EFFECT OF FASTING ON THE URINARY-EXCRETION OF NITROGEN AND PURINE DERIVATIVES BY ZEBU (BOS-INDICUS) AND CROSSBRED (BOS-INDICUS X BOS-TAURUS) CATTLE

Six cattle [3 Boran (Bos indicus) and 3 crossbred (Bos indicus x Bos t aurus)] were fasted for 20 days to estimate the effect of fasting on t he excretion of nitrogen (N) and purine derivatives (PD) in the urine. PD excretion (y) for each genotype was related to time (t in days) by means of an exponential model : Y=A+Be--kt where A is the endogenous excretion, k the fractional rate of excretion of exogenous PD B. The e quations relating y to time (t) were : y = 0.172+0.178(-0.392t), R(2) = 0.86; and y = 0.108+0.146e(-0.108t) R(2) = 0.61 for the zebu and cro ssbred cattle, respectively. The daily fasting urinary nitrogen (N g/k g W-0.75) was related to total daily PD excretion (y mmo/kg W-0.75) by the equations : y = 0.00294+0.7285+/-0.136 N, Adj R(2) = 0.75 (crosse s) y = 0.01099+0.9229+/-0.090 N, Adj R(2) = 0.92 (zebu) The intercepts for both equations were not different from zero (P>0.05). The endogen ous PD excretion (mmols/kg W-0.75 d(-1)) Of the zebu (0.172) was about 1.59 times that of the crossbred (0.108) cattle. The fractional rate of excretion of exogenous PD of the zebu was 3.6 times that of the cro ss bred. The ratios of allantoin to total PD, uric acid to allantoin a nd uric acid to PD did not change with time (P>0.05) but differed (P<0 .01) between genotypes. The present results support the hypothesis tha t Bos indicus cattle and their crosses may have different fasting nitr ogen metabolism from their Bos taurus counterparts, Based on these res ults we suggest endogenous PD values of 0.172 and 0.108 mmol/kg W-0.75 for zebu anal crossbred cattle, respectively.