Impact du chargement d'arrière-saison sur les teneurs en azote potentiellement lessivable en prairie : références établies dans le sud-est de la Belgique

Impact of cattle stocking rate, under grazing, during autumn on potential nitrogen leaching: reference values established in the south-east of Belgium. Grassland, which accounts for half of the agricultural area in Wallonia, forms the basis of many ecosystemic services: cultural, grass production, regulatory services, etc. Nevertheless, the poor ability of cattle to fix ingested nitrogen leads to significant N rejection during grazing. N is excreted in concentrated form via urine, and thus becomes unevenly distributed within the paddock. This phenomenon increases the risk of N leaching in grazed grasslands, especially if urine deposit occurs later during the grazing season, in the autumn. It is therefore necessary to specify good livestock management practices, with particular reference to the livestock stocking rate to be applied during autumn grazing. This ensures the reconciliation of both animal and environmental performance. With this aim, a study was performed over a five-year period, during the autumn, to test the impact of the livestock stocking rate on the risk of nitrate leaching in grazed grassland, in the Jurassic area of south-east Belgium. A gradient of stocking rate was established, ranging between 150 and 350 BLU grazing days·ha-1. Results underlined a highly significant impact of the "year" factor on potentially leached nitrogen (APL), ranging between 9 (2010) and 23 (2005) kg of N-NO3-·ha-1. The impact of the stocking rate was only marginally significant. Whatever the year or the stocking rate, the recorded APL level never represented a significant risk to water resources, and this was in spite of a relatively high stocking rate at the end of the grazing season. This low impact of the stocking rate could be explained by the type of cattle (beef breed heifers and dry cows) mobilized in order to apply these end-of-season stocking rates. A further explanation could lie in the addition of hay to the cattle's diet in order to achieve the highest stocking rates. Higher level of APL would be expected under grazing with dairy cows.