Three rumen fistulated Simmental cows were fed 4 diets (in 4 experimental periods) obtained by substituting part of the basal fescue hay diet (8 kg/d/cow) with extracted soyabean meal (0, 50, 100 and 150 g/kg hay). Including the soyabean meal gave a range of nitrogen (N) intake from 508 to 1448 mg/kg LW0.75/d and of intestinal digestible protein (PDI) from 2.2 to 6.0 g/kg LW0.75/d. The regression of the apparently digested N (y) with the dietary N content (x) was highly significant (r=0.99; P <0.001) and the intercept obtained (-4.25 g N/kg DM) represents the theoretical metabolic loss with zero dietary N. The intercept of the regression between N balance (y) and ingested N (x) (-311 mg N/kg LW0.75/d; r=0.68; P< 0.05) represents an estimate of total endogenous N losses (urinary and faecal). The maintenance requirement for PDI estimated at N equilibrium (3.88 g PDI/kg LW0.75) was slightly higher than the value adopted by INRA (1988). Urinary purine derivatives N excretion was highly correlated (r=0.85; P< 0.001) with the estimated rumen microbial digestible true protein.
