1. Introduction

The cost of many cereal grains and legumes used in making poultry feed is increasing due to the growing markets utilizing them as food and fuel [1]. To counter this, alternative feedstuffs such as agricultural co-products, including wheat middlings, soy hulls, oil cakes, distillers dried grains and solubles (DDGS), and sugar beet pulp, etc., are regularly incorporated in poultry feed. However, these co-products inherently contain a high proportion of dietary fiber (DF) comprising of non-starch polysaccharides (NSP), lignin, and other indigestible plant-based carbohydrates [2]. Poultry lacks endogenous enzymes required for the breakdown of these NSP. The NSP fractions include cellulose and non-cellulosic polysaccharides (NCP), and the NCP portion further consists of pectic polysaccharides and hemicellulose ^[2][3][2,3]. DF is further divided into soluble and insoluble fibers based on their aqueous solubility. Furthermore, a non-digestible fraction of starch that remains resistant to enzymatic digestion is termed as ‘resistant starch’ and has been reported to possess physiological functions similar to other DF [4].

Recently, there has been an increase in the trend of incorporating DF and oligosaccharides in poultry diet to supply substrates for beneficial gut microbes [5]. Previous research on the characteristics of fiber demonstrates both opportunities and challenges in enhancing healthy and efficient poultry production. Fiber can act as an antinutrient, as it often encapsulates nutrients in cell walls of plant-based feed ingredients, negatively influences viscosity of digesta, and impacts mineral absorption through the chelating properties of some fiber moieties ^[6][7][6,7]. It has been noted that DF is utilized by microbes in the lower gut to produce short-chain fatty acids (SCFA) as fermentation metabolites [8]. These SCFA are utilized by the intestinal enterocytes for growth and are transported to the liver to produce ATP. However, because of their low digestibility, NSP reduces the apparent metabolizable energy (AME) value of feed, and consequently increases the viscosity of digesta, which adversely affects the digestibility of other nutrients [5]. Thus, preprocessing and enzyme supplementation to increase the digestibility of fiber will also improve utilization of other nutrients in feed and will increase fermentable resources for the gut microbes ^[5][9][10][5,9,10]. The fermentable substrate can range from complex fragments to simple oligomers that could serve as prebiotics if they could selectively enhance the population of beneficial bacteria leading to immunomodulation and improved gut health ^[11][12][13][11,12,13].

2. Significance of Exogenous Fiber-Degrading Enzymes during Disease Challenge in Poultry

The efficacy of exogenous enzymes becomes more important during disease challenge conditions in poultry flock when the digestive and immune system of the birds are in a compromised state. The disease condition deteriorates the performance and reduces the efficiency of feed utilization that can further increase the cost of production. Amerah et al. ^[14][125] found that in a wheat-based basal diet, xylanase supplementation (2000 U/kg of feed) increased weight gain by 16% and reduced FCR by 6% at day 42 in Salmonella enterica serovar Heidelberg (5 × 10⁵ CFU/mL) challenged broilers. In the same study, xylanase supplementation also reduced the Salmonella-positive cecal samples from 32.5% in the challenged control to 12.5%. Sun et al. ^[15][126] mentioned that the enzyme complex containing xylanase, glucanase, and mannanase as major components supplemented at 500 mg/kg diet decreased Clostridium perfringens from 3.66 to 3.48 log CFU/g of ileal digesta, increased body weight by 4%, improved FCR by 3%, increased villus height by 8%, and villus height to crypt depth ratio by 11% in 3-week broilers.

Likewise, in a study by Jia et al. ^[16][127] on broiler chickens challenged with Clostridium perfringens, the supplementation of carbohydrase enzyme complex at 1 kg/ton of feed (supplying 60 U cellulase, 1400 U pectinase, 1200 U xylanase, 800 U glucanase, 500 U mannanase, 30 U galactanase, and other minor enzyme activities per kilogram of diet) reduced the feed conversion ratio by 5–6% in wheat- and flaxseed-containing diets. The mixture of feed enzymes can also be used in combination with direct-fed microbials to improve feed utilization and compensate for the damage and performance loss if occurred due to a coccidial challenge ^[17][128]. Jackson et al. ^[18][129] supplemented 100 million units of β-mannanase per ton of feed of broilers subjected to necrotic enteritis using a Eimeria sp. and Clostridium perfringens model. The authors reported that the intestinal lesion score was decreased by 16% on day 14, weight gain was increased by 14%, and FCR was improved by 11% on day 21. In a 39-day broiler study, Choct et al. ^[19][130] reported that the inclusion of 2.5 g/kg xylanase enzyme in a wheat-based diet reduced the number of ileal and cecal population of Clostridium perfringens to an insignificant level. Bortoluzzi et al. ^[20][131] confirmed that the addition of β-mannanase at 400 mg/kg in the diet increased Lactobacillus and Ruminococcaceae and reduced Bacteroides in the ceca of 21-day broilers regardless of the Eimeria challenge. Thus, an ideal enzyme or enzyme blend can reduce digesta viscosity, increase available energy, improve nutrient utilization, provide a health benefit, and reduce environmental pollution ^[21][132]. Therefore, such exogenous enzymes can prove effective in circumstances where AGPs are not desired. Further, it would be interesting to focus on research to elucidate the host–immune–diet–microbiome interactions to realize the benefits of additives like prebiotic DF and feed enzymes during restricted use of AGPs. Moreover, the supplementation of potent NSPases along with dietary fiber could improve productive performance and gut health of poultry, and thus increase profitability in both healthy and disease-affected flocks.