The tryptophan requirement of piglets as influenced by the threonine level

SUMMARY: The tryptophan (Trp) requirement was investigated in post weaning piglets from 5 to 10 weeks of age (7-20kg). Three feeding trials were conducted. All piglets were randomly alloted according to age, sex, weight, pre-weaning growth and ancestry to the different dietary treatments. Ad libitum feed intake (per 6-7 pigs) and daily gain (individual) were recorded at 5 and 10 weeks (end of experiment). In the first trial 72 Belgian Landrace (BL) and 60 Piétrain × Seghers Hybrid (P × SH) piglets were used for two dietary treatments. The basal diet, 16% crude protein (CP), 1.06% lysine (Lys) and 0.14% Trp, was supplemented up to 0.19% Trp. This trial revealed a significant improvement in daily gain from 0.14% (0.10% ileal digestible Trp) to 0.19% Trp (0.15% ileal digestible Trp). The second trial was based on a lower CP content (14%) and a similar Lys level. Trp was increased in steps from 0.16% to 0.18, 0.21 and 0.24%. One hundred and fifty-eight BL and 147 P × SH piglets were divided over the four feeding regimes. No improvement in growth was obtained for the levels above 0.16% Trp (0.11% ileal digestible Trp). It was presumed that Thr was the limiting factor. A third trial was conducted, which consisted in a 3 × 3 factorial experiment, using the same CP level as in trial 2, but a lower Lys level (1.03%) and combined three Trp levels (0.18, 0.21 and 0.23%) with three Thr levels (0.65, 0.72 and 0.79%). Nine groups of 49 animals (all P × SH) were used in this experiment. Only a slightly favourable tendency of Trp towards increasing daily feed intake was found and the growth and feed: gain ratio were not affected at all. The Thr, on the contrary, led to increasing growth figures. The optimum Thr level seemed to have been reached by the 0.72% total Thr or 0.57% ileal digestible Thr level, yielding a ratio of 65% of Thr:Lys. For Trp, the requirement was difficult to predict in the conditions of the experiment, as no optimum appeared for Trp from the animal performance results.