ABSTRACT
Summary Forage production basically involves the conversion of environmental inputs to provide digestible energy and other nutrients for ruminant livestock. The plant breeder needs to consider the seasonal inputs or constraints in his particular environment, the efficiency of the crop in responding to these factors, those features of the plant that contribute to this efficiency, and the degree to which those features can be modified by selection and breeding. 70The primary climatic variable determining crop production is the seasonal input of light energy, but in most grassland enviroments growth or even survival can be limited by temperature or water stress, while shortage of soil nutrients, particularly nitrogen, is usually important. Photosynthetic production will be influenced by the photosynthetic rate of the individual leaves, by their arrangement in relation to light perception, and by the extent of respiratory losses. In forage crops, the harvestable yield, including regrowth, is also influenced by the partition of assimilates between shoot and root development and between flowering and continued vegetative growth. Response to seasonal limitations of temperature or water stress often involves two conflicting requirements, the ability to continue active growth under moderate stress and/or survival under more extreme conditions. Furthermore, the physical and chemical constitution of the harvested material is important in determining digestibility and intake, and hence animal production. Considerable genetic variation is available within forage species for many physiological or morphological features. The plant breeder has to consider which features are most important in determining seasonal production and how far reliable selection criteria can be developed that compare favorably with the standard methods of field assessment. Such rapid screening techniques can be of particular value where production is limited by one major but unpredictable climatic factor, such as low temperature or water shortage, or where rapid laboratory techniques can be developed for the assessment of nutritional components.
