The morphology of forage grass, in particular ryegrass, emerges from the combination of many entangled dynamical processes. Although a plant seems to possess an intrinsic (i.e., genetically determined) architecture, its morphogenesis also displays very high plasticity with respect to environmental conditions. This plasticity could be partly mediated by a self-regulatory process. Indeed, the architecture of a shoot can affect subsequent architectural development recursively. For instance, leaf length is directly influenced by the length of the preceding leaves. We present a functional-structural 3D model of ryegrass shoots based on the hypothesis that their morphogenesis obeys such self-regulatory processes. The behaviour and emergent properties of the model are highly compatible with observations regarding plant morphology development, genetic variability and plasticity. Experimental and simulation results therefore suggest that the architectural development of ryegrass could emerge from the collaborative association between genetic programmability and self-organisation, instead of resulting from a centralised control of each architectural trait.