Development of equation based on urinary purine derivatives to estimate rumen microbial protein production in goats

Three experiments were conducted at the farm of the Universiti Putra Malaysia, Serdang, Selangor, Malaysia, to establish a model as an index for estimating rumen microbial protein production. In Experiment 1,six Ferral male goats (wt. 40.2+-4.6 kg) were used to determine the endogenous purine derivatives (PD) excreted in the urine by fasting. In Experiment 2, four Ferral male goats (wt. 39.6+-1.8 kg) were used to measure the proportion of plasma PD excreted in the urine by using (14C)-uric acid as a marker at two levels of feed intake (40 percent and 80 percent voluntary intake), using an incomplete 2*4 Latin square experimental design. The feed consisted of 40 percent oil palm frond and 60 percent concentrate (OPFC). In Experiment 3, four Ferral male goats fed (OPFC) were slaughtered and rumen contents were taken for measurements of purine and total nitrogen contents of mixed rumen microbes. The results showed that endogenous PD (allantoin, uric acid, xanthine and hypoxanthine) excreted in the urine obtained by the fasting trial was 202+-17 micro mol/kg BW sup(0.75)/d. The average percentage recovery of plasma PD excretion in the urine by using (14C)-uric acid as a marker was 83+-2.0 percent (cv=6.88, ranged 76.3-91.4 percent, n=8). Percentage recovery was not affected by levels of feed intake. The ratio of purine N: total N in the mixed rumen liquid associated bacteria (LAB) was 0.085. In this study, a preliminary model for goats was established by using the information from the recovery of labeled PD (14C)-uric acid and the fasting PD excretion. The model obtained was Y= 0.83X+0.202*BW sup(0.75), where Y = PD excretion in the urine (mmol/d) X = PD absorption at small intestine (mmol/d) BW sup(0.75) = Metabolic body weight (kg) Thus the microbial nitrogen based on total PD (MNpd) can be calculated as follows:MNpd = (70*X)/(0.085*0.83 *1000) = 0.992X* (g/d) where 0.085 is the ratio of purine-N: total N in mixed rumen microbes, 0.83 is the average of digestibility of microbial purine from published results, and 70 is the N content of purines (mg N/mmol). The equation can be useful in comparing the nutritive value of diets at farm level.