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 diets containing graded levels of L-threonine (Thr) during the period 10-20 d post-hatching. Doses of Thr represented 5,10,15,40, 55, 70 and 95 % of its ideal level for maximal weight gain and feed efficiency. Other amino acids were maintained at minimized excess levels that were 15 % (of ideal) above the various doses of Thr. Following 10 d of feeding and a 24 h fast, chicks were killed for whole-body protein and amino acid analysis. Using pen accretion means, weight gain (2 0.98), protein accretion (2 0-99), and Thr accretion (2 0.99) were linear (P<O.Ol) functions of Thr intake. Slope of the Thr accretion regression 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 mgld per kg body weight O"'. Increasing 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 increase. Whole-body glycine, proline, serine and cystine concentrations decreased (P < 0.05) as Thr was increased in the diet. The maintenance need for Thr represented 5-5 % of the total need for Thr. The data suggest that efficiency of Thr utilization is constant at all levels of Thr intake between 5 and 95 % of the level required for maximal weight gain and feed efficiency.