Abstract:
Preruminant male crossbred lambs aged 2-5 days at the start of the experiment were used. Plasma urea nitrogen (PUN) and nitrogen balance were significantly correlated (r = - 0.74) in lambs given a wide variety of plant and animal proteins in low protein diets (0.10 of total energy as protein). Nevertheless, lambs having identical intakes of nitrogen and energy from diets containing different proteins could have identical PUN values but significantly different nitrogen balances. It was shown with groups of lambs that PUN could be used to predict the minimum amount of DL-methionine that was required to give maximum nitrogen balance, when used as a supplement to isolated soya bean protein. It was also shown that there were significant daily variations in PUN and in the excretion of urinary nitrogen constituents (urea, ammonia, creatinine) between and within lambs given the same diet under strictly controlled conditions. It was concluded that no further increase in the precision of predicting protein quality from PUN (or urinary nitrogen constituents) was possible, unless an experimental design was used in which the values for individual lambs were compared before and after a change in dietary treatment.