Effects Of Dietary Nutrient Level And Feeding Strategies On Growth,Body Composition And Health Of Large Grass Carp (Ctenopharyngodon Idellus)

Grass carp is one of the major freshwater-farmed fish in China,with a production volume of 575.51 tons in 2021.In actual production,feed nutrition level and feeding strategies are important factors affecting farming efficiency.Different growth stages of farmed fish have different nutritional requirements,and feeding with excessively high or low nutrient levels can have adverse effects on their growth and health,thereby reducing economic benefits.In pursuit of faster growth rates or mixed cultivation with common carp,there is a practice of feeding grass carp with common carp feed.However,the protein,fat,and carbohydrate levels of common carp feed are generally higher than those of grass carp feed,which can have a certain impact on the growth and health of grass carp.Feeding strategies,such as adjusting feeding time,feeding level,and feeding frequency,can regulate fish feeding behavior,and may improve the overnutrition damage caused by feeding grass carp with common carp feed.Therefore,this study explores the specific nutritional requirements of large-sized grass carp,as well as the effects of two feeding strategies,i.e.,feeding-starvation cycle and feeding with low frequency,when feeding grass carp with common carp feed,on the growth and health of large grass carp.The findings provide a scientific basis for the proper cultivation of grass carp.Experiment 1.Effects of feed nutrition level on growth,body composition,and health status of large grass carp.To explore the effects of different nutrient levels on the growth,body composition,and health status of large grass carp,three practical expanded diets were prepared and named L(low protein,low fat,low starch),M(medium protein,medium fat,medium starch),and H(high protein,high fat,high starch),with protein/fat/starch contents(%),respectively: L(31.19/6.3/14.8),M(32.55/6.8/16.2),and H(33.82/7.1/17.3).Two hundred and forty large-sized grass carps with an initial weight of 444.88±0.82 g were randomly divided into three groups(L group,M group,and H group),each with four replicates of 20 fishes,and reared in net cages in a pond for eight weeks with three diets.Results showed that:(1)In terms of growth,the final average weight,specific growth rate,and shell ratio of the M group were significantly higher than those of the H group(P < 0.05),and feed conversion ratio was significantly lower than that of the H group(P < 0.05),but no significant differences were observed between the L group(P > 0.05);(2)In terms of body composition,the crude fat levels in whole fish,liver,and muscle of the M group were significantly higher than those of the L and H groups(P < 0.05),while no significant differences were observed in crude protein and ash level among all groups(P > 0.05);(3)In terms of health status,liver cells of the L group appeared a more pronounced linear shape,while those of the H and M groups appeared more blunt and round;the intestinal villi width of the M group was significantly greater than that of the L group(P < 0.05),while the difference in intestinal muscle layer thickness among groups was not significant(P > 0.05),and the fat cells of the H group were slightly larger than those of the L and M groups.No significant differences were found in liver T-AOC,T-SOD,and MDA among groups(P > 0.05).The results suggest that,compared with the other two diets,the low-nutrient experimental diet(with protein,fat,and starch levels of approximately 31%,6%,and 15%,respectively)is beneficial for the growth and health of grass carp.Experiment 2.Effects of feeding strategies on growth,body composition,and health status of large grass carp.To investigate the impacts of different feeding strategies on grass carp fed with high-nutrient level diets,grass carp of 269.97±0.79 g was used as the experimental object in this experiment,Four experimental groups were set up: FG(grass carp feed)group fed with appropriate nutrient level and normal feeding frequency(3 times/day),FC(carp feed)group fed with high-nutrient level and normal feeding frequency,FCS(carp feed with starve)group fed with high-nutrient level and normal feeding frequency with a 6-day feed and 1-day-starvation cycle,and FCF(carp feed with low frequency)group fed with high-nutrient levels and fed twice a day.An 8-week experiment was conducted at water temperature(30.7±4.9)℃ to investigate the effects of feeding nutrient levels and feeding strategies on grass carp growth,body composition,and intestinal structure.Results showed that:(1)In terms of growth,the final weight and specific growth rate of FG and FC groups were significantly higher than those of FCF and FCS groups(P < 0.05),and the feed conversion ratio was significantly lower than that of the FCF group(P < 0.05).There was no significant difference in feed coefficient between FCS and FCF groups(P > 0.05).(2)In terms of body composition,the whole fish moisture,liver moisture,and crude protein of the liver in FCF and FCS groups were significantly higher than those in FG group(P <0.05)but there was no significant difference compared to FC group(P > 0.05).(3)In terms of health status,the CAT and T-SOD enzyme activities of the FG group were significantly higher than those of the FCF group(P < 0.05)but not significantly different from those of the FC and FCS groups(P > 0.05).The intestinal structure of all groups was intact,with no significant differences in muscle layer thickness between FG,FC,and FCS groups(P <0.05),but significantly higher than that of the FCF group(P > 0.05).The goblet cell numbers of FG and FCF groups were significantly higher than those of the FCS group(P >0.05),while the FCS group was significantly higher than the FC group(P > 0.05).These results suggest that a feeding strategy with a 6-day feed and 1-day-starvation cycle is more effective than reducing feeding frequency when feeding high-nutrient diets.In conclusion,under the conditions of this study,grass carp of large size were found to be suitable for feeding with a diet containing 31%,6%,and 15% protein,fat,and starch,respectively.When feeding a high-nutrient level diet with satiation feeding,the strategy of feeding for 6 days followed by a 1-day-starvation was found to be better.