Abstract :
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 ( mol/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.
