The Dietary Threonine Requirement Of Young Grass Carp(Ctenopharyngodon Idellus)

A total of540young grass carp (Ctenopharyngodon idellus)(255.96±1.44g) were fed six graded levels of threonine (3.6,6.5,9.7,12.6,15.5and18.6g threonine kg-1diet) for seven weeks to determine the threonine requirement of young grass carp and investigate the effects of threonine on growth, intestinal digestive and absorptive capacities, meat quality as well as antioxidant ability in serum, intestine, hepatopancreas and muscle of young grass carp.Percent weight gain (PWG), feed intake (Fl), feed efficiency (FE) and protein efficiency ratio (PER) were all great significantly improved by dietary threonine (P<0.01). Relative analysis found that PWG was positively correlated with FI, FE and PER (r1=+0.985, P<0.01; r2=+0.996, P<0.01; r3=+0.997, P<0.01). Dietary threonine great significantly decreased glutamate-oxaloacetate transaminase (GOT) and glutamate pyruvate transaminase (GPT) activities in hepatopancreas and muscle (P<0.01). Dietary threonine also great significantly increased intestine length, relative gut length, intestine weight, intestosomatic index, intestine protein content, hepatopancreas weight, hepatopancreas protein content, tryspin, chymotrypsin, lipase and amylase activities in whole intestine, alkaline phosphatase (AKP), creatine kinase (CK), y-glutamyl transpeptidase (γ-GT) and Na+, K+-ATPase activities in three intestinal segments (P<0.01), and hepatosomatic index was significantly improved by dietary threonine (P<0.05). The positive correlation existed between FI and intestine weight, intestosomatic index, intestine length, relative gut length, hepatopancreas weight, hepatopancreas protein content, trypsin and amylase activities in whole intestine, CK in mid intestine, as well as γ-GT in proximal and mid intestine (r,=+0.910, P<0.05; r2=+0.842, P<0.05; r3=+0.923, P<0.01; r4=+0.823, P<0.05; r5=+0.900, P<0.05; r6=+0.912, P<0.05; r7=+0.898, P<0.05; r8=+0.955, P<0.01; r9=+0.879, P<0.05; r10=+0.906, P<0.05; r11=+0.928, P<0.01). The positive correlation also existed between FE and intestine weight, intestosomatic index, intestine length, relative gut length, hepatopancreas weight, hepatopancreas protein content, trypsin and amylase activities in whole intestine, AKP in proximal intestine, CK in proximal and mid intestine, as well as γ-GT in proximal and mid intestine (r1=+0.976, P<0.01; r2=+0.924, P<0.01; r3=+0.946, P<0.01; r4=+0.818, P<0.05; r5=+0.946, P<0.01; r6=+0.966, P<0.01; r7=+0.970, P<0.01; r8=+0.891,P<0.05; r9=+0.883, P<0.05; r10=+0.858, P<0.05; r11=+0.900, P<0.05; r12=+0.952, P<0.01; r13=+0.950, P<0.01). Threonine extremely significantly decreased moisture and fat content in muscle of young grass carp (P<0.01), significantly decreased ash content in muscle (P<0.05). and extremely significantly increased protein content and shear force in muscle of young grass carp (P<0.01); whereas muscle pH and cooking loss was not affected by dietary threonine (P>0.05). The trends of malondialdehyde (MDA) in hepatopancreas, serum and muscle were opposite with PWG, while MDA content in intestine was not affected by dietary threonine (P>0.05). The trend of protein carbonyl contents in intestine, hepatopancreas, muscle and serum were opposite with PWG. Anti-superoxide anion capacity and anti-hydroxyl radical capacity in serum, intestine, hepatopancreas and muscle were all great significantly improved by dietary threonine (P<0.01). Superoxide dismutase (SOD), glutathione peroxidase (GPx), catalase (CAT), glutathione-S-transferase (GST) activities, glutathione reducase (GR) activities and glutathione (GSH) content in serum followed a similar pattern to that as observed with PWG. Dietary threonine could great significantly improve CAT, GST activities and GSH content in intestine; meanwhile, GPx activities in intestine was the highest in fish fed diet containing threonine6.5g kg-1. In hepatopancreas, SOD, GPx, CAT GST and GR activities were extremely significantly increased by dietary threonine (P<0.01), and GSH content was also significantly improved (P<0.05). In muscle, CAT and GST activities were extremely significantly improved by threonine (P<0.01), and GPx activities were significantly improved by threonine (P<0.05), and SOD activity and GSH content were the highest in fish receiving threonine6.5and9.7g kg-1, respectively. SOD activity in intestine was no affected by threonine (P>0.01). Whereas GR activity in intestine and muscle was great significantly decreased by threonine (P<0.01). Correlation analysis showed that MDA content in serum was negative correlated to ASA (r=-0.899,.P<0.05); The negative correlation existed between MDA content and GST activity in muscle (r=-0.933, P<0.01). The negative correlation also existed between protein carbonyl content and CAT activity in hepatopancreas (r=-0.932, P<0.01); Correlation analysis showed that protein carbonyl content was negative correlated to GST and GR activities in serum (r1=-0.911, P<0.05; r2=-0.984, P<0.01); Correlation analysis showed that protein carbonyl content was negative correlated to GST and GR activities in muscle (r1=-0.819, P<0.05; r2=-0.923, P<0.01).In conclusion, the present study showed that dietary threonine improved growth, enhanced intestinal digestive and absorptive ability, increased muscle protein content and shear force, decreased moisture, lipid and ash contents in muscle, unaffected mucle pH and cooking loss, and improved antioxidant status of young grass carp. Based on PWG, the dietary threonine requirement for young grass carp (256-480g) was12.18g kg-1diet (4.05g100g-1protein).