Feed additives assist in boosting the monogastric animal immune system through maintaining homeostasis of the gastrointestinal tract. The gastrointestinal tract is a vital immune organ that encompasses roughly 70% of the total immune cells of the body. Apart from the above, feed additives regulate gut flora, reduce weaning stress and other environmental challenges on piglets and pigs respectively. With the increased awareness of potential negative impact on the use of antibiotic growth promoters in diets fed to pigs resulted in an increased interest in producing pigs without using antibiotic growth promoters.
However, to avoid the use of antibiotic growth promoters from diet for pigs, changes in management and nutritional strategies may be required to improve the pigs' ability to prevent pathogenic bacteria from colonizing the intestinal system. This can be achieved through preventing the pathogens from adhering to intestinal mucosa or releasing toxins, and thus, reduce the damaging effects of the pathogens on host. Recently, researched alternatives of antibiotic additives to include in feed are organic/ inorganic acids, high levels of zinc (Zn)/ copper (Cu), essential oils, herbs and spices, different types of prebiotics, probiotics, bacteriophages, antimicrobial peptides, feeding strategies that influence different aspects of health leading to improved productive performances from monogasteic farm animals.
1. Acidifiers
Dietary acidifiers are organic or inorganic acids or its salts. Acidifiers create a favorable gut environment for beneficial microbes that result in increased nutrient digestibility and absorption, increased growth performance, and reduce piglet diarrhea. The following are mechanisms to increased growth performance in pigs-
- Decreased or stabilized gastric pH may lead to increased activity of digestive enzymes (pepsin).
- Modulation and alteration of the gut microflora may inhibit pathogens.
- Improve nutrient digestibility in the intestine (small and large) resulting in increased retention of nutrient(s).
Acidifiers include organic acids, salts of acids and blends of acids. Commonly used organic acids include formic acid, fumaric acid, lactic acid, and citric acids. Supplementation of diets with organic acids usually results in a reduction in stomach pH, improved growth rate and feed conversion ratio when included in diets fed weanling pigs and growing pigs. Supplementation of diets with citric acid had a positive effect on the digestibility of protein, Ca and P in sows. Recently, it was observed that combinations of organic acids and medium-chain fatty acids supplementation reduced pathogenic activity, positive effects on the digestibility of nutrients as well as on growth performance than if acids or fatty acids are fed individually. Furthermore, supplementation of blends of acids found to be beneficial in reducing pathogengs in gut through increasing Lactobacillus spp as well as reducing diarrhea in pigs due to the stressed by changing environmental temperature.
2. Minerals
Minerals are inorganic elements needed for maintenance of body homeostasis, growth, and (re)production. Minerals needed in quantities greater than 100 ppm of feed are called macro minerals (e.g. Ca, P etc) and those minerals required in smaller quantities are called micro minerals or trace minerals (Cu, Zn etc). Except other minerals, Zn and Cu have antimicrobial properties and therefore they are often added to diets in greater quantities than its nutritional requirement. Pharmacological dose of Cu (100 to 250 ppm) in weanling pig diets has been a common practice to improve growth performance, reduce post-weaning scouring, improved average daily gain and average daily feed intake. The growth promoting effects of dietary Cu because of its bacteriostatic and bactericidal properties that alter the growth and community structure of microorganisms from hrmful to useful microorganisms in the cecum and colon. In addition Cu at pharmacological dose in diets of weanling pigs improves intestinal health through increasing villus height and reducing crypt depth.
Deficiency of Zn in weanling/ grower pigs leads to growth retardation, loss of appetite, skeletal abnormalities, and hyperkeratinization of the skin ( parakeratosis). Requirement of Zn in nursery pigs are 80 to 100 ppm. However, use of pharmacological dose (2,000 to 4,000 ppm of inorganic Zn) in diet in the form of ZnO is a common recommendation to improve feed intake by 14% to 17%, improved growth performance and to reduce post-weaning diarrhea.
The mechanism of Zn in enhancing growth performance is related to its function in the intestinal integrity and morphology through increasing the villous height, the villous height to crypt depth ratio and decreased crypt depth. Dietary Zn also assists in stability of the microflora and diversity of the coliform organisms, regeneration of injured intestinal epithelial tissue, reduction of intestinal permeability of and lymphocyte proliferation. High doses of Zn doses also increase mRNA expression of IGF-1 and CDK4. IGF-1 is mediator of the proliferation and differentiation of enterocytes whereas CDK4 regulates apoptosis of enterocytes.
Other experiments also indicated beneficial effects of supplementing pharmacological dose of Cu and Zn in combination. As Zn modify the colonic microbial ecolosystem, whereas Cu reduces microbial diversity in the ileum and colon.
3. Prebiotics
Prebiotics are non digestible oligosaccharides and is defined as “non-digestible food ingredients that beneficially affects the host by selectively stimulating the growth and/or activity of one or a limited number of bacteria in the colon, and thus improves host health”. Gut microflora plays an important role in the health of pigs through increasing the population of helpful bacteria (Bifidobacterium, Lactobacilli, and Eubacteria) that improve the health and decrease the risk of diseases of the animals. Nondigestible carbohydrates (Prebiotics) are the main substrate for gut microbiota thereby reduces the load of pathogens from the gut. Prebiotics helps in the growth of gut microbiota that increases the concentration of lactic and acetic acid production, which will reduce the pH of the intestine to increase fermentation with a subsequent increase in the concentration of short chain fatty acids (volatile fatty acids). Fructo-oligosaccharides, transgalacto-oligosaccharides, inulin, and lactulose are the most common carbohydrates that are commonly used as prebiotics in pigs as these carbohydrates are easily fermentable and therefore will result in reduced gut pH.
Beneficial effects of prebiotics in diets of pigs related to increased production of short chain fatty acids (butyrate) results in reduced intestinal pH. Butyrate regulates epithelial cell growth, differentiation and apoptosis of the small intestine which result in the improved small intestine morphology leading to improved digestive and absorptive capacities as well as decreased the incidence of diarrhea. Studies also demonstrated that the prebiotic feeding in pigs result in optimization in humoral (IgM and IgA) and cell-mediated immune response (IL-1b gene expression, and serum levels of IL-1b, IL-2, and IL-6).
4. Probiotics
Probiotics, are defined as, “live microorganisms which, when administered in adequate amounts, confer a health benefit on the host. The term probiotic only be used when referring to human microbial products as per Food and Drug Administration. Since 1989, the term direct-fed microbials is used in the U.S. feed industry, whereas “probiotic” is used interchangeably with human and animal feed worldwide. Probiotic (Bacillus) are metabolically inactive spores and the pH (6 to 7) small intestine is favorable to germinate, grow, and produce enzymes.
The enzymes produced by the probiotic organisms degrade the feed and produce volatile fatty acids/ short chain fatty acids. These short chain fatty acids are utilized by the pig as an energy source as well as these acids reduces the pH in the gastrointestinal tract causing the intestinal environment unfavorable for growth of pathogenic bacteria. Probiotic decrease in pathogenic bacteria and optimize the gastrointestinal health by establishing colony of beneficial bacteria such as lactobacilli and bifidobacteria. This optimized the gastrointestinal health increases the ability of the pig to digest/ ferment nutrients, enhances utilization of feed, decrease the requirement of maintenance energy associated with immune system stimulation, and thereby promote growth performances. Lactic acid-producing bacteria observed to be more beneficial for pigs that help to stabilize the gastrointestinal tract, whereas Bacillus-based probiotics are more beneficial to increase the digestibility of nutrients leading increase growth and improved carcass characteristics.
Nucleotides requirement for pigs is more important under certain conditions, viz during disease, high stress and growth period. Inclusion of yeast cells/ live yeast cells/ heat-treated yeast cells/ ground yeast cells/purified yeast cell cultures/ yeast extracts in diet are potentially beneficial during weaning period of piglets as yeast contain higher amount of nucleotides. Furthermore, yeast supplementation boost pig growth performance, enhance mucosal immunity, support intestinal development, reduce post-weaning diarrhea, and adjust gut microbiota in grower pigs.
5. Nucleotides
Apart from building blocks for nucleic acids (DNA and RNA), nucleotides also control physiological functions in the body viz as source of energy, cofactors in oxidation and reduction reactions. In addition, nucleotides help in maintenance of small intestinal integrity, microbiota of the intestinal tract and may influence the development of the immune system. Nucleotides stimulate leukocyte production and macrophages of the intra-epithelium of the piglet ileum. Therefore, dietary supplementation of nucleotide is required to boost both humoral and cellular immunity in stressed (grower pigs specifically weaning piglets). It is observed that supplementation of nucleotide have positively influence on gastrointestinal microbiota, lowering of gastric pH, hinders proliferation of pathogenic bacteria species that reduces incidence of piglet diarrhea. It is also observed dietary supplementation of nucleotides enhance the growth and maturation of intestinal epithelial cells, increase height of villi in the small intestine, and increased digestive enzyme (maltase to lactase) ratio.
6. Essential oils
Plant extracts (essential oils) have potential biological functions, such as antiviral, antimicrobial, antioxidant, and anti-inflammatory effects, therefore plant extracts are use as growth promoter in-feed to improve performance and health of animals instead of antibiotics. Supplementation of plant extracts in diet improve animal health through several mechanisms such as it directly suppress the proliferation of pathogens in gastro intestinal tract, alteration of gut microbial populations, and augmentation of immune functions. Anti-microbial effects of essential oils due to the high content of phenolic compounds that prevent the development of virulent structures in bacteria through disturbing the enzyme system of bacteria as well as some plant extracts directly kill the pathogens due to their hydrophobicity. The anti-inflammatory effects of plant extracts are due to its inhibitory properties to inhibition the production of proinflammatory cytokines and chemokines from epithelial cells and immune cells. Plant extracts are also used as antioxidants in animal feed, which will protect animals from oxidative damage of free radicals. The antioxidant properties of extracts is due to the presence of the presence of phenolic OH groups that act as hydrogen donors in lipid oxidation reaction thus retard the hydroxyl peroxide formation. The major bioactive compounds of plant extracts are polyphenols, and their composition/ concentration vary according to the geographical origin of the plant, parts of the plant, harvesting season, environmental factors, processing techniques and storage conditions. Therefore, to be used as potential alternatives to antibiotic growth promoters the variations in active compounds in plants or its products have to be considered.
7. Tannins
Tannins are complex group of polyphenolic compounds having astringency and tanning properties present in a wide range of plant species. Tannins are associated with the higher molecular weight proanthocyanidins, reported to be the molecular weight between 500 and 5000 Da. Tannins have the ability to inhibit extracellular microbial enzymes. Therefore, tannins have been proposed as an alternative to antibiotics. Tannins originate from certain plant species can improve the intestinal micrflora enhancing gut health, and hence increase productive performance when applied appropriately in monogastric diets. Tannins have antiviral, antibacterial and antitumor properties, however, a favorable outcome in the promotion of gut health was observed when used with other antimicrobials as growth-promoting factors such as probiotics. Therefore, natural extracts can be adopted as a valuable alternative to antibiotics in intensive animal farming. However, the effectiveness of tannin supplementation in weaned and finishers are highly related to the dose of administration, duration of supplementation and the presence of other sources of tannins in the basal diet in the feed regimen adopted.
Non-antibiotic growth promoters improve pig growth performance through regulating intestinal environments (modulating GI tract pH), optimoizing immune system and increasing nutrient digestibility. As observed most of the non-antibiotic growth promoters improve pig growth performance through modulating gastrointestinal tract integrity because gastrointestinal tract is a vital immune organ that encompasses around 70% of the total immune cells of the body apart from its role as a digestive and absorptive organ. Thus these feed additives that potentially may be used in diets fed to pigs f no antibiotic growth promoters are used.