A 4 x 5 factorial experiment containing 4
cystine levels (0.325, 0.406, 0.487, or 0.568%) and 5
methionine levels (0.285, 0.385, 0.485, 0.585, or 0.685%) was conducted to evaluate the interrelationship between
methionine and
cystine in corn-peanut meal diet for Peking ducklings from hatch to 21 d of age. Eight hundred 1-d-old male white Peking ducklings were assigned to 20 experimental treatments. All treatments were replicated 4 times using 10 ducklings per pen. As dietary
methionine level increased,
weight gain and feed intake increased and then decreased; the quadratic response of
weight gain was significant (P < 0.05). The
methionine requirement for maximum efficiency of feed utilization (0.585%) was higher than for maximum
weight gain (0.485%). According to the quadratic model, the optimal
methionine requirement of Peking ducklings from hatch to 21 d of age was 0.481% (95% of the level at maximum response). The plasma
uric acid concentration was very low (P < 0.05) when dietary
methionine was 0.485%. When dietary
methionine was excessive (0.685%), the plasma
homocysteine concentration increased (P < 0.05). On the other hand, the
cystine requirement of ducklings from hatch to 21 d of age was not more than 0.325%. A high level of
cystine (0.568%) depressed
weight gain and feed intake (P < 0.05), but
cystine supplementation in the diets lowered the plasma
homocysteine concentration (P < 0.05). There were no significant interactions between
methionine and
cystine on growth performance, plasma
uric acid, and plasma
homocysteine.