Modelling urinary purine derivatives excretion as a tool to estimate microbial rumen outflow in alpacas (Vicugna pacos)

Three experiments were carried out to establish a model to estimate the forestomach microbial yield based on the urinary excretion of purine derivatives (PD: allantoin, uric acid, xanthine and hypoxantine) in alpacas (Vicugna pacos). In Experiment 1, endogenous PD excretion was measured in two fasted adult male alpacas for seven consecutive days. Daily urinary PD excretion (μmol/kg BW0.75) decreased during fasting to a minimum value of 194.8 (s.e. 18.4), ranging from 215.3 to 174.3. In Experiment 2, the relationship between purine bases (PB) input and urinary PD output was defined in two alpacas fitted with an infusion catheter at the terminal third compartment of the forestomach (C3). Animals were fed alfalfa hay at a maintenance level, and four RNA-doses (4.2, 8.3, 12.5 and 16.6 mmol PB/day; RNA from Torula yeast) were continuously infused at random into the C3 in four successive 120 h-periods. Urinary recovery of C3 infused purines averaged 0.615 (s.e. 0.0006) mmol/day. In Experiment 3, urinary PD response to levels of feed intake corresponding to 100, 75, 50 and 20% of the previous voluntary intake was evaluated. The amount of PD excretion in urine increased linearly (r = 0.867) with digestible organic matter intake (DOMI), and the slope of the regression (16.7 mmol PD/kg DOMI) can be assumed as an index of microbial yield.