Abstract: This study evaluated the effects of isobutyrate (IB) supplementation on growth performance, ruminal fermentation, ruminal cellulolytic bacteria abundance and ruminal enzyme activities in pre- and post-weaned dairy calves. Thirty-six Chinese Holstein male calves were randomly allocated to four groups at 15 days of age. The treatments were control, low-IB (LIB), medium-IB (MIB) and high-IB (HIB) with 0, 3, 6 and 9 g isobutyrate per calf per day, respectively. At 60 days of age (weaning) and 90 days of age, body weight of calves was measured and ruminal fluid was collected for analysis. The results showed that dry matter intake, average daily gain and economic benefit were higher (P<0.05) for MIB and HIB than for control in pre- and post-weaned calves. Ruminal pH was lower (P<0.05) for HIB than for LIB and control in calves at 90 days of age. Ruminal total volatile fatty acids and acetate concentrations were higher (P<0.05) for MIB and HIB than for LIB and control. The ratio of acetate to propionate was higher (P<0.05) for MIB and HIB than for control in calves at 60 days of age, and was higher (P<0.05) for MIB and HIB than for control and LIB in calves at 90 days of age. Ruminal isobutyrate concentration was higher (P<0.05) for MIB and HIB than for control in calves at 60 and 90 days of age. At 60 days of age, activities of xylanase and α-amylase were higher (P<0.05) for MIB and HIB than for control, activities of cellobiase and pectinase as well as abundance of Butyrivibrio fibrisolvens, Ruminococcus albus and Fibrobacter succinogenes were higher (P<0.05) for MIB and HIB than for control and LIB. At 90 days of age, all the ruminal enzyme activities and cellulolytic bacterial abundance values were higher (P<0.05) for MIB and HIB than for control and LIB treatments. These results indicated that isobutyrate improved growth performance and ruminal fermentation through stimulation of ruminal cellulolytic bacterial growth and enzyme activity. There was no significant difference in measured prarmeters between MIB and HIB; thus, the optimum supplementary dose of isobutyrate was found to be 6 g per calf per day under the conditions of this experiment.

Key words: isobutyrate, growth performance, ruminal fermentation, cellulolytic bacteria, enzyme activity, calves