Annual variation in weather: its implications for sustainability in the case of optimising nitrogen input in sugar beet

Efficient crop husbandry is crucial in order to prevent unnecessary emissions of environmentally damaging inputs and to maintain economic soundness, but it raises the question of which productivity and efficiency levels should be realised. Agronomists and economists are still debating this issue, as they base their insights on different production functions: economists assume decreasing returns of input use, whereas the response curve used by agronomists is often described as linear with a plateau (LRP). The first objective of this paper is to reconcile these points of view by showing that due to annual variation in weather individual LRP curves turn into a concave function with decreasing returns when aggregated. The second objective is to present empirical evidence on the impact of weather variation on optimal input levels for the case of nitrogen in sugar beet in The Netherlands. Two methods were used: bio-physical simulation and estimation with a regression model using panel data. Optimal crop growth and nitrogen input were simulated with the Water and Agro-chemicals in the soil and Vadose Environment (WAVE) model, using weather data for 43 years. Parameters were assessed with and without additional weather effects. An agronomic crop growth simulation model such as WAVE might underestimate the effect of variation in weather on the agronomic efficiency of nitrogen, because pests, weeds and diseases as well as the effectiveness of biocides, the sowing date and possible harvesting problems are strongly influenced by the weather. To test for this effect, weather indexes that include these indirect weather effects on the occurrence of pests and diseases were derived from panel data by means of a regression model. These weather indexes were used to re-assess the impact of weather variability on the agronomic efficiency of nitrogen in sugar beet cropping. The results of the bio-physical simulation showed that due to annual variation in weather the agronomic efficiency of nitrogen use was reduced by 10% at the current ratio of prices of nitrogen and sugar beet. The variation in weather, including indirect effects, caused the agronomic efficiency to decrease by 13%. This relatively small decrease suggests that at least some of the indirect weather effects might have an opposite effect on the agronomic efficiency of nitrogen use compared with the direct weather effects. This hypothesis seems to be supported by the absence of a significant correlation between the two weather indexes and by the lower estimated variance of the weather indexes based on panel data.