UTILIZATION OF ILEAL DIGESTIBLE AMINO-ACIDS BY GROWING PIGS - TRYPTOPHAN

The ileal digestibility of tryptophan for growing pigs was determined for cottonseed, meat-and-bone and soya-bean meals. Tryptophan in the f ood and digesta was measured by two analytical procedures (NaOH hydrol ysis and colorimetric estimation (method 1) and LiOH hydrolysis and HP LC determination (method 2)). The results were respectively: cottonsee d meal 0.46, 0.81; meat-and-bone meal 0.55, 0.65; soya-bean meal 0.74, 0.90. In the first experiment the values for method 1 were shown to b e inapplicable to pigs. In a second experiment three tryptophan-defici ent diets (0.05 g ileal digestible tryptophan/MJ digestible energy (DE )) were formulated using values from method 2 for cottonseed meal, mea t-and-bone meal plus L-tryptophan and soya-bean meal respectively as t he only sources of tryptophan in the diets. This experiment was termin ated after 28 d as overall growth performance of the pigs was very low . A third experiment was conducted in a similar manner to Expt 2 excep t that the diets were formulated to 0.065 g ileal digestible tryptopha n/MJ DE and growth responses and tryptophan retention were assessed ov er the 20.45 kg growth phase. Growth rates (g/d) of the pigs given the three diets were significantly different (P < 0.01): cottonseed meal 393, meat-and-bone meal plus L-tryptophan 531, soya-bean meal 437 (SED 39.0). Tryptophan retention (as a proportion of ileal digestible tryp tophan intake) was significantly different (P < 0.05): cottonseed meal 0.51, meat-and-bone meal plus L-tryptophan 0.49, soya-bean meal 0.41. These results indicate (1) that the colorimetric technique for assess ing tryptophan was inapplicable and (2) that ileal digestible values f or tryptophan were not suitable for formulating diets containing heat- processed proteins, possibly due to absorption of some of the tryptoph an in a form that was non-utilizable, and/or to underestimation of tot al tryptophan in the protein concentrates.