Advancing plant phenomics requires linking high-resolution phenotypic data to plant performance under environmental stresses like grazing. However, how intrinsic biological factors, specifically individual developmental stage, mediate phenotypic trade-offs in response to management remains poorly quantified. Using a phenomics approach on the dominant grass Stipa bungeana within a two-decade grazing experiment, we integrated multi-year, individual-level trait data to dissect the effects of grazing season (cold/warm), intensity (light/moderate/heavy), climate, and developmental stage (proxied by basal diameter) on vegetative and reproductive tiller phenomes. We show that warm-season grazing increased tiller production, whereas cold-season grazing simplified phenotypic correlation networks. Notably, developmental stage was the dominant driver of vegetative growth and individual biomass, while reproductive investment was primarily governed by external drivers (climate and grazing intensity). Path modeling revealed that developmental stage indirectly enhances sexual reproduction by fueling vegetative investment, jointly determining final biomass. This study suggests that individual developmental stage as a potential internal integrator of grazing signals, reshaping phenotypic architecture. Our findings provide a phenotype-driven framework for precision grassland management. By advocating for the monitoring of developmental stage composition, we bridge phenomics with sustainable practices, enabling dynamic grazing strategies that optimize the balance between productivity and ecosystem resilience.