The effects of body weight and long ahemeral days on early production parameters and morphological characteristics of broiler breeder hens.

Shaver Starbro broiler breeder pullets were used to study the effect of day length (24 and 28 h) on egg production parameters and egg traits from 22 to 30 wk of age, and ovarian and morphological characteristics at 30 wk of age. Floor-housed pullets were raised in a light-tight facility from 1 d of age until housing in individually illuminated cages at time of photostimulation (22 wk). Cages were equipped with hardware to monitor egg laying time. The photoschedule during rearing was 24 h light: 0 h dark for the first 3 d followed by 8 h light: 16 h dark from 3 d to photostimulation. Body weight was monitored throughout the trial to maintain weights at targets set by Shaver Poultry Breeding Farms, Ltd. The experimental design was a 2 x 2 factorial with two day lengths [hemeral (24 h) and ahemeral (28 h)] and two BW groups [high (HBW) and low BW (LBW)], with the LBW weighing 327 g less than the HBW birds at 22 wk of age. The hemeral photoschedule was 14 h light: 10 h dark, and the ahemeral photoschedule was 14 h light: 14 h dark. Differences in BW remained throughout the trial. Egg numbers were not increased by the 28-h day (24 h = 25.4 eggs; 28 h = 23.1 eggs). The 24-h hens laid more double-yolked eggs (0.9% of total egg production) than did the 28-h birds (0.3%). Egg formation time was consistently longer for the 28-h hens (26.8 h at 25 wk of age and 25.8 h at 29 wk of age) compared with the 24-h hens (25.4 h at 25 wk of age and 24.8 h at 29 wk of age). Mean egg weight was higher for the 28-h birds (55.1 g) compared with the 24-h birds (53.0 g). Egg specific gravity was higher in eggs from the 28-h hens than eggs from the 24-h hens. Some of the increase in egg weight between the 28- and 24-h birds was due to shell weight. Absolute and proportional weights of the egg shell were 5.5 g and 10.0% of the total egg weight for the 28-h birds and 4.9 g and 9.3% for the 24-h birds, respectively. The 28-h hens had higher proportional breast muscle weight, smaller livers and oviducts, and lower ovary weight compared with the 24-h birds. The data indicated that, although egg size can be increased with the use of long ahemeral days early in lay, this result may be at the expense of egg numbers. Early in lay, follicular maturation rates are fast, and egg production may be limited to one ovulation per 28 h.