| A study was conducted to investigate the effects of yeast culture? (DVAQUA, Diamond V) supplementation on growth performance, fish body composition, digestibility of nutrients and energy, digestive enzymes activities, gut and liver morphology, and bacterial profiles of intestinal tract of the experiment turbot, Scophthatmus maximus, fed the diets in which fishmeal protein was partially replaced by soybean protein. Using whole fish meal feed as the control, the fishmeal protein was replaced by soybean protein at 30%, 45% and 60%. At each replacement level, three yeast culture levels (DVAQUA, 0%, 0.125% and 0.25%) were supplemented, respectively, to formulate 12 isonitrogenous (crude protein 48.6%) and isoenergetic (20.3kJ·g-1) practical diets. Graded levels of encapsulated crystalline amino acid were added to keep the balance of essential amino acids at each replacement level. The objectives of the present study were to investigate effects of yeast culture? on growth performance, histological structure and microbiology of turbot (Scophthatmus maximus) fed diets with graded levels of fishmeal replaced by soybean meal. The present studies and results were summarized as follows:⑴Effects of experiment diets on growth performance and fish body composition of the turbot. Specific growth rate (SGR) (r=-0.910, P<0.01), dry feed consumption (DFC) (r=-0.860, P<0.01), feed efficiency ratio (FER) (r=-0.856, P<0.01), protein efficiency ratio (PER) (r=-0.861, P<0.01), protein productive value (PPV) (r=-0.910, P<0.01), crud protein content (r=-0.630, P<0.01) and total energy in fish body decreased all significantly with increasing dietary soybean meal levels. Dry feed consumption in the first week (FWDFC) (r=-0.259, P<0.05), crude lipid content (r=-0.431, P<0.05) in fish body, Val (r=-0.479, P<0.05), Phe (r=-0.501, P<0.05) and Arg (r=-0.447, P<0.05) content in fish body decreased significantly with increasing dietary soybean meal levels. While feed intake (FI) (r=+0.786, P<0.01), ash content (r=+0.539, P<0.01) and moisture content (r=+0.587, P<0.01) in fish body increased significantly with increasing dietary soybean meal levels. Supplementation of yeast culture significantly improved PPV, crud protein content and Pro content in the fish body (P<0.05). SGR of the turbot, FER, PER and crude lipid content in the fish body increased slightly with no significant differences by yeast culture addition (P>0.05). On the contrast, as supplementation levels of yeast culture increased, DFC showed an increasing tendency at the first dose level and decreased at the second dose level. FI, ash content, moisture content, Cys and Phe content decreased significantly (P<0.05). Other essential amino acid content were not significantly affected by yeast culture? (P>0.05). Under the combined function of soybean level and yeast culture dose, the SGR of S45X0.00 treatment was significantly lower than that of the control, while the SGR of S45X0.125 treatment showed no significant difference with the control.⑵Effects of experiment diets on turbot digestive physiology. The digestibility of dry matter (r=-0.976, P<0.01), crud protein(r=-0.927, P<0.01), total energy(r=-0.878, P<0.01), Ala (r=-0.878, P<0.01), Phe (r=-0.927, P<0.01), Pro (r=-0.976, P<0.01) in the diets and the enzymes activities of trypsin in the gut (r=-0.678, P<0.01) and pepsin in stomach (r=-0.538, P<0.01) decreased significantly with increasing dietary soybean meal levels. The digestibility of Thr (r=-0.830, P<0.05), Ser (r=-0.830, P<0.05), Val(r=-0.830, P<0.05), Met (r=-0.830, P<0.05), Ile (r=-0.830, P<0.05), Leu (r=-0.830, P<0.05), Tyr (r=-0.830, P<0.05), Lys (r=-0.830, P<0.05), Arg (r=-0.830, P<0.05) and total amino acid (r=-0.830, P<0.05) in the diets and lipase activity in liver (r=-0.463, P<0.05) decreased significantly with increasing dietary soybean meal levels. The digestibility of crude lipid (r=-0.683, P>0.05), Asp (r=-0.439, P>0.05), Glu (r=-0.537, P>0.05), Gly (r=-0.634, P>0.05), Cys (r=-0.634, P>0.05), His (r=-0.634, P>0.05) and the activities of protease in liver (r=-0.307, P>0.05) and protease in intestine (r=-0.081, P>0.05) decreased slightly. On the contrast, the enzymes activities of lipase (r=+0.221, P>0.05) and amylase (r=+0.054, P>0.05) in intestine and amylase (r=+0.269, P>0.05) in stomach increased with increasing dietary soybean mean levels, even though not significantly. Supplementation with yeast culture? significantly increased the energy digestibility of the diet (P<0.05), while the digestibility of dry matter, crude protein and crude lipid increased slightly with no significant change (P>0.05), and the digestibility of other amino acids and the activity of digestive enzymes were not significantly affected (P>0.05). In spite of that, pepsin activity in stomach of 45% soybean level treats showed significant lower than the control without yeast culture supplementation and no significance difference with the control with supplementation of yeast culture.⑶Effects of experiment diets on histological structure and intestinal tract bacterial profiles of the experiment turbot. The results showed that:①The integrity of histological structure of the mid intestine was gradually destroyed as soybean meal replacement levels increased. No significant difference was observed between 30% soybean meal level and the control. All sections showed an integrated normal histological structure with an intact mucosa, normal levels and evenly in size of goblet cell vaculoues, unbroken tidy villus folds and enterocyte with basal nucleus regularly. The 45% soybean meal replacement level showed significant destruction of gut structure in the inner wall and there was an increasing tendency for the destroy of the integrity of the intestine walls at 60% soybean meal level.②Similarly, the liver histological analysis showed differences among fish of different groups concerning the cellular vacuolization degree (VD) of the hepatocytes and widened hepatic sinusoids spaces. At 45% replacement level, nuclei polarization with big vacuolization in hepatocytes was observed. Serious destruction of liver histological structure has been observed in which the cell membranes between hepatocytes was not clear and homogeneous parenchyma of turbot liver showed an tendency of lysis, and only nuclei and hepatic sinusoids spaces can be observed.③Much improvement of the intestine integrity and to a less extent of histological changes were observed with yeast culture supplementation. The morphology of gut and liver were globally intact at relatively low soybean meal levels (30% and 45%) while still out of order at high soybean meal level (60%).④The gut micro flora composition of soybean meal substitution treatment and yeast culture? supplementation treats were much more simplified than that of the control. The predominant genus of bacterial flora was different between treatment, Pseudomonas sp. showed predominance in the control while Vibrio sp and Bacillus sp in the yeast culture? supplementation treats and Vibrio sp absolutely in the soybean meal substitution treats. In conclusion, when dietary soybean meal increased and fish meal decreased, the the dry feed consumption, SGR, and digestive enzymes activities were significantly decreased; the digestive tract histological structure were passively affected, and digestibility of nutrition significantly decreased. Appropriate dose of yeast culture? supplementation increased dry feed consumption and growth performance of turbot, significantly improved the gut and liver histological structure, and increased protein utilization. Replacement level of fish meal by soybean meal could increase from 30% to 45% by the supplementation of yeast culture? in diets of turbot.
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