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METABOLISM AND NUTRITION |
* Department of Animal Science, Iowa State University, Ames 50011; USDA-Agricultural Research Service, Ames, IA 50011; and Degussa Corporation, Kennesaw, GA 30144
1 Corresponding author: kristjan{at}iastate.edu
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Key Words: egg mass • ideal amino acid profile • laying hen
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Unlike the NRC for swine (1998), neither the NRC for poultry (1994) nor the Dutch Centraal Veevoederbureau (CVB, 1996) use the ideal AA profile to determine the AA requirements for laying hens, but report empirically determined AA requirements. These AA requirements can be used to calculate ideal AA profiles for laying hens; however, these profiles were estimated from data compiled from a variety of experiments and, thus, were influenced by genetics and environmental factors as mentioned previously. Coon and Zhang (1999) conducted 5 separate experiments to determine the AA requirements of laying hens and reported the ideal AA profile from averages of the 5 experiments. Although better than the ideal AA profile calculated from NRC (1994) or CVB (1996) recommendations, these experiments were still performed under different experimental conditions with different basal diets, ages, and genetic lines of hens. To ensure a valid measurement of the ideal AA profile, the same basal diet, the same genetic line, and the same assay period should be used in all assays of AA requirements (Baker, 2003). Hence, the objective of experiment (Exp.) 1 was to investigate responses of laying hens, 28 to 34 wk of age, to graded dietary inclusions of the essential AA Arg, Ile, Lys, Met, Thr, Trp, and Val, to determine the ideal AA ratios of the assayed AA relative to Lys. In Exp. 2, the objective was to confirm the Lys, Met, and TSAA requirements (and, therefore, the ideal Met:Lys and TSAA:Lys ratios) determined in Exp. 1.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Animals and Housing
A total of 1,008 white single-comb Leghorn-type hens (Hy-Line W-36, Hy-Line International, Des Moines, IA), 18 wk of age, were placed in a windowless fan-ventilated room. Hens were housed 2 per cage, corresponding to 619 cm2 per hen, in wire-bottomed cages (Chore-Time, Milford, IN), each equipped with a plastic self-feeder and a nipple drinker. Before Exp. 1, all hens were given free access to corn and soybean meal diets following recommendations listed in the W-36 Commercial Management Guide (Hy-Line International). The photoperiod was increased incrementally to 16L:8D at 26 wk of age in accordance with the W-36 Commercial Management Guide. A corn-soybean meal based diet, exceeding the NRC (1994) nutrient recommendations, was fed in the period between Exp. 1 and 2.
Experimental Diets
Before formulation of the assay diets, all protein-supplying ingredients were analyzed for contents of individual AA by ion-exchange chromatography (Llames and Fontaine, 1994). An industry-type control diet (Table 1
) was formulated using corn, soybean meal, and meat and bone meal, a calculated MEn content similar to that of the AA-deficient basal diet, and formulated to meet or exceed nutrient recommendations by the NRC (1994). In addition, a basal diet was formulated to meet or exceed nutrient recommendations by the NRC (1994), except for the 7 assayed AA, using feed ingredients typically used in commercial diets (i.e., corn, soybean meal, and meat and bone meal). In Exp. 1, the basal diet was mixed in 2 separate 2,722-kg batches in a horizontal mixer at a commercial feed mill and bagged in 22.6-kg bags according to batch number. In Exp. 2, a basal diet was mixed in a vertical mixer in 3 separate 500-kg batches at the Iowa State University Poultry Science Research Center. Representative samples of the basal-diet batches were pooled within experiment and analyzed for AA content by ion-exchange chromatography (Llames and Fontaine, 1994) and for true AA digestibility (Table 2) by the cecectomized rooster assay at the Department of Poultry Science, University of Georgia, Athens, GA (Lumpkins and Batal, 2005). The crystalline AA added to the basal diet were assumed to be 100% true digestible (Izquierdo et al., 1988; Chung and Baker, 1992).
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). Hence, K2CO3 and crystalline AA partially replaced cornstarch in the basal diet. To avoid antagonisms and imbalances among AA and to ensure that the assayed AA was first-limiting in the particular assay diet, the basal diet in Exp. 1 was fortified with all AA under investigation—except the one assayed—to contain, on a true digestible basis, 0.81% Arg, 0.64% Ile, 0.87% Lys, 0.63% Met, 0.78% Met+Cys, 0.53% Thr, 0.17% Trp, and 0.74% Val in assay diet 1; 0.87% Arg, 0.78% Ile, 1.05% Lys, 0.85% Met, 1.01% Met+Cys, 0.60% Thr, 0.21% Trp, and 0.88% Val in assay diets 2 and 3; and 0.93% Arg, 0.92% Ile, 1.24% Lys, 1.07% Met, 1.22% Met+Cys, 0.67% Thr, 0.25% Trp, and 1.02% Val in assay diets 4 and 5. In other words, crystalline AA, other than the one assayed, were included in diet 1, 2, 3, 4, and 5 to supply the true digestible AA concentration found in diet 3, 4, 4, 5, and 5, respectively (see Table 3). In Exp. 2, only the responses to Lys and Met were determined; thus, there were 10 assay diets formulated with equal parts of each of the 3 basal-diet batches as described for Exp. 1 (Table 4).
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and 4 (data not shown) and only calculated values are tabulated. Data Collection
Hens were offered free access to the experimental diets from 26 to 34 wk of age (Exp. 1) or from 50 to 58 wk of age (Exp. 2), with the first 2 wk of each experiment considered a depletion period. Thus, only data from the last 6 wk of the experiments were used in the statistical analyses. Egg production was recorded daily and feed consumption (determined as feed disappearance) was measured weekly throughout the 8-wk-long experiments. Consumption of the assay AA (mg/d) was calculated from the mean daily feed consumption (g/d) over the last 6 wk of each experiment and the dietary true digestible AA content (%). This latter content was calculated from the analyzed total AA content of the basal diet multiplied by the analyzed digestibility coefficient plus the inclusion of crystalline AA. Once a week, eggs collected over a 48-h period were weighed and the egg mass calculated by multiplying the week’s egg-production rate by the egg weight. Feed utilization was calculated as grams of egg mass divided by grams of feed consumed. Mortalities were recorded throughout the study with egg production and feed consumption rates adjusted accordingly.
Eggs collected over a 24-h period during wk 4 of Exp. 1 (i.e., hens at 30 wk of age) were saved for measurement of specific gravity using the flotation technique. Briefly, saline solutions were prepared with feed-grade NaCl and tap water to make densities from 1.065 to 1.095 g/cm3 in increments of 0.002 g/cm3. Eggs were placed sequentially in saline solutions, beginning with the lowest density, until the eggs floated for at least 5 s. The density at which each individual egg floated was recorded as its specific gravity. All specific gravity measurements were carried out in a temperature-controlled room set at 18°C. Eggs collected over a 24-h period during wk 5 of Exp. 1 (i.e., hens at 31 wk of age) were saved for measurement of egg contents. Eggs were weighed, broken, and separated into yolk, shell, and albumen fractions. Wet weights were recorded separately for all 3 components from each egg. The corresponding dry weights were recorded after drying in a forced-air drying oven at 70°C for 24 h. The percentage of egg solids was calculated as the sum of dried weights of yolk and albumen divided by the wet weight of the whole egg. Eggs collected over a 24-h period during wk 6 of Exp. 1 (i.e., hens at 32 wk of age) were weighed and used for determination of Haugh units. The albumen height of each egg was measured using an electronic tripod albumen-height gauge (Technical Services and Supplies, York, UK) with the Haugh units subsequently calculated from the records of egg weight (g) and albumen height (mm) as Haugh unit = 100 x Log(albumen height – 1.7 – egg weight0.37 + 7.57). The albumen height was measured in a temperature-controlled room set at 13°C. Hens were weighed at the beginning and end of Exp. 1 and BW change over the 8-wk-long experiment calculated.
Statistical Analyses
In each of the 7 dose-response assays, the requirement for the assayed AA was determined in a randomized complete block design with 6 dietary treatments (i.e., 5 levels of the assayed AA and 1 control diet) and 12 blocks (Morris, 1999). Thus, each block consisted of 6 cages, to which the 5 assay diets and the 1 control diet were randomly distributed. The cage location within the barn served as the blocking criterion and the experimental unit was 1 cage containing 2 hens. The requirements for digestible AA were calculated with the single-slope broken-line regression model (Robbins, 1986) using the nonlinear modeling option in JMP (version 6.0.3; SAS Institute, Cary, NC) with the consumption of the assayed AA (mg/d) as the independent variable (i.e., only data from the 5 cages receiving the amino acid assay diets were used). Block was not included in the broken-line regression model. The R2 values were calculated as 1 – (sum of squared errors/total sum of squares), where the sum of squared errors were derived from the broken-line regression model. Because the nonlinear modeling option in JMP only reports the sum of squared errors, the total sum of squares were calculated after fitting an overall mean to the data using the Fit-Y-by-X option in JMP. Feed consumption data were analyzed by ANOVA with block and dietary treatment as the independent variables; treatment effects were separated using linear, quadratic, and cubic orthogonal polynomial contrasts (Morris, 1999). The maximal response and associated SE (n = 60) from each of the 7 assays, determined using the broken-line model, was compared with the corresponding response from the control-fed hens (n = 12) using a 2-tailed t-test taking the unequal replications into account (Snedecor, 1946). If the broken-line regression model did not converge, ANOVA was performed with block and diet as the independent variables and, if the main effect of diet was not significant, the overall mean was compared with that of the control diet using a 2-tailed t-test. Probability values less than or equal to 0.05 were considered significant in all comparisons.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The hens were generally in good health during the experiment; 3 of the 1,008 hens (0.3%) died of reasons considered unrelated to the dietary treatments. The responses to consumption of Arg, Ile, Lys, Met, TSAA, Thr, Trp, and Val are shown in Figures 1 to 8, respectively. Hens fed the 5 Arg assay diets consumed between 574 and 843 mg/d of true digestible Arg, yet there were no responses to consumption of Arg. Table 5 summarizes the requirements for all the responses as determined using the broken-line regression model. Hens fed the graded levels of AA generally responded by increasing the feed consumption in a linear or curvilinear manner (Table 6). Production parameters of hens fed the AA-supplemented basal diet were inferior (P < 0.05) to those observed from hens fed the industry-type control diet, even when the AA diets supplied the assayed AA above its requirement (Table 7). There was no difference in feed consumption of control-fed hens among the 7 AA assays and an overall mean was calculated (84.2 ± 0.5 g/d of feed; mean ± SEM, n = 84).
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The hens were generally in good health in the period between Exp. 1 and 2 and during Exp. 2; there was no mortality. The responses to consumption of Lys, Met, and TSAA are shown in Figures 9 to 11, respectively. Table 8 summarizes the requirements for all the responses as determined using the broken-line regression model. As observed in Exp. 1, hens in Exp. 2 responded to the AA diets by varying feed consumption in a linear or curvilinear matter (Table 9). Production parameters of hens fed the AA-supplemented basal diet were inferior (P < 0.05) to those observed from hens fed the industry type control diet even when the AA diets supplied the assayed AA above its requirement (Table 10). Across the 2 AA assays,
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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AA Requirements
The AA requirements used to calculate the ideal AA profile were determined with the broken-line regression model. This method is considered the best for obtaining the ideal ratios among AA, whereas curvilinear models such as exponential or quadratic curve fitting are better suited to establish the AA requirements for optimal performance (Mack et al., 1999; Baker et al., 2002; Robbins et al., 2006). Typically, the broken-line regression method results in lower AA requirements than when a nonlinear curve fitting is applied to the same data set (Fisher et al., 1973). However, the broken-line regression model has the advantages of a clearly defined breakpoint (i.e., the requirement) at a dietary AA consumption that marginally limits performance, both necessary to determine the ideal AA profile (Mack et al., 1999). Although the absolute AA requirements (mg/d) are reported herein, they are only valid for the particular hens in the particular experimental settings in the present study and should not necessarily be used in commercial settings, in part because they were determined using the broken-line method and not a curvilinear model (Mack et al., 1999; Baker et al., 2002), and in part because egg production of control-fed hens was superior to that of hens fed the AA assay diets.
Among the AA requirements, Lys is especially important, because it is used as the basis for setting the requirements for all other AA in the ideal AA profile (Baker et al., 2002). In Exp. 1, the true digestible Lys requirement, determined using the broken-line regression model, for maximal egg mass and feed utilization was 538 and 693 mg/d, respectively, similar to the requirements (540 and 720 mg/d apparent digestible Lys, respectively) reported by Schutte and Smink (1998) in 24-to 36-wk-old white Leghorn-type hens (Lohmann LSL) using an exponential model and similar to the 700 mg/ d of digestible Lys recommended by the CVB (1996) for optimal feed conversion. The NRC (1994) recommends a total Lys consumption of 690 mg/d to maximize egg yield (egg mass), corresponding to 593 mg/d of true digestible Lys when applying a mean true Lys digestibility of 86% in corn and soybean meal (NRC, 1994). Coon and Zhang (1999) reported digestible Lys requirements of 705 and 636 mg/d for 33- to 49-wk-old and 35- to 47-wk-old laying hens, respectively, to maximize egg mass of white Leghorn-type hens (Hy-Line W-36). In general, the Lys requirement determined in the present study corresponded well with those reported in literature.
Consumption of Arg did not elicit any responses in Exp. 1, indicating that the basal diet supplied sufficient amounts of Arg to meet the requirement of the hens for the responses measured or that Arg was not the first-limiting AA in the assay diets. Coon and Zhang (1999) demonstrated a digestible Arg requirement of 968 and 791 mg/d for 33- to 49-wk-old and 35- to 47-wk-old laying hens, respectively, whereas the NRC (1994) suggests a requirement for total Arg of 700 mg/d based on a model by Hurwitz and Bornstein (1973). The latter value corresponds to 637 mg/d of true digestible Arg when applying a mean true AA digestibility of 91% in corn and soybean meal (NRC, 1994). Jais et al. (1995) suggested an ideal Arg:Lys ratio of 82% for laying hens, corresponding to a true digestible Arg requirement of 441 mg/d for the hens in Exp. 1 (calculated as 82% of the observed Lys requirement of 538 mg/d). In Exp. 1 of the present study, the basal diet provided 0.69% true digestible Arg, resulting in a mean true digestible Arg consumption of 668 mg/d by hens fed the Arg-1 diet, which was lower than that recommended by Coon and Zhang (1999), but greater than that recommended by the NRC (1994) and Jais et al. (1995). If the NRC (1994) requirement of 637 mg/d of true digestible Arg is accepted, then not only was the Arg requirement met by the Arg-1 diet, but Phe became more limiting than Arg in the Arg-1 diet when compared with the NRC (1994) Phe requirement. However, Phe was not limiting in the Arg assay diets if the Arg and Phe requirements suggested by Jais et al. (1995) or Coon and Zhang (1999) are accepted. Yet another potential cause for the lack of response to Arg is a Lys-Arg antagonism, in which high dietary levels of Lys decrease the Arg utilization (Jones, 1964; Austic and Scott, 1975) and, as a result, reduce feed utilization (Mendes et al., 1997; Brake et al., 1998; Kidd and Kerr, 1998) and egg production. However, all the diets were designed with incrementally increasing contents of the nonassayed crystalline AA to avoid AA antagonisms and imbalances. Hence, the Lys content was similar in the Arg-2 and Arg-3 diets (and in the Arg-4 and Arg-5 diets), and, because the Arg content increased in all Arg assay diets, the Arg:Lys ratio (calculated from analyzed values; data not shown) therefore increased from 74 to 78% in the Arg-2 and Arg-3 diets, respectively (and from 71 to 75% in the Arg-4 and Arg-5 diets, respectively). Despite the increases in Arg:Lys ratios in the 2 pairs of Arg diets, feed utilization or egg production did not differ, indicating that a Lys-Arg antagonism did not occur in the present experiment.
The observed requirements for true digestible Met and TSAA to maximize egg mass (253 and 506 mg/d, respectively) were similar to those recommended by the NRC (1994), when taking digestibility into account. However, results of dose-response assays by Schutte et al. (1994), Waldroup and Hellwig (1995), and Coon and Zhang (1999) indicate that the Met and TSAA requirements for laying hens are well above the NRC (1994) levels.
The requirement for true digestible Thr to maximize egg mass observed in Exp. 1 of the present study (414 mg/d) agreed well with the 453 and 462 mg/d of total Thr reported by Ishibashi et al. (1998) and Faria et al. (2002) for white Leghorn-type laying hens and with the 430 mg/d of digestible Thr reported by Coon and Zhang in their Exp. 3 with 35- to 47-wk-old Hy-Line W-36 hens. However, in their Exp. 2, Coon and Zhang (1999) found the digestible Thr requirement to be 560 mg/d in 33- to 49-wk-old Hyline-W-36 hens, considerably greater than in the present study.
The observed requirement for true digestible Trp (120 mg/d) in the present study was similar to the 122 mg/ d of digestible Trp reported by Coon and Zhang (1999) for 33- to 49-wk-old laying hens and the 149 mg/d of total Trp reported by Harms and Russell (2000b) for 30-to 36-wk-old hens. However, the NRC (1994) recommends 160 mg/d of total Trp, greater than that observed in the present study.
The Ile requirements reported by the NRC (1994) (650 mg/d of total Ile), Harms and Russell (2000a) (601 mg/ d of total Ile), and Coon and Zhang (1999) (555 to 603 mg/d of digestible Ile, depending on age) are greater than that observed in the present study (426 mg/d of true digestible Ile). Yet, the Ile requirement from the present study was comparable to the 469 mg/d of total Ile reported by Shivazad et al. (2002) for 37- to 43-wk-old laying hens when applying a mean true Ile digestibility of 91% in corn and soybean meal (NRC, 1994).
Reported Val requirements for white Leghorn-type laying hens are greater than those observed in the present study (501 mg/d of true digestible Val). The NRC (1994) recommends 700 mg/d of total Val, whereas Coon and Zhang (1999) reported that laying hens needed between 646 and 731 mg/d of digestible Val. Harms and Russell (2001) reported a total Val requirement of 619 mg/d for 41- to 47-wk-old Hy-Line W-36 hens.
Egg Production
The observed mean egg production, egg weight, and egg mass of control-fed hens were comparable to those listed in the 2006–2008 W-36 Commercial Management Guide (i.e., 93.6%, 57.8 g, and 54.1 g/d, respectively for 28- to 34-wk-old hens; and 83%, 62.5 g, and 51.9 g/d, respectively for 52- to 58-wk-old hens). However, the observed egg production parameters from hens fed the AA assay diets were generally inferior to those observed for the control-fed hens, even when the AA requirements were met. Although such observations were discussed by Baker et al. (2002) for broiler chickens, other studies of AA requirements for laying hens mentioned previously have not included a positive control diet enabling direct comparisons. The difference in performance between hens fed the control diet and that of hens consuming adequate AA from the assay diets in the present study cannot readily be explained by a difference in feed or (calculated) ME consumption because the consumption of feed and ME was similar between the 2 groups of hens. A careful comparison of the analyzed dietary AA contents with that of the requirements revealed that the assayed AA were first limiting in their respective assay diets (except perhaps Arg, as discussed previously).
The AA-supplemented basal diet was similar in concept to low-CP diets, in which soybean meal is partially replaced by a mixture of corn and crystalline AA, resulting in a relatively low CP content. The N contributed by true digestible essential AA supplied approximately 44% of the total N in the assay diets when the AA requirements were met (data not shown), indicating an essential:nonessential AA ratio of approximately 44%. In broiler chickens and pigs, the optimal essential:nonessential AA ratio is approximately 50% (Deschepper and Groote, 1995; Heger et al., 1998), suggesting that the assay diets supplied sufficiently nonspecific N, but that energy and Gly may have had to be expended excreting the "excess" nonessential AA and giving a possible explanation for the relatively low production of hens fed the assay diets. Dean et al. (2006) demonstrated that low-CP diets for broilers were deficient in Gly+Ser, even when the NRC-recommended dietary levels were met. The NRC (1994) does not list a minimum recommended dietary content of Gly+Ser for laying hens, but recommends between 0.47 and 0.70% total Gly+Ser for immature Leghorn-type chickens depending on their age. However, the basal diet supplied 1.30% total Gly+Ser in Exp. 1, and consumption of these AA were likely not limiting egg production.
The analyzed content of Phe in the basal diet (0.47%) was similar to that recommended by the NRC (1994). However, because the feed consumption was lower in the present study than that estimated by the NRC (1994), the calculated consumption of true digestible Phe was less than that recommended by the NRC (1994). Nevertheless, Phe was not first-limiting when compared with the assayed AA (except perhaps for Arg as discussed previously). Hence, the difference in performance between hens fed the control and assay diets cannot readily be explained by an AA or nonspecific-N deficiency.
Ideal AA Profile
In contrast to the present study, previous estimates of the ideal AA profile for laying hens were based on experiments with different ages and genetic lines of hens, conducted at different times in different environments. Baker (2003) argued that the same basal diet, the same genetic line, and the same assay period should be used in all assays of AA requirements to ensure a valid measurement of the ideal AA profile. In addition, the ideal AA profile should preferably be defined separately for maintenance and production (NRC, 1998), because the relative AA needs partitioned among these are likely to change as the hen matures. However, in this study, the AA profile for maintenance, BW growth, and egg production were combined into one profile for 28- to 34-wk-old hens (Table 11). The ideal AA profile determined in this study indicated that hens need less true digestible Met, Ile, and Val and more true digestible Thr in relation to Lys than that suggested by Coon and Zhang (1999) and that calculated from requirements published by the NRC (1994). However, the ideal Ile:Lys ratio observed in the present study corresponded well with that calculated from AA recommendations by the CVB (1996). The determined ideal AA profile agrees well with the profile calculated from AA recommendations suggested by Leeson and Summers (2005) for 32- to 45-wk-old hens and is similar to that reported by Jais et al. (1995) with the exception of Trp and Val. The ideal Met:Lys and TSAA:Lys ratios in the present study were higher than those reported by the NRC (1994) and the TSAA:Lys ratio of 75% reported by Liu et al. (2005), but agree well with the ratios suggested by the CVB (1996) and by Leeson and Summers (2005) for 32- to 45-wk-old hens. If the lowest Arg consumption observed in Exp. 1 (574 mg/d) is accepted as meeting or exceeding the requirement of the hen for Arg, the ideal Arg:Lys ratio was no greater than 107%, similar to that calculated from the Arg and Lys recommendations by NRC (1994) and CVB (1996), and less than the 130% recommended by Coon and Zhang (1999) (Table 11).
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ACKNOWLEDGMENTS |
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Received for publication October 4, 2007. Accepted for publication December 21, 2007.
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) represents data (n = 60) collected over 6 wk from 1 cage containing 2 hens.
