Optimum tryptophan: Lysine ratio in diets for growing pigs: Analysis of literature data

Optimum tryptophan supply in diets for growing pigs shows considerable variation between studies; therefore, an attempt was made to analyse all available data reported in the literature. Due to the fact that, in most studies, lysine was the second-limiting amino acid, which means that lysine limits growth at a certain level of tryptophan intake, optimum supply has to be expressed as a ratio to lysine. The database was too small to give optimum supply as a concentration in the diet or as a daily amount. In total, 33 experiments were found, where tryptophan intake was offered in graded levels resulting in a tryptophan-dependent and -independent response phase. An additional 13 experiments showed either no dependent or no independent phase, which did not allow an optimum determination. The optimum tryptophan : lysine ratio (T : Lopt) was defined as the beginning of the plateau response-phase in feed intake or body weight gain. Mean T : Lopt was 0.174 (SD = 0.023; n = 33) and seemed to be unaffected by body weight, growth rate, lysine and protein concentration in the diet, or genetic improvement of the animals. However, precise detection of the optimum ratio is generally restricted due to the relatively large intervals between the ratios tested in the respective studies. Mean difference between T : Lopt and the nearest T : L ratio tested below (T : Lopt − 1) was 0.028 (SD = 0.011; n = 32). Therefore, it is likely that the true optimum ratio is below 0.174, and that the mean of both ratios ((T : Lopt + T : Lopt − 1) / 2) can be taken as a good estimation (mean = 0.160; SD = 0.024; n = 33). With respect to feed formulation for growing pigs, it was concluded that a figure of 0.17 for the optimum T : L ratio includes a safety margin large enough to cover most of the biological variations.