Monensin Level during Grain a D a P T a T I O N and Finishing Performance in Cattle 1

T w o finishing trials were conducted to measure the response of cattle adjusting to high-concentrate diets to dietary monensin level. In trial 1, 54 individually fed Hereford-Angus steers (312 kg), previously fed a two-thirds corn silage: one-third corncob-based diet, were allotted in a 2 • 3 factorial arrangement of treatment with tylosin (0, 11 mg/kg) and monensin (0, 11, 33 mg/kg) fed during a 28<t, grain-adaptation period (fed 75% concentrate for 6 d and then fed 95% concentrate). After 28 d, all steers were continued on their respective levels of tylosin and 33 mg/kg monensin for the remaining 119 d. Daily intake patterns indicated digestive upset in all treatments during adjustment to the 95%-concentrate diet. Blood samples taken during the first 28 d revealed no differences in acid-base status in response to monensin level; however, all steers exhibited reduced (P<.O1) pH and HCO 3 and increased (P<.01) lactate after 4 d on a 75%-concentrate diet. In the initial 28 d, intake decreased (linear P<.05) as the level of monensin increased. In the total finishing period, however, increasing the level of monensin fed during grain adaptation decreased (linear P<.05) intake and tended to decrease (linear P=.20) gain with no effect on feed efficiency. In trial 2, 162 mixed yearling steers (358 kg) and 92 Hereford-Angus heifers (339 kg) were allotted in a 2 • 3 factorial arrangement of treatments with a fast (55, 75 and 95% concentrate for 4, 2 and 15 d, respectively) or slow (55, 65, 75, 85 and 95% concentrate for 4, 2, 5, 2 and 8 d, respectively) step-up period and three monensin levels (0, 11, 33 mg/kg) fed during a 21-d grain adaptation. After 21 d, all cattle were fed 33 mg/kg monensin until slaughter (avg additional days on feed = 113). During the first 21 d, cattle stepped-up slowly had higher gains (P<.10) and improved feed efficiencies (P<.10) than those stepped-up fast. All cattle exhibited a linear (P<.IO) and quadratic (P<.IO) increase in feed efficiency and a trend toward a quadratic (P=.12) increase in gain with higher monensin level. Feeding monensin during grain adaptation improved intake patterns and performance; however, these effects were not manifested in total finishing performance. (