Manures supply nitrogen (N) to crops beyond the year of application. This N must be taken into account for agronomic and environmental reasons. From 2002 to 2006 we conducted a field experiment on a sandy soil in The Netherlands (52°03″N, 6°18″E) to better quantify this residual N effect. Treatments comprised different time series of mineral fertilizer N or cattle manures of different compositions, all applied at a rate of 300 kg total N ha −1 year −1 , whilst compensating for differences in available potassium and phosphorus. Dry matter and N yields of cut grassland responded positively (P<0.05) to both current manure applications and applications in previous years, whereas mineral fertilizer N affected yields in the year of application only. N yields could be reasonably well predicted with a simple N model, adopting an annual relative decomposition rate of the organic N in manure of 0.10-0.33 year −1 during the year of application and 0.10 year −1 in the following years. Subsequent model calculations indicated that the N fertilizer value (NFRV) of injected undigested cattle slurry rises from an observed 51-53% when slurry is applied for the first time, to approximately 70% after 7-10 yearly applications, whereas it took two to four decades of yearly applications to raise the NFRV of surface applied farm yard manure to a similar level from an initial value of 31%. Manures with a relatively high first year NFRV (e.g. anaerobically digested slurry) had a relatively small residual N effect, whereas manures with a low first year NFRV (e.g. farm yard manure) partly compensated for this by showing larger residual effects. Given the long manuring history of most agricultural systems, rethinking the fertilizer value of manure seems justified. The results also imply that the long term consequences of reduced N application rates may be underestimated if manuring histories are insufficiently taken into account.
Dairy farming is one of the main contributors to nitrate leaching to groundwater, particularly on soils that are susceptible to leaching, such as light well-drained sandy soils. In the Netherlands, as in many other European countries, these soils are predominantly used for dairy farming. A prototype dairy farming system that has been implemented in practice in 1989 has continuously been adapted since then to meet environmental standards (i.e. the EU-standard of 50 mg NO 3 À l À1 ) without reducing milk production intensity (11900 kg ha À1 ). After an initial decline in nitrate concentration from 193 mg l À1 to 63 upon implementation, it subsequently 'stabilized' at a level higher than the environmental standard: 55 mg l À1 . The goal of this paper is to examine causes of excessive nitrate leaching. This was done by relating measured nitrate concentrations with management characteristics such as N balances, cropping patterns and grazing intensities. Special attention was paid to aspects that were supposed to be conducive for leaching: crop rotation of grass and maize and grazing. No evidence was found for enhanced nitrate leaching due to the rotation of grass with maize compared to permanent cultivation. This could be ascribed to the reduction in fertilization levels in first and second year maize with 90 and 45 kg N ha À1 , respectively to account for the expected N release from the ploughed-in grass sod. Triticale was found to lead to higher leaching than grass or maize which is attributed to its poor growth in the period that it should function as catch crop in maize. Grazing contributed to a nitrate increase of about 30 mg NO 3 À l À1 on grassland. As grazing management and intensity is already strictly optimized in order to restrict nitrate leaching, this result underpins the need to develop sustainable grazing methods on soil that is susceptible to nitrate leaching.
In the sandy regions of the Netherlands, high nutrient surpluses from dairy farming harm the environment. Government policy aims at reducing nutrient losses to acceptable levels. To explore possibilities and to generate sufficient and accurate information for dairy farmers to reduce surpluses, research was carried out at the experimental dairy farm 'De Marke'. The objective of 'De Marke' is to design and operate a suitable farming system that meets strict environmental standards, taking into account societal objectives with respect to animal welfare, nature and landscape, and economic viability. A dairy farm is characterized as a system with soil, crop, herd and manure as components. Results of animal nutrition, crop yields and nitrogen (N) flows for the period 1993-1998 indicated that intensive farms could attain a N surplus of 158 kg ha-1 • So compared with a 'current average' farm in the middle of the 1990s with the same milk production level (N surplus of 408 kg ha-1 ), a reduction of 62% in N surplus was realized. At 'De Marke', especially the input of purchased feed and chemical fertilizer was much lower. The most important characteristic of the farming system 'De Marke' was the realization of very high N utilization efficiencies in animal nutrition and crop production, allowing a similar milk production but at a much lower input level. With an ammonia volatilization level of 20 kg N ha-1 -which is much lower than the 64 kg ha-1 on the 'current average' farm -the target of 30 kg N ha-1 was attained. Total crop yields (pasture grass, grass silage and silage maize) at 'De Marke' were lower than expected. With a realized N surplus of 156 kg N hll 1 the target of 128 kg N ha-I (including deposition and symbiotic fixation) was not yet attained.
Sandy soils in the Netherlands are mainly used for dairy farming. As a result of intensification of dairy farming in the recent past, valued functions of sandy regions now are threatened by high emissions of nitrogen (N) and phosphorus (P) and by increased water consumption by forage crops. Improved utilization efficiency of resources is proposed as a strategy to realise environmental targets in an economically viable way. Experimental results of the prototype system 'De Marke' indicate that an average intensive commercial farm can halve inputs of fertilizers and feeds at least, without the need to reduce milk yield/ha or to export slurry. Water consumption can be reduced by 13%, increasing groundwater 'production' by 570 msuperscript 3/ha. Nitrate concentration in the upper groundwater decreased from 200 to 50 mg/litre. Changes in soil fertility did not lead to serious agricultural problems but costs of milk production increased by 5%. However, additional costs may be compensated for if the extra groundwater is 'harvested' by water companies, because of high cost of purification of surface water and consumer preference for drinking groundwater instead of river-water. In 1999, the examined strategy of improved resource management was implemented on 12 commercial farms, representing the full range of conditions for dairy farming in the Netherlands.
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