Effect of substrate and feed antibiotics on in vitro production of volatile fatty acids and methane in caecal contents of chickens
Introduction
The poultry caeca are the major site of microbial digestion of carbohydrates and proteins leaving the small intestine. Poultry caeca are colonized by an abundant bacterial flora. Caecal bacteria ferment available nutrients to a mixture of 2–5-carbon volatile fatty acids, ammonia, carbon dioxide and methane. Volatile fatty acids (VFA) are absorbed and utilized by the animal, mainly as an energy source (Annison et al., 1968). The nutritional significance of poultry caeca is limited, considering their small volume (less than 0.5% of the body weight, v/w). The caecal VFA, however, represent an important factor in preventing the caecal Salmonella colonization. Barnes et al. (1979) observed in in vitro tests with S. typhimurium that whilst the bacterium was able to multiply at the VFA concentration found during the first few days after hatching, the rapid increase in VFA concentration during the first 21 days would made this increasingly difficult. Several authors showed that dietary lactose reduced caecal colonization with S. typhimurium (reviewed by Hinton et al., 1991). The control was correlated with decrease in caecal pH and increase in undissociated VFA, especially propionic acid (Ziprin et al., 1991). A significant negative correlation between caecal propionate concentration and Salmonella colonization in young chickens has been reported by Nisbet et al. (1996). Because of its antibacterial properties, propionic acid was proposed as a prophylactic treatment for control of salmonellae infection in poultry. However, the addition of propionic acid has not proven be effective (Hume et al., 1993), presumably because dietary propionic acid was rapidly absorbed or metabolized in the foregut, and did not reach the intestinal tract. Various factors affecting production of caecal metabolites are not well understood and, as recently pointed out by Williams et al. (1997), the caecal fermentation in poultry has not been satisfactorily characterized.
The purpose of our study was to look for factors influencing the molar composition of the caecal VFA. In in vitro experiments, the production of caecal VFA and methane was measured in cultures supplied with substrates fully or partially undigestible in the small intestine (lactose, raffinose and five polysaccharides). Additionally, an attempt was done to modify the VFA molar composition by seven antimicrobial feed additives, known as rumen propionate enhancers.
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Experimental design
The caecal contents were obtained from 7-week-old broiler chickens (Ross) fed an unmedicated commercial concentrate containing 700 g maize/kg, 210 g soyabean meal, extracted/kg, 36 g meat and bone meal/kg, 10 g yeast/kg, 40 g mineral supplement/kg, 4 g vitamin supplement/kg. The caecal contents of 18 chickens were pooled and mixed with warm (39°C) Burroughs buffer (Burroughs et al., 1950) in 1 : 3 (w/v) ratio. The buffer (pH 7.3) contained yeast extract (0.1%, w/v). To prevent the ammonia deficiency
Results
Fermentation of lactose yielded more VFA than fermentation of the mixture of polysaccharides (Table 1). More VFA was produced in the first half than in the second half of the incubation. As expected, lactose was fermented more rapidly than polysaccharides.
Fermentation pattern differed in cultures supplied with different carbohydrates (Table 2). The highest molar percentages of acetate, propionate and butyrate were found in VFA produced from pectin, lactose and inulin, respectively. Fermentation
Discussion
Little information is available on the composition of intestinal digesta entering the poultry caeca. The chicken caecal contents incubated for 20 h without substrate under conditions used in this study produced 18–30 mmol/l VFA, i.e. ca 1.1–1.8 mmol/1 g of caecal dry matter. This suggests that the chicken caecal contents are rich in fermentable substrate, composed of food residues, and carbohydrates and nitrogenous compounds of endogenous origin (uric acid, mucins and digestive enzymes).
Acknowledgements
Our study was funded by grant No. 524/96/0543 of the Grant Agency of the Czech Republic. O. Suchorska is grateful to the Lvov Veterinary Academy for a partial support of her stay.
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