Agronomical and environmental evaluation of a long-term experiment with cattle slurry and supplemental inorganic N applications in silage maize

During 19 years of research on silage maize grown a fertile sandy loam soil in Flanders, we compared the use of only inorganic nitrogen (S−) with a system of an annual application of dairy cattle slurry (S+, corresponding with an average rate of 180 kg N ha−1) supplemented with inorganic fertilizer nitrogen. The objectives were to study the efficiency of slurry-N, to determine the economic optimum of additional fertilizer-N use and to relate this optimum with the local threshold on residual soil nitrate-N. During the last 5 years, the economically optimal fertilizer rates in the S− and the S+ system were centred around 150 and 90 kg N ha−1, respectively. So, saving on fertilizer N amounted to about 60 kg N ha−1. At these N rates, the risk for trespassing the local legal threshold value for residual soil nitrate-N content (90 kg ha−1, 0–90 cm) was small (less than one out of ten seasons); economic and ecological optima of N fertilization concurred well. Meeting the standards of the EU nitrates directive and/or a doubled fertilizer N price would urge to decrease the applied amount of inorganic N by ca. 50 kg ha−1, resulting in yield losses of about 5% on the studied sandy loam soil with an important content of total N (5300 kg ha−1, 0–30 cm). The ratio of N-fertilizer replacement to the total amount of applied slurry-N increased during the experimental period to an average level of 59% during the last 5 years. © 2004 Elsevier B.V. All rights reserved.