Influences of ionophores on microbial development and metabolic aspect of the rumen of calves were assessed in a growth experiment, where 15 male calves were equally allocated to 3 treatments during 3 to 25 wk of age: control with no antibiotic, 20ppm salinomycin (S) and 10ppm portmicin (P). Dietary S and P improved feed conversion ratio based on concentrate intake by 5 and 4%, respectively. Both ionophores shifted ruminal fermentation pattern toward more propionate and less acetate and

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... , and this trend bacame more apparent after weaning. lonophores, in particular S, tended to hinder development of protozoa and cellulolytic bacteria, while they stimulated proliferation of amylolytic, proteolytic, lactate-fermenting and total viable bacteria. Jpn. J. Zootech. Sci., 61(9): 850-857, 1990