Effects of Monensin and Urea in Finishing Steer Rations

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Abstract
In a 120-day finishing trial, 192 steers averaging 763 lb (346 kg) were fed rations containing monensin at 0, 10 or 30 g/ton[0, 11 or 33 ppm-90% dry matter (DM) basis] with supplemental urea withdrawn from rations (80% rolled corn, 15% corn silage, 5% supplement-DM basis) after 0, 42, 84 or 120 days to determine whether cattle should be gradually adapted to monensin and whether moensin has a urea-sparing effect. On a DM basis, rations with urea contained 11.5% crude protein (CP) and rations with no urea (urea replaced with rolled corn) contained 9.5% CP. Steers were allotted to 24 pens (eight steers/pen) and fed rations containing monensin as follows: ration 1, no monensin (120 days); ration 2, 10 g monensin/ton (t) for 7 days, then 30 g monensin/t for 113 days; ration 3, 10 g monensin/t for 21 days, then 30 g monensin/t for 99 days; or ration 4, 30 g monensin/t for 120 days. Feed consumed daily on a DM basis, lb (kg); daily gain, lb (kg); and lb (kg) DM per lb (kg) gain, respectively, for rations 1, 2, 3 and 4 were: 20.97 (9.51), 2.81 (1.27), 7.46; 18.48 (8.38), 2.94 (1.33), 6.30; 18.82 (8.54), 2.98 (1.35), 6.31; 17.40 (7.89), 2.77 (1.26), 6.28. Daily gain from ration 3 was higher (P<.05) than from ration 4. Dry matter required/gain was lower (P<.05) for cattle fed rations 2, 3 or 4 than for those fed ration 1. Daily gain and DM/ gain improvements (%) for cattle fed monensin (rations 2, 3 and 4) compared with those fed no monensin (ration 1) were: ration 2–4.6, 15.5; ration 3–6.0, 15.4; ration 4–1.4, 15.8. Feeding urea with monensin improved daily gains. Daily gain, lb (kg) and lb (kg DM/lb gain for monensin rations containing urea for 0, 42, 84 or 120 days were, respectively: (0) 2.81 (1.27), 6.29; (42) 2.89 (1.31), 6.49; (84) 2.90 (1.32), 6.29; (120) 3.03 (1.37), 6.13. Rumen fluid from steers fed monensin contained more (P<.05) propionate and less (P<.05) butyrate than from steers fed no monensin. Carcass characteristics were not significantly influenced by monensin or urea. Copyright © 1976. American Society of Animal Science. Copyright 1976 by American Society of Animal Science.

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