Effect of dietary pH on amino acid utilization by shrimp ( Penaeus vannamei)

Abstract

A study was conducted to determine the effect of dietary pH on amino acid utilization by juvenile Penaeus vannamei. A basal diet (diet 1), five diets (diets 2–6) supplemented with crystalline amino acids (AA) to simulate the AA pattern of shrimp protein and in which the dietary pH was adjusted from pH 4.8 to 5.0, 6.0, 7.0 and 8.0±0.2, and a 28% shrimp protein control diet (diet 7) were fed to shrimp to satiation 6 times per day for 10 weeks. Growth and feed consumption of shrimp were improved ( P<0.05) by supplementing AA and increasing the levels of dietary pH. Maximum weight gain, which was approximately 85% that of shrimp fed the control diet (diet 7), was obtained when the AA diet was adjusted to pH 8.0 (diet 6). However, this growth improvement was due to increased feed consumption rather than to improvement of nutritional value of diets. Shrimp fed diet 7 also had the best feed conversion ( P<0.05), which was about two-fold better than those obtained with other diets. There were no significant differences among the survival rates of shrimp fed the various diets. The pellet water stability at 1 and 3 h was similar for diets 2–6 and diets 1 and 7. However, the dry matter loss of diets 2–6 was approximately twice that of diets 1 and 7. The crude protein losses were negligible for diet 1, 7.0% for diet 7, and 21.2–22.3% for diets 2–6. The total essential amino acids (EAA) including cystine, at 1 h soaking, slightly increased for diets 1 and 7 but decreased by 27.5% for diet 6. Supplementation of AA and increase in dietary pH had no appreciable effect on moisture and crude protein contents of whole shrimp. Body fat increased ( P<0.05) and ash decreased with increasing levels of dietary pH. Shrimp fed the control diet had the highest content of crude protein and fat and the lowest levels of moisture and ash. This study indicates that growth and feed conversion of P. vannamei fed the AA-supplemented diets were inferior to that of shrimp fed the control diet. This may be attributed to the lower water stability of the AA supplemental diets and the apparent loss of AA from these diets. However, improved growth and feed consumption were obtained with increasing pH values of the AA-supplemented diets. This growth improvement was due to increased feed consumption rather than to improvement of nutritional value of the diets.