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Digestibility has always been a topic of interest to poultry nutritionists. Over the last 50 years, we have come to appreciate the role of various regions of the digestive tract and their impact on the digestion, absorption and maturation of poultry.
Since the development period of broilers has been reduced, the importance of these functions in the start-up phase has increased. Nowadays, it is difficult to reach a weight at normal slaughter age if the growth rate is delayed during the first 7 days of growth, regardless of what the reasons for that may be.
GUT FLORA IN CHICKS
The study of digestion in young birds has revealed that the digestive ability of chicks is still “immature” and therefore, a new industry has emerged, supplementing feed with a variety of enzymes and other additives designed to limit early digestion.
The chick is born with an intestine practically free of microbes, so early colonizers tend to predominate. The undigested nutrients will be available to promote microbial growth in the distal portions of the intestine and cecum, and if they include pathogenic bacteria, the chick will be at a disadvantage.
The “normal” gut microbiota develops rapidly, so that the bacterial load and the species present in the hatchery processing belt, inside the delivery trucks and during the first days on the farm will determine early colonization.
The Nurmi concept of manipulation of the gut microbiota is based on the early introduction of non-pathogenic microbes. Ideally, these microbes will help prevent pathogen colonization and competitive exclusion (CE), which will undoubtedly be one of the future management tools used in broiler production.
Early and rapid development of the intestinal epithelium is another prerequisite for normal digestion. Intestinal villi and microvilli grow rapidly in the first few days, and any delay in this process will lead to reduced nutrient uptake and increased nutrient availability for anaerobic pathogens.
The presence of pathogens, mycotoxins, and animal & plant toxins delay the development of microvilli. The selection of highly digestible ingredients, free of natural toxins, is important to ensure a rapid and early bowel development.
Butyric acid, either in food or as a consequence of fermentation of non-starch polysaccharides, is important for the development of microvilli. As the epithelium develops within the microvilli, the secreted mucosa acts as an important barrier against pathogenic colonization and self-digestion by the bird’s own digestive enzymes.
Some bacteria are able to colonize because they are able to break down this protective mucous layer.
Helicobacter pylori, the bacteria that causes gastric ulcers in people, secretes the urease enzyme that destroys the mucous lining, exposing the gastric wall to the action of hydrochloric acid and pepsin in the stomach. It would be interesting to study the intestinal microbiota of birds fed with a soybean meal rich in urease. Currently, the fate of undigested nutrients is taking on the same importance as that of digested nutrients. In the past, 12-20% of indigestible ingredients were considered to be a problem associated with stool consistency and litter quality.
We now know that indigestible material influences microbial growth, especially in the large intestine and cecum.
Enhancing microbial growth will be of enormous importance in a setting where growth promoting antibiotics are no longer used, especially in the absence of anticoccidial ionophores.
The main factor influencing carbohydrate digestion is the content of complex polysaccharides, such as cellulose and lignin. There is very little lignin in diets based on corn-SBM (corn and soybean meal), so that cellulose becomes the main limitation of digestibility.
Despite the fact that up to 10% of dietary cellulose disappears in the digestive tract, the greatest loss is associated with microbial activity in the large intestine and cecum, where the products of digestion have limited utility for the bird but favor microbial growth.
There are other polysaccharides that are of greatest interest to poultry nutritionists, such as hemicelluloses, pentosans, β-glucans and oligosaccharides (stachyose and raffinose) that are found in oilseed meal, and which together are known as non-starch polysaccharides (or NSPs).
Normally, there is a negative correlation between carbohydrate digestibility and the high-level content of pentosans and β-glucans.
Unfortunately, these undigested polysaccharides have the adverse effect of absorbing large amounts of water from the digesta, creating a more viscous medium.
Consequently, there is less possibility of contact of all substrates with digestive enzymes and all digested products may not reach the intestinal microvilli.
These complex carbohydrates reduce the digestibility of all the nutrients present in the food bolus, not only that of carbohydrates, irremediably leading to bacterial overgrowth.
Fortunately, exogenous enzymes such as xylanase and β-glucanase can currently be added to the feed, eliminating the problems associated with its viscosity, thus improving digestibility of the nutrients and achieving a better balance of the intestinal microbiota.
Alpha-galactosaccharides, commonly known as oligosaccharides, account for up to 12% of the carbohydrates present in soybean meal. The most common components are stachyose, raffinose and cellobiose.
Although they can be extracted with ethanol, these oligosaccharides are not removed from the soybean by chemical extraction of fat with hexane, the residue of which is partly responsible for the low amount of digestible energy in soybean flour for poultry farming.
Due to the absence of β-galactosidase activity in the intestinal mucosa, there is interest in adding exogenous enzymes to the feed and / or extracting the polysaccharides by ethanol.
The proventriculus is the first point of protein degradation thanks to the action of secretions that include hydrochloric acid and the enzyme pepsin.
Before the feed reaches the proventriculus and the gizzard, the pH of the secretions may be as low as 1.5-2, but under the buffering conditions of the food the pH increases to 3.5-5.
An active gizzard with a low pH has great antibacterial properties, although it has less impact on the passage of coccidia oocysts.
A considerable part of the endogenous protein enters the digestive tract in the form of saliva, gastric juices, pancreatic juices and desquamated epithelial cells from the intestinal mucosa and mucins.
This endogenous protein should not be confused with endogenous nitrogen losses through urine, since endogenous protein is digested and used by the animal, while endogenous nitrogen lost through urine must be replenished daily by additional dietary protein.
The increased flow of endogenous nitrogen into the large intestine brings with it a greater potential for microbial overgrowth.
The digestion and absorption of fat occurs mainly in the small intestine. Lipase activity increases rapidly during the first 10 days of life. For example, at the level of the duodenum, lipase activity increases by up to 100 times between 4 and 15 days after birth.
Digestion is enhanced by the emulsifying properties of bile salts, since lipases are only active at an oil-water interface.
Medium chain fatty acids and triglycerides do not seem to need the prerequisite of micelles formation before digestion and absorption, so they are interesting ingredients in the case of starter diets.
Water-insoluble components such as fatty acids and monoglycerides cannot form micelles by themselves but can form stable mixed micelles with conjugated bile salts.
Saturated fatty acids, such as palmitic and stearic acids, are apolar, have high melting points and are only slightly soluble in the bile salt emulsion. However, they are remarkably soluble in the presence of a mixed micelle.
The balance between saturated and unsaturated fatty acids present in the diet and the amount of bile salts are important factors in fat absorption.
A saturated fatty acid will be easily absorbed if it is at position 2 of a triglyceride, since monoglycerides with saturated fatty acids are better absorbed than when they are in the form of free saturated fatty acids.
Balanced feed formulation
In the case of starter diet formulations, the idea is to correct such deficiencies and increase the rate of early growth and/or reduce microbial overgrowth.
The limiting amino acid for clostridia appears to be lysine and serine, and limiting their flow into the large intestine using expensive animal proteins is one strategy for limiting bacterial overgrowth.
In the study of intestinal health we are very limited by our lack of knowledge, with precision, of the normal microbiota present in healthy birds. It has been suggested that, at most, conventional culture techniques manage to isolate only 5% of the bacterial species present in the intestine.
The application of newer techniques, including the identification of microbial DNA, may shed light on the complexity of the microbes, especially how the change in response to different types of diets.
Optimizing digestion, or rather minimizing indigestion, is a viable strategy to limit microbial overgrowth in the gut and cecum. There is no simple substitute for growth-promoting antibiotics, and future management guidelines need to be multifaceted.
Table 1 lists some of the approaches to limiting the flow of nutrients to intestinal pathogens. There is an opportunity for feeding specialized starter diets and, although they are more expensive compared to conventional diets, they have the potential to provide long-term benefits beyond those observed at the time of supply.
Today, there is an extensive list of “alternatives” to antibiotic growth promoters, and many of these are undoubtedly incorporated by the broiler industry.
As we learn more about bacterial populations, we gain knowledge about their nutritional needs and conditions of proliferation.
Our understanding of the interaction between microbial populations and digestion should not divert our attention from the importance of basic principles of broiler management. Incubation practices are increasingly important in optimizing normal intestinal function and minimizing pathogen growth.
Regardless of the strategies employed in preventing the proliferation of pathogens in production, we must remember that microbes are opportunists with a remarkable ability to adapt to new environments.
No strategy is likely to be effective in the long term, so we must be prepared to be flexible with our dietary programs, food additives and management practices.
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