Marginal efficiency of free or protected crystalline L-tryptophan for tryptophan and protein accretion in early-weaned pigs.

We conducted an experiment to determine the efficiency of dietary tryptophan (Trp) for protein and Trp accretion in 4-kg (live weight) pigs. Five Trp-deficient diets were fed for 18 d after weaning. The basal diet contained 23.4% protein and .14% Trp. The four other diets were similar to the basal diet but were supplemented with .06 or .12% free or protected crystalline Trp. No differences were found between the two crystalline Trp forms for all variables under study. Equal amounts of all diets were fed by intragastric tube feeding. Daily weight gain (P < .10), gain:feed (P < .05), and daily protein retention (P < .01) increased linearly as dietary Trp increased. Similarly, retention and gross efficiency (retained: intake) of almost all amino acids increased. Tryptophan retention also increased linearly (P < .05), but gross efficiency was maximal (40.1%) with protein-bound Trp from the basal diet, and it decreased linearly as dietary Trp increased. This important reduction was the result of a low marginal efficiency for crystalline (13.6 +/- 3.3%) free or protected Trp.