1. Introduction
Wheat is widely available together with corn, and the wheat-based diet and corn-based diet are two commonly used commercial swine feed types worldwide [
1,
2]. However, wheat, corn, and co-products from these grains contain considerable quantities of non-starch polysaccharides (NSP), such as arabinoxylans, which may negatively affect the nutrient utilization of feed in pigs [
3]. Usually, the addition of carbohydrases is regarded as an effective strategy to reduce the anti-nutritional effects of NSP and improve the performance of pigs [
4]. To maximize this efficacy, multi-carbohydrase mixtures targeted at various NSP components of different feedstuffs were advised in many studies [
5,
6]. However, due to the great variation of NSP components in cereals and a mass of carbohydrase products in the market, it is usually difficult to select the optimal carbohydrase mixture (OCM) [
7].
Compared with the in vivo method, which is regarded as time-consuming, expensive, and labor-intensive, the in vitro method could provide a simple and rapid technique to evaluate the efficiency of exogenous enzymes and screen the combination of the optimal enzymes [
8]. In our previous studies, two OCMs for corn-soybean meal-based (CSM) diet and wheat-soybean meal-based (WSM) diet were screened, respectively, using a new in vitro method developed by our research group [
9,
10]. Under the effects of these OCMs, the in vitro ileal dry matter digestibility (IVDMD) was improved by 3.26% in the CSM diet and 3.48% in the WSM diet [
11,
12]. However, further in vivo study is still needed to determine if the OCM has similar positive effects on nutrient digestion in pigs.
As previously reported, the responses to exogenous enzymes for corn-based diets were often inconsistent with those of wheat-based diets in pigs [
13]. This possibly attributes to the structure and physicochemical characteristics of the NSP, especially the variations of soluble NSP and insoluble NSP contents between wheat-based diets and corn-based diets [
14,
15]. In addition, the effects of exogenous enzymes originated from various bacteria and fungus may also be inconsistent for pigs fed corn- or wheat-based diets [
16]. However, limited research studies have compared the effects of carbohydrase mixtures for pigs fed the corn-based diet with the wheat-based diet.
Therefore, this study was conducted to test the hypothesis that two OCMs screened by one new in vitro method had positive effects on the nutrient and energy digestibility in pigs, and these positive effects were consistent for both the corn-based diet and the wheat-based diet. Thus, the objectives of this experiment were to investigate the effects of two OCMs on macronutrient digestibility, amino acid (AA) ileal digestibility, and digestible energy in growing pigs fed two types of basal diets.
3. Results
The SNSP, INSP, and TNSP contents in wheat bran and soybean meal were 2.61%, 27.13%, and 29.74% and 1.89%, 18.29%, and 20.18%, respectively, which were greater than those in wheat (1.80%, 8.02%, and 9.82%) and corn (0.60%, 7.04%, and 7.63%;
Table 1). The concentrations of arabinose, xylose, glucose, and arabinoxylans in wheat bran were numerically greater than those in other ingredients, and the soybean meal had the numerically greatest galactose and uronic acid contents. The ratio of SNSP to TNSP in wheat was 0.18 and greater than the other three ingredients, which ranged from 0.08 to 0.09.
The factor of diet had significant effects on the AID, ATTD, and AHD of all macronutrients and energy, IDE content, and DE content (
p < 0.01) in the present study. The AID of DM, NDF, and GE for pigs fed the CSM diet were similar to those of the WSM diet (
Table 4). Pigs fed the CSM diet had greater AID of EE and CHO than those of the WSM diet (
p < 0.05), whereas the AID of ash and ADF for pigs that consumed the WSM diet were greater than those of the CSM diet (
p < 0.05). The IDE content in the CSW diet was 13.03 MJ/kg and similar to that in the WSM diet (12.95 MJ/kg). The AID of DM, ash, CHO, NDF, and GE in the CSM diet was improved by the addition of OCM (
p < 0.05). The OCM addition also increased the AID of NDF and ADF for pigs fed the WSM diet (
p < 0.05). However, the pigs that consumed the WSM diet with the inclusion of OCM had less AID of DM, CP, ash, CHO, and GE (
p < 0.05) than the pigs fed the OCM-free WSM diet. The IDE content of the CSM diet was increased from 13.03 MJ/kg to 13.45 MJ/kg under the effect of OCM (
p < 0.01), whereas the IDE content in WSM was decreased by 9.4% with the OCM addition (
p < 0.01).
The ATTD of DM, EE, ash, CHO, NDF, ADF, and GE of pigs fed the CSM diet were all greater than those of the WSM diet (
p < 0.05,
Table 5). The DE content of the CSM diet was 15.45 MJ/kg and greater than that of the WSM diet (15.03 MJ/kg). There was a 1.71%, 1.35%, and 1.60% increase for the ATTD of DM, CHO, and GE, respectively, for pigs fed the CSM diet under the effect of OCM (
p < 0.01). Furthermore, positive effects were discovered in the ATTD of DM, CP, EE, ash, and GE in the WSM diet under the OCM addition (
p < 0.01). There was a 0.29 MJ/kg and 0.46 MJ/kg increase, respectively, for the DE contents of CSM and WSM diets in pigs under the effects of OCM (
p < 0.05).
Compared with the WSM diet, the CSM diet had greater AHD of DM, EE, ash, NDF, ADF, and GE (
p < 0.05,
Table 6) and similar AHD of CP and CHO. There was no difference in AHD of CP, EE, ash, CHO, NDF, ADF, and GE for pigs fed the CSM diet with or without the OCM addition except the AHD of DM, which was reduced under the effects of OCM (
p < 0.05). On the contrary, the AHD of DM, CP, EE, ash, CHO, and GE in the WSM diet were all improved by the OCM supplementation (
p < 0.05), while the AHD of NDF and ADF were not affected by the OCM addition.
As shown in
Table 7 and
Table 8, most AA ileal digestibility was affected by the factor of diet (
p < 0.05), except for the AID and SID of Arg and Met. The pigs fed the CSM diet had greater AID and SID of Leu, Ala, and Asp, but less AID and SID of Lys, Phe, Cys, Glu, Gly, Pro, and Tyr compared to the WSM diet (
p < 0.05). In addition, the AID of His and Thr of pigs fed the CSM diet were also found greater than those of the WSM diet (
p < 0.05). There were no differences in AID and SID of most AA for pigs fed CSM diet with or without the OCM addition, except that the AID and SID of Ile, Thr, and Cys were improved by the inclusion of OCM (
p < 0.05). On the contrary, the negative effects of OCM addition to the WSM diet were discovered in AID and SID of Ile, Phe, Thr, Val, Ala, Pro, Ser, and Tyr (
p < 0.05).
4. Discussion
As reported by Bach Knudsen et al., [
14] and Chen et al., [
32], the INSP contents of the ingredients used in this study were greater than the SNSP content, and the arabinose, xylose, and glucose were also the main NSP components. The TNSP and INSP contents in corn or wheat were less than values reported by Abelilla and Stein [
13], but the soybean meal or wheat bran had greater TNSP and INSP contents in this study than those in Abelilla and Stein’s [
13] study. As expected, the wheat bran had the greatest SNSP, INSP, and TNSP contents than those of three other ingredients and the ratio of SNSP to TNSP in wheat was the greatest in this study, which agreed with previous reports [
13,
14].
Dietary fiber is not only difficult to be degraded by endogenous enzymes but is also reported to encapsulate other nutrients, and then negatively affects nutrient and energy digestibility [
32]. Therefore, in the present experiment, the total tract digestibility of most macronutrients and energy was found greater in pigs fed the CSM diet than the WSM diet. These results are in agreement with Jaworski et al., [
33] and Zhao et al., [
34] who reported an increased level of fiber in the diet decreased the ATTD of nutrients and energy. The AID of ADF in the WSM diet was greater than that in the CSM diet and the AID of NDF also had this similar tendency. This was likely a result of the fact that there were more substrates of ADF and NDF in the wheat-based diet than the corn-based diet [
13,
16]. The AHD of DM, EE, ash, NDF, ADF, and GE were found to be greater in the CSM diet than those of the WSM diet, which agreed with Abelilla and Stein [
13]. This indicated that the dietary fiber components in the CSM diet were more fermentable than those in the WSM diet and could release more nutrients in the hindgut section. The observation that the DE content in the CSM diet (15.45 MJ/kg) was greater than that in the WSM diet (15.03 MJ/kg) was possibly due to the fact that the corn-based diet contained less NDF and ADF compared with the wheat-based diet (NRC, 2012) [
15]. Additionally, the greater DE content could have been attributed to the greater EE content and increased ileal and total tract digestibility of EE in the CSM diet [
35,
36].
In the present experiment, the OCM addition to the CSM diet increased the IDE and AID of most macronutrients except EE and ADF; meanwhile, the ATTD of DM, CHO, and GE was improved by the OCM addition. The DE content in the CSM diet was also increased from 15.45 MJ/kg to 15.74 MJ/kg under the effect of the OCM. This is consistent with most previous studies that reported that exogenous fiber-degrading enzymes had positive effects on nutrient digestibility in pigs fed the corn-soybean meal diet [
37,
38]. The mechanism might be that the OCM broke the long-chain structure of the polymer, removed the side chain groups, released nutrients in the feed, and thus improved the digestibility of nutrients in the CSM diet [
39].
In the present study, it is interesting to find that the in vitro method provided an improvement in ileal digestibility of DM (3.48%), while in vivo data show 7% reduction in AID of DM in WSM diet inclusion of OCM. The reason may be that in vitro methods cannot completely simulate the physiological structure and digestive enzymes of animals, so there are certain differences between in vivo and in vitro results [
40]. Therefore, in vitro experiments cannot completely replace animal tests and need to be verified in vivo. These results also suggest that in vitro enzyme screening methods may not be applicable to WSM diets. Additionally, the AID of CP, ash, CHO, and GE in the WSM diet was decreased with the inclusion of OCM. Similar results were also found in some previous studies, which found that the exogenous enzyme addition had negative effects on nutrient and energy digestibility in the foregut section [
13,
41]. However, on the contrary, more studies reported that the exogenous enzymes supplementation could increase the nutrient and energy digestibility for pigs fed wheat and/or wheat by-products diets [
42]. As previously reported, the differences in the fiber component and structural arrangement, and the type and quantity of wheat or wheat by-products used may lead to the inconsistent efficacy of carbohydrases [
24,
39]. Usually, INSP could increase the digesta passage rate, reduce the endogenous enzymes reaction time, and lead to the decrease in nutrient digestibility [
43,
44]. On the contrary, SNSP could increase the viscosity of digesta and slow down the digesta passage rate [
45]. In the present study, the AID of NDF and ADF were all increased under the effect of OCM addition for pigs fed the WSM diet, which might decrease the digesta viscosity in the foregut section and lead to the speedup of digesta flow [
43]. Subsequently, that might lessen the reaction time of nutrients with endogenous digestive enzymes and reduce the nutrient ileal digestion for pigs fed the WSM diet in this study [
32,
36]. The starch content may also be one of the factors affecting the digestibility. The starch source and amylose/pullulan ratio all affect the digestibility of starch and other nutrients in animals [
46]. Starches from different grains also respond differently to enzymes. Yang et al., [
47] have reported that in wheat and wheat by-product-based diets, the multi-carbohydrase and phytase complex supplementation did not significantly increase the AID and (or) ATTD of starch. In addition, different sources and dosages of exogenous enzymes applied in diets may also have various efficacies [
13,
16], which also could induce the decrease in nutrient ileal digestion of the WSM diet in this study. Different from the foregut section digestion, the AHD of DM, CP, EE, ash, CHO, and energy in the WSM diet were all improved by the OCM greatly, which also led to the increase in the ATTD of DM, CP, EE, ash, and GE. The DE content was also increased from 15.03 MJ/kg to 15.49 MJ/kg, which was similar to most previous reports [
48]. This might be attributed to the possibility that some undigested nutrients in foregut under the effect of OCM could have transferred to hindgut for fermentation by microorganisms [
49].
Usually, there are negative effects of high-level fiber on the AA utilization in pigs, which probably occurs due to the increased endogenous AA loss, and low availability of AA in fiber-rich diets [
39]. However, the AID and SID of Lys, Phe, Cys, Glu, Gly, Pro, and Tyr, and the AID of Val were found to be decreased in the CSM diet than the WSM diet, whereas the AID and SID of Leu, Ala, and Asp, and the AID of His and Thr were found greater. The AID of CP for pigs fed the CSM diet was also found to be less than those in the WSM diet. As mentioned above, the ileal digestibility of fiber in the WSM diet was greater than those in the CSM diet, which might release more dietary protein in the foregut. Moreover, the greater SNSP content in the WSM diet might slow down the gastric and intestinal emptying process and give the dietary protein increased exposure time to proteolytic enzymes [
50]. Furthermore, there was a greater starch content in the WSM diet compared with the CSM diet, which could affect the utilization of AA [
39]. Therefore, most AA and CP digestibility for pigs fed the WSM diet was greater than the CSM diet in the current study, although the WSM diet contained greater fiber.
The carbohydrase addition could increase the fiber decomposition, reduce the endogenous losses, enhance the access to protein for endogenous proteases, and improve the hydrolysis of dietary protein [
32,
39]. In the current experiment, the OCM addition to the CSM diet increased the AID and SID of Ile, Thr, and Cys, and these results are consistent with previous studies [
38]. The low-level of dietary fiber in the CSM diet might be not enough for OCM to cause a significant increase in the ileal AA digestibility in this study [
51]. Same with the tendency of AID of CP in the WSM diet, the AID and SID of most AA for pigs that consumed the WSM diet were decreased by the addition of OCM, which agreed with Ji et al., [
50], in which the AID and SID of Met, Ala, and Ser of pigs were found to be decreased with the supplementation of exogenous enzymes. This result was extremely different from previous studies that used wheat or wheat by-products as the fiber source [
42]. As discussed above, possible reasons for the difference might be due to the various fiber component and structural arrangement, the type and quantity of wheat or wheat by-products used, and sources and dosages of exogenous enzymes applied in diets.