Effect of lighting programs during the pullet phase on skeletal integrity of egg-laying strains of chickens

Egg-laying strains of chickens are highly susceptible to osteoporosis, a noninfectious disease characterized by a decrease in structural bone as hens age. To minimize the onset of osteoporosis, it was hypothesized that a delay in sexual maturity may allow a pullet to develop a stronger skeletal frame before egg laying, leading to improved skeletal mineralization at end of lay. One management tool that can easily be implemented by pullet growers to delay sexual maturity is length of photoperiod. The objective of the current study was to determine whether lighting programs used during the pullet phase of egg-laying strains of chickens can be manipulated to allow for improved skeletal mineralization in laying hens at end of lay. Two experiments were conducted in which 1,000 pullets/experiment were exposed to 1 of 3 varying step-down lighting programs (2 to 17 wk of age), referred to as rapid, moderate, and slow. For both experiments, 2 strains of chickens were used. Experiment 1 compared the Hy-Line W-36 with the Hy-Line W-98, and experiment 2 compared the Hy-Line Brown with the Hy-Line W-98. At 66 wk of age, all hens remaining in the study were weighed individually and the drum stick and wing were retrieved for determination of bone mineralization and bone size traits. Bone data were analyzed using an analysis of covariance with BW as the covariant, and BW was analyzed as an ANOVA. Skeletal frame development was affected by lighting regimen. Pullets exposed to the slow lighting photoperiod had longer bones and more bone area (experiment 2) than those exposed to the rapid photoperiod, most likely because of a delay in bone growth plate closure, which occurs at sexual maturity. However, this delay in sexual maturity, as indicated by longer bones, did not improve bone mineralization at 66 wk of age. It was concluded that pullet lighting regimen had little effect on bone mineralization at end of lay.