This study aimed to evaluate the effects of oral supplementation of a multifunctional additive on the fecal microbiota, feeding behavior, and growth of calves raised in groups in pasture-based systems. The study was carried out at Embrapa Pecuária Sudeste, Brazil. Sixteen Kiwi-Cross dairy calves were randomly assigned to two groups: treated (n=8): animals (3 g/animal/day) that received a compound of probiotics, prebiotics, essential fatty acids, organic minerals, and amino acids; and control (n=8): animals (3 g/animal/day) that received a placebo. From birth to weaning (0-66 days), the calves' fecal samples (D0, D15, D30, D60, and D66), feeding behavior (grazing and rumination), and growth parameters (average daily gain—ADG and thoracic circumference—TC) were taken. Fecal microbiota were analyzed by 16S rRNA sequencing (Illumina MiSeq), while behavior and growth were analyzed by Generalized Linear Models. Regardless of treatment, there was an increase (~16%) in α-diversity over time (p<0.001). The treated group presented a greater (p = 0.001) relative abundance of the families Lactobacillus and Muribaculaceae than the control group. The treated group (grazing: 43 d; rumination: 36 d) began grazing and rumination behaviors later than the control group (grazing: 35 d; rumination: 28 d). The ADG (p = 0.008) and TC (p = 0.003) were higher in the treated group (ADG: 0.69 kg/day; TC: 90.3 cm) than control group (ADG: 0.52 kg/day; TC: 86.7 cm). In conclusion, the fecal microbiota changed over time, probably due to rumen development during the experimental period. Although supplementation with a multifunctional additive delayed grazing and ruminating behaviors in dairy calves, it improved their growth.