The effect of age at photostimulation on carcass composition at photostimulation and at sexual maturity as well as its effect on subsequent reproductive performance in broiler breeders was investigated. A total of 600 broiler breeder pullets were reared in light-tight facilities. Skip-a-day feeding was used after 2 wk of age to limit body weight to levels suggested by the breeder. Limited daily feeding was initiated at the time of caging (time of photostimulation). The photoperiods used during rearing and lay were 8L:16D and 14L:10D, respectively. Forty birds were selected for study and moved to laying cages and photostimulated at 120, 130, 140, 150 or 160 d of age. Individual egg production records were kept until 60 wk of age. Twenty additional birds were killed for study of carcass composition and reproductive organ, morphology at the time of photostimulation and 20 others were killed at sexual maturity (first oviposition).As age at photostimulation increased, body weight and percent carcass lipid content increased while percent water and ash content decreased. No significant differences were found among treatment groups at photostimulation for carcass protein content. No significant differences were found in any carcass composition parameter at sexual maturity. All groups showed an increase in lipid content from photostimulation to sexual maturity, with the 120 d treatment group showing the largest increase. As age at photostimulation increased, the time from photostimulation to sexual maturity decreased. Total egg production, first egg weight, total egg mass produced, average fertility, hatchability and hatch of fertile did not differ significantly among treatment groups. Chick production was significantly higher in the 140-, 150- and 160-d treatment groups compared with the 120- and 130-d treatment groups. One explanation for the high rate of egg production seen in birds photostimulated at the older ages was that these birds reached sexual maturity with less between-bird variation. To take advantage of this improved flock uniformity, delaying photostimulation to 140 d of age or later is recommended. Key words: Chicken, broiler breeder, sexual maturity, egg production, hatchability, carcass composition

Increasing day length is the primary cue for the timing of sexual maturation in poultry. Typically, Single Comb White Leghorn (SCWL) pullets are reared under conditions of short days (8L:16D) until 18 wk of age. In this study, the effects of varying the age at photostimulation from 16, 18 or 20 wk of age were studied in terms of carcass composition at photostimulation and at sexual maturity, as well as reproductive performance.A flock of 1600 SCWL pullets (Shaver 288 strain) were housed in floor pens in light-tight facilities (8L:16D). All birds were provided with ad-libitum access to both feed and water. At 16 wk, 18 wk and 20 wk, 50 birds per treatment were moved to laying cages and provided to a photoperiod of 14L: 10D. An additional 288 birds per treatment were housed two birds per cage, and group egg production and feed consumption data were collected to 64 wk of age. At photostimulation (n = 20 birds per treatment) and on the day following the first oviposition, (n = 20 birds per treatment) birds were killed by cervical dislocation. All surviving hens were killed at 64 wk of age. All carcasses were studied in terms of selected organ weights and reproductive organ morphology. Birds killed at photostimulation and at sexual maturity were further studied for whole body carcass composition.Significant differences were found in dry matter, water, lipid and ash content among treatment groups at the age of photostimulation. As age at photostimulation increased, absolute weight of carcass fat, protein and ash increased. The percent carcass lipid content increased for all groups between photostimulation and sexual maturity with the 16 wk treatment group showing the largest increase. The time required to reach sexual maturity from the age of photostimulation decreased as the age at photostimulation increased. No significant differences were found in total egg production. The 16 wk group laid significantly smaller eggs. Delaying sexual maturity until 20 wk of age resulted in an increased prime sequence length and fewer pause days to produce approximately the same number of eggs as birds that were photostimulated at 16 wk of age. Key words: Chicken, egg-type hens, sexual maturity, egg production, carcass composition

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