The utilization of urea in the bovine rumen. 1. Methods of analysis of the rumen ingesta and preliminary experiments in vivo

Abstract

I. Three methods of estimating N.P.N. in rumen contents were investigated. They involved precipitation by (1) trichloracetic acid, (2) alcohol and (3) Na tungstate in N/6 H2SO4. No. 3 was found to be the most efficient procedure. Ammonia and urea N were readily estimated by its means. The heterogeneous nature of rumen contents is illustrated by total N and N.P.N. values for samples all taken at the same time but from different positions of the rumen. Differences as great as 20% were encountered. This heterogeneity and the consequent difficulty in obtaining true samples of rumen contents render the type of in vivo expt. which was originally contemplated of little value for proving whether urea is converted to protein in the paunch. Recent work by Wegner, Booth, Bohstedt and Hart (Jour. Dairy Sci. 24: 51. 1941), in which they claim the conversion of urea to protein, is discussed in the light of this finding.[long dash]II. The urease activity of rumen contents is high. 23 to 52 mg. urea-N was found to be converted to ammonia-N in 1 hr. by 100 g. rumen ingesta at 39[degree] C. This is probably equivalent to the conversion of 40-80 g. urea in the intact rumen per hr. The influence of various factors on this conversion rate was studied. The opt. temp. was 49[degree] C; the opt. pH was between 7 and 9; the conc. of urea in the medium made little difference. Unsuccessful attempts were made to separate the urease principle from bacteria. The effect of inhibitors such as NaF, quinone and boric acid was studied. The results suggest that the urease of rumen ingesta is similar to urease from other sources such as soya or jack beans.[long dash]III. On incubating rumen liquor with urea and starch for 2-4 hrs. at 39[degree] C, the N.P.N. diminished while the total N remained unchanged, indicating synthesis of protein from N.P.N. Usually about 8 mg. N.P.N. were converted to protein in 2 hrs. per 100 g. rumen liquor. This is probably equivalent to a synthesis of some 450 g. protein from 72 g. N per day in the intact rumen. Incubations in the presence of NaF, quinone and boric acid and also at different temps. showed that this synthesis was biological in origin. Both synthesis and hydrolysis of protein appeared to proceed simultaneously, but synthesis predominated when conditions were suitable. Carbohydrates such as starch, galactose and maltose favored synthesis; lactic acid and glycerol tended to favor hydrolysis. In the presence of the insoluble protein of blood meal synthesis predominated but with the soluble protein gelatine much hydrolysis occurred. These results suggest that urea will only be utilized for protein synthesis provided the whole diet is properly balanced with regard to its starch and potential protein equivalents; and that the biol. value of proteins for ruminants may depend not only on their amino acid content but also on their fate in the rumen.