| Grass carp(Ctenopharyngodon idellus)belongs to Cypriniformes,Cyprinidae,Yarosuinae and grass Carp,which is one of the "four major fish" in our country,and the breeding production has ranked the first in the world for many consecutive years.Soybean meal is an important protein source in grass carp diet.Due to the limited cultivated land area,soybean yield can not meet the aquaculture demand,increasing demand,dependence on import and rising price,restricting the application of soybean meal in aquatic feed.Single-celled algae are rich in high protein,essential amino acids and polyunsaturated fatty acids,among which Chlorella meal(CM)belongs to chlorophyta,Chlorella order and Chlorella genus.It is an efficient photosynthetic plant with extremely fast growth and reproduction speed and high nutritional value,and can be used as a high-quality protein source in feed industry.In this paper,grass carp as the research object,to explore the effects of Chlorella on the growth performance,antioxidant performance and quality of grass carp and its mechanism after replacing soybean meal in the diet and adding selenium yeast,in order to provide reference for the reasonable replacement of soybean meal in grass carp compound feed and the application of Chlorella.Experiment 1.Effects of soybean meal replacement with Chlorella meal on growth performance,health status and flesh quality of grass carp.The soybean meal of 0%,15%,30%,45%,60%,75% and 100% in grass carp diet were replaced by chlorella,which were recorded as CM0,CM15,CM30,CM45,CM60,CM75 and CM100 groups,respectively,and 7 kinds of iso-nitrogen and isolipid diets were prepared.420 grass carp(540.07±1.00 g)were fed in pond cages for 90 days.The results showed as follows:(1)Compared with CM0 group,Chlorella replacement group had no significant effects on final weight,specific growth rate and feed conversion ratio of grass carp(P > 0.05).(2)In terms of nutrient composition,muscle crude protein content in CM30,CM45 and CM75 groups was significantly higher than that in CM0 group,and whole fish ash content in CM30 and CM45 groups was significantly higher than that in CM75 group(P < 0.05);The histamine content in CM100 group was significantly lower than that in CM0 group,but the proline content in CM100 group was significantly higher than that in CM0 group(P < 0.05).The content of linolenic acid,EPA and DHA in muscle of grass carp increased with the replacement level of Chlorella.Compared with the CM0 group,∑n-3PUFA and n-3/n-6ΣPUFA were significantly increased in the CM60,CM75 and CM100 groups(P<0.05).(3)In terms of antioxidant,T-AOC in muscle of CM45 and CM60 groups was significantly higher than that of CM0 group(P < 0.05),MDA content in muscle showed a decreasing trend with the increase of replacement ratio,and that in CM30,CM45,CM75 and CM100 groups was significantly lower than that in CM0 group.(4)In terms of body color,the back redness value a*,abdomen yellow value b* and saturation value c* in Chlorella replacement group were significantly higher than those in CM0 group(P < 0.05),and the back yellow value b* and saturation value c* in CM30 and CM100 groups were significantly higher than those in CM0 group(P < 0.05).(5)In terms of muscle flavor,K value,Ki value and Ino content reflecting freshness were the highest in CM0 group,and K value and Ki value were significantly higher than that in Chlorella replacement group,Ino content was significantly higher than that in CM30 and CM60 groups(P<0.05).(6)In terms of texture characteristics,the muscle cohesion of CM100 group was significantly higher than that of CM0 group,but the muscle hardness and muscle fiber density were significantly decreased(P < 0.05).The results showed that Chlorella micrococcus(supplemental level: 18.29%)completely replaced soybean meal(24%)in grass carp diet,and had no negative effects on growth performance and feed utilization of grass carp.The replacement of 75% soybean meal(6%)with Chlorella microalgae(13.97%)in grass carp diet improved muscle antioxidant performance,crude protein content,n-3/n-6ΣPUFA and K value.Experiment 2.Effects of soybean meal complete replacement Chlorella meal addition of yeast selenium on growth performance,health status and flesh quality of grass carp.In this study,five treatments were designed,namely,soybean meal group,Chlorella complete replacement of soybean meal group,Chlorella complete replacement of soybean meal plus0.3mg/kg,0.6mg/kg,0.9mg/kg yeast selenium group,and recorded as SM,CM,0.3SE,0.6SE,0.9SE group,respectively.Three hundred grass carp(370.13 ± 0.07g)were reared in pond cages for 10 weeks.The results showed as follows:(1)In terms of growth,dietary supplementation of selenium yeast could significantly improve growth,feed utilization and protein efficiency of grass carp(P < 0.05);Compared with SM group,spleen index in0.3SE group was significantly decreased(P < 0.05).(2)In terms of nutrient composition,crude lipid content of whole fish in 0.9SE group was significantly decreased,crude lipid content of liver in 0.6SE group was significantly decreased,and crude protein content of muscle in 0.3SE group was significantly increased(P < 0.05).The histidine content in yeast selenium supplementation group was decreased(P < 0.05).The content of EPA,∑n-3PUNA and n-3/n-6 Σ PUFA were significantly increased in CM group and yeast selenium supplementation group(P>0.05).(3)In terms of antioxidant,MDA content in muscle was significantly decreased in CM group and yeast selenium supplementation group(P<0.05),CAT activity in 0.9SE group was significantly increased(P<0.05),and GSH-px activity in muscle in 0.6SE and 0.9SE groups was significantly increased(P<0.05).(4)In terms of body color,compared with SM group,CM group and yeast selenium supplementation group could significantly increase the redness value a*,flesh color yellow value b* and saturation value c* of the back of fish body(P < 0.05).(5)In terms of muscle flavor,the K value and Ki value of muscle in CM group and yeast selenium supplementation group were significantly increased(P>0.05).(6)In terms of texture characteristics,muscle elasticity,shear force,adhesiveness,cohesiveness,recovery and adhesion of grass carps were not affected among all groups(P>0.05).Muscle hardness in0.3SE group was significantly higher than that in SM group(P < 0.05),and muscle mastication in selenium yeast group was significantly higher than that in SM and CM groups(P < 0.05).The results of principal component analysis(PCA)showed that 0.3SE could improve the muscle texture characteristics of grass carp.The muscle fiber diameter in0.3SE group was the lowest and significantly lower than that in other experimental groups(P < 0.05),and the muscle fiber density in 0.3SE group was the highest,and the selenium yeast supplementation group was significantly higher than that in SM and CM groups(P <0.05).At the same time,the expression of myod and myf5 genes was significantly up-regulated and the expression of mstn gene was downregulated in 0.3SE group.The results showed that the supplementation of 0.3mg/kg selenium yeast could significantly improve the growth performance,feed utilization,antioxidant performance and muscle quality of grass carp after the complete replacement of soybean meal by Chlorella meal.In conclusion,18.29% Chlorella meal in grass carp diet does not affect the growth performance of grass carp,and 10.09% Chlorella meal can improve the health status and muscle quality of grass carp.Supplementation with 0.3mg/kg selenium yeast(dietary selenium content 0.89mg/kg)on the basis of complete replacement of soybean meal by Chlorella meal can improve the health status of grass carp,increase the content of crude protein,selenium content and hardness of muscle,and improve the quality of muscle. |
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