[en] Nutritional status is some measure of the extent to which current feeding meets the animal's needs. Those needs depend in the long term on the animal's genetically programmed potential path of growth and reproductive performance and in the short term on the extent to which its current condition diverges from its inherent target. To make responses of animals to inputs of feed more predictable, quantitative assessment of genotype and current condition is required, which calls for a more precise definition of genotype than has been conventional in most current nutritional systems. If properly calibrated, simple methods such as condition scoring of animals can be invaluable for assessing an animal's current state and, if combined with a quantitative description of potential growth, would be likely to enhance greatly the accuracy of predictions of food intake, diet selection and nutrient partition. Within this framework, the influence of the balance of glucogenic, lipogenic and aminogenic nutrients to sustain growth and lactation is considerable. The efficiency with which feed resources are used to sustain production depends both on supplying an appropriate balance of different major nutrient classes and on the animal's inherent potential to perform. Evidence is presented that animals selected for high genetic merit for milk production in a temperate environment are both more biologically and economically efficient in terms of feed resource use in systems of production relevant to that environment. Future improvements in matching available feed resources to the needs of indigenous livestock will depend on greater attention to the assessment and improvement of the genotype and condition of animals as well as to better understanding of the qualities of feed. (author). 69 refs, 3 tabs