INFLUENCE OF RUMINALLY DISPENSED MONENSIN AND FORAGE MATURITY ON INTAKE AND DIGESTION

Eight ruminally cannulated crossbred steers (average weight 336 kg) gr azing native blue grama (Bouteloua gracilis [H.B.K.]) range-land were used in a repeated measures design to evaluate effects of monensin rum inal 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 s teers via the ruminal cannula 21 days before each period. Intake was e stimated using total fecal collections. Diet samples were collected us ing 3 esophageally fistulated steers. Ruminal fill was measured by rum inal 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 matte r digestibility were estimated using indigestible neutral detergent fi ber. 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 grea ter (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 expe cted (100 mg day-1). Intake, organic matter digestibility, ruminal pas sage 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, respective ly). As the grazing season progressed, intake declined (P<.10), wherea s 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 leas t (P<.05) in Nov.