The Influence of Differences in Dietary Amino Acids During the Early Growing Period on Growth and Development of Nicholas and British United Turkey Toms1

The Influence of Differences in Dietary Amino Acids During the Early Growing Period on Growth and Development of Nicholas and British United Turkey Toms1 MICHAEL S. LILBURN and DEREK EMMERSON2 Department of Poultry Science, Ohio Agricultural Research and Development Center, The Ohio State University, Wooster, Ohio 44691

1993 Poultry Science 72:1722-1730

at increasing the growth potential of commercial strains concomitant with inTurkey consumption in the United creases in breast yield (Nestor et al, 1988). States has increased from 2.77 kg (6.1 lb) In 1979, commercial toms weighed 14.5 kg in 1960 to over 8 kg (18 lb) in 1990 (32 lb) at 24 wk (Jensen, 1979) compared (Anonymous, 1990). To keep pace with the with a similar weight at 20 wk in 1991 needs of an expanding industry, commer- (Sell, 1991). Associated with the improvecial turkey breeders have been successful ments in growth and conformation of commercial turkeys has been a renewed interest in experiments dealing with genetic effects on BW, carcass, and skeletal Received for publication January 11, 1993. development (Bacon et al, 1986; Nestor et Accepted for publication May 18, 1993. al, 1987,1988; Larsen et al, 1988; Anthony Salaries and research support provided by State and Federal Funds appropriated to the Ohio Agricul- et al, 1991; Emmerson et al, 1991; Lilburn tural Research and Development Center, The Ohio and Nestor, 1991; Lilburn et al, 1992). State University. Manuscript Number 320-92. With respect to the interaction of diet 2 Present address: Hybrid Turkeys Inc., 9 Centen- and genotype, there is considerable comnial Drive, Kitchener, ON, Canada, N2B 3E9. INTRODUCTION

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ABSTRACT From 0 to 4, 4 to 8, and 8 to 12 wk of age, Nicholas (NIC) and British United Turkeys (BUT) toms were fed one of two series of experimental diets. All toms were fed the same diet, appropriately adjusted for age period, from 12 to 16 and 16 to 24 wk. The first series was formulated to 1984 NRC specifications whereas the second series contained higher concentrations of lysine and total sulfur amino acids (NRC+). The NIC toms were heavier (P < .05) at 4,8,12, and 16 wk, but there were no strain differences at 20 wk. The NRC+ diets significantly increased BW in both strains at 12 wk, but there were no diet effects at 16 or 20 wk. At 12 wk, BUT toms had significantly more carcass protein and less carcass lipid compared with NIC toms, but there were no significant dietary effects on these variables. In both strains, the NRC+ diets significantly increased the relative weight of the eviscerated carcass and both the absolute and relative weights of the Pectoralis major muscles at 12 wk. At 20 wk, the absolute and relative weight of the Pectoralis minor muscles was significantly heavier in BUT toms and in toms fed the NRC+ diets. The tibia and femur were longer in BUT toms compared with NIC toms. From 1 to 24 wk, weekly BW from 50 toms per strain and treatment combination were used to calculate growth curve parameters according to the Gompertz equation. The slope of the growth curve was significantly less in BUT toms, but the BW and age at the point of inflection were increased compared with NIC toms. There were no significant diet effects on any growth curve parameters. (Key words: turkeys, toms, growth curves, carcass composition, yield)

DIETARY AMINO ACIDS AND GROWTH OF COMMERCIAL TOMS

tributed to the significant differences in carcass composition observed when hens from both strains were compared at the same age (16 wk). The objectives of the present study were twofold. The first objective was to compare the growth curves of BUT and NIC toms, as Lilburn et al. (1992) did for hens. The second objective was to compare the response of BUT and NIC toms to diets that differed in their concentration of lysine and total sulfur amino acids during the initial stages of growth. Unlike the experiments of Moran et al. (1984) or Leeson and Caston (1991), no attempt was made to alter energy concentrations together with the changes in essential amino acids. MATERIALS AND METHODS

Commercial hatching eggs from similar aged BUT and NIC breeder flocks were purchased.3 The eggs were hatched at the Ohio Agricultural Research and Development Center, Wooster, OH. All the birds were sexed, wing-banded, and beak trimmed in the hatchery prior to placement. Due to large differences in the number of poults that hatched from each strain, there were unequal numbers of BUT and NIC toms available for use in the experiment. The BUT toms were randomly allotted to 10 litter floor pens, and the NIC toms were randomly allotted to 6 litter floor pens. The area in each pen was 9.44 m2, and there were 35 toms per pen at the start of the experiment. The poults in half the experimental pens allotted to each strain were fed one of two experimental diet series. One diet was formulated to NRC (1984) specifications and the other experimental diet (NRC+) was formulated to contain increased concentrations of lysine and total sulfur amino acids from 0 to 4 and 4 to 8 wk and increased total sulfur amino acids from 8 to 12 wk (Table 1). The increased amino acid levels in the NRC+ diets were in accordance with the dietary specifications recommended by British United Turkeys, Inc.4 for their particular turkey 3Cooper Hatchery, Inc., Oakwood, OH 45873. 4 British United Turkeys, Inc., Hockenull Hall, strain. From 12 to 16 and 16 to 24 wk, all birds were fed similar diets, which were Tarvin, Cheshire, England CH3 8LE.

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mercial interest because of the availability of European strains in North America and the potential for differing nutritional requirements. Moran et al. (1984) reported on the responses of Hybrid, Nicholas (NIC), and British United Turkeys (BUT) strain toms to diets that were formulated to European or North American specifications. The European diet sequences had higher protein levels during early growth and higher protein and lower energy levels over the latter stages of growout than the North American diets. The European diets improved growth through 14 wk in all three strains. From 14 to 24 wk, however, only the BUT toms showed a positive, significant response to the European diets, and this resulted in a significant strain by diet interaction. There were strain differences but no diet effects on percentage breast, thigh, or drum. Leeson and Caston (1991) used a similar approach of comparing European and North American diets in an experiment in which they studied the comparative responses of NIC and BUT hens. The NIC hens fed the North American diets consumed significantly more feed from 0 to 28 and 28 to 56 days of age compared with BUT hens and were significantly heavier at both 28 and 56 days. There were no significant strain or diet carryover effects at 105 days. Likewise, there were no significant strain differences in hens fed the European diets. The genetic studies referred to previously were conducted primarily with selected and randombred control turkey strains. Anthony et al. (1991) reported that the growth curves of a strain selected for 16-wk BW (Line F) and the randombred control strain (RBC2) from which it was developed were best described by the Gompertz equation. Using this equation, Lilburn et al. (1992) compared the growth curves of BUT and NIC hens. The slope of the growth curve was significantly greater for NIC hens, but the BW and age at the point of inflection (POI) was significantly greater for BUT hens. These changes in the form of the growth curve probably con-

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LILBURN AND EMMERSON

Each poult was subsequently spraypainted on the tail feathers for easy identification within its pen the following week. At weekly intervals thereafter through 24 wk, the same toms were weighed. The weekly BW data were fitted for Gompertz growth analysis as described by Anthony et al. (1991). The slope or magnitude of the growth curve is represented by the conversion factor, a value that when substituted for BW and expressed over time will result in a straight line (Ricklefs, 1967). Beginning at 4 wk of age, and at 4-wk intervals thereafter through 20 wk, all the birds in each pen were weighed. At

TABLE 1. Composition of experimental diets1 Ingredients and composition

4 to 8 wk

0 to 4 wk NRC

NRC+

CP

NRC

NRC+ •

46.20 Corn (8.5% CP) 44.5 Com gluten meal (60% CP) 3.80 Alfalfa (17% CP) 4.00 Soybean meal (44% CP) 42.00 Soybean meal (48% CP) 37.90 6.70 Fishmeal, Menhaden 5.00 Animal and vegetable fat .20 Salt .20 .35 Ground limestone .50 2.15 Deflourinated phosphate 2.05 .20 DL-methionine .05 .20 LysineHCl Vitamin and mineral 2.00 premix2-3'4 2.00 Total 100 100 Composition CP 28.8 Calculated 28.4 Analyzed 27.2 27.6 2,803 ME, kcal/kg 2,796 1.93 Lysine 1.62 .71 Methionine .58 1.19 TSAA 1.05

8 to 12 wk

(%)

CP

NRC+

NRC

CP

—

9.1

8.43 55.7 15.4

46.00 2.30 4.00

45.00 7.73 2.00 58.4 4.00 16.0

61.00

61.00

46.7 61.8

38.00 3.00 1.80 .30 .60 2.00

38.00 47.3 4.30 63.3 1.95 .30 .40 2.00 .10 .05

30.80 2.50 1.00 .20 .60 1.80

30.80 49.7 2.50 66.5 1.00 .20 .55 1.70 .15 .10

2.00 100

2.00 100

26.4 27.1 25.7 26.2 2,890 2,895 1.52 1.61 .47 .59 .90 1.03

.10 2.00 100

2.00 100

21.8 21.7 22.4 22.6 2,994 2,994 1.32 1.32 .54 .39 .89 .74

J NRC = formulated to recommendations of NRC (1984): NRC+ = formulated to contain increased concentrations of lysine and TSAA from 0 to 4 and 4 to 8 wk and increased TSAA from 8 to 12 wk. 2 The premix contributed the following in grams per 45.4 kg feed: ground corn, 490; choline chloride, 54.5; amprolium (25%), 22.7; selenium premix (200 mg Se/kg), 45.4; bacitracin MD, 22.7; vitamin premix, 227; and trace mineral premix, 45.4. 3 The vitamin premix contributed the following per kilogram of diet: vitamin A, 8,745 IU; cholecalciferol, 3,745 IU; vitamin E, 60 IU; vitamin K (menadione sodium bisulfite), 2.91 mg; thiamine HCl, 2.2 mg; riboflavin, 6.6 mg; niacin, 99 mg; pantothenic acid, 15.4 mg; folic acid, 1.2 mg; pyridoxine, 2.2 mg; and biotin, 165 mg. 4 The trace mineral premix contributed the following per kilogram of diet: zinc oxide (72% Zn), 147 mg; manganous oxide (55% Mn), 152 mg; copper sulfate (25.2% Cu), 35 mg; ferrous sulfate monohydrate (31% Fe), 72 mg; and potassium iodide, 1.5 mg.

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formulated to NRC (1984) specifications for protein and amino acids (Table 2). During each age period, both diets provided ME in accordance with NRC (1984) specifications. Samples of each ingredient for a particular set of diets together with samples of each diet were analyzed for crude protein (Kjeldahl nitrogen) after diet manufacturing. At 7 days of age, 50 poults from each strain and diet combination were randomly selected, identified by wing-band, and individually weighed. An equal number from each pen was represented in the total number weighed from each strain (n = 100).

DIETARY AMINO ACIDS AND GROWTH OF COMMERCIAL TOMS

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TABLE 2. Composition of experimental diets 16 to 24 wk

12 to 16 wk Ingredients

Amount

Corn Soybean meal (44% CP) Soybean meal (48% CP) Animal and vegetable fat Salt Ground limestone Deflourinated phosphate DL-methionine Vitamin and mineral premix1-2-3

65.50

9.5

28.0 2.00 .25 .50 1.70 .05 2.00

50.2

CP

Amount

CP

68.00 23.50

9.4 46.4

(%)

4.00 .25 .50 1.70 .05 2.00

12 wk of age, 12 toms from each strain and viscera included all internal organs except treatment that were not part of the growth the kidneys and lungs. Whole eviscerated curve study were randomly selected for carcasses were stored in an ice-water slush processing and carcass analysis. Each torn overnight and individually reweighed was weighed, killed by exsanguination, prior to dissection (chilled weight). From deplumed, and eviscerated. The eviscer- one half of each carcass, the P. major and ated carcass was reweighed and the Pec- P. minor muscles were individually distoralis major and Pectoralis minor muscles sected and weighed. The tibia plus aswere individually dissected from one half sociated muscles (drum) and femur plus of each carcass and weighed. The carcass associated muscles (thigh) were dissected was subsequently cut in half lengthwise from the carcass and weighed. The lengths on a band saw and the undissected of the tibia and femur were also recorded. portion was frozen at -20 C for determina- One person was responsible for each stage tion of total carcass protein and lipid. Each of the cut-up process. Relative carcass half was thawed, ground, and several parts data were expressed as a percentage subsamples were mixed, frozen, and of live weight prior to processing. lyophilized as described by Bacon et al. All data were analyzed by analysis of (1986). The composite samples weighed variance using the least squares General approximately 500 g prior to drying. Two Linear Models (GLM) procedures of SAS® subsamples from each dried carcass sam(SAS Institute, 1985). Strain, dietary treatple were analyzed for protein and three subsamples were analyzed for total lipid ment, and the strain by diet interaction were the effects tested. Main or interaction as described by Bacon et al. (1986). effects were considered significant at P < At 20 wk, 30 toms per strain and diet combination that were not part of the .05. 24-wk growth curve study were randomly chosen for processing. Each torn was RESULTS individually weighed (live weight) and The slope of the growth curve was killed by exsanguination. After scalding, significantly greater for NIC compared depluming, and evisceration, the carcasses were reweighed (eviscerated weight). The with BUT toms although the POI or point

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1 The premix contributed the following in grams per 45.4 kg feed: ground corn, 490; choline chloride, 54.5; amprolium (25%), 22.7; selenium premix (200 mg Se/kg), 45.4; bacitracin MD, 22.7; vitamin premix, 227; and trace mineral premix, 45.4. 2 The vitamin premix contributed the following per kilogram of diet: vitamin A, 8,745 IU; cholecalciferol, 3,745 IU; vitamin E, 60 IU; vitamin K (menadione sodium bisulfite), 2.91 mg; thiamine HC1,2.2 mg; and biotin, 165 mg. 3 The trace mineral premix contributed the following per kilogram of diet: zinc oxide (72% Zn), 147 mg; manganous oxide (55% Mn), 152 mg; copper sulfate (25.2% Cu), 35 mg; ferrous sulfate monohydrate (31% Fe), 72 mg; and potassium iodide, 1.5 mg.

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LILBURN AND EMMERSON TABLE 3. The influence of diet and strain on Gompertz growth curve parameters in commercial turkeys1 Strain2

Calculated parameter 4

Age at POI, days BW at POI, g Slope at POI, ACF/dayW Age at 90% asymptote, days BW at asymptote, g

BUT

Diet3

NIC

98.84" 88.97b 10,472" 9,321b .01017b .01018" 135.9" 122.4b 28,466" 25,337b

NRC

NRC+

94.84 93.38 9,942 9,899 .01018 .01018 128.9 129.9 27,025 26,908

SEM 2.03 235 .00001 2.7 639

at which linear growth declines was reached at an earlier age and lighter BW for NIC toms (Table 3). There were no significant diet effects or diet by strain interactions for any of the growth curve measurements. Among the birds individually weighed for growth curve analysis, there was a significant difference in BW between the mean of the heaviest 50% and lightest 50% of selected birds at 5 wk but not at 20 wk when the mean BW of the

TABLE 4. The relationship between body weight at 5 and 20 wk of age in British United Turkey (BUD and Nicholas (NIC) toms Strairi Group

5 wk

BUT Top 33% (n = 30) Bottom 66% (n = 55) SEM NIC Top 33% (n = 28) Bottom 66% (n = 56) SEM BUT Top 50% (n = 42) Bottom 50% (n = 43) SEM NIC Top 50% (n = 42) Bottom 50% (n = 42) SEM

1.608* 1.325 .020 1.758* 1.478 .020 1.567* 1.287 .018 1.714* 1.428 .020

20 wk flrrl

16.390 16.196 .153 17.093 16.780 .174 16.421 16.285 .153 16.898 16.871 .174

Significant differences between groups (P < .05).

same birds were compared (Table 4). The same was true when comparisons were made between the heaviest 33% and lightest 66% at 5 and 20 wk, respectively. The NIC toms were significantly heavier at 4, 8, 12, and 16 wk, but there were no significant strain differences at 20 wk (Table 5). The NRC+ dietary treatment significantly increased BW at 4 and 12 wk but not at subsequent ages. In those toms randomly selected for processing at 12 wk, there were no significant strain or diet differences in live or eviscerated weight (Table 6). The relative weight of the eviscerated carcass was significantly greater for the BUT than NIC toms, however. The NRC+ diets significantly increased the absolute and relative weights of the P. major muscle in both strains. The BUT toms had significantly more carcass protein and significantly less there were no significant diet effects or strain by diet interactions. In the toms selected for processing at 20 wk, there were no significant diet or strain effects on BW (Table 7). The BUT toms from the NRC+ treatment, however, weighed less than those toms from the NRC treatment and the opposite was true for the selected NIC toms, resulting in a significant strain by diet interaction. There were also significant strain by diet interactions for eviscerated and chilled carcass weights. There were no main effects or

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"-bMeans within main effects and parameters with no common superscripts differ significantly (P < .05). 1 Gompertz equation was used to fit growth curves. 2BUT = British United Turkeys; NIC = Nicholas. 3 NRC = formulated to recommendations of NRC (1984); NRC+ = formulated to contain increased concentrations of lysine and TSAA from 0 to 4 and 4 to 8 wk and increased TSAA from 8 to 12 wk. 4 POI = The point at which linear growth rate declines. sSlope was expressed as the change in the conversion factor (ACF) over time. The conversion factor is that value that when substituted for BW and expressed over time will yield a straight line (Ricklefs, 1967).

DIETARY AMINO ACIDS AND GROWTH OF COMMERCIAL TOMS

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TABLE 5. The effect of differences in lysine, methionine, and total sulfur amino acids on body weight in two strains of commercial turkeys Variable

4 wk

12 wk

16 wk

20 wk

fl-r-1 (*%l

.83 .93

3.61 3.96

7.91 8.31

12.23 12.73

16.40 16.84

.82 .93 .02

3.74 3.83 .05

8.00 8.22 .06

12.45 12.52 .08

16.70 16.54 .13

.003 .002 .652

.0001 .141 .754

Probabilities .0008 .026 .642

.0014 .564 .417

.131 .594 .464

iBUT = British United Turkeys; NIC = Nicholas. NRC = Formulated to recommendations of NRC (1984); NRC+ = Formulated to contain increased concentrations of lysine and total sulfur amino acids from 0 to 4 and 4 to 8 wk and increased total sulfur amino acids from 8 to 12 wk. 2

interactions for the relative weights of the eviscerated and chilled carcasses. There were no significant diet or strain effects on the absolute or relative weights of the P. major muscle, but there was a significant strain by diet interaction for absolute weight of this muscle. The absolute and relative weight of the P. minor muscle was significantly heavier in BUT toms. The mean weight of this muscle in toms fed the NRC+ diets was also significantly heavier, and there were no significant strain by diet interactions. There were no significant strain or diet effects on the absolute or relative weights of the drum or thigh (Table 8). There was a diet by strain interaction for the relative weight of the thigh and strain difference in relative drum weight. The femur (P < .069) and tibia were longer in BUT toms compared with NIC toms. DISCUSSION The growth curve analysis data support the conclusions of Moran et al (1984) and Leeson and Caston (1991) that BUT and NIC turkeys have different growth characteristics. Lilburn et al. (1992) reported that the slope of the growth curve was significantly greater for NIC hens compared with BUT hens, but the age and BW at the

POI were significantly greater for BUT hens. This is also true for the BUT and NIC toms studied in the present experiment. Growth curve analysis comparisons among selected and randombred populations of Japanese quail and turkeys have also shown that where the slope of the growth curve or magnitude of growth is increased, the faster growing strains also exhibit a decreased age at the POI (Anthony et al, 1986, 1991). In the present study, the age at POI was significantly earlier for NIC toms (88.9 days), and this was close to the age at which carcass composition comparisons were made (84 days). This probably contributed to the increased carcass lipid observed in NIC toms at this age. Hurwitz et al (1991) reported that the growth curve of BUT toms was best described when fit for two growth phases, one occurring early (< 50 days) and the other occurring at a later age, closer to sexual maturity. In the present experiment, the primary objective was to compare potential strain and diet effects on growth and not to compare alternative growth curve models. It is interesting, however, that the age at POI for BUT toms (98.4 days) in the present experiment was close to the age (100.3 days) that Hurwitz et al (1991) reported as the POI for their second phase of maximal gain.

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Strain1 BUT NIC Diet2 NRC NRC+ Pooled SEM Analysis of variance Source of variation Strain Diet Strain x diet

8 wk

.1 .1 .0

. .2

(

P

— Probabilities .387 .2 .009 .6 .352 .5

.798 .260 .952

.568 .220 .732

.410 .016 .812

7.79 8.20 .29

.609 .658 .037

79.4 79.8 .8

6.23 6.41 .29

7.81 8.02 .33

.050 .307 .919

7.93 8.06

(%LW)

.625 .641

(kg)

80.0 79.2

(%LW3)

Pectoralismajor

6.30 6.34

(kg)

Eviscerated weight

7.86 7.96

(kg)

Live weight

Diet2

(kg) 13.07 12.60 12.75 13.15 .10

.591 .881 .047

.334 .781 .023

.105 .686 .567

C/oLW3) 80.8 81.1 80.5 80.4

Eviscerated weight

(kg) 16.17 15.53 15.84 16.35 .12

Live weight

.568 .865 .040

(kg) 13.26 12.78 12.93 13.34 .10

(%LW) 81.9 82.2 81.7 81.6

Probabilities .144 .729 .624

Chilled weight

(g) 1,532 1,468 1,496 1,616

. . .

Pector

TABLE 7. Influence of diet and strain on carcass and breast muscle weights in comme

2

iBUT = British United Turkeys; NIC = Nicholas. NRC = Formulated to recommendations of NRC (1984); NRC+ = formulated to contain increased concentrations o 4 to 8 wk and increased total sulfur amino acids from 8 to 12 wk. 3 LW = Live weight at processing.

NRC NRC+ NIC NRC NRC+ SEM Analysis of variance Source of variation Strain Diet Strain x diet

BUT

Strain1

2

1BUT = British United Turkeys; NIC = Nicholas. NRC = Formulated to recommendations of NRC (1984); NRC+ = Formulated to contain increased concentr from 0 to 4 and 4 to 8 wk and increased total sulfur amino acids from 8 to 12 wk. 3 LW = Live weight at processing.

Strain1 BUT NIC Diet2 NRC NRC+ Pooled SEM Analysis of variance Source of variation Strain Diet Strain x diet

Variable

TABLE 6. The influence of diet and strain on carcass, breast muscle weight, and carcass compositi

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DIETARY AMINO ACIDS AND GROWTH OF COMMERCIAL TOMS TABLE 8. Influence of diet and strain on thigh and drum characteristics in commercial toms at 20 wk of age Strain1 BUT

Diet2

(kg) .961 .972 .970 1.010 .011

(%BW) 6.17 6.01 5.93 6.37 .04

.306 .255 .516

.502 .131 .001

Femur length

Drum weight

(cm) 13.84 13.84 13.50 13.72 .06

(kg) .817 .849 .819 .808 .008

Probabilities .254 .069 .526 .394 .192 .389

Tibia length (%BW) 5.26 5.24 5.01 5.10 .03

.001 .569 .388

(cm) 22.6 22.5 21.6 21.8 .1

.0001 .779 .263

iBUT = British United Turkeys; NIC = Nicholas. NRC = Formulated to recommendations of NRC (1984); NRC+ = Formulated to contain increased concentrations of lysine and total sulfur amino acids from 0 to 4 and 4 to 8 wk and increased total sulfur amino acids from 8 to 12 wk. 2

The weekly weighing of individual birds for the growth curve analysis also allowed for individual comparisons of the heaviest and lightest birds in the flock at two ages within the production cycle. There was a small increase in the mean BW difference between the heaviest 33% and lightest 66% at 20 wk compared with 5 wk, but these differences were not significant at the former age. When the heaviest 50% and lightest 50% of the birds at 5 wk were compared at 20 wk, the respective BW in both segments of the population were almost identical in both strains. This suggests that within a normal population of turkeys, the performance of toms between 5 and 20 wk will be different, depending upon their position in the bottom or top 50% of the flock through 5 wk of age. For the entire experimental population, the NIC toms were significantly heavier early in the experiment with no significant differences at the end of the study. Similar BUT and NIC strain by age interactions have been previously reported for toms (Moran et al, 1984) and hens (Leeson and Caston, 1991; Lilburn et al, 1992). In the toms that were randomly chosen for processing at 12 wk there were no significant differences in BW due to diet or strain, but the differences were in similar directions to what was observed for the

entire experimental population at this age. There were significant increases in the absolute and relative weights of the P. major, however, so it appears that the higher levels of lysine and total sulfur amino acids may enhance carcass development in young turkeys. Leeson and Caston (1991) had previously reported that higher protein and amino acid levels significantly increased BW through 70 days in BUT and NIC hens, but this was only observed in one of two experiments. In the study in which growth was improved by the higher protein and amino acid levels, the experiment was terminated at 70 days, so the possible carryover effects on carcass development through later ages was not determined. In their initial experiment, Leeson and Caston (1991) did not observe any beneficial effects of higher protein and amino acid levels on BW of BUT and NIC hens at 105 days. In toms, Moran et al (1984) likewise reported that higher protein diets had no effect on growth or carcass development in BUT or NIC toms, although as mentioned previously, there were significant strain by age effects. The processing data from the 20-wk-old toms were somewhat inconclusive and illustrate some of the problems encountered when a subsample of the entire experimental population is not truly

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NRC NRC+ NIC NRC NRC+ SEM Analysis of variance Source of variation Strain Diet Diet x strain

Thigh weight

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ACKNOWLEDGMENTS

The authors wish to thank Elizabeth Stamp for her help in organization of the weekly BW data collection and processing and to Wanda Acord for help in handling and preparation of the manuscript.

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reflective of overall treatment effects. There were no significant strain or diet effects on live weight, eviscerated weight, chilled carcass weight, or the weight of the P. major muscle. In the subsample of processed toms, however, there was a significant strain by diet interaction for each of these traits, so interpretation of the data is difficult relative to the objectives of the experiment. The increased absolute and relative weights of the P. minor muscle in BUT toms, independent of any significant differences in the P. major muscle, is consistent with what was observed in hens (Lilburn et al., 1992). The increased weight of this muscle in toms fed the NRC+ diets is more difficult to interpret. The greater relative weight of the total drum but not total thigh in BUT toms compared with NIC toms is consistent with what was reported for hens at 16 wk (Lilburn et al, 1992). Moran et al. (1984) reported that both the thigh and drum were significantly heavier in BUT toms compared with NIC toms. Lilburn and Nestor (1991) had reported that total thigh weights were similar, but the relative weight of the drum was significantly lighter at 16 wk in NIC toms compared with a line selected for BW at 16 wk (F line). The decreased weight of the total leg parts is consistent with the significant decrease in the length of the tibia and femur in NIC toms. Lilburn and Nestor (1991) also reported a decrease in the lengths of the femur and tibia in NIC toms compared with the F line.