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

作为季节性种鸡，蛋鸡依靠光周期（白天长度）的变化来开始和结束繁殖。在商业条件下，人工照明允许生产者全年光刺激小鸡，尽管负责调节光周期对生殖轴影响的机制已经被研究和建模，但我们最近的数据表明，代谢线索可能和光周期信号一样重要。由于蛋鸡幼崽在饲养过程中可以自由获取食物，资源的可用性不是一个限制因素，如果它们的体重超过饲养员的推荐值，商业品种的蛋鸡通常在光刺激的年龄之前就进入产蛋期。然而，这种代谢控制的起源、作用方式和具体目标仍有待确定。对于小鸡来说，一旦性成熟被激活，生理机能就从生长转向产蛋，纵向骨生长停止，有利于髓质形成，肠道和肝脏生理机能改变，以支持蛋前体的合成，生殖系统和性别特征发育。不幸的是，如果这种情况发生在小鸡达到成年尺寸之前，骨骼框架将保持不发达，使母鸡容易骨折和缺钙。我们推测，最可能的原因不是体重，而是身体成分和相关的特定代谢信号。这一点具有重要意义，因为蛋鸡的生长和身体组成受到饲养环境的影响，在过渡到非笼养系统时可能需要进行营养调整。因此，本项目旨在确定在模仿工业实践的不同环境下饲养的两种蛋鸡（棕色蛋鸡和白色蛋鸡）的体重和体成分阈值，这些体重和体成分阈值负责启动性成熟。

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.