Amino acids are responsible for the most parts of physiological responses for growth and maintenance in the body such as nutrient metabolism, immunity, and protein deposition [19,20]. In particular, nursery pigs would be susceptible from multiple stressors during postweaning periods, leading to reduced feed intake and growth retardation with impaired intestinal functions, especially barrier function and nutrient utilization [21,22]. According to previous studies, AA balance and some specific AA could be significantly engaged in the modulation of immunity and recovery the intestinal damage induced from weaning in nursery pigs [23–25]. Thus, estimation of specific AA availability in feedstuff would be important to efficiently supply and utilize the AA for growth and health of nursery pigs.

Soybean meal is the main protein supplement in swine feeds, but it has been used at less than 20% in nursery feeds because the immature intestine of nursery pigs do not tolerate to digest the high levels of soybean meal [2,26,27]. Fermented soybean meal could be an effective source of soy protein and it is produced by mixing conventional soybean meal with water and a bacterial culture achieved from human food production, microbial fermentation, and then drying it on a plate or drum drier at a specific temperature to avoid heat damage to the protein [28]. Microbial fermentation by beneficial microbes such as Apergillus oryzae and Bacillus subtillits could effectively reduce anti-nutritional factors such as the trypsin inhibitor by 84%, glycinin by 40%, beta-conglycinin by 40%, and phytic acid by 35% with increasing protein content and producing smaller peptide size in conventional soybean meal [2,3,28]. Previous studies have shown that supplementation of FSBM positively affects growth performance and protein digestibility of nursery pigs compared with conventional soybean meal [3–5].

In this study, the analyzed compositions of indispensable and dispensable AA in FSBM were similar or slightly higher than those in conventional soybean meal described in NRC [7]. These difference would be possibly due to the inclusion rate of soybean hulls during the fermentation process, which may affect the AA concentrations in FSBM [29]. Microbial fermentation in soybean meal could decrease the composition of DM contents and trypsin inhibitors with increase in smaller peptide and fat contents and it would be accounted for by carbohydrate fermentation by Apergillus oryzae [2]. Thus, the use of fermented soybean would be beneficial to efficiently supply high AA through changing the AA profiles and reducing the antinutritional factors by microbial fermentation compared with conventional soybean meal.

In this study, the SID of AA in CBD and CFD diets for nursery pigs were relatively low compared with the SID of AA reported in previous studies. The CBD and CFD diets in this study were used to estimate the SID of AA in FSBM using a difference procedure. In contrast to the SID of AA in FSD, which was relatively similar to the SID of AA from previous literature [8,9,30], the possible reason for this observation would be related to the inclusion of corn as a major feedstuff in CBD and CFD diets. According to Oliveira et al [15], the SID of AA in the corn-basal diet and corn-SBM diet was over 85% for growing pigs. However, considering the growth phase of pigs, Trindade Neto et al [31] showed that the SID of AA in corn for nursery pigs was relatively low with the SID of most AA at below 30%. Sauer et al [32] also showed that the SID of AA in corn for nursery pigs was also relatively similar to the SID of AA in this study. It can be explained by the high variable digestibility of AA in corn [33–35]. However, the possible reason for this observation could also be related to the capacity of nursery pigs to digest the plant feedstuff during the post-weaning period, although corn is one of the most used cereals as an energy source containing less antinutritional factors. According to Mahan [36], starch from cereal grains is low palatable and less digestible for newly weaned pigs rather than lactose, because their digestive tract would be adapted to lactose digestion by milk consumption during lactation. Moeser et al [37] showed that intestinal damage by weaning stress would be sustained for about 14 d after weaning with impaired intestinal functions, resulting in a reduction in the digestive and absorptive capacity in the small intestine of nursery pigs. Lindemann et al [38] also showed the activities of digestive enzymes in nursery pigs could be reduced during the first 2 wks after weaning. In this study, although the pigs had an adaptation period for 10 d after weaning, their intestine would be limited to digesting the corn as a plant feedstuff in CBD and CFD diets. Therefore, the results in this study indicate that the inclusion level of a test feedstuff that affects the inclusion of other typical feedstuffs such as corn in test diets for a difference procedure could be important to determine the SID of AA in feedstuff for nursery pigs.

Amino acid digestibility in feed ingredients have been de termined either by direct procedure or difference procedure [13,14]. The direct procedure can be used for determining digestibility of nutrients in the feed ingredients with high feeding value such like palatable feedstuff and low content of antinutrients [13,14]. In difference procedure, when the test feedstuff would not be solely used as the major ingredient due to poor palatability, high content of protein, or anti-nutritional factors. For a difference procedure, testing diets are required to be formulated with both the test feedstuff and cereal feedstuff. A direct procedure has been widely used to evaluate the SID of protein feedstuff rather than a difference procedure [4,5,8,9,15], because having poor palatability or high content of proteins and antinutritional factors can interrupt the evaluation of AA digestibility in a feedstuff using a difference procedure [13]. Based on the findings in previous studies [2,3,28], it could suggest that FSBM at a practical inclusion level can have additivity of SID AA because microbial fermentation leads to the change in protein profile and the reduction of the concentration of antinutritional compounds in conventional soybean meal. The inclusion level of soy protein source below 20% in early weaner diets also has been considered the palatability and digestive capacity of nursery pigs [2,26,27].

In this study, the AID of dispensable AA including His, Leu, and Thr in FSBM from direct procedure were less than from difference procedure, but the SID of AA in FSBM were not different except for Thr. The possible reason for the non-additivity on AID and SID of these AA would be related to the contribution of digestible AA in corn for a difference procedure. This study considered the practical inclusion level of FSBM in nursery feeds and thus the experiment diets contained the relatively lower inclusion levels of FSBM at 20% in the testing diets compared with previous studies using about 30% [4,5,8–10,12]. It could also bring an increase in the portion of the use of corn in the test diets for a difference procedure. This may indicate that digestibility of AA in corn could result in relatively high variation in AID and SID of AA in corn and thus it may affect the estimation for a difference procedure to measure AID and SID of several AA in FSBM for nursery pigs. Previous studies have shown the cereal grains including corn could have high variable digestibility of AA [33–35] that were not also in accordance with NRC [7]. According to Stein et al [33], the AID of AA in feedstuff could not be additive depending on the AA composition in the testing diets. Stein et al [39] also showed that the low concentration of AA in feed ingredients may be potential to cause variation on the AID of AA due to the relatively greater contribution of AA of endogenous origin to the ileal output of AA in feed ingredients. Xue et al [40] also showed that the predicted AID of AA based on the values from mixed diets containing a high proportion of corn as low CP feedstuff be likely to be lower than the determined values by a direct method. Therefore, an increase in the portion of corn in the testing diets for a difference procedure may influence to be underestimated values for the AID and SID of several AA in FSBM using a difference procedure and thus resulted in the lack of additivity.

Most of the previous literature reported the SID of AA in FSBM for pigs by a direct procedure [4,5,8,9], but there is limited information about those from a difference procedure. Although this study shows the SID of AA in FSBM using both a direct procedure and a difference procedure, comparing the results with literature within the same method would be necessary. Comparing the SID of AA in previous reports with consideration of the type of FSBM, the BW of pigs, and the measuring procedure, interestingly, the SID of Lys in FSBM for pigs was lower than the values in conventional soybean meal. Cervantes-Pahm and Stein [4] also showed 9% lower SID Lys in FSBM with Aspergillus oryzae (77% vs 85%) than in conventional soybean meal for nursery barrows during 10.9 to 22.2 kg BW. The SID Lys in FSBM was 16% lower (75% vs 89%) than in conventional soybean meal in NRC [7]. The SID Lys in FSBM with Aspergillus oryzae and Bacillus subtilis was 2% lower (82% vs 84%) for nursery pigs at 10.4 kg initial BW in Rojas and Stein [5]. The SID Lys in FSBM with Streptococcus thermophiles and Saccharomyces cerevisiae was 5% (83% vs 88%) lower for growing barrows at 26.8 kg initial BW in Wang et al [9]. The SID Lys in FSBM with Aspergillus oryzae and Bacillus subtilis was 4% lower (86% vs 90%) for grower-finisher barrows at 30.4 kg initial BW in Yáñez et al [8]. Cervantes-Pahm and Stein [4] demonstrated that the reduced the SID of lysine would be results from the heating processing in FSBM production which is possibly causing the Maillard reaction with decreased the available Lys content. Besides, based on previous findings, the SID of AA in FSAM was not related to the BW of pigs or the procedure, because SID of AA in various types of FSBM would be influenced by the bacterial species as previously reported by Kim et al [11]. Indeed, the SID Lys in FSBMs seems to be getting increased as times go on. Based on the published years of studies, the initial studies showed underrated SID Lys for pigs compared with the latest studies. This study also shows that the SID Lys in FSBM was about 83 (difference procedure) to 88% (direct procedure) for nursery pigs. These values are relatively accordance in the values from the latest reports [8,9,30] rather than the initial reports [4,7,10]. It may be possibly due to the advances in the processing technology using microbial fermentation by specific bacteria to consider the impacts on available AA content in soybean meal for growing pigs [8,11,41]. Unfortunately, conventional soybean meal was not tested in this study indicating that it would not be available to directly comparison in the AA digestibility between FSBM and conventional soybean meal. Another possible reason could be suggested by the inclusion levels of FSBM in testing diets. Previous studies using a direct procedure with around 30% FSBM showed relatively low SID Lys (average 77%) in FSBM for nursery pigs [4,5,10]. In contrast, previous studies using a direct procedure with around 25% FSBM showed SID Lys at an average of 84% in FSBM for nursery pigs [9,12]. Due to possibly causing the low palatability or the content of antinutritional factors when a test feedstuff is added at high levels in test diets [13,14], the SID AA in FSBM would be variable by the inclusion level of FSBM in experimental diets.