Effects of dietary protein on threonine requirements of Pekin ducks from hatch to 21 days of age

• Thr deficiency resulted in poor growth performance of Pekin ducks regardless of dietary protein levels.
• Dietary protein influenced the Thr requirements of Pekin ducks.
• Thr requirements were a constant proportion when expressed as a percentage of dietary lysine.

A 2 × 6 factorial experiment with two dietary crude protein (CP) levels (17.73% with 0.46% Thr and 20.89% with 0.54% Thr) and six supplemental threonine (Thr) levels (0, 0.06, 0.12, 0.18, 0.24, and 0.30%) was conducted to study the effects of dietary CP on the levels of Thr required for the optimal growth of white Pekin ducks from hatch to 21 d of age. A total of 576 one-d-old male white Pekin ducks were assigned to one of 12 experimental treatment groups. Each treatment group consisted of six replicate pens with eight ducks per pen. All ducks were reared in raised wire-floor pens from hatch to 21 d of age. Birds that were fed basal diets not supplemented with crystalline Thr had the worst weight gain and feed intake at both dietary CP levels. Quadratic broken-line regression analysis was used to estimate the Thr requirement of ducks. When expressed as a percentage of diet, the estimated Thr requirement was 0.610% at 20.89% CP and 0.556% at 17.73% CP. When expressed as a percentage of dietary CP, the Thr requirement was 2.931% at 20.89% CP and 3.146% at 17.73% CP. When expressed as a percentage of dietary lysine, the estimated Thr requirement was 53.0% at 20.89% CP and 52.0% at 17.73% CP. The difference between the Thr requirement at 17.73 and 20.89% CP was examined by t-test. This analysis demonstrated that dietary CP influenced the Thr requirement when it was expressed as a percentage of diet or as a percentage of dietary CP (P < 0.05). In contrast, Thr requirement was not affected by dietary CP when it was expressed as a percentage of dietary lysine (P > 0.05). It was concluded that dietary CP can cause variations in the Thr requirement of Pekin ducks, but expressing the Thr requirement as a percentage of dietary Lys can reduce this variation.