1. Introduction
The occurrence of mycotoxins is one of the major concerns in the swine feed industry, and mycotoxins are generated by fungal activity as secondary metabolites [
1,
2]. These metabolites are commonly found in grains such as barley, corn, and wheat, which are major ingredients of swine diets, resulting in deleterious effects on pig growth. Deoxynivalenol (DON), generated by the
Fusarium fungus, in pig diets typically induces vomiting and reduces feed intake in young pigs, which could cause subsequent growth retardation of pigs [
3,
4,
5]. Additionally, dietary DON can affect the morphology of the intestinal mucosa [
6], the immune system [
7], and the health of pig organs [
8].
There have been efforts to alleviate the adverse effects of dietary DON on the growth performance of pigs by supplementing feed additives to swine diets. Clay products are well known to adsorb mycotoxins in diets [
8,
9,
10] and therefore are often used in swine diets. However, limited efficacy of clay products for adsorbing DON and compensating for the reduced growth performance of pigs has been reported in previous in vitro studies [
9,
10] and in vivo studies [
11,
12], respectively. Moreover, clay products potentially bind nutrients including minerals and amino acids (AA) in addition to mycotoxins because clays’ binding capacity is not specific [
13]. While Li and Kim [
14] and Van Le Thanh et al. [
11] reported that a supplemental clay product did not affect phosphorus (P) and calcium (Ca) digestibility, Schell et al. [
15] observed decreased absorption of P and Ca as well as other minerals including magnesium (Mg) and zinc (Zn) by supplemental clay product in pigs fed an aflatoxin-contaminated diet. Considering these inconsistent results among the literature, more information is needed on the influence of clays on mineral digestibility. Additionally, the influence of clays on AA digestibility is also unknown, in spite of the fact that clays potentially adsorb AA.
Similar to clays, blend products consist of preservatives, and various nutrients have shown to partially restore the growth retardation in pigs fed DON-contaminated diets in previous studies [
11,
16]. However, limited data for the effects of blend products containing preservatives and other nutrients on the digestibility of minerals and AA are available. To bridge these gaps, therefore, we aimed to determine the effects of supplementing a bentonite (BEN) product and a blend product containing preservatives and nutrients on the ileal digestibility of minerals and AA in pigs fed diets naturally contaminated with DON.
4. Discussion
Deoxynivalenol is one of most prevalent mycotoxins in swine diets and naturally occurs from
Fusarium fungus [
2]. Ingested DON by pigs causes inflammation and necrosis in the gastrointestinal tract of pigs, leading to a disturbance of gut function for nutrient absorption [
6] and consequently, growth retardation of pigs [
3,
4,
5]. Although a DON-contaminated diet can be supplemented with feed additives such as clay products and preservatives to alleviate negative influences of DON, the effects of supplementing these additives on ileal and total tract digestibility of minerals and ileal digestibility of AA are not well documented. Particularly, the potential adsorbing action of supplemental clays against minerals and AA is anticipated, but the regarding data for the negative effects on minerals and AA digestibility of pigs are scarce.
In the present study, the analyzed DON concentrations in diets containing DON-contaminated wheat were reasonably close to the expected values, except for the PB diet, which had a less analyzed DON concentration compared with the expected value. The low DON concentration in the PB diet is consistent with Van Le Thanh et al. [
11], who reported a relatively low DON concentration in the diet. The low DON concentrations in DON-contaminated diets supplemented with the PB may be attributed to the influence of the preservative components in the PB that potentially inactivate DON [
24,
25]. These findings are supported by results from Dänicke et al. [
26], who observed a dose-dependent reduction in DON concentration when supplementing sodium metabisulphite to the diets containing triticale kernels contaminated with 6.6 mg/kg of DON. The PB used in the present study also contained sodium metabisulphite.
The lack of difference in the apparent digestibility of DM and CP between the UCD and CD groups agrees with previous studies [
24,
27]. In contrast, Holanda et al. [
28] reported decreases in the AID of DM, GE, and nitrogen in nursery pigs fed the DON-contaminated diet, but this may be due to the effects of other mycotoxins including fumonisin and zearalenone, as well as DON. Additionally, the response to dietary DON may vary depending on the growth stages of pigs. While dietary DON resulted in decreased DM and GE digestibility in nursery pigs [
11,
28], dietary DON did not affect DM or CP digestibility in growing pigs in the present work and other studies [
24,
27]. Older pigs may be less responsive to dietary DON compared with nursery pigs. The lack of effects of supplemental BEN or the PB on nutrient digestibility in the present work may be at least partially due to a lack of impaired digestibility of DM and CP by dietary DON.
The lack of the effects of dietary DON on the apparent digestibility of Ca and P in the present study is in agreement with Kwon et al. [
12], who reported no effects of DON on Ca digestibility. In contrast, Bouchard et al. [
24] reported an increase in AID of Ca and P in pigs fed a DON-contaminated barley diet (4.49 mg/kg of DON). In addition, Van Le Thanh et al. [
11] also observed an increased retention-to-intake ratio of Ca and P by dietary DON at the level of 4.61 mg/kg in pigs. Dietary DON has been reported to reduce serum concentrations of Ca and P [
29], likely leading to an increased absorption of Ca and P [
30]. However, the reason for no response in Ca and P digestibility by dietary DON observed in the present work may be partially attributed to the relatively low dietary DON concentration compared with other studies.
The limited effect of supplementing BEN on the digestibility of Ca and P in pigs fed the DON-contaminated diet is consistent with previous studies [
12,
31,
32], in which various clay minerals including BEN and maifanite were employed. These observations indicate that BEN may not largely adsorb Ca or P in the gastrointestinal tract of pigs. On the other hand, the observation that supplementing the PB did not affect the AID of Ca and P in the present work contrasts with Van Le Thanh et al. [
11]. It is suggested that supplementation of the PB can alleviate the deterioration of health in pigs caused by the consumption of DON. Preservative components can inactivate DON in diets [
24,
25], and other components in the PB such as AA, vitamins, and microorganisms may improve pig health impaired by DON. However, the lack of effects of the supplemental PB on Ca and P digestibility remains unclear, although the relatively low DON concentration may partially explain the present results.
Very limited information is available for the effects of DON on mineral digestibility. The AID of K and Mg was not affected by dietary DON in the present study. Similarly, Zeebone et al. [
33] fed a diet contaminated by fumonisin at 30 mg/kg to nursery pigs and found no changes in K and Mg digestibility due to fumonisin. In addition, aflatoxin B
1 did not affect the absorption of Mg in pigs [
15]. These results suggest that although mycotoxins in diets can negatively affect the growth performance of pigs, these toxins may not interrupt or stimulate the absorption of K or Mg. While the addition of BEN did not affect the apparent digestibility of Mg in the present study, Schell et al. [
15] reported decreased Mg absorption by adding clays to diets regardless of DON contamination. The decrease in Mg absorption was also observed in sheep fed diets containing a clay product [
34]. The reason for this discrepancy may include different sources of clay products, experimental conditions, and animals in the experiments.
For the effects of mycotoxins on Na absorption, inconsistent results have been reported in previous studies [
13,
31]. The increased Na absorption by consuming aflatoxin B
1 in pigs was observed by Schell et al. [
15], whereas no effect of dietary fumonisin on Na digestibility was observed by Zeebone et al. [
33]. In the present work, the AID of Na was decreased by dietary DON. These observations suggest that the mechanism of the influence of mycotoxins on Na absorption in animal bodies is still unclear. However, different mycotoxins may have their own specific mechanism of interaction with Na. Additionally, as well as Na, the interaction of mycotoxins with the metabolism of K can also affect Na absorption in animal bodies because they serve as a part of the Na–K pump to balance anions intracellularly [
19]. Clay products potentially degrade chlorinated compounds [
35] and they might partially liberate Cl ions from the chlorinated compounds originating from diets and endogenous losses, increasing the absorption of Cl. Additionally, changes in Na metabolism and acid-base balance could affect the metabolism of Cl in pigs because Cl is associated with maintaining osmotic pressure in body fluids and acid-base balance with Na [
19]. However, the reason for the lack of changes in the apparent digestibility of Cl due to BEN in the present study remains unclear. Further research is warranted to investigate the effects of dietary DON and supplementing mycotoxin-sequestering agents on macromineral digestibility.
The limited effects of dietary DON on the apparent digestibility of Fe of pigs observed in the present study agree with previous studies [
15,
36,
37]. The increased digestibility of Zn by dietary DON observed in this study is in agreement with Yunus and Bohm [
38], who reported that broilers fed diets contaminated with an increasing DON concentration (0, 1.7, and 12.2 mg/kg) for 5 weeks showed a linear increase in serum Zn concentration. A similar result was reported by Schell et al. [
15], who fed an aflatoxin B
1-contaminated diet to pigs and found an increase in Zn absorption. Exposure to mycotoxins has been suggested to alter the demands for Zn or the efficiency of Zn absorption in animal bodies [
15,
38]. As observed in the present work, the ability of clay products to decrease Zn digestibility or absorption is relatively clear. Schell et al. [
15] reported that the Zn absorption was decreased by supplementing BEN in diets fed to pigs regardless of aflatoxin contamination. Chestnut et al. [
34] also reported an approximately 50% decrease in Zn absorption due to supplementation of aluminosilicate in sheep. Because clay minerals have a large surface area and high structural charge, the divalent transition metals such as Zn can be adhered by clays [
39], resulting in reduced absorption of Zn in animals.
Whereas no effect of dietary DON on the AID of all indispensable AA was observed in the present study, supplementing cultured DON to diets at the level of 10 mg/kg tended to decrease the digestibility of Lys, Thr, Trp, and Val compared with control diets fed to pigs [
27]. These inconsistent results are likely due to the difference in dietary DON concentrations or the source of DON. High concentrations of dietary DON have the potential to destruct barriers of the intestinal epithelium, which has been confirmed by previous studies [
40,
41]. The decreased AID of some AA by supplementing BEN is likely because AA molecules can be adsorbed to the clay surface, which has been identified by previous studies [
42,
43]. It is likely that dietary AA, undigested AA, or both are adsorbed to clay products and excreted in the fecal form, leading to decreased digestibility. However, further research is warranted to identify the reasons for the effects of BEN on AA digestion and absorption in pigs.