This study's aim was to assess how l- threonine (Thr) affected duck breeders' reproductive and productive performance by examining how it affected the amino acid composition of their eggs and plasma and how well ducklings performed a week after hatching. A total of 648 Longyan duck breeders (23 wk) were randomly allotted to six groups with six replicates of 18 ducks. Ducks were fed a basal control diet deficient in Thr and dietary treatments consisted of the basal diet supplemented l-Thr at 0.0 %, 0.07 %, 0.14 %, 0.21 %, 0.28 %, and 0.35 %, constituting total Thr content of 0.41 %, 0.45 %, 0.51 %, 0.60 %, 0.66 %, and 0.72 %, respectively. At 43 wk of age, the addition of l-Thr at a concentration of 0.28 % had superior (P < 0.05) values (P < 0.05) for egg production, egg weight, egg weight, egg mass, feed conversion ratio, yolk weight and its ratio to total egg weigh compared to the control diet, which yielded the lowest values. Over the entire experimental period, feed intake was not affected by dietary treatments. Plasma tyrosine elevated (P = 0.03 and quadratic trend = 0.02) at 0.28 % l-Thr. In addition, plasma citrulline increased linearly (P = 0.06, linear trend P = 0.003) at 0.28 % l-Thr as compared to the control diet. The contents of aspartic acid, Thr, tyrosine, and proline in egg yolk exhibited a linear increase (P < 0.05) corresponding to increased l- Thr levels. In addition, the contents of serine and cysteine were significantly enhanced at 0.35 % l-Thr, whereas the content of glycine experienced a decrease (P < 0.05) as l- Thr levels increased. The amino acid profile in the egg albumen demonstrated a reduction at 0.28 % and 0.35 % l-Thr, whereas feeding l-Thr at 0.21 % led to an increase in proline contents (P = 0.03, quadratic trend P = 0.002). The addition of l-Thr at a concentration of 0.28 % recorded the heaviest body weight of ducklings after hatching, while the control or 0.07 % Thr group recorded the lowest values. These findings suggest that l-Thr supplementation at 0.28 % (constituting total Thr content of 0.66 %) is an effective nutritional strategy to optimize the performance of duck breeders and the quality of their offspring, providing valuable insights for dietary formulations in poultry production and emphasizing the importance of balanced amino acid nutrition for maximizing breeder performance and offspring quality.