ESTIMATION OF MINIMUM CRUDE PROTEIN-LEVELS IN DIETS FOR HIGH-LEAN GROWTH PIGS

Balance experiments on female pigs comprising two stages of growth (me an body weights 51 kg and 83 kg) were conducted to estimate the minimu m crude protein (CP) level allowing maximum rate of protein accretion in amino acid-supplemented, wheat and barley-based diets. In Experimen t 1, dietary CP was gradually decreased from 210 g/kg and 178 g/kg to 123 g/kg and 110 g/kg for Stage 1 and Stage 2, respectively while the concentrations of lysine, threonine, methionine + cystine and tryptoph an were kept constant, The critical CP level below which N retention s tarted to decrease was estimated to be 157 g/kg and 142 g/kg for Stage 1 and Stage 2, respectively. CP levels corresponding to the breakpoin ts in N excretion and plasma urea concentration were slightly lower th an those found for N retention. Reducing dietary CP to minimum concent ration for maximum N retention resulted in a decrease in total N exret ion by 0.4 and 0.32 for Stage 1 and Stage 2, respectively. In Experime nt 2, methionine and tryptophan supplements were successively deleted from diets with the minimized CP contents to study the degree of their deficiency. The results suggested that methionine was the third-limit ing amino acid while tryptophan was not limiting or was co-limiting wi th other amino acid(s). It is concluded that dietary CP and N excretio n can be substantially reduced without affecting the rate of protein d eposition as long as the diets are supplemented with lysine, threonine and methionine.