Introduction

The improvement of grassland productivity depends upon the recognition of elite plant germplasm which in local environments optimizes the conversion of carbon dioxide, water and minerals to digestible herbage acceptable to livestock and which protects environmental resources from degradation (chapter 2). The definition and rectification of deficiencies of mineral nutrition and the effective management of biological N fixation (chapter 3) constitute the second line of approach, which includes the definition of genotypes adapted to the soil conditions as modified. The third strand is the manipulation of the leaf surface by management of defoliation which optimizes the sustained harvesting of herbage nutrients and maintains protective cover of the soil. This depends upon insights from plant physiology, which describes and quantifies processes occurring in plants. These insights are used in meeting other objectives of grassland improvement, and managing the defoliation of grassland is selected as the most significant physiological theme and one which has excited controversy in the past four decades.

Non-structural carbohydrate ‘reserves’

Earlier phases of grassland science, for example at the IV International Grassland Congress in 1937, emphasized two aspects of plant response to defoliation: (1) the significance of total non-structural carbohydrate (TNC) in the roots and crown of the plant in controlling persistence and the rate of recovery growth after defoliation; and (2) the effects of frequency and height of cutting on pasture yield.