THE OPTIMUM DIETARY PROPORTIONS OF TRYPTO PHAN AND LYSINE FOR GROWING-FINISHING PIGS

The purposes of the trial were to evaluate the optimum tryptophan/lysi ne ratio for growing pigs and to investigate the effect of low protein diet supplemented with glutamic acid compared with a high protein die t on growth performances of growing pigs. Forty-eight growing pigs wer e randomly assigned to eight treatments with tryptophan/lysine ratios of 16,18.5,20,22,24, and 26. Six diets (Treatments 1-6) were based on corn and soybean meal with the same nutrient composition (crude protei n 11.5%, DE 3,370 kcal/kg digestible lysine 0.60%) except tryptophan c oncentration. Crystalline lysine, threonine, and methionine were added to each of six diets. The other 2 diets with the same tryptophan/lysi ne ratio of 26 were (1) 3.5% glutamic acid added to the corn-soybean m eal diet of treatment 6 to increase crude protein to 15% (Treatment 7) and (2) a corn-soybean meal diet formulated with 15% crude protein le vel (Treatment 8). Pigs were slaughtered after attaining a live weight of 95 kg and carcass characteristics were measured. Sixteen gilts (40 kg live weight) were twice randomly assigned to eight treatment diets as mentioned above, and fecal and urinal nitrogen measurements were t aken to calculate nitrogen retention. The results revealed that daily gain and feed/gain ratio of Treatments 1-6 were different significantl y (P<0.05) on finishers (50-95kg) and the whole period (20-95kg), but no differences could be seen for nitrogen retention or carcass charact eristics. There were no significant differences in growth, carcass, or nitrogen response of pigs among Treatments 6-8, although the low prot ein diet with 3.5% glutamic acid supplement (Treatment 7) slightly imp roved daily gain. From the results of Treatments 1-6, regression analy sis with Y as daily gain on finishing period, and X as tryptophan/lysi ne ratios, tryptophan %, or daily tryptophan intake, the optimum trypt ophan/lysine ratios for growing pigs were 22.8, 22.7, and 23.1, respec tively. When Y was feed/gain ratio, and X was the same as mentioned ab ove, the optimum tryptophan/lysine ratios for growing pigs were 23.6, 23.6, and 22.5, respectively. With Y as daily gain of the whole period , the optimum tryptophan/lysine ratios were 23.4, 23.3, and 24.6, resp ectively. Using Y as feed/gain ratio of the whole period, the optimum tryptophan/lysine ratios were 23.4, 23.6, and 24.5, respectively. When ''bent-stick'' model was used to interpret the results, the optimum t ryptophan/lysine ratios ranged from 20 to 24. From an economic viewpoi nt, the optimum tryptophan/lysine ratio was 20.