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Performance and Plasma Amino Acids of Steers Fed Soybean Meal, Urea or no Supplemental Nitrogen in Finishing Rations¹

University of Kentucky, Lexington 40506

Abstract

Yearling steers receiving soybean meal supplementation to ear corn finishing rations (11.0% CP) gained (P<.05) more than those fed the basal ear corn ration (7.0% CP). Gains made by steers fed the soybean meal ration were similar regardless of whether supplement was provided the entire finishing phase (112 days) or during only the first 56 days. Replacing soybean meal with urea and corn on an isonitrogenous, isocaloric basis resulted in a 11.0% reduction in gains. Daily gains made by steers fed the urea supplemented ration were only 0.04 kg greater than for those fed no supplemental nitrogen.

Supplementation, as well as source of supplemental nitrogen, significantly (P<.05) influenced the plasma amino acid concentrations. The amino acids generally most responsive were valine, isoleucine, leucine and lysine. However, in some instances the phase of the trial during which the samples were collected influenced the results. Correlations between growth response and plasma amino acid were discussed in relation to protein adequacies of the ration fed.

Postprandial plasma urea levels were significantly higher for steers fed either soybean meal or urea supplements (P<.05).

From the growth measures and various blood parameters discussed it appears that (1) yearling steers (332 kg) need a preformed protein supplement during the early phase of the feeding period and (2) such requirements cannot be met entirely by microbial protein synthesis from urea.

¹ This investigation reported in this paper (72-5-87) is in connection with a project of the Kentucky Agricultural Experiment Station and is published with approval of the Director.

² Department of Animal Sciences.