EFFECT OF NON-IONOPHORE FEED ANTIBIOTICS ON IN-VITRO FERMENTATION IN THE OVINE RUMEN AND RABBIT CECUM

Experiments were carried out at Uhrineves (Czech Republic) in 1996 and 1997 to evaluate the effects of avoparcin, bacitracin, flavomycin, ty losin and virginiamycin on in vitro fermentation of ovine ruminal cont ents and rabbit caecal contents. In incubations of the rumen contents supplied with starch, hemicellulose and pectin, all antibiotics at 20 mu g/ml decreased production of fermentation gas and changed molar com position of fermentation products in favour of propionate. Flavomycin, tylosin and virginiamycin significantly decreased total production of volatile fatty acids (VFA). In incubations of the rabbit caecal conte nts, attempts to modify composition of microbial metabolites failed. B acitracin was the only antibiotic which significantly increased the mo lar percentage of propionate and decreased acetate. This fermentation shift was accompanied by a decrease in total VFA production. The net p roduction of propionate was thus similar in control and bacitracin-tre ated cultures. Rumen in vitro fermentation yielded more fermentation g as, methane and propionate than rabbit caecal fermentation of the same substrate. There was no apparent effect of antibiotics tested on meth anogens in either microbial system, as the ratios of methane to VFA pr oduction were similar in control and treated cultures.