Synergistic effects of dietary phytase and protease on the zootechnical, antioxidant, and immune indices of African catfish (Clarias gariepinus) fingerlings fed on low-phosphorus diets

This study aimed to evaluate the interactive effects of dietary phytase and/or protease in low-P diets (8.6 g/kg) on the growth, gut morphometry and welfare status of African catfish, Clarias gariepinus. Six experimental diets (40 % crude protein; 17 kJ/g gross energy) were formulated in a 3 × 2 factorial design containing three phytase (Ph) levels (0, 500, and 1000 FTU/g) and two protease (Pr) levels (0 and 0.5 g/kg feed) expressed by Ph0Pr0 (T1), Ph500Pr0 (T2), Ph1000Pr0 (T3), Ph0Pr0.5(T4), Ph500Pr0.5 (T5), and Ph1000Pr0.5(T6). Fish (mean initial weight 7.0 ± 0.3 g; n = 20 fish per tank) were fed on the experimental diets three times daily for 56 days. Significant interactive effects of treatments were found for growth performance, where the fish fed on the Ph1000Pr0.5 (T6) diet produced highest growth indices compared to other treatments. These results evoked that the inclusion of protease to phytase-containing diets increased intestinal proteases and lipase activities, resulting in a significant increase in the fish growth in the Ph1000Pr0.5 (T6) compared with other groups. Additionally, feeding the fish on the Ph1000Pr0.5 (T6) diet had greater intestinal villi width, greater absorption area, and greater muscularis thickness compared to the control fish fed on the Ph0Pr0 (T1) diet. Significant interaction (P < 0.05) was observed for intestinal villi height, propria width, and goblet cell number and the Ph1000Pr0.5 fed fish exhibited the best intestinal morphometry values compared to the Ph0Pr0 (T1) fish group. Dietary phytase and protease; particularly at T6 group, had significant effects on blood parameters. Lower levels of serum superoxide dismutase, catalase, glutathione peroxidase, reduced glutathione, lysozyme, and respiratory burst activity were observed in the Ph0Pr0 (T1) fish, while those parameters were significantly (P < 0.05) improved by dietary phytase and protease; especially in the Ph1000Pr0.5 (T6) treatment, with no interaction between phytase and protease (P > 0.05). It is worth noting that the P content and P retention in the fish body were significantly (P < 0.05) influenced by dietary phytase and by phytase-protease interaction, but not by dietary protease. Regardless of the dietary protease, fish body P content and P retention increased with increasing dietary phytase levels, and their highest values were found in T3 and T6 (Ph1000Pr0 and Ph1000Pr0.5, respectively). This study showed that the inclusion of phytase and protease to diets of African catfish, C. gariepinus (particularly Ph1000Pr0.5) improved their performance, increased the secretion of digestive enzymes, and stimulated the antioxidant and immune responses.