Chickens, kept under commercial conditions aim to efficiently convert feed into meat. Particularly the interaction between nutrition and the microbiology of the gut ecosystem is a means to enhance health, performance and ultimately, profitability.
By Dr Salah H. Esmail , Cairo, Egypt
Newly hatched chicks are quite often exposed to a 24-72 hour transition between hatching and placing them on the farm, during which time chicks that hatch first are without feed or water. The lack of access to feed and water during this holding time in the incubator leads to depression of intestinal function by reducing the number of enterocytes available for villous growth, and hence reducing the villous absorptive surface area. This may limit the nutrient uptake capacity and contributes to decreased growth, which may not be overcome at a later stage in life.
The early supply of feed in the hatching trays and/or the transportation boxes should, therefore, be considered for improving intestinal functions and growth performance of chickens. As a better approach, nutrients may be supplied to the growing embryo through injection of arginine and -hydroxy–methyl-butyrate into the amniotic fluid at 17-18 days of incubation. In this way, the in ovo-fed chicks were functionally equivalent to 2-days old chicks fed in a conventional manner, and had also developed a strongerimmune system in the intestinal site, exhibiting protection functions against a variety of pathogens later in their life.
Physical form of the diet In broiler chickens fed whole wheat, the relative gizzard weight increased by 37%. Similar results were observed with turkeys fed whole barley and coarsely ground corn, where gizzard weights increased by 34% and 27% respectively compared to the birds fed finely ground corn/SBM-based diets. As the gizzard grows larger, it will more effectively maintain normal intestinal motility, and will have a greater influence on digestion and absorption of nutrients by increasing digesta retention time.
The increased retention time in the gizzard exposes the contents to greater peptic digestion, which is particularly important for enhancing protein digestion efficiency in the small intestine. It was further reported that feeding whole grain to broiler chickens causes a significant reduction in the gizzard content pH, probably due to mechanical stimulation of the proventriculus which leads to increased hydrochloric acid production. Each time the gizzard contracts, some of its contents are refluxed back into the proventriculus for further exposure to hydrochloric acid and pepsin, thereby cleaving proteins into multiple peptide fragments, which are then further digested by pancreatic trypsin and chymotrypsin.
Whole wheat inclusion It was also found that feeding whole wheat to broiler chickens reduced the number of Salmonella typhimurium and C. perfringens in the intestinal tract of birds, and hence reduced mortality associated therewith. For example, the use of wheat finely ground with a hammer mill has in some cases increased mortality to 28.9% compared to a mortality of 18.1% when using wheat coarsely ground with roller mill. In other studies, however, the feeding of whole wheat or coarsely ground wheat to broiler chickens experimentally infected with coccidiosis has enhanced development of Eimeria tenella in the cecum. This resulted in significantly lower weight gain in the whole wheat group compared with ground wheat-fed broilers. From these studies, it can be concluded that when the GI tract is healthy, inclusion of whole wheat into the diet may help improve digestive tract function and immunity, but when integrity of the GI tract is already impaired through microbial invasion, then inclusion of whole grains may decrease performance.
Influence of cereal type A study was conducted to compare gut morphology and performance of broiler chickens fed a maize-based diet and on diets based on wheat/rye which are known to contain considerable amounts of non-starch polysaccharides (NSP). As shown in Table 1, the villous fusion score was greater for the wheat/rye diet compared with the maize-based diet, probably due to the fact that NSP in wheat/rye diet stimulate proliferation of C. perfringens in the chicken gut.
This bacterium can induce epithelial cell damage and enhances fusion of the villi due to toxin production and intestinal inflammation. The thickness of tunica muscularis was also greater for the wheat/rye diet, suggesting increased number and activity of the goblet cells in the epithelial lining. In addition, the increased intestinal viscosity observed with the wheat/rye diet due to presence of NSP often leads to a decreased rate of diffusion of substances and digestive enzymes and hinder their effective interaction.
Such changes of gut morphology and functions induced by NSP in wheat/rye diet have obviously decreased feed utilisation and performance of chickens. This situation may, however, be improved by using NSP-degrading enzymes such as xylanase, which enhances digestibility of most cereal-based broiler diets and thus also body weight gain and feed conversion.
Dietary protein The increased protein level of the diet can be a contributing factor to necrotic enteritis in broiler chickens, which usually occur 2-6 weeks post hatching. This is due to the over growth of C. perfringens in the small intestine, increasing from a normal level of 104 CFU to 107 or 109CFU per gram of digesta and causing clinical disease.
With the increased dietary protein level, there will also be an increased activity of the enzyme trypsin in the small intestine. This will, in turn, lead to faster release of coccidia from their oocytes which eventually become so active as to be less responsive to vaccination.
In cases where such bacterial and protozoan agents are likely to prevail, it may then be beneficial, among other measures, to reduce the supply of protein and maintain it below the recommended range. It is also important to consider the amino acid balance of the protein source to be used. Methionine and glycine, for example, have long been known to stimulate growth and establishment of C. perfringens and other pathogens in the gut. Therefore, the use of protein sources having excessive amounts of these amino acids should be minimised. Given in Table 2 are the methionine and glycine contents of some common protein sourcesto be considered when formulating diets for chicken subject to disease outbreaks.
Also, there are some protein sources such as raw soybean, cottonseed meal and flax cakes which contain varying amounts of anti-nutritional factors such as trypsin inhibitors, gossypol, and glucosides, respectively. When ingested by the bird, these factors would then exert some damaging effects on the small intestine, thereby impairing the immune apparatus having not only local but also systemic protective functions. Excessive use of such protein sources in the diet should, therefore, be avoided as well.
Dietary fat A study was conducted to compare the effects of two different fat sources (10% lard and tallow vs 10% soy oil) on bacterial community in the ileum of broiler chickens receiving diets supplemented with antibiotics containing avilamycin (10 mg/kg of feed) and salinomycin (40 mg/kg of feed). Results are given in Table 3.
The improved response of chickens to antibiotic treatment with the soy oil over the animal fat diet could be attributed to the higher content of unsaturated fatty acids in the soy oil, which increases solubility in the micellar phase and hence facilitates dispersion of antibiotics in the small intestine. The type of fat source may also indirectly influence gut microflora through its impact on viscosity of digesta, intestinal transit time, bile salt hydrolase activity and digestion in the small intestine.
Dietary fibre Fibre has an effect on the microbial profile of the gut through production of short-chain fatty acids (SCFA). In studies with diets containing corn, wheat and soybean meal (3.1% crude fibre), the production of SCFA in the cecum was only 22 mg/g of cecal sample, and has increased to 51 mg/g when adding soybean hulls to bring the fibre level up to 5.7%. The increased production of SCFA due to addition of the fibre source to the diet was linked with a bacteriostatic effect on some enteric bacteria such as C. perfringens, which has, in turn, improved health and performance of birds.
Fibre may also improve intestinal digestion by reducing the number of goblet cells present on the villous structures in the small intestine, and hence reduce the amount of goblet mucin which acts as a luminal barrier against absorption. This, however, may not always be the case, especially with fibre sources of high molecular weight or those having high methoxyl contents such as citrus pomace, apple pomace, tomato pomace, etc.
Excess feeding of such fibre sources may lead to enlargement of intestinal villi arising from physical stimulation of the villous growth similar to that observed with ruminants fed on high-fibre diets, where rumen papillae are also enlarged through the physical action of fibre. The increased size of the villi is often coupled with about two-fold increase in goblet cell numbers which adversely affects absorption. The excessive use of such fibre sources in the diet may also increase viscosity of the intestinal content, with a resulting decrease in bio-availability of vitamin A and utilisation of dietary fat, which adversely affects body weight gain and carcass quality. It is therefore, recommended to use such sources of fibre in limited amounts when better performance is to be achieved.
Mineral and vitamin supplements Some mineral elements have the potential to regulate microbial population in the gut. For their antimicrobial properties, copper and zinc are routinely fed at levels greater than the requirement to promote health and growth. The effects of different sources of these minerals may, however, vary. Dietary copper and zinc sources that are more soluble in the gastrointestinal tract affect intestinal physiology in the anterior parts of the small intestine (i.e., duodenum), whereas sources that are less soluble affect the posterior parts (i.e., jejunum). Therefore, selection of dietary copper and zinc sources to modulate gut physiology may be based upon the type and frequency of expected intestinal disease challenge.
Vitamins also have an effect on the intestinal integrity and its associated immune system. Studies on vitamin A, for example, have reported higher mortality from E. acervulina and E. tenella in chickens fed vitamin A-deficient diets compared to those fed on diets supplemented with 8,000 IU of vitamin A per kg of feed.
This result was attributed to the decreased number of T-helper cells in the intestinal epithelium of chickens when fed on diets deficient in vitamin A. The T-helper cells are responsible for orchestrating many other immune responses through cytokine secretion and interaction with other immune cells. This may provide a clue for the increased mortality of birds as the number of these cells decreases with vitamin A deficiency.
* References are available from the author upon request