Effects of Stand Density and N Fertilization on the Performance of Maize ( Zea mays L.) Intercropped with Climbing Beans ( Phaseolus vulgaris L.)

Maize is Germany’s most important fodder and energy crop. However, pure maize cultivation has ecological disadvantages. Moreover, its yield is low in crude protein, an important feed quality parameter. Maize–bean intercropping can potentially address both issues. A bean variety specially developed for intercropping was first introduced in 2016. Using this variety, a network of institutions conducted 13 field trials from 2017 to 2020 on four sites in Germany. We sought to determine the effects of stand density and nitrogen (N) fertilization on dry matter yield, crude protein yield, and soil mineral N content (N min ) at harvest of intercropped vs. pure maize. The three intercropping bean densities we tested (7.5, 5.5, and 4 plants/m 2 ) produced non-significantly different yields of dry matter or crude protein, given a maize density of 7.5–8 plants/m 2 . Intercropping was inferior to pure maize in dry matter yield, but non-significantly different in crude protein yield. Under neither cropping strategy were significant losses in dry matter or crude protein yield recorded with reduced compared to full N fertilization. At full fertilization, however, both pure maize systems and the 8/4 maize–bean intercrop system left significantly higher N min at harvest than the other variants of the corresponding system or N fertilization level and thus an increased risk of nitrate leaching. We encourage further optimization of yield performance in maize–bean intercropping, e.g., through breeding or promotion of biological N fixation via rhizobia inoculation. Furthermore, we recommend reducing N fertilization levels in maize cultivation.