1. Introduction
In modern poultry production, feed costs cover about 70–75% of total costs of production, with maize and soybean meal being the mostly used conventional feed ingredients [
1,
2]. Future predictions on use of soybean meal use as a protein source for both humans and animals pointed out potential problems mainly due to factors such as availability, the risk of over-reliance on a single ingredient and production costs [
3]. The use of non-conventional feed materials could sustain the poultry industry by alleviating the shortage of feed materials [
4]. The need for such conventional feed materials have never been so real than the present era that humankind world-over finds themselves in. Since the outbreak of the coronavirus disease (COVID-19) pandemic, we have experienced a 2% decrease in production; as well as global chicken meat reduction of 1% [
5]. Among other reasons, these reductions may be due fluctuations of poultry supplies and feeds. Consequently, nutritionists are constantly in search of alternative feed ingredients that are readily available, affordable and nutritious [
1]. Sunflower meal (SFM), an inexpensive by-product of agro-industry origin is one promising alternative feed ingredient that can partially replace the inclusion of soybean meal in poultry diets [
6]. It is of broad availability globally, due to its wide adaptability to a range of soil and climatic conditions, it is rich in crude protein content, methionine, and have limited antinutritional factors [
7,
8,
9]. However, its use in broiler feed has been limited by low levels of lysine and high crude fibre, high non-starch polysaccharides concentrations, low metabolizable energy content and some presence of chlorogenic acid [
10,
11,
12,
13].
Some studies indicated an inclusion level of 15% SFM in broiler diets without any negative effects on broiler performance and/or other measured parameters [
14,
15,
16], whereas some studies reported that it can be used at higher levels with no adverse effects on utilization and growth performance of broiler chickens [
11,
17,
18], especially with the addition of enzymes [
6].
On the contrary, there were studies which reported that the use of 10% and more SFM in broiler diets negatively affected the growth performance [
19,
20,
21,
22]. Additionally, Ref. [
23] reported that SFM can be used up to 20% in growing quail diets without negatively affecting performance. In their study [
24], postulated that the inclusion of SFM at higher levels may necessitate the need of supplementation with synthetic lysine and oils in the diet in-order to compensate for the low metabolizable energy (ME) associated with this ingredient. However, supplementation with extra fats to rectify the low ME associated with SFM use must be done with caution, and huge attention in terms of fats storage must be exercised due to rancidity and poor pelleting quality; which may in turn necessitate the need for additional supplementation with antioxidants [
14,
24].
According to [
11], supplementation of exogenous enzymes in poultry diets with SFM can decrease their deleterious effects and stimulate fibre digestion. Amongst other benefits of enzyme supplementation, Ref. [
25] reported that enzymes functions in the breakdown of NSP’s, reduction of gut viscosity thereby improving nutrients digestibility and gut performance.
Exogenous enzymes have been used with mixed results as reported in previous studies. In some studies, growth performance parameters were worsened as a result of enzyme supplementation [
8,
26]; in others, it led to no improvements [
6,
27,
28,
29], whereas in others, it led to an improvement in growth performance parameters [
22,
23,
24,
26,
30]. These contrasting findings created a knowledge gap and inconsistency in the industry regarding the optimum inclusion rates of SFM in poultry diets and the best enzyme combinations for optimum results.
To our knowledge, there is still a knowledge gap that exists in the optimum inclusion levels of sunflower meal in broiler diets supplemented with exogenous enzymes. Hence, the present study was conducted to determine if inclusion levels of de-hulled sunflower meal can be increased, and if the efficacy of exogenous enzymes (EE) on broiler performance is altered by sunflower meal inclusion rates.
4. Discussion
The current study investigated de-hulled sunflower meal inclusion rate and the effect of exogenous enzymes on growth performance of broiler chickens grown to 35 d. Previous studies show that responses of broiler chicks to SFM depend on its chemical composition, as well as the type of diet and enzymes supplemented. In the current study, during the pre-starter phase, increasing SFM inclusion did not affect any of the studied parameters. Our results are partially in agreement with those of [
6], who only observed significance difference for the FCR, but not for BWG and FI in their study. Likewise, Ref. [
22] did not find any significance difference in FI as influenced by SFM or enzyme addition, whereas enzyme supplementation influenced FCR and BWG in the grower phase. This differs to our study because during the grower phase, no significant differences were observed amongst all studied parameters; even FCR was not influenced by enzyme supplementation. The results on FCR may be an indication of the implication of non-starch polysaccharides on FCR [
26]. Lower metabolizable energy tend to reduce growth performance of broiler chickens due to high concentrations of NSPs, which has an inverse effect on dietary energy levels. To counter that effect, in this study, metabolizable energy was added to some of the dietary treatments to see if there would be any variations in response. Treatments diets with additional energy performed better, placing our findings in sync with those reported by [
35] who improved the nutritional value of high SFM meal for laying hens by adding oil and correcting for energy deficiencies to improve growth performance.
XAP enzyme supplementation to de-hulled SFM overall improved growth performance. We speculate that the observed increase in BWG, FI and FCR of the birds fed SFM supplemented with XAP may have been observed as a result of enhancement of other physiological and metabolic processes [
30]. In addition to increasing nutrient digestibility, carbohydrases have been suggested to depolymerize complex NSPs, hence releasing fermentable xylo-, galacto-, manno-, or gluco-oligomers that have prebiotic effects from health promoting bacterial (
Lactobacillus,
Bifidobacterium) proliferation [
30,
36]. The consequence would be further increases in energy concentration and the enhancement of nutrient utilization and absorption [
30,
36].
Similarly, Ref. [
16] observed an improvement in BWG and FCR in the finisher phase due to enzyme supplementation. Our study differs in terms of the results on FI, in that we also observed a significance difference on the diets because of SFM type and enzyme supplementation. Interestingly, Ref. [
16] used a multi-enzyme consisting of xylanase, protease and amylase, a combination that we also adopted in the current study. It is therefore conceivable that the addition of XAP and X in our study resulted in higher BWG, FI and FCR’s in the post-finisher phase, like what was recorded to the overall study by the same authors, except for FI, which was not significantly affected in their study.
Additionally, an improvement in BWG and FCR was recorded elsewhere due to the addition of enzyme blend (cellulase, β-glucanase and xylanase) in diets containing 6% and 8% SFM in the grower and 10% SFM in the finisher, where enzyme supplementation started from the grower phase to the finisher phase [
22]. In our study, enzyme supplementation resumed at the pre-starter phase. Body weight gain was only significantly (
p < 0.05) improved when de-hulled sunflower meal was included in the diet at 13%, with the supplementation of enzyme.
In our study, the inclusion of dehulled-SFM at a higher levels (13–13.5%) resulted in higher BWG (13% and 13.5%), and FI (13.5%) due to enzyme supplementation. According to [
37], higher levels of insoluble fibre in poultry diets has always been associated with negative effects of reduction of nutrient digestibility, absorption and feed intake, thus feeding diets rich in fibre in poultry tend to increase feed intake as a way of compensation for the reduced concentration of nutrients, mainly energy level, in the diets. Enzyme addition in the de-hulled SFM based diet was intended for the degradation of NSP and thereby facilitate the nutrient absorption and improve weight gain. These appeared to be true in our study because there was an improvement in BWG due to enzyme supplementation. The improvement could be a result of exogenous enzymes eliminating adverse and/or deleterious effects or impacts of anti-nutritional compounds, thus enhancing the nutrients utilization by the birds, particularly energy and amino acids which in turn translated into better broiler production performance observed in our study [
16].
Chickens are generally unable to breakdown phytate and NSP that is present in most raw materials of plant origin due to their inability to secrete phytase and NSP-hydrolyzing enzymes [
38]. Thus, they need exogenous enzymes to act upon the fibrous material more effectively by breaking the polymeric chain, thereby improving their nutritive value and reducing gut viscosity. In view of this, our study incorporated phytase enzyme with additional NSP-hydrolyzing enzymes (X, XB, XBP and XAP) to see their efficacy in breaking down the polymeric chains found in de-hulled SFM based diets. The activity was only determined by growth performance in our study, hence based on these, the results suggest that the effect of enzymes was not similar, only xylanase and XAP showed beneficial effect over the others overall. Xylanase showed significant improvements (
p < 0.05) for all studied parameters in the post finisher phase. Enzyme effects differ according diet composition (target substrate), the age of the chickens and the dose of the enzymes [
39]. Our study did not analyze the fractions of the substrates; hence, we could not ascertain the specific enzymes to target predominant substrates. According to [
40], a consortium or a combination of enzymes offers greater benefits than when a single enzyme is used due to their synergistic effect on the various substrates. Our choice of enzymes was informed much by literature about the common substrates found in SFM (arabinoxylans, pectins, beta-glucans etc) [
11,
40]. Improved BWG and FCR were observed in birds grown of 1–21 d suggesting that more benefits are achieved at the early starter phase [
39]. However, these studies administered their enzymes in drinking water whereas enzymes in the current study enzymes were administered in the feed and we observed benefits at the post finisher phase.
Successful use of enzymes in broiler diets is restricted by the cost-benefit ratio [
39]. However, such analysis was not the basis for our study. Actually, reducing the cost of feed offers the reserve for cost reduction, and this is achieved by reducing the cost of the protein component of the diet [
13]. By increasing the levels of dehulled SFM (13–13.5%) in the diet, SBM inclusion was partially replaced as the most expensive traditional protein source. Therefore, we inherently realized the economics of using enzymes as evidenced through overall improvements in production performance of our study. Factoring the prices of enzymes, we could still realize the beneficial effects since using SFM in the diet implies that it must be supplemented with synthetic oil and lysine, thus making the feed not cost effective [
13].
Nevertheless, there are many cases in which enzyme supplementation to SFM containing diets did not improve of all studied growth parameters [
6,
27,
28,
29]. In some instances, exogenous enzymes use resulted in worse performance parameters [
26], whereas in some studies, including ours, it resulted in improved growth performance parameters [
16,
22,
23,
24,
30]. Such discrepancies in the results recorded in literature are multifaceted, and may be attributed to the quality of SFM processing, variety of experimental birds, experimental periods (21, 35 or 42 d), and diet composition [
30]. In our study we used Ross 308 males whereas some of the studies on literature used Hubbard [
16], Cobb 500 [
3], Arbor Acres [
39], layers [
41] and in some instances, quails [
29]. Our experimental period was 35 d and enzyme supplementation began at day-old with birds on iso-caloric and iso-nitrogenous diets.
According to [
42], exogenous enzymes helps in the disruption of cell wall integrity in plant based ingredients, and the disruption of the cage effect, results in the subsequent release of nutrients that were previously housed by the cell, thus making them available to the animal for use. Additionally, enzymes use has been shown to be responsible for the breaking down of NSP’s, improvement of nutrient digestibility due to a reduction of the gut viscosity and thereby improving the growth performance and uniformity of birds flock [
25].
The non-starch polysaccharides consists of a portion that is water-soluble that is responsible for the formation of a viscous texture in the gastro-intestinal tract (GIT), hence decreasing gut performance of the bird [
1]. β-glucans mostly possess a negative effect on nutrients, especially protein and starch utilization, leading to highly viscous condition in the small intestines which are associated with nutrient absorption reduction due to negative changes in the GIT microflora [
1,
43]. According to [
44], cereal β-glucans are not digested by the monogastric animal’s endogenous enzymes and have a negative effect on bird performance and health. There was no significant differences amongst the PC, NC, XB and XBP treatments in terms of overall BWG. However, numerically, our results show that supplementation with XBP enzyme led to poor growth performance, suggesting that the cereal β-glucans were not hydrolyzed efficiently by the enzyme combination. Probably these may be because enzyme XBP did not promote improvement in overall epithelial cell arrangement and villus height, thereby limiting nutrient absorption [
43]. Worth mentioning is that we did not do intestinal morphology in our study, so we are making assumptions on the possibilities of such occurrence.