EC legislation requires the land to which free-range poultry have access to be ‘mainly covered with vegetation’, but the nature of this vegetation is not specified. In practice, most free-range land will be grass pasture. Modern poultry nutrition is a highly-developed science, and to house poultry in an environment in which they may consume an unknown quantity of vegetation, itself of unknown nutritive value, introduces uncertainty into a predictable system. In recent years the contribution of grazing to the nutrition of the birds has been considered negligible. However, birds on range appear to ingest a little grass, and therefore the quality of the pasture may affect their performance. If poultry consumed largely grass, then the nutritional value derived from it would be relatively poor. The grass would constitute a source of energy and fibre, but would make little contribution in terms of protein. The likely approach that poultry nutritionists would take is a precautionary one. They would need to know the full nutritive value of the pasture and the likely quantitative intake by the birds, and they would then make adjustments to feed formulations at what they considered to be a ‘safe’ level, in order to minimise the risks of impairing performance. A small (0–5) percentage contribution is probably the most that could be achieved, but given that feed accounts for about 70% of the variable costs of poultry production, such a contribution is economically important.