Determination and prediction of the apparent and standardized ileal amino acid digestibility in soybean meal fed to the pullets

Abstract

This experiment was conducted to evaluate the chemical composition, apparent ileal digestibility (AID) and standardized ileal digestibility (SID) of amino acid (AA) in 10 different sources of soybean meal (SBM) fed to the pullets and established prediction equations based on the chemical composition of SBM. 792 Hy Line Brown hens aged 12-15 weeks were divided into 11 diet groups, the diets included one nitrogen-free diet (NFD) used to measure the basal endogenous losses of AA and 10 SBM test diets containing 35.2 % SBM as the sole source of AA. The 0.5 % titanium dioxide (TiO₂) was used as an indigestible marker. The results shown that there was considerable variation in crude ash (Ash), ether extract (EE), and acid detergent fiber (ADF) content in the 10 SBM samples with a coefficient of variation greater than 10 %. The AID and SID of both indispensable and dispensable AA except for the Cys and Ala among the 10 SBM differed (P < 0.05). The AID of AA was correlated with the dry matter (DM), gross energy (GE), crude protein (CP), EE, Ash, and reactive Lys (rLys) (P < 0.05), and SID of AA was correlated with the GE, CP, EE, CF, neutral detergent fiber (NDF) and rLys (P < 0.05). With R² values exceeding 0.39, significant linear regression equations for the AID of Met, Phe, Lys, Asp, and Gly, as well as SID of Met, Phe, Lys, Ser and Tyr were obtained. The best fit equation for the AID of Asp was the DM, GE, Ash and rLys (R² = 0.820, P = 0.042), and the SID of Lys was the GE, CP, CF, Ash, ADF and rLys (R² = 0.989, P = 0.005), respectively. In conclusion, this experiment suggested that the rLys can be used to predict the AID and SID of AA in SBM with reasonable accuracy together with the basal chemical composition, and prediction equations for AID and SID could be used to evaluate the digestibility of SBM in the pullets.

Introduction

Accurate evaluation of amino acids (AAs) digestibility of feed is pivotal to obtain the optimal feed formula, reduce the cost, and alleviate the shortage of feed grains in the present poultry industry (Adedokun et al., 2014; Adedokun et al., 2015; Cowieson et al., 2019; Siegert et al., 2023; Yun et al., 2023). Particularly worth mentioning is that the different types or ages of birds at different physiological stages have different digestive statues, such as laying hens (Adedokun et al, 2014; Barua et al., 2021a; Qiu et al., 2023). The growing pullets, as the growing stage of laying hens, are the key time for organ development and body growth, and affecting the stability of egg quality during laying period (Murugesan et al., 2013; Qiu et al., 2021; Wei et al., 2022). However, There is an extremely lack of relevant data in the nutritional value of feed materials, especially the digestibility of AA in the pullets now. Therefore, the accurate evaluation of AA digestibility of the pullets feed is necessary to carry out.

The use of standardized ileal amino acid digestibility (SIDAA) has now become commonplace because it exhibits greater additivity, which is adjusted for the basal endogenous AA flow originating from different digestive secretion in practical feed formulations in birds or pigs (An et al., 2020; Kong et al., 2013a; Ma et al., 2019; Ravindran, 2021; Wang et al., 2023b). A series of reports are available on the apparent ileal digestibility (AID) and SID coefficients of AA in feed ingredients (Barua et al., 2021b; Barua et al., 2020). Soybean meal (SBM) is the dominant protein source used in poultry diets worldwide, due to beneficial attributes such as high crude protein (CP) content, an excellent AA profile that complements cereal and high AA digestibility (Ravindran et al., 2014a). However, owing to the fact that data on AA digestibility determined in one type or age of birds should not be applied in another type (Adedokun et al., 2009). Frikha et al. (2012), who showed that SBM from different regions of the world are different in nutrient composition as well as their AA digestibility potential in broilers. Previous studies in our laboratory compared the AID and SID of SBM of 21 day broilers and 32 week laying hens and found that both AID and SID were significantly lower in laying hens than in broilers for most of the AA (Qiu et al., 2023). Barua et al. (2023) reported the broiler age effect on SIDAA in different SBM sources. The above research work indicated that SBM of different physiological stages and different breeds of poultry are different, so it is very necessary to carry out accurate AID and SID AA of SBM in the pullets. Therefore, the hypothesis of this study was that the AID and SID of AA in the pullets are not different from those in SBM. The study aimed to compare the chemical composition of different SBM and determine the AID and SID of AA of SBM fed to the pullets to test the above hypothesis. In addition, the results of chemical analysis were used to establish prediction equations for AID and SID that could be applied in future commercial practice.

Materials and methods

Animal care

This study was approved by the Animal Care and Use Committee of the Feed Research Institute of the Chinese Academy of Agricultural Sciences, Beijing. The 792 laying hen 12-15 weeks of age were obtained from a commercial farm (Hy-Line Brown laying hens, China) and housed 240 metal cages (55 cm×40 cm×40 cm), with three birds each. Each replicate consisted of four adjoining metal cages, and 66 replicate hens were randomly allocated to one of eleven groups. Ambient temperature and humidity in the barn were maintained at 14°C±2°C and 50 % to 65 %, respectively. The lighting was consistent with conventional commercial operations. The pullets were exposed to a 16-h photoperiod throughout the experiment. The animals had ad libitum access to feed and water.

Experimental diets

Ten representative samples of SBM from SBM-producing areas of China were obtained, and the chemical composition of the samples was analyzed (Table 1). Eleven experimental diets were examined in this study (Table 2). Ten SBM diets included SBM as the only source of crude protein (CP) in the semi-purified diets; A semi-purified nitrogen-free diet (NFD) was formulated to determine the endogenous AAs flow (Adedokum et al., 2014). The pullets were fed a standard corn-soy-based starter diet that the met or exceeded the NRC (1994) recommendations, respectively, before the feeding of the experimental diets. Each of experimental diet contained 0.5 % titanium dioxide (TiO₂) as an indigestible marker. The same diet batch was fed to the pullets. The analyzed CP and AA contents of the diets are presented in Table 3.

Table 1.

Analyzed chemical properties of soybean meal from different origins ( %, DM basis)

Item	Soybean meal 1										Mean	Min	Max	CV
	SBM1	SBM2	SBM3	SBM4	SBM5	SBM6	SBM7	SBM8	SBM9	SBM10
DM	89.26	89.47	88.47	90.42	88.64	90.72	90.03	90.67	89.31	91.20	89.82	88.47	91.20	1.03
GE, Kcal/Kg	4689	4629	4706	4682	4752	4680	4668	4715	4649	4713	4688	4680	4752	0.76
CP	42.70	47.52	46.41	43.95	43.77	45.24	44.24	46.78	44.96	45.50	45.11	42.70	47.52	3.32
EE	1.84	1.82	1.97	1.91	1.85	1.87	1.60	1.81	1.89	2.02	1.86	1.60	2.02	6.04
Ash	7.22	6.90	6.22	5.85	6.51	5.83	5.90	5.18	6.15	7.61	6.34	5.18	7.22	11.55
CF	6.40	5.03	5.70	7.55	5.02	6.16	7.25	4.82	6.62	5.09	5.96	4.82	7.55	16.52
NDF	22.40	22.13	19.24	26.04	20.87	24.94	22.99	25.62	24.38	21.27	22.99	19.24	26.04	9.68
ADF	6.24	3.67	5.16	7.06	3.25	5.96	6.57	4.45	5.90	4.28	5.25	3.67	6.24	24.58
rLys	2.11	2.35	2.47	2.15	2.17	2.35	2.41	2.39	2.37	2.44	2.32	2.11	2.47	5.58
Indispensable AA, %
Met	0.70	0.78	0.72	0.72	0.70	0.74	0.72	0.74	0.72	0.71	0.73	0.70	0.78	3.27
Thr	1.65	1.82	1.78	1.69	1.66	1.79	1.73	1.69	1.77	1.70	1.73	1.65	1.82	3.41
Val	2.06	2.26	2.17	2.11	2.09	2.19	2.12	2.17	2.14	2.23	2.15	2.06	2.26	2.90
Ile	1.93	2.14	2.08	2.14	2.00	2.00	2.09	1.98	2.03	2.02	2.04	1.93	2.14	3.41
Leu	3.98	4.36	4.22	4.08	4.10	4.26	4.03	4.14	4.11	4.06	4.13	3.98	4.36	2.78
Phe	2.02	2.42	2.09	2.03	2.28	2.28	2.40	2.24	2.32	2.36	2.24	2.02	2.42	6.59
His	1.13	1.21	1.19	1.13	1.13	1.14	1.17	1.15	1.16	1.18	1.16	1.13	1.21	2.42
Lys	2.41	2.71	2.84	2.46	2.48	2.70	2.77	2.72	2.77	2.82	2.67	2.41	2.84	5.92
Arg	3.15	3.15	3.38	3.28	3.16	3.21	3.15	3.22	3.24	3.32	3.23	3.15	3.24	2.48
Trp	1.01	1.08	1.11	1.09	1.08	1.09	1.05	1.12	1.08	1.04	1.08	1.01	1.11	3.08
IAA	20.04	21.93	21.58	20.73	20.68	21.40	21.23	21.17	21.34	21.44	21.15	20.04	21.93	2.55
Dispensable AA, %
Cys	0.83	0.91	0.92	0.91	0.85	0.85	0.88	0.85	0.86	0.93	0.88	0.83	0.93	4.06
Asp	5.22	6.69	5.38	5.27	5.23	5.58	5.48	6.22	6.32	6.28	5.77	5.22	6.69	9.55
Ala	1.70	1.98	1.90	1.89	1.94	1.90	1.79	1.73	1.96	1.79	1.86	1.70	1.98	5.30
Glu	6.68	8.89	8.58	8.06	8.23	8.20	8.10	8.29	6.79	7.69	7.95	6.68	8.89	8.99
Gly	1.65	1.89	1.86	1.82	1.67	1.89	1.87	2.29	1.90	1.92	1.88	1.65	1.92	9.25
Pro	2.24	2.99	2.98	2.25	2.34	2.79	2.34	2.55	2.58	2.86	2.59	2.24	2.99	11.43
Ser	2.12	2.30	2.48	2.18	2.29	2.36	2.47	2.29	2.45	2.34	2.33	2.12	2.48	5.13
Tyr	0.95	1.28	1.12	1.08	0.97	1.02	1.01	1.24	1.31	1.30	1.13	0.95	1.30	12.64
DAA	21.39	26.93	25.22	23.46	23.52	24.59	23.94	25.46	24.17	25.11	24.38	21.39	26.93	6.07
TAA	41.43	48.86	46.8	44.19	44.2	45.99	45.17	46.63	45.51	46.55	45.53	41.43	48.86	4.37

Abbreviations: ADF, acid detergent fiber; Ala, alanine; Arg, arginine; Asp, aspartic acid; EE, crude fat; CF, crude fiber; CP, crude protein; Cys, cysteine; CV, coefficient of variation; DAA, dispensable amino acids; DM, dry matter; GE, gross energy; Glu, Glutamic acid; Gly, Glycine; His, histidine; Ile, isoleucine; Lys, lysine; Leu, leucine; Max, maximum; Met, methionine; Min, minimum; NDF, neutral detergent fiber; Phe, phenylalanine; Pro, proline; rLys, reactive Lysine; Ser, serine; TAA, total amino acids;Thr, threonine; Trp, trptophan; Tyr, tyrosine; Val, Valine.

¹sources of SBM are: SBM1, Beijing (China, regular, solvent process); SBM2, Shandong (USA, dehulled, solvent process); SBM3, Anhui (USA, dehulled, solvent process), SBM4, Henan (China, regular, solvent); SBM5, Fujian (Brazil, regular, solvent process); SBM6, Shandong (Argentitina, regular, solvent process); SBM7, Fujian (Argentitina, regular, solvent process); SBM8, Jiangshu (Brazil, dehulled, solvent process); SBM9, Neimenggu (China, regular, solvent process); SBM10, shandong (Brazil, regular, solvent process).

Table 2.

Ingredient composition of the experimental diets (g/kg, as-fed basis)

Ingredient	NFD diet	SBM diets
Dextrose	69.67	57.70
Corn starch	17.00	-
Cellulose	6.00	-
Soybean meal	-	35.20
Soybean oil	1.60	2.30
Dicalcium phosphate	2.10	1.72
Limestone	1.10	1.00
NaCl	-	0.30
DL-methionine	0.30	0.10
Vitamin-trace mineral premix1	0.50	0.50
NaHCO3	0.60	-
KCl	0.23	-
Mgo	0.40	-
Zeolite powder	-	0.68
TiO2	0.50	0.50
Total	100.00	100.00

Abbreviation: NDF, nitrogen-free diet; SBM, soybean meal; TiO₂, titanium dioxide

¹Premix supplied per kilogram of diet: vitamin A, 12,500 IU; vitamin D3, 4125 IU; vitamin E, 15 IU; vitamin K3, 2 mg; thiamine, 1 mg; riboflavin, 8.5 mg; pyridoxine, 8 mg; vitamin B12, 0.04 mg; biotin, 0.1 mg; folic acid, 1.25 mg; Ca-pantothenate, 50 mg; niacin, 32.5 mg; Cu, 8 mg; Zn, 65 mg; Fe, 60 mg; Mn, 65 mg; Se, 0.3 mg; I, 1 mg.

Table 3.

Analyzed crude protein and amino acid composition ( %) of the experimental diets (on an as-fed basis)

Item	Diet
	NFD	SBM1	SBM2	SBM3	SBM4	SBM5	SBM6	SBM7	SBM8	SBM9	SBM10
CP	0.32	18.26	16.93	15.23	17.26	17.24	17.66	16.13	19.66	16.67	17.88
Indispensable AA
Met	0.003	0.216	0.202	0.209	0.272	0.260	0.263	0.248	0.245	0.236	0.254
Thr	0.003	0.568	0.558	0.382	0.658	0.656	0.570	0.495	0.635	0.565	0.681
Val	0.035	0.829	0.802	0.718	0.837	0.874	0.899	0.813	0.879	0.852	0.880
Ile	0.008	0.753	0.737	0.657	0.761	0.803	0.820	0.745	0.808	0.774	0.784
Leu	0.012	1.520	1.494	1.331	1.558	1.634	1.647	1.502	1.630	1.574	1.605
Phe	0.013	0.848	0.837	0.746	0.854	0.903	0.917	0.835	0.906	0.866	0.897
His	0.005	0.430	0.412	0.369	0.437	0.449	0.462	0.417	0.447	0.441	0.455
Lys	0.008	1.003	0.962	0.862	1.032	1.061	1.086	0.988	1.049	1.045	1.086
Arg	0.009	1.178	1.139	1.017	1.198	1.244	1.273	1.145	1.237	1.205	1.256
Trp	0.035	0.207	0.187	0.174	0.188	0.196	0.207	0.192	0.214	0.206	0.197
Dispensable AA
Cys	0.012	0.329	0.318	0.288	0.326	0.343	0.356	0.318	0.342	0.341	0.350
Asp	0.013	1.917	1.877	1.694	1.961	2.037	2.237	2.017	2.040	2.135	2.448
Ala	0.012	0.651	0.635	0.457	0.707	0.717	0.689	0.618	0.716	0.670	0.322
Glu	0.035	3.031	2.964	2.430	3.140	3.267	3.251	2.934	3.256	3.102	2.408
Gly	0.008	0.664	0.628	0.487	0.712	0.701	0.698	0.628	0.704	0.677	0.500
Pro	0.141	2.272	2.213	1.663	2.379	2.471	2.424	2.181	2.475	2.336	2.232
Ser	0.007	0.831	0.800	0.666	0.853	0.878	0.871	0.792	0.873	0.838	0.861
Tyr	0.009	0.362	0.328	0.278	0.333	0.356	0.388	0.331	0.365	0.354	0.347

Abbreviation: AA, amino acids; Ala, alanine; Arg, arginine; Asp, aspartic acid; CP, crude protein; Cys, cysteine; CV, coefficient of variation; DAA, dispensable amino acids; Glu, Glutamic acid; Gly,Glycine; His, histidine; Ile, isoleucine; Lys, lysine; Leu, leucine; Met, methionine; NDF, nitrogen-free diet; Phe, phenylalanine; Pro, proline; Ser, serine; TAA, total amino acid; Thr, threonine; Trp, trptophan; Tyr, tyrosine; Val, Valine,SBM, soybeal meal; sources of SBM: Beijing(SBM1), Shandong(SBM2,SBM6,SBM10), Anhui(SBM3), Henan(SBM4), Fujian(SBM5, SBM7), Jiangshu(SBM8), Neimengu(SBM9).

Experimental procedure

A total of 792 laying hens 12-15 weeks of age were used in this study. The 11 experimental diets (10 feed ingredient diet and 1 NFD) were fed to 6 replicates of 12 birds per replicate. The birds were fed the 11 diets consecutively for 4 d, and then 4 birds, respectively, from each replicate, were selected and euthanized using CO₂ asphyxiation, and the ileum contents (portion of the small intestine from Meckel's diverticulum to approximately 1 cm proximal to the ileocecal junction) were removed, gently flushed with distilled water, pooled for per-replicate cage (Ghazaghi et al., 2023), and then stored at -20 °C until processing.

Sample preparation and analyses

Samples of SBM, diet, and the frozen ileal digesta were finely ground using an electric grinder and filter through a 3-mm screen to ensure a homogeneous mixture for analysis. The samples were further analyzed as follows: SBM for dry matter (DM), CP, gross energy (GE), ether extract (EE), crude ash (ash), crude fiber (CF), neutral detergent fiber (NDF), acid detergent fiber (ADF), AA contents and reactive Lysine (rLys); ileal digesta and diets for AAs and TiO₂ content. The DM, CP, Ash, CF, and EE were analyzed according to AOAC International (2000) and lytical methods (934.01, 990.03, 920.39, 978.10, 942.05, respectively). The NDF and ADF analysis were performed as described by Van Soest et al. (1991) and Yun et al (2023). The rLys analysis was performed as described by Boucher et al. (2009) and Brestensk et al. (2018).

Samples for AA analysis was prepared by acid hydrolysis method (AOAC, 2016). Briefly, after a cold performic acid oxidation overnight and hydrolyzing using 7.5 N HCl at 110°C for 24 h, Met and Cys were measured as methionine sulfone and cysteic acid using an amino acid analyzer (model L-8900; Hitachi High Technologies Corp.) Trp was analyzed according to the GB/T15400-2018. TiO₂ concentrations were determined in duplicate for samples; samples to be analyzed were ashed and digested in accordance with the procedures described by Myers et al. (2004).

Calculations

The ileal endogenous amino acid (IEAA) flow in the pullets fed the NFD calculated as milligrams of AA content per kilogram of DM intake (DMI) using the formula proposed by Adedokun et al. (2008) and Yun et al. (2023):

IEAA, mg/kg of DMI=[AA in ileal digesta, mg/kg×(TiO₂ in diet, mg/kg/ TiO₂ in digesta, mg/kg)].

The AID and SID of each AA were calculated following the methods described by Wang et al. (2020):

AIDAA, %=[1-TiO₂ in diet/TiO₂ in digesta)×(AA in digesta/AA in diet)]; SIDAA, %= AIDAA, %+ [(IEAA flow, g/kg of DMI)/ (AA content of the raw material, g/kg of DM)] ×100.

Statistical analysis

Data from the present study were subjected to one-way ANOVA using the GLM procedure of SAS 9.4 (SAS Institute Inc., Cary, NC). The treatment comparisons for significant differences were tested by the least significant difference (LSD) method. The replicate cage was regarded as an experimental unit. The relationships between chemical composition and AIDAA as well as SIDAA values were analyzed using bivariate correlation analysis by SPSS procedure. The forward stepwise regression method was used to establish linear regression equations for predicting the AIDAA and SIDAA values from the chemical composition of SBM. The statistical significance was set at P ≤ 0.05.

Results

Chemical compositions and AA contents of SBM samples

As shown in Table 1, there was considerable variation in Ash, CF and ADF levels among the 10 SBM samples, with the coefficient of variation (CV) greater than 10 %. On a DM basis, the averaged CP, EE, Ash, CF, NDF, ADF, and rLys concentrations in SBM were 45.11 %, 1.86 %, 6.34 %, 5.96 %, 22.99 %, 5.25 %, and 2.32 %, with ranges of 42.70 % to 47.52 %, 1.60 % to 2.02 %, 5.18 % to 7.22 %, 4.82 % to 7.55 %, 19.24 % to 26.04 %, 3.67 % to 6.24 %, and 2.11 % to 2.47 %, respectively. The average GE value of SBM was 4688 kcal/kg, with a range of 4680 to 4752 kcal/kg.

The AA concentrations of the 10 SBM samples were also presented in Table 1. The concentrations of all AA were relatively stable with CV less than 10 % except for the Pro and Tyr. The average Lys, Met, Thr and Trp concentrations in SBM were 2.67 %, 0.73 %, 1.73 % and 1.08 %, with ranges of 2.41 % to 2.84 %, 0.70 % to 0.78 %, 1.65 % to 1.82 %, and 1.01 % to 1.11 %, respectively. The average IAA, DAA and TAA concentrations in SBM were 21.05 %, 24.38 %, and 45.53 %, with ranges of 20.04 % to 21.93 %, 21.39 % to 26.39 %, and 41.43 % to 48.86 %, respectively.

Endogenous AA losses in the pullets

The CP content of the NFD in this study was only 0.32 % (Table 3), and the ileal endogenous AA flows for all AA were determined in Table 4. The IEAA of indispensable AA ranged from 38.63 mg/kg DMI for Met to the 178.88 mg/kg DMI for Val, and dispensable AA ranged from 39.47 DMI mg/kg for Tyr to the 338.57 DMI mg/kg for Gly, respectively.

Table 4.

Ileal endogenous amino acid flow (mg/kg of DM feed intake)in the pullets fed a nitrogen-free diet¹ (NFD)

Item	Average	SD
Indispensable AA
Met	38.63	24.53
Thr	150.03	26.81
Val	178.88	27.70
Ile	105.81	25.11
Leu	147.96	136.00
Phe	110.05	29.17
His	55.76	26.22
Lys	123.38	44.37
Arg	133.11	36.57
Trp	38.84	12.26
Dispensable AA
Cys	63.96	6.20
Asp	213.45	50.63
Ala	102.70	89.73
Glu	338.57	21.46
Gly	142.48	40.28
Pro	208.40	82.10
Ser	166.35	40.55
Tyr	39.47	17.85

Abbreviation: AA, amino acids; Ala, alanine; Arg, arginine; Asp, aspartic acid; Cys, cysteine; Glu, Glutamic acid; Gly,Glycine; His, histidine; Ile, isoleucine; Lys, lysine; Leu, leucine; Met, methionine; NDF, nitrogen-free diet; Phe, phenylalanine; Pro, proline; Ser, serine;Thr, threonine; Trp, trptophan; Tyr, tyrosine; Val, Valine; SBM, soybeal meal; sources of SBM:Beijing(SBM1), Shandong(SBM2,SBM6,SBM10), Anhui(SBM3), Henan(SBM4), Fujian(SBM5, SBM7), Jiangshu(SBM8), Neimengu(SBM9).

¹Data represent the means of 6 replicates (n =6).

Digestibility of amino acids

The AID of AA in the SBM samples were presented in Table 5. The AID of indispensable AA was different (P < 0.03) among the 10 sources of SBM, the lowest was Thr (47.73 %), and the highest was Arg (73.16 %), and the average was 62.62 %. The AID of dispensable AA was different (P < 0.03) among the 10 sources of SBM, except for the Cys and Ala (P > 0.14), the lowest was Cys (34.83 %), and the highest was Pro (74.00 %), and the average was 60.99 %. The AID of IAA, DAA and TAA were different (P < 0.03) among the 10 sources of SBM. In indispensable AA, the AID of AA in SBM1 were significantly greater than SBM2, SBM3 and SBM4 (P < 0.05), except for Met in SBM1, which was significantly greater than the SBM2 and SBM3 (P < 0.05). In dispensable AA, the AID of Asp, Glu and Tyr were significantly greater than SBM2, SBM3 and SBM4 (P < 0.05), while the AID of Pro and Ser were significantly greater than SBM3 and SBM4 (P < 0.05). The AID of IAA, DAA, and TAA in SBM1 were significantly greater than SBM2, SBM3 and SBM4 (P < 0.05).

Table 5.

Apparent ileal amino acid digestibility in the pullets fed soybean meal from different origins ¹

Item	Diet										SEM	P value	Mean	CV
	SBM1	SBM2	SBM3	SBM4	SBM5	SBM6	SBM7	SBM8	SBM9	SBM10
Indispensable AA
Met	54.89abc	40.51de	37.08e	47.56cd	58.35ab	61.34a	58.91ab	45.54cde	50.74abce	49.77bcd	2.83	0.0001	50.47	15.95
Thr	53.95a	43.08bc	40.33c	40.83c	54.34a	51.46ab	48.22abc	49.15abc	45.92abc	50.04abc	2.57	0.021	47.73	10.62
Val	72.26a	58.32bcd	48.74d	52.60cd	60.55bcd	65.78ab	59.12bcd	62.55abc	56.77bcd	56.64bcd	3.25	0.003	59.33	11.13
Ile	72.07a	61.04c	60.75c	60.72c	70.48ab	72.04a	63.62abc	67.68abc	62.78bc	63.72abc	2.38	0.005	65.49	7.11
Leu	73.94a	61.14cd	56.34d	60.63cd	70.54ab	71.67ab	65.01abcd	67.57abc	62.76bcd	58.58cd	2.64	0.001	64.82	9.15
Phe	76.01a	66.20bc	63.06c	65.95bc	73.90ab	73.15ab	68.01abc	70.08abc	67.83bc	68.35abc	2.35	0.013	69.25	5.82
His	76.58a	66.53bc	62.10c	64.04bc	72.23ab	72.54ab	67.92bc	70.41abc	68.45abc	68.98abc	2.53	0.011	68.98	6.16
Lys	76.95a	63.69bc	60.73c	66.54bc	72.49ab	72.53ab	66.69bc	67.67abc	65.70bc	67.83abc	2.87	0.017	68.08	6.95
Arg	80.67a	70.39bc	67.75c	67.63c	75.15abc	78.19ab	71.90bc	73.76abc	72.22bc	73.90abc	2.51	0.012	73.16	5.70
Trp	67.73a	52.42bcd	49.77cd	44.71d	58.48abc	61.12ab	60.34ab	61.14ab	59.94ab	53.26bcd	2.88	0.0002	56.89	11.86
IAA	70.51a	58.70cde	57.14de	56.62e	64.75abc	68.18ab	63.31bcd	63.98bc	61.31cde	61.70bcde	2.01	0.0001	62.62	7.22
Dispensable AA
Cys	35.85	34.73	27.18	31.57	35.48	37.17	34.37	39.73	35.72	36.49	2.15	0.141	34.83	9.75
Asp	70.35ab	58.05cd	57.65d	59.10cd	64.24bcd	67.01abc	62.20bcd	67.26abc	62.93bcd	75.15a	2.77	0.001	64.39	8.77
Ala	69.34	60.07	57.56	57.82	79.24	72.27	62.97	62.42	60.85	42.73	6.12	0.155	62.53	15.71
Glu	79.34a	67.62bcd	65.46cd	68.47bcd	75.31abc	77.17ab	74.34abc	74.76abc	71.65abc	61.02d	2.96	0.004	71.51	8.04
Gly	59.17abc	53.14bcd	45.10de	48.33cd	66.20a	62.88ab	59.30abc	57.58abc	54.33bcd	36.83e	2.89	<0.0001	54.29	16.26
Pro	79.65ab	70.93bcd	66.35d	68.78cd	81.28a	78.85ab	76.06abc	74.10abcd	73.68abcd	70.33bcd	2.85	0.009	74.00	6.70
Ser	63.48ab	58.23abc	47.82d	48.80cd	67.17a	64.54ab	57.11abcd	62.45ab	55.44bcd	59.35ab	2.92	0.001	58.44	11.01
Tyr	74.09a	61.06cd	52.65d	61.07cd	69.19abc	71.29ab	61.53bcd	62.24bcd	62.38bcd	60.34cd	2.90	0.0004	63.58	9.83
DAA	66.41a	57.82bc	53.73c	57.03bc	65.85a	66.79a	61.14abc	63.37ab	58.90abc	58.89abc	2.47	0.004	60.99	7.32
TAA	68.68a	57.64cd	55.50d	56.86cd	65.19ab	67.55a	62.39abc	63.67abc	59.15bcd	60.33bcd	2.11	0.0002	61.70	7.37

Abbreviation:AA, amino acids; Ala, alanine; Arg, arginine; Asp, aspartic acid; Cys, cysteine; CV, coefficient of variation; DAA, dispensable amino acids; Glu, Glutamic acid; Gly,Glycine; His, histidine; IAA, indispensable amino acids; Ile, isoleucine; Lys, lysine; Leu, leucine; Met, methionine;Phe, phenylalanine; Pro, proline; Ser, serine; TAA, total amino acids; Thr, threonine; Trp, trptophan; Tyr, tyrosine; Val, Valine; SBM, soybeal meal; sources of SBM: Beijing (SBM1), Shandong (SBM2, SBM6, SBM10), Anhui(SBM3), Henan (SBM4), Fujian (SBM5, SBM7), Jiangshu (SBM8), Neimengu (SBM9).

¹Data represent the means of 6 replicates (n =6). ^a-d Means within a row lacking a common superscript differ (P < 0.05).

The SID of AA in the SBM samples were presented in Table 6. The SID of indispensable AA was different (P < 0.05) among the 10 sources of SBM, and the lowest was Met (66.69 %), and the highest was Arg (84.39 %), and the average was 77.69 %. The SID of dispensable AA was different (P < 0.05) among the 10 sources of SBM, except for the Cys, Ala (P > 0.05), the lowest was Cys (54.21 %), and the highest was Pro (83.32 %), and the average was 75.38 %. The AID of IAA, and TAA were different (P < 0.03) among the 10 sources of SBM, but the DAA were not affected (P = 0.066). In indispensable AA, the SID of AA in SBM1 were significantly greater than SBM2, SBM3 and SBM4 (P < 0.05), except for the Thr, which was only greater than SBM4 (P < 0.05). In dispensable AA, the SID of Asp, Glu, and Tyr in SBM1 were significantly greater than SBM2, SBM3 and SBM4 (P < 0.05), whereas the SID of Gly, Pro and Ser in SBM1 were significantly greater than SBM3 and SBM4 (P > 0.05). In addition, the SID of IAA and TAA in SBM1 were significantly greater than SBM2, SBM3 and SBM4 (P < 0.05).

Table 6.

Standardized ileal amino acid digestibility in the pullets fed soybean meal from different origins ¹

Item	Diet										SEM	P value	Mean	CV
	SBM1	SBM2	SBM3	SBM4	SBM5	SBM6	SBM7	SBM8	SBM9	SBM10
Indispensable AA
Met	72.78a	59.63cd	55.56d	61.76cd	73.21ab	76.03a	74.48ab	61.31cd	67.11abc	64.98bcd	2.83	0.0006	66.69	10.69
Thr	80.36a	69.96ab	79.61a	63.63b	77.21a	77.78a	78.53a	72.78ab	72.47ab	72.07ab	2.57	0.009	74.44	7.03
Val	89.06a	80.62bc	73.66c	73.98c	81.02abc	85.68ab	81.12abc	82.90abc	77.76bc	76.97bc	2.79	0.026	80.28	6.09
Ile	86.12a	75.40cd	76.85bcd	74.62d	83.66abc	84.94ab	77.83abcd	80.77abcd	76.45cd	77.22bcd	2.61	0.016	79.39	5.26
Leu	83.67a	71.04cd	67.46d	70.12cd	79.60abc	80.65ab	74.86abcd	76.65abcd	72.16bcd	67.80d	2.64	0.0020	74.40	7.55
Phe	88.99a	79.35b	77.82b	78.84b	86.09ab	85.15ab	81.19ab	82.23ab	80.53b	80.62b	2.35	0.046	82.08	4.35
His	89.55a	80.06b	77.21b	76.80b	84.65ab	84.61ab	81.29ab	82.88ab	81.09ab	81.24ab	2.53	0.042	81.94	4.59
Lys	89.26a	76.52b	75.04b	78.50b	84.12ab	83.90ab	79.18b	79.43b	77.51b	79.19b	2.87	0.049	80.26	5.32
Arg	91.97a	82.08bc	80.84bc	78.74c	85.85abc	88.65ab	83.52bc	84.52abc	83.27bc	84.50abc	2.51	0.035	84.39	4.50
Trp	86.49a	73.19bc	72.09bc	65.36c	78.30ab	79.89ab	80.56ab	79.29ab	78.80ab	72.97bc	2.88	0.002	76.69	7.66
IAA	85.75a	74.61cd	74.44cd	71.20d	79.11abc	82.17ab	78.41bcd	78.37bcd	76.71bcd	76.11bcd	2.22	0.003	77.69	5.32
Dispensable AA
Cys	55.29	54.85	49.39	51.19	54.12	55.13	54.48	58.43	54.48	54.77	2.15	0.598	54.21	4.48
Asp	81.48a	69.42d	70.25cd	69.99cd	74.72abcd	76.55abcd	72.78bcd	77.72abc	72.93bcd	79.36ab	2.37	0.008	74.52	5.60
Ala	85.11	76.24	80.03	72.35	93.56	87.18	79.59	76.76	76.18	74.62	6.12	0.587	80.16	8.19
Glu	90.51a	79.05bc	79.40bc	79.25bc	85.67ab	87.58ab	85.88ab	85.16abc	82.56abc	75.08c	2.96	0.028	83.01	5.72
Gly	80.63ab	75.83abcd	74.35bcd	68.34cd	86.53a	83.29ab	81.98ab	77.82abc	75.38abcd	65.33d	2.89	0.002	76.95	8.57
Pro	88.83ab	80.34abcd	78.89cd	77.54d	89.72a	87.44abc	85.62abcd	82.52abcd	82.60abcd	79.67bcd	2.85	0.039	83.32	5.22
Ser	83.50ab	79.03abc	72.80cd	68.30d	86.12a	83.64ab	78.11abcd	81.51abc	75.29bcd	78.67abc	2.92	0.0098	78.70	6.90
Tyr	84.99a	73.10bc	66.85c	72.92bc	80.28ab	81.46ab	73.45bc	73.05bc	73.53bc	71.71bc	2.90	0.004	75.13	7.19
DAA	81.29	71.90	71.54	70.55	78.59	79.98	76.20	76.60	73.37	73.77	2.67	0.066	75.38	4.95
TAA	83.75a	72.36cd	73.09cd	70.94d	78.80abc	81.16ab	77.52abcd	77.58abcd	74.10bcd	74.98bcd	2.35	0.0064	76.43	5.33

Abbreviation: AA, amino acids; Ala, alanine; Arg, arginine; Asp, aspartic acid; Cys, cysteine; CV, coefficient of variation; DAA, dispensable amino acids; Glu, Glutamic acid; Gly,Glycine; His, histidine; IAA, indispensable amino acids; Ile, isoleucine; Lys, lysine; Leu, leucine; Met, methionine; Phe, phenylalanine; Pro, proline; Ser, serine; Thr, threonine; TAA, total amino acids; Trp, trptophan; Tyr, tyrosine; Val, Valine; SBM, soybeal meal; sources of SBM:Beijing (SBM1), Shandong (SBM2, SBM6, SBM10), Anhui (SBM3), Henan (SBM4), Fujian (SBM5, SBM7), Jiangshu (SBM8), Neimengu (SBM9).

¹Data represent the means of 6 replicates (n =6). ^a-d Means within a row lacking a common superscript differ (P < 0.05).

Correlations between chemical composition and AID or SID of AA in SBM

The AID of all indispensable AA, including IAA were not corrected with the DM, GE, CF, ash, NDF and ADF, and the AID of all dispensable AA, including DAA and TAA were not corrected with the CF, NDF, ADF and rLys (P > 0.05) (Table 7). The CP was negatively correlated with the AID of the indispensable AA (Met, Ile, Leu, Phe, His, Lys, Arg), IAA, the dispensable AA (Glu, Pro, Tyr), DAA and TAA (P < 0.05), the DM and GE were positively correlated with the AID of Asp (P < 0.05), the EE was negatively corrected with the AID of the indispensable AA (Met, Leu, Trp), and the dispensable AA (Glu, Gly, Pro) (P < 0.05), the ash was negatively corrected with the AID of Asp (P < 0.05), and the rLys was negatively corrected with the AID of Arg (P < 0.05).

Table 7.

Pearson correlation coefficients between proximate chemical composition of soybean meal and AID AA of soybean meal of the pullets

Item	DM	GE	CP	EE	CF	Ash	NDF	ADF	rLys
Indispensable AA
Met	-0.012	0.103	-0.349**	-0.227*	0.138	-0.004	-0.006	0.112	-0.074
Thr	-0.078	0.128	-0.123	-0.076	-0.082	0.085	-0.129	-0.050	-0.053
Val	0.018	0.002	-0.149	-0.149	-0.032	0.041	0.070	0.021	-0.055
Ile	-0.004	0.171	-0.209*	-0.050	-0.065	0.034	0.026	-0.005	-0.026
Leu	-0.058	0.096	-0.323**	-0.217*	0.028	-0.037	0.128	0.080	-0.109
Phe	0.018	0.097	-0.244*	-0.079	-0.018	0.071	0.066	0.015	-0.066
His	0.020	0.076	-0.202*	-0.090	-0.067	0.107	0.036	-0.027	-0.087
Lys	0.037	0.117	-0.320**	-0.072	0.031	0.104	0.077	0.062	-0.072
Arg	0.038	0.083	-0.192*	-0.056	-0.072	0.129	0.011	-0.011	-0.076
Trp	-0.035	0.02	-0.180	-0.213*	-0.028	0.034	0.035	0.028	-0.206*
IAA	0.009	0.075	-0.198*	-0.124	-0.033	0.062	0.021	0.021	-0.095
Dispensable AA
Cys	0.123	0.004	-0.002	-0.063	-0.071	-0.018	0.123	-0.044	0.007
Asp	0.219*	0.246**	-0.170	0.091	-0.132	0.195*	0.002	-0.036	-0.053
Ala	-0.172	0.125	-0.165	-0.098	-0.057	-0.014	-0.066	-0.067	0.018
Glu	-0.107	0.043	-0.263**	-0.263**	0.100	-0.141	0.117	0.128	-0.157
Gly	-0.175	0.022	-0.140	-0.295**	-0.012	-0.141	0.030	-0.024	-0.029
Pro	-0.063	0.073	-0.268**	-0.228*	-0.010	0.007	0.035	-0.017	-0.080
Ser	0.050	0.102	-0.092	-0.106	-0.185	0.090	0.004	-0.167	0.062
Tyr	0.040	0.010	-0.349**	-0.137	0.065	0.083	0.165	0.091	-0.087
DAA	0.030	0.113	-0.193*	-0.129	-0.039	0.001	0.062	-0.009	-0.016
TAA	0.031	0.125	-0.214*	-0.129	-0.027	0.026	0.039	0.020	-0.045

Abbreviations: AA, amino acids; ADF, acid detergent fiber; Ala, alanine; Arg, arginine; Ash, crude ash; Asp, aspartic acid; EE, crude fat; CF, crude fiber; CP, crude protein; Cys, cysteine; DAA, dispensable amino acids; DM, dry matter; GE, gross energy; Glu, Glutamic acid; Gly, Glycine; His, histidine; Ile, isoleucine; IAA, indispensable amino acid; Lys, lysine; Leu, leucine; Met, methionine; NDF, neutral detergent fiber; Phe, phenylalanine; Pro, proline; rLys, reative Lysine; Ser, serine; TAA, total amino acids; Thr, threonine; Trp, trptophan; Tyr, tyrosine; Val, Valine.

*P<0.05, **P<0.01

The SID of all indispensable AA, including IAA were not corrected with DM, GE, CF, ash, and ADF, and the SID of all dispensable AA, including DAA and TAA were not corrected with the DM, ash, NDF, ADF and rLys (P > 0.05) (Table 8). The CP was negatively correlated with the SID of the indispensable AA (Met, Ile, Leu, Phe, His, Lys), IAA, the dispensable AA (Glu, Pro, Tyr), DAA and TAA (P > 0.05), the GE was positively corrected with the SID of Asp (P < 0.05), the EE was negatively corrected with the SID of Met, Trp, Glu and Gly (P < 0.05), the CF was negatively corrected with SID of Ser (P < 0.05), and the rLys was negatively corrected with SID of Trp (P < 0.05).

Table 8.

Pearson correlation coefficients between proximate chemical composition of soybean meal and SID AA of soybean meal of the pullets

Item	DM	GE	CP	EE	CF	ASH	NDF	ADF	rLys
Indispensable AA
Met	-0.056	0.104	-0.455**	-0.315*	0.169	0.032	-0.044	0.140	-0.123
Thr	-0.277	0.184	-0.205	-0.214	-0.122	0.139	-0.344*	-0.052	-0.16
Val	0.005	-0.015	-0.188	-0.260	-0.060	0.031	0.090	0.019	-0.061
Ile	-0.045	0.244	-0.293*	-0.068	-0.09	0.058	-0.002	-0.002	-0.046
Leu	-0.088	0.107	-0.374**	-0.257	0.038	-0.04	0.131	0.098	-0.133
Phe	-0.016	0.134	-0.343*	-0.111	-0.015	0.105	0.059	0.031	-0.108
His	-0.013	0.099	-0.265*	-0.126	-0.091	0.150	0.012	-0.036	-0.128
Lys	0.010	0.145	-0.397**	-0.093	0.042	0.134	0.066	0.082	-0.098
Arg	0.014	0.102	-0.238	-0.074	-0.089	0.17	-0.017	-0.010	-0.109
Trp	-0.088	0.028	-0.259	-0.315*	-0.039	0.061	0.010	0.038	-0.300*
IAA	-0.035	0.098	-0.280*	-0.183	-0.051	0.104	-0.010	0.033	-0.165
Dispensable AA
Cys	0.200	-0.006	0.017	-0.146	-0.135	-0.041	0.206	-0.084	0.005
Asp	0.188	0.291*	-0.234	0.057	-0.139	0.191	0.010	-0.015	-0.102
Ala	-0.171	0.186	-0.164	-0.051	-0.109	0.064	-0.158	-0.114	0.045
Glu	-0.13	0.066	-0.309*	-0.299*	0.111	-0.138	0.099	0.151	-0.192
Gly	-0.294	0.069	-0.178	-0.421**	-0.071	-0.153	-0.069	-0.083	-0.023
Pro	-0.113	0.093	-0.296*	-0.253	-0.015	0.018	-0.011	-0.02	-0.098
Ser	0.016	0.166	-0.112	-0.160	-0.299*	0.143	-0.066	-0.267	0.096
Tyr	0.015	0.013	-0.413**	-0.158	0.081	0.106	0.164	0.109	-0.106
DAA	0.006	0.162	-0.260*	-0.165	-0.047	0.023	0.028	0.004	-0.050
TAA	0.004	0.183	-0.303*	-0.179	-0.035	0.051	0.007	0.041	-0.091

Abbreviations: AA, amino acids; ADF, acid detergent fiber; Ala, alanine; Arg, arginine; Ash, crude ash; Asp, aspartic acid; EE, crude fat; CF, crude fiber; CP, crude protein; Cys, cysteine; DAA, dispensable amino acids; DM, dry matter; GE, gross energy; Glu, Glutamic acid; Gly, Glycine; His, histidine; Ile, isoleucine; IAA, indispensable amino acid; Lys, lysine; Leu, leucine; Met, methionine; NDF, neutral detergent fiber; Phe, phenylalanine; Pro, proline; rLys, reative Lysine; Ser, serine; TAA, total amino acids; Thr, threonine; Trp, trptophan; Tyr, tyrosine; Val, Valine.

*P<0.05, **P<0.01

Prediction equation for amino acids digestibility

Stepwise regression equations for predicting the AID and SID of AA in growing layer based on the chemical composition of SBM are presented in Table 9. Several equations were developed to predict the AID for Met, Phe, Lys, Asp, and Gly, as well as the SID for Met, Phe, Lys, Ser, and Tyr, following a stepwise regression procedure. The best fit equation for AID of Asp were the DM, GE, ash and rLys (R² = 0.820, P = 0.042), and the SID of Lys were the GE, CP, CF, ADF and rLys (R² = 0.954, P=0.009), respectively. The least equation for AID of Gly were the EE (R² = 0.402, P=0.049), and SID of Tyr was CP (R² = 0.400, P = 0.050), respectively. Adding more indicators to predict the AID of Phe, SID of Lys increased the R² of the equations. For the AID of Phe, the R² increased from 0.631 to 0.776, when the NDF added. For the AID of Lys, the R² increased from 0.482 to 0.811, when the CF, and NDF added gradually. For the SID of Lys, the R² increased from 0.498 to 0.989, when the CF, GE, ADF, rLys, and ash added gradually.

Table 9.

Prediction equations of apparent and standardized ileal digestibility of some amino acids based on the chemical properties of soybean meal in the pullets

Amino acids	Basis	Prediction equations	R2	P-value
Met	CP	AIDMet= 215.219-3.652CP	0.462	0.031
Phe	CP, CF	AIDPhe=202.466-2.590CP-2.744 CF	0.631	0.030
	CP, CF, NDF	AIDPhe= 202.608-2.861CP-3.706CF+0.774NDF	0.776	0.022
Lys	CP	AIDLys= 167.018-2.193CP	0.482	0.026
	CP, CF	AIDLys= 227.639-3.190CP-2.626 CF	0.682	0.018
	CP, CF, NDF	AIDLys= 227.796-3.492CP-3.694CF +0.861NDF	0.811	0.014
Asp	DM, GE, Ash, rLys	AIDAsp= -725.034+4.243DM +22.664GE+3.754Ash -24.350 rLys	0.820	0.042
Gly	EE	AIDGly = 146.945-49.870EE	0.402	0.049
Met	CP	SIDMet= 210.727-3.193CP	0.450	0.034
Phe	CP, CF	SIDPhe=202.313-2.352CP-2.372CF	0.656	0.024
Lys	CP	SIDLys=170.991-2.011CP	0.498	0.023
	CP, CF	SIDLys=225.086-2.901CP-2.343CF	0.693	0.016
	CP, CF, ADF	SIDLys= 240.786-3.086CP-6.306CF+3.098ADF	0.799	0.016
	GE, CP, CF, ADF	SIDLys =561.204-13.768GE-3.964CP-8.990CF+4.073ADF	0.888	0.013
	GE, CP, CF, ADF, rLys	SIDLys= 637.180-18.279GE-4.534CP-9.949CF+4.871ADF+16.170rLys	0.954	0.009
	GE, CP, CF, Ash, ADF, rLys	SIDLys=764.325-22.719GE-5.133CP-10.939CF-1.404 Ash+4.875ADF+16.881rLys	0.989	0.005
Ser	CP, EE, CF	SIDSer= 265.032-2.424CP-22.728EE-5.831CF	0.768	0.025
Tyr	CP	SIDTyr= 178.011-2.281CP	0.400	0.050

Abbreviations: ADF, acid detergent fiber; Ash, crude ash; Asp, aspartic acid; EE, crude fat; CF, crude fiber; CP, crude protein; DM, dry matter; GE, gross energy; Gly, Glycine; Lys, lysine; Met, methionine; NDF, neutral detergent fiber; Phe, phenylalanine; rLys, reative Lysine; Ser, serine; Tyr, tyrosine.

Discussion

The importance of adequately determining AID and SID of AA values of different SBM in hen nutrition cannot by overemphasized, especially in the light of increased demand for corn and SBM. The SBM is sourced from the beans, and different properties of SBM can be attributed to the soybean cultivars (Ibáñez et al., 2020), growing environments (Goerke et al., 2012; Lagos et al., 2017; Sotak-Peper et al., 2017), the processing technic (Woyengo et al., 2010), and the assay method (Siegert et al., 2023), and so on. In the present study, ten SBM samples were collected from the major production areas in China, and their chemical compositions were analyzed. The averaged GE value of those samples was close to the values reported by Sheikhhasan et al. (2020) and Siegert et al. (2023), and the averaged CP contents of SBM were close to Lagos et al. (2017). Wang et al. (2023a) reported that 13.18 % NDF and 6.14 % ADF, which were certain difference to our values, and the averaged ash values is lower than our values, which could be attributed to the various planting environments of soybeans used in different studies. In addition, some studies roported that the rLys was a good predictor for the concentration of Lys in feed ingredients (Almeida et al., 2014; Ma et al., 2019). The variation coefficient of 10 rLys values is greater than 5 % in our study, which may be related to different heat treatments (Ma et al., 2019) . The AA contents also measured and found to be stable among the 10 SBM sample, the results were consistent with those reported by Wang et al. (2023a). In all, this indicated that the SBM samples were representative in China.

A typical NFD diet and management were used in the present study, and the basal losses of almost all AA were lower than ileal EAA losses in both 21 day broilers and 30-and 50-wk-old laying hens (Adedokun et al., 2014; Adedokun et al., 2018; Qiu et al., 2023), which is the reason that the influenced by the age of the bird, the presence or absence of dietary protein in the test diet, the length of feeding of the diet, the level and type of dietary fiber (Ravindran, 2021), dietary electrolyte balance and differing ratios of dextrose to corn starch (Zhou et al., 2022).

An accurate assessment of AA digestibility in feed ingredients is essential for its effective utilization in hens feed formulations. The AID is measured based on the net disappearance of ingested dietary amino acid from the proximal digestive tract to the distal ileum (Stein et al., 2007). Nevertheless, AID underestimates the actual digestibility of AA by neglecting the IEAA losses (Kong et al., 2013b), and correcting the AID by accounting for IEAA losses determines the SID of AA can obtain to the more accurate diet formulation (Adewole et al., 2017). Barua et al. (2023) reported that age influences on the AID and SID AA of SBM, depending on the protein source and AA. In our study, the AID and SID AA of SBM values were relatively less than reported by Siegert et al. (2023) for the AA digestibility of 18 SBM in cecectomized laying hens of 55 to 80 wk. The physiological stage of birds, the origin of SBM, the processing conditions of SBM, and the methodology used might affect the differences (Hernández et al., 2012; Kaewtapee et al., 2023; Ravindran et al., 2014a; Yun et al, 2023). Similarly the results of AA digestibility of SBM were also different from broilers and hens (Qiu et al., 2023; Adedokun et al., 2014; Ravindran et al., 2014b), and all both indicating that the AA digestibility of broilers was higher than that of laying hens, which could be attributed to the differences in breed and physiological stage. Additionally, in our study, the AID and SID of most AA in SBM1 were both higher SBM2, SBM3, and SBM4, which may be related to the chemical composition, origin, and processing of SBM, indicating that accurate evaluation of AA digestibility is essential in hens'formula. In addition, We also found that amino acid digestibility values are very low and much lower than typical for properly processed soybean meal (Yun et al., 2023). The diet we used for the typical SIDAA determination for feed ingredients may be not accurate for soybean meal because the included soybean meal level was 35.2 % and the overall CP content of the diet was about 17.3 %, which was higher than the CP requirement of pullets. The higher CP availability in the diet may lead to the lower value of amino acid digestibility. Moreover, the difference between AID and SID is much larger than normal in our study, it may be that diverse endogenous amino acid flow values obtained from different studies due to the special digestive physiological characteristics of laying hens at different physiological stages (Siegert et al., 2023; Barua et al., 2021), and contributing to the big variations. The specific reasons need to be further studied.

In our study, the correlation between fiber content and amino acid digestibility and betwen rLys and Lys digestibility in the practical diet were not observed in the semi-purified diet for soybean meal SID AA evaluation. It is possible that the excess of crude protein in the diet may compromise or mask the correlation which was usually observed in the practical diet. Furthermore, the CF in the SBM diets (35.2 % SBM, with a range of crude fiber from 4.82 % to 7.55 %) varied from 1.70 % to 2.66 %, and the small fluctuation in CF level may not trigger the decrease of amino acid digestibility. It was reported that the productive performance of laying hens improved with 40 or 50 g/kg more CF in feed, mostly from sunflower meal (SFM) supplementation, and the performance increase was attributed mostly to the increase in fat content, rather than DF level, when high-fibre SFM was included in layer diets (Koçer et al., 2021). Based on the mentioned study, it can be speculated that the 2.3 % soybean oil in the SBM diets may be also involved in the absence of the expected correlation between fiber content and amino acid digestibility.

To improve the accuracy of diet formulation in meeting the requirements for AID and SID of AA in the pullets, it is necessary to have a better evaluation of the digestible AA content of feedstuffs and to establish prediction for AID and SID of AA for ingredients. To the best of our knowledge, there is desperate lack of the evaluation the AID and SID of AA in SBM based on the chemical composition in the pullets. In recent years, many studies have reported that prediction equations for AID and SID of AA in ingredients fed to broilers or pigs can be established based on the chemical characteristics of the raw materials (Feng et al., 2023; Liu et al., 2015; Ma et al., 2019). We also calculated multiple linear regression aiming to predict AA digestibility based variables analyzed in the SBM samples, as performed previously (Siegert et al., 2023; Siegert et al., 2022). In the present study, we found that the CP values could be a reasonable predictor of AID and SID for some AA, and the CP was negatively correlated with AID or SID of Met, Phe, Lys, Asp, Gly, Ser, and Tyr, which is not consistent with the reported by Sheikhhasan et al. (2020), and similar to the reported by Wang et al. (2020), these differences may be related to the breed and physiological stage of poultry, feed sources, and etc., and the specific reasons need to be further analyzed. The predictive quality of these multiple regressions was neither consistent among AA nor sufficiently accurate in most cases, as indicated by the R² of the relationships between the cross-validated predictions and observed values. In our present study, adding more indicators to predict the AID and SID of AA, such CF, NDF, ash, rLys, the R² increased, which is similar to previous study reported by the Liu et al. (2015) and Ma et al. (2019). The rLys values could be a reasonable predictor of AID and SID for some AA in some feed ingredients (Almeida et al., 2014; Liu et al., 2015). In our study, adding the rLys indicators can increase the AID of Asp and SID of Lys, indicating that the rLys can be a reasonable predictor of AID and SID for some AA in SBM, although there was no correlation with AID and SID of AA. In addition, we also found that other factors (CF, ADF, NDF, ash, EE, etc) can predict the equation of the AID or SID of Phe, Lys, Asp, Gly, Ser, and even in some predict equations, and the R² increases with the addition of factors, which is similar with Sheikhhasan et al. (2020). However, this study also has some disadvantages, only some AA digestibility was predicted, especially SIDAA of SBM relatively few, may be added more factors such as trypsin inhibitor activity (Wang et al., 2023a), etc. in the future.

Conclusions

In summary, the result of this study indicated that chemical composition and the AID and SID of most AA in SBMs showed a huge difference, and the samples of SBMs were representative. The best fit equation for AID of Asp were the DM, GE, Ash and rLys (R² = 0.820, P = 0.042), and the SID of Lys were the GE, CP, CF, Ash, ADF and rLys (R² = 0.989, P = 0.005), respectively. The rLys can be used to predict the AID and SID of AA in SBM with reasonable accuracy together with the basal chemical composition, and prediction equations for AID and SID could be used to evaluate the digestibility of SBM in the pullets.

Disclosures

We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the content of this paper.