Effect of substrate and feed antibiotics on in vitro production of volatile fatty acids and methane in caecal contents of chickens

Abstract

An attempt was done to identify some factors influencing the caecal fermentation pattern in poultry. Experiments were conducted to study effects of carbohydrate substrates (feed components and supplements) and antibiotics on the formation of volatile fatty acids (VFA) and methane in in vitro incubations of the caecal contents of 7-week-old chickens. Stoichiometry of fermentation differed in cultures with different carbohydrates. Fermentation pattern characterized by high propionate and low acetate production was found in cultures with lactose (0.447 and 0.376 mol/1 mol of VFA produced, respectively) and, to a lesser extent, also in cultures with raffinose. Acetate was the predominant metabolite of starch, pectin and xylan (0.727, 0.773 and 0.685 mol/1 mol of VFA produced, respectively). Fermentation of inulin resulted in high proportion of butyrate, 0.221 mol/1 mol of VFA. Other polysaccharides produced only 0.060–0.111 mol of butyrate per 1 mol of VFA. Oligosaccharides (lactose, raffinose) were fermented more rapidly than polysaccharides. Fermentation of inulin yielded more VFA than fermentation of starch, pectin and xylan. No production of VFA from carboxymethylcellulose was observed. On average, 11 mols of VFA were produced per mol of methane. Lasalocid significantly increased molar proportion of propionate, which is potentially beneficial from the point of view of salmonellae control. The magnitude of improvement, however, was small. Other feed antibiotics tested (avoparcin, bacitracin, lincomycin, spiramycin, tylosin, virginiamycin) produced only non-significant or marginal fermentation shifts. Formation of valerate, isoacids and methane was not significantly influenced by the substrate or by antibiotic treatment.

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.