Validity of accessible critical nitrogen dilution curves in perennial ryegrass for seed production

The objectives were to test if accessible critical nitrogen dilution curves (NDCs) in rapeseed, pea, alfalfa, tall fescue, wheat, annual ryegrass and linseed could be used in grass species for seed production and to develop a critical NDC especially in grass species for seed production. The critical NDC is based on critical plant nitrogen concentration (%Nc) which is defined as the minimum plant nitrogen (N) concentration in plants needed for maximum growth rate of the crop. Critical NDC is a sequence of %Nc values starting at approximately 1 t ha−1. Field experiments in perennial ryegrass (Lolium perenne L.) and red fescue (Festuca rubra L.) for seed production were carried out at different locations in Denmark from 1996 to 2005. At least three different N application rates were applied in each experiment. Shoot biomass (t ha−1) and N concentration expressed in percentage of dry matter were measured for calculation of %Nc and critical NDC.The %N decreases during the growing season from a maximum of 4.6 to a minimum of 0.76 while shoot biomass increases from 2.3 to 13.8 t ha−1. Therefore it was not possible to calculate a %Nc at a biomass lower than 2.3 t ha−1 and the NDC then has to be extrapolated to estimate α. Based on this we suggest that the NDC developed in grass species for seed production should be further improved. The conclusion from the use of accessible NDCs was that NDCs in linseed, wheat, and annual ryegrass were better to describe %Nc in grass species for seed production than the NDCs in tall fescue, alfalfa, pea and rapeseed. These findings should be used to continue the interesting and necessary work on developing a NDC in grass species for seed production.