Among the challenges of agriculture in the coming decades is the necessity to increase crop yields and crop nutrient use efficiency while reducing the ecological footprint. Biostimulants can provide a solution to reach these goals. The aim of this study was to conduct a field test of the effects of foliar applications of a zeolite-based biostimulant on maize (three applications in 2022) and winter wheat (four applications in 2023) along a gradient of nitrogen fertilisation (50–155 kg N ha–1). Maize silage biomass and wheat straw biomass, grain yields, thousand-kernel weights, and grain nutrient content were measured. Further, measurements of chlorophyll content and normalized difference vegetation index (NDVI) were taken during the growth of the two crops. We observed that the biostimulant increased wheat and maize yields at the lowest doses of nitrogen addition, particularly by increasing the number of grains per m2. The increase in the harvest index seems to indicate an intensification of production. No effect on NDVI was observed, whereas chlorophyll content increased occasionally. The results also indicate an increase in the nitrogen use efficiency of wheat and maize, particularly at the lowest nitrogen fertilisation levels.