Ten ruminally and duodenally cannulated (326 +/- 28 kg) and four esophageally fistulated (394 +/- 23 kg) steers grazing cool-season pasture throughout the growing season were used to evaluate the influence of Aspergillus oryzae fermentation extract (AO) supplementation on intake, forage nutrient utilization, and duodenal amino acid flow. Steers grazed a predominantly smooth brome (Bromus inermis L.) pasture, and measurements were taken in three periods (June, July, and August). Steers were dosed daily at 0700 via the ruminal cannula with AO (2 g of AO per steer daily; DM basis) or not supplemented with AO. Each period consisted of 18 d for adaption to AO and 7 d for collection. Forage N was greater, and ADF was lower (P < .10), in June than in July and August. Ruminal pH, ammonia, total VFA concentration, and VFA proportions were not affected (P > .10) by AO supplementation. In vitro DM digestibility (percentage) and forage OM intake (grams/kilogram of BW) were greater (P < .10) for steers supplemented with AO. Ruminal and total tract NDF and ADF digestibilities were lower in June and greater during July (P < .10) in steers supplemented with AO. Total tract percentage of N disappearance was lesser during June and greater (P < .10) during July for steers receiving AO. Total, essential, and nonessential amino acid flows were increased (P < .10) by both AO supplementation and advancing season. In situ rate of ruminal CP degradation was not affected (P < .10) by treatment during any period.(ABSTRACT TRUNCATED AT 250 WORDS)