A field experiment was conducted in 2022–2023 and repeated in 2023–2024, estimating plant coverages using digital processing of autumn and spring aerial images to determine fertilizer rates. Three fixed and two variable manure and mineral N rates were applied in early spring and after the first cut. Dry matter yield (DMY) and agronomic efficiency (AE) were evaluated over two seasons. A low or variable N rate based on spring coverage led to DMY and AE comparable to high N rates. Autumn coverage in the second season improved slurry application decisions, offering a valuable tool for grassland management.