Doctoral Dissertations

Date of Award

12-1972

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Animal Science

Major Professor

K. M. Barth

Committee Members

S. L. Hansard, J. B. McLaren, M. J. Montgomery, T. P. Salo

Abstract

Two successive 7-day total-collection digestion and nitrogen (N) metabolism trials were conducted with beef steers to compare biuret with urea and to determine if other ration constituents influence utilization of N furnished by biuret. Twelve medium grade Hereford steers averaging 221 kg in body weight were allotted to one of the following treatments: 1) urea control; 2) biuret control; 3) biuret-molasses; and 4) biuret-corn. All animals received a basal ration of poor-quality fescue hay and were allowed 42 days to adapt to the NPN source before the start of the first trial. A comparison of the data from the two trials indicated nutrient digestibility (dry matter, crude protein and gross energy) was not affected by NPN source. However, the addition of readily-fermentable energy (molasses, cornmeal) increased digestibility of all nutrients. Dry matter and gross energy digestibility of rations containing biuret and additional energy were significantly (P < .10) higher than of the urea and biuret control rations. Nitrogen digestibility was non-signif-icantly higher in rations containing additional energy than in both control rations. The percent absorbed N retained by animals consuming all types of biuret rations was lower than that by animals consuming the urea ration. The difference was from 6 to 8 percentage units, but was not significant (P < .10). Net protein utilization and percent intake N retained, which take into account digestibility, followed essentially the same pattern as percent absorbed N retained. They did, however, reflect the advantage of additional energy on digestibility. Another experiment was conducted to determine the pattern of peripheral blood ammonia and urea as a function of time after feeding. Six animals, used previously in metabolism trials, and six unadapted to NPN, were allotted to one of the following treatments; 1) urea-adapted; 2) biuret-adapted; 3) urea-unadapted; or 4) biuret-unadapted. Rations consisted of long timothy hay, blackstrap molasses and either urea or biuret. NPN comprised approximately 1.5 percent of the ration. Jugular blood samples were taken at intervals of 15 minutes, 30 minutes, 1 hour, 2 hours, 4 hours and 6 hours after feeding. An incomplete Latin Square design was used for sampling. Adapted animals consuming urea had the highest blood ammonia levels. Blood from animals of other treatments all contained approximately the same level of ammonia within all treatments, peak ammonia-N levels appeared at one hour after feeding. Plasma urea levels were fairly constant for all animals, averaging approximately 12-14 mg per ICQ ml plasma. There was, however, a gradual increase in plasma urea from 2 to 6 hours Data from six total-collection digestion and N-metabolism trials with steers were compiled to examine some factors affecting ration digestibility and N-utilization of urea-corn silage based rations. F-max tests indicated homogeneity of variances among the trials. Multiple regression equations were developed to determine the effect of various ration constituents and intake parameters on nutrient digestibility and efficiency of N-metabolism. Calorie to protein ratio, grams N intake, percent digestible NFE and ration dry matter content together accounted for 51, 46 and 74 percent of the total variability in the apparent digestibility of dry matter, crude protein and crude fiber, respectively. Increasing calorie to protein ratio resulted in a decrease of apparent digestibilities of dry matter and nitrogen, and an increase in diges-tibility of crude fiber. Nitrogen intake was associated negatively with digestibility. Increasing ration dry matter content increased dry matter and crude fiber digestibility and decreased that of crude protein. When percent absorbed N retained, percent intake N retained and net protein utilization were the dependent variables, their regression on the four independent variables: 1) percentage of ration N supplied by urea; 2) energy supplied by concentrates; 3) N intake; and 4) fiber intake accounted for 34, 18 and 20 percent respectively, of the total variability in the three dependent variables. Percentage of N supplied by urea and N intake had no significant effect on any measure of N-metabolism. When energy supplied by concentrates increased from 3.8 to 11.3 mcal/day, retention of absorbed N, retention of intake N and net protein utilization increased 15.7 (significant, P < .001), 5.7 and 7.5 (significant, P < .05) percentage units, respectively. In-creased fiber intake was associated with significantly (P < .05) decreased retention of intake N and net protein utilization. There was little effect of fiber intake on retention of absorbed N.

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