Increased nitrogen (N) inputs via fertiliser and animal manure have played a large role in the intensification of grassland-based dairy farming in the Netherlands during the second half of the 20th century. However, the increased N inputs have also contributed to large increases in N losses to the environment. In response, the government has implemented a series of environmental legislation to restrict the use of N inputs and thereby to minimise N losses. Consequently, management goals in dairy farming have shifted from mainly economic to a combination of economic and environmental.
The general objective of the research prsented in this thesis was to gain insight into the effects of various management decisions at operational, tactical and strategic levels on herbage DM yield and N content per cut, N losses via leaching and N2O and CO2 emissions, using multi-site and many-years experiments, statistical analyses and empirical models. New criteria for environmentally sound N recommendations and tools for operational grassland management were derived, so as to improve the decision making in grassland management on intensive dairy farms.
Relationships between growth time, N application, herbage DM yield and N content and Soil Mineral N (SMN) have been quantified in single cuts during the whole growing season. Using critical levels formarginal N response, herbage N content and unrecovered N, combinations of economically optimum and environmetally sound N applications per cut were established for the complete growing season. Residual effects of the previously applied N may show up as SMN, but results indicate that SMN is not a useful tool for fine-tuning N-application per cut. Growth time per cut has a large effect on herbage DM yield and N use eficiency. It is shown that grazing at a young growth stage leads to low productivity and N use efficiency and to high leaching losses. In practice, grazing at a young stage is probably related to a risk-averse attitude of farmers, which in turn is caused by the lack of accurate data on herbage DM yield and quality during the growing season. A combination of the developed relationships with quick and accurate herbage yield and N content measurements in a 'Dynamic Decision Support System' is suggested to be the way to further improve operational grassland management.