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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.

Environmental pressures posed through human activities are expected to further increase due to growing population numbers and increasing per capita consumption. It will be crucial that the sum of all pressures leaves the planet within sustainability thresholds. The huge challenge for agriculture is to double its food production without further deteriorating the environment, but there is little consensus on how to do this. At the global scale, 'sustainable intensification' is seen as an important strategy. At best, intensification improves the utilization of resources, but it also increases emissions per ha and may go hand in hand with specialisation, increases in the scale of farming and regional concentration. A typical example of a sector characterised by intensification, scale enlargement and regional concentration is the Dutch livestock sector. To consolidate and strengthen the Dutch position as second agricultural exporter in the world, this process is still continuing, linked with constant efforts to further improve economic and environmental efficiencies through farm size enlargement and adoption of additional technologies. However, the industrial and inherently resource-intensive character of this livestock production leaves numerous sustainability issues unaddressed, provoking new questions and controversy in Dutch society. Sound policies start with the acknowledgement of trade-offs between population size, food consumption patterns and land spared for nature. Therefore, a legitimate, but seldom asked question is which part of the total effort needed to feed the human population should be on more production and which part on limiting population growth, changing human diets and global redistribution of wealth.

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