A blood sample (about 10 mL) was collected from the jugular vein at the same time as rumen fluid sampling into tubes containing 12 mg of EDTA, and plasma was separated by centrifugation at 500 × g for 10 min at 4 °C and stored at −20 °C until analysis of plasma urea N according to Crocker (1967). Urine samples were analyzed for total N (AOAC, 1995; ID 984.13) and allantoin in urine was determined by HPLC as described by Chen and Gomes (1995). The amount of microbial purines absorbed was calculated from purine derivative excretion based on the relationship derived by Chen and Gomes (1995).