Further understanding the physiological functions of the small intestine in broilers is essential for industry professionals. How does the small intestine develop at different ages? When is intestinal villus growth complete? Do the nutritional requirements of the intestine differ at different ages? Only by understanding these factors can farming risks be reduced.

I. Effects of Age on Intestinal Development

The small intestine grows rapidly during the late stages of incubation, increasing from 1% of body weight on day 17 of incubation to 3.5% at hatch (day 21). In the first 12d after hatching, intestinal length increases by 2–4 times. Villus length increases near hatching, and villus size and number increase after hatching.

By day 7 after hatching, duodenal villus growth is nearly complete, while jejunal and ileal villus growth can continue until after 14d. In mammals, most intestinal cell proliferation occurs in the crypts. Unlike mammals, in birds, intestinal cell proliferation can occur throughout the villi. In chicks aged 4 days, intestinal cells take 3d to migrate to the villus tips; in older chicks, this takes 4 d.

Post-hatch growth rate is influenced, at least in part, by the allometric growth of various organs. The intestine grows rapidly after hatching, with its relative growth rate reaching its peak at 5~7 days of age. At 12 days of age, intestinal length increases by 2–4 times, while the small intestine increases by 7–10 times during this period.

II. Effects of Age on Nutrient Absorption

　　Regarding the activity of intestinal digestive enzymes, trypsin, protease, and amylase activity in the small intestine increases rapidly within 21d after hatching. However, lipase activity does not begin to rise until after 21 days of age. Feeding a high-fat diet before 21 days of age does not significantly increase lipase activity. At least during the first few weeks, lipase activity appears to be a limiting factor in digestion.

　　During broiler development, the nutrient transport system undergoes a series of significant changes. During the first week after hatching, the small intestine takes up more proline than glucose. Because broilers have their highest relative growth rate during week 1, amino acid absorption matches this growth pattern.

　　Glucose uptake briefly increases during week 2. This increase is thought to result from the depletion of yolk reserves and the broiler's shift from lipid metabolism to carbohydrate metabolism. Due to the allometric growth of the intestine, the proportion of body weight accounted for by the intestine decreases during week 2 after hatching, which may be a second reason for the increase in glucose intake during this period. Proline uptake briefly increases during week 6, coinciding with the first molt after the juvenile stage and an increase in absolute growth rate.

　　The intestinal absorptive capacity of broilers matches their nutritional requirements. This contrasts sharply with findings in mammals, whose intake greatly exceeds their needs. Whether nutrient absorption is a potential constraint on broiler growth or broilers are better able to allocate nutritional resources remains to be clarified.

　　Feeding immediately after hatching stimulates intestinal development. Delaying feeding by 24–48h can reduce villus length, the rate of enterocyte migration, and the number of enterocytes, but increases the number of goblet cells that produce acidic and neutral mucus. Currently, commercial broiler chicks are prepared for placement on the placement day: water and feed are provided at the same time to ensure that chicks can drink and eat as soon as they enter the house. The crop fill rate of the chicks is observed within 12 hours; crop fill rate is a very important production indicator for assessing whether the chicks are healthy and whether the house environment is suitable.