Assessment of the Nitrogen Correction Factor in Evaluating Metabolizable Energy of Corn and Soybean Meal in Diets for Broilers

Assessment of the Nitrogen Correction Factor in Evaluating Metabolizable Energy of Corn and Soybean Meal in Diets for Broilers G. Lopez and S. Leeson1 Department of Animal and Poultry Science, University of Guelph, Guelph, Ontario, Canada, N1G 2W1 varying inclusion levels of test ingredients in the diet, the extrapolated AME and AMEn of corn were estimated more precisely (R2 = 0.90 to 0.95) than those of SBM (R2 = 0.57 to 0.85), suggesting that the variability of AME and AMEn is better explained by a linear regression of AME or AMEn on percentage of inclusion. For corn, AME and AMEn were little affected by age, and the effect of N correction was consistent at around 3%. Determined energy values of SBM were more variable. Experiment 3 was conducted to assess the effect of formulating diets based on either AME or AMEn on broiler performance. A 2-sample t-test was implemented examining AME vs. AMEn formulation. The analyses for numerous production and carcass traits were nonsignificant except for the case of less abdominal fat in birds fed diets formulated to AME rather than AMEn (P < 0.01). These results showed that the use of the N correction imposed a penalty to corn of 3 to 5% and SBM of 7 to 12%.

Key words: metabolizable energy, ingredient, broiler 2008 Poultry Science 87:298–306 doi:10.3382/ps.2007-00276

corrected for N retention (AMEn) to convert all data to a basis of N equilibrium for comparative purposes. In the case of growing broilers, correction for N retention accommodates the effect of differential growth rate inherent across birds in any assay. Correction for N retention is expected to greatly influence the ME of ingredients such as soybean meal (SBM) compared with corn (Dale and Fuller, 1984) because of associated higher protein accretion. A similar effect is expected from diets high in CP, such as broiler starter vs. finisher diets. The effect of the N correction has also been used to decrease the variability of estimates of ME of ingredients varying in protein content (Leeson et al., 1977). However, the argument against the N correction is that it is not a biological norm, especially for modern broilers with large feed intakes, fast growth, and the associated accretion of protein and fat. On the other hand, N correction allows strain comparison and accommodates any age-related effects. Because broiler nutrition is now very specialized, the need for cross-strain comparison is less relevant, although within poultry, age and strain may still be influential on AME (Mollah et al., 1983; Ha¨rtel, 1986; Bourdillon et al., 1990b; Carre´ et al., 1995; Farrell et al., 1997).

INTRODUCTION Energy systems are used to predict values of feeds relative to energy requirements for maintenance and production. In poultry, AME has been commonly accepted and extensively used to define energy values of feedstuffs and diets. Several procedures have been used to measure the ME content of individual ingredients (Hill and Anderson, 1958; Potter et al., 1960; Sibbald and Slinger, 1963; Lockhart et al., 1966; Miller, 1974; Farrell, 1978; Bourdillon et al., 1990a; Farrell et al., 1991; McNab, 2000). In these studies, both nutrient balance and palatability determine the maximum inclusion of an ingredient within assay diets. Differences in the AME values assigned to different ingredients are caused by differences in their chemical composition as well as factors such as bird age and strain. The AME values of ingredients (or diets) are commonly

©2008 Poultry Science Association Inc. Received July 5, 2007. Accepted October 22, 2007. 1 Corresponding author: [email protected]

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ABSTRACT Three experiments were carried out to determine AME and AMEn of corn and soybean meal (SMB) in diets for growing broilers. In experiments 1 and 2, ingredient-specific basal diets or a combination of these basal diets with corn and SBM were prepared. For corn, the substitution was 25, 50, or 75% of the total diet, whereas SBM substitution was at 10, 20, or 30%. In experiment 1, birds were fed the experimental diets continuously from 0 to 33 d, and AME and AMEn were determined during 9 to 12 d and 30 to 33 d of age. In experiment 2, birds were fed the experimental diets only around the time of the collection period. The AMEn of corn was 95 to 97% of corresponding AME, whereas for SBM, AMEn was 93 to 88% of AME. Linear regression was used as an alternative method of calculating ingredient energy values resulting in a significant regression of diet AME and AMEn content on inclusion level, for each period of time and for each ingredient (corn and SBM). Based on

299

CORN AND SOYBEAN FOR SOYBEAN MEAL 1

Table 1. Ingredients and diet composition of experiments 1 and 2 (g/kg) Ingredients

C1

S1

CB

C25

C50

C75

SB

S10

S20

S30

615 304 13.5 0.0 15 35 4.0 10.0 1.0 2.5

531 414 — 52 — — — — 2.080 — 999.08

639 259 — 42 — 54 — — 1.78 4.50 1,000.28

957 — — 15 — 14 — 4.0 10.0 — — 1,000

707 250 — 15 — 14 — 4.0 10.0 — — 1,000

457 500 — 15 — 14 — 4.0 10.0 — — 1,000

207 750 — 15 — 14 — 4.0 10.0 — — 1,000

955 — — 16 — 15 — 4.0 10.0 — — 1,000

855 — 100 16 — 15 — 4.0 10.0 — — 1,000

755 — 200 16 — 15 — 4.0 10.0 — — 1,000

655 — 300 16 — 15 — 4.0 10.0 — — 1,000

2,809 24.0 0.56 1.33 0.95 0.42 0.18

2,913 19.9 0.46 1.05 0.92 0.41 0.18

3,016 15.7 0.36 0.77 0.89 0.40 0.18

3,120 11.6 0.26 0.48 0.86 0.38 0.18

3,200 18.0 0.45 1.25 0.96 0.42 0.18

3,108 20.9 0.47 1.41 0.98 0.43 0.18

3,017 23.8 0.49 1.58 0.99 0.44 0.18

2,926 26.7 0.51 1.74 1.0 0.45 0.18

3,100 20.1 0.41 1.25 0.95 0.42 0.18

1 Starter = standard starter diet; C1 = corn 1; S1 = soybean meal (SBM) 1; CB = corn basal; C25 = 25% corn inclusion; C50 = 50% corn inclusion; C75 = 75% corn inclusion; SB = SBM basal; S10 = 10% SBM inclusion; S20 = 20% SBM inclusion; S30 = 30% SBM inclusion. 2 Cl diet, Cl as an ingredient. Sl diet, Sl as an ingredient. 3 Provided the following per kilogram of diet: vitamin A, 8,800 IU; vitamin D3, 3,300 IU; vitamin E, 40 IU; vitamin K3, 3.3 mg; thiamine, 4.0 mg; riboflavin, 8.0 mg; pantothenic acid, 15.0 mg; niacin, 50 mg; pyridoxine, 3.3 mg; choline, 600 mg; folic acid, 1.0 mg; biotin, 220 ␮g; vitamin B12, 12 ␮g; antioxidant, 120 mg; Mn, 70 mg; Zn, 70 mg; Cu, 10 mg; Fe, 60 mg; I, 1.0; and Se, 0.3 mg.

For modern broilers, the energy deposited as protein is a biological norm, and so the N correction system heavily penalizes dietary protein from ingredients such as SBM, because the system assumes zero protein retention, inferring that all protein is used as a source of energy. The

question therefore arises as to whether AME is more relevant than AMEn within the confines of just commercial broiler nutrition and that AME values more equitably describe energy available from protein concentrates vs. cereals. The objective of these studies was to determinate

Table 2. Diet composition of experiment 3 (g/kg) Starter Ingredients Corn, 8.5% CP Soybean, 48% CP Limestone, 38% Ca Dicalcium phosphate, 20% P Animal-vegetable fat Salt Premix1 DL-Met L-Lys HCl Stafac Coban Total Calculated analysis AME (kcal/kg) AMEn (kcal/kg) CP (%) Met (%) Met + Cys (%) Lys (%) Ca (%) Available P (%) Na (%)

Grower

AME

AMEn

AME

AMEn

618.1 325.7 16.50 11.00 11.22 4.00 10.00 2.06 0.43 0.50 0.50 1,000

590.28 330.91 16.50 11.00 33.91 4.00 10.00 2.06 0.31 0.50 0.50 1,000

640.89 281.22 15.68 9.63 35.78 4.00 10.00 1.87 0.00 0.50 0.50 1,000

610.81 286.47 15.68 9.72 60.55 4.00 10.0 1.87 0.00 0.50 0.50 1,000

3,100 (2,965)2 21.00 0.55 0.78 1.20 0.95 0.42 0.20

(3,215) 3,100 21.00 0.55 0.78 1.20 0.95 0.42 0.20

3,250 (3,144) 19.00 0.51 0.71 1.03 0.88 0.38 0.20

(3,394) 3,250 19.00 0.51 0.71 1.03 0.88 0.38 0.20

1 Provided the following per kilogram of diet: vitamin A, 8,800 IU; vitamin D3, 3,300 IU; vitamin E, 40 IU; vitamin K3, 3.3 mg; thiamine, 4.0 mg; riboflavin, 8.0 mg; pantothenic acid, 15.0 mg; niacin, 50 mg; pyridoxine, 3.3 mg; choline, 600 mg; folic acid, 1.0 mg; biotin, 220 ␮g; vitamin B12, 12 ␮g; antioxidant, 120 mg; Mn, 70 mg; Zn, 70 mg; Cu, 10 mg; Fe, 60 mg; I, 1.0 mg; and Se, 0.3 mg. 2 For comparison.

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C1 or S12 Corn, 8.5% CP Soybean, 48% CP Limestone, 38% Ca Oats, 11% CP Dicalcium phosphate, 20% P Animal-vegetable fat Salt Premix3 DL-Met L-Lys HCl Total Calculated analysis AMEn/kcal CP (%) Met (%) Lys (%) Ca (%) Available P (%) Na (%)

Starter

300

LOPEZ AND LEESON Table 3. Effect of inclusion level of corn and soybean meal (SBM) on their AME and AMEn values in experiment 1 Period 9 to 12 d 30 to 33 d SEM

Corn (%)

AME (kcal/kg)

AMEn (kcal/kg)

AMEn/ AME

SBM (%)

25 50 75 25 50 75

3,485 3,384 3,461 3,509 3,450 3,489 32.4

3,459 3,245 3,378 3,398 3,287 3,383 56.0

0.99 0.95 0.97 0.96 0.95 0.96

10 20 30 10 20 30

AME

AMEn

2,575 2,307 2,172 2,208 2,474 2,662 151.1

1,998 2,037 1,962 1,986 2,286 2,477 174.5

AMEn/ AME 0.77 0.88 0.90 0.89 0.92 0.93

1 For both corn and SBM, AME and AMEn contents were not influenced (P > 0.05) by inclusion level or bird age.

MATERIALS AND METHODS Two experiments were designed to determine AME and AMEn of corn and SBM in growing broilers, fed continuously from 0 to 33 d (experiment 1) or fed at 2 different periods of time (9 to 12 d and 30 to 33 d, experiment 2), whereas experiment 3 utilized these results to study broiler response to diets with corn and SBM formulated to AME or AMEn.

Experiment 1 Two hundred eighty-eight 1-d-old male broiler chicks of a commercial cross (Hubbard × Hubbard) were randomly allocated to 1 of 8 diets in a completely randomized design. Birds were placed in groups of 6 in 48 growing cages (50 × 60 cm) at the University of Guelph Arkell Research Center facilities, which are managed and cared for according to guidelines established by the University of Guelph Animal Care Committee. The 8 treatments (4 for each of corn and SBM) were each replicated 6 times, with each replication consisting of a cage (50 × 60 cm) group of from 1 to 6 broilers. There were 6 broilers per

Figure 1. Experiment 1: 95% confidence interval for the expected mean of AME and AMEn at different levels of corn substitution in broiler diets (period 9 to 12 d).

cage during the 9 to 12-d excreta collection, and numbers reduced to 1 bird per cage during collection from 30 to 33 d. Room temperature was maintained at 32°C from 0 to 5 d and gradually reduced according to normal brooding practices to 22°C with 24 h/d of lighting. Water and feed were available ad libitum. Treatments were a corn or SBM basal diet (CB or SB, respectively, Table1) or a combination of these basals with corn or SBM. For corn, the substitution was at 25, 50 or 75% using the C1 basal (Table 1), whereas for SBM, substitution was at 10, 20 or 30% using the S1 basal (Table 1). In this experiment, birds were continuously fed the experimental diets from 0 to 33 d. Energy and N balance were determined from 9 to 12 d and from 30 to 33 d of age. For each period, the reported values are the corresponding response variables based on 3-d data, whereas the value of the predictor variable was set as the average day for each collection period. Total excreta were collected over the 3-d periods, being captured on aluminum trays directly beneath each cage. During the collection period, pooled 3-d excreta samples were wrapped in aluminum foil and then oven-dried (Hotpac, Waterloo, Ontario, Canada) at 45°C to constant weight and then ground to a consistent particle size (Willey Co., Chicago, IL). Feed samples were ground using a commercial food blender. Samples of the diets and excreta were assayed for gross energy (GE) by C5003 IKA adiabatic oxygen bomb calorimeter (GMBIT and Co., Dresden, Germany). Diets and excreta were assayed for total N

Figure 2. Experiment 1: 95% confidence interval for the expected mean of AME and AMEn at different levels of soybean meal (SBM) substitution in broiler diets (period 9 to 12 d).

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AME and AMEn of corn and SBM for growing broilers and then to assess the effectiveness of these values in formulating broiler diets.

301

CORN AND SOYBEAN FOR SOYBEAN MEAL Table 4. Effect of inclusion level of corn and soybean meal (SBM) on their AME and AMEn values in experiment 2 Period 9 to 12 d 30 to 33 d

Corn (%) 25 50 75 25 50 75

SEM

AME (kcal/kg)

AMEn (kcal/kg)

AMEn/ AME

SBM (%)

AME

AMEn

3,352 3,283 3,362 3,453 3,322 3,347 43.3

3,372 3,162 3,313 3,450 3,232 3,258 45.8

1.01 0.96 0.98 0.99 0.97 0.97

10 20 30 10 20 30

2,478 2,568 2,352 2,353 2,648 2,527 154.0

2,171 2,383 2,186 2,180 2,327 2,170 137.0

AMEn/ AME 0.88 0.93 0.93 0.93 0.88 0.86

1 For both corn and SBM, AME and AMEn contents were not influenced (P > 0.05) by inclusion level or bird age.

Experiment 2 Growth was below normal in experiment 1, likely due to the fact that the higher levels of corn and SBM substitution resulted in imbalanced diets relative to requirements over a prolonged period of time. For this reason, experiment 2 was modified such that the experimental diets were only fed around the times of the balance periods. Three hundred thirty-six 1-d-old male broiler chicks of a commercial cross (Hubbard × Hubbard) were randomly

allocated to 1 of the 48 growing cages (50 × 60 cm) and fed a standard broiler starter diet (Table 1). Birds were housed and managed as described in experiment 1. At 7 and 28 d, birds were randomly allocated to 1 of the 4 diets (Table 1) in a completely randomized design, allowing birds at least 2 d adaptation to each diet before any excreta collection for energy balance studies. The 4 treatments were each replicated 6 times, with each replication consisting of a cage (50 × 60 cm) group of from 2 to 5 broilers. There were 5 broilers per cage during the excreta collection from 9 to 12 d and 2 birds per cage during collection from 30 to 33 d. Birds were also weighed at the beginning and end of each collection period. Treatments were a corn or SBM basal diet (Table 1) or a combination of these basal diets with corn or SBM. Corn and SBM substitution levels and diets were as described for experiment 1. Bird management and measurements and chemical analyses were performed as described in experiment 1.

Table 5. AME and AMEn for corn and soybean meal (SBM) determined by regression analyses in experiment 1 Period

R2

β0

β1

SE β1

Corn1

9 to 12 d AME

SE β0

0.95

2,856***

5.850***

13.077

0.279

0.92

2,669***

6.751***

19.364

0.414

AME

0.94

3,014***

4.648***

11.814

0.252

AMEn

0.92

2,854***

5.051***

14.615

0.312

AMEn 30 to 33 d

9 to 12 d

SBM 0.78

3,261***

−10.836***

22.930

1.225

0.77

3,115***

−11.386***

24.638

1.317

AME

0.57

3,389***

−7.414***

25.442

1.359

AMEn

0.60

3,236***

−7.715***

25.096

1.341

AME AMEn 30 to 33 d

1

At 100% inclusion2 3,442 ± 19.14 (3,401, 3,481) 3,345 ± 28.34 (3,280, 3,403) 3,479 ± 17.29 (3,443, 3,514) 3,359 ± 21.39 (3,315, 3,403) 2,177 ± 105.07 (1,960, 2,395) 1,976 ± 112.90 (1,742, 2,210) 2,648 ± 116.58 (2,406, 2,889) 2,465 ± 114.97 (2,226, 2,703)

Model linear in X; β0, β1 = regression coefficients; SE β0, SE β1 = SE. 95% confidence interval for the expected mean of the response variable AME and AMEn for different levels of corn and SBM substitution at 9 to 12 d and 30 to 33 d. Each entry consists of the estimated mean ± its SE and the corresponding interval. ***P < 0.001. 2

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using a Leco FP-428 Nitrogen Analyser (Leco Instuments, Stockport, Cheshire, UK). Nitrogen retention was calculated as the difference between N intake and N in the excreta. The AMEn values were calculated by subtracting GE excreted (adjusted to zero N balance) from GE intake and dividing this value by DM feed intake. For correction to zero N retention, a value of 8.22 kcal/g of N retained was used (Hill and Anderson, 1958).

302

LOPEZ AND LEESON

Experiment 3

Statistical Analyses Experiment 1 and 2. Diet AME values were uncorrected, whereas diet AMEn values were corrected to zero N retention using a value of 8.22 kcal/g of N retained, with ingredient values determined according to the equation: AME =

⎛ AME test diet ⎜ ⎝ % substitution

± AME basal ⎞ ⎟ + AME basal. test ingredient⎠

Treatments means were tested by ANOVA, and response variables were further analyzed using Tukey test. The statistical model used was: Yij = ␮ + τi + εij, where Yij = the average of AME or AMEn, obtained from cage j under treatment i, i = 1,...6, j = 1,...6; ␮ = general mean; τi = effect of the ith treatment, i = 1,...6, on the

Yi = β0 + β1Xi1 + εi, where Yi = AME (or AMEn) measured in kilocalories per kilogram on the ith sample; β0, β1 = regression coefficients; Xi = percentage of ingredient (corn or soybean); εi = random variable assumed to be normally distributed with mean 0 and variance σ2. The SAS GLM procedure (SAS Institute, 2000) was used to fit the regression models and PROC. To obtain a 95% confidence interval for the expected mean of the response variable AME or AMEn given a fixed level of ingredient substitution, the following equation was used. ⎡

⎛ ⎞⎤

α ␮ˆ y/x = (βˆ 0 + βˆ 1 x) ± t ⎢n − 2,1 − ⎜ ⎟ ⎥ (SE␮ˆ Y/X), ⎣ ⎝2⎠ ⎦ where βˆ 0 and βˆ 1 = the point estimates; x = the value of ⎡ ⎛ α⎞ ⎤ ⎛ α⎞ the ingredient substitution; t⎢n − 2,1 − ⎜ ⎟⎥ = the 1 − ⎜ ⎟ ⎣ ⎝ 2 ⎠⎦ ⎝2⎠ quantile of the tn − 2 distribution; and SE␮ˆ Y/X= the SE of the mean of y given x, which is obtained as: ⎧ ⎡ 1 (x − X)2⎤ ⎫1/2 ⎥⎬ SE␮ˆ Y/X = ⎨S2 ⎢ + Sxx ⎦ ⎭ ⎩ ⎣n where S2 = the residual MS error from the ANOVA table; n = number of observations; x = percentage of inclusion level under study; X = mean percentage of inclusion level; and Sxx = corrected sum of squares of the test ingredient level (CB, C25, C50, C75, SB, S10, S20, and S30). Experiment 3. To test whether the means of groups 1 and 2 can be considered statistically different, a t-test was performed. The hypothesis of interest is: Ho: ␮1 = ␮2 vs. Ha: ␮1 ≠ ␮2, where the observations from group 1 are Y11,...Y1n and those from group 2 are Y21,...Y2n. The test statistic is:

t=

X 1 − X2 Sp

√

,

1 1 + n1 n2

where X1 and X2 = the sample average, and the pooled SD, Sp, is defined as

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This experiment was designed to evaluate broiler performance when fed diets formulated using either AME or AMEn values for corn and SBM as determined in the previous experiments. Four hundred twenty broiler males (Ross × Ross) were weighed and randomly allocated to 1 of the 2 diet treatment groups based on the system of energy evaluation, namely AME and AMEn. Each treatment group was replicated 6 times, with each replication consisting of a floor pen (2.44 × 1.83 m) containing 35 birds. Room temperature was maintained at 32°C from 0 to 5 d and was gradually reduced according to normal brooding practices to 22°C with 23 h/d of lighting. From 0 to 21 d, birds were fed 1 of 2 starter diets (AME or AMEn, Table 2). Using AME and AMEn values for corn and SBM (experiment 2), both diets were formulated to be isoenergetic and contain similar levels of protein, amino acids, and other major nutrients (3,100 kcal of ME/kg and 21% CP, Table 2). As shown in Table 2, formulation to AME, rather that AMEn, results in less supplemented fat, because the energy value of SBM is increased with this system of evaluation. On d 21, birds were offered corresponding AME and AMEn grower diets providing 3,250 kcal of ME/kg and 19% CP to 42 d (Table 2). All birds were weighed individually at 21 and 42 d of age, and feed intake was measured over these times. Birds were allowed free access to water and feed. Mortality was recorded daily throughout the experiment. At 42 d of age, 10 birds were randomly selected from each pen for processing at the University’s plant. After processing, the abdominal fat pad was removed and weighed (Griffiths et al., 1977). Remaining carcasses were chilled in water at 4°C for 1 h, then weighed, and the breast meat on both left and right side of the carcass, consisting of pectoralis and supracoracoideus muscles, was removed and weighed.

response variable; and εij = random variable with mean 0, variance σ2 independent and normally distributed. To look for significant differences between a pair of means, the Tukey test was used at the 5% level of significance. Because the treatment factor is quantitative, to study the effect of the percentage of inclusion of corn and SBM on AME or AMEn, a linear regression analysis was performed. For each combination of period of time and ingredient (corn or SBM), a separate regression analysis was conducted. The statistical model used was:

303

CORN AND SOYBEAN FOR SOYBEAN MEAL Table 6. AME and AMEn for corn and soybean meal (SBM) determined by regression analyses in experiment 2 R2

Period

β0

β1

SE β0

SE β1

Corn1

9 to 12 d AME

0.91

2,889***

4.526***

14.13

0.302

0.90

2,735***

5.393***

17.44

0.370

AME

0.90

2,891***

4.496***

14.33

0.306

AMEn

0.90

2,717***

5.532***

18.01

0.385

AMEn 30 to 33 d

9 to 12 d

SBM 3,260***

−8.681***

18.749

1.002

0.79

3,116***

−8.859***

18.196

0.972

AME

0.79

3,194***

−6.303***

12.966

0.693

AMEn

0.85

3,066***

−8.628***

14.392

0.769

AMEn 30 to 33 d

3,341 ± 20.69 (3,298, 3,384) 3,275 ± 25.53 (3,222, 3,328) 3,340 ± 43.52 (3,296, 3,383) 3,269 ± 54.69 (3,251, 3,324) 2,394 ± 85.92 (2,216, 2,572) 2,230 ± 83.40 (2,057, 2,403) 2,564 ± 59.41 (2,441, 2,687) 2,203 ± 65.94 (2,067, 2,340)

1

Model linear in X; β0, β1 = regression coefficients; SE β0, SE β1 = SE. 95% confidence interval for the expected mean of the response variable AME and AMEn for different levels of corn and soybean meal substitution at 9 to 12 d and 30 to 33 d. Each entry consists of the estimated mean ± its SE and the corresponding interval. ***P < 0.001. 2

Sp =

√

(n1 − 1)s21 + (n2 − 1)s22 , n 1 + n2 − 2

where n1 and n2 = the sample sizes of groups 1 and 2, respectively. The test was applied to each of the variables of interest (e.g., BW, feed intake, feed conversion, BW gain, abdominal fat, carcass weight, and breast weight).

RESULTS Energy values determined by the substitution method are given in Tables 3 and 4 for experiment 1 and 2, respectively. The AME and AMEn of corn were very consistent and not affected by the level of substitution or bird age. Method of feeding the diets (experiment 1 vs. 2) also resulted in similar overall values. The AMEn of corn was around 95 to 99% of AME, implying a N correction penalty of 1 to 5%. The energy values for SBM were much more variable. The energy values for SBM were more consistent in experiment 2 (Table 4, P > 0.05) where the imbalanced diets were not fed for the duration of the trial, rather being introduced only at time of collection. For SBM, the N correction caused a 7 to 12% decline in available energy. An alternative method of calculating ingredient energy values is to use linear regression analyses, comparing energy values of individual diets to the corresponding inclusion level of the ingredient. Extrapolation of energy to the equivalency of 100% inclusion predicts the energy value of the ingredient (Figures 1 and 2). These results for experiments 1 and 2 are shown in Tables 5 and 6, respectively. From these tables, it can be seen that the estimated models are significant (P < 0.001). Corn

AME and AMEn are little affected by bird age, and the effect of N correction is consistent at around 3%. Results for the energy values of SBM are again more variable. In experiment 1, there was an increase in both AME and AMEn with age (Table 5). In experiment 2, values for SBM are less consistently affected by bird age (Table 6). In both experiments 1 and 2, the variance of SBM energy values was almost double that of corresponding corn values. Performance of broilers fed diets formulated using either AME or AMEn of corn and SBM values is shown in Table 7. Results were very similar for birds fed the 2 test diets, and only for abdominal fat pad size at 42 d was there a significant (P < 0.01) difference. Abdominal fat pad was smaller in birds fed diets formulated to AME rather that AMEn.

DISCUSSION Metabolizable energy is the standard used in describing requirements and formulating diets in the poultry industry. Metabolizable energy has been used extensively to compare energy values of feedstuffs (or diets), and many attempts have been made to increase the precision of the ME evaluation for each of the many types of ingredients available in the industry. However, ME is a function of both the diet and the animal, so variability of ME values are influenced not only by methodology (Sibbald, 1989; Farrell et al., 1991) but also bird age (Lessire and Leclercq, 1982; Sibbald, 1982) and species and strain (Leeson, 1974; Spratt and Leeson, 1987; Bourdillon et al, 1990b; our unpublished data). Several methods have been developed to evaluate ME of individual ingredients (Hill and Anderson, 1958; Potter et al., 1960; Sibbald and Slinger, 1963; Lockhart et al.,

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0.77

AME

At 100% inclusion1

304

LOPEZ AND LEESON Table 7. Broiler performance when fed diets formulated using AME or AMEn values for corn and soybean meal (SBM) 0 to 21 d Period

2

BW (g) AMEn AME FI AMEn AME FC AMEn AME DBWG3 AMEn AME

Mean

SE

856 846

6.27 3.90

1,106 1,104

5.31 9.75

1.38 1.39 38.6 38.1

Difference

t

Significance

10

1.40

NS2

2

0.16

NS

0.01 0.01

−0.01

−0.82

NS

0.29 0.18

0.5

1.40

NS

1,763 1,754

20.44 23.97

9

0.61

NS

3,249 3,268

19.67 44.48

−19

−0.39

NS

0.02 0.02

−0.01

−0.56

NS

83.9 82.5

0.97 1.14

0.4

0.28

NS

57.6 50.4

0.95 1.50

7.2

4.03

**

1.88 1.90

1,996 1,985

10.28 25.99

11

0.38

NS

475 492

4.92 7.71

−17

−1.91

NS

0.61

NS

−1.08

NS

0 to 42 d FBW AMEn AME FFC AMEn AME

2,619 2,600 1.67 1.69

19.71 24.68 0.01 0.01

19 −0.02

1 FI = feed intake (g); FC = feed conversion; DBWG = daily BW gain (g/bird per d); AF = abdominal fat (g); CWT = carcass weight (g); BWT = breast weight (g); FBW = final BW (g); FFC = final feed conversion. 2 NS = no significant difference (P > 0.05). 3 Obtained from difference of final BW at 21 d and their initial BW at 0 d. 4 Obtained from difference of final BW at 42 d and their initial BW at 21 d. **P < 0.01.

1967; Miller, 1974; Sibbald, 1976; Farrell, 1978; Farrell et al., 1991). These methods vary in their procedures, the simplest being feeding only the test ingredient (Lockhart et al., 1967). However, in most cases, procedures are based on substituting ingredients into a basal diet (Miller, 1974). Usually, nutrient balance and palatability are factors dictating maximum inclusion level. The use of such substitution inevitably includes an additional confounding factor in that diets can be potentially imbalanced as occurred in experiment 1. It has been proposed that values derived using this approach may not be a true representation of actual values within balanced diets as used commercially (Miller, 1974; Sibbald, 1982). However, there is little information on the ability of the bird to derive energy from such unbalanced diets, for either short or prolonged peri-

ods of time during an assay. For example, in the case of proteins, it is expected that imbalanced amino acids relative to requirements imply an energetic cost related to its excretion (Van Milgen et al., 2001) in addition to a diet-induced protein turnover (Kita et al., 1993), which increases the energy cost to the animal and concomitantly lowers the energy value of the protein source. The ME values of ingredients and diets are commonly corrected for N retention to convert all data to a basis of N equilibrium for comparative purposes. Because N retention differs with bird age, type of birds, and possibly strain, a correction factor is essential if comparisons of ME values for the same ingredient are to be made (Leeson et al., 1977). Species comparison (and their age-related cofactors) has been the main argument in favor of the use

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21 to 42 d BW AMEn AME FI AMEn AME FC AMEn AME DBWG4 AMEn AME AF AMEn AME CWT AMEn AME BWT AMEn AME

CORN AND SOYBEAN FOR SOYBEAN MEAL

greater available energy in birds fed to AMEn, because there was greater fat accumulation. Information from this experiment indicates that within the confines of commercial broiler nutrition, the uncorrected AME values determined with broilers describe more equitably the energy available from cereals and protein concentrates. From a practical commercial point of view, the study shows that birds fed a diet formulated to similar AME and AMEn contents performed equally. However, because feed price is around $10/ton less with formulation to AME contents, the effect of dietary energy evaluation requires further study. If these results are confirmed, then data suggest that the energy value of SBM is currently being undervalued.

ACKNOWLEDGMENTS This work was supported by the Ontario Ministry of Agriculture Food and Rural Affairs, Guelph, Ontario, Canada.

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of N correction. This concept is of questionable value for broilers (our unpublished data), because species-age comparison is rarely critical for broiler nutritionists, and broilers are considered to be relatively uniform in protein accretion over time. Obviously, methodology influences AME and AMEn evaluations. Within the same experiment, the use of the substitution method, which is based on only 6 observations, showed large variability in values (Tables 2 and 3). On the other hand, the results obtained by the regression analysis (extrapolation to 100% inclusion), which used 24 observations, seems a more reliable estimator based on reduction in the SEM. These results show that the SE of the predicted value of AME or AMEn for SBM is noticeably higher than the value for corn. This difference is due to the fact that with SBM, lower inclusion levels necessarily mean a greater degree of extraplation and the fact that SE diverges the further the prediction is away from actual analyses levels. This indicates that the precision of the ME value obtained is dependent on the proportion of the test ingredient substituted. Leeson et al. (1977) explained the need for high proportions of the test ingredients in the test diets and that this phenomenon explains some of the variability associated with determination of ME of fats, for example, where inclusion level is necessarily low (Miller, 1974). There is evidence that AME and AMEn values vary according to the age of the birds used in the assay. Results from our current data indicate that AME and AMEn are influenced by bird age, and this is especially so for SBM. In experiment 2, the variability in correction for N retention with SBM decreases as birds get older, agreeing with Leeson et al. (1977), who used N correction to decease the variability of estimates of ME in which ingredients vary in protein content. Nitrogen retention is expected to have a greater influence on the ME of ingredients such as SBM compared with corn because of expected higher protein accretion (Dale and Fuller, 1984). For modern broilers, the use of the N correction will penalize the biological characteristic of depositing ME as protein during growth, again especially with high-protein ingredients such as SBM. Corn and SBM are considered the 2 major ingredients in formulation for broiler diets. Due to its chemical composition, corn is considered a primary source of energy (Cowieson, 2005), whereas SBM is considered a protein (or amino acid) source, with lower ME contribution (Coon et al., 1990; Dale, 2000). These 2 ingredients together constitute approximately 85 to 90% of the total energy content in a typical broiler diet (Leeson and Summers, 2005). Broilers fed diets formulated using AME or AMEn values for corn and SBM grew at remarkably similar rates. Feed efficiency was also comparable, which is an indication that diets were of comparable energy value or that the sensitivity of the response criteria was insufficient to detect diet differences. On this basis, formulating to AME values within starter and grower diets is more economical. It would be interesting to conduct further studies to measure the NE of these diets. There is an indication of

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