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METABOLISM AND NUTRITION

Modeling of Threonine Requirement in Fast-Growing Chickens, Depending on Age, Sex, Protein Deposition, and Dietary Threonine Efficiency

Samadi and F. Liebert1

Institute for Animal Physiology and Animal Nutrition, Georg-August-University, D-37077 Goettingen, Germany

1 Corresponding author: flieber{at}gwdg.de


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
In addition to dose-response studies, modeling of N utilization, depending on intake of the first limiting amino acid in the diet, is one of the tools for assessing amino acid requirements in growing animals. Based on a verified nonlinear N-utilization model and following the principles of the diet dilution technique, N-balance experiments were conducted to estimate the Thr requirement of fast-growing chickens (genotype Cobb), depending on age, sex, CP deposition. and efficiency of dietary Thr utilization. Different predictions were made for the feed intake to conclude the optimal Thr concentration in the feed. The results are based on N-balance experiments with a total of 144 male and 144 female growing chickens within 4 age periods (I: 10 to 25 d; II: 30 to 45 d; III: 50 to 65 d; IV: 70 to 85 d), using diets with graded protein supply (6.6, 13, 19.6, 25.1, 31.8, and 37.6% CP in DM) from high-protein soybean meal with a constant amino acid ratio and Thr as the first limiting amino acid (3.87 g of Thr/100 g of CP; dietary Lys:Thr = 1:0.54). The observed optimal Thr concentration (% of feed) was influenced by age, sex, level of CP deposition, dietary efficiency of Thr utilization, and predicted feed intake. For male chickens, assuming an average CP deposition (60% of the potential) and average efficiency of Thr utilization, 0.78% (10 to 25 d), 0.73% (30 to 45 d), 0.65% (50 to 65 d), and 0.55% (70 to 85 d) total dietary Thr were observed as optimal total Thr concentration in the diet (corresponding to 60, 135, 160, and 180 g of daily feed intake, respectively). Data are discussed in context with the main factors of influence like age, sex, level of daily CP deposition, efficiency of dietary Thr utilization, and predicted feed intake.

Key Words: threonine • efficiency of utilization • requirement • modeling • broiler


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
Inadequate formulation of dietary amino acids impairs protein utilization and increases total N excretion. It has been well documented that several factors such as genotype, age, and sex influence the requirement in growing chickens (Hurwitz et al., 1980; Moran and Bilgili, 1990; Zuprizal et al., 1992; Webel et al., 1996).

Threonine may be considered as the third limiting amino acid (LAA) in chicken diets based on corn, wheat, and soybean meal, following Met and Lys (Han et al., 1992; Fernandez et al., 1994). Kidd (2000) reported that Thr deficiency resulted in decreasing utilization of TSAA and Lys. Several studies about Thr requirements in growing chickens have been published with varying results, depending on diet composition, age, sex, and breed. In early studies, Krautmann et al. (1958) proposed 0.55 and 0.60% of the diet, or 2.6 and 2.9% of dietary CP, as the optimal Thr concentration using diets with 21% CP. Hewitt and Lewis (1972) proposed 0.48% Thr in diets with 16.2% CP, corresponding to 3% Thr in the dietary CP. Penz et al. (1991) recommended 0.68% Thr for optimal feed efficiency from 21 to 42 d of age. NRC (1994) recommendations are 0.80% Thr for 0 to 3 wk, 0.74% for 3 to 6 wk, and 0.68% for 6 to 8 wk of age, respectively. Webel et al. (1996) observed maximal feed efficiency at 0.61% digestible Thr (3 to 6 wk) and 0.52% (6 to 8 wk), respectively. Rosa et al. (2001) suggested 0.69% Thr for growth and 0.68% for the feed conversion ratio (FCR) for classic strains of growing chickens in the starter period. Kidd et al. (2004) concluded 0.74% Thr for growth (95% of maximum growth response) and 0.71% for breast meat yield, using 3 commercial lines of growing chickens. It is indicated that the procedure for assessing amino acid requirements is an important factor influencing the established requirement data. Factorial approaches as well as dose-response studies are mostly utilized for amino acid requirement studies in growing chicken. Actual experiments are based on principles of the "diet dilution technique" (Fisher and Morris, 1970) and its application within an exponential N-utilization model (Thong and Liebert, 2004a,b,c; Samadi and Liebert, 2006a,b). Based on this modeling procedure, the Thr requirement of fast-growing chicken should be examined. Diets with a graded protein supply but a similar amino acid ratio and Thr as the first LAA were applied as described by Samadi and Liebert (2006a).


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 REFERENCES
 
The modeling of Thr requirement is based on the results of N-balance studies with a graded dietary protein supply from high-protein (HP) soybean meal (Table 1Go) as the single protein source but a similar amino acid ratio (Lys:Met + Cys:Thr = 1:0.85:0.54). The experimental diets were formulated to maintain Thr as the first LAA in all age periods under study (Table 2Go), comparing to the recommendations of the NRC (1994). The energy content of the pelleted feed was calculated in terms of ME, according to WPSA (1984). Diet formulation should avoid a strong decrease of ME, mainly in the high-protein diets (N5, N6), by graded supplementation of soybean oil within the physiological limitations.


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  		Table 1. Composition of the experimental diets (% as fed)
 

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  		Table 2. Analyzed nutrient content of the experimental diets (DM basis)
 
Nitrogen-balance data from 144 male and 144 female growing chickens (genotype Cobb 500) were utilized. Nitrogen-balance experiments were divided into 5-d adaptation and 2 consecutive collection periods (5 d each) within 4 age periods (I: 10 to 25 d; II: 30 to 45 d; III: 50 to 65 d; IV: 70 to 85 d). Additional details about the experimental procedures were already reported (Samadi and Liebert, 2006a)

Chemical Analyses
Dietary ingredients, mixed diets, and excreta were analyzed according to the regulations of German Verband Deutscher Landwirtschaftlicher Untersuchungs- und Forschungsanstalten standards (Naumann and Bassler, 1976–1997). The N content was determined using the Dumas method (LECO LP-2000, LECO Instrumente GmbH, Kirchheim, Germany), and CP was calculated by the factor 6.25. The ether extract was determined following HCl hydrolysis of the feed samples. Amino acid analyses (except tryptophan) were done by ion-exchange chromatography (LC 3000; Biotronik, Eppendorf-Netheler-Hinz GmbH, Hamburg, Germany), following acid hydrolysis of the proteins. An oxidation step for quantitative determination of TSAA was utilized. The model calculation of Thr requirement data was based on the analyzed Thr concentration in the feed protein.

Statistical Analyses
For analysis of the N-balance data, principles of an exponential N-utilization model (Gebhardt, 1966; Liebert, 1995a,b; Thong and Liebert, 2004a,b,c; Samadi and Liebert, 2006a) were applied


Formula 1([1])


Formula 2([2])

where NR = daily N retention (ND + NMR; mg/BWkg0.67); ND = daily N balance or N deposition (mg/BWkg0.67); NMR = daily N maintenance requirement (mg/BWkg0.67); NRmaxT = theoretical maximum for daily N retention (mg/BWkg0.67); NI = daily N intake (mg/BWkg0.67); b = model parameter for the slope of the function between NI and NR, depending on the dietary protein quality; and e = basic number of natural logarithm.

Earlier model applications in growing animals (Liebert and Gebhardt, 1980, 1986, 1988; Thong and Liebert, 2004a,b,c; Wecke and Liebert, 2005a,b) were based on similar procedures. However, some abbreviations were actually modified (Samadi and Liebert, 2006a,b) to improve the accordance with the physiological meaning. In addition, the genotype-dependent parameters daily N maintenance requirement and NRmaxT were taken from actual experiments with similar genotype (Samadi and Liebert, 2006a). For modeling the Thr requirement, an extended model application is necessary. Generally, the concentration of the LAA in the feed protein and the resulting dietary protein quality are linearly correlated (Gebhardt, 1980; Liebert and Gebhardt, 1980, 1988). The slope of the linear function (quotient bc–1) indicates the efficiency of utilization of the LAA in the diet (Liebert, 1995a,b). Furthermore, parameter (bc–1) summarizes the efficiency within the processes digestion and absorption and postabsorptive utilization. Following logarithmization and several transformation steps of Equation [1], the amino acid requirement is established (Liebert and Gebhardt, 1986; Liebert et al., 2000; Thong and Liebert, 2004a,b,c; Samadi and Liebert, 2006b) due to Equation [3]


Formula 3([3])

where ln = natural logarithm; NR = daily N retention (ND + daily N maintenance requirement; mg/BWkg0.67); bc–1 = slope between c and b (model parameter, indicating the efficiency of utilization of the dietary LAA); b = dietary protein quality; c = concentration of the LAA in the feed protein (g/100 g of CP); LAAI = daily intake of the LAA, depending on performance and efficiency of LAA utilization (mg/BWkg0.67; amino acid requirement in terms of total amino acids but defined efficiency).

Due to this procedure, the amino acid requirement is established depending on performance (daily N retention) within the estimated growth potential (Thong and Liebert, 2004a,b,c; Samadi and Liebert, 2006a). Furthermore, genotype under study (NRmaxT) and the efficiency of LAA utilization (slope between LAA-concentration and dietary protein quality) are taken into account. Within the variation of observed efficiency of amino acid utilization, modeling of amino acid requirements may run for graded dietary amino acid efficiency. Due to logarithmization and transformation of Equation [1], Equation [4] was applied to establish a model parameter from N-balance studies with graded protein supply (Samadi and Liebert, 2006a)


Formula 4([4])

where b = model parameter of dietary protein quality; ln = natural logarithm; NR = daily N retention (ND + daily N maintenance requirement; mg/BWkg0.67); NI = daily N intake (mg/BWkg0.67).

Efficiency of Thr utilization (bc–1) is calculated based on the analyzed Thr concentration. The observed parameter is only for use within the modeling procedure; further applications are described earlier (Liebert, 1995a,b).


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
REFERENCES
 
Analysis of Experimental Diets
Summarized data of diet composition are presented in Table 1Go. Analyzed CP values ranged from 6.6 to 37.6% of DM, respectively (Table 2Go). The calculated dietary ME content was slightly decreased with increasing the protein content and varied from 14.5 to 15.5 MJ of ME/kg of DM. Within physiological limitations for dietary fat supplementation in chicken diets, completely isoenergetic diets cannot be achieved. Indispensable amino acids rationed to Lys indicate Thr as the first limiting dietary amino acid. Relative amino acid pattern of Thr (Lys:Met + Cys:Thr = 1:0.85:0.54) was below the recommendations of the NRC (1994). Consequently, Thr can be generalized as the first limiting dietary amino acid. New ratio estimates for Thr based on true digestible AA and using semipurified diets (Baker, 2003) have modified Lys:Thr (1:0.56, 0 to 21 d; 1:0.58, 21 to 56 d) but are not directly comparable with our total Thr basis.

Model Parameters and Determined bc–1
Table 3Go summarizes the model parameters used and the observed efficiency of bc–1. Parameters NRmaxT and NDmaxT decreased with increasing age (Samadi and Liebert, 2006a). Male chickens exhibited a slightly higher potential for protein deposition as compared with females. The observed age-depending efficiency of Thr utilization was applied for modeling the Thr requirement, depending on daily protein deposition.


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  		Table 3. Model parameters and summarized data of N-balance experiments with fast-growing chickens (genotype Cobb 500), depending on age and sex
 
Modeling Thr Requirement Data for Growing Chickens
Requirement data depending on sex, age and varying percentage of the growth potential (NDmaxT) are summarized in Tables 4Go to 6GoGo. We suggested approximately 50, 60, and 70% of the theoretical maximum of daily N deposition of (NDmaxT) as not too far from practical conditions. Assessing requirement data per metabolic BW, the daily requirement decreased with increasing age. The absolute daily requirement (mg/d) of the older chickens increased. For practical applications, the recommended optimal dietary Thr concentration is of special interest. Several assumptions were made for the daily feed intake, depending on age. However, assumptions for feed intake restrict the generalization of resulting modeling procedures, indeed. The prediction of true feed intake under varying circumstances remains difficult.


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  		Table 4. Model calculation of the Thr requirement for male and female fast-growing chickens (Cobb 500) in age period I (10 to 25 d), depending on daily CP deposition and predicted daily feed intake (mean BW: males, 500 g; females, 500 g)
 

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  		Table 5. Model calculation of the Thr requirement for male and female fast-growing chickens (Cobb 500) in age period II (30 to 45 d), depending on daily CP deposition and predicted daily feed intake (mean BW: males, 2,000 g; females, 1,800 g)
 

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  		Table 6. Model calculation of the Thr requirement for male and female fast-growing chickens (Cobb 500) in age period III (50 to 65 d), depending on daily CP deposition and predicted daily feed intake (mean BW: males 3,500 g; females, 3,100 g)
 
In age period I (10 to 25 d), for 8 g of daily protein deposition (expected BW gain of 47 g/d), 0.78 and 0.80% Thr was yielded as optimal concentrations, assuming 60 g of daily feed intake in male and female chickens, respectively (Table 4Go). However, varying the predicted feed intake provides remarkable changes for optimal dietary Thr concentration. By using approximately 60% of NDmaxT (15 g of daily protein deposition; expected BW gain of 78 g/d) and 140 g of realized daily feed intake in age period II (30 to 45 d), male and female growing chickens need 0.70% (979 mg/d) and 0.73% (1,027 mg/d) of total Thr, respectively (Table 5Go). The optimal Thr concentration is further decreasing in older chickens with similar use of the NDmaxT (Tables 5Go to 7GoGo). It is well documented that amino acid requirements of growing animals depend on many factors such as genotype, age, and sex (Siegel et al., 1984; Lehmann et al., 1997; Smith et al., 1998; Rosa et al., 2001; Kidd et al., 2004). Furthermore, the method for requirement assessment is an important factor of influence (Ishibashi et al., 1998; Barkley and Wallis, 2001). Our study has examined Thr requirement data based on N-balance experiments with a graded protein supply and Thr as the first limiting dietary amino acid, based on principles of the diet dilution technique (Fisher and Morris, 1970).


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  		Table 7. Model calculation of the Thr requirement for male and female fast-growing chickens (Cobb 500) in age period IV (70 to 85 d), depending on daily CP deposition and predicted daily feed intake (mean BW: males, 4,700 g; females, 4,100 g)
 
Optimal Thr concentration decreased with increasing age. The NRC (1994) recommended 0.80% (1 to 3 wk), 0.74% (3 to 6 wk), and 0.68% (6 to 9 wk) total Thr, respectively. Our conclusions for 60% of NDmaxT and average predicted feed intake were 0.79% (10 to 25 d), 0.72% (30 to 45 d), 0.65% (50 to 65 d), and 0.58% (70 to 85 d) for male and female mixed chickens (1:1). Çiftci and Ceylan (2004) reported 0.68 to 0.75% (1 to 3 wk) and 0.65 to 0.68% (3 to 6 wk) as optimal Thr concentrations. Rangel-Lugo et al. (1994) revealed that part of the variation of Thr requirement data may be due to the variations in dietary CP level. Webel et al. (1996) reported 0.70% (3 to 6 wk) and 0.60% (6 to 8 wk) Thr, assuming 87% Thr digestibility. This result is lower than the NRC (1994) recommendations but was in close agreement with ideal protein calculations (70% of Lys requirement, 3 to 4 wk). Rangel-Lugo et al. (1994) concluded 0.77% (based on gain) and 0.67% (based on FCR) total Thr (0 to 2 wk) using 20% CP but 0.86 and 0.82% Thr in 25% CP diets, respectively. Furthermore, 0.63% (based on gain) and 0.69% (based on FCR) Thr (3 to 4 wk) were recommended using 20% CP diets. Kidd et al. (2004) observed 0.74% total Thr (based on gain), corresponding to 0.65% digestible Thr for male chickens from several commercial lines (21 to 42 d). For high breast yield, 0.71% total Thr (0.62% digestible Thr) was recommended and was in accordance with our data (30 to 45 d) for 60% of NDmaxT and 140 g of daily feed intake (0.70 and 0.73% for males and females, respectively). In the finishing period (42 to 56 d), Kidd et al. (2003a) observed 0.63 and 0.68% total Thr (0.56 and 0.61% digestible Thr) for chickens in a clean environment as optimal for growth and breast yield, respectively. These data are in agreement with our results in age period III (50 to 65 d) for 60% of NDmaxT and 170 g of daily feed intake (0.61 and 0.69 % for males and females, respectively).

The estimated potential for protein deposition is slightly higher in male than in female growing chickens (Table 3Go). However, at similar levels of daily protein deposition, the difference between both sexes is marginal. Dozier and Moran (2001) reported 0.74 and 0.63% total Thr for male and female chickens (42 to 56 d), respectively. However, our modeling was based on an equal level of daily protein deposition in male and female chickens. Additionally, different model parameters (NRmaxT) for both sexes were observed (Table 3Go) and utilized within the model calculations. Consequently, the noticed higher optimal Thr concentration for female chickens is summarizing these factors and provides evidence for a slightly impaired utilization of dietary Thr for protein deposition. However, due to the described procedure, it is difficult to conclude a distinction between both sexes in optimal dietary Thr concentration. Additionally, differences in the predicted feed intake are also a factor of influence. According to 60% of NDmaxT and 140 g of daily feed intake (30 to 45 d), Corzo et al. (2003) found 0.69 and 0.71% total Thr optimal for female chickens (30 to 42 d), based on growth and feed conversion, respectively. Kidd et al. (2003b) reported 0.67% Thr for female chickens (42 to 56 d). Rosa et al. (2001) reported marginal differences in optimal Thr concentration for males and females (1 to 18 d), based on gain and feed conversion.

Furthermore, the actual modeling of the Thr requirement (Tables 4Go to 7GoGoGo) only applied the observed bc–1 of HP soybean meal as a single feed protein source. However, the varying bc–1 in feed ingredients is an important factor of influence. Actually, no reliable experimental data due to the variability of efficiency of dietary Thr utilization are available. Consequently, several assumptions were made for a further example of modeling (Table 8Go). Assuming 10% higher or lower efficiency of dietary Thr utilization demonstrates that varying this dietary factor may yield in 0.1% lower or higher optimal Thr concentration in the feed. It can be concluded that mistakes in evaluating the dietary efficiency of LAA utilization may have strong effects on assessment of requirement data.


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  		Table 8. Model calculation of the Thr requirement for male and female fast-growing chickens (Cobb 500) in age period I (10 to 25 d) with varying dietary efficiency of Thr utilization (mean BW: males, 500 g; females, 500 g)
 
Based on the described procedure, it is possible to conclude requirement data graduated and within a given variation of the LAA efficiency. Taking into account this important dietary factor, the applied procedure is indeed a step in the direction of assessing the bioavailability of feed amino acids (Sibbald, 1987). Results of the model calculation are summarized in Figures 1Go and 2Go, demonstrating Thr requirement curves for female and male growing chickens, depending on age and reflecting the bc–1 of Thr as measured in HP soybean meal. For further research, requirement curves have to be established for graded efficiency of amino acid utilization (Thong and Liebert, 2004c).


Figure 1
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  		Figure 1. Daily N deposition (ND), depending on Thr intake (ThrI) and age of female fast-growing chickens. NRmaxT = theoretical maximum for daily N retention (mg/BWkg0.67); e = basic number of natural logarithm; bc–1 = efficiency of Thr utilization; and NMR = daily N maintenance requirement (mg/BWkg0.67).

 

Figure 2
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  		Figure 2. Daily N deposition (ND), depending on Thr intake (ThrI) and age of male fast-growing chickens. [Arrow on x-axis indicates the necessary ThrI for ND, according to approximately 60% of the theoretical maximum of daily N deposition (mg/BWkg0.67) in the age period I]. NRmaxT = theoretical maximum for daily N retention (mg/BWkg0.67); e = basic number of natural logarithm; bc–1 = efficiency of Thr utilization; and NMR = daily N maintenance requirement (mg/BWkg0.67).

 
In conclusion, the results of our investigations in growing chickens give an example for modeling Thr requirement data, depending on genotype, sex, age, and efficiency of Thr utilization in the diet. According to present knowledge, the effect of age is clearly reflected. Furthermore, Thr requirements of male and female broilers were only slightly different, assuming similar levels of daily protein deposition. Optimal Thr concentration in the feed is influenced by the level of feed intake. The reliable prediction of feed intake is very important for concluding precise amino acid concentrations in the feed, but generalized feed intake patterns depending on genotype and age are not available. Finally, our reported data are in the scope of published recommendations, but growth studies are initiated to support the results of modeling before application under real conditions. Particularly, the real variation of the bc–1 of Thr in practical diets needs more detailed investigations.

Received for publication March 30, 2006. Accepted for publication July 4, 2006.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
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Samadi and F. Liebert
Threonine Requirement of Slow-Growing Male Chickens Depends on Age and Dietary Efficiency of Threonine Utilization
Poult. Sci., June 1, 2007; 86(6): 1140 - 1148.
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