Studies on the tryptophan requirement of piglets

Three experiments were conducted to determine the requirement of tryptophan for piglets. In the first two experiments dose-response relationships between the dietary tryptophan concentration and performance criteria of piglets were examined in order to determine the dietary tryptophan concentration required for maximum performance. In the first experiment dietary tryptophan levels ranged from 1.09 to 2.32 g per kg, in the second experiment they ranged from 1.70 to 2.60 g per kg. In both experiments a close correlation was observed between the dietary tryptophan concentration and piglet performance criteria (feed intake, daily gains, feed efficiency). In the first experiment, regression analysis using a non-linear model revealed that the optima, defined as 95% of the asymptotic response in the model used, for feed intake and daily gains were achieved at tryptophan levels in excess of the highest concentration of 2.32 g per kg feed. The performance level of the piglets in this experiment was generally very low, however. In the second experiment feed intake, the optimum tryptophan concentration, defined as 95% of the asymptotic response, for daily gains and feed efficiency were achieved within a small range between 2.07 and 2.14 g tryptophan per kg feed, corresponding to 1.84 to 1.91 g precaecal digestible tryptophan per kg feed or 0.153 to 0.159 g tryptophan per MJ ME. These results suggest that the tryptophan concentration for maximum performance of piglets is probably higher than has been implied in numerous studies to date. The third experiment was set up to investigate the effect of the reduced feed intake and the effect of an inadequate tryptophan supply per se on the animals' growth. Here a two-factorial experimental design was used also by varying the energy density of the diet (13 vs. 14 MJ ME per kg feed). In addition to tryptophan deficient groups (1.5 g tryptophan per kg feed), this experiment contained conventional control groups (2.6 g tryptophan per kg feed, ad libitum feeding) and pair-fed control groups (2.6 g tryptophan per kg feed, feed intake identical to that of the tryptophan deficient group). The energy density had no significant effect on the animals' performance and increasing the energy density of the diet did not significantly affect feed and energy intake or daily gains of the tryptophan deficient animals. Feed intake, daily gains and feed efficiency of the tryptophan deficient groups were markedly poorer (by 30, 35 and 10%) than in the ad libitum control groups. When compared with the pair-fed control groups, on the other hand, the performance of the tryptophan deficient groups in terms of daily gains, feed conversion and energy efficiency was only slightly and not significant lower by 1, 4 and 3%, respectively. These results demonstrate conclusively that the growth depression in tryptophan deficiency is almost entirely due to the marked reduction in feed intake rather than to a direct limitation of protein accretion caused by an inadequate tryptophan supply.