Summary -An in vitro study was carried out to evaluate the effect of different ionophore antibiotics and some of their derivatives on rumen fermentation and on the degradation of peanut meal nitrogen. The increase in the production of propionic acid at the expense of acetic acid, observed with lonomycin, nigericin, cationomycin and lysocellin, was identical to that noted with monensin. The decrease in methanogenesis observed in the presence of monensin was also found with cationomycin and lysocellin. With the exception of lysocellin, which greatly reduced protein degradation of peanut meal, and of nigericin, which had no effect on this parameter, the 2 other molecules presented the same action as monensin. The negative effect of monensin on microbial ammonia uptake was demonstrated with the same intensity in the presence of cationomycin; it was slightly higher with nigericin and particularly accentuated with lonomycin and lysocellin. Three ester derivatives of monensin (monensin acetate, monensin propionate and monensin butyrate) had a similar action to that of monensin on the orientation of rumen fermentations. The monensin isobutyrate derivative appeared to be more active than monensin and only weakly altered microbial ammonia uptake. The oxolonomycin and hydroxolonomycin derivatives behaved identically to lonomycin with respect to microbial metabolism and protein nitrogen degradation. Unlike the molecules from which they derive, the deacylated cationomycin and nigericic acid had no effect on the orientation of rumen fermentations. Of the compounds tested and presenting a potential 'growthpromoting action' at least comparable to that of monensin, and which demonstrated lower toxicity on mice, three molecules (oxolonomycin, lysocellin and

We investigated the capacity of a new ionophore antibiotic, abierixin, to modify fermentations in the rumen using a semi-continuous fermenter (Rusitec). As in the studies carried out on a "batch" fermenter (HILLAIRE et al., 1989a), abierixin failed to alter volatile fatty acids and gas productions but, in contrast, it limited the degradation of dietary nitrogen without affecting microbial synthesis. This molecule which has a low level of toxicity and is capable of improving the use of dietary proteins by ruminants, was found to be more effective when used at very low dose levels.

An in vitro study on a "batch" type fermentation system was carried out in order to evaluate the effect of different doses of a new ionophore antibiotic, abierixin, on the rumen fermentation parameters. The reaction of microorganisms to the antibiotic were determined according to the nature of the nitrogen introduced into the fermenter: ammonium sulfate alone or complemented with protein nitrogen (peanut meal or a mixed pelleted feed). This molecule had a protective effect with respect to dietary protein degradation in the rumen, at the lowest dose tested (13.5 ppm). The dose (8 ppm) used with the pelleted feed led to a decrease in the protein degradability but the significant threshold was not reached. Abierixin failed to modify bacterial ammonia uptake with the nitrogen sources used in our experiment. In addition, it had no effect on rumen fermentations. It neither led to any alteration in the total VFA and gas productions during 6 hours of fermentation, nor on their molar composition, whatever the dose of antibiotic used and the nitrogen source considered.

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(groundnut meal or urea). Our results suggest that nitrogen from groundnut meal is used more efficiently by rumen microbes than urea, which agrees with growth trials conducted simultaneously on bulls.Certains travaux ont montré que la synthèse microbienne était plus importante à partir d'acides aminés provenant de protéines dégradables que de l'ammoniaque issue d'azote uréique (Blake et al., 1983