Influence of grain and monensin supplementation on ruminal fermentation, intake, digesta kinetics and incidence and severity of frothy bloat in steers grazing winter wheat pastures

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Abstract
Three 10-d collection periods (April 4 to 14, early April, EApr; April 23 to May 3, late April, LApr; May 10 to 20, 1984, mid-May, MMay) were conducted to evaluate effects of no supplement (C), .5 kg·head−1·d−1 (as-fed basis) supplemental grain (steam-flaked milo, G) or G plus 170 mg monensin·headhead−1·d−1 (M) on forage intake and digestion by 12 ruminally cannulated beef steers (four/treatment; avg initially BW = 393 kg) grazing irrigated winter wheat pasture. Ruminal pH was greater (P < .01) for M than for C or G during EApr but was not altered by treatments in LApr or MMay. Compared with C, ruminal NH3 was decreased (P < .10) by G and M (5 h after supplementation) in EApr, decreased (P < .05) by G (2h) and increased (P < .05) by M (8 h) in LApr and decreased (P < .10) by G (−1h) in MMay. Treatments had little influence on total VFA concentrations or on molar proportions of acetate and propionate. Butyrate molar proportion was decreased (P < .10) by M during EApr and LApr, but not during MMay. Monensin increased (P < .05) fluid passage rate compared with C and G in EApr but not in other periods. Particulate passage measurements did not differ (P > .10) among treatments within periods. Forage DM intake was not influenced (P > .10) by supplementation during any period. Extent of in situ forage DM disappearance was greater (P < .10) for M than for C or G during EApr (12 and 30 h of incubation) but was not different (P > .10) in LApr or MMay. Incidence of frothy bloat was decreased (P < .05) by M during EApr; this reduction may have been related to effects of M on ruminal pH, forage digestion and fluid passage.