Abstract
Two experiments were conducted to evaluate the use of plasma urea nitrogen (PUN) as a response criterion for determining the amino acid requirements of growing pigs keptunder a practical ad libitum group-feeding condition. 1) In experiment1, involving 20pigs weighing approximately 12 kg each, we measured the response of PUN to crudeprotein (CP) levels, at the 14% and 26% level in diets.A new equilibrium of PUN wasreached within 1-2 days of changing CP levels.This suggests that the period ofmetabolic adjustment necessary before PUN can be used to evaluate amino acidrequirements is less than 2 days. 2) In experiment 2 we measured the response of PUNand plasma-free lysine to the graded addition of L-lysine to a rice-soybean meal basaldiet (CP 12.7% and lysine 0.6%) with pigs weighing approximately 25 kg.A lineardecrease in PUN was observed until reaching a 0.9% level of dietary lysine, and plasmafree lysine was gradually increased to 0.8% dietary lysine at which point it increasedrapidly as dietary lysine was increased.Using PUN and plasma free lysine as theresponse criteria, the lysine requirement was estimated to be 0.8-0.9%. 3) The resultsobtained in experiments 1 and 2 suggest that PUN could be used efficiently as a responsecriterion for the determination of amino acid requirements.
