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(日间长度)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对蛋鸡进行光刺激，尽管负责调节光周期对生殖轴影响的机制已经被研究和模拟，但我们最近的数据表明，代谢信号可能与光周期信号一样重要。由于蛋鸡在饲养期间可以随意获得食物，资源的可获得性不是一个限制因素，如果商业品系的蛋鸡体重超过育种者的建议，则通常在光刺激年龄之前很久进入产蛋期。然而，这种代谢调控的来源、作用方式和特异性靶点(S)仍有待确定。对于蛋鸡来说，一旦性成熟被激活，生理从生长转换到产卵，纵向骨骼生长停止，有利于髓的形成，肠道和肝脏的生理变化，以支持卵前体的合成，生殖道和性别特征的发展。不幸的是，如果这种情况发生在小鸡达到成体大小之前，骨骼框架将保持不发达，使母鸡容易骨折和缺钙。我们推测，最有可能涉及的不是体重，而是身体成分和相关的特定代谢信号。这一点很重要，因为蛋鸡的生长和身体成分受到饲养环境的影响，当过渡到非笼养系统时，可能需要进行营养调整。因此，该项目旨在确定在模仿行业惯例的不同环境下饲养的两个品系(棕色和白色)蛋鸡的体重和身体组成阈值，这些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.