1. Introduction
In the quest to reduce feed cost, improve animal uniformity, and minimize nitrogen excretion into the environment, non-ruminant animal nutritionists have embraced diet formulation on ileal digestible amino acid basis, especially in light of the increasing use of alternative feed ingredients. Diet formulation on a standardized ileal amino acid digestibility (SIAAD) basis is important in diet formulation on apparent digestibility basis because it accounts for basal ileal endogenous amino acid (EAA), hence the use of SIAAD values is more attractive. The relative advantages of diet formulation on SIAAD basis have been previously discussed [
1,
2,
3,
4,
5].
Several factors, including dietary protein and fiber levels, could influence ileal EAA losses in non-ruminant animals [
1,
4]. Dietary fiber contributes to an increase in EAA losses because of its effects on mucous secretion and cell proliferation [
6]. In order to obtain consistent and reliable EAA losses data for standardization of apparent ileal amino acid digestibility values, it is important to evaluate the effect of different dietary components, such as fiber type (soluble vs. non-soluble) and viscosity, of feed ingredients on basal ileal EAA flow. For instance, high levels of dextrose compared with high levels of cornstarch in a typical nitrogen-free diet (NFD) resulted in significantly higher ileal EAA losses in broiler chickens [
7]. Similarly, ileal EAA losses in mature broiler chickens were significantly higher in birds fed an NFD with a high dietary electrolyte balance (108 vs. 219 mEq/kg, [
8]). Additionally, the influence of fiber [
9] and fiber levels in healthy and challenged broiler chickens [
10], phytase and phytic acid [
11,
12,
13] have been reported. The method of estimating basal ileal EAA losses is also important. The total basal ileal EAA losses obtained by the regression method was higher in younger broiler chickens compared with values from the NFD method (day 5; 11,492 vs. 8692 mg/kg DM intake [
14]); however, this difference disappeared by day 15 [
14]. It has been reported [
12] that there was minimal difference in basal ileal EAA losses obtained through NFD, low casein diet, and regression methods in pigs.
The importance of estimating EAA losses and the effects of different dietary components on EAA losses in poultry is well documented [
3,
4,
9,
14,
15,
16,
17]. Despite the availability of information on the role of dietary fiber on ileal EAA losses, the effect of the inherent characteristics of the different components of feed ingredients using the regression methods has not been extensively studied in broiler chickens.
Dietary fiber affects EAA losses in a variety of ways. First, highly soluble fiber with high water-holding capacity (WHC) may result in high ileal EAA losses as a result of its effect on the epithelial wall of the gastrointestinal tract (GIT). Secondly, highly soluble fiber reduced feed intake in pigs [
18], consequently resulting in a higher proportion of amino acids of endogenous origin in the digesta. However, high solubility does not always translate to high WHC [
19]) and EAA flow changes with the level of fiber inclusion in the diet. An increase in EAA flow in pigs fed a NFD with an increase in dietary fiber was reported [
20], whereas a high level of solkafloc (75 vs. 25 g/kg diet) in the NFD did not significantly influence basal ileal EAA flow in 26-d old broiler chickens [
10]. Although it has been reported that different fiber sources do influence ileal EAA flow, there is little information on the effect of different fiber sources on ileal EAA flow in broiler chickens using the regression method.
Despite the plethora of available information on ileal EAA losses in broiler chickens, there is a need to further investigate the role of different dietary components (fiber type and viscosity) on ileal EAA losses in broiler chickens and cannulated growing pigs. The hypothesis of this study was that the feed ingredient with the highest WHC will result in a higher ileal EAA flow. Hence, the objective of this study was to determine the effect of CF, WB, and PEC on ileal EAA losses in 26-d old broiler chickens and cannulated growing pigs.
4. Discussion
The objective of this study was to investigate the effect of different test feed ingredient (WB, CF, and PEC) on ileal EAA losses in broiler chickens and cannulated pigs using the regression method. The level of fiber in CF and WB was high compared to that of PEC. In nonruminant animals, fiber, especially cellulose, is regarded as an antinutritional factor because of the inability of the animal to effectively utilize fiber due to their inability to digest it. At the same time, however, dietary fiber could affect the health of the GIT, especially the hindgut. Fiber in feed ingredients have generally been classified as soluble (ability to absorb moisture) or insoluble (resistant to moisture absorption). In general, fiber influences digesta passage rate and, as in the case of fiber with high WHC, the bulkiness of the digesta could result in an increase in the size (or volume) of the GIT and this could lead to an increase in the level of interaction between the digesta and the wall of the GIT as the digesta travels along the GIT. Corn fiber is a classic example of insoluble fiber with a low WHC (2.9 g of water retained/g of sample), while the PEC used in this study was soluble with relatively high WHC (>5.0 g/g). Based on our observation, the WHC of the WB (3.1 g/g) used in this study was higher than that of CF (2.9 g/g) but lower than that of PEC (<5.0 g/g). The effect of dietary fiber on feed intake, digesta passage rate, gut health, and EAA losses in poultry and swine has been well-documented [
18,
19,
29,
30]. The characteristics of the feed ingredient fiber that may influence EAA flow include its potential for bulkiness, which may result in abrasion and adsorption as well as its tendency for high viscosity arising from the potential for high WHC, especially for soluble fiber. The significance of the nature of dietary fiber (soluble or insoluble) on ileal EAA losses has not been extensively documented. Available information [
9] shows that soluble fiber is more likely to result in increased ileal EAA losses, while insoluble fiber (cellulose such as solkafloc) may not [
9,
10]. Results from the current study, however, could not be explained solely from the perspective of high viscosity, dietary crude protein level, and dietary crude fiber contents.
Increasing level of dietary nitrogen, protein, peptides, and amino acids have been shown to increase basal ileal EAA losses in non-ruminant animals [
1,
14,
31,
32]. The crude protein levels of the different ingredients evaluated in the current study were 10.8, 16.5, and 10.8%, respectively, for CF, WB, and PEC. However, a substantial quantity of the nitrogen in the PEC was not from protein origin compared with the remaining two ingredients (CF and WB). For instance, only about 10% of the nitrogen in the PEC was from amino acids (protein), which means about 90% of the nitrogen in the PEC was from non-protein source(s). The respective values for CF and WB were 91 and 85%. Furthermore, the analyzed fiber content in the PEC was about 2% of what it was in CF and WB. The ADF value for PEC was about 0.5% of that of CF and WB and the NDF was not detected in the PEC. Based on this, while CF and WB have similar chemical compositions, the chemical composition of PEC was different.
The higher ileal His, Glu, and Pro and the tendency for higher nitrogen, total amino acid, Ile, and Met losses in broiler chickens fed diets containing CF and WB compared with those fed diets containing PEC could be attributed to either the higher level of amino acids, crude fiber or both in the CF and WB diets. It has been reported in broiler chickens that higher dietary fiber level could result in higher ileal EAA losses [
17], while [
10] reported no difference in ileal endogenous total amino acid losses in the non-challenged broiler chickens fed low and high dietary fiber levels. The former study [
17] employed complete diets with fiber from corn, wheat, soybean meal, and cellulose while the later study [
10] used semi-purified diets with cellulose as the only source of fiber. The differences in the results of the two studies could be attributed to the type and level of fiber used. Even for the amino acids that showed no significant effect of ingredient type, the EAA losses from broiler chickens fed the PEC diets were consistently lower (PEC values for nitrogen, threonine, and total amino acid were 64, 76, and 68% of the average values for CF and WB). It is important to note that when values from birds fed CF were compared with values from birds fed WB, birds fed CF had higher ileal endogenous nitrogen, total amino acid, His, Ile, Met, Glu, Pro, and Tyr with six other amino acids showing tendency for higher losses. This finding is interesting based on the fact that both fiber types had similar crude fiber (10.0 vs. 9.4%) and ADF (11.3 vs. 11.4%) levels. Crude protein (10.8 for CF vs. 16.5% for WB), ash (0.8 vs. 6.3%), crude fat (5.6 vs. 3.5%), and NDF (49.8 vs. 44.4%) were the only components of the two ingredients that were different. Based on the available information on the effect of dietary amino acid peptides on ileal EAA flow, the additional dietary amino acids coming from WB were expected to have resulted in a higher level of EAA flow for birds on WB-based diets compared with birds on CF-based diets (9.4-percentage point difference for nitrogen). More importantly, WB contained a relatively higher level of nitrogen coming from non-protein source(s) compared with CF (15 vs. 9%). Hence, it is likely that the higher EAA and nitrogen losses from the CF-fed birds could not be explained by its crude protein level alone.
Seven amino acids and total amino acid showed higher (or tendency to be higher) endogenous losses when diet containing WB was fed to 26-d old broiler chickens compared with the PEC diet. This difference in EAA losses becomes even greater when losses from birds fed diets containing CF are compared with flow from birds fed PEC diets. This observation could be attributed to differences in the crude protein (especially the proportion of nitrogen from protein) and crude fiber levels of the two ingredients. The high level of nitrogen from non-protein sources in PEC resulted in lower basal ileal EAA losses. The effect of high viscosity of the digesta as a result of feeding PEC-containing diets was not sufficient to offset the effect of the quantity of nitrogen from non-protein sources on ileal EAA losses in 26-d old broilers.
The effect of CF on ileal EAA losses in cannulated growing pigs was different (higher) in only three amino acids (His, Leu, and Tyr) with similar endogenous nitrogen (4192 vs. 3997 kcal/kg DMI) and total amino acid (21,971 vs. 20,280 mg/kg DMI) values. Unlike in broilers, these data suggest that the nitrogen from PEC which is mainly from non-protein source(s) had a much higher effect on ileal EAA losses compared with what was observed in the broiler chickens. One of the questions that needs to be addressed is the possibility that the presence of nitrogen from non-protein nitrogen sources could have served as the nutrients for microbes in the distal section of the small intestine, the ileum, and hence might have resulted in the growth and proliferation of these microbes, leading to an increase in ileal EAA losses. This becomes a possibility because ileal endogenous nitrogen loss was 39% higher in broiler chickens fed CF compared with those fed PEC. However, in the cannulated pigs, the increase was less than 7%. The difference in these values (CF vs. PEC) in the pig was definitely associated with the interaction of PEC with the gastrointestinal tract (e.g., mucin, microbes). Ileal digesta bacteria contents have been shown to be influenced by the viscosity of the diet [
33] and dietary fiber types [
34]. The level of
Lactobacillus bacteria has been shown to be significantly increased by pea fiber compared with the control and soybean fiber [
35]. Furthermore, soybean fiber significantly increased the level of
E. coli in the ileal digesta compared with wheat bran. The implication for this study is unclear because the levels of non-protein nitrogen in the different diets used were not measured.
Another factor that could be considered is the effect of dietary fiber on gizzard development and its implications on ileal EAA losses. Although it has been established that high levels of dietary insoluble fiber could result in a reduction in the length of the small intestine [
36,
37] as well as an increase in gizzard weight [
37,
38,
39,
40], the effects of this on gizzard weight are not expected to influence ileal EAA losses in this study for a number of reasons. First, the level of fiber in the diets used in this study was less than 1% of the diet. Secondly, the particle size of all the ingredients used in the current study were similar, as already discussed. Thirdly, irrespective of what goes on within the gizzard, this organ is not known to produce any secretions that could influence the digestive capacity of the birds. More research needs to be conducted to further understand the effect of dietary insoluble fiber level on ileal EAA losses, as it related to the length of the mid gut.
This study showed that the effect of WHC on ileal EAA losses may not be as significant as the influence of crude fiber type and concentration. However, between CF and WB, it may be concluded that the effect of crude protein or total amino acid may not be as significant in ileal EAA losses as that of NDF. Finally, CF, the least soluble among the three samples evaluated in this study resulted in significantly higher EAA losses in 26-d old broilers compared with WB, which had higher crude protein and total amino acid concentration, and PEC, which had the highest WHC but lowest total amino acid and crude fiber. In order to fully understand the results from this study, it is important to estimate the effect of different source of nitrogen (protein nitrogen and non-protein nitrogen) on basal ileal EAA losses in broiler chickens and cannulated pigs. Furthermore, quantification of the type and relative quantities of microbes in the ileal digesta could answer some of the questions that arose from these studies.