Determination of the metabolic triggers responsible for sexual maturation in layer chickens and their relation to rearing environment

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As seasonal breeders, layer chickens rely on changes in photoperiod (daytime length) to initiate and terminate reproduction. Under commercial conditions, artificial lighting has allowed producers to photostimulate pullets all year long and although the mechanisms responsible for mediating the effects of photoperiod on the reproductive axis have been studied and modelled, our recent data suggest that metabolic cues may be as important as photoperiodic signals. Since layer pullets have ad libitum access to food during rearing, resources availability is not a limiting factor and commercial strains of layers often enter lay well before the age at photostimulation if their body weight exceeds the breeder's recommendation. However, the origin, mode of action, and specific target(s) of this metabolic control remain to be determined. For a pullet, once sexual maturation is activated, physiology switches from growth to egg production and longitudinal bone growth stops in favor of medullary formation, the gut and liver physiology changes to support the synthesis of egg precursors, and the reproductive tract and sex characteristics develop. Unfortunately, if this occurs before the pullet reached its adult size, the skeletal frame will remain underdeveloped, predisposing the hen to fractures and calcium deficiency. We speculate that rather than weight, body composition and associated specific metabolic signals are most likely involved. This is of significance as growth and body composition of layer pullets are influenced by rearing environments which may require nutritional adjustment when transitioning to non-cage systems. Thus, this project aims at determining the body weight and body composition thresholds responsible for initiating sexual maturation in two strains of layers (brown and white) reared under different environments mimicking industry practices.