULTS
Line Comparisons in Fourth Generation of the FM Line As expected, muscle and bone weights of the RBC2 population were consistently lower than those from the other lines surveyed (Table 1). Body weights of the F and FL lines were higher than in the FM line. Absolute muscle weights of the F line were consistently higher than the FL and FM lines. However, relative breast and thigh muscle weights of the F line were not different from the FM line, and relative drum muscle weight was not different from the FL or FM lines. Although total leg muscle weight of the F line was greater than that of the FM and RBC2 lines, the relative leg muscle weight of the F line was not different from FL. The FL line had heavier shank bones than the other lines
surveyed. In addition, the relative shank bone weight of the FL line was greater than that of the F, FM, and RB C2 lines and the relative thigh and drum bone weights of the FL line were greater than those of the F and FM lines. Total leg bone weights followed the same pattern as BW except that leg bone weight of the FL line was greater than the other lines tested. Relative total leg bone weights of the RBC2 and FL lines were greater than for the F and FM lines. The RBC2 population had a lower leg score than the F, FL, and FM lines. Males had higher values than females for all traits except percentage breast, thigh, and total leg muscle (Table 1). There was no sex difference for relative breast muscle weight. Relative thigh, drum, and total leg muscle weights were greater for females than for males. Line Comparisons in Fifth Generation of the FM Line Absolute muscle and bone weights of the RBC2 population were lower than for the F, FL, and FM lines for all traits measured (Table 2).
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Body weight, kg Muscles Breast, g Breast, % Thigh, g Thigh, % Drum, g Drum, % Total leg, g Total leg, % Bones Shank, g Shank, % Drum, g Drum, % Thigh, g Thigh, % Total leg, g Total leg, % Leg scores2
Sex
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EMMERSON ET AL.
literature. Males had greater absolute muscle and bone weights, as would be expected based on BW differences due to sexual dimorphism. Body weight was distributed differently in males and females as evidenced by die greater relative tiiigh, drum, and total leg muscle for females (Generation 4) and me greater relative shank, drum, and diigh bone for males (Generations 4, 5, and 6). Clayton et al. (1978) observed a greater percentage of leg bone in male turkeys but also found me percentage dark meat to be greater for males. This difference is apparendy related to me lines tested. Sex differences in me present experiment are consistent with previous observation in these lines (Nestor et al., 1988). The sex difference for leg ratings has also been previously observed (Nestor, unpublished data).
As expected, selection for increased BW successfully increased not only BW but produced consistent increases in muscle mass. In contrast, changes in leg bone weights were negligible. These observations were consistent Line Comparisons in Sixth Generation with die high genetic correlation between BW of the FM Line and breast, drum, and tiiigh muscle (HavenAll absolute weight measures were lower for stein et al., 1988a) and the smaller genetic the RBC2 population than for the selected lines correlation between BW and skeletal measures (Table 3). In addition, the RBC2 population had (Havenstein et al., 1988b). Based on differlower percentages of thigh, drum, and total leg ences in the magnitude of increase in muscle muscle. The F line had a greater B W and greater and bone, the F line had a greater proportion of weight of breast, thigh, drum, and total leg muscle tissue and a smaller proportion of bone muscles man die FL and FM lines. However, man die RBC2 population from which it was relative tiiigh, drum, and total leg muscle derived. This was in agreement with a previous weights were not different among the selected report by Nestor et al. (1988). However, populations. The F line also had greater diigh Nestor et al. (1988) also reported disproporbone weight than the FM line and a greater drum tionate increases in breast and leg muscle of bone weight than both FL and FM. However, the percentage diigh bone for die F line was smaller weight-selected turkeys. Although relative man for die odier selected populations and die breast muscle weights were generally higher relative drum bone weight was less for the F line for die F line than for RBC2 in the present tiian for me FL line. As expected, die FL line had study, relative leg muscle weights were also a greater absolute and relative shank bone equal to (one generation) or greater (two weight man me other lines. Total leg bone generations) in die F line man in the RBC2 weights of the F and FL lines were greater tiian population. In addition, deviations between die for the FM line. Relative total leg bone weight F line and me RBC2 were generally greater for was greater for FL man for RBC2 and bom were percentage leg muscle than for percentage greater than FM and F. The leg score of RBC2 breast muscle. Therefore, it appears that me was lower man for die selected populations. previously reported pattern of a decreased There were sex differences for all traits except proportion of leg muscle in the F line has for the percentages of breast, tiiigh, drum, and reversed in more recent generations. The reason for this change in response is unknown. total leg muscle. Relative leg bone, however, remained lower in die F line than in the RBC2. DISCUSSION
Sex differences in body composition were generally consistent with diose reported in die
Selection for increased shank width produced a general increase in leg bone development. Body weight has continued to
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Body weights and thigh muscle weights of the F and FM lines were higher than those from die FL line. Breast weight of the F line was greater than the FM line, which was greater than die FL line. The F line had a greater percentage breast muscle than the FL line but neither line was different from the FM line. The FM line had a greater drum muscle weight than the FL line but these values were not different when expressed relative to BW. The FL line had a heavier thigh bone than die F line and a heavier shank bone than botii F and FM. Relative thigh and shank bone weights were also greater for die FL line than for the F and FM lines. Altiiough drum bone weights were not different for die F, FL, and FM lines, percentage drum bone was greater for me FL line man for the F line. Although RBC2 had a lower leg score than die selected lines, there was no difference between die leg scores of me selected lines. There were significant sex differences for all traits except percentage breast, migh, drum, and total leg muscle.
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LEG MUSCLE SELECTION AND BODY COMPOSITION
TABLE 2. Body, muscle, and bone weights of a randombred control population (RBC2), a line selected for increased 16-wk body weight (F), a line selected for increased shank width (FL), and a line selected for increased leg muscle weight (FM) in the fifth generation of selection in FM1 Line Measurement
Leg score
Sex
FL 9.32°
FM 9.74*
Male 10.19*
Female 7.37°
6.27°
10.05*
538.5d 17.5* 319.3° 10.4* 231.9C 7.6*b 551.2C 17.9*
878.6* 17.4* 505.5* 10.0* 365.7ab 7.3 b 871.1* 17.3*
753.3° 16.4b 474.0b 10.3* 353.3b 7.7* 827.6b 18.0*
823.3b 17.0ab 504.7* 10.4* 366.0* 7.6 ab 870.7* 18.0*
854.5* 16.8* 518.5* 10.3* 377.9* 7.5* 896.4* 17.8*
632.0 b 17.3* 376.9b 10.3* 276. l b 7.6* 653.0b 17.9*
50.5° 1.6* 83.8b 2.7* 58.4C 1.9b 192.7° 6.2*b 1.42b
73.7b 1.4b 128.9* 2.5b 85.6b 1.7° 288.2b 5.6° 2.20*
76.7* 1.6* 128.3* 2.7* 96.8* 2.1* 303.0* 6.4* 2.22*
75.6*b 1.5»b 125.7* 2.6*b 87.5b 1.9b 288.8*b
88.0* 1.8* 148.9* 3.0* 103.2* 2.1* 340.1* 6.8* 2.67*
48.8 b 1.3b 81.7b 2.3 b 58.9 b 1.6b 189.3b 5.2 b 1.39b
2.37*
*_cMeans within line and sex subgroups with no common superscripts are significantly different (P<.05). Absolute weights measured on one-half of each carcass. Absolute weights were multiplied by 2 prior to calculation of percentage. Subjective rating of 1 (no difficulty walking) through 5 (extreme lateral deviation of legs and extreme difficulty walking).
increase as a correlated response to selection for increased shank width as evidenced by the linear regression coefficient of response on generation of .26 kg (P<.01) and .15 kg (P<.01) for males and females, respectively (Nestor and Emmerson, 1990). However, this increase in BW of the FL line did not keep pace with me increase in the F line. Increased leg bone development in the FL line resulted in relative thigh, drum, shank, and total leg bone weights, which were greater than those of the F line. Consequently, the FL line has proportions of leg bone which are more similar to the RBC2 population than to the F line, its line of origin. Nestor et al. (1988) reported an increased proportion of leg muscles in the FL line as compared with the F and RBC2 lines in the fifth generation of selection. Although relative leg muscle weights were generally higher for the FL line than for the F line in the present study, these values were not significantly different. However, it is possible that the relative proportion of leg muscle in the F line has increased in recent generations, as evidenced by the lack of a difference between
relative leg muscle weights of the F and RBC2 lines in the present study. Thus, an increase in the relative leg muscle of the F line might have eliminated the difference in leg muscle development between the F and FL lines that was previously reported. The linear regression coefficients of total mean leg muscle weight on generation for the FM line were 11.6 g (P<05) and 3.42 g (P>.05) for males and females, respectively (Nestor and Emmerson, 1990). Although total leg muscle did show a small increase in the FM line, the relative proportion of leg muscle did not significantly increase. Therefore, family selection for increased leg muscle mass was ineffective in increasing the proportion of leg muscle in the FM line and, consequently, the FM line could not be used to test the hypothesis that the relative amount of leg muscles influences walking ability. The relative ineffectiveness of selection in me FM line appears to be related to the type of selection that was employed. Out of necessity, the FM line was developed using family selection. Prior to the establishment of the FM
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Body weight, kg Muscles Breast, g Breast, % Thigh, g Thigh, % Drum, g Drum, % Total leg, g Total leg, Bones Thigh, g Thigh, % Drum, g Drum, % Shank, g Shank, % Total leg, g Total leg,
RBC2
F
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EMMERSON ET AL.
TABLE 3. Body, muscle, and bone weights of a randombred control population (RBC2), a line selected for increased 16-wk body weight (F), a line selected for increased shank width (FL), and a line selected for increased leg muscle weight (FM) in the sixth generation of selection in FM Sex
Line Measuremenit
6.37°
FL
F 9.99*
8.77°
Male
FM 8.87
b
9.72*
Female 7.45b
518.5d 16.3b 314.8° 9.9b 230.4C 7.2 b 545.2° 17.1b
900.2* 18.0* 518.8* 10.3* 378.3* 7.6* 897.1* 17.9*
729.8° 16.6b 458.1 b 10.4* 338.0b 7.7* 796.1 b 18.2*
781.5b 17.6b 470.5 b 10.6* 3399b 7.7* 810.4b 18.3*
835.7* 17.1* 505.7* 10.4* 368.1* 7.6* 873.8* 17.9*
644.9b 17.2* 386.2b 10.3* 281.7b 7.6* 667.9b 17.9*
55.8° 1.7* 92.9° 2.9* 60.6d 1.9b 209.3° 6.4" 1.68b
77.9* 1.5° 133.0* 2.6b 88.9b 1.7° 299.7* 6.0° 2.74*
75.3*b 1.7* 125.7b 2.9* 96.0* 2.2* 296.4* 6.8* 2.72*
72.7b 1.6b 120.3b 2.7 b 88.9 b 1.9b 276.3 b 6.2° 2.61*
89.8* 1.9* 148.5* 3.1* 104.1* 2.2* 342.2* 7.1* 2.81*
53.5 b 1.4b 90.6b 2.5b 63.0 b 1.7b 207.1 b 5.6b 2.18b
^ M e a n s within lines and sex subgroups with no common superscripts are significantly different (P<05). Absolute weights measured on one-half of each carcass. Absolute weights were multiplied by 2 prior to calculation of percentages. Subjective rating of 1 (no difficulty walking) through 5 (extreme lateral deviation of legs and extreme difficulty walking).
line, attempts were made to correlate live bird measures with the amount of leg muscle development. Unfortunately, none of diese measures had a high genetic correlation with the actual amount of leg muscle (Nestor, unpublished data). Family selection was the only viable alternative. Because of the nature of the selection program, it was necessary to save 50% of the families in order to minimize inbreeding. Li addition, some families were lost as potential breeders because they produced insufficient numbers of offspring to adequately evaluate the family mean, which effectively increased the proportion of the families that were saved as breeders. In contrast, only 18% and 21% of the measured individuals were selected this past year in the F and FL lines, respectively. Thus, the intensity of selection was much higher in the F and FL lines than for the FM line. Commercial turkey breeders could overcome this constraint by increasing the number of families. Another factor that appeared to have limited the response in the FM line was the small number of offspring measured to estimate the
family mean. Due to the poor reproductive performance of heavy turkeys, a maximum of four offspring per family was available for the evaluation of family means. A single outlier for leg muscle from a family could potentially bias the family mean such that the family would be incorrectly selected or rejected for breeding. Because the reproductive rate of these experimental populations does not differ greatly from that observed in a commercial sire line (Nestor, unpublished data), commercial breeders would probably be unable to increase family size. Therefore, this constraint would be difficult to overcome commercially. The lack of line differences among the selected lines for walking ability is not fully understood. The FL line has been previously reported to have major increases in BW without associated losses in walking ability (Nestor et al., 1988). Li contrast, the F line has shown consistent losses in walking ability with increases in BW (Nestor, 1984). Li die present experiment, all birds were raised in confinement housing rather man under the range conditions that were used in the development
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Body weight, kg Muscles Breast, g Breast, % Thigh, g Thigh, % Drum, g Drum, % Total leg, g Total leg, % Bones Thigh, g Thigh, % Drum, g Dram, % Shank, g Shank, % Total leg, g Total leg, % Leg score2
RBC2
LEG MUSCLE SELECTION AND BODY COMPOSITION
REFERENCES Burton, R. W., A. K. Sheridan, and C. R. Howlett, 1981. The incidence and importance of tibial dyschondroplasia to the commercial broiler industry in Australia. Br. Poult. Sci. 22:153-160. Clayton, G. A., C. Nixey, and G. Monaghan, 1978. Meat yield in turkeys. Br. Poult Sci. 19:755-763. Harshaw, H. M, and R. R. Rector, 1940. The composition of turkeys as affected by age and sex. Poultry Sci. 19:404-411. Havenstein, G. B., K. E. Nestor, V. D. Toelle, and W. L. Bacon, 1988a. Estimates of genetic parameters in turkeys. 1. Body weight and skeletal characteristics. Poultry Sci. 67:1378-1387. Havenstein, G. B., V. D. Toelle, K. E. Nestor, and W. L. Bacon, 1988b. Estimates of genetic parameters in turkeys. 2. Body weight and carcass characteristics. Poultry Sci. 67:1388-1399. Larsen, J. E., R. L. Adams, I. C. Peng, and W. J. Stadelman, 1986. Growth, feed conversions, and yields of turkey parts of three strains of hen turkeys as influenced by age. Poultry Sci. 65:2076-2081. Leach, R. M., Jr., and M. C. Nesheim, 1965. Nutritional, genetic and morphological studies of an abnormal cartilage formation in young chicks. J. Nutr. 86: 236-244. Leach, R. M., Jr., and M C. Nesheim, 1972. Further studies on tibial dyschondroplasia (cartilage abnormality) in young chicks. J. Nutr. 102:1673-1680. Leenstra, F. R., A. van Voorst, and U. Haye, 1984.
Genetic aspects of twisted legs in a broiler sire strain. Ann. Agric. Fenn. 23:261-270. Marsden, S. J., 1940. Weights and measurements of parts and organs of turkeys. Poultry Sci. 19:23-28. Mercer, J. T., and W. G. Hill, 1984. Estimation of genetic parameters for skeletal defects in broiler chickens. Heredity 53:193-203. Miller, B. F., 1968. Comparative yield of different size turkey carcasses. Poultry Sci. 47:1570-1574. Naber, E. C , and S. P. Touchbum, 1970. Ohio poultry rations. Ohio Cooperative Extension Service Bulletin 343, The Ohio State University, Columbus, OH. Nestor, K. E., M. G. McCartney, and N. Bachev, 1969. Relative contributions of generics and environment to turkey improvement Poultry Sci. 48:1944-1949. Nestor, K. E., 1977a. The use of a paired mating system for the maintenance of experimental populations of turkeys. Poultry Sci. 56:60-65. Nestor, K. E., 1977b. Genetics of growth and reproduction in the turkey. 5. Selection for increased body weight alone and in combination with increased egg production. Poultry Sci. 56:337-347. Nestor, K. E., 1984. Genetics of growth and reproduction in the turkey. 9. Long-term selection for increased 16-week body weight. Poultry Sci. 63:2114-2122. Nestor, K. E., W. L. Bacon, Y. M. Saif, and P. A. Renner, 1985. The influence of genetic increases in shank width on body weight, walking ability, and reproduction of turkeys. Poultry Sci. 64:2248-2255. Nestor, K. E., W. L. Bacon, G. B. Havenstein, Y. M. Saif, and P. A. Renner, 1988. Carcass traits of turkeys from lines selected for increased growth rate or increased shank width. Poultry Sci. 67:1660-1667. Nestor, K. E., and D. A. Emmerson, 1990. Genetics of leg strength in turkeys. Pages 57-82 in: Proceedings of the 39th National Breeders' Roundtable, St Louis, MO. Peng, I. C, R. L. Adams, E. J. Furumoto, P. Y. Hester, J. E. Larsen, O. A. Pike, and W. J. Stadelman, 1985. Allometric growth patterns and meat yields of carcass parts of turkey toms as influenced by lighting programs and age. Poultry Sci. 64:871-876. Riddell, C , 1976. Selection of broiler chickens for a high and low incidence of tibial dyschondroplasia with observations on spondylolisthesis and twisted leg (perosis). Poultry Sci. 55:145-151. Serfontein, P. J., and L. F. Payne, 1934. Inheritance of abnormal anatomical condition in the tibial metatarsal joints. Poultry Sci. 13:61-63. Sheridan, A. K., C. R. Howlett, and J. A. Burton, 1974. Genetic factors influencing dyschondroplasia in Australian meat chickens. Pages 34-35 in: Proc. XV World's Poultry Congress, New Orleans, LA. Sheridan, A. K., C. R. Howlett, and R. W. Burton, 1978. The inheritance of tibial dyschondroplasia in broilers. Br. Poult Sci. 19:491-499. Somes, R. G., Jr., 1969. Genetic perosis in the domestic fowl. J. Hered. 60:163-166. Steel, R.G.D., and J. H. Torrie, 1960. Principles and Procedures of Statistics. McGraw-Hill Book Co., New York, NY. Walser, M. M, F. L. Cherms, and H. E. Dziuk, 1982. Osseous development and tibial dyschondroplasia in five lines of turkeys. Avian Dis. 26:265-270.
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of these lines. Leg ratings have been observed to differ under confinement and range conditions (Nestor, unpublished data). If an interaction exists between line and growout environment for leg ratings, line differences might have been masked in the present experiment by confinement rearing. The lack of a change in the leg rating for the FM line is consistent with the general absence of selection response. Li summary, selection for increased BW in the F line increased the relative proportion of breast muscle and decreased the relative proportion of leg bone. These changes are consistent with patterns already reported in the literature. Selection for increased shank width reversed some of these composition changes by increasing the proportion of leg bones. Body weight also increased as a correlated response in the FL line. Selection for increased leg muscle mass was largely ineffective in changing body composition. Technological advances allowing for the direct measurement of leg muscle development on the live bird would appear to be necessary before such a selection program could be effectively instituted.
745
1991 Poultry Science 70:739-745 INTRODUCTION
Leg weakness in broilers and turkeys is a problem of increasing importance for the poultry industry. Leg abnormalities are responsible for poor growth performance, increased downgrading, and mortality, all of which reduce the efficiency of commercial production. There is evidence for a strong genetic component to leg weakness. In broilers, most types of leg weakness have been shown to be highly heritable (Sheridan et al., 191A; Burton et al, 1981; Mercer and Hill, 1984) or to be under the influence of major genes (Somes, 1969; Sheridan et al, 1978). Consequently, leg abnormalities are very responsive to selection (Serfontein and Payne, 1934; Leach and
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 258-90. 'Present address: Department of Animal and Poultry Sciences, University of Arkansas, Fayetteville, AR 72701.
Nesheim, 1965, 1972; Riddell, 1976; Leenstra et al, 1984). Although not as extensively studied, line differences for the incidence of leg problems have been reported in turkeys (Walser et al, 1982; Nestor, 1984). Similar contributory factors and types of leg problems are observed in broiler chickens and turkeys (Nestor and Emmerson, 1990). Therefore, leg weakness in turkeys would also be expected to respond to selection. Indeed, selection for shank width has been shown to improve walking ability in large-bodied turkeys (Nestor et al, 1985). A close relationship exists between body size and the relative proportion of body parts. In general, as BW increases, there is an increase in the proportion of breast muscle and a decline in the proportion of leg muscle and bone. These changes in proportion accompany both developmental (Harshaw and Rector, 1940; Clayton et al, 1978; Peng et al, 1985; Larsen et al, 1986) and genetic (Marsden, 1940; Miller, 1968; Clayton et al, 1978; Walser et al, 1982; Nestor, 1984; Nestor et
139
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ABSTRACT Body compositions of a randombred control population (RBC2), a line selected for increased 16-wk BW (F), a line selected for increased shank diameter (EL), and a line selected for increased leg muscle mass (FM) were compared at 16 wk of age. The F line originated from the RBC2 population and the FL and FM lines were developed as sublines of the F line. The F and EL lines were developed through mass selection for BW and shank width, respectively. The FM line was developed through family selection for leg muscle mass. Selection for B W in the F line has resulted in an increase in the proportion of breast muscle and a decrease in the relative amount of leg bone in comparison with the RBC2 population. Selection for increased shank width in the FL line has increased not only shank weight but has produced concomitant increases in the weight of the tibiotarsal and femur bones. Although the FL line has also shown a correlated increase in BW in association with increases in shank width, the increase in leg bone weight was relatively greater than the increase in BW. Therefore, the relative weights of leg bones in the FL line increased such that they were more similar to the RBC2 population than to the F line. Although the amount of leg muscle increased slightly in the FM line, the relative proportion of leg muscle did not increase significantly. The selection program used was probably ineffective in increasing the proportion of leg muscle because of the high percentage (low selection intensity) of families selected and small family size for estimating family means. There were no differences in the walking ability of F, FL, and FM lines. These selected lines did have poorer walking ability than RBC2 as indicated by higher leg scores for these lines. (Key words: body composition, leg bones, leg muscles, leg weakness, turkeys)
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EMMERSON ET AL.
MATERIALS AND METHODS
Four lines of turkeys (RBC2, F, FL, FM) were surveyed for carcass traits in each of six generations. The RBC2 control line was established through the crossing of two commercial strains of large white turkeys and has been maintained for 23 generations under random mating (Nestor et al, 1969). The F line was derived from the RBC2 line by mass selection for increased 16-wk BW (Nestor, 1977b). The FL line is a subline of the F line that has been mass selected for increased shank width at the dewclaw (Nestor et al., 1985). The FM line, also a subline of F, was developed through family selection for increased leg muscle mass. Assuming the genetic correlation between leg muscle mass and BW is less than 1, direct selection for leg muscle mass would be expected to increase leg muscle mass to a greater extent than the correlated trait BW. Thus, selection for increase muscle mass would also be expected to increase the relative proportion of leg muscle. This selection program was initiated after 13 generations of selection for 16-wk BW in the F line. In the
base generation of subline FM, two groups of 36 F-line females were mated to a single group of F-line males. These groups of hens were assembled such that the average selection differentials for 16-wk BW were similar for the two groups. One group of females was used to reproduce the F line and the other group served as the base population for the FM line. Each generation, family selection of the FM line was based on thigh and drum muscle weights from two male and two female offspring. Fifty percent of the families were then selected based on the total family weight of thigh and drum muscle. Leg muscle weights for families with fewer than four offspring were appropriately weighted to avoid unnecessary loss of families. Due to poor reproduction in the F and FM lines, it was impossible to sample more than four offspring per family and still adequately reproduce the line. All lines were maintained under a paired mating system with 36 parental pairs in each line (Nestor, 1977a). In each of six generations, lines were surveyed for carcass composition. Approximately 150 birds per line and 300 birds per sex were used for the determination of carcass component weights in each generation. Poults were grown to 16 wk of age in floor pens in an enclosed building under standard management practices (Nestor et al., 1988). All lines were fed a six-ration declining protein system throughout the growout period (Naber and Touchburn, 1970). At 16 wk of age, birds were weighed and subjectively scored for walking ability as described by Nestor (1984). Birds were fasted overnight, killed by severing the jugular vein, scalded at 68 C, and defeathered. Carcasses were then split lengthwise and onehalf of the carcass was chilled overnight in cold water for later dissection. Legs were removed from the carcass halves and separated into drumsticks and thighs by cutting through the knee joint. Thigh, drum, and breast muscles were dissected away from the bone and weighed. The femur, tibiotarsus, and tarsometatarsus (skin and scales included) were then individually weighed. Data were analyzed using ANOVA in a 4 x 2 factorial with line and sex as main effects. When appropriate, means were separated using Tukey's test (Steel and Torrie, 1960). Percentage data represent the weight of individual body parts relative to live BW. Weight
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al, 1985) increases in BW. Changes in the relative proportions of body parts are further compounded as a result of selection for increased breast development in commercial poultry stocks. It has been suggested that changes in the relative proportions of body component parts may be partially responsible for the increased incidence of leg problems in modern commercial turkeys. Nestor et al. (1985) hypothesized that increases in BW and breast muscle development without concomitant increases in leg muscle and bone is biologically incompatible with the maintenance of normal walking ability. As a result, two selection experiments have been initiated with the goal of increasing the relative proportions of leg muscle and bone in turkeys which have been previously selected for increased BW. The development of a subline (FL) selected for increased leg bone development has been previously reported (Nestor et al, 1985). A second subline (FM) has been selected for increased leg muscle. The objective of the present study is to report the establishment of the FM subline and to investigate the effect of these selection programs on carcass composition traits and walking ability over six generations of selection.
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LEG MUSCLE SELECTION AND BODY COMPOSITION
TABLE 1. Body, muscle, and bone weights of a randombred control population (RBC2), a line selected for increased 16-wk body weight (F), a line selected for increased shank width (FL), and a line selected for increased leg muscle weight (FM) in the fourth generation of selection in FM Line RBC2
Measurement
EL
F 9.37"
6.24°
FM
9.08"
8.73b
Male 9.46"
Female 7.30b
524.1° 16.8b 296.6° 9.6° 212.1° 6.8 b 508.8° 16.4b
818.8" 17.4" 482.1" 10.4" 338.7" 7.2" 820.9" 17.5"
733.8 b 16.2° 438.5 b 9.$* 317.0b 7.0"b 755.6b 17.3"
757.2b 17.4" 439. l b 10.0"b 314.9b 7.2" 754.0b 16.7b
802.9" 17.0" 464.4" 9.8 b 334.4" 7.0 b 798.9" 16.8b
620.9 b 17.0" 368. l b 10.0" 260.0b 7.2" 628.1 b 17.2"
62.9d 2.0 b 91.6° 2.9" 57.7° 1.8" 212.2d 6.7" 1.64b
87.6 b 1.8° 127.0" 2.7 b 78.5" 1.6b 293.1 b 6.2b 2.73"
97.0" 2.2" 131.7" 2.9" 81.3" 1.8" 310.0" 6.8" 2.70"
80.8° 1.8° 115.6b 2.6 b 73.0 b 1.6b 269.9° 6.1 b
102.2" 2.2" 144.3" 3.1" 89.8" 2.0" 336.3" 7.2" 3.05"
61.0 b 1.6b 87.2 b 2.4 b 54.6 b 1.6b 202.7 b 5.6b 1.87b
2.71"
""^Means within lines and sex subgroups with no common superscripts are significantly different (P<05). Absolute weights measured on one-half of each carcass. Absolute weights were multiplied by 2 prior to calculation of percentages. 2 Subjective rating of 1 (no difficulty walking) through 5 (extreme lateral deviation of legs and extreme difficulty walking).
measures were log transformed and percentage data were arc sine square root transformed prior to analysis. RESULTS
Line Comparisons in Fourth Generation of the FM Line As expected, muscle and bone weights of the RBC2 population were consistently lower than those from the other lines surveyed (Table 1). Body weights of the F and FL lines were higher than in the FM line. Absolute muscle weights of the F line were consistently higher than the FL and FM lines. However, relative breast and thigh muscle weights of the F line were not different from the FM line, and relative drum muscle weight was not different from the FL or FM lines. Although total leg muscle weight of the F line was greater than that of the FM and RBC2 lines, the relative leg muscle weight of the F line was not different from FL. The FL line had heavier shank bones than the other lines
surveyed. In addition, the relative shank bone weight of the FL line was greater than that of the F, FM, and RB C2 lines and the relative thigh and drum bone weights of the FL line were greater than those of the F and FM lines. Total leg bone weights followed the same pattern as BW except that leg bone weight of the FL line was greater than the other lines tested. Relative total leg bone weights of the RBC2 and FL lines were greater than for the F and FM lines. The RBC2 population had a lower leg score than the F, FL, and FM lines. Males had higher values than females for all traits except percentage breast, thigh, and total leg muscle (Table 1). There was no sex difference for relative breast muscle weight. Relative thigh, drum, and total leg muscle weights were greater for females than for males. Line Comparisons in Fifth Generation of the FM Line Absolute muscle and bone weights of the RBC2 population were lower than for the F, FL, and FM lines for all traits measured (Table 2).
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Body weight, kg Muscles Breast, g Breast, % Thigh, g Thigh, % Drum, g Drum, % Total leg, g Total leg, % Bones Shank, g Shank, % Drum, g Drum, % Thigh, g Thigh, % Total leg, g Total leg, % Leg scores2
Sex
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EMMERSON ET AL.
literature. Males had greater absolute muscle and bone weights, as would be expected based on BW differences due to sexual dimorphism. Body weight was distributed differently in males and females as evidenced by die greater relative tiiigh, drum, and total leg muscle for females (Generation 4) and me greater relative shank, drum, and diigh bone for males (Generations 4, 5, and 6). Clayton et al. (1978) observed a greater percentage of leg bone in male turkeys but also found me percentage dark meat to be greater for males. This difference is apparendy related to me lines tested. Sex differences in me present experiment are consistent with previous observation in these lines (Nestor et al., 1988). The sex difference for leg ratings has also been previously observed (Nestor, unpublished data).
As expected, selection for increased BW successfully increased not only BW but produced consistent increases in muscle mass. In contrast, changes in leg bone weights were negligible. These observations were consistent Line Comparisons in Sixth Generation with die high genetic correlation between BW of the FM Line and breast, drum, and tiiigh muscle (HavenAll absolute weight measures were lower for stein et al., 1988a) and the smaller genetic the RBC2 population than for the selected lines correlation between BW and skeletal measures (Table 3). In addition, the RBC2 population had (Havenstein et al., 1988b). Based on differlower percentages of thigh, drum, and total leg ences in the magnitude of increase in muscle muscle. The F line had a greater B W and greater and bone, the F line had a greater proportion of weight of breast, thigh, drum, and total leg muscle tissue and a smaller proportion of bone muscles man die FL and FM lines. However, man die RBC2 population from which it was relative tiiigh, drum, and total leg muscle derived. This was in agreement with a previous weights were not different among the selected report by Nestor et al. (1988). However, populations. The F line also had greater diigh Nestor et al. (1988) also reported disproporbone weight than the FM line and a greater drum tionate increases in breast and leg muscle of bone weight than both FL and FM. However, the percentage diigh bone for die F line was smaller weight-selected turkeys. Although relative man for die odier selected populations and die breast muscle weights were generally higher relative drum bone weight was less for the F line for die F line than for RBC2 in the present tiian for me FL line. As expected, die FL line had study, relative leg muscle weights were also a greater absolute and relative shank bone equal to (one generation) or greater (two weight man me other lines. Total leg bone generations) in die F line man in the RBC2 weights of the F and FL lines were greater tiian population. In addition, deviations between die for the FM line. Relative total leg bone weight F line and me RBC2 were generally greater for was greater for FL man for RBC2 and bom were percentage leg muscle than for percentage greater than FM and F. The leg score of RBC2 breast muscle. Therefore, it appears that me was lower man for die selected populations. previously reported pattern of a decreased There were sex differences for all traits except proportion of leg muscle in the F line has for the percentages of breast, tiiigh, drum, and reversed in more recent generations. The reason for this change in response is unknown. total leg muscle. Relative leg bone, however, remained lower in die F line than in the RBC2. DISCUSSION
Sex differences in body composition were generally consistent with diose reported in die
Selection for increased shank width produced a general increase in leg bone development. Body weight has continued to
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Body weights and thigh muscle weights of the F and FM lines were higher than those from die FL line. Breast weight of the F line was greater than the FM line, which was greater than die FL line. The F line had a greater percentage breast muscle than the FL line but neither line was different from the FM line. The FM line had a greater drum muscle weight than the FL line but these values were not different when expressed relative to BW. The FL line had a heavier thigh bone than die F line and a heavier shank bone than botii F and FM. Relative thigh and shank bone weights were also greater for die FL line than for the F and FM lines. Altiiough drum bone weights were not different for die F, FL, and FM lines, percentage drum bone was greater for me FL line man for the F line. Although RBC2 had a lower leg score than die selected lines, there was no difference between die leg scores of me selected lines. There were significant sex differences for all traits except percentage breast, migh, drum, and total leg muscle.
743
LEG MUSCLE SELECTION AND BODY COMPOSITION
TABLE 2. Body, muscle, and bone weights of a randombred control population (RBC2), a line selected for increased 16-wk body weight (F), a line selected for increased shank width (FL), and a line selected for increased leg muscle weight (FM) in the fifth generation of selection in FM1 Line Measurement
Leg score
Sex
FL 9.32°
FM 9.74*
Male 10.19*
Female 7.37°
6.27°
10.05*
538.5d 17.5* 319.3° 10.4* 231.9C 7.6*b 551.2C 17.9*
878.6* 17.4* 505.5* 10.0* 365.7ab 7.3 b 871.1* 17.3*
753.3° 16.4b 474.0b 10.3* 353.3b 7.7* 827.6b 18.0*
823.3b 17.0ab 504.7* 10.4* 366.0* 7.6 ab 870.7* 18.0*
854.5* 16.8* 518.5* 10.3* 377.9* 7.5* 896.4* 17.8*
632.0 b 17.3* 376.9b 10.3* 276. l b 7.6* 653.0b 17.9*
50.5° 1.6* 83.8b 2.7* 58.4C 1.9b 192.7° 6.2*b 1.42b
73.7b 1.4b 128.9* 2.5b 85.6b 1.7° 288.2b 5.6° 2.20*
76.7* 1.6* 128.3* 2.7* 96.8* 2.1* 303.0* 6.4* 2.22*
75.6*b 1.5»b 125.7* 2.6*b 87.5b 1.9b 288.8*b
88.0* 1.8* 148.9* 3.0* 103.2* 2.1* 340.1* 6.8* 2.67*
48.8 b 1.3b 81.7b 2.3 b 58.9 b 1.6b 189.3b 5.2 b 1.39b
2.37*
*_cMeans within line and sex subgroups with no common superscripts are significantly different (P<.05). Absolute weights measured on one-half of each carcass. Absolute weights were multiplied by 2 prior to calculation of percentage. Subjective rating of 1 (no difficulty walking) through 5 (extreme lateral deviation of legs and extreme difficulty walking).
increase as a correlated response to selection for increased shank width as evidenced by the linear regression coefficient of response on generation of .26 kg (P<.01) and .15 kg (P<.01) for males and females, respectively (Nestor and Emmerson, 1990). However, this increase in BW of the FL line did not keep pace with me increase in the F line. Increased leg bone development in the FL line resulted in relative thigh, drum, shank, and total leg bone weights, which were greater than those of the F line. Consequently, the FL line has proportions of leg bone which are more similar to the RBC2 population than to the F line, its line of origin. Nestor et al. (1988) reported an increased proportion of leg muscles in the FL line as compared with the F and RBC2 lines in the fifth generation of selection. Although relative leg muscle weights were generally higher for the FL line than for the F line in the present study, these values were not significantly different. However, it is possible that the relative proportion of leg muscle in the F line has increased in recent generations, as evidenced by the lack of a difference between
relative leg muscle weights of the F and RBC2 lines in the present study. Thus, an increase in the relative leg muscle of the F line might have eliminated the difference in leg muscle development between the F and FL lines that was previously reported. The linear regression coefficients of total mean leg muscle weight on generation for the FM line were 11.6 g (P<05) and 3.42 g (P>.05) for males and females, respectively (Nestor and Emmerson, 1990). Although total leg muscle did show a small increase in the FM line, the relative proportion of leg muscle did not significantly increase. Therefore, family selection for increased leg muscle mass was ineffective in increasing the proportion of leg muscle in the FM line and, consequently, the FM line could not be used to test the hypothesis that the relative amount of leg muscles influences walking ability. The relative ineffectiveness of selection in me FM line appears to be related to the type of selection that was employed. Out of necessity, the FM line was developed using family selection. Prior to the establishment of the FM
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Body weight, kg Muscles Breast, g Breast, % Thigh, g Thigh, % Drum, g Drum, % Total leg, g Total leg, Bones Thigh, g Thigh, % Drum, g Drum, % Shank, g Shank, % Total leg, g Total leg,
RBC2
F
744
EMMERSON ET AL.
TABLE 3. Body, muscle, and bone weights of a randombred control population (RBC2), a line selected for increased 16-wk body weight (F), a line selected for increased shank width (FL), and a line selected for increased leg muscle weight (FM) in the sixth generation of selection in FM Sex
Line Measuremenit
6.37°
FL
F 9.99*
8.77°
Male
FM 8.87
b
9.72*
Female 7.45b
518.5d 16.3b 314.8° 9.9b 230.4C 7.2 b 545.2° 17.1b
900.2* 18.0* 518.8* 10.3* 378.3* 7.6* 897.1* 17.9*
729.8° 16.6b 458.1 b 10.4* 338.0b 7.7* 796.1 b 18.2*
781.5b 17.6b 470.5 b 10.6* 3399b 7.7* 810.4b 18.3*
835.7* 17.1* 505.7* 10.4* 368.1* 7.6* 873.8* 17.9*
644.9b 17.2* 386.2b 10.3* 281.7b 7.6* 667.9b 17.9*
55.8° 1.7* 92.9° 2.9* 60.6d 1.9b 209.3° 6.4" 1.68b
77.9* 1.5° 133.0* 2.6b 88.9b 1.7° 299.7* 6.0° 2.74*
75.3*b 1.7* 125.7b 2.9* 96.0* 2.2* 296.4* 6.8* 2.72*
72.7b 1.6b 120.3b 2.7 b 88.9 b 1.9b 276.3 b 6.2° 2.61*
89.8* 1.9* 148.5* 3.1* 104.1* 2.2* 342.2* 7.1* 2.81*
53.5 b 1.4b 90.6b 2.5b 63.0 b 1.7b 207.1 b 5.6b 2.18b
^ M e a n s within lines and sex subgroups with no common superscripts are significantly different (P<05). Absolute weights measured on one-half of each carcass. Absolute weights were multiplied by 2 prior to calculation of percentages. Subjective rating of 1 (no difficulty walking) through 5 (extreme lateral deviation of legs and extreme difficulty walking).
line, attempts were made to correlate live bird measures with the amount of leg muscle development. Unfortunately, none of diese measures had a high genetic correlation with the actual amount of leg muscle (Nestor, unpublished data). Family selection was the only viable alternative. Because of the nature of the selection program, it was necessary to save 50% of the families in order to minimize inbreeding. Li addition, some families were lost as potential breeders because they produced insufficient numbers of offspring to adequately evaluate the family mean, which effectively increased the proportion of the families that were saved as breeders. In contrast, only 18% and 21% of the measured individuals were selected this past year in the F and FL lines, respectively. Thus, the intensity of selection was much higher in the F and FL lines than for the FM line. Commercial turkey breeders could overcome this constraint by increasing the number of families. Another factor that appeared to have limited the response in the FM line was the small number of offspring measured to estimate the
family mean. Due to the poor reproductive performance of heavy turkeys, a maximum of four offspring per family was available for the evaluation of family means. A single outlier for leg muscle from a family could potentially bias the family mean such that the family would be incorrectly selected or rejected for breeding. Because the reproductive rate of these experimental populations does not differ greatly from that observed in a commercial sire line (Nestor, unpublished data), commercial breeders would probably be unable to increase family size. Therefore, this constraint would be difficult to overcome commercially. The lack of line differences among the selected lines for walking ability is not fully understood. The FL line has been previously reported to have major increases in BW without associated losses in walking ability (Nestor et al., 1988). Li contrast, the F line has shown consistent losses in walking ability with increases in BW (Nestor, 1984). Li die present experiment, all birds were raised in confinement housing rather man under the range conditions that were used in the development
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Body weight, kg Muscles Breast, g Breast, % Thigh, g Thigh, % Drum, g Drum, % Total leg, g Total leg, % Bones Thigh, g Thigh, % Drum, g Dram, % Shank, g Shank, % Total leg, g Total leg, % Leg score2
RBC2
LEG MUSCLE SELECTION AND BODY COMPOSITION
REFERENCES Burton, R. W., A. K. Sheridan, and C. R. Howlett, 1981. The incidence and importance of tibial dyschondroplasia to the commercial broiler industry in Australia. Br. Poult. Sci. 22:153-160. Clayton, G. A., C. Nixey, and G. Monaghan, 1978. Meat yield in turkeys. Br. Poult Sci. 19:755-763. Harshaw, H. M, and R. R. Rector, 1940. The composition of turkeys as affected by age and sex. Poultry Sci. 19:404-411. Havenstein, G. B., K. E. Nestor, V. D. Toelle, and W. L. Bacon, 1988a. Estimates of genetic parameters in turkeys. 1. Body weight and skeletal characteristics. Poultry Sci. 67:1378-1387. Havenstein, G. B., V. D. Toelle, K. E. Nestor, and W. L. Bacon, 1988b. Estimates of genetic parameters in turkeys. 2. Body weight and carcass characteristics. Poultry Sci. 67:1388-1399. Larsen, J. E., R. L. Adams, I. C. Peng, and W. J. Stadelman, 1986. Growth, feed conversions, and yields of turkey parts of three strains of hen turkeys as influenced by age. Poultry Sci. 65:2076-2081. Leach, R. M., Jr., and M. C. Nesheim, 1965. Nutritional, genetic and morphological studies of an abnormal cartilage formation in young chicks. J. Nutr. 86: 236-244. Leach, R. M., Jr., and M C. Nesheim, 1972. Further studies on tibial dyschondroplasia (cartilage abnormality) in young chicks. J. Nutr. 102:1673-1680. Leenstra, F. R., A. van Voorst, and U. Haye, 1984.
Genetic aspects of twisted legs in a broiler sire strain. Ann. Agric. Fenn. 23:261-270. Marsden, S. J., 1940. Weights and measurements of parts and organs of turkeys. Poultry Sci. 19:23-28. Mercer, J. T., and W. G. Hill, 1984. Estimation of genetic parameters for skeletal defects in broiler chickens. Heredity 53:193-203. Miller, B. F., 1968. Comparative yield of different size turkey carcasses. Poultry Sci. 47:1570-1574. Naber, E. C , and S. P. Touchbum, 1970. Ohio poultry rations. Ohio Cooperative Extension Service Bulletin 343, The Ohio State University, Columbus, OH. Nestor, K. E., M. G. McCartney, and N. Bachev, 1969. Relative contributions of generics and environment to turkey improvement Poultry Sci. 48:1944-1949. Nestor, K. E., 1977a. The use of a paired mating system for the maintenance of experimental populations of turkeys. Poultry Sci. 56:60-65. Nestor, K. E., 1977b. Genetics of growth and reproduction in the turkey. 5. Selection for increased body weight alone and in combination with increased egg production. Poultry Sci. 56:337-347. Nestor, K. E., 1984. Genetics of growth and reproduction in the turkey. 9. Long-term selection for increased 16-week body weight. Poultry Sci. 63:2114-2122. Nestor, K. E., W. L. Bacon, Y. M. Saif, and P. A. Renner, 1985. The influence of genetic increases in shank width on body weight, walking ability, and reproduction of turkeys. Poultry Sci. 64:2248-2255. Nestor, K. E., W. L. Bacon, G. B. Havenstein, Y. M. Saif, and P. A. Renner, 1988. Carcass traits of turkeys from lines selected for increased growth rate or increased shank width. Poultry Sci. 67:1660-1667. Nestor, K. E., and D. A. Emmerson, 1990. Genetics of leg strength in turkeys. Pages 57-82 in: Proceedings of the 39th National Breeders' Roundtable, St Louis, MO. Peng, I. C, R. L. Adams, E. J. Furumoto, P. Y. Hester, J. E. Larsen, O. A. Pike, and W. J. Stadelman, 1985. Allometric growth patterns and meat yields of carcass parts of turkey toms as influenced by lighting programs and age. Poultry Sci. 64:871-876. Riddell, C , 1976. Selection of broiler chickens for a high and low incidence of tibial dyschondroplasia with observations on spondylolisthesis and twisted leg (perosis). Poultry Sci. 55:145-151. Serfontein, P. J., and L. F. Payne, 1934. Inheritance of abnormal anatomical condition in the tibial metatarsal joints. Poultry Sci. 13:61-63. Sheridan, A. K., C. R. Howlett, and J. A. Burton, 1974. Genetic factors influencing dyschondroplasia in Australian meat chickens. Pages 34-35 in: Proc. XV World's Poultry Congress, New Orleans, LA. Sheridan, A. K., C. R. Howlett, and R. W. Burton, 1978. The inheritance of tibial dyschondroplasia in broilers. Br. Poult Sci. 19:491-499. Somes, R. G., Jr., 1969. Genetic perosis in the domestic fowl. J. Hered. 60:163-166. Steel, R.G.D., and J. H. Torrie, 1960. Principles and Procedures of Statistics. McGraw-Hill Book Co., New York, NY. Walser, M. M, F. L. Cherms, and H. E. Dziuk, 1982. Osseous development and tibial dyschondroplasia in five lines of turkeys. Avian Dis. 26:265-270.
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of these lines. Leg ratings have been observed to differ under confinement and range conditions (Nestor, unpublished data). If an interaction exists between line and growout environment for leg ratings, line differences might have been masked in the present experiment by confinement rearing. The lack of a change in the leg rating for the FM line is consistent with the general absence of selection response. Li summary, selection for increased BW in the F line increased the relative proportion of breast muscle and decreased the relative proportion of leg bone. These changes are consistent with patterns already reported in the literature. Selection for increased shank width reversed some of these composition changes by increasing the proportion of leg bones. Body weight also increased as a correlated response in the FL line. Selection for increased leg muscle mass was largely ineffective in changing body composition. Technological advances allowing for the direct measurement of leg muscle development on the live bird would appear to be necessary before such a selection program could be effectively instituted.
745