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Protein Levels with and without Monensin for Finishing Steers¹

Kansas State University, Manhattan 66506

Abstract

Thirty yearling Hereford steers were individually fed one of the following diet crude protein (CP) treatments with and without 200 mg monensin daily in a completely randomized design with a 2 x 5 factorial arrangement of treatments: 9%, 11% or 15%, continuously; 12% for 63 days, then 10.5% or 13% for 42 days, 11% for 42 days and then 9% until slaughter. Soybean meal was used as the supplemental nitrogen source in all but the 9% CP diet, which contained no supplemental protein. Steers continuously fed the 9% CP diet gained the least (P<.05) and were the least efficient (P<.05). Daily gain and efficiency of gain were similar for the other four protein treatments. Protein treatment did not significantly affect feed intake. Averaged across all crude protein treatments, monensin did not affect steer performance. However, among steers fed 11%, 12-10.5% and 13-11-9% CP, monensin improved feed efficiency an average of 7.4%. Dry matter and CP digestibilities were lowest (P<.05) for steers fed 9% CP. Crude fiber digestibility was lowest for the 9% and the 13-11-9% CP levels. Monensin increased crude fiber digestibility (36.9 vs 42.3%) but had no effect on the other digestibility coefficients. The percentage starch in the feces was not affected by protein treatment or monensin. Fecal pH tended to be higher for steers fed 15% CP and lower for steers fed 9% CP. The correlation coefficient between fecal pH and fecal starch was –.23. Monensin lowered the acetate to propionate ratio for all steers except those fed 15% CP. Rumen NH₃ and plasma urea nitrogen (PUN) correlated positively with protein intake (.60 and .74, respectively). PUN ranged from 7.0 mg/100 ml for steers on the 9% CP treatment to 19.1 mg/100 ml for steers on the 15% CP treatment. Steers fed monensin had lower (P<.05) rumen NH₃ (2.74 vs 3.18 mg/100 ml) and higher (P<.05) PUN (12.66 vs 12.21 mg/100 ml) than steers that received no monensin.

¹ Contribution No. 79-353-J, Dept. of Anim. Sci. and Ind., Agr. Exp. Sta., Kansas State Univ., Manhattan 66506.

² Present address: Dept. Anim. Sci., Univ. of Minnesota, St. Paul 55108.

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