Effects of light intensity from photostimulation in four strains of commercial egg layers. 1. Ovarian morphology and carcass parameters

We examined the effects of light intensity (LI) from photostimulation to 45 wk of age on egg production parameters and egg size characteristics and on ovarian and carcass morphology at sexual maturity and 45 wk of age in four layer strains. Floor housed pullets were raised in a light-tight facility from 1 d of age until housing in individually illuminated cages at 17 wk of age. Two white egg strains, ISA-White (ISA-W) and Shaver 2000 (S2000), and two brown egg strains, ISA-Brown (ISA-B) and Shaver 579 (S579), were used. Pullets were randomly assigned to a processing group that was killed at sexual maturity (first oviposition) (Group 1) or kept to 45 wk (Group 2). Birds were photostimulated at 18 wk of age using a LI of 1, 5, 50, or 500 lx (4 x 4 factorial design). One bird from Group 1 and one from Group 2 were caged together in individually lit cages (one brown and one white egg layer). Carcass and ovarian morphology data were examined as related to Strain, LI, or the interaction of Strain and LI. The time from photostimulation to sexual maturity did not differ due to LI, but was shorter for brown egg strains (ISA-B = 19.9 d, S579 = 20.2 d) than for white egg strains (ISA-W = 26.6 d, S2000 = 28.1 d). Body weight at sexual maturity differed among all strains, with the white egg strains having the lowest BW. Ovary weight was the greatest in ISA-W birds, in which 8.0 large yellow follicles (LYF) were present compared to 6.8 in S2000 birds. The LI affected ovary development, as birds with the 1 lx exposure had lower ovary weights and fewer LYF than did 50 lx birds, suggesting that the 1 lx LI did not result in an adequate photostimulatory cueing of sexual maturation. The threshold LI for a complete morphological response to photostimulation in this study was 5 lx. Strain differences in BW observed at sexual maturity continued to 45 wk of age. Light intensity affected 45 wk BW, with 5 lx LI birds weighing 7.2 and 8.7% more than the 50 and 500 lx birds, respectively. On an absolute basis, brown egg stains carried significantly more breast muscle at 45 wk of age than did white egg strains. The fatpad was heavier on a relative basis for brown egg layers than white egg layers. The 1 lx hens had lower 45-wk ovary weights than did the other three LI treatments. These data support the conclusion that with the development of highly specific genetic strains, it is increasingly important to match the environmental management practices to a particular hen's genotype.