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Effects of Level of Urea and of Urea Plus Limestone on N Metabolism of Corn Silage-Based Rations by Cattle¹

University of Tennessee, Knoxville 37916^,4

Abstract

Three total collection digestion and nitrogen (N) metabolism trials were conducted to determine the effect of three levels of urea, and of urea and urea-limestone treatment of corn silage on nutrient digestion and N metabolism of steers. In the first experiment, urea was added to a concentrate at feeding time to supply 24, 36, or 48% of the total N. Increasing the percentage of total N as urea from 24 to 48% resulted in a significant (P<.05) increase in crude protein digestibility and in total digestible nutrients and in a significant (P<.05) decrease in retention of absorbed N and net protein utilization. The second experiment consisted of two trials in which untreated corn silage, urea (0.5%) treated corn silage and urea (0.5%)-limestone (0.5%) treated corn silage were compared. The data from the two trials were pooled and N digestion and metabolism data were adjusted by covariance to a constant N intake. The addition of urea or urea plus ground limestone at ensiling time did not affect digestibility of any constituents except ether extract when the average of the two were compared to rations containing untreated corn silage. However, apparent crude protein digestibility of the urea-limestone ration was significantly (P<.05) higher than that of the ration containing only urea. The mean of N retention, percent absorbed N retained, percent of intake N retained and net protein utilization for the two rations containing urea were significantly (P<.05) lower than for the ration containing untreated silage, but differences were due primarily to lower N utilization of the ration containing urea only. Grams of N retained per day, retention of absorbed N, percent of intake N retained and net protein utilization were increased when limestone was added to the urea ration.

¹ Published with the permission of the Dean of the University of Tennessee Agricultural Experiment Station, Knoxville.

² Present address: Georgia Agricultural Extension Service, Zebulon, Georgia.

³ Present address: Agricultural Research and Education Center, Post Office Drawer A, Belle Glade, Flordia 33430.

⁴ Department of Animal Science.