Link between the number of grazing days and the mineral nitrogen fluxes in grazing systems

The Moorepark St. Gilles grass growth (MoSt GG) model has been developed to predict grass growth taking account of the water and nitrogen (N) fluxes at the paddock level. At grazing, the animal plays a specific role in the N fluxes due to urine and faeces deposition directly on the field. The MoSt GG model has been used to evaluate the effect of N fertilisation on the grazed grass N fluxes. The study was conducted using 10 weather years (2006 to 2015) from the INRA Le Pin experimental farm in Normandy at 4 different N fertilisation rates (0, 100, 200 and 300 kg N ha-1 yr-1). Results show a clear link between the number of grazing days, yearly rainfall and the amount of N leached. The range of N leaching varies between 0 and 135 kg N ha-1 yr-1. The number of grazing days during the year seems to be the best indicator of the contribution of N leaching risk.