Prececal amino acid digestibility of soybean cake in fast- and slow-growing broiler chickens

Prececal amino acid digestibility of soybean cake in fastand slow-growing broiler chickens C. Ganzer,∗ W. Siegert,†,1 H. Kluth,∗ J. Bennewitz,† and M. Rodehutscord† ∗

Institut f¨ ur Agrar- und Ern¨ ahrungswissenschaften, Martin-Luther-Universit¨ at Halle-Wittenberg, 06120 Halle ur Nutztierwissenschaften, Universit¨ at Hohenheim, 70599 Stuttgart, Germany (Saale), Germany; and † Institut f¨ colonic junction. The prececal amino acid digestibility of soybean cake was calculated by linear regression simultaneously for both strains. There was no significant interaction between broiler strain and inclusion level of soybean cake with respect to the prececal CP and amino acid digestibility of complete diets; there was a significant strain effect for 5 out of the 16 measured amino acids. The prececal CP and amino acid digestibility of soybean cake did not differ significantly between strains and was numerically almost identical. The results of the present study provide evidence of the transferability between broiler strains of prececal amino acid digestibility data, determined using the regression approach, thus improving the accuracy of diet formulation without drawbacks.

ABSTRACT The objective of the present study was to determine whether there are differences in prececal amino acid digestibility between commonly used slow- and fast-growing broiler strains when the regression approach is applied. ISA J-275 and Ross 308 were selected as common representatives of slow- and fastgrowing broiler strains, respectively. The experimental diets with soybean cake at levels of 0, 100, and 200 g/kg were offered for ad libitum consumption between 22 and 29 d post-hatch. Titanium dioxide was used as an indigestible marker. Each treatment was tested with six pens comprising 10 birds each. Digesta samples were collected on a pen basis from the distal two-thirds of the intestine section between Meckel’s diverticulum and 2 cm anterior to the ileocecal-

Key words: broiler strain, amino acid digestibility, soybean cake, regression approach 2017 Poultry Science 0:1–7 http://dx.doi.org/10.3382/ps/pex090

INTRODUCTION

there is an increasing proportion of slow-growing broiler strains with increasing relevance in organic poultry meat production in some regions such as North America (USDA-NASS, 2016) and Europe (Eurostat, 2016). Differences in the digestive capacity among strains appear possible owing to differences in their digestive physiology. The digestibility of AA might be influenced, for instance, by a genetic variation in AA uptake, jejunal and ileal villus areas, intestine length, mucosal properties, and digestive enzyme activity (Mitchell and Smith, 1990, 1991; O’Sullivan et al., 1992; Uni et al., 1995). Some studies have found marginal to considerable differences in apparent pc AA digestibility among unspecified broiler strains (Ravindran et al., 1999; Al-Marzooqi et al., 2010, 2011; Kim and Corzo, 2012). However, basal endogenous AA losses are not excluded when the apparent AA digestibility is determined (Adedokun et al., 2011). There is a general agreement that exclusion of basal endogenous AA losses from pc AA digestibility increases the accuracy of feed formulation (Adedokun et al., 2011). The regression approach, for instance, enables the simultaneous determination of pc AA digestibly and the exclusion of basal endogenous AA losses (Rodehutscord et al., 2004). The objective of the present study was, therefore, to investigate whether there are differences in pc AA

An adequate supply of amino acid (AA) to animals can be achieved by formulating feed based on the contribution of prececal (pc) AA digestibility from each feed ingredient. This is advantageous because reducing the CP level in broiler diets without performance loss has been shown to be effective in decreasing environmental impacts caused by broiler meat production. When feed is formulated, detailed knowledge of the nutritive value of the feed ingredients for the targeted species is essential to prevent malnutrition of animals. These advantages are offset by the necessity of conducting digestibility experiments involving animals, the related public resentment, and the high effort in terms of time and cost. The number of required digestibility experiments depends, among other factors, on transferability between avian species. The pc AA digestibility, for instance, was found to be different between broilers, turkeys, and Pekin ducks (Kluth and Rodehutscord, 2006). Differences might also occur among broiler strains. This aspect is gaining importance since  C 2017 Poultry Science Association Inc. Received December 13, 2016. Accepted March 22, 2017. 1 Corresponding author: [email protected]

1

2

GANZER ET AL. Table 1. Composition of the experimental diets containing different levels of soybean cake (g/kg). Diet Corn Solvent-extracted soybean meal Wheat gluten Soybean oil Monocalcium phosphate Premix1 TiO2 NaCl l-Lys·HCl l-Arg dl-Met l-Val l-Thr l-Ile l-Trp Cornstarch Soybean cake2

I

II

200 0

480 100 100 50 25 10 5 3 7 4.3 3.5 3.5 2.5 2.0 0.2 100 100

Table 2. Analyzed concentrations of proximate nutrients, gross energy, and amino acids in the experimental diets and in soybean cake (g/kg DM unless otherwise stated).

III

0 200

1 BASU Mineralfutter GmbH, Bad Sulza, Germany; provided per kg of diet: 2.4 g Ca, 0.7 g Mg, 60 mg Mn, 50 mg Zn, 60 mg Fe, 5 mg Cu, 12,000 IU vitamin A, 3,000 IU vitamin D3, 42 mg vitamin E, 2 mg vitamin K3, 2 mg vitamin B1, 7 mg vitamin B2, 5 mg vitamin B6, 20 μ g vitamin B12, 36 mg nicotinic acid, 15 mg pantothenic acid, 1 mg folic acid, 150 μ g biotin, 700 mg choline chloride. 2 Obtained from Mischfutter- und Landhandel GmbH, Edderitz, Germany; organically cultivated; cold pressed without extracting agent; heat-treated; urease activity below 0.2 mg/g DM according to the supplier.

digestibility between commonly used slow- and fastgrowing broiler strains when applying the regression approach. Soybean cake was used as an example test component.

MATERIALS AND METHODS The present study was conducted at the experimental station of the Martin-Luther-Universit¨ at HalleWittenberg in Merbitz, Germany. It was approved by the animal welfare authorities in the state of SachsenAnhalt in accordance with the German Welfare Legislation.

Experimental Diets Feed was mixed in the certified feed mill facilities of the experimental station of the university. Three diets were used. The basal diet consisted of corn, solventextracted soybean meal, wheat gluten, and cornstarch (Table 1). The concentration of nutrients was calculated to meet or to exceed the recommendations of the Gesellschaft f¨ ur Ern¨ ahrungsphysiologie (1999). Organically produced soybean cake (CP 452 g/kg DM and gross energy 21 MJ/kg DM) was substituted for half or all of the cornstarch, so that the differences in the AA content of the diets resulted only from soybean cake (Table 2). Titanium dioxide (TiO2 ) was included as an indigestible marker at a level of 5 g/kg. All ingredients except for cornstarch and soybean cake were mixed in one batch to ensure uniformity of all diets. This mixture was divided into equal parts and complemented with

Item

Diet I

Diet II

Diet III

Soybean cake

DM (g/kg) CP Crude fat Crude fiber Crude ash TiO2 Gross energy (MJ/kg DM) Ala Arg Asx2 Cys Glx3 Gly Ile Leu Lys Met Phe Pro Ser Thr Trp Val

936 195 81 28 52 4.9 19.0 8.4 12.0 11.9 3.7 50.7 6.7 8.8 16.4 11.3 6.3 9.4 16.9 8.9 8.4 1.8 11.7

926 253 95 41 62 5.2 19.7 10.9 16.5 18.0 4.6 64.1 9.2 11.4 21.1 14.8 7.3 12.5 21.6 12.2 11.1 2.5 14.9

929 299 103 46 66 5.2 20.0 13.1 20.2 24.0 5.5 73.2 11.4 13.5 24.6 18.0 7.9 15.0 23.7 14.7 13.0 3.1 17.1

945 452 72 77 63 n.a.1 21.0 21.1 35.4 56.6 7.3 89.1 21.0 21.3 37.0 29.3 6.5 24.5 24.9 24.8 19.5 6.1 22.7

1 2 3

n.a. = not analyzed. Aspartic acid and asparagine together. Glutamic acid and glutamine together.

the respective amount of cornstarch and soybean cake. The diets were pelleted without using steam through a 3-mm die to prevent de-mixing or feed selection.

Animals and Housing ISA J-257 and Ross 308 were used as common representatives of slow- and fast-growing broiler strains. Day-old male broilers were obtained from local hatcheries (ISA J-257 from Br¨ uterei RoBert’s BioGefl¨ ugel GmbH & Co. KG, Sch¨ oneck, Germany and Ross 308 from Gefl¨ ugelhof M¨ ockern, M¨ ockern, Germany) and allocated in groups of 12 birds each to 36 pens (1.7 m2 ) on a straw bedding. Lighting was continuous during the first two days and the day before slaughtering. In between, a lighting regimen was maintained at 16 h of light and 8 h of dark. Illumination during the light phases was ∼25 lux. The temperature was set to 37◦ C during the first two days, reduced stepwise to 25◦ C on d 21, and maintained at this temperature until the end of the experiment. The birds had free access to drinking water from nipple drinkers and feed from one trough per pen.

Experimental Procedures The Ross and ISA broilers received commercial conventional and organic starter diets, respectively, until d 22. On d 22, the number of birds in each pen was reduced from 12 to 10 to decrease the variation in BW.

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COMPARISON OF AMINO ACID DIGESTIBILITY BETWEEN BROILER STRAINS

The experimental diets were then offered for ad libitum consumption until the end of the experiment. All birds were weighed again and asphyxiated by CO2 exposure at the end of the experiment on d 29. The section of the small intestine between Meckel’s diverticulum and 2 cm anterior to the ileocecal-colonic junction was removed. Digesta samples were obtained by flushing the terminal two-thirds of the removed section of the small intestine with deionized water as described by Kluth and Rodehutscord (2006). The digesta were pooled on a pen basis, immediately frozen at −20◦ C, and freeze-dried for further analysis.

Chemical Analyses Undried diet and freeze-dried digesta samples were ground through a 0.5-mm screen using a centrifugal mill (Fritsch Pulverisette 14.702, Idar-Oberstein, Germany). All analyses were performed in duplicate. The official methods for nutrient analysis in Germany (Verband Deutscher Landwirtschaftlicher Untersuchungsund Forschungsanstalten, 2007) were used for DM (no. 3.1), CP (no. 4.1.1), crude ash (no. 8.1), crude fat (no. 5.1.1), and crude fiber (no. 6.1.1). An automatic distillator was used for Kjeldahl digestion (Kjeltec 20300 Analyzer Unit, Foss Analytical, Hillerød, Denmark). The gross energy content was measured using a bomb calorimeter (IKA-Calorimeter C7000 isoperibolic, Janke & Kunkel IKA Analysentechnik, Staufen, Germany). The concentrations of TiO2 in the diet and digesta samples were determined photometrically after acid hydrolysis as described by Boguhn et al. (2009). Amino acids except for Trp were analyzed as described by Rodehutscord et al. (2004). The samples were oxidized in an ice bath in a mixture of hydrogen peroxide and phenolic formic acid solution and then hydrolyzed in 6 N HCl and norleucine as an external standard. Separation and detection of AA were carried out by an AA analyzer (Biochrom 30, Biochrom Ltd., Cambridge, UK) with various buffer solutions with different pH. Quantitative acquisition of AA was conducted photometrically after adding ninhydrin. All AA were measured at 570 nm except for Pro, which was determined at 440 nm. As Asn and Gln lose the amide residue in the side group during acid hydrolysis and form Asp and Glu, respectively (Fontaine, 2003), Asn and Gln were determined together with Asp and Glu and are presented as Asx and Glx. For Trp analysis, samples were saponified under alkaline conditions with barium hydroxide solution in the absence of air in an autoclave. Then, α-methylTrp was added to the mixture as the internal standard. The internal standard and Trp were separated by reverse-phase chromatography (Agilent 110, Agilent Technologies, Waldbronn, Germany) with fluorescence detection at excitation and emission wavelengths of 280 and 355 nm (Fatufe et al., 2005).

Calculations and Statistical Analyses The pc digestibility of CP, AA, and gross energy for each pen was calculated according to the following equation: Digestibility (%) = 100 − [(TiO2Diet × ItemDigesta ) / (TiO2Digesta × ItemDiet )] × 100 (1) where TiO2Diet , TiO2Digesta , ItemDiet , and ItemDigesta are the concentrations of the marker and CP, the respective AA, or gross energy in the diet and the digesta samples (g/kg DM or MJ/kg DM). The daily intake of CP and each AA (mg/d) or gross energy (MJ/d) was calculated as the product of feed intake (g DM/d or MJ/d) and the analyzed CP and AA concentrations or the gross energy concentration in the diet (mg/g DM or MJ/g DM). The daily digested amount of CP, AA, or gross energy (mg/d or MJ/d) was calculated as the total intake of CP, AA, or gross energy (mg/d or MJ/d) multiplied by the pc digestibility determined for the respective diet. The daily intake of CP and AA from the supplemented soybean cake was calculated as the difference between total intake and intake attributable to the basal diet. The CP and AA digestibility values of the experimental diets were statistically analyzed by two-way ANOVA with interactions considering the effect of diets and strains as fixed effects using the MIXED procedure of SAS for Windows (version 9.3, SAS Institute, Cary, NC). If necessary, a different variance for the ISA J257 and Ross 308 broilers was considered by assessing the heterogeneity in the covariance structure using the REPEATED statement. A model similar to the one described by Siegert et al. (2017) was applied to determine the effect of strain on the pc digestibility of CP or AA in soybean cake: digi = αi + ingi × βi + ei

(2)

where digi is the daily amount of digested CP or AA of strain i, αi is the intercept of strain i, ingi is the daily amount of ingested CP or AA of strain i, β i is the digestibility of CP or AA of strain i and ei is the residual error. The assumption of linearity between the intake and digested amounts of AA is based on previous studies (Rodehutscord et al., 2004; Rezvani et al., 2008a; Kluth et al., 2009). Data were analyzed using the MIXED procedure of SAS. The strains were compared by Student t tests using the ESTIMATE statement.

RESULTS No significant interactions (P ≥ 0.311) between the strains and the inclusion level of soybean cake were determined for growth performance, intestine characteristics, and pc digestibility of CP, AA, and gross

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GANZER ET AL. Table 3. Growth performance, intestine length, and digesta quantity determined for the experimental treatments (Estimates and SE; n = 6 pens per diet). Broiler strain Diets

ISA J-257

1

BW on d 22 (g/bird) SE BW on d 29 (g/bird) SE Daily feed intake (g/bird) SE Intestine length (cm)4 SE Intestine length per BW (cm/kg) SE Digesta (g/bird)5 SE

Ross 308

ANOVA S3

D×S

0.448

< 0.001

0.960

0.057

< 0.001

0.874

0.130

< 0.001

0.684

0.032

< 0.001

0.924

0.384

< 0.001

0.728

0.197

< 0.001

0.647

I

II

III

I

II

III

D

427 14 820 9 66 4.1 55b 1.0 67 1.0 0.59 0.03

441 20 867 17 66 4.8 56ab 0.8 65 2.0 0.74 0.05

426 12 853 8 59 4.7 58a 0.7 68 1.0 0.74 0.04

991 44 1,667 24 149 2.5 76b 1.4 45 0.6 1.95 0.11

1,002 44 1,716 28 143 2.8 76ab 0.8 45 0.6 1.93 0.19

983 40 1,717 21 139 2.5 78a 0.7 45 0.6 2.16 0.13

2

1 Diet I: 0 g/kg soybean cake, 200 g/kg cornstarch; Diet II: 100 g/kg soybean cake, 100 g/kg cornstarch; Diet III: 200 g/kg soybean cake, 0 g/kg cornstarch. 2 Effect of the diets. 3 Effect of the strains. 4 Segment of the small intestine between Meckel’s diverticulum to 2 cm anterior to the ileocecal-colonic junction. 5 Freeze-dried material obtained from the terminal two-thirds of the small intestine between Meckel’s diverticulum and 2 cm anterior to the ileocecal-colonic junction. a,b Different superscript letters within each strain indicate significant differences (P < 0.050).

Table 4. Crude protein, amino acid, and gross energy digestibility determined for the experimental treatments (%, estimates and SE; n = 6 pens per diet). Broiler strain Diet1 CP Ala Arg Asx4 Cys Glx5 Gly Ile Leu Lys Met Phe Pro Ser Thr Trp Val Gross energy

ISA J-257

Ross 308

ANOVA

I

II

III

I

II

III

pooled SE

D2

S3

D×S

81b 77b 87b 70c 70 90 70b 83 83b 84 91 83b 86b 77b 75b 72b 84 81a

83a 80a 88ab 75b 70 91 74a 85 85a 85 91 85ab 88a 80a 78a 75ab 85 79a

83a 80a 89a 77a 70 90 75a 85 85a 86 91 86a 87a 81a 78a 77a 85 77b

80b 72b 88b 70c 71 89 72b 83 79b 86 90 82b 85b 76b 75b 75b 83 81a

83a 77a 89ab 75b 71 90 76a 84 82a 87 91 84ab 87a 80a 78a 77ab 85 81a

83a 78a 89a 77a 71 90 76a 85 83a 87 90 85a 86a 81a 78a 78a 84 78b

0.7 1.2 0.6 1.0 1.0 0.5 1.0 0.7 0.8 0.8 0.6 0.9 0.7 0.8 0.9 1.3 0.8 0.7

0.007 0.004 0.012 < 0.001 0.928 0.471 < 0.001 0.068 0.021 0.187 0.892 0.030 0.040 < 0.001 0.011 0.014 0.147 < 0.001

0.682 0.005 0.016 0.945 0.174 0.013 0.057 0.325 0.002 0.061 0.239 0.232 0.076 0.484 0.728 0.043 0.401 0.063

0.984 0.558 0.732 0.986 0.948 0.740 0.726 0.992 0.493 0.837 0.906 0.991 0.986 0.889 0.979 0.772 0.978 0.311

1 Diet I: 0 g/kg soybean cake, 200 g/kg cornstarch; Diet II: 100 g/kg soybean cake, 100 g/kg cornstarch; Diet III: 200 g/kg soybean cake, 0 g/kg cornstarch. 2 Effect of the diets. 3 Effect of the strains. 4 Aspartic acid and asparagine together. 5 Glutamic acid and glutamine together. a-c Different superscript letters within each strain indicate significant differences (P < 0.050).

energy of the complete diets (Tables 3 and 4). The BW of the ISA broilers at the beginning and the end of the experimental phase was significantly lower (P < 0.001) than that of the Ross broilers. Correspondingly, the daily feed intake of the ISA broilers during the experimental phase was lower (P < 0.001). The sampled intestine length and the obtained digesta quantity of the ISA broilers were lower (P < 0.001) than those of the Ross broilers. The diets had no significant effect (P > 0.050) on growth performance.

The pc AA digestibility of the complete diets was significantly affected by the strains in 5 out of the 16 AA under study. The determined pc digestibility of complete diets of the ISA broilers was significantly higher (P < 0.050) for Ala, Glx, and Leu and lower for Arg and Trp than that of the Ross broilers. The pc AA digestibility of the complete diets significantly increased with increasing soybean cake inclusion in all AA (P < 0.050) except for Cys, Glx, Ile, Lys, Met, and Val. The pc digestibility of gross energy was not significantly

COMPARISON OF AMINO ACID DIGESTIBILITY BETWEEN BROILER STRAINS Table 5. Digestibility of CP and amino acids in soybean cake measured for different broiler strains (%).1,2 Broiler strain CP Ala Arg Asx3 Cys Glx4 Gly Ile Leu Lys Met Phe Pro Ser Thr Trp Val

ISA J-257

Ross 308

Pooled SE

P value

84 83 89 79 69 92 77 86 86 87 92 87 89 82 79 81 87

86 83 91 81 72 92 79 87 86 89 92 88 89 84 81 83 88

1.0 1.4 0.7 1.5 1.6 0.5 1.5 0.9 1.0 1.0 0.7 0.9 0.9 1.1 1.3 1.5 1.0

0.391 0.994 0.118 0.424 0.159 0.797 0.279 0.464 0.705 0.250 0.651 0.367 0.779 0.343 0.179 0.312 0.527

1

Calculated applying the regression approach. Coefficient of determination of all presented regressions ≥ 0.992. Aspartic acid and asparagine together. 4 Glutamic acid and glutamine together. 2 3

influenced (P = 0.063) by the strains and decreased with increasing inclusion of soybean cake. The digestibility of CP and AA of soybean cake did not differ significantly between the broiler strains (P ≥ 0.118) (Table 5). The highest numerical deviation in pc digestibility between the broiler strains of three percentage points was observed for Cys.

DISCUSSION The main objective of the present study was to investigate whether pc AA digestibility is affected by the genetic background of the broilers. Ross 308 and ISA J-257 were selected for this experiment as commonly used representatives of fast- and slow-growing broiler strains in Europe, respectively. The results from the present study indicate that specific AA digestibility experiments for fast- and slow-growing broiler strains are not necessary because the digestibility estimates were almost identical for both broiler strains. This outcome of the present study appears to be different from the results of some previous studies determining apparent pc AA digestibility. Al-Marzooqi et al. (2010, 2011) found significant differences in the apparent pc AA digestibility of barley, soybean meal, and several fish meals in Cobb 500 broilers and an unspecified local Omani strain. In both studies, the numerical differences among the strains ranged from 5 to 16 percentage points. Ravindran et al. (1999) also described significant differences in apparent pc AA digestibility in three broiler strains commonly used in Australia. There, the average deviation between the strains with the lowest and highest digestibility values was 6.7 percentage points across all measured AA. Comparing the apparent pc AA digestibility between Ross 308 broilers and a strain designated as commercially not available

5

in the test phase, Kim and Corzo (2012) described no significant strain effects for most AA (based on the results of the one-way-ANOVA presented in their study). The average numerical deviation in pc AA digestibility of 21-d-old broilers in the present study was 2.3 percentage points. The reasons for differences among strains in previous studies and the almost identical digestibility estimates in the present study are not clear. In the studies described above, the apparent pc AA digestibility based on the digestibility of complete diets with the test ingredient as the single AA providing ingredient was compared. This approach is similar to the digestibility of the complete diets presented in Table 4, where significant differences were determined for some AA, and numerical differences of up to 3.3 percentage points between the strains were observed. The apparent pc AA digestibility based on the digestibility of complete diets does not account for endogenous AA losses (Adedokun et al., 2011). The regression approach applied in the present study, however, provides digestibility estimates that encompass specific and exclude basal endogenous AA losses (Rodehutscord et al., 2004). It appears probable that the differences in pc AA digestibility among broiler strains in the literature can be explained by endogenous AA losses. Explaining the differences of the complete diet digestibility between the strains shown in Table 4 and the differences described in previous studies would fit the high variability regarding the amount and composition of basal endogenous AA losses (Adedokun et al., 2007, 2011). AA digestibility estimates regarding specific endogenous amino acid losses are usually assumed to increase the accuracy of feed formulation compared with feed formulation based on apparent pc AA digestibility (Kong and Adeola, 2013). This would mean that, according to the results of the present study, it is not necessary for pc AA digestibility to be determined for slow- and fastgrowing broiler strains separately when the regression approach is applied. This experiment was conducted with soybean cake as an example test component. Further studies should investigate whether this conclusion can be generalized. There were no significant differences in daily feed intake between the diets within each strain. This means that the flow of basal endogenous AA losses within each strain was similar provided that the basal endogenous AA losses depend on DM intake as generally assumed (Adedokun et al., 2011). In consequence, the determined digestibility estimates actually inherit the nondigested dietary AA and specific endogenous AA losses but not the basal endogenous AA losses. Regarding the length of the small intestine between Meckel’s diverticulum and 2 cm anterior to the ileocecal-colonic junction, neither the absolute length (77 vs. 52 cm; P < 0.001), length per BW (45 vs. 66 cm/kg; P < 0.001), length per metabolic BW (51 vs. 64 cm/kg0.75 ; P < 0.001), nor the length per daily feed

6

GANZER ET AL.

intake (53 vs. 88 cm/100 g; P < 0.001) were similar between sampled segments of the Ross and ISA broilers. This indicates that the length of this segment of the small intestine might not be the factor limiting the pc AA digestibility despite the crucial importance of the section between Meckel’s diverticulum and the ileocecal-colonic junction for the determination of AA digestibility in broilers (Kluth et al., 2005) and laying hens (Rezvani et al., 2008b). The results of the present study further indicate that a limited AA digestibility probably is not the reason for the lower growth performance of slow-growing broilers than that of fastgrowing broiler strains. An evaluation of pc gross energy digestibility similar to the evaluation of pc AA digestibility in soybean cake is not reasonable because such a calculation would also reflect the pc gross energy digestibility of cornstarch. The effect of the gross energy content of cornstarch in the diets was considered in a model calculation assuming 98.6% pc gross energy digestibility (Yuan et al., 2016) and 17.4 MJ/kg DM gross energy content (D’Alfonso, 2005) for cornstarch. According to that, the pc gross energy digestibility of the complete diets was not significantly different between the strains and diets (P > 0.050). The pc gross energy digestibility values in soybean cake were 77% (SE 3.8) and 84% (SE 3.2) for the ISA and Ross broilers, respectively, and not significantly different between the strains (P = 0.222). Given that the CP content and pc CP digestibility in soybean cake were high, the pc gross energy digestibility in other nutrients than CP must have been lower (with a total of ∼73% and ∼83% for the ISA and Ross broilers, respectively). The difference of 10 percentage points in gross energy digestibility of nutrients other than CP is suggestive of a possible reason for the lower growth performance of slow-growing broilers than that of fast-growing broiler strains. However, differences in the genetic growth potential still appear a more likely reason for differences in growth. To our knowledge, no other study exists describing the pc AA digestibility of soybean cake in broilers. Another study published in the 1930s investigated the total tract CP digestibility of soybean cake in cockerels (Suzuki, 1931), but comparisons are not reasonable due to differences in methodology and sample material. In the present study, the pc AA digestibility of soybean cake was on a rather high level. The high level of pc AA digestibility of soybean cake in the present study probably is a consequence of inactivation of trypsin inhibitors in the raw material during heat treatment. This is indicated by the low level of urease activity (Heger et al., 2016) in the batch used for this experiment. It is concluded that there is no difference in pc AA digestibility between fast- and slow-growing broiler strains in soybean cake as estimated using the regression approach. Thus, experimentally determined pc AA digestibility estimates can be applied for both types of broiler strains.

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