Effects of nitrogen and light in grass-legume pastures-a systems analysis approach.
A simple mathematical model is presented describing the effects of N and light on the vegetative growth of grass/legume swards, consisting of a set of first-order differential equations based on the mechanisms involved and providing a conceptual framework for further study of these swards, as well as enabling their behaviour to be simulated on a digital computer. The model was found to be capable of reproducing experimental results in the literature; however, proper validation required more detailed information. During the modelling process, qualitative knowledge of the system was given a quantitative and explicit form, while areas where knowledge was poor were exposed. Areas of particular uncertainty were the processes transferring legume N to an associated grass and the mechanisms by which lack of N limited grass growth. Comparison of the model with experimental data suggested that the delay in converting photosynthetic products to leaf area could have an important effect on growth in some situations. The sensitivity of the model to changes in parameter values suggested that leaf area produced/unit of photosynthate and the lifetime of leaves were of major importance as well as parameters controlling rates of photosynthesis.