1. Introduction
Protein (amino acids) is one of the expensive components of poultry ration. The efficiency of a protein feedstuff for poultry depends on its ability to provide a sufficient amount of essential amino acid prerequisite to the bird, as modern-day broilers are more sensitive to dietary protein (amino acids) than energy [
1]. Sustainable, efficient and ecofriendly poultry production requires highly digestible amino acids, resulting in an unremitting competition amongst poultry and humans for protein sources, including soybean meal (SBM). This competition leads to increased costs of SBM, creating a simulated food shortage among the poorest societies worldwide. Canola meal (CM) is the most commonly used plant protein source in poultry feed following SBM [
2], with lower CP than SBM (38 vs. 48%), although it is comparable with SBM in terms of amino acid content expressed per 100 g of CP. Canola meal is also a quality source of sulfur-containing amino acids, such as methionine + cystine (0.74 vs. 0.65), with a higher fat content than SBM (3.6 vs. 1.3% of DM) due to presence of triglycerides, phospholipids, glycolipids and free fatty acids. However, the CP contents in CM are less digestible (62%) than those in SBM (72%) [
2]. There is a limited use of CM in poultry feed, owing to its less metabolizable energy (2000 vs. 2230 kcal/kg) and the presence of various antinutritional factors containing fiber, glucosinolates, phytate, sinapine, non-starch polysaccharides and tannins [
3,
4] that may reduce the digestibility of AA and energy, leading to poor growth performance.
Whole wheat (WW) feeding in poultry, a common practice in wheat-dominant producing countries, can be an effective nutritional tool to enhance the availability of nutrients in low-digestible protein sources, including CM. These whole grains are crushed into small particles in the gizzard, resulting in a longer retention time leading to an increased gizzard weight as a result of prolonged mechanical stimulation [
5]. A well-functioning gizzard increases gut motility by enhancing cholecystokinin secretion [
6], which can induce the excretion of pancreatic enzymes and gastro-duodenal refluxes [
7]. This prolonged retention time in the gizzard consequently increases the efficacy of digestive enzymes and HCl released from the proventriculus into the gizzard [
5], which promotes efficient nutrient digestion and absorption, resulting in an improved growth performance in broilers.
Supplementation of organic acids, including butyric acid (BA), as a feed additive is an alternative way to enhance the availability of nutrients in broilers [
8]. The BA stimulates the multiplication and differentiation of gut epithelial cells, as it is an easily accessible energy source for these cells, improving mucosal immune response by increasing the growth of gut-associated lymphoid tissue and anti-inflammatory impact and reducing the detrimental microbiota population in the gut [
9,
10,
11,
12]. Supplementation of BA enhances functional growth of the gut due to an improved villus height (VH) and reduced crypt depth (CD), leading to increased availability of nutrients in broilers [
13]. This increased VH results in an increased surface area for nutrient absorption in the small intestine and improved nutrient utilization, resulting in better live performance of broilers [
11,
12].
It was postulated that BA supplementation, along with WW, in a less digestible protein source-based diet, may counterbalance the negative consequences of the protein source with respect to overall growth performance of broilers. Therefore, the present study was executed to explore the interacting impact of protein source (PS), diet structure (DS) and BA supplementation on live performance, carcass characteristics, gut development and health and apparent ileal digestibility (AID) of nutrients (CP and AA) in broilers.
4. Discussion
The present study was executed to analyze the interacting effects of PS, DS and BA supplementation on live performance, carcass characteristics, gut development and health and AID of protein and AA in broilers. We hypothesized that a WW based diet supplemented with BA may counterbalance the negative consequences of poorly digestible protein sources, including CM, on growth performance in broilers.
The reduced live performance (lower FI, reduced BWG and poorer FCR) and a decreased carcass yield in the broilers fed CM diets relative to those consuming SBM diets are in agreement with the results of previous studies reported in broilers [
14,
18,
19]. The observed compromised live performance in broilers consuming diets with increasing levels of CM may be related to various antinutritional factors present in CM, including tannins, glucosinolates, sinapine, NSPs and phytate [
20]. These antinutritional factors, including tannins and phytic acid, make protein–enzyme complexes in the GIT, resulting in a reduction in protein digestion [
2,
18]. This diminished digestion may lead to a reduced absorption and eventually compromised growth performance, which may also be explained by a poorer gut health (shorter villi, deeper crypts and reduced VCR;
Table 4) than broilers fed SBM based diet. This poorer gut health in broilers fed CM diets is in accordance with the literature on broilers [
14,
19], leading to reduced availability of protein and amino acids (
Table 4 and
Table 5). Longer villi and shorter crypts are associated with increased absorption of nutrients in the small intestine. Live performance results reported in this study also support the existence of shorter villi in broilers fed CM diets. Furthermore, extra maintenance energy and nutrients, are required for gut repair and liver metabolic functions when broilers are fed diets containing high levels (>20%) of CM, resulting in a reduced growth performance.
Poor gut health and reduced digestibility of nutrients (CP and AA) in CM consuming broilers indicate a decreased relative yield of carcass relative to those fed SBM diets, which corresponds to the results of previous research [
19,
21,
22]. Breast meat yield is believed to be hyper-sensitive to the amount and composition of dietary amino acids [
22]. A reduced digestibility of most AAs, including lysine, methionine, cysteine, threonine, arginine and glutamic acid, in CM fed broilers therefore led to diminished carcass characteristics, especially the breast muscles, because lysine is essential for the growth of the pectoralis major muscle (a breast muscle composed entirely of fast-twitch glycolytic fibers) [
23]. Similarly, via the formation of glutamate, arginine proliferates the quantities of proline and hydroxyproline that are essential for connective tissue synthesis [
24].
The improved growth performance (FI, BWG and FCR) in the broilers fed WW based diets compared to those fed GW based diets can be attributed to a well-functioning gizzard, which enhances gut motility, in particular, gastro-duodenal reflux [
7]. This reflux, along with a reduced passage rate of digesta, results in a prolonged time budget available for mixing of enzymes with feed particles in the gut. The reduced gizzard pH in broilers consuming WW relative to those fed GW may be explained by chyme reflux between the proventriculus and gizzard and increased HCl production. Pancreatic enzymes more efficiently denature and hydrolyze dietary protein in this low-pH environment, resulting in an enhanced protein digestion [
25,
26], which is confirmed by outcomes reported in the present study.
Lower crop and proventriculus relative empty weights in WW fed broilers may be a result of the short residence time of the feed in these organs relative to broilers consuming GW diets because a well-functioning gizzard acts as a FI controller and prohibits overconsumption of feed by vagal signals activated by stretch and muscular movement [
27] or by humoral signals comprising cholecystokinin, ghrelin and gastrin [
28]. This improvement in the gizzard weights may be the result of larger particle size of the diet, resulting in increased mechanical stimulations of the gizzard muscles, which lead to improved enzyme–substrate interaction, resulting in an improved nutrient digestion and gut health [
5,
29], as indicated by improved digestion of proteins and amino acids (
Table 4 and
Table 5) and improved gut health data (
Table 4). A higher relative weight of the pancreas may indicate increased pancreatic secretion in WW fed broilers relative to those fed GW based diets. The lower relative empty weights of the duodenums, jejunums and ileum in WW fed broilers can be attributed by their reduced activity due to a well-developed and functional gizzard, resulting in an appropriate pre-digestion in the foregut [
25]. Improved digestibility of most of AAs, including threonine, may also lead to improve gut health because higher digestibility results in an increased availability of threonine in the gut. This greater availability of threonine, for instance, stimulates mucin synthesis, leading to improved intestinal defense, its healing with an improved gut health [
30]. Threonine is a component of mucin, which accounts for approximately 40% of total digestive tract proteins [
31], whereas mucin itself contains about 30% threonine [
32], which protects the intestine from chemical secretions and pathogens, leading to an improved gut morphology.
Improved carcass yield in broilers consuming WW relative to those fed GW can be attributed to improved gut health, resulting improved nutrient digestion, especially protein and amino acids. A healthy and well-functioning gut may require fewer nutrients (protein and energy) for its maintenance and; therefore, more nutrients will be available for growth and carcass development. Decreased abdominal fat weight in broilers fed WW diets is in agreement with the results of a previous study of broilers [
33]. The improved digestibility of lysine and threonine also supports improved carcass characteristics in broilers fed WW based diets because the mentioned AAs are responsible for muscle development in broilers [
23,
24].
The improved live performance in the broilers consuming BA supplemented diets is in agreement with previously published data on broilers [
11,
12,
13,
14]. Supplementation of 0.004 g/g of BA glycerides resulted in an 8% improvement in BW and a 6% in FCR relative to broilers consuming diets without BA [
34]. Qaisrani et al. [
14] reported 7 and 4% improved BWG and 5 and 3% improved FCR in starter and grower phases, respectively, in broilers consuming diets with BA. The reduction in gizzard pH in broilers fed BA diets could be the result of BA absorption in the foregut, as about 60% of the BA is absorbed in this organ [
35]. The improved nutrients digestion and absorption in broilers fed diets with BA observed in the present study can be attributed to reduced gizzard pH, increased pancreatic secretion and the positive influence of BA on gut mucosal integrity and repair; furthermore, antimicrobial activity may have led to an improved growth performance. Improved gut health (increased VH, decreased CD and enhanced VCR) in broilers fed diets with BA could be the result of the availability of energy to enterocytes, as BA is the main energy source for these cells [
5]. The improved VH and VCR and decreased CD in broilers reared on BA supplemented diets are in line with literature reports of broilers [
11,
13]. Increased CD may have a deleterious impact on the performance of birds, as it increases the mucosa turnover rate, which is involved in increasing maintenance requirements in broilers [
36]. Morphometric changes in the gut as a result of BA supplementation extends the surface area for nutrient penetration, leading to enhanced growth performance, as confirmed by the data (
Table 2). As expected, BA supplementation enhanced the AID of protein and AAs in broilers relative to those fed diets without BA, in agreement with previous studies of broilers [
13,
37,
38]. Increased AA and protein digestibility may be the result of improved gut health (increased VH and VCR and reduced CD) in broilers fed BA supplemented diets relative to those fed diets without BA.
Decreased duodenal weight in broilers consuming diets with BA may be the result of reduced activity due to a functional muscular gizzard, which ensures maximum digestion, resulting in less work available for duodenum, leading to its decreased weight. The improved breast meat yield in broilers fed BA supplemented diets may be the result of improved ileal digestibility of nutrients (CP and AA), especially lysine and threonine, as discussed above. Lysine is the most critical essential amino acid involved in protein synthesis and breast muscle deposition in broilers. Nasr and Kheiri [
39] reported a 14% increased breast meat yield in broilers fed 10% extra dietary lysine relative to those consuming diets with NRC-endorsed dietary levels of lysine.
The results of the present study suggest that CM can lead to diminished live performance, compromised gut morphology and reduced AID of CP and AA. In contrasts, the inclusion of WW in broiler diets, resulted in a heavier gizzard, improved gut development and morphology and better AID of CP and AA, leading to improved FCR and carcass characteristics. Furthermore, supplementation of BA has additional positive impacts on overall performance of broilers fed WW based diets. In conclusion, WW based diets with BA enhanced the live performance of broilers consuming even less digestible protein source (CM).