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Growth and Blood Plasma Amino Acid Patterns in the Bovine Fed Different Sources of Supplemental Nitrogen¹

Department of Animal Sciences, University of Kentucky, Lexington, Kentucky 40506

Sixty Aberdeen Angus steers averaging 311 kg were fed for 156 days in a growth trial. The steers were fed corn silage free choice plus 3.63 kg of a supplement containing the following nitrogen sources: 1) soybean meal, 2) urea, and 3) a low nitrogen supplement. The three supplements analyzed 16.51, 16.19 and 6.44% crude protein, respectively. The soybean meal protein provided 42.2% of the total daily crude protein intake of the steers in group 1 and the urea provided 41.8% of the total daily nitrogen intake of group 2. The daily crude protein intake for the three treatment groups, respectively, was 1.02 kg, 1.00 kg and 0.63 kg. The average daily gains of the steers for the growth trial were: soybean meal 0.92 kg, urea 0.76 kg and low nitrogen 0.59 kg, and were significantly (P < 0.05) different from each other. Plasma samples were taken via jugular puncture 2 to 4 hours after the morning feeding on the day prior to the termination of the study. Plasma protein (P < 0.05) and urea nitrogen (P < 0.05) were significantly reduced in the low nitrogen group of steers. Of the individual amino acids measured, the concentrations (µmoles/100 ml) were not significantly influenced by treatment. Glycine and aspartic acid were significantly (P < 0.05) increased in the plasma of the steers in the low nitrogen group on a molar percentage basis. These data indicated a significant (P < 0.05) improvement in growth response due to the utilization of urea as compared to the steers fed the low nitrogen diet. Growth rate was less in the urea group than the rate observed in the soybean meal group. The relationship of dietary treatment to plasma nitrogen constituents is discussed.

KEY WORDS: • amino acids • plasma • dietary nitrogen • growth

¹ This paper (No. 71-5-106) is published with the approval of the Director of the Kentucky Agricultural Experiment Station.

Manuscript received 19 August 1971.