Plasma urea nitrogen as an indicator of protein quality. I. Factors affecting the concentration of urea in the blood of the preruminant lamb

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Abstract
Preruminant male crossbred lambs, aged between 2 days and 5 weeks, were given milk replacers of low (0.10 of total energy as protein) or medium (0.28) protein content. Plasma urea nitrogen (PUN) values were unaffected by: the time of blood sampling within 9 hr after feeding; the age of the lamb, between 3 and 33 days; the lamb's intake of the medium protein diet (range, 500–1180 kJ gross energy/day per kg0.73 storage of plasma samples at –15°C for 5 months before analysis; fasting of lambs for 4 days; feeding of lambs on a nitrogen-free diet for 7 days. PUN values were significantly increased by: an increase in the dietary protein concentration; a reduction in the lamb's intake of the low protein diet below 840 kJ gross energy/day per kg0.73 (negative nitrogen balance); an increase in the dry matter content of the medium protein diet from 0.10 to 0.25; the addition of urea to the low protein diet (peak values 4–6 hr after feeding). PUN values estimated on successive days reflected a change in the protein content of the diet within 3–4 days, regardless of whether the change in protein concentration was from low to medium, or medium to low. It is concluded that PUN values can be used to evaluate protein quality only when experimental conditions are strictly controlled. Blood samples taken without regard to the above factors may give misleading results.

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