Purine Derivatives/Creatinine Ratio as an Index of Microbial Protein Synthesis in Lactating Holstein Cows

Three experiments were conducted to study creatinine excretion in milking cows, and its relationship to food intake and purine derivatives (PD) excretion as an index of microbial N synthesized in the rumen. In the first experiment, differences in creatinine determination method were studied: creatinine excretion was 26.8 ±1.2 and 27.4 ±2.2 mg/kg live weight, or 1,175.39 ±74.69 and 1,199.9 ±103.9 mmol/kgW0.75 respectively for HPLC and colorimetry analysis. In the second experiment, the effect of concentrate supplement on the PDC index was studied. The inclusion of different levels of concentrate (0, 16, 33 and 50%) increased the PDC index significantly. The PDC index (Y) also showed a significant relationship with DOMI (X, kg/d): Y = 16.85 X + 6.23; R2=0.84. Total PD excretion was calculated assuming constant daily excretion of creatinine (1.199 mmol/kgW0.75). In the third experiment the effect of different roughage on the PDC index was studied. The substitution of a 23% of rye grass (Lolium sp.) silage by alkali treated wheat straw did not modify DOMI and the PDC index although among groups there was a significant decrease in milk production (1.5 L/cow/d) and in growth rate (−380 g/cow/d)