Dr. Majdood Ahmad, Retd. Dy. Commissioner Poultry (Govt. Of India)
For almost eight decades,Antibiotic Feed Additives (AFAs) have been used in poultry for increased productivity and efficiency. A great deal of research has been focused on the development of antibiotic substitutes to maintain or improve poultry health and performance. Optimal combinations of various substitutes coupled with good management and husbandry practices will be the key to maximize performance and maintain poultry productivity. This review article describes the potential for the various substitutes available to increase poultry productivity and help poultry birds to perform their highest genetic potential under existing commercial conditions. An ideal substitute should have the same beneficial effects of Antibiotic Growth Promoters (AGPs); ensure optimum poultry performance and increase nutrient availability (Huyghebaert et al., 2011). Considering the proposed mechanism of action of AGPs (microbiome and immune-modulating activities), a practical substitute should possess both of these properties in addition to having a positive impact on feed conversion and/or growth (Huyghebaert et al., 2011; Seal et al., 2013).
Classes of Substitutes:
Several classes of substitutes have been proposed and tested in poultry production; including probiotics, prebiotics, synbiotics, organic acids, enzymes, phytogenics and trace metals. Novel substitutes such as hyperimmune egg yolk IgY, AntiMicrobial Peptides (AMP), bacteriophages and clay have come into existence in recent years.
Probiotics help establish a micro-environment in the gut that favors beneficial microorganisms and reduces the colonization of pathogenic bacteria (competitive exclusion) by:
(i) Creating a hostile environment for harmful bacterial species through production of lactic acid, Short Chain Fatty Acids (SCFAs) and reduction in pH;
(ii) Competing for nutrients with undesired bacteria;
(iii) Production and secretion of antibacterial substances (e.g. bacteriocins by Lactobacillus, Bacillus spp.)
(iv) Inhibition of bacterial adherence and translocation (Nurmi and Rantala, 1973; Fuller, 1989; Netherwood et al., 1999; Schneitz, 2005; Ng et al., 2009; Brown, 2011).
Probiotics are mono or mixed cultures of live organisms which when administered in adequate amounts confer a health benefit to the host. Probiotics may contain one or more strains of microorganisms and may be given either alone or in combination with other additives in feed or drinking water (Thomke and Elwinger, 1998). A variety of Bacteria (Bacillus, Bifidobacterium, Enterococcus, Lactobacillus, Streptococcus and Lactococcus spp.) and in some cases Yeast (Saccharomyces spp.) have been tested as probiotics in poultry birds (Simon et al., 2001; Patterson and Burkholder, 2003; Griggs and Jacob, 2005; Kabir, 2009). Supplementation of diets with a single strain of Lactobacillus sp. (L. casei, L. fermentum, L. bulgaricus, L. reuteri) showed to improve the body weight and feed efficiency in broilers (Yeo and Kim, 1997; Khan et al., 2007; Apata, 2008; Nakphaichit et al. 2011; Salim et al., 2013).
Prebiotics are macromolecules that are either derived from plants or synthesized by microorganisms. A number of characteristics should be taken into consideration when selecting prebiotics including – resistance to gastric acidic environment, intestinal/ pancreatic enzyme hydrolysis and absorption across intestinal epithelium (Hume, 2011; Heo et al., 2013; Ricke, 2015). The most important characteristic of an ideal prebiotic is the ability to selectively enrich beneficial microorganisms associated with health and well-being (Simmering and Blaut, 2001; Patterson and Burkholder, 2003; Heo et al., 2013; Samantha et al., 2013).
Mannan Oligo Saccharide (MOS), derived from the outer cell-wall layer of Saccharomyces cerevisiae has been studied extensively as a prebiotic supplement in poultry diets. The addition of various levels of MOS to the broiler diets significantly increased their body weight and improved feed conversion efficiency (Benites et al., 2008; Bozkurt et al., 2008; Hooge et al., 2003; Yang et al., 2007; Mohamed et al., 2008) with increased intestinal villi height (Baurhoo et al., 2007; Yang et al., 2007), also it improved immune-competence in the intestine (Janardhana et al., 2009; Shanmugasundaram and Selvaraj, 2012), altered jejunal gene expression (Xiao et al., 2012; Brennan et al., 2013) and influenced intestinal microbiota (Geier et al., 2009; Corrigan et al., 2011; Kim et al., 2011; Pourabedin et al., 2014). Fructo OligoSaccharide (FOS) which is derived from plants has also showed to possess significant prebiotic effect and improve performance in broiler chickens (Xu et al., 2003; Kim et al., 2011).
Another class of prebiotics includes Inulo OligoSaccharide (IOS) showing promise as an antibiotic substitute owing to their efficacy in improving weight gain and FCR when fed to broilers (Mookiah et al., 2014). Calik and Ergün (2015) showed that lactulose supplementation in broiler diets not only improved body weight and FCR but also increased villi height, goblet cell numbers, total SCFA concentrations and Lactobacillus counts.
Synbiotics are additives that combine the use of probiotics and prebiotics such that they act synergistically (Alloui et al., 2013). The use of synbiotics was based on the concept that a mixture of probiotics and prebiotics beneficially affect the host by improving the survival and implantation of probiotic organisms and by selectively promoting the growth or metabolism of beneficial bacteria in the intestinal tract (Gibson and Roberfroid, 1995). Supplementation of diets with a synbiotic product was shown to significantly improve body weight, average daily gain, feed efficiency and carcass yield percentage compared with controls or probiotic-fed broilers (Awad et al., 2009). There is a great potential for synbiotics to be used as antibiotic substitutes for improving performance and reducing pathogenic load in the intestines of poultry birds. But still careful consideration must be given when selecting the combinations of various prebiotics and probiotics to be used as symbiotic substitutes and proper research based trials must be conducted to demonstrate their synergistic effect compared with the use of either product alone because till date mostly all studies were done on Broilers leaving space for research in other poultry birds too.
4. Organic acids:
Dietary organic acids have been considered as potential substitutes to AGPs, owing to their antibacterial nature. Chemically, organic acids used in poultry production can be described as either simple monocarboxylic acids like formic, acetic, propionic and butyric acids or carboxylic acids bearing hydroxyl group like lactic, malic, tartaric and citric acids (Dibner and Buttin, 2002).
They are widely distributed in nature as normal constituents of animal or plant tissues and some of them (specifically SCFA) are produced in the hind gut of food animals and humans through microbial fermentation of carbohydrates (Van Der Wielen et al., 2000; Ricke, 2003; Huyghebaert et al., 2011). They can be administered in the feed or drinking water and can be used either individually as organic acids or as their salts (sodium/ potassium/ calcium) or as blends of multiple acids or their salts (Huyghebaert et al., 2011).
Dietary enzymes are biologically active proteins that facilitate chemical breakdown of nutrients to smaller compounds for further digestion and absorption (Thacker, 2013). The different classes of enzymes commonly employed include phytase, carbohydrases (xylanase, cellulase, α-galactosidase, β-mannanase, α-amylase, pectinase) and proteases. The effect of various in-feed enzymes in improving the growth and feed efficiency in poultry is already well documented and reviewed by several scientists’ time & again (Bedford and Schulze, 1998; Choct, 2006; Selle and Ravindran, 2007; Adeola and Cowieson, 2011; Slominski, 2011; Woyengo and Nyachoti, 2011).
Phytogenic Feed Additives (PFAs), also referred as phytobiotics or botanicals are natural bioactive compounds that are derived from plants and incorporated into poultry feed to enhance productivity (Windisch et al., 2008). A wide range of plants and their products fall under this category and based on their origin (part of the plant), they can be broadly classified as Herbs – flowering, non-woody, non-persistent plants of which leaves and flowers are used or Spices (non-leaf parts of plants, including seeds, fruits, bark or root with intensive taste/ smell (Windisch et al., 2008; Van Der Klis and Vinyeta-Punti, 2014). They can be used in solid, dried ground form or as extracts (crude or concentrated).
Depending on the process used to derive the active ingredients, PFA can also be classified as Essential Oils (EOs- volatile lipophilic substances obtained by cold extraction or by steam or alcohol distillation) and Oleoresins extracts derived by non-aqueous solvents (Windisch et al., 2008; Van Der Klis and Vinyeta-Punti, 2014). The main bioactive compounds of the PFAs are polyphenols while their composition and concentration vary according to the plant, parts of the plant, geographical origin, harvesting season, environmental factors, storage conditions and its processing techniques (Windisch et al., 2008; Applegate et al., 2010).
In recent years, PFAs have been used as natural growth promoters in the pig and poultry industries (Windisch et al., 2008; Franz et al., 2010). A wide variety of herbs and spices (e.g.- thyme, oregano, rosemary, marjoram, yarrow, garlic, ginger, green tea, black cumin, coriander, and cinnamon) have been used in poultry for their potential application as AGP substitutes. Guo et al. (2004) showed a significant increase in body weight gain and improvement in feed efficiency when broilers were given diets supplemented with a mixture of 14 herbs.
7. Hyperimmune egg yolk antibodies:
Pimentel and Cook (1988) and Pimentel et al. (1991) showed that progeny from hens injected with jack bean urease had improved body weight at 3 weeks of age. It was proposed that urease antibodies maternally transferred to the progeny decreased ammonia production in the intestinal tract by inhibiting bacterial urease enzyme and improving growth.
The use of egg yolk antibodies offers several advantages. Large quantities of antibodies can be produced in laying hens and can be non-invasively collected. Their use is environmentally friendly, less toxic and does not select for resistance.
8. Antimicrobial peptides:
AMPs are widely distributed, small, gene-encoded peptides that have germicidal properties. They have been seen in all kingdoms of life and have shown activity against a wide range of pathogens such as Gram-negative and Gram-positive bacteria, fungi, enveloped viruses and parasites (Koczulla and Bals, 2003; Li et al., 2012; Kim et al., 2016b). The studies that have been done on AMPs and their applications in poultry have been mostly focused on their protective potential against diverse pathogens causing infectious diseases rather than growth promoting activities. They reported that the birds that were given naturally synthesized AMPs showed improvement in growth performance, intestinal ability to absorb nutrients and mucosal immune parameters such as intraepithelial lymphocytes or mast cell counts and in secretory IgA levels when compared with un-supplemented or non-inoculated birds (Liu et al., 2008; Bao et al., 2009; Wang et al., 2009).
One of the most reported bacteriocins as a dietary supplement in poultry is divercin AS7, which is produced by Carnobacterium divergens AS7, a lactic acid-producing bacterium isolated from fish, which has been extensively studied by Józefiak and colleagues. Dietary supplementation of AMPs in poultry seems to affect the birds in a positive way by improving their intestinal balance and creating gut micro ecological conditions that suppress harmful microorganisms like Clostridium spp. and coliforms while favoring beneficial microorganisms like Lactobacillus spp. (Ohh et al., 2009).
Bacteriophages can be considered safe antibiotic substitutes as they exhibit no activity against animal and plant cells. Increased body weight gain and reduced FCR were reported in broilers given diets supplemented with 0.10% and 0.15% (Kim et al., 2013c) or 0.5g kg-1 of bacteriophages, respectively (Wang et al., 2013b).
The mechanism by which clays and clay minerals influence growth is unclear, but it depends largely on their ability to physically bind and remove toxins, anti-nutritional components and pathogenic organisms. This results in reducing microbial metabolites, toxins, enzymes in the intestine and thus preventing irritation and damage with improving morphological characteristics of the intestinal mucosa (Xia et al., 2004; Jorge de Lemos et al., 2015) hence improving performance. The inclusion of clay was also shown to improve nutrient digestibility by reducing digest transit time and also decreasing litter moisture (Olver, 1997; Jorge de Lemos et al., 2015).
Clays based products are the most effective mycotoxin adsorbents. However they are diverse aluminosilicates with a variety of properties. Many types of clays do not capture mycotoxins; some can absorb water, others can absorb ammonia and only certain clays can adsorb mycotoxins. Scientific studies have demonstrated that some aluminosilicates are very effective in preventing aflatoxicosis at an inclusion rate of 5 or 10 kilos/mt of feed; and only few can do it at 2.5 kg/mt.
11. Trace minerals:
The use of trace minerals to increase poultry productivity and performance has been gaining importance in the recent years and they are being substituted in levels beyond the recommended nutritional requirements. Copper, an essential trace mineral, plays a significant role in hemoglobin synthesis, angiogenesis, connective tissue, bone development and more importantly serves as a cofactor for many metabolic enzymes (Brainer et al., 2003; Richards et al., 2010; Vasanth et al., 2015).
Nutritional strategies and feed additives:-
i. Diet Digestibility and Enzyme Supplementation:
The digestibility of wheat, barley, rye, triticale and even corn-based diets can be significantly improved through use of exogenous enzymes including xylanases, phytases and ß – glucanases.
Rosen (2001) concluded that the effect of enzymes was nearly equivalent to the effects of antibiotics on gain and FCR, while that in combination there was further improvement but less than the sum of the two. Enzymes are perhaps the most extensively reviewed products that seem to be capable of limiting the performance losses associated with removal of antibiotic growth promoters.
ii. Acidifiers and Organic Acids:
As organic acids have strong bacteriostatic effects, they have been used as salmonella-control agents in feed and water supplies for livestock and poultry. But the benefit for poultry seems to be less conclusive.
iii. Herbs, Spices and Essential Oils:
Plant-based antimicrobial compounds, which function in a fundamentally similar way to antibiotic compounds produced by fungi, could be used to replace some antibiotic growth promoters. To be most effective as growth promoters, these herbal antimicrobial compounds must be supplemented to the feed in a more concentrated form than found in their natural source. As with antibiotics, continued use of these plant-based antimicrobials may result in the development of resistance in some pathogenic bacteria. However, more research is necessary to confirm this risk.
Essential oils from oregano are showing the greatest potential as a substitute to antibiotic growth promoters. Oregano contains phenolic compounds, such as carvacrol, that have antimicrobial activity (Akagul and Kivanc, 1988). Like antibiotics, oregano essential oils modify the gut microflora and reduce microbial load by suppressing bacteria proliferation.
There are some claims that oregano oil can replace anticoccidial compounds, not because they inactivate coccidia, but because they increase the turnover of the gut lining and prevent coccidial attack by maintaining a more healthy population of gut cells (Bruerton, 2002).
iv. Oligo Saccharides:
Oligosaccharides are promising substitutes to antibiotic growth promoters because they facilitate and support the symbiotic relationship between host and micro flora. FOS and MOS are two classes of oligosaccharides that are beneficial to enteric health, but they do so by different means.
These compounds are inulin-type oligosaccharides of D-fructose attached by ß (2-1) linkages that are attached to a D-glucosyl residue at the end of the chain (Yun, 1996). A sucrose unit attached to one additional fructose residue is commonly referred to as 1-kestose. Nystose contains two additional fructose units and three additional fructose units are designated as 1F- ß-fructofuranosyl (Hidaka and Hirayama, 1991). FOS is found in numerous plants such as the onion, Jerusalem artichoke, garlic, banana, chicory, asparagus and wheat. Ammerman et al. (1988) demonstrated that the addition of either 0.25% or 0.50% dietary FOS improved feed efficiency from 1 to 46 days of age and reduced mortality when fed at the higher level (0.50%). FOS treated birds also had less air sac lesions at day 46.
Bio-Mos® (Alltech Inc., Nicholasville, KY) is the commercial source of MOS that has been used in most of the published research literature. Based on the scientific literature, Bio-Mos® enhances resistance to enteric disease and promotes growth by the following means: 1) inhibits colonization of enteric pathogens by blocking bacterial adhesion to gut lining; 2) enhances immunity; 3) modifies microflora fermentation to favor nutrient availability for the host; 4) enhances the brush border mucin barrier; 5) reduces enterocyte turnover rate; and 6) enhances the integrity of the gut lining.
Owing to the rise in consumer demand for livestock / poultry products from antibiotic-free production systems, there exists a great need for the development of antibiotic substitutes that can help improve performance and maintain optimal health of the poultry birds. Several products have been evaluated in poultry over the past several years for their potential to replace antibiotics. Though the beneficial effects of many of the substitutes tested have been well demonstrated, there is the general consensus that these products lack consistency, also as results vary greatly from farm to farm. Hence care must be taken in the choice of substitutes, such that they fit the needs of the individual production program.