Abstract: In order to assess the effects of different concentrations of gibberellic acid on the growth of bacteria added in feed additives, two different concentrations （10 mg/L and 20 mg/L） of gibberellic acid solutions were added to the culture medium of lactic acid bacteria strains of A4+A7 and cellulose-decomposing bacteria strains of Nf+Y6 separately for a 52-hour-long cultivation. The OD_{600 nm} value was determined every 4 hours and the growth curve was drawn. The results showed that when the adding concentration of gibberellic acid was 10 mg/L, the OD_{600 nm} values of the different groups were 0.64, 0.70, 0.84, 0.78 and 0.72, respectively. The OD_{600 nm} value of the experimental group 2 were obviously higher than that of the control group and the other experimental groups. The total viable count of bacteria in experimental group 2 was 11.6×10⁸ CFU/mL, which was 7 times higher than that of the control group （1.63×10⁸ CFU/mL）. When the concentration of gibberellic acid increased to 20 mg/L, the OD_{600 nm} values of the different groups were 0.64, 0.60, 0.59, 0.59 and 0.63, respectively. There was no significant （P>0.05） difference in OD_{600 nm} values between the experimental groups and control group, and there was no significant （P>0.05） difference in total viable count of bacteria between the experimental group （1.60×10⁸ CFU/mL） and control group （1.63×10⁸ CFU/mL）. The experimental data and growth curve showed that the growth of lactic acid bacteria and cellulose-decomposing bacteria could be promoted when the concentration of gibberellic acid was 10 mg/L, while the growth rate of lactic acid bacteria and cellulose-decomposing bacteria was decreased significantly when the concentration of gibberellic acid was 20 mg/L. The combined data suggested that proper addition of gibberellic acid could obviously promote the growth of the bacteria added in feed additives, but the growth of bacteria was inhibited when the concentration of gibberellic acid was exorbitant.

Key words: gibberellic acid, lactic acid bacteria, cellulose-decomposing bacteria, silage