Dietary interactions between lysine and threonine in broilers

Dietary Interactions Between Lysine and Threonine in Broilers M. T. KIDD,1 B. J. KERR, and N. B. ANTHONY2 Nutri-Quest, Inc., Chesterfield, Missouri 63017

(Key words: lysine, threonine, broiler, performance, breast meat yield) 1997 Poultry Science 76:608–614

essential amino acids are deficient. It is important, therefore, to determine whether essential amino acids may be formulated in a specific ratio with lysine in broilers. The TSAA, lysine, and threonine are considered to be the first, second, and third limiting amino acids in broilers fed corn-soybean meal diets, respectively. Essential amino acids in broilers may interact with one another to improve a certain production function. For example, diets fortified with lysine (106% of the NRC, 1994) and methionine (112% of the NRC, 1994) interacted to optimize carcass yield, but not growth, in male broilers compared to diets fortified with lysine and methionine at recommended levels (Hickling et al., 1990). Supplementing arginine to a high lysine diet in chicks improves growth due to lysine-arginine antagonism (O’Dell and Savage, 1966). Broiler research concerning lysine interactions with threonine is scarce. Much research has been conducted evaluating the lysine requirement of broilers (Edwards et al., 1956; Bornstein, 1970; Boomgaardt and Baker, 1973; Twining et

INTRODUCTION The NRC (1994) for poultry has lowered the recommended level of dietary lysine for 1- to 21-d-old chicks from 1.20 to 1.10% of diet. Formulating broiler diets that are not deficient in lysine is crucial because lysine is the second limiting amino acid and the reference amino acid for the ideal protein concept (Baker, 1994). The ideal protein concept expresses all essential amino acid requirements as a percentage of lysine. However, commercial diets may utilize lysine levels above that considered adequate by the NRC (1994) due to consumer requests for further processed white meat products. Increasing dietary lysine without consideration of other amino acids may limit performance if other

Received for publication August 19, 1996. Accepted for publication November 19, 1996. 1To whom correspondence should be addressed. 2Department of Poultry Science, University of Arkansas, Fayetteville, AR 72701.

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In Experiment 1, increasing dietary lysine from 1.10 to 1.20% from 1 to 18 d in broilers improved (P < 0.001) BW gain (453 vs 488 g) and feed:gain (1.39 vs 1.33). No interactions between lysine and threonine were observed in Experiment 1. Differences in immune parameters or mortality were not observed. In Experiment 2, an interaction in 18 to 54 d weight gain occurred with the highest gain in broilers receiving dietary lysine and threonine levels equivalent to 100 and 83%, respectively, of NRC (1994) or lysine and threonine at levels of 105% and 100% of NRC (1994), respectively (P ≤ 0.05). Supplemental lysine (105% of the 1994 NRC) improved (P ≤ 0.01) 18 to 54 d feed:gain (2.30 vs 2.26). No differences in mortality occurred. Supplemental lysine increased preslaughter weight (P ≤ 0.05), but differences in carcass yield were not observed. Breast fillet yields were the highest (P ≤ 0.03) in broilers receiving 100% of NRC lysine and 83 or 92% of NRC threonine or 105% of NRC lysine and 100 or 108% of NRC threonine. In conclusion, additional lysine improved feed:gain independent of threonine from 1 to 54 d of age. However, lysine and threonine interact to increase weight gain and breast fillet yields.

ABSTRACT Two experiments were conducted to evaluate the effects of two dietary levels of lysine and four dietary levels of threonine in a factorial arrangement on broiler growth, carcass traits, and immunity. In both experiments, 120 broilers were allocated to each of 56 floor pens (6,720 total broilers). In Experiment 1, two levels of lysine (1.10 and 1.20% of diet) and four levels of threonine (0.68, 0.74, 0.80, and 0.86% of diet) were fed to broilers from 1 to 18 d of age in a sorghum-peanut meal diet. Body weight gain, feed:gain, mortality, and cellular and humoral immunity were measured. In Experiment 2, all broilers received a common basal diet up to 18 d of age. Experimental diets were fed from 18 to 34, 34 to 44, and 44 to 54 d of age. Two levels of lysine [100 and 105% of NRC (1994) recommendations] and four levels of threonine [83, 92, 100, and 108% of NRC (1994) recommendations] were included in the experimental diets for each age group (seven replications per treatment). The diets consisted of wheat (soft), corn gluten meal, soybean meal, and meat and bone meal. Weight gain, feed:gain, mortality, and carcass traits were measured at 54 d of age.

609

LYSINE AND THREONINE TABLE 1. Total dietary calculated and analyzed percentages of Lys and Thr and their respective ratios Experiment 1

Experiment 2 18 to 34 d

Thr: Lys

Lys

Thr

1.10 1.10 1.10 1.10 1.20 1.20 1.20 1.20

0.68 0.74 0.80 0.86 0.68 0.74 0.80 0.86

62 67 73 78 57 62 67 72

1.10 1.10 1.10 1.10 1.17 1.17 1.17 1.17

0.65 0.71 0.77 0.83 0.65 0.71 0.77 0.83

1.09 1.10 1.13 1.08 1.18 1.17 1.18 1.19

0.68 0.76 0.81 0.84 0.70 0.77 0.80 0.85

62 69 72 78 59 66 68 71

1.06 1.04 1.08 1.06 1.16 1.17 1.16 1.11

0.63 0.71 0.77 0.81 0.63 0.72 0.80 0.84

34 to 44 d Lys Calculated values 1.00 1.00 1.00 1.00 1.06 1.06 1.06 1.06 Analyzed values 0.97 1.00 1.00 1.04 1.06 1.00 1.05 1.08

al., 1973; Thomas et al., 1977; Sibbald and Wolynetz, 1987; Han and Baker, 1991, 1993; Bilgili et al., 1992; Holsheimer and Veerkamp, 1992). Furthermore, several studies demonstrate that the lysine requirement for optimal meat yield is above that considered adequate for growth (Jackson et al., 1989; Moran and Bilgili, 1990; Hickling et al., 1990; Bilgili et al., 1992; Acar et al., 1991). Dietary additions of methionine and lysine to commercial rations usually allow a decrease in dietary CP, thereby decreasing levels of other essential amino acids. Broilers consuming diets fortified with lysine above suggested recommendations (NRC, 1994) may have higher requirements for threonine. To determine whether lysine affects a broilers’ response to threonine, experiments were conducted to determine whether lysine and threonine interact to affect weight gain, feed conversion, carcass traits, and cellular and humoral immunity.

MATERIALS AND METHODS

Animals and Experimental Design Two experiments were conducted utilizing Ross × Ross broiler cockerels. One-hundred twenty broilers were allocated to each of 56 floor pens measuring 3.66 × 3.05 m. Treatments consisted of two levels of lysine and four levels of threonine (seven replications per treatment) and are described in Table 1. Each pen was equipped with one gas-heated brooder, two bell waterers, three tube feeders, and pine shavings. House temperature was maintained at approximately 32 C from 1 to 7 d of age, 29 C from 8 to 14 d

3Beckman

Systems Inc., Fullerton, CA 92634-3100.

44 to 54 d

Thr

Lys

Thr

18 to 54 d Thr:Lys

0.60 0.66 0.72 0.78 0.60 0.66 0.72 0.78

0.85 0.85 0.85 0.85 0.90 0.90 0.90 0.90

0.55 0.61 0.67 0.73 0.55 0.61 0.67 0.73

61 68 74 80 58 64 69 75

0.61 0.68 0.73 0.79 0.62 0.65 0.73 0.81

0.85 0.84 0.85 0.84 0.84 0.89 0.87 0.86

0.55 0.61 0.55 0.61 0.55 0.61 0.67 0.74

62 70 70 75 59 65 72 89

of age, 26 C from 15 to 21 d of age, and 22 C thereafter. Broilers were reared on continuous incandescent lighting. At 1 d of age, all birds were vaccinated by neck injections with Marek’s virus and infectious bursal disease virus. In addition, Newcastle and infectious bronchitis vaccinations were administered to all broilers at 1 d of age by coarse spray. In Experiment 2, all broilers were revaccinated at 10 d of age with infectious bursal disease virus and at 14 d of age with Newcastle and infectious bronchitis. Broilers consumed feed and water at will. Amino acid specifications used in dietary formulation for protein-contributing ingredients in the basal diets were analytical data from their respective ingredient amino acid analysis. To prepare feed samples for amino acid analysis, composite samples were analyzed for Kjeldahl nitrogen and DM (Association of Official Analytical Chemists, 1984). After acid hydrolysis, amino acid compositions were determined with a high performance cation exchange resin column.3 Each composite feed sample was analyzed twice.

Experiment 1 Treatments were allocated to a peanut meal and grain sorghum basal diet (Table 2) from 1 to 18 d of age. Treatments consisted of two levels of lysine (1.10 and 1.20% of diet) and four levels of threonine (0.68, 0.74, 0.80, and 0.86% of diet) (Table 1). Treatment additions of crystalline L-lysine·HCl and L-threonine were made at the expense of washed builders sand. Amino acid analysis of all diets is presented in Table 1. The basal diet consisted of 22.43% CP and contained 3.2 kcal ME/g of feed. All amino acids, except lysine and threonine, were formulated to a minimum of 100% of suggested recommendations (NRC, 1994). Pen weights were obtained at 1 and 18 d of age. Feed consumption and weight gain were calculated at 18 d

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Lys

1 to 18 d Thr

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KIDD ET AL. TABLE 2. Ingredients and calculated contents of the threonine-deficient experimental diet fed from 1 to 18 d in Experiment 1

Item

62.45 15.85 9.40 5.00 2.65 1.50 1.20 0.55 0.56 0.35 0.21 0.09 0.06 0.06 0.05 0.02 100.00 3,201 22.43 0.61 0.99 0.88 1.10 0.97 0.68 0.57 1.38 0.20 0.99 1.92 0.68 1.00 133

1Variable ingredient represents additions of crystalline L-lysine·HCl and L-threonine made at the expense of washed builders sand. 2Vitamin premix provided per kilogram of diet: vitamin A (source unspecified), 11,894 IU; cholecalciferol (D-activated animal sterol), 2,379 IU; vitamin E (source unspecified), 7.9 IU; niacin, 27.8 mg; pantothenic acid, 17.2 mg; riboflavin, 6.6 mg; thiamine, 1.32 mg; pyridoxine, 1.32 mg; vitamin K, 0.72 mg; folic acid, 0.70 mg; biotin, 0.12 mg; cobalamin, 0.01 mg; selenium, 0.12 mg. 3Mineral premix provided per kilogram of diet: manganese, 146 mg; zinc, 120 mg; iron, 48 mg; copper, 4.8 mg, iodine, 2.9 mg; calcium, 278 mg. 4Digestible amino acids were calculated using percentage coefficients from Heartland Lysine, Inc., Chicago, IL 60631. 5DEB = dietary electrolyte balance and is defined as: (sodium + potassium) – chloride in mEq/kg of diet.

of age. Birds that died during the experiment were weighed and used to adjust feed consumption data. Three chicks per pen were randomly chosen to evaluate cell mediated immunity. Toe web swelling in response to phytohemagglutinin-P was measured using a previously described cutaneous basophil hypersensitivity test (Corrier and DeLoach, 1990). Briefly, at 10 d of age, the right toe web thickness was measured in millimeter prior to injection of 100 mg of phytohemagglutinin-P. Twenty-four hours after injection the toe web was measured again. Swelling was deemed an indicator of the cellular immune response. Cellular immune results are presented as millimeter increase in toe web thickness.

Experiment 2 All broilers received a common basal diet from 1 to 18 d of age. Basal diets containing wheat, corn gluten meal, soybean meal, and meat and bone meal were fed to broilers from 18 to 34, 34 to 44, and 44 to 54 d of age (Table 3). To each of these basal diets treatments consisted of two levels of lysine [100 and 105% of NRC (1994) recommendations] and four levels of threonine [83, 92, 100, and 108% of NRC (1994) recommendations] (Table 1). The basal diets were formulated to a lysine and threonine level to average 100 and 83% of the NRC (1994) recommendations, respectively. Treatment additions of crystalline Llysine·HCl and L-threonine were made at the expense of washed builders sand. Basal CP levels in the 18 to 34, 34 to 44, and 44 to 54 d periods were 20, 18, and 16%, respectively. The TSAA, tryptophan, and valine were formulated to a minimum of 100% of NRC recommendations; other amino acids were not specified during formulation. Amino acid analysis3 was conducted for each treatment within each basal diet (Table 1). Pen weights were obtained at 18 and 54 d of age. Weight gain was measured for the 18 to 54 d period. Feed consumption was measured throughout the experiment and calculated for the 18 to 54 d period. Chicks that died during the experiment were weighed and used to adjust feed consumption data. At 54 d of age eight broilers per pen were randomly selected for processing (56 broilers per treatment). Bird selection for processing was random; however, careful selection of birds was made so that birds chosen had no visual abnormalities. Feed was withdrawn 8 h before processing; however, broilers were allowed free access to water until they were placed in coops for transportation to the pilot processing plant. Broilers were weighed, stunned with an electric knife, bled for 90 s by severing the jugular vein, scalded for 2 min, and defeathered in a rotary picker. Eviscera and abdominal fat were removed manually and hot carcass and fat pad weights were obtained. Broilers were then chilled in an ice bath at 0 C for 15 h. Carcass traits measured were chill weight, fillets, tenders, wings (bone in), thighs (bone in), and drumsticks (bone in).

Statistical Analysis The eight treatments of the factorial arrangements of two levels of lysine and four levels of threonine were analyzed using the General Linear Models procedure of the SAS Institute (1985) by the following model:

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Grain sorghum, ground Peanut meal Corn gluten meal Poultry meal (60% CP) Poultry fat Dicalcium phosphate Limestone Sodium bicarbonate L-lysine·HCl Variable1 DL-methionine Monensen sodium Vitamin premix2 Mineral premix3 Sodium chloride L-tryptophan Total Calculated contents ME, kcal/kg CP Methionine TSAA TSAA, digestible4 Lysine Lysine, digestible Threonine Threonine, digestible Arginine Tryptophan Valine Glycine + serine Phosphorus, available Calcium DEB, mEq/kg5

Percentage inclusion

Humoral immunity was evaluated by measuring the primary antibody response to SRBC in two chicks per pen. At 11 d of age, a 5% suspension of SRBC in physiological saline solution was injected in the peritoneal cavity. Seven d after injection chicks were bled and a serum hemagglutination assay quantitated antibody titers to SRBC (Toivanen et al., 1972). Humoral immunity results are presented in SRBC titer levels and transformed to log3.

611

LYSINE AND THREONINE

Experiment 1

Yijk = m + Li + Tj + LTij + eijk where m is the common mean; Li is the effect of the ith lysine; Tj is the effect of the jth threonine; LTij is the effect of the interaction of the ith lysine with the jth threonine; and eijk is random error. Pen was the experimental unit for all analyses. Percentage data were transformed using square root prior to analysis. When significant differences among means were found, means were separated using repeated t test.

Calculated lysine and threonine contents of diets and their respective analyzed values are presented in Table 1. Analyzed values were generally in agreement with calculated levels.

TABLE 3. Ingredients and calculated contents of the threonine deficient experimental diets fed from 18 to 54 d, Experiment 2 Percentage inclusion Item Wheat, soft Corn gluten meal Soybean meal Meat and bone meal Poultry fat Limestone L-lysine·HCl DL-methionine Sodium chloride Vitamin premix1 Mineral premix2 Coban, 60 g Copper sulfate Choline chloride, 70% Variable ingredient3 Total Calculated contents ME, kcal/kg CP Methionine TSAA TSAA, digestible4 Lysine Lysine, digestible Threonine Threonine, digestible Arginine Tryptophan Valine Phosphorus, available Calcium DEB, mEq/kg5

18 to 34 d

34 to 44 d

44 to 54 d

71.73 10.25 5.85 6.85 3.35 0.50 0.45 0.24 0.20 0.06 0.06 0.09 0.05 0.05 0.27 100.00

72.64 3.65 10.30 6.90 4.75 0.50 0.30 0.21 0.20 0.06 0.06 0.08 0.05 0.05 0.25 100.00

75.47 0.00 13.05 4.65 5.25 0.75 0.12 0.04 0.20 0.06 0.06 0.00 0.05 0.05 0.25 100.00

3,200 20.0 0.59 1.01 0.92 1.10 0.95 0.65 0.56 1.14 0.22 0.98 0.40 0.90 132

3,200 18.0 0.53 0.88 0.76 1.00 0.89 0.60 0.51 1.17 0.23 0.93 0.36 0.90 151

3,200 16.00 0.30 0.62 0.54 0.85 0.74 0.55 0.47 1.07 0.23 0.85 0.30 0.82 151

1Vitamin premix provided per kilogram of diet: vitamin A (source unspecified), 11,894 IU; cholecalciferol (Dactivated animal sterol), 2,379 IU; vitamin E (source unspecified), 7.9 IU; niacin, 27.8 mg; pantothenic acid, 17.2 mg; riboflavin, 6.6 mg; thiamine, 1.32 mg; pyridoxine, 1.32 mg; vitamin K, 0.72 mg; folic acid, 0.70 mg; biotin, 0.12 mg; cobalamin, 0.01 mg; selenium, 0.12 mg. 2Mineral premix provided per kilogram of diet: manganese, 146 mg; zinc, 120 mg; iron, 48 mg; copper, 4.8 mg; iodine, 2.9 mg; calcium, 278 mg. 3Variable ingredient represents additions of crystalline L-lysine·HCl and L-threonine made at the expense of washed builders sand. 4Digestible amino acids were calculated using percentage coefficients from Heartland Lysine, Inc., Chicago, IL 60631. 5DEB = dietary electrolyte balance and is defined as: (sodium + potassium) – chloride in mEq/kg of diet.

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RESULTS AND DISCUSSION

No significant interactions between lysine and threonine occurred (Table 4). Moreover, threonine levels ranging from 0.68 to 0.86% of diet failed to improve any parameter measured. These results are in disagreement with others (Smith and Waldroup, 1988; Kidd et al., 1996), who demonstrated that the threonine requirement for feed:gain is near the NRC (1994) recommendation of 0.80% of diet. Increasing dietary lysine from 1.10 to 1.20% improved (P ≤ 0.001) 1 to 18 d BW gain (453 vs 488 g). Feed: gain response was decreased (P ≤ 0.001) from 1 to 18 d with higher dietary lysine (1.39 vs 1.33). Thus, the present data suggest that the 1994 NRC recommendation of lysine at 1.10% of diet for 1 to 21 d of age is too low. These results are in agreement with Latshaw et al. (1993) who evaluated lysine levels from 0.99 to 1.44% of diet from 1 to 21 d of age

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KIDD ET AL. TABLE 4. Weight gain, feed conversion, mortality, cutaneous basophil hypersensitivity response, and primary antibody response of 1- to 18-d-old chicks as influenced by total percentage dietary Lys and Thr, Experiment 1 Treatment Lys

Thr

1.10 1.20 0.68 0.74 0.80 0.86

Feed:gain

(g) 453 488 463 474 473 471 4

(g:g) 1.39 1.33 1.36 1.36 1.36 1.35 0.01

0.001 0.297 0.126

Mortality (%) 2.46 2.88 2.97 2.63 2.74 2.34 0.42 Probability 0.360 0.810 0.639

0.001 0.880 0.742

and determined that 1.20% dietary lysine was required to optimize gain, feed consumption, and feed:gain. Continuous incandescent lighting and the strain of chickens used in the present study may have contributed to the increased lysine requirement. No differences in cellular immunity, humoral immunity, or mortality were observed (Table 4). Bhargava et al. (1971) fed graded levels of L-threonine (0.3 to 1.1% of diet) to chicks and observed significant improvements in hemagglutination titers to Newcastle disease virus as the level of L-threonine was increased. Lotan et al. (1980) fed wheat-gluten diets deficient in lysine and threonine to rats and evaluated growth, immune organ weights, antibody proliferation, and graft-vs-host reaction. Additions of lysine and threonine to the wheat-gluten diets restored growth and immunity to the level of the control. However, these authors concluded that lysine’s improvement in immunity was due to restoring growth, whereas

Toe web thickness

Titer

(mm) 0.72 0.83 0.71 0.80 0.76 0.84 0.05

(log3) 4.9 5.7 5.3 4.8 5.6 5.4 0.4

0.092 0.475 0.118

0.068 0.527 0.495

threonine’s improvement in immunity was independent of growth. Therefore, Lotan et al. (1980) concluded that the immune system of the rat has a specific requirement for threonine that is much higher than that of growth. Results of Experiment 1 (Table 4) indicate that basal diets fed to chicks up to 18 d of age may contain 1.10% lysine and 0.68% threonine without significantly lowering responses on cellular or humoral immunity. Moreover, nutrient limitation during the 1 to 18 d period may be advantageous for optimal livability to slaughter (Renden et al., 1994).

Experiment 2 Increasing dietary lysine from 100 to 105% of the NRC (1994) resulted in optimal (P ≤ 0.01) feed conversion (2.30 vs 2.26) (Table 5). No differences from lysine or threonine occurred in BW, weight gain, or mortality. Results of

TABLE 5. Growth, feed efficiency, and mortality as influenced by dietary Lys and Thr, Experiment 2 Dietary treatments Lys

Thr (added)

(% of NRC) 100 83 (0.00) 100 92 (0.06) 100 100 (0.12) 100 108 (0.18) 105 83 (0.00) 105 92 (0.06) 105 100 (0.12) 105 108 (0.18) SEM Source of variation Lys Thr Lys × Thr a,bMeans

Weight gain BW

Feed:gain ratio

45 d

54 d

18 to 45 d

45 to 54 d

18 to 54 d

18 to 45 d

45 to 54 d

18 to 54 d

Mortality 18 to 54 d

2,321 2,302 2,310 2,293 2,286 2,299 2,357 2,292 23

2,878 2,845 2,791 2,779 2,788 2,806 2,884 2,778 36

(g/bird) 1,785 1,757 1,764 1,752 1,739 1,758 1,811 1,746 22

557 542 481 486 501 508 527 486 21

2,342a 2,300ab 2,245b 2,238b 2,241b 2,266ab 2,338a 2,232b 34 Probability 0.63 0.29 0.05

2.03 2.04 2.01 2.03 2.01 1.98 1.97 1.97 0.02

(g:g) 3.16 3.27 3.43 3.38 3.32 3.25 3.19 3.38 0.11

2.29 2.31 2.29 2.31 2.29 2.25 2.23 2.26 0.02

(%) 4.20 5.69 7.25 6.32 5.20 4.76 4.86 5.51 1.52

0.01 0.43 0.53

0.75 0.59 0.33

0.01 0.54 0.36

0.47 0.79 0.74

0.91 0.32 0.38

0.72 0.34 0.08

0.97 0.32 0.21

0.46 0.15 0.09

with no common superscript differ significantly (P ≤ 0.05).

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Pooled SEM Source of variation Lys Thr Lys × Thr

Weight gain

LYSINE AND THREONINE

Experiment 2 (Table 6) demonstrate that increasing lysine from 100 to 105% of the NRC (1994) requirement significantly increased the amount of white meat (fillet, 358 vs 370 g; tender, 91 vs 95 g; bone in wings, 232 vs 237 g) independent of threonine. However, differences in the main effect of lysine in fillet or tender percentage yields and carcass fat were not observed. Gous and Morris (1985) increased lysine from 0.6 to 1.6% in the starter diet of male broilers and observed reductions in carcass fat. Dietary lysine increases lean carcass and decreases carcass fat (Sibbald and Wolynetz, 1986). Furthermore, 50% of all edible protein in broilers is breast meat (Summers et al., 1988). It must be pointed out that increasing lysine from 100 to 105% of the NRC (1994) increased 54-d preslaughter weight, but not 54-d pen weights. Although selection of birds for processing was random, sample selection technique may have resulted in birds, and the weight of their respective carcass parts, from the 105% lysine treatment being heavier and not representative of the population. Interactions of lysine and threonine occurred in BW gain (P ≤ 0.05), but not BW or feed conversion (Table 5). Body weight gain is corrected for the initial weight at 18 d of age. At the 100% NRC lysine level, optimal weight gain was obtained by feeding threonine at 83 or 92% of the NRC. At the 105% NRC lysine level, optimal weight gain was obtained by feeding threonine at 92 or 100% of the NRC. No interactions occurred in feed conversion. These results suggest that the threonine:lysine ratio required to optimize BW gain in finishing broilers is at or below 69% and are in agreement with past findings (Baker, 1994). Lysine and threonine interacted to affect breast fillet yields (P ≤ 0.03). At the 100% NRC lysine level, optimal fillet yields were obtained by feeding threonine at 83 or 92% of the NRC. However, at the 105% NRC lysine level, optimal fillet yields were obtained by feeding threonine at 100 or 108% of the NRC. In addition, lysine and threonine interactions were observed for thigh yields (P ≤ 0.05) and drumstick yields (P ≤ 0.05). It is generally accepted that diets limiting in essential amino acids decrease breast yield; however, all essential amino acids may not be required to optimize breast yields. Sibbald and Wolynetz (1986) demonstrated that essential amino acid requirements for optimal breast yields are higher than that of growth. Optimal breast tissue accretion in finishing broilers can be obtained by feeding levels of methionine (Summers et al., 1988; Schutte and Pack, 1995; Gorman and Balnave, 1995) and lysine (Jackson et al., 1989; Moran and Bilgili, 1990; Hickling et al., 1990; Bilgili et al., 1992; Acar et al., 1991) above suggested recommendations (NRC, 1994). However, results of Experiment 2 (Table 6) indicate that feeding high dietary lysine without consideration to dietary threonine may limit breast fillet yields. Moreover, if low CP diets with supplemental methionine and lysine are desired, careful evaluation of the dietary threonine minimum (or the threonine:lysine ratio) may be necessary for optimal breast fillet yields. Furthermore, if formulating diets to essential amino acid minimums

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6226, table 6

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KIDD ET AL.

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rather than CP results in optimal breast yields, economic analysis of low CP diets plus supplemental methionine, lysine, and threonine vs high CP diets is the remaining determinant. Consumer demands in the U.S. market have resulted in breast meat yield becoming the primary concern for production of broilers. Genetic selection by primary poultry breeder companies has increased absolute and relative breast meat yield and decreased relative offal and abdominal fat yields in commercial broilers. Sex, age, and nutrition also affect salable product yields. In the current study, lysine and threonine interacted (threonine:lysine of approximately 70%) to increase breast fillet yields. Increasing dietary lysine without consideration to other essential amino acids may allow expression of a marginal threonine deficiency. Future research on other essential amino acid ratios to lysine (i.e., arginine) may reveal optimum carcass traits in conjunction with optimum growth parameters.