A 60-day feeding trial was conducted to evaluate the effects of supplemental nucleotides in diets with graded
levels of soybean protein on growth, immune responses and intestinal morphology of juvenile turbot,
Scophthalmus maximus L. (mean initial body weight, 9.18±0.02 g). Nine isonitrogenous (50% crude protein)
and isolipidic (12% crude lipid) practical diets were formulated to contain 30%, 40% and 50% soybean protein,
and each soybean protein level was supplemented with three levels of nucleotides (0.0 g kg−1
, 0.3 g kg−1
and 1.0 g kg−1
) from a mixed-nucleotides (sigma). Quadruplicate groups of fish were randomly fed with
each diet by 20–30 g kg−1 of their body weight per day. The results showed that not dietary nucleotide supplementation
but soybean protein level significantly affected specific growth rate (SGR) following the 60 -day
feeding trial. The immune assay showed that activity of serum lysozyme in fish decreased with increasing dietary
soybean protein (Pb0.05) only in the treatment at nucleotide supplementation level of 0.3 g kg−1
, while
activity of serum superoxide dismutase (SOD) in fish increased (Pb0.05) with increasing dietary soybean protein
and then decreased (P>0.05) at each dietary nucleotide supplementation level. The activity of lysozyme
in fish fed diets containing 30% soybean protein increased with increasing dietary nucleotide level (P>0.05)
and then remarkably decreased (Pb0.05). At each dietary soybean protein, activity of serum SOD was not significantly
different among fish fed diets with graded supplementation levels of nucleotides (P>0.05). The
higher soybean protein and nucleotide supplementation level showed a significantly higher activity of respiratory
burst (Pb0.05). There was a significant decrease in enterocyte height (HE), microvillus height (HMV)
and fold height (HF) in distal-intestine when the replacement level increased from 40% to 50% (Pb0.05). HE
in distal-intestine of fish fed diets at nucleotide supplementation level of 1.0 g kg−1 was higher than those
without nucleotides at each soybean protein level. These results suggested that up to 40% of fish meal protein
replaced by soybean protein with essential amino acid supplementation did not significantly reduce growth
and feed utilization. Nucleotide supplementation did not significantly influence growth of fish fed diets
with 30% to 50% soybean protein but could be helpful to improve the non-specific immune responses and
the intestinal structure of turbot
A 60-day feeding trial was conducted to evaluate the effects of supplemental nucleotides in diets with gradedlevels of soybean protein on growth, immune responses and intestinal morphology of juvenile turbot,Scophthalmus maximus L. (mean initial body weight, 9.18±0.02 g). Nine isonitrogenous (50% crude protein)and isolipidic (12% crude lipid) practical diets were formulated to contain 30%, 40% and 50% soybean protein,and each soybean protein level was supplemented with three levels of nucleotides (0.0 g kg−1, 0.3 g kg−1and 1.0 g kg−1) from a mixed-nucleotides (sigma). Quadruplicate groups of fish were randomly fed witheach diet by 20–30 g kg−1 of their body weight per day. The results showed that not dietary nucleotide supplementationbut soybean protein level significantly affected specific growth rate (SGR) following the 60 -dayfeeding trial. The immune assay showed that activity of serum lysozyme in fish decreased with increasing dietarysoybean protein (Pb0.05) only in the treatment at nucleotide supplementation level of 0.3 g kg−1, whileactivity of serum superoxide dismutase (SOD) in fish increased (Pb0.05) with increasing dietary soybean proteinand then decreased (P>0.05) at each dietary nucleotide supplementation level. The activity of lysozymein fish fed diets containing 30% soybean protein increased with increasing dietary nucleotide level (P>0.05)and then remarkably decreased (Pb0.05). At each dietary soybean protein, activity of serum SOD was not significantlydifferent among fish fed diets with graded supplementation levels of nucleotides (P>0.05). The
higher soybean protein and nucleotide supplementation level showed a significantly higher activity of respiratory
burst (Pb0.05). There was a significant decrease in enterocyte height (HE), microvillus height (HMV)
and fold height (HF) in distal-intestine when the replacement level increased from 40% to 50% (Pb0.05). HE
in distal-intestine of fish fed diets at nucleotide supplementation level of 1.0 g kg−1 was higher than those
without nucleotides at each soybean protein level. These results suggested that up to 40% of fish meal protein
replaced by soybean protein with essential amino acid supplementation did not significantly reduce growth
and feed utilization. Nucleotide supplementation did not significantly influence growth of fish fed diets
with 30% to 50% soybean protein but could be helpful to improve the non-specific immune responses and
the intestinal structure of turbot
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