Dr. Amit Kumar Das
In past few decades, tremendous improvements in the genetic potential of both broiler and layer chicken breeds had been made. To leverage the maximum genetic potential of modern chicken breeds, intensive rearing system was adopted by poultry producers. Under such type of rearing system, the birds are exposed to various stresses due to rapid growth, environmental challenges, immunosuppression, etc. This makes the birds more vulnerable to infectious pathogens leading to illness, decrease in performance and mortality. To minimize these losses and enhance the growth and productivity, antibiotics have been used by the poultry producers as in-feed medication since a long time. Constant use of antibiotics can possibly lead to the emergence of resistant strains of bacteria which is a global concern. Several countries have already banned the usage of antibiotics in animal feed for growth promotion, and awareness is growing in other countries too. This necessitates usage of alternatives to antibiotics for reducing microbial population, particularly in the intestine. Several products have shown encouraging results in replacing antibiotics for improving growth and performance. Prebiotics, probiotics, organic acids, essential oils, etc., are commonly used as alternatives to antibiotics. Organic acids, especially short chain fatty acids (like butyric acid, formic acid, propionic acid, etc.) has shown encouraging results in improving growth, reducing the pathogen load in gut and improving intestinal morphology.
In birds, short chain fatty acids are produced mostly by microbial fermentation of undigested carbohydrates in the hind gut. These short chain fatty acids are rapidly absorbed by colonic epithelial cells and is used as energy source. Among the short chain fatty acids produced in the intestinal lumen, butyric acid is found to be more effective in improving intestinal health. Butyric acid in native form is liquid and has a pungent smell. For animal feed, it is available as salts of sodium, potassium, calcium or glycerides, which helps in turning it to solid form and reduces the odor. Most of the dietary butyrate is absorbed in upper gastrointestinal tract. So, the benefits of butyric acid don’t reach lower intestine. To overcome this, coated form of butyric acid is used. Coated form of butyric acid has advantage over other forms due to its slow releasing capability. This ensures butyric acid to be released effectively throughout the gastrointestinal tract.
Mostly, antibiotic growth promoters (AGP) improve performance by regulating the intestinal microbiota leading to increased nutrient utilization and reduction in maintenance costs of the gastrointestinal system. Many studies have been conducted to demonstrate antimicrobial activity of butyric acid. The antibacterial property of butyric acid is high in undissociated form. Butyric acid enters the bacterial cell by diffusion. In the neutral pH of bacterial cytoplasm, butyric acid dissociates into butyrate and hydrogen ions. The butyrate ion has toxic effect on the bacterial cell. The hydrogen ion formed, lowers the cytoplasmic pH. This reduction in cytoplasmic pH affects the purine bases and denatures enzymes (inactivating bacterial decarboxylases, catalases, etc.,). The bacterial cell requires additional energy to pump out the excess hydrogen ion. This depletes the bacterial cell of energy. The combined effect of all these actions ultimately causes exhaustion and death of the bacterial cell. The antimicrobial action of butyric acid on Salmonella sp. and Clostridium perfringens is well documented.
Figure-1: Multiple effects of butyric acid
Figure-2: Bactericidal action of butyric acid
Another way butyric acid works against pathogens is by reducing intestinal pH. Butyric acid upon entering intestine, dissociates and liberates hydrogen ion, which reduces the intestinal pH. This lower pH of the intestine favors growth of certain beneficial bacteria like Lactobacillus sp., Bifidobacterium sp. etc. These beneficial bacteria help in reducing intestinal pathogen load by various mechanisms like stimulation of the immune system, competition for nutrients, competitive exclusion and production of antimicrobial substances.
The epithelium of intestinal mucosa plays an important role as a barrier between the animal and intestinal microflora as well as harmful substances in the lumen. There is constant immunological challenge in the intestine to fight against the pathogens. Damage to this epithelial layer can incite inflammatory responses which can affect absorption of nutrients. The integrity of this epithelial barrier depends on tight junctions between the epithelial cells. Any damage to these tight junctions will cause a condition called ‘leaky gut’, letting antigenic or toxic loads to “leak” into the general circulation and lymphatic system. Several microbes and toxins are known to disrupt tight junctions. It has been found that administering butyric acid enhances this epithelial barrier by regulating the assembly of tight junctions.
It has been found that in small intestine butyric acid helps in improving villi development, gut morphology and function. Increase in villus morphology can be attributed to increased epithelial turnover. Intestinal epithelium constantly keeps on shedding and regenerating. This turnaround takes around 24-96 hours. This regeneration of epithelium requires energy and other nutrients. Butyric acid is the preferred energy source for the enteric cells. After absorption of butyric acid by enterocytes, it undergoes β-oxidation in mitochondria and converts into acetyl-CoA, which enters citric acid cycle to produce adenosine triphosphate (ATP).
Butyric acid also has antidiarrheal property. In diarrhea, water and sodium are less absorbed, leading to excretion of more fluid and electrolytes through feces. Administration of butyric acid promotes absorption of sodium and potassium uptake by the intestinal epithelial cells. This causes osmotic changes in enteric cells, leading to more absorption of water from the intestinal lumen.
Several experiments have been conducted to study the effect of butyric acid in improving bird’s performance and is well documented. It has been found that feeding butyric acid to commercial broiler birds improved body weight gain (Fig. 3) and feed conversion ratio (Fig. 4). The improvement in performance of commercial broiler birds fed butyric acid is comparable to what is achieved by using AGP alone (Fig. 5 and Fig. 6), which advocates its potential to be used as substitute for AGP. Heat stress is another important factor which has deleterious effect on intestinal integrity, making birds more susceptible to enteric pathogens. Supplementing feed with butyric acid has shown to improve intestinal integrity in heat stressed birds, thereby improving performance, intestinal morphology and microflora composition in heat stressed broiler birds.
Fig. 3: Effect of feeding encapsulated butyric acid on body weight of broiler birds
Fig. 4: Effect of feeding encapsulated butyric acid on feed conversion ratio of broiler birds broiler birds
Fig. 5: Performance of broiler chickens (body weight) fed diet containing butyric acid and antibiotic in the feed
Fig. 6: Performance of broiler chickens (FCR) fed diet containing butyric acid and antibiotic in the feed
Thus, it is apparent that butyric acid helps in improving intestinal integrity by various mechanisms. Antibacterial property, as an energy source for enterocytes, improving intestinal morphology and facilitating mineral absorption are few among them. Butyric acid checks proliferation of enteric pathogens and absorption of toxins into systemic circulation, thereby improving carcass characteristics and performance of broiler birds. Hence, butyric acid can be a potential ingredient among antibiotic alternatives for improving intestinal integrity and performance of chickens.
