Influences of dietary protein source and extrusion temperature on growth performance, tissue composition, and gut microbiota of grass carp Ctenopharynodon idellus

This study aimed to investigate the effects of dietary protein source (DPS) and extrusion temperature (ET) on the physical qualities of extruded grass carp feed and the growth performance, tissue composition, and gut microbiota of these animals. We used a 2 × 2 factorial experimental design, with DPSs consisting of mostly plant or fishmeal proteins extruded at 120 °C (DP1 and DA1 diets) or 140 °C (DP2 and DA2 diets) for grass carp. The growth trial was performed for 9 weeks, and the gut microbiota trial lasted 4 weeks. Compared with the fishmeal-based diets, the plant protein-based diets produced higher expansion ratios (P < 0.001). A high ET also improved the expansion ratio (P < 0.001) and significantly decreased the feed pellet bulk density (P < 0.001). The DP2 diet had the highest hardness and water solubility index and the lowest water absorption index. The DP1 diet exhibited the lowest protein and phosphorus apparent digestibility coefficients (ADCs). The ET significantly affected the ADCs for the plant protein-based diets, but not for the fishmeal-based diets. Grass carp fed the DP1 diet presented the lowest weight gain and specific growth ratio. The interactions between DPS and ET affected the final body weight (P = 0.030), protein efficiency ratio (P = 0.011), and feed efficiency (P = 0.016), and these indices were significantly greater in grass carp fed the DP2 diet than in those fed the DP1 diet. However, DPS and ET did not affect the concentrations of crude protein or amino acids in the dorsal muscle. Compared with grass carp fed plant-based diets, those fed fishmeal-based diets presented greater fat contents in the muscle (P = 0.011) and liver P = 0.037), and a greater hepatosomatic index (P = 0.003). The interaction of DPS and ET influenced the viscerosomatic index (VSI, P = 0.001), with the highest VSI found after feeding the DA2 diet. DPS (P = 0.001) but not ET (P = 0.172) significantly affected the gut microbial communities, although the plant protein-based diets with increased ET resulted in more differential taxa and greater differences in the abundances of the core microbiota in grass carp compared to the fishmeal-based diets. These results indicate that improving the ET of plant protein-based diets can alter the physical qualities of the diets, further affecting the gut microbiota, and enhancing the growth performance of grass carp. These findings reveal that higher ETs are needed for plant protein-based diets than for fishmeal-based diets, which is a valuable guideline for feed production.