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

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Abstract
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-laclating, 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 W0·75at 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.