Reducing the dietary starch:protein ratios in low-protein diets enhanced the growth performance of goslings from 1 to 28 days of age.

The current study aimed to explore the suitable starch: protein ratios under different dietary protein levels for goslings. A total of 360 male 1-day-old Jiangnan White goslings were randomly divided into 6 groups with six replicates containing ten goslings each. The experimental design consisted of a 3 × 2 factorial array of treatments. Three protein levels (18%, 16%, 14%) and two starch: protein ratio (S: P ratio) types (standard, reduced) were formulated. The results showed that: reducing the S: P ratio at the same dietary protein level increased weight gain (WG), average daily gain (ADG), and average daily feed intake (ADFI) of goslings (P < 0.05). Lowering the protein level increased feed-to- gain ratio (F/G) at the same dietary S: P ratio type. Both decreasing dietary protein levels and reducing S: P resulted in an increase (P < 0.05) in the serum albumin (ALB) content of goslings. Protein at 18% level, minimized serum total cholesterol (TC) in goslings. Reducing the dietary S: P ratio elevated serum lipid concentration. In reduced S: P ratio diets, serum Leucine (Leu) decreased and Threonine (Thr) concentration increased. The reduction in dietary protein level and S: P ratio significantly affected the amino acid composition of muscles. The varied levels of protein and S: P ratio types interacted to influence the starch digestibility of distal jejunum. In addition, the reduced S: P ratio attenuates α-amylase activity of jejunal chyme. Moreover, SGLT1 and GLUT2 genes expression were generally down-regulated, and SLC7A5 gene expression was up-regulated in reduced S: P ratio groups. In summary, the diet with 14% protein level, and 2.97 starch: protein ratio is recommended to use in gosling's growth phase of 1 to 28 days.