Effects Of Soybean Meal And Tenebrio Molitor As Protein Sources On Feeding,Growth,Flesh Quality And Health Of Grass Carp

Due to the shortage of grain protein sources in aquaculture,evaluating the effect of Tenebrio molitor in grass carp culture can increase the candidate protein sources in grass carp feed and provide support for developing other protein sources.Soybean meal is one of the main protein sources in grass carp feed.Further accurate soybean meal protein requirements for grass carp and the impact of soybean meal protein levels on muscle growth and health can reduce unnecessary waste and improve farming efficiency.Therefore,the purpose of this experiment is to determine the protein requirement of soybean meal for grass carp and to evaluate the feasibility of Tenebrio molitor as a dietary protein source for grass carp,as well as the effect of protein level on improving the flesh quality and health of grass carp.The experimental design follows: carried out a 56 d culture experiment with the juvenile grass carp(initial weight is 6.80±0.10g).Six experimental diets were compounded only using soybean meal protein to obtain six protein levels(20.95%,24.83%,27.68%,32.26%,35.98%,and 39.76%,named SM1,SM2,SM3,SM4,SM5,and SM6);and six experimental diets were compounded using Tenebrio molitor as a protein source to obtain six protein levels(20.94%,24.97%,29.00%,33.03%,37.06%,and 41.09%,named MW1,MW2,MW3,MW4,MW5,and MW6,respectively).The response of the olfactory system of grass carp to Tenebrio molitor(MW3,MW6)and soybean meal feed(SM3,SM6)was detected by olfactory electrophysiology.After the breeding experiment,combined with the growth performance,feed utilization,and olfactory electrophysiological results,the protein demand of grass carp for soybean meal and Tenebrio molitor,and the utilization potential of Tenebrio molitor feed were comprehensively analyzed.Then,the physiological and biochemical indexes,texture,muscle development genes,and histopathology of grass carp were analyzed.At the same time,combined with muscle and spleen transcriptomes,the potential influence mechanism of dietary protein level on flesh quality and health was revealed.1.Effects of soybean meal and Tenebrio molitor on the growth and feeding of grass carpIn the soybean meal groups,the final body weight,specific growth rate,feeding rate,and apparent digestibility of dry matter/protein were significantly the highest in the SM5group(P < 0.05).Based on the double broken line result of specific growth rate,the protein requirement for grass carp for soybean meal was 38.63%;In the Tenebrio molitor groups,the final body weight,specific growth rate,and protein efficiency ratio were significantly the highest in MW6 group(P < 0.05),but there was no significant difference in feeding rate(P > 0.05).In addition,the feed protein sources and protein levels interacted to affect electro-olfactory signals response amplitude and the m RNA levels of agrp and pomc.Among them,soybean meal feeds significantly affected the expression level of some feeding genes and the response amplitude of electro-olfactory signals(P <0.05),but there was no significant difference in Tenebrio molitor groups(P > 0.05).Considering the growth and feeding behaviors,Tenebrio molitor is not suitable as a dietary protein source for grass carp alone,and the dietary protein level and protein source can affect the olfactory system and the expression of appetite-related genes to control the feeding behavior of grass carp.2.Effects of dietary protein level on texture,myosin heavy chain gene and muscle transcriptome of grass carpThe results showed that appropriate dietary protein levels improved the hardness and p H of muscle while decreasing muscle crude lipid content and cooking loss and altering the antioxidant capacity and metabolic enzyme activities.Based on the regression curve analysis of hardness,the dietary protein requirement for grass carp was estimated to be31.66%.In addition,appropriate dietary protein promoted the gene expression of myhc-1,myhc-4,myf5,myod,myog,and fgf6 a,whereas inhibited that of myhc-7,myhc-2,mrf4,and mstn.Transcriptome profiling of muscle revealed that flesh quality-specific differences were related to tight junctions and intramuscular fat accumulation.GSEA showed that fatty acid metabolism and oxidative phosphorylation were down-regulated in SM5 compared with SM1.To conclude,appropriate protein levels improved the growth and flesh quality by regulating muscle antioxidant capacity and gene expression of myhcs and fat metabolism-related signaling pathways.3.Effects of dietary protein level on health and spleen transcriptome of grass carpThe results showed that appropriate dietary protein levels increased serum lysozyme activity and antioxidant capacity and improved the serum biochemical indexes.In the hepatopancreas,the hepatopancreas index and lipid content decreased linearly with the increase of protein level in the diet,while the activities of aspartate aminotransferase and alanine aminotransferase increased linearly.Low-protein diet resulted in varying degrees of edema,cytoplasmic vacuoles,and abnormal nuclei of grass carp in SM1,SM2,and SM3 groups,which were significantly improved with the increase in dietary protein gradient.In the intestine,mesenteric fat index,mucosal fold length,muscle layer thickness,lamina propria width,and goblet cell number were affected by protein level.In addition,adequate-protein intake significantly improved the spleen index(P < 0.05).KEGG analysis of the spleen showed that the phagosome pathway,hematopoietic cell lineage pathway,and protein digestion and absorption were critical in resisting the adverse health effects of a low-protein diet.These results showed that dietary protein level affects the health of the hepatopancreas and intestine of grass carp and spleen immune function.This study provided accurate reference data for the use of soybean meal in grass carp feed and considered that Tenebrio molitor should not be used as a protein source for grass carp.At the same time,proper protein levels can improve flesh quality and health.