Effect of a new ionophore antibiotic, abierixin, on the digestion of a roughage-based diet measured in sheep; comparison with results obtained in Rusitec

Abstract

Four adult sheep fitted with rumen, simple duodenal and ileal cannulas, were continuously fed 980 g of a roughage-based diet supplemented (R2) or not (R1) with abierixin (22 ppm). Digesta flow at duodenum and ileum were measured using Yb as digestive marker. Cr-EDTA solution was infused to estimate the volume of liquid content in the rumen and the outflow rate. The microbial nitrogen flow at the duodenum was calculated with diaminopimelic acid (DAP) and nucleic-acid purine bases (PB) as microbial markers. Digestive balances and pattern of rumen fermentation were measured after 3 weeks of adaptation to the diets.

Abierixin had no effect on the end products of fermentation nor on feed digestion measured in the rumen, the small intestine and the large intestine. Only a weak increase in rumen organic matter digestion was observed at the statistical threshold P < 0.06. Estimated from DAP, the synthesis of microbial proteins and its efficiency were significantly reduced by abierixin. Using purine bases as microbial marker, these parameters showed no modification. Abierixin had no effect on the mean retention time of digestive contents in the forestomachs and in the intestines. We observed that abierixin had no effect on nitrogen digestion which is markedly different from the results previously noted in Rusitec where abierixin induced a decrease in the dietary nitrogen breakdown by rumen microorganisms without affecting the microbial proteosynthesis. The hypothesis that abierixin could lead to a higher supply of amino acids in the duodenum by increasing dietary nitrogen flow was not confirmed by this in vivo study. The differences in behaviour of abierixin in Rusitec compared with our in vivo study could be explained by differences in concentration of the antibiotic in the two systems. This concentration was calculated from equations proposed to model the kinetic evolution. Our models show that abierixin concentration was two to five times greater in vivo compared with Rusitec, which indicates either that the optimum dose of abierixin is 5 ppm rather than 22 ppm, or that Rusitec selects a flora which is more sensitive to the ionophore antibiotic than the normal mixed rumen population. If the latter is true, then Rusitec is unsuitable to test the activity of such molecules, and in vitro results should be analysed with care when extrapolated to animals.