Abstract: This experiment was conducted to evaluate the effects of dietary metabolizable energy (ME) levels during rearing on growth performance and the development of Rugao yellow chicken at the initiation of the laying period. Three hundred and sixty 8-week-old Rugao yellow chicken with similar body weight were assigned to 3 groups with 8 replicates per group and 15 hens per replicate. The pullets were fed one of three diets that were nutritionally equal with the exception of ME levels of 10.88, 11.30, and 11.72 MJ·kg^-1 from 8 to 18 weeks of age. From 18 to 20 weeks of age, all pullets were fed a diet with the same nutritional level. The trial period lasted for 12 weeks (from 8 to 20 weeks of age). The results showed as follows:1) The body weight of hens at 18 weeks of age, the average daily weight gain (ADG) and the average daily ME intake (MEi) of hens aged from 8 to 18 weeks increased linearly (P<0.05), while the ratio of feed to gain (F/G) and the ratio of CPi to gain (CPCR) of hens aged from 8 to 18 and 8 to 20 weeks decreased linearly (P<0.05) with the increase of dietary ME levels from 8 to 18 weeks of age.2) The development of carcass, internal organ and digestive tract at 18 and 20 weeks of age were not significantly affected by dietary ME level from 8 to 18 weeks of age (P>0.05). 3) The tibia weight and tibia index showed a decreasing and then increasing response to increasing dietary ME level from 8 to 18 weeks of age (P<0.05), while there was no significant difference in the development of comb and femur at 18 weeks of age and the development of comb, femur and tibia at 20 weeks of age. 4) The ovary stroma index and small yellow follicle index at 18 weeks of age increased linearly with the increase of dietary ME levels from 8 to 18 weeks of age, while there was no significant difference in the other reproductive organs at 18 weeks of age and all the reproductive organs at 20 weeks of age. It was indicated that the body weight of hens at 18 weeks of age and the ADG of hens aged from 8 to 18 weeks were increased, the F/G and CPCR of hens aged from 8 to 18 weeks were improved, and the development of ovary stroma and small yellow follicle was promoted when the dietary ME levels was 11.72 MJ·kg^-1 from 8 to 18 weeks of age. But no significant difference was observed in the indexes of growth performance and development among the treatments at the initiation of the laying period (20 weeks of age). The results of the present study showed that it has an important practical importance to study the combination of growing period and preparatory laying period of laying hens.

Key words: Rugao yellow chickens, metabolizable energy, rearing period, initiation of the laying period, growth performance, development