EFFECTS OF N APPLICATION ON AGRONOMIC AND ENVIRONMENTAL PARAMETERS INSILAGE MAIZE PRODUCTION ON SANDY SOILS

The current nitrogen (N) use in silage maize production can lead to co nsiderable N losses to the environment. Maize growers fear that a redu ction of N inputs needed to minimize N losses might depress yields. Th e objective of this study was therefore to quantify: (1) the response of silage maize dry matter (DM) yields to N, (2) the economically opti mal N reserve, and (3) the trade-off between silage maize DM yield and N losses. The indicators of N losses used in this study were the diff erence between N input and N uptake and the post-harvest residual soil mineral N. Regression models were used to fit DM yields and N uptakes of silage maize measured in 25 experiments on sandy soils in the Neth erlands to the sum (SUMN) of the soil mineral N reserve (SMNearly) in March-April, plus mineral N in fertilizer, plus ammonium N in spring-a pplied slurry. The values obtained for the economically optimal SUMN i n the upper 30 and 60 cm of soil were, respectively, 173 and 195 kg N ha(-1), when we assumed that the value of 1 kg fertilizer N equals the value of 5 kg silage DM. The economically optimal SUMN was not signif icantly related to the attainable DM yield. The apparent N recovery (A NR) of maize averaged 53% at the economically optimal SUMN. The ANR ro se considerably, however, when N was applied at lower rates, indicatin g that N losses may be considerably smaller in less intensive maize cr opping. When maize was fertilized at 100 kg N ha(-1) below the economi c optimum, the ANR was 73%, the difference between the mineral N input and the N crop uptake decreased by 57 kg N ha(-1) and the soil minera l N residue at the end of the growing season (0-60 cm) decreased by 24 kg N ha(-1). The associated reduction in DM yield averaged 16%. Ferti lizer prices would have to be as much as four times higher to make mai ze growers spontaneously reduce the application rates by a 100 kg N ha -l, however. It is concluded that adjusting the N input to a level bel ow the economically optimal rate can reduce the risks for N losses to the environment associated with conventional maize production, with a limited effect on silage yields. (C) 1998 Published by Elsevier Scienc e B.V. All rights reserved.