This study aimed to evaluate the effect of replacement of ground corn by soybean hulls associated or not to whole soybean grain in the feed supplement on intake, nutrient digestibility, rumen microbial population, and fermentation parameters of growing Nellore steers grazing Brachiaria brizantha cv. Xaraés during rainy season. Were used eight castrated Nellore steers (425 ± 36 kg of body weight (BW)) fitted with ruminal and duodenal cannulas in a replicated 4 × 4 Latin square with a 2 × 2 factorial arrangement, allocated into 4 paddocks of 0.25 ha each, consisting of Brachiaria brizantha cv. Xaraés and supplemented with: (1) ground corn combined with whole soybean grain (SG); (2) ground corn without SG; (3) soybean hulls (SH) combined with SG; and (4) SH without SG. Supplement intake was no affected by SG or SH (mean 2.12 kg-1 d, P > 0.05). There were no interactions between SH and SG on DM and nutrients intake (P > 0.05). The addition of SG reduced the dry matter (DM) intake expressed as % of BW and Kg-1 d, as well forage DM, organic matter (OM), crude protein (CP), neutral detergent fiber (aNDF), and gross energy (GE) intake (P ≤ 0.01). Animals supplemented with SH without SG had greater digestibility of DM (74.52), OM (77.62), CP (77.51), NDF (71.93) and GE (72.90) than animals supplemented with SH with SG (DM =69.01, OM = 71.92, CP = 72.81, NDF = 66.01, GE =68.01) expressed as % (P ≤ 0.01). The addition of SG in the supplements declined the ruminal pH and NH3-N (P=0.02). Animals supplemented without SH without SG showed greater Entodinium counts (6.01 n x 104 ml-1, P=0.04), and SG supplementation decreased the numbers of Dasytricha, Isotricha, and ruminal total protozoa (P < 0.01). The abundance of Ruminococcus albus, R. flavefaciens, and Archaeas were higher for SH without SG supplement. Additionally, animals supplemented with SG had lower numbers of Fibrobacter succinogenes. The use of soybean hulls without whole soybean grain in the supplement may be effective to increase fiber digestibility, N retained, R. albus and R. flavefaciens in the rumen of Nellore steers grazing Brachiaria brizantha cv. Xaraés during the rainy season.