The objective of this study was to evaluate the effects of feeding gallic acid on the growth, nutrient digestibility, plasma metabolites, rumen fermentation, and bacterial community in the rumen fluid and feces of preweaning calves. Thirty-six female Holstein calves with similar ages (means ± SD; 3.1 ± 1.39 d) and body weights (40.8 ± 2.87 kg) were randomly assigned to receive 3 treatments. Calves were fed 1 of 3 treatments as follows: basal diet with no gallic acid (control), 0.5 g/kg gallic acid in starter diet (low), and 1 g/kg gallic acid in starter diet (high). The results showed that feeding gallic acid increased growth by improving the starter intake and average daily gain of the calves. The fecal score tended to decrease in a linear manner with the addition of gallic acid. Total-tract apparent protein digestibility tended to increase linearly with feeding gallic acid. Feeding gallic acid led to a linear increase in the plasma total protein and β-hydroxybutyrate levels. In addition, feeding gallic acid linearly increased catalase and total antioxidant capacity levels and decreased malondialdehyde and tumor necrosis factor-α concentrations. The concentrations of total volatile fatty acids, propionate, butyrate, and valerate in the rumen fluid increased linearly with the addition of gallic acid, resulting in a linear pH reduction. Feeding gallic acid linearly increased the relative abundances of Prevotella_1, Saccharofermentans, and Prevotellaceae_ UCG-001 and linearly decreased the relative abundance of Prevotella_7 in the rumen fluid. The Shannon index of ruminal bacterial communities linearly increased by feeding gallic acid. Feeding gallic acid linearly increased the relative abundances of Ruminococcaceae_UCG-005, Bacteroides, and Christensenellaceae_R-7_group in the feces. In summary, feeding gallic acid improved growth, antioxidant function, and rumen fermentation and altered the bacterial community in the rumen fluid and feces of preweaning dairy calves.

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