Abstract:To investigate the effects of lysozyme as a green feed additive on the development of the digestive tract and digestive utilization of nutrients in GIFT tilapia, a 60-day feeding trial was conducted with graded levels of dietary lysozyme (0, 18, 36, 54, 72, and 90 mg/kg, marked as L0, L18, L36, L54, L72, and L90 respectively). The results were as follows: The fish had a different feedback response on the digestive enzyme activity in the liver and the gastrointestinal tract among groups; the protease activity in the stomach, and the anterior and middle intestine of L36~L72 groups was significantly higher than that of the control group (P<0.05); the lipase activity in the liver and the anterior intestine of the L54 and L72 groups was significantly higher than that of the control group (P<0.05); and the amylase activity in the digestive tracts (anterior and middle intestine excluded) of the L72 and L90 groups was significantly higher than that of the control group (P<0.05). The intestinal morphology showed that the values of villus density, villus height, and villus width of different intestinal parts in L36~L72 groups were higher than those of the control group, while the muscular thickness of the anterior and distal intestine was reduced in dietary lysozyme groups. The thickness of the middle intestines of the L18, L36, and L90 groups was significantly lower than that of the control group (P<0.05), and there was a firstly increased and then decrease tendency. The thickness of the middle intestines of L36~L72 groups were all higher than the control group with no significant difference (P>0.05). The goblet cell numbers were more in the L54 and L72 groups than in the control group (P<0.05). The liver morphology showed that the liver cells were more voluptuous and denser in the L36 and L54 groups than in the control group, while worse health condition was found in the L90 group. With regards to apparent nutrient digestibility, the crude protein digestibility was significantly higher in the L36 and L540 groups than in the control group in periods I and Ⅱ (P<0.05), and in periods Ⅲ and Ⅳ. The digestibility of dry matter, crude protein, and crude lipid were significantly higher in L36~L90 groups than in the control group (P<0.05). The results above indicated that 36 and 54 mg/kg dietary lysozyme had the most stable efficacy, which could improve GIFT tilapia dry matter, crude protein, and crude lipid digestibility by promoting liver and intestine development and digestive enzyme activity.
