The study was carried out using a herd of standard nutria. Animals were fed with feeds with various flavomycin supplements depending on the group (gr. I - control; gr. II - 3 mg; gr. III - 4 mg; gr. IV - 5 mg). Animals in each group were weighed once a week. The data ob-tained were used to calculate weight gains and growth rate in particular weeks. Results were analysed using three-way analysis of variance (time, ration and sex). Body weight of individu-als of each sex differed highly significantly from the beginning of the experiment. The increase in nutria body weight at 7 months of age was highly significant (p < 0.01). The final means of body weight in particular group were not statistically significant (gr. I - 4401 g, SD=585; gr. II - 4904 g, SD = 875; gr. III - 5021 g, SD = 1134; gr. IV - 4593 g, SD = 645). The data ob-tained did not show statistically significant differences in the body weight gains of nutrias from 7 to 8.5 months of age. The data obtained did not show statistically significant differences in the growth rate of animals receiving different flavomycin supplements.

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