Digestibility and nitrogen utilization of three beef cattle rations containing corn silage ensiled with limestone and varying levels of urea

The purpose of this experiment was to compare digestibility and nitrogen utilization of three beef cattle rations containing one of three levels of urea in limestone corn silage. The experiment was designed to determine if adaptation to urea nitrogen utilization occurs when these rations are fed, and if diethylstilbestrol has an effect on the rate of this adaptation. Two digestion and nitrogen metabolism trials were conducted separately. The experimental silage treatments were as follows:Treatment 1—0.5 percent urea silage: corn green chop plus5 kilograms of urea (281% crude protein) and 5 kilograms ground limestone per metric ton. Treatment 2--0.75 percent urea silage: corn green chop plus 7.5 kilograms of urea (281% crude protein) and 5 kilograms ground limestone per metric ton. Treatment 3 -- 1.0 percent urea silage: corn green chop plus10 kilograms of urea (281% crude protein) and 5 kilograms ground limestone per metric ton. In both trials, there were four steers per treatment and each steer was offered a constant amount of concentrate mixture (corn and cottonseed meal) together with approximately 80 percent of the amount of urea corn silage which was consumed by the animal with the lowest feed intake toward the end of the preliminary period. The three rations were calculated to supply the recommended crude protein requirements. As the nitrogen content in the urea silages increased, the proportion of cottonseed meal in the concentrate mixture was reduced to make the threerations approximately isonitrogenous. The first two steers of each treatment group were implanted with 2k mg. of diethylstilbestrol.During the period between the two trials, the experimental steers were continued on their respective rations. The second trial was conducted the same as the first.The results from this study were as follows: 1. The digestion coefficients of dry matter and energy were slightly higher for the 0.5 percent urea ration but no significant differences were observed among treatments in both trials. 2. Digestion coefficients of crude protein were significantly higher (P\u3c.05) for the 1.0 percent ration in the first trial, and higher in the second trial but not significantly so. Crude protein digestibility between the 0.75 and 0.5 percent urea silage rations for both trials was not significantly different. The higher apparent crude protein digestibility of the 1.0 percent silage ration may have been caused by the higher crude protein equivalency in that ration. 3. Estimated total digestible nutrient values in the 0.5 percent\irea silage ration were significantly higher (P\u3c.05) than in the 0.75and 1.0 percent silage rations in both trials. There were no significant differences in TDN values between the 0.75 and 1.0 percent urea silage ration in both trials. The higher estimated TDN values of the 0.5 percent silage ration was probably not related to its lower urea content but to the decreased dry matter intake of that ration. 4. In both trials, absorbed nitrogen retained data indicated that the 1.0 percent urea silage ration was utilized significantly-better (P\u3c.Q5) than the 0.5 percent urea silage ration. The general trend in both trials was an increase in retention of absorbed nitrogen with increased urea levels in the ration. This increase of retention of absorbed nitrogen with increased urea levels in the rations was probably not related to the amo-unt of urea in the rations. 5 In both trials, net protein utilization was significantly higher in the 1.0 percent urea silage ration and these values generally followed the same trend as absorbed nitrogen retained values. 6 The data from both trials indicated that no adaptation response for nitrogen utilization occurred in rations where urea supplied30 percent or less of the crude protein equivalency. 7 Since there was no adaptation response, diethylstilbestrol had no effect upon shortening the adaptation period. However, thosesteers implanted with diethylstilbestrol showed improved weight gains