Eight ruminally cannulated crossbred steers (average weight 336 kg) grazing native blue grama (Bouteloua gracilis  [H.B.K.]) rangeland were used in a repeated measures design to evaluate effects of monensin ruminal delivery devices (MRDD) and forage phenology on ruminal digestion.  Three periods were assessed: mid-August (Aug.), early October (Oct.), and mid-November (Nov.).  One MRDD was placed in the reticulum of 4 steers via the ruminal cannula 21 days before each period.  Intake was estimated using total fecal collections.  Diet samples were collected using 3 esophogeally fistulated steers. Ruminal fill was measured by ruminal evacuation; rate and extent of in situ ruminal neutral detergent fiber disappearance were estimated before ruminal evacuations.  Ruminal passage rates, retention time, and apparent total tract organic matter digestibility were estimated using indigestible neutral detergent fiber.  In vitro organic matter disappearance of esophageal masticate did not differ (P>.05) in Aug. and Oct. (average of 53.7%), but declined (P<.05) in Nov. (48.7%), whereas organic matter digestibility was greater (P<.10) in Aug. (62.3%) than in either Oct. (55.2%) or Nov. (53.9%).  Release of monensin from the bolus (68 mg day^-1) was less than expected (100 mg day^-1).  Intake, organic matter digestibility, ruminal passage rates, retention time, pH, and ammonia were not affected (P>.10) by MRDD.  In situ neutral detergent fiber disappearance at 96 hours was decreased (P<.10) by MRDD (68 vs 65% for control and MRDD, respectively).  As the grazing season progressed, intake declined (P<.10), whereas ruminal fill and retention time increased (P<.05), and passage rate of indigestible neutral detergent fiber decreased (P<.05). At 48 hours in situ neutral detergent fiber was greatest (P<.05) in Aug. and least (P<.05) in Nov.