Performance and Carcass Composition of Large White Toms as ...

Performance and Carcass Composition of Large White Toms as Affected by Dietary Crude Protein and Threonine Supplements M. T. KIDD,* B. J. KERR,* J. A. ENGLAND,† and P. W. WALDROUP† *Nutri-Quest, Inc., Chesterfield, Missouri 63017 and †Poultry Science Center, University of Arkansas, Fayetteville, Arkansas 72701 ABSTRACT This experiment evaluates the effects of decreasing dietary CP, in addition to the effects of dietary supplements of L-Thr to low CP diets, in Large White Nicholas toms from 0 to 18 wk of age. Toms were fed dietary treatments consisting of four levels of dietary CP as a percentage of NRC (1994) recommendations (100, 92, 84, and 76% of NRC recommendations). Additional treatments consisted of supplements of L-Thr (0.1 and 0.2% of diet) added to the 92 and 84% NRC CP treatments. All eight dietary treatments were formulated to meet a minimum of 105% of NRC (1994) recommendations for Met, TSAA, Lys, Thr, and Trp. Body weight, feed conversion, mortality, and carcass composition responses were measured. Decreasing CP to 84% of NRC resulted in 18-wk BW lower than that (P ≤ 0.001) of toms fed diets containing 100 or 92% of NRC CP; however, toms fed 84% of NRC CP diet supplemented with 0.1% L-Thr had 18-wk BW equal to (P ≤ 0.001) that of the 100 and 92% NRC CP treatments. Toms fed diets containing 76% of

NRC CP had depressed BW and feed:gain in comparison to all other treatments. No adverse effects in cumulative feed:gain (0 to 18 wk) were noted by decreasing CP from 100 to 84% of the NRC recommendations. Mortality did not differ among treatments. Treatments had no effect on carcass fat expressed as a percentage of hot carcass weight. Breast meat yield (deboned Pectoralis major and Pectoralis minor) was highest (P ≤ 0.001) in toms fed the 100 and 92% NRC CP treatments. The 84 and 76% NRC CP treatments resulted in decreased breast meat yield regardless of L-Thr supplements. These results indicate that diets containing Met, TSAA, Lys, Thr, and Trp at a minimum of 105% NRC recommendations may support favorable breast meat yield when CP is decreased to 92% of the NRC (1994) recommendation. If growth and feed conversion are the desirable traits, rather than breast meat yield, CP levels below 92% of the NRC (1994) recommendation may support favorable responses.

(Key words: turkey, dietary protein, growth, breast meat, threonine) 1997 Poultry Science 76:1392–1397

INTRODUCTION The dietary CP requirement of commercial turkeys is relatively high in comparison to CP requirements of some other poultry (e.g., broiler chickens, layer chickens, geese, and ducks). Indeed, the rapid growth of Large White turkeys is attributed to the high CP requirement, representing a delicate balance of essential amino acids and nonessential nitrogen supporting maintenance and tissue accretion needs. Feeding turkeys diets low in CP decreases live performance (Carter et al., 1957; Summers et al., 1968; Potter et al., 1981; Bowyer and Waldroup, 1986; Blair and Potter, 1988) and carcass and breast yields (Sell et al., 1989; Sell, 1993). However, marginal reductions in dietary CP may support live performance equal to high CP diets depending on the extent to which CP is reduced [approximately 90% of NRC (1994) recommendations] and which essential amino acids are

Received for publication January 2, 1997. Accepted for publication May 22, 1997.

supplemented to the low CP diets (Spencer, 1984; Sell et al., 1989; Sell, 1993). Sell et al. (1994) demonstrated that turkeys receiving diets marginally reduced in CP (93% of 1984 NRC recommendations) and supplemented with essential amino acids to contain a minimum of 100% of NRC (1984) recommendations had carcass composition equal to that of turkeys receiving the high CP positive control diet. More research concerning low CP diets for turkeys is needed to determine the extent to which CP can be decreased and to determine which essential amino acids are needed for favorable turkey production. Beneficial effects of early CP restriction in turkeys include increasing the efficiency of protein utilization, minimizing amino acid excesses, reducing nitrogen excretion, minimizing leg weakness, reducing ammonia in litter, and maximizing profitability depending on the price of oilseed meals vs crystalline amino acids. To take advantage of these benefits, low CP turkey rations must be balanced with the most limiting essential amino acids. Stas and Potter (1982) and Jackson et al. (1983) conducted experiments to determine which amino acids

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are limiting in poults fed a 22% CP corn and soybean meal diet adequate in Met. These authors measured poult weight gain and determined that the order of most limiting to least limiting amino acids were Val, Lys, Thr, and Ile. Corn and soybean meal turkey diets with marginal levels of CP are fortified with commercially available supplements of Met and Lys: the most limiting amino acids in turkey rations (Waibel et al., 1995). As a result, live performance and carcass yield of turkeys fed low CP diets plus Met and Lys may be compromised by marginal deficiencies of Val, Thr, Ile, and Arg. Threonine may be limiting in low CP corn and soybean meal diets with supplements of Met and Lys, in addition to being limiting in diets containing Thr limiting grains, e.g., milo and wheat. However, determining Thr specifications for turkeys in linear programming is difficult because requirement estimates beyond 3 wk of age were determined using computer models (Hurwitz et al., 1983), not requirement experiments. Moreover, few studies have addressed essential amino acid requirements in growing turkeys beyond Met and Lys. It is important, therefore, to determine the extent to which CP can be decreased and which essential amino acids are needed so that edible meat portions are not decreased. The objective of this experiment was twofold: 1) to determine the effect of low CP diets (100, 92, 84, and 76% of 1994 NRC) containing a minimum of 105% of NRC (1994) levels of Met, TSAA, Lys, Thr, and Trp on live performance and carcass composition of Large White Nicholas toms from 0 to 18 wk of age; and 2) to determine the effect of supplements of L-Thr (0.1 and 0.2%) to the diets containing 92 and 84% of NRC CP recommendations.

MATERIALS AND METHODS One-day-old male Large White Nicholas poults were obtained from a commercial hatchery on July 10, 1995 and randomly allotted to 48 floor pens, 15 poults per pen (0.37 m2 floor space per tom). Each pen was equipped with one bell waterer, one hanging tube feeder, and built up pine shavings. At 10 d of age, all poults were wing-banded and poults with leg disorders were removed. Toms were provided incandescent light for 23 h/d. The experimental facility was a curtain-sided house with hallways containing infrared brooding lamps. House temperature was maintained at 32 C for Week 1, 29 C for Week 2, 27 C for Week 3, and approximately 24 C thereafter. The experimental design consisted of eight dietary treatments (six replications per treatment) from 0 to 18 wk of age. All dietary treatments had Met, TSAA, Lys, Thr, and Trp levels formulated to meet a minimum of 105% of NRC (1994) recommendations. The experimental diets were formulated to meet four CP levels of 100,

92, 84, and 76% of NRC (1994) recommendations (Table 1). Diets were fed from 0 to 3, 3 to 6, 6 to 9, 9 to 12, 12 to 15, and 15 to 18 wk of age. Supplements of L-Thr (0.1 and 0.2% of diet) were added to the experimental diets containing 92 and 84% of NRC CP levels at the expense of a nonnutritive filler (sulka flock). The dietary treatments consisted of: 1) 100% NRC CP; 2) 92% NRC CP; 3) 92% NRC CP plus 0.1% L-Thr; 4) 92% NRC CP plus 0.2% L-Thr; 5) 84% NRC CP; 6) 84% NRC CP plus 0.1% L-Thr; 7) 84% NRC CP plus 0.2% L-Thr; and 8) 76% NRC CP. Composite samples of experimental diets differing in CP for each time period were analyzed in duplicate for DM and CP (Association of Official Analytical Chemists, 1984). Also, composite samples were analyzed in duplicate for all amino acids after acid hydrolysis by a high performance cation exchange resin column.1 The antibiotic bacitracin methylene disalicylate2 was added (55 mg/kg) to all diets for each time period. The anticoccidial monensin sodium3 was added (66 mg/kg) to all diets up to 15 wk of age. Toms consumed feed and water at will. Pen weights were obtained at 0, 3, 6, 12, and 18 wk of age. Feed consumption was determined at 3, 6, 12, and 18 wk of age. Mortality was recorded throughout the experiment. Toms that were culled or died during the experiment were used to correct feed consumption data. Feed conversion was calculated by dividing pen feed consumption data by pen live weight data including dead and cull weights minus initial tom weight at 1 d of age. At 18 wk of age, two toms per pen (12 toms per treatment) closest to the average pen weight were selected for processing. Feed was removed 12 h before processing. Water was provided until birds were transferred in coops to the pilot processing plant. Birds were weighed immediately before processing. Each bird was killed with an electric knife that severed the jugular vein and allowed to bleed for approximately 3 min. Birds were scalded in hot water for 3 min and placed in a rotary picker for 40 s. Neck, feet, and viscera were removed manually. Hot carcass weight and fat (abdominal and outer gizzard fat) were recorded. Carcasses were placed in an ice bath at 0 C for 20 h. Breast skin, breast (deboned Pectoralis major and Pectoralis minor), wings (bone in and skin on), thighs (bone in and skin on), drumsticks (bone in and skin on), and the remaining carcass rack were removed manually and weighed. In addition, shank and keel length were recorded. All data from dietary treatment means were analyzed as a completely randomized block design using the General Linear Models procedure of the SAS Institute (1985). The SEM and probability values are presented. When differences (P < 0.05) among means were found, means were separated using repeated t test.

RESULTS 1Beckman Systems Inc., Fullerton, CA 2Alpharma Inc., Ft. Lee, NJ 07024. 3Elanco Animal Health, Eli Lilly and

46285.

92634. Co. Inc., Indianapolis, IN

Analyzed CP and amino acid values from the experimental diets containing the four levels of CP (100, 92, 84, and 76% of the 1994 NRC) are in good agreement

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(± 10%) with calculated values. Thus, only calculated total amino acid values are presented (Table 1). The BW of toms was affected by dietary treatments at 3, 6, 12, and 18 wk of age (Table 2). At 3 wk of age, toms fed the diet containing 84% of NRC CP had BW equal to

that of the toms fed the diet containing 100% of NRC CP. However, 3-wk BW of toms receiving 92% of NRC CP was depressed (P ≤ 0.037) when 0.2% L-Thr was added to the diet. At 6 wk of age, BW of toms was depressed (P ≤ 0.001) in all low CP treatments regardless

TABLE 1. Diet series of 100, 92, 84, and 76% of NRC (1994) CP levels with 105% of suggested (NRC, 1994) minimum amino acid recommendations for Met, TSAA, Lys, Thr, and Trp 0 to 3 wk of age Ingredients Yellow corn Soybean meal Poultry oil Dicalcium phosphate Limestone Vitamin premix1 Sodium chloride DL-methionine Trace mineral2 L-lysine·HCl L-threonine L-tryptophan Calculated composition ME, kcal/kg Crude protein Met + Cys Met Lys Thr Trp Val Arg Ile Gly + Ser Phe + Tyr

Yellow corn Soybean meal Poultry oil Dicalcium phosphate Limestone Vitamin premix1 Sodium chloride DL-methionine Trace mineral2 L-lysine·HCl L-threonine L-tryptophan Calculated composition ME, kcal/kg Crude protein Met + Cys Met Lys Thr Trp Val Arg Ile Gly + Ser Phe + Tyr

100 CP 92 CP 36.27 54.13 4.30 2.56 1.42 0.50 0.40 0.26 0.10 0.06 0.00 0.00

44.04 47.18 3.03 2.61 1.44 0.50 0.40 0.33 0.10 0.29 0.08 0.00

84 CP 51.83 40.18 1.75 2.66 1.46 0.50 0.40 0.40 0.10 0.53 0.19 0.00

3 to 6 wk of age 76 CP 59.65 33.17 0.46 2.71 1.48 0.50 0.40 0.47 0.10 0.77 0.29 0.00

100 CP 92 CP 41.97 48.66 4.82 2.04 1.20 0.50 0.40 0.21 0.10 0.10 0.00 0.00

84 CP

(%) 52.41 56.43 39.28 35.66 3.11 2.45 2.10 2.13 1.23 1.24 0.50 0.50 0.40 0.40 0.30 0.34 0.10 0.10 0.42 0.55 0.15 0.20 0.00 0.00

6 to 9 wk of age 76 CP 63.70 29.15 1.25 2.17 1.25 0.50 0.40 0.40 0.10 0.77 0.30 0.01

100 CP 92 CP 53.79 37.64 4.47 1.67 1.07 0.50 0.40 0.15 0.10 0.20 0.01 0.00

59.91 32.14 3.46 1.71 1.09 0.50 0.40 0.21 0.10 0.39 0.09 0.00

84 CP 66.04 26.65 2.46 1.74 1.10 0.50 0.40 0.26 0.10 0.57 0.18 0.00

76 CP 72.19 21.12 1.44 1.78 1.11 0.50 0.40 0.32 0.10 0.76 0.26 0.02

2,900 2,900 2,900 2,900 29.00 26.68 24.36 22.04 1.14 1.14 1.14 1.14 0.69 0.72 0.76 0.80 1.74 1.74 1.74 1.74 1.12 1.09 1.09 1.09 0.42 0.38 0.33 0.28 1.35 1.22 1.10 0.98 2.02 1.81 1.60 1.38 1.25 1.13 1.00 0.88 2.70 2.44 2.18 1.91 2.57 2.32 2.08 1.83 9 to 12 wk of age

3,000 3,000 3,000 3,000 26.90 23.79 22.59 20.44 1.03 1.03 1.03 1.03 0.61 0.66 0.67 0.71 1.63 1.63 1.63 1.63 1.03 1.03 1.03 1.03 0.34 0.32 0.30 0.26 1.25 1.08 1.02 0.90 1.85 1.57 1.46 1.26 1.15 0.99 0.92 0.80 2.49 2.14 2.00 1.76 2.37 2.04 1.91 1.68 12 to 15 wk of age

3,100 3,100 3,100 3,100 22.73 20.92 19.10 17.28 0.87 0.87 0.87 0.87 0.50 0.53 0.56 0.59 1.41 1.41 1.41 1.41 0.87 0.87 0.87 0.87 0.31 0.27 0.24 0.22 1.05 0.95 0.86 0.76 1.51 1.35 1.18 1.01 0.95 0.86 0.76 0.66 2.07 1.87 1.66 1.46 1.98 1.79 1.59 1.40 15 to 18 wk of age

100 CP 92 CP

100 CP 92 CP

100 CP 92 CP

61.46 30.00 4.91 1.50 0.94 0.50 0.40 0.07 0.10 0.05 0.07 0.00

66.74 25.27 4.04 1.53 0.95 0.50 0.40 0.11 0.10 0.21 0.15 0.00

84 CP 72.00 20.53 3.18 1.57 0.97 0.50 0.40 0.16 0.10 0.37 0.22 0.00

76 CP 77.36 15.71 2.28 1.60 0.98 0.50 0.40 0.21 0.10 0.53 0.30 0.03

3,200 3,200 3,200 3,200 19.61 18.04 16.48 14.91 0.71 0.71 0.71 0.71 0.38 0.40 0.43 0.46 1.08 1.08 1.08 1.08 0.81 0.81 0.81 0.81 0.26 0.23 0.20 0.20 0.91 0.83 0.74 0.66 1.28 1.13 0.99 0.84 0.81 0.73 0.64 0.56 1.78 1.61 1.43 1.25 1.71 1.54 1.37 1.20

69.89 22.73 4.31 1.18 0.86 0.50 0.40 0.02 0.10 0.00 0.01 0.00

84 CP

(%) 74.38 78.92 18.69 14.63 3.57 2.82 1.21 1.24 0.87 0.88 0.50 0.50 0.40 0.40 0.06 0.10 0.10 0.10 0.14 0.27 0.08 0.14 0.00 0.00

76 CP 83.48 10.51 2.06 1.27 0.89 0.50 0.40 0.14 0.10 0.42 0.20 0.03

3,250 3,250 3,250 3,250 16.76 15.42 14.08 12.74 0.59 0.59 0.59 0.59 0.30 0.32 0.34 0.36 0.86 0.85 0.85 0.85 0.64 0.64 0.64 0.64 0.21 0.18 0.16 0.16 0.78 0.71 0.64 0.57 1.06 0.93 0.81 0.68 0.69 0.61 0.54 0.47 1.51 1.36 1.21 1.06 1.45 1.31 1.16 1.02

76.02 16.24 4.94 1.02 0.73 0.50 0.40 0.02 0.10 0.02 0.01 0.00

79.83 12.84 4.32 1.04 0.74 0.50 0.40 0.04 0.10 0.13 0.06 0.00

84 CP 83.68 9.38 3.68 1.06 0.75 0.50 0.40 0.06 0.10 0.25 0.12 0.02

76 CP 87.55 5.90 3.04 1.09 0.76 0.50 0.40 0.08 0.10 0.37 0.17 0.04

3,350 3,350 3,350 3,350 14.21 13.08 11.94 10.80 0.52 0.50 0.49 0.48 0.27 0.27 0.27 0.28 0.69 0.69 0.69 0.69 0.53 0.53 0.53 0.53 0.17 0.14 0.14 0.14 0.67 0.60 0.54 0.48 0.85 0.75 0.65 0.54 0.57 0.50 0.44 0.38 1.27 1.14 1.01 0.88 1.21 1.09 0.97 0.85

1Vitamin premix provides per kilogram of diet: vitamin A, 33,040 IU; cholecalciferol, 7,158 IU; vitamin E, 50 IU; vitamin B , 0.022 mg; 12 menadione, 3.85 mg; riboflavin, 13.75 mg; pantothenic acid, 30.25 mg; thiamin, 3.3 mg; niacin, 105 mg; pyridoxine, 5.5 mg; folacin, 2.2 mg; biotin, 0.181 mg; ethoxyquin, 125 mg; Se, 275 mg. 2Trace mineral mix provides per kilogram of diet: Mn (MnSO ·H O) 100 mg; Zn (ZnSO ·7H O) 100 mg; Fe (FeSO ·7H O) 50 mg; Cu 4 2 4 2 4 2 (CuSO4·5H2O) 10 mg; 1 (Ca(IO3)2·H2O) 1 mg.

1.411 1.864 2.050a 3.278ab 1.719 1.971ab 2.450a 2.22

1.693 1.992ab 2.556a 1.11

548a 1,741bc 7,377ab 12,719a

1.411 1.798 2.103ab 3.557bc

538a 2,011a 7,499a 12,735a

0% Thr

1.792 2.000ab 2.519a 5.56

1.414 1.967 2.067ab 3.354ab

551a 1,744bc 7,270abc 12,766a

0.1% Thr

92% CP

1Represents

a–eValues

1.735 1.958a 2.429a 6.67

1.359 1.902 2.035a 3.156a

499b 1,730bc 7,063bcd 12,731a

0.2% Thr

1.741 2.022ab 2.549a 6.67

1.418 1.885 2.119ab 3.488b

530ab 1,733bc 6,888cd 11,681b

0% Thr

1.753 2.067b 2.497a 3.33

1.518 1.860 2.181b 3.166a

524ab 1,732bc 6,870d 12,324ab

0.1% Thr

84% CP

1.641 1.995ab 2.514a 5.56

1.415 1.736 2.122ab 3.435ab

541a 1,815b 6,933cd 12,032b

0.2% Thr

0% Thr 12,075ab 78.85a 9,531ab 80.27a 9,699ab 1.98 193 29.44a 2,864ab 17.30 1,681ab 13.15cd 1,272ab 11.56b 1,115abc 28.55 12.58 18.75bcde

100 % CP 0% Thr 12,390ab 77.41a 9,584a 79.16a 9,802a 1.79 172 28.17abc 2,766abc 17.36 1,699ab 13.98b 1,371a 11.78b 1,155ab 28.69 12.75 19.59a

79.06a 9,858a 80.79a 10,073a 1.77 174 28.32ab 2,955a 17.49 1,762a 13.10d 1,318ab 11.55b 1,164a 28.54 12.83 19.19abc

12,492a

0.1% Thr

92% CP

78.13a 9,668a 79.72a 9,862a 1.99 193 28.96abc 2,860ab 17.57 1,732a 13.17cd 1,298ab 11.70b 1,153ab 28.61 12.71 19.45ab

12,375ab

0.2% Thr

77.48a 8,922b 79.32a 9,134b 2.06 184 27.97bc 2,553c 17.56 1,602b 13.65bcd 1,245b 11.85b 1,082c 28.98 12.38 18.36de

11,525b

0% Thr

77.98a 9,349ab 79.40a 9,517ab 2.20 206 27.87bc 2,657bc 17.79 1,692ab 13.33bcd 1,266ab 11.49b 1,092bc 29.52 12.50 19.11abcd

11,992ab

0.1% Thr

84% CP

78.89a 9,262ab 80.36a 9,431ab 2.10 196 27.67c 2,609bc 17.58 1,656ab 13.88bc 1,307ab 11.39b 1,072c 29.45 12.79 18.54cde

11,742ab

0.2% Thr

TABLE 3. Effect of CP1 level and supplemental L-Thr2 on 18-wk carcass of male Large White Turkeys

in rows with no common superscript differ significantly (P ≤ 0.05). CP levels as a percentage of NRC (1994) recommendations. 2Represents percentage L-Thr added to the basal diets at the expense of sulka flock. 3Represents abdominal and outer gizzard fat.

Live BW, g Hot carcass, % BW Hot carcass, g Cold carcass, % BW Cold carcass, g Fat, % Hot carcass3 Fat, g Breast, % Cold carcass Breast, g Thighs, % Cold carcass Thighs, g Drums, % Cold carcass Drums, g Wings, % Cold carcass Wings, g Rack, % Cold carcass Shank length, cm Keel length, cm

Variable

1Represents

in rows with different superscript differ significantly. CP levels as a percentage of NRC (1994) recommendations. 2Represents percentage L-Thr added to the basal diets at the expense of sulka flock.

a–eValues

Body weight, g 3 wk 6 wk 12 wk 18 wk Period feed:gain, g:g 0 to 3 wk 3 to 6 wk 6 to 12 wk 12 to 18 wk Cumulative feed:gain, g:g 0 to 6 wk 0 to 12 wk 0 to 18 wk Mortality, %

Variable

100 % CP 0% Thr

TABLE 2. Effect of CP1 level and supplemental L-Thr2 on performance of male Large White Turkeys

74.75b 6,915c 76.40b 7,068c 2.34 161 25.01d 1,771d 17.37 1,226c 14.82a 1,048c 12.57a 887d 30.23 12.64 18.13e

9,291c

76% CP 0% Thr

1.757 2.195c 2.774b 7.78

1.524 1.860 2.375c 3.821c

499b 1,650c 5,679e 9,769c

76% CP 0% Thr

0.001 0.010 0.001 0.013 0.001 0.394 0.613 0.001 0.001 0.937 0.001 0.001 0.001 0.001 0.001 0.116 0.369 0.003

P > F

0.441 0.001 0.001 0.299

0.198 0.284 0.001 0.001

0.037 0.001 0.001 0.001

P > F

272 0.797 204 0.787 202 0.177 17 0.518 85 0.264 40 0.262 34 0.170 22 0.464 0.147 0.277

SEM

0.045 0.036 0.049 2.20

0.047 0.060 0.045 0.103

13 41 136 228

SEM


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of L-Thr supplements. Toms receiving the 76% NRC CP treatment had depressed (P ≤ 0.001) BW at 12 wk of age in comparison to all other dietary treatments. Also, BW of toms receiving the 92% NRC CP treatment was equal to (P ≤ 0.001) the 100% NRC CP treatment and additions of L-Thr showed no beneficial effects. Body weights of 18-wk-old toms receiving the 84 and 76% NRC CP treatments were depressed (P ≤ 0.001). However, adding 0.1% L-Thr, but not 0.2% L-Thr, to the 84% NRC CP treatment restored BW equal to (P ≤ 0.001) the 100 and 92% NRC CP treatments. Dietary treatments did not alter 0 to 3 wk or 3 to 6 wk period feed:gain (Table 2). Statistically equivalent feed: gain responses in the 6–12-wk period were observed in the 100, 92, and 84% NRC CP treatments and supplements of L-Thr showed no beneficial effects. Decreasing CP from 92% of NRC to 84 and 76% of NRC CP increased (P ≤ 0.001) 12 to 18-wk feed:gain. However, supplementing the 84% NRC CP diet with 0.1% L-Thr restored the feed:gain response equal to (P ≤ 0.001) the toms receiving diets containing 92% of NRC CP. Cumulative feed:gain (0 to 12 and 0 to 18 wk of age) was statistically equivalent in the 100, 92, and 84% NRC CP treatments. However, cumulative feed:gain was increased (P ≤ 0.001) in the 0 to 12-wk and 0 to 18-wk periods when dietary CP was decreased to 76% of NRC. Differences among treatments for mortality did not occur. Carcass composition results are presented in Table 3. Hot and cold carcass as a percentage of live BW or actual weight were decreased (P ≤ 0.010 and P ≤ 0.013, respectively) when dietary CP was decreased to 76% of NRC. Also, decreasing CP to 84% of NRC without additional L-Thr decreased hot and cold carcass weights below those of toms receiving diets containing 100% NRC CP (P ≤ 0.001). Differences in dietary treatments for fat, in grams or as a percentage of hot carcass, were not observed. Toms receiving the diet containing 92% of NRC CP had absolute and relative breast meat responses that were higher (P ≤ 0.001) than toms receiving diets containing 84 and 76% NRC CP. Thigh yield did not differ among treatments. Thigh weight was decreased when CP was reduced to 84 and 76% NRC (P ≤ 0.001). However, supplements of L-Thr to the diets containing 84% NRC CP restored thigh weights equal to the 100% NRC CP diet (P ≤ 0.001). Drumstick and wing yield were highest (P ≤ 0.001) in the 76% NRC CP treatment. Drumstick and wing weights were decreased in toms receiving diets containing 84 and 76% NRC CP (P ≤ 0.001). Supplements of L-Thr (0.1 or 0.2%) restored drumstick weight, but not wing weight, equal to the diet containing 100% of NRC CP (P ≤ 0.001). Shank length was not affected by dietary treatments, but keel length was increased (P ≤ 0.003) in the 100% NRC CP treatment. Decreasing CP to 92 or 84% of NRC required supplements of 0.1% dietary L-Thr to restore keel length statistically equivalent to the 100% NRC CP treatment.

DISCUSSION The experimental diets consisted of corn, soybean meal, poultry oil, vitamin and mineral fortifications, and amino acids (Table 1). Diets were least-cost formulated using NRC (1994) feed composition data, adjusted to moisture and CP content of local ingredients. Decreasing the CP content of the experimental diets was achieved by increasing corn and decreasing soybean meal. The experimental diets were formulated to provide a minimum of 105% of NRC (1994) recommendations, adjusted to the dietary ME content, for Met, TSAA, Lys, Thr, and Trp. As dietary CP decreased from 100 to 76% of NRC levels, the dietary inclusion of DL-Met, LLys·HCl, L-Thr, and L-Trp increased to meet minimum levels specified during linear programming. These amino acids were chosen for dietary fortification because they are nutritionally limiting and commercially available. Indeed, the extent to which CP is decreased deems Val and Ile as nutritionally limiting (Stas and Potter, 1982; Jackson et al., 1983). In addition, it has recently been suggested that Arg becomes important as turkeys mature in a specific ratio to Lys, rather than a function of CP (Brake et al., 1994). Decreasing CP from 100 to 84% of NRC resulted in suppressed BW at 18 wk of age (Table 2). However, the addition of 0.1% L-Thr to the 84% NRC CP treatment restored BW to the 100 and 92% NRC CP treatments. The beneficial effects of L-Thr on BW in the 84% NRC CP treatment occurred between 12 and 18 wk of age. Lilburn and Barbour (1996) conducted experiments evaluating Thr requirements of growing turkeys and stated that the NRC (1994) Thr requirements for the two oldest ages appear to be underestimated. Cumulative feed:gain results (0 to 18 wk of age) indicate that CP can be decreased to 84% of the NRC recommendation. Thus, live performance results from this experiment suggest that feeding 84% of NRC CP, with higher levels of some essential amino acids, is adequate for commercial toms. Spencer (1984) found that turkeys fed diets containing 90% of NRC (1977) recommendations plus added Met and Lys support growth and feed conversion equal to the 100% CP positive control. Similarly, Sell et al. (1989) found that low CP diets (90% of 1984 NRC) with supplemental essential amino acids resulted in live performance equal to high CP diets. In the present experiment, it may be that supplements of L-Thr and LTrp, in addition to DL-Met and L-Lys, allowed good performance to be attained at a CP level of 84% of NRC, as opposed to 90% of the NRC (Spencer, 1984; Sell et al., 1989). The effects of dietary CP were most pronounced for carcass composition (Table 3). Results of carcass data show that decreasing CP below 92% of NRC recommendations limit breast meat weight and yield, but not carcass yield. Sell (1993) conducted two experiments evaluating low, adequate, and high CP diets on turkey growth and carcass composition. Toms fed diets containing 93% of NRC CP plus essential amino acids had equal breast meat yield to the 100% CP treatment in only one


experiment (Sell, 1993). It was concluded that low ambient temperatures resulted in rapid growth and the 93% CP diet could not support optimal breast tissue accretion in one experiment (Sell, 1993). In the present experiment, yields of breast meat for the toms receiving the 92% NRC CP treatment were equal to those receiving the 100% NRC CP treatment (29.44 vs 28.17%). Results from this experiment demonstrate that marginal reduction in CP (down to approximately 90% of the NRC), provided that the most limiting amino acids are supplemented, may support good breast meat yield. However, more research is needed on the effect of limiting amino acids (Val, Ile, and Arg) on breast meat yield to determine the extent to which CP can be reduced. For example, Waibel et al. (1996) demonstrated that large BUT turkeys have suppressed breast weight and yield when fed diets containing 77.5% NRC CP with supplements of DL-Met, L-Lys·HCl, L-Thr, and L-Trp. However, Waibel et al. (1996) noted that significant improvements (equal to that of turkeys fed the 100% NRC CP diet) in breast weight and yield occurred when supplements of Val, Ile, and Arg were added in addition to the other essential amino acids. Dietary protein is one of the most expensive ingredients. Early protein restriction of Large White toms has been shown to result in adequate growth and fewer leg disorders (Ferket and Sell, 1989). Leg disorders were not evaluated in this experiment, but toms fed diets containing 76% of NRC CP had the largest drumstick yield, presumably as a result of the slow growth. Nevertheless, this experiment demonstrates that dietary CP can be decreased to approximately 90% of NRC recommendations, which represents a potential savings depending on the price of protein contributing ingredients. Broiler research suggests that essential amino acids may be expressed as a percentage of dietary CP (Abebe and Morris, 1990; Robbins, 1987), which represents a tremendous potential cost savings by decreasing CP and limiting the additions of crystalline amino acids due to lower requirements. However, due to the sensitivity of breast muscle of Large White toms for amino acids, research should be conducted evaluating the most limiting amino acids in diets varying in CP.

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