MAINTENANCE THREONINE REQUIREMENT AND EFFICIENCY OF ITS USE FOR ACCRETION OF WHOLE-BODY THREONINE AND PROTEIN IN YOUNG CHICKS

Broiler chicks were fed on chemically-defined crystalline amino acid d iets containing graded levels of L-threonine (Thr) during the period 1 0-20 d post-hatching. Doses of Thr represented 5, 10, 15, 40, 55, 70 a nd 95 % of its ideal level for maximal weight gain and feed efficiency . Other amino acids were maintained at minimized excess levels that we re 15 % (of ideal) above the various doses of Thr, Following 10 d of f eeding and a 24 h fast, chicks were killed for whole-body protein and amino acid analysis. Using pen accretion means, weight gain (r(2) 0.98 ), protein accretion (r(2) 0.99), and Thr accretion (r(2) 0.99) were l inear (P < 0.01) functions of Thr intake. Slope of the Thr accretion r egression line indicated that 82 % of the Thr intake was recovered in whole-body protein. At zero Thr intake, chicks lost 11.9 mg Thr/d. The Thr maintenance requirement was 45.7 mg/d per kg body weight(0.75). I ncreasing doses of Thr resulted in increased (P < 0.05) concentrations of methionine, isoleucine, histidine and lysine in whole-body protein . Other indispensable amino acids, including Thr, also tended to incre ase. Whole-body glycine, proline, serine and cystine concentrations de creased (P < 0.05) as Thr was increased in the diet. The maintenance n eed for Thr represented 5 5 % of the total need for Thr, The data sugg est that efficiency of Thr utilization is constant at all levels of Th r intake between 5 and 95 % of the level required for maximal weight g ain and feed efficiency.