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

作为季节性繁殖者，蛋鸡依靠光周期（白天长度）的变化来启动和终止繁殖。在商业条件下，人工照明允许生产者全年光刺激母鸡，虽然负责调解光周期对生殖轴的影响的机制已经研究和建模，我们最近的数据表明，代谢线索可能是一样重要的光周期信号。由于蛋鸡在饲养过程中可以随意获取食物，因此资源的可用性不是限制因素，如果蛋鸡的体重超过饲养员的建议，则蛋鸡的商业品系通常在光刺激年龄之前进入产蛋。然而，这种代谢控制的起源、作用方式和具体靶点仍有待确定。对于一只小母鸡，一旦性成熟被激活，生理学从生长切换到产蛋，纵向骨生长停止，有利于髓质形成，肠道和肝脏生理学改变，以支持卵前体的合成，生殖道和性特征发育。不幸的是，如果这发生在母鸡达到成年大小之前，骨骼框架将保持不发达，容易导致母鸡骨折和缺钙。我们推测，而不是体重，身体成分和相关的特定代谢信号最有可能参与。这一点很重要，因为蛋鸡的生长和身体组成受到饲养环境的影响，在过渡到非笼系统时可能需要进行营养调整。因此，本项目旨在确定体重和身体组成阈值负责启动性成熟的两个品系的蛋鸡（棕色和白色）饲养在不同的环境下模仿行业惯例。

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.