Effects of Dietary Methionine and Lysine Balance on Intestinal Function and Immune Organ Development in Wanxi White...

(1) Background: This experiment investigated the effects of balancing methionine or lysine under different dietary protein levels on intestinal function and immune organ development in Wanxi white geese during the brooding period. (2) Methods: A total of 180 one-day-old Wanxi white geese were randomly divided into six groups using a 2 × 3 factorial design, with amino acid balance (lysine or methionine) and dietary crude protein levels (20%, 18%, and 16%) as experimental factors. Each group consisted of five replicates with six geese per replicate. The feeding trial lasted 28 days. (3) Results: The lysine-balanced diet significantly increased jejunal lipase activity in 14-day-old geese (p < 0.05). In contrast, the methionine-balanced diet significantly enhanced ileal trypsin, maltase, and amylase activities at 14 days, as well as ileal trypsin activity at 28 days (p < 0.05). Lysine balance markedly improved jejunal weight and muscularis thickness, ileal weight, villus height, and villus-to-crypt (V/C) ratio in 14-day-old geese, and further increased jejunal villus height and ileal crypt depth and muscularis thickness at 28 days (p < 0.05). Methionine balance significantly elevated total antioxidant capacity (T-AOC) in the jejunum and ileum of geese at both 14 and 28 days (p < 0.05). Moreover, the methionine-balanced group exhibited a significantly higher thymus index and spleen index in 28-day-old geese compared with the lysine-balanced group (p < 0.05). Notably, geese fed the 18% crude protein diet showed significantly greater follicular and medullary areas of the bursa of Fabricius at 14 days than those fed 20% or 16% protein diets (p < 0.05). (4) Conclusions: Appropriate dietary protein reduction combined with balanced lysine or methionine supplementation effectively enhances intestinal digestive function, antioxidant capacity, and immune organ development in Wanxi white geese. Among the tested treatments, a dietary protein level of 18% produced the most favorable overall outcomes during the brooding period.