Nitrogen utilization by forage grasses

The efficient utilization of nitrogen (N) in grass production is essential to reduce the risks of water and air pollution, and the costs of production . Recent findings in grass physiology and agronomy should help in developin g new tools to improve N utilization efficiency. A model of N dilution desc ribing the decrease in plant N concentration with increasing shoot biomass under non-limiting N supply is used to define a critical N concentration in grasses required to reach maximum shoot growth and yield. The index of N n utrition (INN) is then calculated as the measured N concentration in a give n situation divided by the critical N concentration. The INN is a diagnosti c tool to quantify the level of N deficiency during growth cycles, and can also be used in crop modelling and in the interpretation of results from st udies conducted over many sites and yeats. The "universality" of the model of N dilution is based on the increased proportion of structural to metabol ic components during crop growth combined with the fact that the structural component has a lower N concentration. Inter- and intra-species difference s in N concentration at a given shoot biomass can be related to differences in the proportion of leaves which are assumed to be equivalent to the meta bolic component. Under N-deficient conditions, the reduction in grass growt h is due to a reduction in the interception of solar radiation primarily th rough reduced leaf extension, and to a reduction in the conversion efficien cy of intercepted radiation into shoot biomass primarily through an effect on biomass partitioning between roots and shoots. The concept of the critic al N concentration based on the relationship between plant N concentration and shoot. biomass is used to derive general and synthetic expressions of t he effect of plant N nutrition on crop growth and crop growth processes. Th ese recent findings on the relationship between N nutrition and the growth of forage grasses should result in the improvement of the efficiency of N u tilization by a more precise fertilizer management and the development of m ore N efficient cultivars.