Crop growth is modelled using a set of mathematical equations, based on physiological processes and environmental variables. The growth models may allow analysis of production components, providing an integrated overview of production. In order to adjust models of biomass accumulation as a function of photosynthetically active radiation absorbed (PAR(a)) in an annual ryegrass pasture (Lolium multiflorum Lam.), four N fertilization rates were applied (zero, 50, 100 and 200 kg ha(-1)) and arranged in a complete block design with three replicates. Grazing was done each time canopy intercepted 95% of incident light and stopped at 30% of interception. The biomass accumulation, as a function of PAR(a) during the establishment, was adjusted to linear plateau model for N-zero and linear model for the other doses. In regrowth, there were also different models for N-50 (linear plateau), N-100 (linear) and N-200 (linear plateau). Biomass accumulation as a function of PAR(a) presented different models depending on sward condition (establishment and regrowth) and N fertilization. Therefore, the accumulation of biomass can be estimated using parameters describing the time-evolution of LAI, the absorption efficiency of PAR and the local ratio between the global solar radiation and PAR.