Effect Of Deficiency Or Excess Of Dietary Threonine On Growth Performance And Physiological Metabolism Of Broilers

Threonine is considered as the third-limiting amino acid, which functions in regulating physiological metabolism and immune activity of poultry. However, Threonine is not be paid much attention on formulating diets. This experiment was conducted to study the effects of deficiency and excess of dietary threonine levels on growth performance, serum biochemical indices, antioxidant capacities and immune function of broiler chickens. Four hundred and thirty-two 1-day-old commercial broilers were randomly assigned into four treatments and each of them included 6 replicates, a replicate with 18 birds. The 6- weeks experiment was divided into starter phase(0 to 21 d) and finisher phase(22 to 42 d). The amount of dietary threonine in four treatments(T1~T4) respectively reached 85%, 100%, 125%, and 150 % of NRC. Different treatments effects on growth performance, serum biochemical indices,antioxidant capacities and immunity performance of broilers were determined. The results were as follows:(1)The level of dietary threonine significantly(P<0.01) affected average daily gain(ADG) of broilers at age of 0-42 days. Among them, T1 group was obviously lower than the rest(P<0.01). T2 group was higher than others(P<0.01). However, it had no significant differences on average daily feed intake(ADFI). And deficient dietary threonine level could significantly(P<0.05) increased feed conversion ratio(F/G). For the broilers of 0-21 age days,dietary threonine significantly(P<0.05) affected F/G.(2) Excess dietary threonine level triggered the activity of alanine aminotransferase(ALT)in plasma(P<0.01). Obviously, T1 group was lower than others. And it influenced the activity of aspartate aminotransferase(AST) for broilers in the whole period. T1 and T4 group were lower than the other two(P<0.05). And Dietary threonine level significantly(P<0.01)affected serum uric acid(UA). T1 and T4 groups were higher than T2 and T3. Whereas, there were no significant differences on serum triglyceride(TG) and total cholesterol(TC) among the four groups(P>0.05).(3) Glutathione peroxidase(GSH-Px) activities were significantly influenced by imbalanced-dietary threonine level(P<0.05). T1 group was inferior to the rest(P<0.05). Italso influenced the activity of SOD for 42-day broilers(P<0.05). T1 and T4 groups were lower than T2 and T3. But serum malonyldialdehyde(MDA) was not affected by dietary threonine level(P>0.05).(4)During the starter phase(0 to 21 d),The bursal index of broilers in the 100%threonine level treatment(T2) was 12.8% higher(P<0.05) than the 85% threonine level treatment(T1) and the 150% threonine level treatment(T4); There was no significant(P>0.05) difference on spleen index and thymus index among four treatments. But compared with T1 and T4 groups, index of T2 and T3 increased.(5) Dietary threonine level significantly influenced globulin(GLO) in broilers and Serum total protein(TP) in 42-day-old broilers(P<0.05). And T2 was highest group in these two indexes. Whereas, there were no significant differences on albumin(ALB) concentrations and albumin globulin ratio(A/G) among the four groups(P >0.05).(6)The antibody concentrations of Newcastle disease virus was highest at 28 d. Dietary threonine level significantly affected antibody titer of broilers at 35 d(P<0.05) and 42 d(P<0.01). The T3 treatment group obtained the highest antibody concentration.By reference to the Dietary threonine level of NRC(1994), we summed up three points:(1)These results indicated that both deficiency and excess of dietary threonine reduced the average daily gain and F/G of broilers. Excess dietary threonine couldn’t bring sound economic benefit for broilers production.(2)Deficiency dietary threonine influenced protein metabolism of broilers by the decrease of AST and ALT activity in plasma. Both deficiency and excess of dietary threonine increased serum UA concentrations, and depressed plasma TP and GLO concentrations. Antioxidant capacity of broilers was significantly influenced by the threonine-imbalanced diets. To appropriately raise the level of threonine of dietary can improve Newcastle disease antibody concentration and improve the immune function of broiler chickens. Furthermore, this study suggested that an optimum dietary threonine could play an important role in promoting growth, enhancing antioxidant function, improving immunity performance and maintaining health of broilers.