The effect of increasing photoperiod and food restriction in sexed, broiler‐type birds. II. plasma thyroxine, triiodothyronine, insulin‐like growth factor‐I and insulin

Abstract

1. Sexed broiler‐type chicks were raised either under a continuous (CON) 23 h light (L) and 1 h dark (D) schedule or an increasing photoperiod (INC). From 5 to 11 d of age birds were fed either: ad libitum (AL), energy intake (kJ ME) restricted to 9.414×g BW^0.67 (R1) or energy intake (kJ ME) restricted to 6.276×g BW^0.67 (R2).

2. Blood samples were taken at 4, 7, 11, 14 d of age and weekly thereafter to 49 d of age. Plasma thyroxine (T₄), triiodothyronine (T₃), insulin‐like growth factor‐I (IGF‐I) and insulin were determined.

3. CON birds had elevated plasma T₃ concentrations to 21 d of age, and greater plasma T₄ concentrations at 11 and 21 d of age concurrent with greater food intake. Elevated plasma T₃ concentrations in INC birds at 28 d of age coincided with lower plasma IGF‐I concentrations at a time when growth and food intake were greater than CON birds.

4. Food restriction elevated plasma insulin and T₄ but depressed plasma T₃ and IGF‐I. Plasma T₃ was greater for food‐restricted birds at 21 d of age, but subsequently, was generally lower than ad libitum‐fed birds which may account for a lack of complete ‘catch‐up’ in growth.

5. Plasma T₃ was higher in females at 11 d of age when growth was equivalent for both sexes. From 28 to 42 d, when sex differences in growth became most apparent, plasma T₃ was greater in males.