ABSTRACT
The practice of agriculture is the management of biological systems with the object of economically producing one or more products. A great many biological systems, or more correctly sub-systems, are involved in the overall system called agriculture. The complexity of agricultural systems is well illustrated by the diagrammatic representation of sheep production given by Spedding (1975). To study such a system in its entirety in a traditionally designed experiment would be impractical if not impossible. However, it is theoretically possible to build mathematical models that describe the system, and which can be manipulated to predict the outcome of changing one or more of the input variables. This practice of systems analysis requires a comprehensive and quantitative description of the system before a model can be built (Smith, 1982). Generally, and particularly for models of for-age-livestock production systems, the lack of knowledge of the plant-animal interface (Minson, 1983) and the traditional division of research areas between plant and animal scientists has hampered model development (Dougherty, et al., 1985).
