Has the genetic selection of commercial layers impacted the control of the reproductive axis?
In the laying hen, intensive genetic selection for earlier sexual maturation and laying persistency has led to rapid improvements in reproductive capacity. While literature demonstrates that layers require photostimulation to initiate maturation, recent evidence has reported modern commercial hens entering lay without this stimulatory cue. Additional cues under consideration include metabolic influences, with body weight (BW) believed to influence the hypothalamic-pituitary gonadal (HPG)-axis during maturation. In terms of persistent lay, skeletal health must be considered due to its association with reproductive capacity. Therefore, this thesis aims to provide novel insights into the integration of cues responsible for the activation of the HPG-axis and, determine physiological alterations resulting in sustained production rates. To study these effects, three strains of laying hens, Lohmann LSL-lite as the modern commercial strain, heritage Shaver white-leghorns as the 2000s-commercial equivalent, and white-leghorn derived Smoky Joe as the 1960s-commercial equivalent, were used. Results demonstrated an earlier pituitary sensitivity to stimulatory neuropeptides in the modern hens, along with an earlier rise in estradiol (E2) and age of first egg (AFE). Meanwhile, the non-selected strain demonstrated a delayed AFE and initial rise in E2. Interestingly, regardless of strain, age or photostimulation, all hens laid their first egg within a 64-g BW window. Furthermore, five recurrent elevations in E2 were observed in modern hens, possibly contributing to their sustained high rate of production. We also demonstrated a correlation between orexigenic peptides, responsible for stimulating feed intake, during the period of maturation in Lohmann and Shavers, but not in Smoky Joes. Finally, cortical and medullary bone mineral density were not altered by earlier AFE or persistent lay, as density was maintained in all strains regardless of cumulative production. In conclusion, this thesis provides critical revisions of the HPG-axis throughout the laying cycle in modern commercial layers. This knowledge aims to improve management guidelines, as BW must be considered in the pullet phase to synchronize metabolic and photostimulatory triggers, optimizing entry into lay. This study is the first to consider the effects of persistent lay from the standpoint of three different strains of layers, highlighting alterations of the HPG-axis which make this possible.