The determination of the minimal nitrogen excretion in steers and dairy cows and its physiological and practical implications.

1. Cattle were maintained by intragastric infusion to see how much nitrogen was excreted on protein-free diets. 2. Minimal N excretion was estimated with two dairy cows in three periods, i.e. when they were non-pregnant and non-lactating, when they were between 117 and 133 d pregnant and when they were between 220 and 233 d pregnant. The minimal N excretion was also estimated on two occasions with two steers when their average live weights were 200 and 350 kg. 3. Average urinary N excretion without protein infusion was 298, 305 and 283 mg/kg metabolic live weight (W0.75) for the non-pregnant cows and for cows during the first and second periods of pregnancy respectively; total N excretion including the faecal N was 340, 329 and 319 g/kg W0.75. 4. For steers the urinary N values were 403 and 295 mg/kg W0.75 at 200 and 350 kg live weight respectively and total N excretion including faecal N was 408 and 320 mg/kg W0.75. 5. Urinary excretion of creatinine was the same for animals given casein via the abomasum as a source of protein or given no protein with mean values for the cows of 13.6 and 14.9 g/d for the first and second stages of pregnancy respectively. Mean values for the steers were 6.5 and 7.6 g creatinine/d at 200 and 350 kg live weight respectively. 6. It is suggested that the so-called metabolic faecal N in ruminants, estimated with N-free diets, is mainly, endogenous N derived from tissue breakdown of protein but incorporated in microbial debris and excreted in the faeces.