Effect of dietary L-malic acid supplementation on growth, feed utilization and digestive function of juvenile GIFT tilapia Oreochromis niloticus (Linnaeus, 1758)

Two feeding trials (FTs) were conducted in 2013 and 2014, respectively, to determine the optimal L-malic acid (LMA) level for juvenile GIFT (Genetically Improved Farmed Tilapia) Oreochromis niloticus. Except for the LMA level, the FT1 and FT2 had a similar diet formulation. In FT1, LMA was included at 0 (basal diet), 1, 4, 8, 16 and 32gkg(-1), respectively. After 20weeks, daily weight gain and feed conversion ratio were improved but not differentiated with 1-8gkg(-1) LMA. Further increasing the LMA supply initially decreased the feed intake (16gkg(-1)), and then decreased both feed intake and feed utilization (32gkg(-1)), thus impairing the fish growth. FT2 was subsequently conducted with a smaller LMA range (0, 0.5, 1, 2, 4 and 8gkg(-1), respectively) but was unfortunately terminated at the end of 8weeks because 20% of the fish were badly injured during weighing. Unexpectedly, growth and feed utilization were still improved but not differentiated with 0.5-8gkg(-1) LMA. In FT2, beneficial effects of LMA inclusion on the digestive function (pepsin, foregut amylase and foregut lipase), the activities of serum lysozyme and hepatic superoxide dismutase, and liver lipid peroxidation (malondialdehyde concentration) were found. Taking the results of FT1 and FT2 together, it could be concluded that dietary LMA supplementation at low concentrations (0.5-8gkg(-1)) could improve growth and feed utilization, but excess LMA (16gkg(-1)) might compromise feed intake and/or feed utilization, thus impairing fish growth. To reduce feed costs in commercial practice, 0.5gkg(-1) LMA is recommended in the feed of juvenile GIFT tilapia based on the results of this study.